Training the Young Standardbred

Yearling STBs are introduced to the harness and bridle in the stall and then are line driven at a walk for several days before being hitched and jogged on the track. Once the horses can go to the track with a single driver, trainer, or groom, they are jogged 1 to 2 miles each morning, usually in a clockwise direction (wrong way) first. Young STBs are encouraged to jog at the intended gait as soon as possible. STBs are usually jogged 6 days a week at slow speed. This is usually natural for trotters, but many pacers need hobbles or special shoeing. Weight usually is added to hind shoes of pacers, especially to the outside web, whereas the front feet may be left unshod. In trotters, weight is added to the front shoes. Shoeing and equipment changes begin early to produce the best gait possible.

Jogging is increased 1 mile every few weeks, up to 4 to 5 miles daily. Horses are taught to jog both ways of the track. Most young horses are jogged in groups, so horses overcome fear of traveling in close proximity. In large stables, horses are trained in sets of three to five to teach passing and racing in tight quarters and to simulate racing. In addition to jogging, horses are trained fast every 3 to 4 days initially, but later twice weekly, going counterclockwise (the race or right way on the track). Horses are trained 2 and sometimes 3 fast miles on training days but usually go back to the barn for a breather (to blow out) between fast miles (trips). STBs develop tremendous stamina compared with TBs, but fatigue and strain cycles predispose them to stress-related injury of the subchondral bone, not commonly of cortical bone unlike TBs. When the fastest miles approach racing speed, young horses are ready for baby or schooling races to learn how to leave the starting gait. It is interesting that most STBs are trained, schooled, and qualified in the morning daylight and then make the first parimutuel start at night under the lights.

Retraining a Racehorse

The classic retraining approach involves jogging 2 to 5 miles a day and additional training one to two times a week. A fast workout usually consists of two to three fast trips, which are simulated race miles taken at increasing speed. Horses usually are allowed to blow out between trips. Some are given double headers, which are two 1-mile workouts without returning to the barn, but they walk or jog slowly between trips. A break is when a horse leaves the expected gait and breaks into a gallop. Many training miles are given to obtain maximum physical fitness and to condition the horse to travel at speed without breaking. Many training miles are considered necessary to evaluate and adjust equipment and shoeing. Diagnostic analgesia can be performed between trips.

Alternative training methods produce good results and are customized to the disposition and physical condition of the horse. Trainers now use interval training, straight tracks, inclines and hills, swimming, towing behind a vehicle, and mechanical walkers or joggers. With relocation of European trainers to North America, more alternative techniques have been introduced, including riding trotters between races.

Once racing, work is customized to each horse’s needs. Hard training using the classic training method consists of one race and two training trips per week, but in this system horses are under considerable stress. Now that the breed is refined and heat races are fewer, racing horses are given much less training between races, and fast work may not be given at all. Reducing exercise intensity between races is important in managing STBs with chronic lameness.

Palpation

Careful palpation is the art of diagnosis in the racehorse but often is sacrificed because palpation is time consuming. Palpation is critical in STBs because of numerous compensatory lameness problems. The veterinarian needs to be able to “read” the horse to make a diagnosis. A successful lameness detective respects what the horse is trying to say. The veterinarian should move over the entire horse with a light touch to see whether a withdrawal response is elicited. A light touch along the neck and gluteal regions may elicit pain from secondary muscle soreness or painful previous intramuscular injection sites. Horses often exhibit a withdrawal response when a limb is first picked up, indicating a problem even before the specific region is manipulated. Horses may exhibit false-positive responses in areas that have been recently painted, blistered, or freeze fired. Once horses have been freeze fired for splints and curbs, many trainers assume the problem has been solved and are incredulous if the veterinarian suggests the area is still the source of pain. Cryotherapy is not a panacea and may have to be repeated, or another form of management may be needed (see Chapters 78 and 89).

