Chapter 106The North American Thoroughbred
Description of the Sport
King James I of England established horse racing as the Sport of Kings in the early seventeenth century, and the earliest English immigrants brought the sport to the American colonies. Colonials imported fine-blooded horses and staged informal race meets almost as soon as they hit land. Richard Nicolls took command of New Amsterdam, renamed New York, became the first governor of the colony in 1664, and established a racecourse.
Many of the founding fathers of the new country, including George Washington and Thomas Jefferson, raced horses. John Wickham, Aaron Burr’s attorney at his treason trial, lost one of the top American stallions of the emerging Thoroughbred (TB) breed, named Boston, in a card game in 1835. Nathaniel Rives won the difficult 2-year-old colt and ignored advice to geld him. Boston not only won 40 of 45 starts, but he also sired Lexington. A foal of 1850, Lexington became the top racehorse in the country by winning from New York to New Orleans.
Lexington was named for the Kentucky town that became the center of TB breeding. Horsemen quickly realized that healthy horses could be raised on the limestone base of Central Kentucky, and the horse Lexington helped establish the area by becoming the top sire in the United States for 16 years.
Racetracks sprang up in many cities, and although the industry suffered during the Civil War, Saratoga was the exception. The New York social elite took in the waters at the Spa to escape the heat and humidity of the city, attended the races during the day, and partied through the summer evenings in spite of the war. A few years after the conflict, Col. M. Lewis Clark founded the Louisville Jockey Club to conduct race meets in Kentucky. In 1875 the Louisville Jockey Club opened a new racetrack, later called Churchill Downs, and that year Aristides won the first edition of the Kentucky Derby (the Derby).
The Derby did not begin as America’s most famous race. Col. Matt Winn’s genius at marketing in the early twentieth century was required to create the legend the Derby has become. When the filly Regret won the race in 1915, the Derby rose to prominence quickly as a premier American sporting event and a major stop for 3-year-old TBs.
Fans were disappointed when owner Sam Riddle and trainer Louis Feustel deemed the Derby too early in the year for their strapping colt Man o’ War, the horse that had broken many records the previous year, winning 9 of 10 races. However, Man o’ War added to his reputation without the Derby, going through a perfect season of 11 wins in as many races. To many he remains the finest TB racehorse the United States has ever produced.
A new moral climate was sweeping the nation after the turn of the century. Gambling, including on horse racing, was banned in many states as a precursor to Prohibition, which began in 1919. Kentucky was the exception, and horsemen and racing fans found many ingenious ways to skirt the law. The need for more state revenue from the Great Depression in the 1930s prompted the ban on racehorse wagering to be lifted in many states. Until the 1970s horse racing was often the only legalized form of gambling in most states. From the 1930s to the 1960s horse racing, boxing, and baseball were the most popular sports in the United States.
War Admiral, sired by Man o’ War, gained such a following in the 1930s that the whole country listened to the Seabiscuit–War Admiral match race. Because the older Seabiscuit had lost his first 17 races as a 2-year-old, everyone expected War Admiral to win. Yet Seabiscuit, “the people’s horse,” beat War Admiral that day, and in 1938 Seabiscuit was said to have had more newspaper space than Hitler.
As a 3-year-old, War Admiral won the Derby, the Preakness, and the Belmont Stakes, races that became known as the Triple Crown. Only 11 horses have ever won all three races. In 1948 Citation so impressively captured the series that some thought the horse replaced Man o’ War as the greatest racehorse. That argument persisted for the next 25 years, despite the achievements of horses such as Native Dancer, Swaps, Kelso, and Dr. Fager, none of which won the Triple Crown. However, in 1973 a bright chestnut colt, Secretariat, so captured the public imagination when he won the Triple Crown that Time, Newsweek, and Sports Illustrated all put him on the cover. He won the 
-mile Belmont Stakes by a spectacular 31 lengths.
Although the Triple Crown increased in stature, California developed a strong racing circuit. Santa Anita, east of Los Angeles, opened in 1934 and gained immediate attention by offering the then unheard-of sum of $100,000 (U.S.) for the inaugural Santa Anita Handicap. Across town, Hollywood Park began attracting movie stars and established its own fixture, the Hollywood Gold Cup. Bing Crosby and friends built Del Mar about 100 miles south.
As transportation by air became more popular in the last half of the 20th century, horses from the East and West began meeting regularly. In fact, Affirmed, the latest horse to win the Triple Crown, in 1978, prepared for racing at Santa Anita and Hollywood Park.
Regional rivalries became popular, which added to the success of the Breeders’ Cup series, inaugurated at Hollywood Park in 1984. Unlike the Triple Crown, which is solely for 3-year-olds, Breeders’ Cup Day offers races for various divisions. Two-year-olds show what might emerge at the next year’s Kentucky Derby. European horses often invade for the grass events and more recently have achieved notable success on synthetic surfaces. The Breeders’ Cup Classic attracts elite runners on the dirt from coast to coast and from abroad.
The Breeders’ Cup epitomizes the tenacity of the racing TB. Spectacular victories—including the triumph of Sunday Silence over archrival Easy Goer in the 1989 Classic, the victory of Personal Ensign over the Kentucky Derby–winning filly Winning Colors in the 1988 Distaff, and the devastating move Arazi made to capture the 1991 Juvenile after shipping to the United States from France—are legendary. American TBs regularly compete in Europe, Japan, Australia, and Dubai. Not even King James I could have envisioned the Sport of Kings developing into the international phenomenon it is today.
TB racing always has been an expensive sport. Since the introduction of pari-mutuel gambling, financial support and revenue have been derived from two groups: racehorse owners and the gambling public. The third leg of the horse racing industry is the racetrack operator. Racetracks are generally for-profit corporations, with the exception of Del Mar, Keeneland, Oak Tree at Santa Anita, and a few others. Racehorse ownership was once the domain of the wealthy, but now opportunities and partnership options are available for many. In the past, most owners maintained breeding operations to supply horses for their personal racing stables; commercial breeders were few. The focus of breeding was for successful racehorses, and rigorous selection for soundness was as important as racing ability. Commercial breeders have now become a major source of racehorses and have a different goal. They produce horses to satisfy the commercial market, an entirely different objective, often unrelated to producing a racehorse.
Commercial breeders offer horses at public auction, primarily as yearlings but also as weanlings and 2-year-olds in training. The auction market is fashionable and in many ways fickle and trendy. Buyers favor precocious yearlings, relatively more mature than others. Yearlings with sprinting bloodlines and well-muscled yearlings sell better than those with a classic distance pedigree and conformation for all but the most expensive horses. Little is known of soundness of the sire and dam. Although these trends are worrisome, one positive consequence has been the ever-improving quality of horse offered at public auctions. Now some of the best-pedigreed horses in the world are sold at public auction, a fact not true 30 years ago, and even traditional owners and breeders have entered the lucrative commercial market.
The commercial market is a straightforward avenue for owners to buy a high-quality racehorse without maintaining broodmares. The September Keeneland yearling sale offers nearly 5000 yearlings over a 10-day period. Numerous other sales are conducted throughout the year. Presale examination is an important part of veterinary practice at racetracks (see Chapter 99). The auction environment can be an extremely high-pressure setting for the veterinarian because the knockdown price may reach millions of dollars.
Owners can also buy horses in claiming races. In claiming races the prospective owner buys the horse for a predetermined price before the race, and the horse is considered sold once the starting gate opens, regardless of whether the horse wins or even finishes the race. Purses and monies from purchase (claim) go to the previous owner. Potential buyers of claiming horses cannot examine the horse, but they can watch it walk into the saddling paddock. Buying a horse by claiming gives an owner a ready-made racehorse, whereas owners and breeders have to wait at least 3 years before racing a homebred horse, and yearling buyers have to wait at least a year. Unfortunately, the risks in claiming are considerable, because the claiming game has no honor and the new horse may already be at the end of its career.
Racetrack operators have changed to meet demands of the racing fans. The state, racetrack operator, and horsemen, through purse distributions, obtain income from a percentage of the moneys wagered, called handle. Revenue from simulcasting races across the United States and internationally is now an important source of handle, and Internet and at-home wagering are already realities. There was a time in the 1970s when Santa Anita and Hollywood Park had a higher daily average attendance than the Los Angeles Dodgers baseball stadium. On-the-track attendance has dwindled and contributes only a fraction to purses and commissions compared with what it did 40 years ago, when a fan had to come to the track to place a wager. Unfortunately, for reasons the racing industry is now regretting, the revenue sharing from off-track sites is much less than when the same amount of money is wagered on-track. The amount of revenue generated for purses and racetrack operators has not kept up with inflation, thereby placing greater economic stress on all industry participants. The importance of simulcasting to the future of horse racing has led to consolidation of racetrack ownership.
Because track operators emphasize handle, races are run in a manner to encourage betting. Races must be competitive, and the more horses the better. An anathema to the nongambling race fan and many horsemen is that gamblers bet more money on a large field of poor-quality horses than on a small field of high-quality horses. In the United States, field size rarely exceeds 12 horses, because dirt racetracks are relatively narrow and have tight turns, and turf courses, usually inside of the main dirt track, are even narrower with tighter turns.
Races are made to be competitive by restricting which horses are eligible to run in a particular race. Most important are age, gender, distance, and surface (turf or dirt). Sex and age restrictions limit which horses are allowed to compete in a race. Age is a major factor, and 2-year-olds are never raced against older horses, because they would not be competitive. Distaff races are those restricted to fillies and mares, and although the distaff side is not excluded from open races, such races are seldom competitive, except early in the 2-year-old year.
Distance and surface are conditions selected to suit the horse. Another set of conditions, handicaps and allowances, can be confusing but works well. Handicap and allowance races are set up to even the race by varying the weight carried by the horse. Weight variation is a subjective value, determined by racing officials for handicap races. In allowance races, weight variation is determined by a set of published criteria. For example, 3-year-olds carry less weight than older horses, fillies less than colts, and nonwinners may get additional weight off.
Most races in the United States are claiming races. In claiming races, horses of similar ability are pitted together. Because of the risk of losing a horse by claim, owners and trainers are discouraged from running more valuable horses to steal a purse. The claiming aspect of American racing is popular, but it has several negative effects on racing.
Stakes races are for the best horses. Stakes races require an entry fee and some require an additional starting fee. Stakes races are raced according to sex, age, distance, and surface. Stakes races are graded, listed, or restricted. Restricted stakes races, popular in recent years, restrict eligibility to the state of foaling or to conditions similar to allowance races. Stakes races are run even if only one horse is entered, but this is rare and is called a walkover. Stakes can be handicaps, allowances, or weight-for-age races. In weight-for-age races all horses carry the same weight, except for the well-established allowances for age and gender. The Kentucky Derby is a weight-for-age race for 3-year-olds. All horses are assigned the same weight (126 lb [57.3 kg]), but a filly receives a weight allowance and carries 123 lb (55.9 kg). The best stakes races are graded by a national committee and classified as Grade 1, Grade 2, or Grade 3; Grade 1 races are the top races. The Kentucky Derby and Breeders’ Cup Classic are examples of Grade 1 races. The North American graded stakes races are analogous to the European group or pattern races.
Nonstakes races fall into several categories. Maiden races are for horses that have never won a race and can be allowance or claiming races. Races can be restricted to horses that have not won a certain number of lifetime starts. Other specific conditions for races can include eligibility for horses that have not won a race in a certain time period or over a certain distance. Claiming races also are restricted by age, gender, distance, and surface. Although races are devised to maximize competitiveness, the uncertain offering of races for nonstakes horses greatly complicates training and veterinary care, because timing for future races can be difficult to judge.
Stakes races are predominantly for horses 2 and 3 years of age, whereas horses may continue to race in claiming races up to 10 to 12 years of age. Claiming horses may drop progressively in class and value, and the lowest level of TB racing in the United States is considerably below that of the United Kingdom. Much of the following discussion reflects our dealings with high-quality, younger horses. These situations are easier for the veterinarian to be in control and to dictate the diagnostic approach and therapeutic plan, working with the trainer. With low-value claiming horses the trainer is more likely to dictate what is treated and with what. Frequently joints are treated with corticosteroids alone, rather than in combination with hyaluronan. Regardless of level of competition, it is important to establish and maintain a good owner-trainer-veterinarian relationship. This helps ensure the health of the racehorse and the success of the racing venture (JAB).
Conformation
Many mild conformational defects appear to be well tolerated by TB racehorses (MWC, PJM). Mild toe-out and toe-in conformation causes few problems, but horses with moderate or severe toe-out conformation develop interference injuries. Toe-out conformation may predispose the horse to subsolar bruising of the medial heel. The most severe conformational defect is calf-knee or back-at-the-knee conformation. This defect leads directly to carpal lameness. Short, straight pasterns increase concussion, and long, sloping pasterns are undesirable because they may lead to soft tissue injuries or fractures of the proximal sesamoid bones (PSBs). Offset (bench) knee conformation leads to splint disease, injury of the antebrachiocarpal joint, and proximal suspensory desmitis (PSD). Horses that are tied in behind the knee develop tendonitis. Sickle-hocked conformation leads directly to lameness of the distal tarsal region. Horses with straight hindlimbs develop lameness of the stifle region and are prone to upward fixation of the patella.
Track Surface and Lameness
Track surface is an important factor in the development of lameness and frequently is overlooked or neglected in lameness discussions (PJM, MWC) (see Chapter 105). Track surface dictates frequency and type of lameness. Injuries are often blamed on track surfaces, but musculoskeletal injuries are multifactorial. Shoeing, medication, training patterns, commercial breeding, and many other factors have all been implicated in the high injury rates in race-horses, and as often as not injuries cannot be fairly blamed on track surface (RMA, JAB). The most important factor in fatal racing injuries is not the track surface, but the horse (RMA). Almost all horses that develop fatal musculoskeletal racing and training injuries have evidence of preexisting pathology at the site of the fatal injury. They are repetitive stress injuries. Regardless, track surfaces play an important part in injury development, and the veterinarian needs to appreciate the types of injuries that are likely to develop in the track surfaces in his or her practice area.
Surfaces that favor speed also predispose horses to catastrophic breakdowns. Quality of racing surface is much more important than whether the surface is dirt or turf. Hard racetracks with little cushion are the worst surfaces. One of us (MWC) observed 38 horses with bone injury in a 4-week period when horses were training and racing on a hard racetrack. When sand was added to the track and the cushion increased from 6 to 11 cm, over the ensuing 4-week period only four horses developed bone injury. However, race times increased substantially. Horses need not train on hard surfaces to be able to race on them. Training on a forgiving surface then racing on a fast surface is safer than the opposite approach or working on a hard surface constantly. Consistent and uniform surfaces are the safest, allowing horses to remain sound and reducing incidence of injury (JAB). Track surfaces can change dramatically, especially in inclement weather.
Banking around turns can influence how horses negotiate turns and lameness distribution and expression. The hindlimbs of horses negotiating flat turns appear to slip out from under them, and in some instances horses nearly go down.
Track surface can make a difference in how horses with injuries are managed or rehabilitated. Horses rehabilitating after fractures or other bone injuries should not train on hard tracks. Likewise, horses coming back after soft tissue injuries or racing with minor infirmities should not be trained on deep or muddy tracks.
Many European horses that previously trained on grass now race in North America. Most race and continue to train on grass, but some make the switch to training and racing on dirt successfully (see Chapter 107). Our impression is that European horses appear to have fewer forelimb and more hindlimb lameness problems than North American TBs. European horses often train on long straightaways, and this, combined with a forgiving surface (grass), is a situation different from training on dirt in North America. Horses bred to race on the turf appear to have an inherent conformational difference that allows superior performance on grass. Synthetic racing surfaces fall somewhere between dirt and turf relative to horse surface preferences. Turf horses are much more likely to be competitive on synthetic surface, even if they do not handle dirt tracks. Just as there are horses that do well or poorly on dirt or turf, there are horses that handle synthetic surfaces well and horses that do not.
In recent years more attention has been placed on track surfaces than ever before. The major change has been the introduction of synthetic surfaces in North America. The first Breeders’ Cup events run over a synthetic surface were at Oak Tree at Santa Anita in 2008 and 2009. Synthetic surfaces are primarily sand, some fiber material, and a wax or polymer used to coat the sand and bind the material together. Unlike dirt tracks, which are usually cambered to drain horizontally, synthetic tracks are designed to drain vertically and require a drainage system built into the track. The racing results from synthetic surfaces have been mixed. In California, where the racing commission mandated synthetic surfaces by the beginning of 2008, racing fatalities dropped 40% when main track racing fatalities were compared with the same main tracks when they were dirt from 2004 to 2007. The training fatalities at the same tracks have not significantly improved with the synthetic surfaces. The distribution of racing and training injuries resulting in fatalities has changed. There are many more hindlimb injuries with synthetic surfaces compared with dirt (RMA). There are also numerous reports of increases in soft tissue injuries, primarily involving the proximal aspect of the suspensory ligament (SL). Although synthetic surfaces appear promising, the technology is relatively new, and the synthetic surfaces have had installation and maintenance problems.
A better understanding of the relationship between track surfaces and injury has been woefully neglected in the past. A number of research efforts have begun to better characterize the important relationship between track surfaces, biomechanics, and ultimately injuries. A safe racing surface has become of paramount importance to racing as the highly public injuries to Barbaro in the 2006 Preakness and Eight Belles in the 2008 Kentucky Derby have made equine welfare concerns a public debate. Marketing surveys by the racing industry have shown racing fatalities are of major concern to the general public. There can only be increasing emphasis on equine safety in the future.
Medication and Drug Testing Considerations
Horse racing has always regulated the use of medications, but more recently the relatively permissive North American medication regulatory scheme has come under greater scrutiny. Outside North America most racing jurisdictions operate under International Federation of Horseracing Authorities (IFHA) rules. IFHA rules prohibit the use of nonsteroidal antiinflammatory drugs (NSAIDs) within several days of racing, whereas all North American racing jurisdictions allow some use of at least one NSAID the day before racing. Whether the permissive NSAID regulations and other medication policies contribute to the higher racing fatality rates in North America than most other racing jurisdictions using IFHA rules is debatable. Further complicating the issue, racing regulatory veterinarians, the veterinarians responsible for the prerace examinations have begun to question whether North American NSAID regulations are compromising their examinations by masking clinical signs of inflammation. Currently, an approximate 4-mg/kg dose of phenylbutazone about 24 hours before racing is permitted in North America and conforms to the blood level of lower than 5 mcg/mL on raceday.
The policies and regulations governing the use of corticosteroids, particularly intraarticular corticosteroids, are also under scrutiny. Most racetrack veterinarians believe intraarticular corticosteroids are most effective if used 5 to 10 days before racing (RMA, JAB). Unfortunately, changes in the way races are carded today have made planning for races problematic except for stakes races. Trainers cannot confidently plan for races on a certain date and feel forced to wait until the horse is in a race to perform intraarticular injections, sometimes as little as 2 days before the a race. The American Association of Equine Practitioners is recommending that racehorses should not receive intraarticular corticosteroid injection within 5 days of racing. Racing regulatory veterinarians have called for an outright prohibition within 5 to 7 days of racing (RMA). Most corticosteroids are Association of Racing Commissioners International (ARCI) Class 4 drugs, and recent research efforts have been directed toward increasing the sensitivity of drug screening for corticosteroids. Research suggests triamcinolone acetonide and betamethasone have chondroprotective properties when used prudently. On the other hand, one of the most commonly used intraarticular corticosteroids, methylprednisolone acetate, does not fare as well in laboratory studies but is still a preferred intraarticular corticosteroid of many racetrack practitioners because of potency and consistency of clinical response. The pharmacodynamics of intraarticular corticosteroids is just now being studied in horses. With a better understanding of the clinical response to these drugs, combined with advances in laboratory detectability, the way corticosteroids are used may change dramatically within a few years.
Withdrawal time information is not readily available in all jurisdictions for many common therapeutic drugs used for equine lameness. The state-by-state regulation of racing in the United States and the large number of drug-testing laboratories complicates racetrack veterinary practice. Prudence dictates that all veterinarians treating horses intended to race at state-sanctioned racetracks be familiar with the drug testing and medication regulations in that jurisdiction. Resources that may be of use to racetrack veterinarians are the ARCI and the Racing Medication and Testing Consortium (RMTC). Both organizations have websites with information on medication and veterinary practice regulations as well as drug testing.
Lameness Examination
A racetrack clinician has distinct advantages over other practitioners. A racetrack veterinarian is involved intimately in the day-to-day operation of a racing stable and becomes familiar with the normal gait, disposition, and general health care factors of each horse, such as appetite, coat condition, weight, training and racing status, and performance. Numerous examinations can be performed easily if needed. Horses can be examined before and after training to see if lameness worsens or improves.
History
The most important piece of information is the training history. What did the horse do today, yesterday, and the day before? How far is the horse from racing? When was the last fast work or race? Did any problems occur? Did the horse want to train? Did the horse lug in or out? Did the horse cool out normally? When did the trainer or exercise rider notice a problem? When was the last time the horse was shod? Is the horse receiving any medication? NSAIDs and corticosteroids can easily compromise the veterinarian’s ability to properly evaluate lameness (RMA, JAB). The entire veterinary medical history is important for the veterinarian to know, but the past medical history of claiming horses is usually unavailable.
Most fractures occur in horses with a history of a recent hard workout or race. Sometimes lameness may not become apparent until the horse returns to the track several days later, but the injury occurred in the previous workout or race. Humeral stress fractures are the exception, because they usually occur in horses returning to work after not having worked for 45 to 90 days. Horses that lug in (drift toward the inside rail) or lug out (drift toward the outside rail) usually are moving away from the source of pain, but not always. The gallop is a complicated gait, and some horses may drift toward the side of pain while galloping at high speed, perhaps reflecting the lack of power on the affected side (MWR). Most horses in North America finish a race on the right lead. Finishing a race on the left lead may be normal for a horse, but in some horses finishing on the left lead may be a sign of high-speed lameness. A left-lead finish should prompt careful investigation of the left forelimb and right hindlimb as potential sites of lameness (MWR). Many lameness conditions are insidious in onset. Subtle signs, such as a horse that is unusually nervous and prefers to break into the gallop instead of jogging, are easy to overlook. Lameness the day after a shoeing change may implicate a close nail or a drastic change in hoof angle.