Palpation should be done in a quiet place, before the horse is trotted in hand, so that all potential lameness problems are detected. Careful and detailed examination of the front feet should be performed. Hoof tester examination is critical, but most horses show a painful response for 1 to 2 days after racing, particularly if the track was hard. Effusion of the distal interphalangeal (DIP) joint capsule often accompanies early OA and sore feet. Shoes on the front and back feet should be evaluated critically for wear, type, weight, and the presence of additives. The proximal, dorsal aspect of the proximal phalanx should be palpated for pain associated with midsagittal fractures of the proximal phalanx. Palpation findings in the metacarpophalangeal and metatarsophalangeal joints often do not correlate well with degree of lameness, response to diagnostic analgesia, and results of scintigraphic examination. The presence of mild warmth (heat) on palpation, a subtle clinical finding, is an important observation for detection of subchondral bone pain in the distal aspect of the McIII or the MtIII. Pain on palpation of the PSBs may indicate sesamoiditis. Normally a horse responds little to compression of the suspensory ligament (SL), deep digital flexor tendon (DDFT), and superficial digital flexor tendon (SDFT). Pain is often the first sign of desmitis or tendonitis. Chronic enlargement of the SL and SDFT is common in lower-class horses, and although a horse may not react to palpation, it may have associated high-speed lameness. Splint exostoses are most painful after training or racing but may be nonpainful several days later. Standing and flexed subcarpal palpation may reveal pain associated with PSD, longitudinal and avulsion fractures of the McIII, proximal superficial digital flexor tendonitis, and a dorsal medial articular fracture of the McIII.

No palpable abnormalities may be associated with carpal lameness. The veterinarian should carefully assess for warmth and compare the limb with the contralateral limb. If the horse has been clipped for the application of a paint or blister, the affected area will feel warm compared with the surrounding unclipped skin. Effusion occurs in 2-year-old horses early in training, but it often is minimal in horses with subchondral bone pain. The antebrachiocarpal joint is rarely a site of carpal lameness in older racehorses, but in young horses, especially trotters, effusion may occur. The medial aspect of the carpus and the distal aspect of the antebrachium are common sites for interference injury. Carpal tenosynovitis is unusual to rare, but occasionally hyperextension injury and hemorrhage occur. Rarely, almost always in trotters, desmitis of the accessory ligament of the SDFT occurs, associated with carpal tenosynovitis. Pacers develop hobble burns and cellulitis in the proximal aspect of the antebrachium. Elbow and shoulder joint lameness is rare, but horses may exhibit pain when the biceps brachii, intertubercular bursa, and other muscles are palpated or the shoulder joint is flexed, from muscle soreness secondary to primary carpal lameness. The neck, back, and rump regions should be palpated for symmetry and muscle soreness. Young horses will occasionally manifest sore withers from an improperly fitted harness. Because intramuscular injections of counterirritant and antiinflammatory solutions are common in STBs, careful palpation is necessary to discover deep abscesses causing pain, lameness, and fever. Disease of the thoracic dorsal spinous processes is rare. In a 16-year period, only one of nearly 2400 STBs undergoing scintigraphic examination had increased radiopharmaceutical uptake (IRU) in the dorsal spinous processes or the intervertebral synovial joints, and no bony abnormalities were found in the pelvis.⁵

Secondary muscle soreness of the gluteal region is a common problem, and the diagnosis of trochanteric bursitis often is made based on a painful response to compression. Trochanteric bursitis (whorl bone disease) may be overdiagnosed and difficult to authenticate, but injections often improve hindlimb gait and performance (see Chapter 47). Rhabdomyolysis also can cause pain on palpation of the rump region. Young horses with loose stifles often knuckle behind, but palpation is often negative. Pain associated with the medial aspect of the stifle may be present, but abnormalities when manipulating the patella are absent. Femoropatellar effusion in young horses can signal osteochondrosis. The medial femorotibial joint capsule should be palpated because OA is the most important cause of lameness of the stifle. The crus is a rare site of pain. Hobble burns on the caudal aspect can cause soreness in young pacers.