The age of the horse is important. Dorsal third metacarpal bone (McIII) disease and other stress-related bone injuries are unusual to diagnose in older horses if they do not have problems at 2 or 3 years of age. Older claiming horses are much more likely to have chronic osteoarthritis (OA), osteochondral fragmentation, or tendon injuries.
Palpation
The physical examination should be routine and complete. Horses are first examined in the stall, where they are comfortable and are more willing to tolerate manipulation. One of us (PJM) feels strongly enough about thorough examination that if trainers want to start with a visual examination while horses are tacked up and proceed directly to injection, they should find another veterinarian. Many lameness conditions are acute, and signs of inflammation are clearly useful. Horses with chronic lameness can be challenging, but long-term association with a horse is of great benefit. Regardless, the examination is the same. Palpation is critically important in evaluating racehorses, possibly the single most important clinical skill a racetrack veterinarian can develop. Very subtle abnormalities or changes from previous examinations can often be the key to developing the most effective diagnostic plan (JAB). With the horse standing squarely, all limbs are palpated for heat, pain, and swelling. During individual flexion of the front fetlock joints and carpi, pain response is important. Pain during carpal flexion almost always indicates a problem in that region. The clinician should remember that during lower limb flexion tests, joints other than the fetlock are being stressed. If concerned, we flex joints independently if possible. Whenever a pain response is elicited, the same palpation technique should be used to compare the response with that of the contralateral limb. There is considerable individual variation from one horse to another, particularly with more subtle responses. All aspects of the PSBs should be examined with the limb in flexion. SL branches, body, and origin and digital flexor tendons are carefully palpated along the entire length. The dorsal aspect of the McIII is palpated by placing the palm of the hand around the tendons and applying firm finger pressure. The forelimb is then brought forward while elevated, and the dorsal cortex of the McIII is palpated with firm thumb pressure. Careful palpation of the dorsal and palmar aspects of the McIII is important in detecting fracture of this bone (MWC).With the palm of the hand on the dorsal aspect of the McIII, the fingers can be used to palpate the proximal aspect of the SL and each splint bone. Care must be taken when examining the dorsal and palmar metacarpal regions, because the clinician may inadvertently cause pain on the dorsal cortex by examining the palmar aspect of the metacarpal region and vice versa (PJM). Proximal palmar metacarpal pain is important to detect, but false-positive results can occur (MWC). Any suspicious response should be evaluated further with ultrasonography (PJM). The carpus is flexed and the dorsal surfaces of the antebrachiocarpal and middle carpal joints are palpated with thumb pressure. An effort should be made to stretch the joint capsule around the borders of each carpal bone, because stretching often elicits pain if a lesion is present. To evaluate the elbow and shoulder regions, the limb is pulled backward and forward. With the limb extended, a jerking, upward motion sometimes causes pain in horses with humeral stress fractures. The shoulder joint and intertubercular (bicipital) bursa should be palpated with firm digital pressure.
The hands should be run lightly over the back to assess for sore or tense muscles. The lumbar and sacroiliac regions, gluteal muscles, and greater trochanter of the femur should be palpated from each side. Although the process appears hazardous, the gluteal muscles and greater trochanter also should be palpated from behind. If necessary a forelimb can be elevated. Firm pressure should be used to detect gluteal myositis or trochanteric bursitis. Standing behind the horse is the best way to compare effusion of the femoropatellar joints. Each hindlimb should be examined in the standing and flexed positions. The Churchill test should be performed bilaterally (see Chapter 6). A positive response suggests distal hock joint pain. Negative findings on palpation and manipulation do not eliminate a joint from consideration, but positive findings often point to the source of pain causing lameness.
Movement
It is mandatory to observe the horse during the first few steps out of the stall. Then the horse is usually walked down the shed row and back. Occasionally the horse is trotted immediately. If a horse appears “stiff” initially, it is useful to walk the horse a couple of times around the shed row to help discern if the lameness is real or if mild stiffness is a normal characteristic of that horse. This is important with older horses that may have chronic problems. Horses with painful conditions do not generally warm out of lameness (JAB). It is important to trot the horse at a comfortable speed for the horse, not the handler, and the horse must be trotted far enough to reach an even speed. The head must be free, without allowing the horse to throw the head. Surface is important, and the harder, the better; soft surfaces may hide lameness. The horse is trotted in a straight line and while circling. Trotting the horse in a circle is the best way to differentiate diagonal or ipsilateral lameness (i.e., left forelimb from right or left hindlimb) and exacerbates many lameness conditions. Horses with tibial stress fractures are much worse while circling. Horses with third carpal bone pain and medial foot lameness are worse with the affected limb on the outside of a circle. Horses with lameness of the fetlock joint are usually worse with the affected limb innermost. Although characteristics of lameness while trotting are important, they are subjective.
At times, observing a horse under tack is useful. Often horses with hindlimb lameness are best examined on the racetrack, because many horses do not use themselves behind. Only when horses are absolutely sound in hand is examination at speed warranted, and even then, with the exception of understanding the complaint of the rider, obtaining useful diagnostic information is rare. However, one of us (PJM) likes to examine horses with obscure lameness on the track, under tack at the trot.
The usefulness of flexion or other manipulative tests is debatable. Lameness may be exacerbated, but disagreement exists over what a positive response means. For instance, when performing fetlock flexion, a positive response is common even in horses with lameness unrelated to the fetlock joint (RMA). One of us (PJM) finds fetlock and lower limb flexion tests worthwhile, but not carpal flexion, and finds hindlimb flexion tests nonspecific. One of us (MWC) finds forelimb flexion tests useful but finds all but the lower limb flexion test in the hindlimbs questionable. Finally, one of us finds the carpal flexion test the most specific and accurate flexion test; horses with a positive response generally have carpal region pain, and there are few false-positive results (MWR; see Chapter 8). Hindlimb flexion tests lack specificity (MWR). Each clinician needs to develop a protocol and be consistent.
Diagnostic Analgesia
Diagnostic analgesia is important to localize sources of pain, and sequential, distal-to-proximal, intraarticular, and perineural blocks should be performed. One of us (PJM) feels strongly that shortcuts, such as skipping immediately to a low palmar or plantar block rather than performing a palmar or plantar digital block first, lead only to misdiagnoses. Another (MWC) feels confident in clinical examination and blocks only when unsure of his diagnosis but admits that when a trainer or owner wants a definitive answer, this approach is unacceptable. However, to diagnose proximal plantar metatarsal pain definitively, a block must be performed. Blocking horses with long winter coats can be difficult. For distal McIII or third metatarsal bone (MtIII) bone disease, one of us (MWC) uses specific palmar or plantar metacarpal or metatarsal analgesia. A novel approach is taken to desensitize the distal 50% to 75% of the McIII. Five milliliters of local anesthetic solution are injected into the nutrient foramen. This block is not specific for bony disease, because the nearby accessory ligament of the deep digital flexor tendon can be affected; however, injury of this ligament is rare.
Whether to clip for intraarticular injections or analgesia is personal preference, but two of us do not clip (MWC, PJM). Although medicating and performing intraarticular analgesia simultaneously is acceptable, one of us (MWC) prefers to wait at least 1 day after blocking to medicate.
Horses may be difficult to handle during analgesic procedures or during intraarticular injections, but usually a lip chain or twitch provides adequate restraint. A good handler must be in control of the horse during the procedure. Gentle, confident, sure hands with painless technique are best (PJM). If sedation is necessary, a combination of 5 mg butorphanol and 200 mg xylazine or 2.5 mg (0.25 mL) detomidine is useful (MWC). However, sedation for diagnostic analgesia should be kept to a minimum. Acepromazine (5 to 10 mg intravenously [IV]) is useful, because the lameness may be exacerbated as a horse trots more quietly. This amount is usually adequate during the analgesic procedure (JAB).
Diagnostic analgesia is often necessary to identify the exact location of pain causing lameness. It is quite useful when two or more limbs or areas are affected. In the forelimb, intraarticular analgesia of the fetlock joint is an excellent and preferred starting point. Palmar disease of the fetlock joint (subchondral bone pain) is common, and if there is a positive response, this block identifies pain in this region. However, if degree of lameness does not change, low palmar or low plantar and palmar metacarpal or plantar metatarsal perineural analgesia (four-point block) should be performed, because subchondral bone pain of the distal aspect of the McIII or the MtIII may not be eliminated using intraarticular analgesia (MWR). Intraarticular analgesia of the fetlock joint will abolish pain associated with an articular fracture of the PSB, pain from which may be abolished using palmar digital analgesia. When the carpus is being blocked, intraarticular analgesia of each joint is useful to determine location of the pain causing lameness. A dorsal approach to the carpus is beneficial to prevent diffusion of local anesthetic solution to the proximal aspect of the SL. It is possible to abolish pain from a fracture of the third carpal bone with subcarpal analgesia. Time of response to intraarticular analgesia is important. A positive response may be seen with an intraarticular problem within minutes. When a positive response to intraarticular analgesia of the middle carpal joint is seen only after 30 minutes, it is important to evaluate the SL. In the hindlimb, a plantar digital nerve block is the preferred starting point.
Approaches and techniques have been well published. A sterile preparation should be performed for intraarticular injection. Gloves are not required but are a personal preference. After injection the limb is bandaged with “foam air” or Vetrap; alcohol and clean cotton bandages are used for the next 12 hours. Concurrent intraarticular medication is often administered but may be more beneficial to the joint if administered 24 hours later.
Diagnostic analgesia is a critical piece of the lameness puzzle. Proper allotment of time is necessary. Consistent interpretation of lameness is essential during the procedure (JAB).
Imaging Considerations
Routine radiographic images often are supplemented with views looking specifically at problem areas of the TB racehorse. For instance, for the metacarpophalangeal and metatarsophalangeal joints, flexed dorsopalmar or dorsoplantar (DPa, DPl), dorsal 35° distal-palmaro(plantaro)proximal oblique, standard horizontal DPa or DPl, lateromedial (LM), flexed LM, and dorsolateral-palmaro(plantaro)medial oblique and dorsomedial-palmaro(plantaro)lateral oblique images should be considered routine. Additional images of the PSBs may be useful in evaluating fractures of the PSBs or palmar erosions of the McIII or the MtIII. The cassette is placed on the ground and a 45° (or 60°) oblique skyline image of each PSB is obtained (JAB). For the carpus the four standard images and a skyline image of each of the proximal and distal rows of carpal bones are routine. A flexed dorsal 45-degree lateral–palmaromedial oblique image should be considered when evaluating a distal radial carpal bone fracture (JAB).
Scintigraphic examination is important, because TB racehorses frequently develop cortical and subchondral stress-related bone injuries. Horses with obscure or undiagnosed lameness and those suspected of having stress-related bone injuries are candidates for scintigraphic examination. Because complete fracture often results from preexisting stress-related bone injuries in cortical and cancellous bone, horses with signs consistent with pelvic, tibial, humeral, and McIII or MtIII stress fractures should undergo scintigraphic examination before continuing in work. However, interpretation is not always straightforward. False-negative results have been found in horses with preexisting tibial stress fractures (PJM) and false-positive results in those with modeling reflecting changes in work intensity. Therefore experienced interpretation is essential, and follow-up examinations are required if findings do not concur with clinical observations.
For ultrasonographic examination, horses are routinely clipped. Ultrasonographic examination is valuable in diagnosing and staging tendonitis, suspensory desmitis, carpal tenosynovitis, lateral branch superficial digital flexor tendonitis, and other swellings of the pastern, swellings of the medial stifle region, and distal sesamoidean desmitis. Digital ultrasonography is the gold standard (JAB).
Shoeing
Trimming and shoeing of the TB racehorse are extremely important. In many practices, veterinarians are not involved in trimming and shoeing, and decisions often are made exclusively by trainers and farriers. The veterinarian should make every effort to work with the farrier regarding shoeing changes when a musculoskeletal problem renders them necessary (JAB). Because recent evidence has linked catastrophic injury to toe grabs on shoes, now a general trend is to see horses shod with low grabs, or none at all. Toe grabs are thought by many to cause forelimb lameness. Hindlimb toe grabs are often associated with hindlimb injuries on synthetic surfaces. There is virtually no slide of the foot on synthetic surfaces, and toe grabs can be a contributor to lameness (JAB). The rationale for using grabs is questionable, because the forelimbs are propelled by the hindlimbs, and traction on the front feet makes little sense. Toe grabs may shorten the cranial phase of the stride, prevent forward sliding of the forelimbs, and cause lameness. One of us (MWC) has observed many horses that are stiff and short in front, and by simply removing the grabs and applying Queens plates (aluminum shoes with a low toe grab; Victory Racing Plate Company, Baltimore, Maryland, United States), lameness abates in 2 to 3 days. Steel shoes are a good alternative when horses resume training after a lay off. Training barefoot is often beneficial for horses that continue to develop sole bruises as a result of track surfaces. Training and racing without shoes may be beneficial for horses with distal hindlimb pain such as that caused by subchondral bone injury of the distal aspect of the MtIII (MWR). Lameness associated with toe grabs appears most commonly on dirt tracks with hard bases and cushions of less than 7.5 cm. For racing on dirt the Queens plates or variations are popular, and for turf the Queens plates and Queens XT shoes are required. Turndown shoes have received negative press, but they may be advantageous in horses with low heels. Slight turndowns without toe grabs on hind feet may actually benefit horses with hock and stifle pain. Many lameness problems may be created or exacerbated by poor shoeing, including PSD, tendonitis, and fractures of the PSBs. One of the major problems is shoeing horses with small shoes. Overzealous use of acrylic can cause severe foot lameness. Horses with thin walls are predisposed to nail bind, and those with sheared heels sustain repeated heel bruising. Glue-on shoes may be beneficial in the short term but can be detrimental if used long term. Using a three-quarter shoe is an alternative for bruised quarters. Cut-out or half aluminum plates are useful for a bruised or sore heel. Accurate diagnosis is essential (JAB).
Shoeing for synthetic racing surfaces is a hotly debated issue. In California many trainers are using Queens plates or Queens XT shoes all around for the synthetic surfaces. Rarely are traction devices used behind because of the concern that the natural slide for hindlimbs on dirt is lost on synthetic surfaces when shod with toe grabs, calks, or other traction devices. Many trainers and farriers keep flat or low toes on the hind feet when horses are training and racing on synthetic surfaces. California changed regulations permitting horses to race barefoot on synthetic surfaces. Although this is still fairly unusual, horses perform quite well without shoes on synthetic tracks. More commonly, horses are trained barefoot, especially the hind feet.
Inability to Make a Diagnosis
In many horses, despite a methodical and time-consuming approach to diagnostic analgesia, the source of pain cannot be identified (MWC). These horses are referred for additional clinical and scintigraphic examinations (MWC, PJM). Horses with stress-related bone injuries may show few clinical signs, and scintigraphy is the imaging modality of choice. Lameness in young horses in training may be more difficult to diagnose than that in horses that are racing (MWC). Many horses with undiagnosed lameness are confined to at least 6 weeks of walking. The advances of ultrasonography, digital radiography, computed tomography, and magnetic resonance imaging (MRI) have made the inability to make a diagnosis a rarity (JAB).
Specific Lameness Conditions
Lameness of the Foot
The most common cause of lameness in the TB racehorse is foot pain. Horses may be observed in the stall to pile bedding under the feet, presumably to cushion the feet or to change angles. Lameness is often worse with the affected limb on the inside of a circle. Heat at the coronary band and increase in digital pulse amplitude are frequent signs. TB racehorses have little hoof. Many are shod at least monthly, but most are shod more frequently, with thin, lightweight shoes. When shoes are changed frequently, little natural hoof is available with which to work if adjustments are necessary. The tendency is for the farriers to remove too much foot, particularly toward the heel, resulting in low heels. Thin feet have prompted the use of acrylics to augment the hoof wall, but overzealous use may itself result in foot soreness (MWC). Soreness and bruising are common, particularly on the medial quarter, just forward of or at the bar. The center of load distribution for a TB racehorse is medial and palmar to the anatomic center of the foot, close to the medial bar.
Most racehorses react positively to the application of hoof testers because only a thin covering of horn protects the sensitive structures of the foot. Degree of sensitivity to hoof testers is important and is learned through experience. Hoof tester examination is best done when an assistant holds the foot (RMA). Hoof testers can be placed carefully on front and hind feet, and slight differences in positioning make a substantial difference. Left and right feet should be compared at least twice before conclusions are reached. The diagnosis of palmar foot pain can be confirmed readily by selective medial or lateral palmar digital analgesia. Horses with a naturally wide gait, or with carpal pain, bruise the medial quarters and can have concomitant foot pain. Acquired bruises of this nature are difficult to treat in horses with wide gaits without lameness and without managing primary lameness in those with carpal pain. In most horses with foot lameness, diagnosis is made with clinical findings and diagnostic analgesia. If abnormalities of the sole or hoof wall cannot be found, radiographic examination is occasionally necessary (PJM). In most horses without fracture, lameness resolves in 2 to 3 weeks with specific therapy and the administration of NSAIDs.
Bruised Heels or Quarters
Lameness caused by bruised heels or quarters is often mild, and horses usually are kept in training. More often than not, little heat is associated with a bruised quarter. Mild dilation of one or both digital arteries and hoof tester sensitivity are the most common signs. The best treatment is rest, however. The foot should be examined for any conformational or shoeing faults; sheared heel or unbalanced feet are common causes of bruised quarters. Horses with toed-out conformation often bruise the medial aspect of the heel (MWC). Corrective shoeing is really the only treatment option. Steel shoes distribute force along the hoof wall evenly and can be applied to horses in training but not racing. Bonded shoes can be applied to horses that are racing. If the foot is warm, the horse should stand in ice water, but if the foot is normal in temperature, the horse should stand in a foot tub of hot water with Epsom salts. A poultice of warm, cooked flaxseed or commercially available products such as Animalintex (3M Animal Care Products, St Paul, Minnesota, United States) should be applied. A common practice of applying hoof-hardening agents such as iodine and turpentine should be avoided, because hard hooves bruise easily.
Abscesses
Foot abscesses are a common problem, particularly during the wet seasons. A foot abscess is the most common eventual diagnosis when a horse is found three-legged in the stall in the morning without a recent history of hard work or racing. TBs are prone to foot abscesses because of the manner and frequency of shoeing. Foot abscesses occur most commonly in the medial quarter, where bruising is also most common. Sole abscesses not involving the quarter are uncommon, take more time to resolve, and cause different clinical signs than do abscessed quarters. Although abscesses in the heel or quarter usually are located easily and precisely with hoof testers, a sole abscess can produce a large area of soreness. Identifying the specific area of pain is often difficult.
Horses with foot abscesses are best managed with hot water baths with Epsom salts and poulticing. The longer the horse stands in hot water with Epsom salts, the better. Ideally, the shoe should be removed, and the abscess should be allowed to open naturally. Usually more damage is done to the hoof wall by attempting to open the abscess manually than by allowing it to open naturally. Probing the white line to identify an abscessed area can be worthwhile. Sole abscesses are more likely to require manual opening and debridement than are abscessed quarters. The integrity of the hoof wall needs to be evaluated and preserved, if possible, but wet, infected areas need to be exposed, opened, and dried. Foot abscesses or infections also can originate from grabbed quarters, which occur frequently in horses that stumble out of the starting gate. When grabbed, the hoof wall of the quarter may separate at the heel bulb or quarter, well down the foot. The opened area becomes contaminated with track dirt, and infection can occur several days to weeks after the original injury. Fortunately a pathway for drainage is already present.
Quarter Cracks
Quarter cracks are common for the same reasons as bruising and abscessation. The quarters and heels take continual pounding, particularly in a TB racehorse with poor hoof wall support. Quarter cracks may be caused by poor hoof balance but most commonly result from innate hoof weakness. Farrier-incurred hoof imbalance can contribute to the development of quarter cracks, but most commonly a single horse in a stable has numerous cracks. Horses with an initial crack are prone to develop subsequent cracks in the same hoof or other hooves. A horse ran successfully with seven quarter crack patches at one time (RMA). If identified early, quarter cracks can be patched readily with acrylic or epoxy resin. The goal is to stabilize the hoof wall and eliminate uneven movement. Mild cracks should be patched if any movement occurs at all. As little patching material as possible should be used to stabilize the hoof wall. If a quarter crack is infected, a drainage path needs to be established. If the quarter crack is infected systemic antibiotics are warranted. An acrylic patch should never be applied until the infection has resolved (JAB). Some epoxy resins are much harder than the natural hoof wall, and trapped infection eventually drains through natural tissues rather than the patch. The incidence of quarter cracks is markedly reduced on synthetic surfaces (JAB).
Osteoarthritis of the Distal Interphalangeal Joint
Most lameness conditions of the foot seen in other sports horses also occur in racing TBs. Considerable controversy exists regarding the frequency of osteoarthritis (OA) of the distal interphalangeal (DIP) joint. Some veterinarians diagnose the problem regularly, whereas others never recognize it. In our experience, OA of the DIP joint is uncommon. Effusion is the most common clinical sign. Osteophytes may be seen on the extensor process of the distal phalanx. Frequently, horses with bilateral OA of the DIP joint have poor performance rather than overt lameness, and lameness is worse at the end of a race (MWC).
Navicular Syndrome
Navicular disease and navicular bursitis are unusual or rare conditions in the TB racehorse (PJM). Lameness can be localized using palmar digital analgesia, but most often horses have bruises, cracks, and abscesses, and those with navicular syndrome are unusual. One of us (MWC) diagnoses navicular bursitis commonly. Rarely, fractures of the navicular bone, bipartite formation, or other anomalous conditions of the navicular bone can occur in young TBs, either in the forelimbs or hindlimbs. Successful management of this type of palmar foot pain has been achieved by injecting the digital cushion with orgotein (Palosein) and Sarapin and methylprednisolone acetate, applying a bar shoe, increasing heel angle, and giving NSAIDs. Intraarticular injection of the DIP joint with hyaluronan and corticosteroids affects numerous structures in the foot and improves the condition of horses with navicular syndrome or OA of the DIP joint.
Distal Phalanx Fractures
Fractures of the distal phalanx occur occasionally. Most fractures are nonarticular palmar process (wing) fractures, and horses do well with the application of a bar shoe. Articular wing fractures are more serious. Extensor process fragments are rare and can be removed arthroscopically.