The tarsocrural joint is the most common site for osteochondrosis, and effusion in young horses should prompt radiographic examination. Periarticular pain in the distal aspect of the tarsus and proximal metatarsal region often accompanies distal hock joint pain. The Churchill test is done routinely, but in addition to those with distal hock joint pain, STBs often respond positively with other common distal limb lameness. Bony enlargement (bone spavin) is rare. Plantar soft tissue swelling and pain associated with curb is a common cause of lameness (see Chapter 78). The plantar metatarsal region commonly is overlooked, but it should be palpated carefully for the presence of suspensory desmitis. Signs of inflammation may be difficult to detect because the SL is enclosed within the bony confines of the metatarsal bones. The first evidence of suspensory desmitis is mild enlargement and pain of the suspensory body in the midmetatarsal region. Compression of the splint bones puts indirect pressure on the SL and explains why the Churchill test is nonspecific. The dorsal aspect of the MtIII is a common site for interference injury in trotters. Palpation of the fetlock region is critical because the region is an important source of lameness, but often no localizing clinical signs are apparent. Mild warmth may signal the presence of subchondral bone injury. Pain from interference injury should be differentiated from that associated with midsagittal and dorsal plane fracture of the proximal phalanx. The hind foot is an unusual source of lameness, but fractures of the distal phalanx, bruising, navicular disease, and OA of the DIP joint are diagnosed occasionally. STBs normally respond positively to hoof testers placed across the heel.

Movement

The horse should be examined at a walk and trot in hand. The pace is a forgiving gait, and head and neck nod and pelvic hike can be difficult to see. Evaluating lameness in pacers is easier while they are trotting, but correlation between lameness seen in hand and during pacing at high speed is questionable. Forelimb lameness is less obvious at the pace. Degree of pelvic hike is comparable at the pace and trot. Baseline lameness should be determined before the harness is applied because horses that are fracture lame should not be taken to the track, and a good correlation exists between lameness seen in hand and in harness in horses with obvious lameness. The ability to trot horses in hand at the racetrack may be limited because of too much activity, lack of room, and often poor or slippery surfaces. Flexion tests are performed but lack specificity. The carpal flexion test is the most reliable and specific of all flexion tests. Direct compression of a painful splint, the proximal aspect of a SL, and a curb followed by trotting is useful. Subtle hindlimb lameness is best evaluated in harness because little correlation may exist with lameness seen in hand. Pulling a cart and different surface likely explain this observation. Subtle (less than grade 1 of 5) hindlimb lameness at the trot in hand should be taken with a grain of salt. Observation on the track puts lameness into perspective and allows evaluation of compensatory lameness. Although there may be a lack of correlation between lameness seen in hand and that observed on the track or at speed, any lameness condition may be an important contributor to poor performance. Lameness is most pronounced when horses are jogged slowly and is much less obvious with speed. High-speed lameness is evaluated by communication with the trainer and by resolving signs such as being on a line by using diagnostic analgesia.

Radiography

Routine radiographic examination of STBs does not differ from that of other sports horses. However, a few specific images should be kept in mind. Down-angled oblique (dorsal 60° proximal 45° lateral-palmarodistal medial oblique and dorsal 60° proximal 45° medial-palmarodistal lateral oblique) and horizontal oblique (dorsolateral-palmaromedial oblique [DL-PaMO] and dorsomedial-palmarolateral oblique) images should be obtained to evaluate the DIP joint and palmar processes of the distal phalanx. The tangential (palmaroproximal-palmarodistal oblique) image of the navicular bone is less valuable in the STB than in other older sports horses. To evaluate the distal aspect of the McIII (or MtIII) for subchondral stress-related bone injury and the space between the proximal palmar (or plantar) aspect of the proximal phalanx and the base of the PSBs for the presence of fragments, down-angled oblique (dorsal 30° proximal 45° lateral-palmarodistal medial oblique and dorsal 30° proximal 45° medial-palmarodistal lateral oblique) images should be obtained (see Chapter 42). The most important images of the carpus are the DL-PaMO image and the tangential (skyline) image of the distal row of carpal bones. In STBs it is essential to have a well-exposed and well-positioned skyline image to evaluate the third carpal bone for increased radiopacity, radiolucency, and osteochondral fragments (see Chapter 38). Radiological changes of OA of the CD and TMT joints and slab fractures of the third tarsal bone are seen best on the lateromedial and dorsomedial-plantarolateral oblique (DM-PlLO) images. A common misconception is that changes associated with bone spavin occur medially but not in young STBs. The DM-PlLO image is essential in evaluating the tarsocrural joint for osteochondrosis fragments because two common locations—the cranial aspect of the intermediate ridge of the tibia and the lateral trochlear ridge of the talus—are evaluated best using this image. The caudocranial image is essential when evaluating the medial femorotibial joint for narrowing, osteophytes, and subchondral bone cysts. Inadequate exposure and positioning often result in images that cannot be interpreted.