Palmar digital neurectomy is permitted in most racing jurisdictions, but it must be reported to the race office. Indications for neurectomy are few, but occasionally a horse with a distal phalangeal fracture and chronic pain is a candidate. Occasionally, palmar digital neurectomy is performed in horses with recurrent quarter cracks to eliminate pain, but the procedure does nothing to solve the original problem. Surgical neurectomy should be performed because percutaneous techniques are often ineffective.
Lameness of the Fetlock Joint
The metacarpophalangeal or metatarsophalangeal (fetlock) joint, carpus, and tarsus are the most important joints associated with lameness in racing TBs, especially the fetlock joints. Clinical findings of fetlock joint lameness usually include effusion, heat, a positive response to flexion, and elimination of lameness using intraarticular analgesia. However, in some horses with subchondral bone pain, clinical signs such as effusion are often lacking, and perineural blocks are most effective. The most common lameness conditions of the fetlock can be divided into four categories: synovitis, distal palmar McIII or MtIII disease, fractures, and OA.
Synovitis
Fetlock joint pain associated with effusion, pain on joint manipulation, a positive response to a distal limb flexion test, and moderate lameness is common, but horses generally respond well to intraarticular treatment with hyaluronan and corticosteroids (repeated if necessary), combined with systemic treatment with polysulfated glycosaminoglycans (PSGAGs). Intravenous hyaluronan may be useful in these horses. However, recurrence of clinical signs indicates that the horse should be removed from training, because continued training may result in chronic OA (MWC).The use of autologous conditioned serum (interleukin-1 receptor antagonist, IRAP) has gained popularity for management of synovitis (JAB).
An increase in forelimb tenosynovitis of the digital flexor tendon sheaths (DFTSs) has been noted in horses trained and raced on synthetic surfaces. Lameness is acute. The DFTS is distended and warm to palpation, and there is a marked response to lower limb flexion. Ultrasonographic and radiographic examinations are often normal and unrewarding. These horses typically become sound in 7 to 10 days with the administration of NSAIDs, walking, and the application of a topical poultice (JAB). Firing and blistering may be useful.
Distal Third Metacarpal or Metatarsal Bone Disease
Stress and maladaptive or nonadaptive remodeling, a form of stress-related bone injury, of the distal aspects of the McIII and the MtIII is a common cause of lameness. During training and racing, these areas undergo considerable modeling and remodeling, change shape, and are at risk to develop fracture and cartilage damage. Scintigraphic examination reveals focal areas of increased radiopharmaceutical uptake (IRU) and is the imaging modality of choice for diagnosis, because radiographs are often negative. Ultimately, damage to the distal aspect of the McIII and the MtIII leads to OA and fracture. Early in the course of the disease pain originates from subchondral bone, without effusion or heat. However, intraarticular analgesia is usually effective in partially eliminating pain. Perineural analgesia of the palmar or plantar and palmar metacarpal or plantar metatarsal nerves (low four-point block) or a modification of the diagnostic technique is usually quite successful in abolishing pain. In a hindlimb the lateral plantar metatarsal nerve can be blocked independently of the other three nerves in the conventional low four-point block; in most horses improvement is noted, or a horse will then show contralateral hindlimb lameness. Often contralateral limb lameness is quite prominent and lameness grade can be 1 or 2 degrees higher than in the original limb (MWR). If lateral plantar analgesia produces only partial improvement or if no improvement is noted, the conventional low four-point technique should then be completed. In some horses all fetlock joints are affected simultaneously, and the horse appears sore all over. This disease is often referred to as “bone bruising” but is a form of chronic, stress-related bone injury characterized by the development of dense, painful sclerotic bone, rather than acute, subchondral bone injury from a single-event form of trauma. Distal McIII or MtIII disease is so prevalent that some veterinarians first perform intraarticular analgesia of the fetlock joint rather than beginning with palmar or plantar digital analgesia. As the disease progresses, radiological changes can include flattening of the distal palmar aspect of the McIII or the MtIII, increased radiopacity and radiolucency, and, when severe, large areas of subchondral lucency of the McIII or the MtIII. A form of severe flattening of the distal palmar aspect of the McIII has been recognized and may result in severe OA by the end of the 3-year-old year (MWC). Negative radiological findings do not eliminate this disease as a source of pain, and scintigraphy is more sensitive for its detection. Areas of IRU can represent subchondral bone damage from nonadaptive remodeling or fracture, and a combination of imaging modalities is necessary to differentiate these conditions. Increased radiopacity representing sclerosis of the distal palmar or plantar aspects of the McIII and the MtIII may be visible. Interpretation of sclerosis is subjective and should correlate to scintigraphic findings and clinical signs. Subtle fracture lines do occur in the subchondral bone plate of the distal condyle of the McIII and the MtIII. These become evident on successive radiographs. Surgical fixation of a visible fracture in the subchondral bone plate is the preferred treatment (JAB). Rest, a minimum of 4 months, to allow healing of sclerotic subchondral bone is recommended in horses without fracture, the most common situation. Bisphosphonate therapy has become popular, but there is little evidence to date to support the use of bisphosphonates unless administered in horses that are resting or in a reduced training program (see Chapter 42, page 490). One author (MWR) has used subchondral forage in two TBs with unilateral hindlimb lameness as a result of subchondral bone injury, and although both horses returned to racing, one developed recurrent lameness as a result of the same condition. Recurrence of lameness is a common problem in 40% to 50% of horses with conservative management.
Osteochondral Fragmentation
Osteochondral fragments (chip fractures) of the proximal, dorsal aspect of the proximal phalanx are common in TB racehorses. Fragments are more often medial than lateral, but they can occur biaxially and bilaterally. Fragments are more common in forelimbs but do occur in hindlimbs as well. Often disease of the fetlock joint occurs before fragments are recognized. Subtle radiological changes in the silhouette of the dorsal rim of the proximal phalanx are important and indicate that this area is experiencing stress. The distal dorsal aspect of the McIII at the joint capsule attachment can be injured at the same time that chip fractures are seen.
Once fractures are recognized, arthroscopic surgery should be performed. After surgery the horse can be back in training in as little as 6 weeks if cartilage damage is minimal. Horses with small fragments can be managed conservatively using local ice therapy, NSAIDs, and a reduction in exercise intensity. Inflammation (effusion) subsides within 7 to 10 days, and training can be resumed. Small osteochondral fragments appear to demineralize and no longer irritate the joint. If inflammation and lameness recur, the horse should be taken out of training, and surgery is recommended. Intraarticular injection of corticosteroids and hyaluronan is one option and is effective at reducing inflammation, but if training and racing continue, deep scoring of the articular cartilage of the distal aspect of the McIII occurs and leads to OA. If the goal is long-term racing, surgery should always be recommended.
Osteoarthritis
OA is a common problem and usually is associated with fractures of the proximal phalanx or palmar or plantar McIII or MtIII disease, but it can develop as a primary condition. Arthroscopic examination often reveals considerable articular cartilage scoring and thinning, or complete erosions through the articular cartilage, without any evidence of an associated fracture. Because OA is chronic and cartilage cannot regenerate, the disease must be identified early so preventative measures can be taken. Judicious use of NSAIDs and intramuscular PSGAGs is the best long-term treatment for OA of the fetlock joint. If present, fractures need to be repaired or fragments removed. Young horses should be taken out of training and allowed to mature. Immature horses are particularly prone to overextension injury of the fetlock joint. Some horses may require PSGAGs throughout the racing career. Oral supplements may be beneficial, but intramuscular administration is best. Repeated use of intraarticular corticosteroids exacerbates chronic OA, but whether this is a direct result of the medication or simply allowing a horse to continue in training and racing is unknown. Unfortunately, heat, effusion, positive response to flexion, and lameness resulting from OA need to be treated, because horses are expected to perform. Intraarticular injections are unavoidable, and a combination of hyaluronan and corticosteroids is preferred (RMA). Horses with recurrent synovitis may need additional injections in combination with 2 to 3 weeks of rest (MWC). In spite of information to the contrary, methylprednisolone acetate is the drug of choice, because if used prudently at well spaced intervals, the drug is preferable to repeated injections of shorter-acting corticosteroids (RMA). Intraarticular injections of PSGAGs every 2 weeks are beneficial in horses with chronic OA (PJM). IRAP therapy is gaining popularity for treatment of OA in the fetlock joint. It should be used in conjunction with rest or as a postoperative treatment for better efficacy (JAB).
Other Conditions
Distal McIII or MtIII condylar fractures, PSB fractures, sesamoiditis, and proximal phalangeal fractures are discussed in Chapters 35, 36, and 42. Distal McIII or MtIII condylar fractures often result from maladaptive or nonadaptive remodeling and may be more common in certain racetrack practices than others. One of us (PJM) includes these fractures as a top 10 lameness condition. Prognosis is much better in horses with incomplete fractures, with or without surgery. One author (PJM) recommends surgery in horses with fractures longer than 4 to 5 cm above the joint surface. Surgical fixation may yield a better long-term prognosis than conservative management in horses with short condylar fractures of the McIII and the MtIII since quality of healing may be better and recurrence of the injury less (MWR).
Lameness of the Carpus
The carpus is similar to the fetlock joint in several ways. The third carpal bone undergoes considerable modeling and remodeling with associated subchondral bone pain, similar to the distal palmar or plantar aspect of the McIII or MtIII. Fractures and OA are often the end-stage result of this process of stress-related bone injury. The response of the third carpal bone to stress-related bone injury is similar to that of cortical bone.
Nonadaptive Remodeling of the Third Carpal and Radial Carpal Bones
The third carpal bone and other carpal bones such as the radial carpal bone become sclerotic (model) to withstand the stress of training and racing. The change in the third carpal bone is seen on a skyline radiograph as increased radiopacity of the radial fossa. The mere presence of increased radiopacity does not establish a diagnosis because a certain amount of increased radiopacity reflects a normal, adaptive response. At what point the sclerotic process becomes pathological is not known. Diagnostic analgesia should be used to establish the authentic source of pain. Sclerotic bone eventually becomes painful if the condition of stress remodeling becomes pathological, called maladaptive or nonadaptive remodeling. If training and racing continue, bone loss, lysis, or fracture occurs. Third carpal bone subchondral bone pain is common, particularly in young horses, but because pain involves bone, no treatment other than reducing training and racing intensity is available.
Diagnosis of third carpal bone subchondral bone pain can be challenging, because many young horses do not develop effusion. Heat may be present over the dorsal aspect of the carpus, and horses move wide and tend to abduct the limb during advancement (MWC). Response to flexion varies, but in horses with subchondral bone pain, response is often negative. Diagnostic analgesia is essential for diagnosis. Careful interpretation of the results of diagnostic analgesia is necessary because inadvertently blocking the proximal palmar metacarpal region with middle carpal analgesia and vice versa is easy (MWC). The middle carpal and antebrachiocarpal joints always should be blocked separately. Radiological changes of increased radiopacity and later radiolucency can be seen with good-quality skyline images, but they are missed easily with poorly positioned and exposed images. To assess radiographic exposure, the veterinarian should evaluate the second and fourth carpal bones. These bones rarely have alterations in trabecular pattern, and both bones should be clearly visible on the skyline image. Scintigraphy is an excellent tool with which to diagnose stress-related bone injuries of the third carpal bone and other carpal bones, and although not needed in many horses, scintigraphy assists in early diagnosis and helps convince skeptical trainers and owners. Sclerosis of the third carpal bone changes the mechanical properties of bone, and sclerotic bone is brittle and at risk of fracturing. Few osteochondral fragments or small (chip) or large (slab) fractures of the third carpal bone occur in normal bone. At the time of arthroscopic surgery to repair a third carpal bone frontal slab fracture, a wedge-shaped piece of sclerotic and often necrotic bone often can be found between the fracture fragment and parent third carpal bone.
Once painful sclerosis of the third carpal bone develops, treatment is difficult. Rest may not help remodel dense bone, so prevention is important. The training programs of young TBs with evidence of third carpal bone sclerosis should be modified, but trainers are often unwilling. Although little evidence supports using isoxsuprine or aspirin to improve blood flow, some veterinarians prescribe these medications. Bisphosphonate therapy has become popular but there is little conclusive evidence regarding efficacy. Extracorporeal shock wave therapy (ESWT) is useful in treating horses with third carpal bone remodeling and pain. The benefits of therapy will be achieved only in conjunction with rest or a substantial reduction in training. There will be no radiological changes; however, there will be a reduction in pain when full training resumes (JAB).
Carpal Fractures
Osteochondral fragmentation of the carpus is a common problem. Clinical signs are unique for each fracture type, and radiographic examination is most important. Horses with distal radial carpal bone fractures exhibit pain on direct palpation of the fracture site and often have effusion of the middle carpal joint. Horses with third carpal bone pain and fractures are unwilling to rock on the limb, evaluated by picking up the contralateral limb and forcing the horse to rock on the affected limb by putting side-to-side pressure against the horse’s body. Horses with distal, lateral radius fractures generally have pain on direct palpation. Horses with proximal intermediate carpal and radial carpal bone fractures may be surprisingly lame, without obvious palpable abnormalities.
A rare fracture, but one to keep in mind, is a sagittal fracture of the proximal aspect of the intermediate carpal bone (MWC). Horses train and race but come back to the barn obviously lame (grade 3 of 5). When walking and jogging, they exhibit a stiff-legged gait and obvious abduction during advancement. Horses respond positively to carpal flexion, and lameness is abolished using antebrachiocarpal analgesia. Intraarticular treatment does not improve lameness. The skyline image of the proximal row of carpal bones is mandatory for diagnosis, because fractures will be missed on other views. Early arthroscopic surgery is recommended, because OA can develop if horses are continued in work with a fracture.
Arthroscopic surgery has improved the management of horses with carpal joint lameness greatly. Removal of small osteochondral fragments and repair of large fragments substantially reduces the severity of OA. PSGAGs can be useful in limiting the development of OA, particularly if pain is originating from the third carpal bone. After surgery in horses with carpal osteochondral fragmentation, one of us (MWC) recommends hyaluronan injection 2 to 3 weeks later. Horses with fractures of the middle carpal joint need additional rest compared with horses with antebrachiocarpal joint fractures (MWC). One of us (PJM) prefers to manage horses with incomplete or nondisplaced third carpal bone slab fractures conservatively by giving 60 days of rest and reevaluating the horse. The prognosis for TB racehorses with displaced third carpal bone slab fractures even with surgery is only guarded. Horses with fractures extending into the weight-bearing surface of carpal bones more than 0.5 cm have a poor prognosis. Horses with recurrent and chronic OA appear to benefit from topical blisters and 3 or more months of rest.
Osteoarthritis
In young horses with early signs of OA, rest is recommended. In horses with chronic OA that are able to race, intraarticular injection of a combination of hyaluronan and PSGAG produces better results than does injection of either product alone (MWC). In horses with mild or moderate lameness, intraarticular injection of hyaluronan and triamcinolone acetonide a minimum of 5 to 7 days before racing (depending on racing commission rules) decreases clinical signs for 6 to 8 weeks. Most horses exhibiting carpal lameness appear to benefit from the intramuscular administration of PSGAG once every 4 days for seven or eight treatments, and intravenous injection of hyaluronan appears beneficial before racing.
Rest should be recommended for young horses with early OA even though recurrence of signs is common. NSAIDs are useful in managing pain in horses with OA of the carpus.
Intercarpal Ligament Damage
Because the complex carpal joint is highly dynamic and moveable, fractures and soft tissue damage can lead to OA. Although fractures are more common, injury of the medial palmar intercarpal ligament has been recognized (see Chapter 38).
Another rarely seen lameness of the carpus is related to an osteochondroma of the distal aspect of the radius (see Chapter 75). Rarely, exostoses of the caudal perimeter of the distal radial physes are seen. These horses are acutely lame after exercise and sound within a few hours. Marked carpal sheath swelling and resistance to carpal flexion are usually obvious. Radiologically, there is an osteochondroma proximal and medial to the distal radial physis. There is usually marked carpal sheath effusion seen on ultrasonographic examination. The osteochondroma structure and a lesion in the DDFT are often evident. Surgery to remove the osteochondroma or exostoses is the treatment of choice. Prognosis depends on the degree of damage to the DDFT and the tendency for the osteochondroma to reappear (JAB).
Suspensory Desmitis
Proximal Suspensory Desmitis
Suspensory desmitis is a very important cause of lameness in the TB racehorse and previously has been underestimated. Suspensory desmitis is much more common in forelimbs than in hindlimbs (PJM). Hindlimb PSD is difficult to diagnose because horses have a stiff, hopping-type gait similar to that in horses with tibial stress fractures, and heat and swelling are unusual (MWC). Lameness in horses with forelimb PSD is similar to that in horses with carpal disease. Horses travel wide, with a shortened cranial phase of the stride, and are worse with the limb on the outside of a circle. Conformational faults such as offset knees and upright pasterns and fetlock joints predispose horses to PSD. PSD is most commonly a 2-year-old lameness condition, perhaps caused by immaturity, and can be diagnosed by manual palpation in some horses, but that is the exception rather than the rule (MWC). An increase in occurrence of PSD has been noted in horses that race and train on synthetic surfaces. This may be related to the inability of the foot to slide, causing increased stress on the SL (JAB). More often, horses with PSD require diagnostic analgesia, and one author (RMA) uses the lateral palmar block, the site for which is just below the accessory carpal bone. With the lateral palmar block it is unlikely to inadvertently cause analgesia of the carpometacarpal and middle carpal joints. In many horses, lameness thought to originate from PSD, after a positive response to high palmar analgesia, was later found to originate from the carpus (MWC).
Considerable variation occurs in the ultrasonographic appearance of the proximal aspect of normal and diseased SLs. Overt tears are easy to diagnose. Chronic tears have an abnormal ultrasonographic appearance despite absence of clinical signs (JAB). Hypoechoic areas can be identified in sound horses. Furthermore, bony pathological conditions of the proximal, palmar aspect of the McIII can be seen in horses without obvious tearing of the SL. Avulsion fractures of the McIII are diagnosed in horses that develop acute, severe lameness abolished by lateral palmar analgesia. Scintigraphy is the best way to diagnose bony injury of the proximal aspect of the McIII in horses without ultrasonographic evidence of PSD and negative radiological findings. Avulsion fracture of the McIII is associated with focal, intense IRU, but many horses have diffuse IRU of the proximal aspect of the McIII. Whether these horses have chronic avulsion injury or whether this represents stress reaction or other injury of the proximal aspect of the McIII is unknown. Regardless, horses with bony injury of the proximal, palmar aspect of the McIII have a good prognosis if given a minimum of 120 days of rest. Horses with PSD often can be managed and allowed to continue racing. Local infiltration with corticosteroids or other products is of questionable benefit, and systemic therapy is more effective and rewarding (RMA). Others disagree and recommend local injection with triamcinolone acetonide in horses with mild PSD (PJM). The combination of Sarapin, methylprednisolone acetate, and in some horses Palosein has been successful, and 2-year-old horses may never take another lame step (MWC). Systemic corticosteroids and NSAIDs reduce inflammation enough to relieve the lameness. Where regulations permit, phenylbutazone and triamcinolone acetonide (12 to 18 mg intramuscularly) can be administered 3 to 5 days before racing. A good alternative is two Naquasone (a combination of the diuretic trichlormethiazide [200 mg] and dexamethasone [5 mg]) 48 hours before racing, if permitted by the rules of racing. Triamcinolone acetonide lasts longer, but Naquasone contains a diuretic that is also useful for reducing swelling. An ice boot or an ice tub is beneficial. Platelet-rich plasma (PRP) may be beneficial for treating horses with lesions of the SL. A single treatment or treatment in combination with autogenous stem cells appears promising, when combined with removal from training for 4 to 6 months. Prognosis is good. ESWT is also useful in horses with PSD. Repeated treatments every 10 days for 6 weeks is ideal. The horse should be jogging during the treatment (JAB).
Horses in which lameness is mild when trotted in hand or even under tack but that refuse to give a full effort toward the end of a race should be given rest.
Corrective shoeing can make a difference in horses with PSD. In fact, PSD may be secondary to lameness of the foot (MWC). The toe should be shortened, and the shoe should have only minimal toe grabs. Horses with PSD do not develop catastrophic disruption of the suspensory apparatus but return from a race sore.
Suspensory Branch Desmitis
Suspensory branch desmitis is a different situation and can easily lead to catastrophic breakdown if horses are managed improperly. Early detection is critical because suspensory branch desmitis is a predisposing factor for condylar fractures of the McIII. If there is any detectable change or swelling of a suspensory branch, an ultrasonographic examination should be performed. Subtle clinical changes can predispose the horse to catastrophic breakdown and must be identified early (JAB). Management of suspensory branch desmitis with infiltration of corticosteroids in a TB racehorse is a high-risk treatment, with no therapeutic benefit to the horse. Systemic antiinflammatory therapy similar to the approach described for PSD is useful for horses with suspensory branch desmitis, but the horse should not be raced if the SL remains hot and painful. Suspensory branch desmitis is a dangerous injury.
In horses with suspensory branch desmitis the splint bones should be evaluated carefully. Radiographs should be obtained to check for fracture, flaring, or thickening of the second and fourth metacarpal bones. A clear relationship exists between suspensory branch desmitis and pathological conditions of the splint bone. Splint ostectomy is recommended in horses with fractured, flared, or thickened splint bones, not just in those with fracture.
Bucked Shins: Dorsal Metacarpal Disease
Bucked shins or dorsal metacarpal disease (periostitis) is common but readily manageable (see Chapter 102). This condition is simply the mismatching of exercise with modeling and remodeling of the McIII necessary for the bone to withstand the rigors of training and racing. Bucked shins are most commonly seen in young horses early in training. The simplest and best treatment is to modify the training program to match the horse. With bucked shins, horses are usually only mildly lame after exercise, unilaterally or bilaterally (PJM). Bucked shins are common in young horses with other primary lameness conditions, particularly of the hindlimbs (MWC). Heat and pain over the dorsal cortex of the McIII are commonly found, but care must be taken not to place pressure on the palmar metacarpal structures while palpating the dorsal cortex (PJM). One of us (MWC) feels that careful palpation of the palmar cortex is useful in identifying McIII disease. Clinical signs are usually sufficient for diagnosis, but low or high palmar nerve blocks, infiltration of the nutrient foramen of the McIII, or local infiltration of local anesthetic solution occasionally is required (MWC). Radiographs reveal a typical modeling response, but in some horses with fracture, computed or digital radiographs should be obtained, because fractures may be missed on conventional films (PJM). Scintigraphic examination can be useful, mostly in horses with dorsal cortical fracture (MWC).
Great strides have been made in changing training techniques to manage TB racehorses with bucked shins (see Chapter 102). This requires close monitoring of the dorsal cortex of the McIII. Overall, the incidence of authentic dorsal cortical fractures has decreased greatly. Although all trainers have horses with bucked shins, those that still have major problems train in the old-fashioned style. There is a noticeable reduction of bucked shins in horses racing and training on synthetic surfaces (JAB).
Once inflammation is noticed, ice, NSAIDs, and topical cooling muds, such as Uptite poultice (Uptite, Lawrence, Massachusetts, United States), are often all that is necessary if training intensity is reduced and modified. Once the periosteum becomes hot, thickened, and sore over a large portion of the dorsal cortex of the McIII, modifying training is usually too late. Training should be stopped, and when necessary thermocautery may still be the best treatment, once inflammation has subsided (RMA, PJM, MWC). When done properly thermocautery is humane and effective. An old-time racetrack practitioner was asked a few years ago if he still fires horses, and if so, why? His response was simple: “I can tell you in two words: It works.” The real problem with thermocautery is perception. Most persons who adamantly are opposed to thermocautery generally have no experience with the procedure. Regardless, alternative treatments are available, but none in one author’s opinion work as well as thermocautery (RMA). All treatments require extended periods of rest or light training. Others prefer a relatively new technique called periosteal scratching. A 16-gauge needle is used to create linear incisions of the periosteum using aseptic technique. This procedure, as with thermocautery, is combined with rest and is done after the inflammation has subsided (PJM). Prognosis after periosteal scratching is estimated to be 70% (MWC). ESWT is an effective therapy for bucked shins. The success of treatment relies on training modification. Treatment is performed every 10 days for 3 to 5 treatments. During that time the horse should only walk and jog (JAB).
Bucked shins can progress to dorsal cortical (stress) fracture if horses with periostitis are forced to continue training. Stress fractures occur most commonly on the dorsolateral aspect of the McIII, but those located in the distal third to distal fourth of the McIII are most problematic. Fractures in this location under the extensor tendons can be difficult to palpate, but they may be extensive.
Horses with bucked shins should not be lame at the jog 1 to 2 days after the last workout, but if lameness persists, a dorsal cortical fracture should be suspected. Radiographs should be obtained. Success in treating horses with dorsal cortical fracture relates directly to the duration of fracture. Horses with acute fractures are treated more easily than those with chronic fractures. In horses with acute fractures, rest, with or without thermocautery, is successful. Chronic dorsal cortical fractures are essentially nonunions and heal slowly or not at all and require surgery. Cortical drilling (forage, osteostixis) using numerous small-diameter holes drilled across the fracture line is effective and in one author’s opinion is the treatment of choice (RMA). Rarely a cortex bone screw placed in lag fashion or as a positional screw is needed. The question of whether compression using lag screw technique is necessary or if screws are best placed as set or positional screws remains unanswered. Implants must be removed before training begins. ESWT in combination with rest appears promising for the management of dorsal cortical fractures of the McIII (MWR).
Superficial Digital Flexor Tendonitis
Superficial digital flexor tendonitis usually is advanced before lameness is observed and diagnosis is made. Lameness is one of the last clinical signs observed. Prognosis for racing is poor when lameness is present (JAB). Superficial digital flexor tendonitis is usually a career-threatening or career-ending injury, and estimates indicate that superficial digital flexor tendonitis is the single most common injury ending a TB racehorse’s career. However, many horses with superficial digital flexor tendonitis return to racing or, on occasion, race through the injury. One author (RMA) feels strongly that local infiltration of corticosteroids and continued racing are contraindicated. Intralesional injection of corticosteroids can cause tendon necrosis and result in complete rupture of the superficial digital flexor tendon (SDFT). Some horses may be able to race after treatment with systemic corticosteroids, NSAIDs, and ice therapy. The SDFT will set up on occasion, but that is certainly an exception, and horses fall in value.
Recognizing superficial digital flexor tendonitis before overt fiber tearing occurs has distinct advantages. To recognize tendonitis early, horses must be monitored closely, and ultrasonographic examination must be performed at the earliest indication of inflammation. Heat and swelling are early clinical signs but may not indicate necessarily that the tendon is involved, because peritendonous injury can cause similar signs. The key is pain on palpation of the suspect tendon. Pain, even without obvious heat or swelling, is an important clinical finding. Ultrasonographic examination must include cross-sectional area measurements, because subtle enlargement of the SDFT precedes fiber tearing. Once early superficial digital flexor tendonitis has been recognized, horses should be given rest, or at least the training intensity should be reduced.
One author (RMA) prefers to manage horses with core lesions of the SDFT with tenoplasty (tendon splitting). The surgery can be done with the horse standing and is inexpensive, and aftercare is minimal. To perform tendon splitting, a core lesion must be present, and the earlier the procedure is done, the better. A double-edged tenotome is used. The tendon fibers are split longitudinally with five rows of about 15 percutaneous stab incisions each. After surgery horses are surprisingly sound. Horses with superficial tearing along the edge of a SDFT have a poor prognosis with any treatment and may be the only candidates for peritendonous injection of corticosteroids, provided the horse is removed from training and rested. PRP and stem cell therapy appear promising in treating SDFT core and peripheral lesions. Ongoing research is needed to determine long-term efficacy. Lesions are typically resolved ultrasonographically at 60 days. Rehabilitation is critical to success, and the horse should not have a saddle on its back for 8 to 10 months. ESWT has been unrewarding in getting horses back to racing after an acute tendon injury. It does, however, have some merit in treating horses with chronic tendonitis (JAB). One author prefers the use of surgical management (MWR; see Chapter 69). A combination of desmotomy of the accessory ligament of the superficial flexor muscle (superior check ligament) and tenoplasty is recommended. Prognosis in the TB racehorse, however, is no higher than 50% to 60%, and recurrence of tendonitis is common.
Horses with superficial digital flexor tendonitis should have as much time off as possible. However, if owners and trainers are unwilling to give 6 to 8 months of rest before the horse returns to training, any treatment is a waste of time. One of us (PJM) has stopped recommending any form of surgery or injections and simply recommends walking rest for 4 to 16 weeks. A blister is applied at least twice, and tendon healing is monitored with ultrasonography. Even with this approach, recurrence rate is 60% to 80%. The trainer plays a major role in rehabilitating horses with superficial digital flexor tendonitis. Those that are successful give the horse the most time off and are patient getting the horse fit before starting hard work.
Tibial Stress Fractures
Tibial stress fractures are the most common cause of acute, pronounced, unilateral hindlimb lameness in the TB racehorse. Lameness from tibial stress fractures usually is recognized after a hard workout or after breaking from the gate. Lameness can be severe, and the horse may not bear weight on the limb. Tibial stress fractures are usually unilateral, but bilateral fractures can cause unusual hindlimb lameness (PJM). Similar to humeral stress fractures, tibial stress fractures can occur after a horse has had 60 to 90 days of training, often after a period of rest for an unrelated cause. Tibial stress fractures can occur anytime during training and racing, and the association between a layup period and the early development of a tibial stress fracture is not as well established as for horses with humeral stress fractures (MWR). Two of us (RMA, PJM) do not find manual palpation of the tibia useful diagnostically, but one of us (MWC) finds deep palpation and tibial percussion helpful. Scintigraphy is the diagnostic modality of choice. Less than 50% of horses diagnosed with tibial stress fractures using scintigraphy have radiologically apparent fractures, even if follow-up examination is performed after 2 weeks. If scintigraphy is unavailable, even small areas of periosteal, endosteal, or cortical change should be considered important, if clinical signs suggest tibial stress fracture. Tibial stress fractures are located most commonly on the caudolateral cortex, proximally or midshaft, but can occur caudolaterally and caudomedially distally. Bilateral tibial stress fractures are occasionally seen, even in horses with unilateral hindlimb lameness. One of us (PJM) has examined a number of horses with clinical signs consistent with tibial stress fracture but in which scintigraphic examination findings were negative. Horses returned to the track and developed complete tibial fractures. Now this author (PJM) recommends reexamination in 3 weeks. False-negative results are unusual in horses with stress-related bone injuries unless fractures are located medially and only lateral scintigraphic images are obtained (MWR). Horses that are trained on synthetic surfaces with hindlimb toe grabs appear to have an increased incidence of tibial stress fractures. Whether synthetic surfaces increase the incidence of tibial stress fracture as compared with dirt surfaces is unclear and will not be answered without clinical research (JAB). A recent trend is for practitioners to take digital radiographic images of a tibia in a TB racehorse suspected of having a tibial stress fracture in lieu of obtaining a scintigraphic examination. There are many normal irregularities of the cranial tibial cortex and care must be taken not to make an erroneous diagnosis; the practitioner should keep in mind the common distribution of tibial stress fractures (see above). One of the authors (MWR) has never recognized an authentic tibial stress fracture involving the cranial cortex.
Horses with tibial stress fractures are given 90 to 120 days of rest. The prognosis is excellent. The crus is well muscled and has a good blood supply, and callus formation of the tibia does not interfere with nearby structures. Complications develop when horses have spiral fractures and severe lameness or when horses with unrecognized tibial stress fractures are trained or raced. Catastrophic failure can result. Tibial stress fractures can recur if horses are given inadequate time for healing, but this is otherwise unusual.
Some horses with only mild lameness at the time of initial diagnosis and in which lameness rapidly resolved can be kept in light work. In these horses scintigraphic examination reveals mild IRU, and radiographs are negative. Horses are given a minimum of 60 days without hard work, but if lameness recurs, additional rest for 90 to 120 days is recommended. Aspirin at 60 grains orally (PO) sid is thought to be of benefit during rest (JAB). When the horse becomes sound, light training may begin (JAB).
Tibial stress fractures were misdiagnosed as stifle lameness for years at some racetracks, and the same error is still made today. Even with the introduction of high-quality nonportable radiography equipment at referral centers, the diagnosis of a tibial stress fracture has been difficult to make because radiographs are often negative and clinicians have to isolate pain, a difficult task in many TB racehorses. Scintigraphy has taught us that tibial stress fracture is an important and frequent cause of hindlimb lameness in TB racehorses.
Distal Hock Joint Pain
Distal hock joint pain, or distal tarsitis, is a common cause of hindlimb lameness. One of us (MWC) feels strongly that the hock joint is the most important articular structure in equine locomotion. Compensatory lameness caused by primary distal hock joint pain is important. The skeleton of the TB foundation sire Eclipse at the Racing Museum in Newmarket and the skeleton from an extinct breed from the La Brea Tar Pits in Southern California reveal extensive osteoarthritic changes. Radiological changes in the tarsometatarsal joint of young horses are common and can be found in yearlings before training begins. The importance of osteophyte formation involving the dorsal proximal aspect of the MtIII, often called juvenile spavin, is difficult to evaluate without signs of lameness.
Horses with distal hock joint pain have a typical gait. The hindlimbs travel close together and may even cross midline. Horses with bilateral lameness may not show overt lameness, but they are observed to be not right behind (PJM). This has been described as a “bicycling” gait because the hindlimbs appear to mimic the action of peddling a bicycle (JAB). However, this gait is not pathognomonic for distal hock joint pain, and diagnostic analgesia should always be used to confirm the source of pain causing lameness. Some horses may break from the gait slowly, may not switch leads properly behind, or may crossfire and develop abrasions on the medial aspect of the tarsus (MWC). Horses often wear the lateral aspect of the toe of the shoe. Horses that have a positive Churchill test response likely have distal hock joint pain involving the tarsometatarsal joint (RMA). Some clinicians place emphasis on upper limb flexion, and certainly the response rate is high, but this test lacks specificity. Diagnostic analgesia should be performed to localize pain causing lameness to the distal hock joints. Distal hock joint pain can cause lameness without radiological changes, particularly in young horses, but radiological changes can be present in horses with pain originating elsewhere. Involvement of the centrodistal (distal intertarsal) joint is difficult to assess without radiological and scintigraphic examinations. Nuclear scintigraphy is useful to identify slab fractures of the third and central tarsal bones. Oddly, horses with radiologically evident periarticular osteophyte formation may show little IRU.
The simplest and best treatment is intraarticular injection of methylprednisolone acetate (100 mg) into each affected joint. The centrodistal and tarsometatarsal joints are small, and injecting more than 3 mL of medication without encountering resistance is often difficult. Using hyaluronan gives little advantage, because the joints are low-motion joints (RMA). However, two of us (PJM, MWC) use hyaluronan (20 mg) and methylprednisolone acetate (40 mg), and one of us (MWC) feels strongly that the combination of hyaluronan and corticosteroids prolongs the racing careers of affected horses by delaying progression of OA. Rarely the cunean bursa is injected. There appear to be few complications following repeated injections of corticosteroids into low-motion joints, such as the centrodistal and tarsometatarsal joints. If the diagnosis is correct, the response to therapy is rewarding. However, lameness recurring rapidly after one or two injections may indicate that the primary source of pain is elsewhere or that subchondral pain is a substantial component of lameness. Corrective shoeing may help horses with distal hock joint pain. A high hoof angle (≥54 degrees) and the application of slight turndown shoes may help horses push off and may prevent sliding. In horses with primary, chronic distal hock joint pain, turnout or complete rest actually may worsen lameness, and better results are achieved in these horses by maintaining a low level of training (MWC).
The tarsocrural joint usually is not involved, but if effusion exists, one author (RMA) injects the joint with hyaluronan and corticosteroids, because the joint is a high-motion joint. If dark synovial fluid is obtained during arthrocentesis of the tarsocrural joint, cartilage damage should be suspected (MWC). Repeated injections of corticosteroids into the tarsocrural joint should be avoided.
Myositis
Exertional rhabdomyolysis (ER) and nonspecific localized myositis frequently are overlooked sources of lameness in TB racehorses. ER is a common problem, especially in young horses, and primarily is seen in fillies. Male horses can be affected, but much less frequently. Diagnosis usually is not a challenge if a veterinarian observes a hot, sweaty horse with obvious muscle cramps. Lameness is mostly bilateral, is transient after exercise, and resolves within a few hours. A horse with ER should be sound at the trot even when lameness is present at the walk. If ER is suspected and the horse jogs lame, a lameness examination should be performed (JAB). Some horses have unilateral lameness with ER. Colts often develop unilateral forelimb ER, resembling other causes of upper limb pain. Horses may have generalized whole body stiffness if ER is unrecognized and untreated. Elevated serum levels of creatine kinase and aspartate transaminase are necessary to diagnose ER definitively. Horses at risk are given acetylpromazine (5 to 15 mg IV) before exercise to prevent ER. Dantrolene (500 mg PO) given 4 hours before exercise is used in horses that have had serious episodes of ER and have missed training.
Interestingly, some horses are found to have IRU during bone (delayed) phase scintigraphic imaging, and serum muscle enzyme levels may be normal or only mildly elevated. In some horses a history of previous ER is known, but others have no known history of ER. Large muscles or individual muscle bellies within muscles can be affected. Lameness associated with IRU in skeletal muscles may or may not be seen, depending on the muscles involved. These horses typically require rest for a period of 6 weeks (JAB). Muscles of the hindlimb most commonly are affected, such as the gluteal, biceps femoris, and semitendinosus muscles, but forelimb or trunk muscles can be involved, rarely.
Gluteal myositis occurs in TB racehorses. Lameness is characterized by a reluctance to reach forward during the cranial phase of the stride and to push off to extend the hindlimbs at the end of the stride. Lameness may result from a loose, cuppy racetrack that breaks away from the horse. Whether the trochanteric bursa is involved is unclear. Pain is best identified by using direct digital pressure over the greater trochanter of the femur. Local pain is eliminated with injection of corticosteroids (methylprednisolone acetate, 100 mg/site) with an 8-cm, 18-gauge spinal needle into the middle gluteal muscle. The needle is directed horizontally at a 45-degree angle to the sagittal plane toward the opposite tuber coxae. One author (RMA) does not make any effort to inject the trochanteric bursa.
The lumbar region is prone to myositis. Diagnosis is made by palpation of pain and recognition of hindlimb stiffness. An effective treatment is methocarbamol (10 g PO tid). This high dose is effective with no substantial side effects. The withdrawal time for methocarbamol can be long in some racing jurisdictions. Alternatively the affected muscles can be injected with corticosteroids diluted with Sarapin (2.5 mL of a mixture of 50 mL Sarapin and 200 mg methylprednisolone acetate per site).
Other Stress Fractures
TB racehorses are prone to stress-related bone injuries of several sites other than the tibia, including the humerus, pelvis, and scapula. Humeral stress fractures account for an estimated 5% to 8% of the fatal musculoskeletal injuries in any given year in California. Horses typically develop lameness 45 to 90 days after returning to training, following rest for an unrelated cause. Acute lameness quickly abates within 1 to 2 days, typical of many horses with stress fractures. The cranial phase of the stride is shortened, and the horse may drag the forelimb. This fracture is dangerous. When humeral stress fractures displace, they spiral and invariably cause the destruction of the horse on humane grounds. However, when identified before displacement, fractures heal well without complications. Fractures are nonarticular and rarely recur. Horses are given 90 to 120 days of rest. Scintigraphic examination is the best method to diagnose humeral stress fractures, because location precludes adequate radiographic examination in many horses. Humeral stress fractures occur medially and involve the proximal caudal cortex immediately below the humeral head; the caudal distal cortex (rare); and the cranial, distal medial cortex. Distal-medial humeral stress fractures may be more common in horses training on synthetic surfaces (JAB). Radiographs may show proliferative changes and fracture lines in horses with fractures of the distal, craniomedial aspect of the humorus, but often in other sites neither the fracture line nor proliferative changes can be seen. Proximally, any loss of distinction in the proximal caudal aspect of the humerus should be considered diagnostic for a humeral stress fracture until proved otherwise.
Stress fractures of the scapula are rare but do occur in the TB, and clinical signs are similar to those caused by humeral stress fractures. Nuclear scintigraphy is the only way to diagnose this fracture. Most scapular fractures occur where the spine of the scapula meets the neck, in the same location as scapular fractures that necessitate destruction on humane grounds.
Pelvic stress fractures are underestimated grossly, overlooked, and misdiagnosed as a source of lameness in racehorses. Pelvic stress fractures most commonly involve the ilium, anywhere from the tuber sacrale to the tuber coxae. Stress fractures at the base of the tuber sacrale frequently are misdiagnosed as strains of the sacroiliac ligament. Fractures involving the base of the tuber coxae can be incomplete or complete (knocked-down hip).
Pelvic stress fractures can be catastrophic injuries, if a fracture becomes complete and displaced and results in laceration of the iliac artery. All horses with stress fractures usually have a history of transient recent lameness. Considerable evidence indicates that catastrophic pelvic fractures develop from preexisting stress-related bone injuries in horses that were continued in training. The incidence is markedly higher in fillies. Nuclear scintigraphy is useful for identifying pelvic stress fractures, but several factors decrease sensitivity, including the overlying muscle mass. Even a minor area of IRU should be considered important. Motion-correction software improves accuracy and image quality. Ultrasonographic examination can be used to identify fracture if displacement exists but is less sensitive than scintigraphy. Ultrasonography is most useful in horses that cannot be moved. Pelvic stress fractures should be included in the differential diagnosis of any TB with hindlimb lameness. Horses returning from training with severe, undiagnosed hindlimb lameness should be tied and not allowed to become recumbent until a pelvic fracture can be eliminated as a source of pain. Horses with a pelvic stress fracture may not exhibit severe lameness and may be confused with those suspected of having sacroiliac pain or pelvic muscle pain.
Sacroiliac strain is diagnosed in horses when lameness results from slipping while the horse is coming out of the starting gate (MWC). Horses with sacroiliac pain resent pressure applied between the tubera sacrale. Certainly, sacroiliac strain needs to be differentiated from pelvic fracture, because management may include local, deep injections of corticosteroids and Sarapin. Injecting the back or hip muscles in a horse with a pelvic fracture gives no benefit and incurs considerable risk.
The prognosis for horses with pelvic stress fractures is good, unless displacement exists. Even horses with complete, displaced fractures of the tuber coxae can return to racing if the displacement is not severe.
Other Lameness Conditions
Stifle
Stifle lameness causes unilateral and occasionally bilateral lameness in TB racehorses (MWC, PJM). Horses have mild or moderate lameness and appear to swing the limb. Other characteristics include a stiff hindlimb gait, not using the hindlimbs or weak behind, getting out in the straightaway, slipping behind, and locking up behind. The most common clinical finding is effusion of the medial femorotibial joint. Lameness is exacerbated by upper limb flexion. Pain on palpation of the medial collateral ligament is found occasionally.
Diagnostic analgesia is important, but eliminating the lameness totally in most horses is difficult. Radiological findings are generally negative, unless osteochondritis dissecans or subchondral bone cysts are present. Horses with osteochondrosis usually are treated surgically as young horses before arriving at the racetrack. Ultrasonographic examination is useful in identifying soft tissue lesions such as collateral desmitis, and when identified, affected horses are given 7 to 10 days or more of handwalking.
Most horses with stifle lameness appear to have synovitis or early OA. Differences of opinions exist regarding the type of intraarticular injections to use in the stifle. One of us (MWC) uses hyaluronan and betamethasone but avoids using methylprednisolone acetate, because synovitis may worsen. On the other hand, one of us (PJM) uses hyaluronan (50 to 200 mg) and methylprednisolone acetate (160 to 200 mg) in the medial and lateral femorotibial joints.
Secondary Shoulder Region Pain
Horses with forelimb lameness characterized by a shortened cranial phase of the stride may have some degree of secondary muscle soreness in the shoulder region or mild intertubercular (bicipital) bursitis (MWC). Horses often have primary lameness in the foot or fetlock joint, but management of the primary lameness conditions does not alleviate all clinical signs. Injection of a combination of methylprednisolone acetate, isoflupredone acetate, and hyaluronan into the bicipital (intertubercular) bursa and 2 days of handwalking resolves remaining clinical signs in some horses.