Digital and computed radiography are useful in evaluating stress-related bone injury of subchondral bone and incomplete fractures. Computed tomography is useful to evaluate subchondral bone and fracture configuration. Magnetic resonance imaging is useful to evaluate subchondral bone and soft tissues of the distal aspect of the limb, but scintigraphic examination is still used routinely for investigation of high-speed lameness and poor performance and continues to have great value in lameness diagnosis in STBs.

Ultrasonographic Examination

Suspensory desmitis, superficial digital flexor tendonitis, curb, and desmitis of the distal sesamoidean ligaments are common in the STB, and ultrasonographic examination is essential for diagnosis and assessing prognosis (see Chapter 16).

Scintigraphic Examination

Referral for scintigraphic examination is common, and horses with poor performance often have numerous areas of IRU, indicating high-speed lameness is a part of the problem. Common areas of IRU include the distal phalanx, metacarpophalangeal joint (medial condyle of the McIII and PSBs), proximal aspect of the McIII, carpus, especially the third and radial carpal bones, metatarsophalangeal joint (proximal aspect of the proximal phalanx and distal plantarolateral aspect of the MtIII and PSBs), the proximal plantar aspect of the MtIII, subchondral bone of the TMT and CD joints and the medial femorotibial joint (see Chapter 19).

Front Foot Lameness

Front foot lameness is the most common lameness condition. Palmar foot pain abolished using palmar digital analgesia is a common finding, but the veterinarian should keep in mind that palmar digital analgesia also abolishes most of the pain associated with the DIP joint and toe and lower pastern regions. Occasionally, palmar digital analgesia can abolish pain associated with the fetlock joint. Palmar foot pain can be caused by a bruised heel, corns, sheared heel, hoof cracks, contracted heel, wall separations and gravel, stress remodeling and stress fractures, traumatic fractures, OA, or various combinations of these conditions.

Fractures of the Distal Phalanx

Fractures of the distal phalanx occur most commonly in the LF lateral and RF medial palmar processes and are usually articular (Figure 108-4).⁷ Clockwise training and racing may account for asymmetrical location of fractures. Nonarticular palmar process and midsagittal fractures are rare. Predisposing factors include hard racing surfaces (most fractures occur during a race, more often in winter months), foot imbalance, impact on an uneven surface, and overloading. Fractures of the distal phalanx are unlikely single-event injuries and are the sequelae to maladaptive or nonadaptive remodeling of the distal phalanx that results from stress-related bone injury. IRU often is seen in the lateral aspect of the LF and medial aspect of the RF distal phalanx in horses with lameness abolished by palmar digital analgesia but in which radiographs are negative (Figure 108-5).⁸ Most fractures of the distal phalanx occur in aged horses. Treatment includes application of a wide web, concave inner surface steel straight or egg bar shoe with two side clips on each side. Horses are given 3 months of stall rest and then 3 months of turnout in a small paddock. Most fractures are complete but nondisplaced; mild displacement results in a step in the articular surface. Fractures take months to heal, and some appear to develop chronic nonunions, although many horses return to racing soundness even with a radiologically apparent nonunion. Distal phalangeal fractures in the hind feet are unusual to rare but generally involve the medial plantar process and are articular in location.⁵ OA of the DIP joint often occurs, particularly in horses with displacement or nonunion, but unless severe, prognosis for racing is good. Osteochondral fragments of the extensor process of the distal phalanx are unusual, but arthroscopic removal of small fragments and repair of large ones are indicated. Prognosis is fair, but OA of the DIP joint can affect prognosis negatively.

Fig. 108-4 Dorsomedial proximal-palmarolateral distal oblique xeroradiographic image of the right front foot of a Standardbred racehorse. There is a typical articular fracture of the medial palmar process (arrows) of the distal phalanx. The horse is shod with a bar shoe with clips.

Fig. 108-5 A, Dorsal delayed (bone phase) scintigraphic image showing a typical distribution of increased radiopharmaceutical uptake of the Standardbred front feet. The left forelimb is on the right. B, Lateral delayed (bone) phase scintigraphic image of the left forelimb. This Standardbred was lame in the left forelimb, and lameness was localized to the lateral aspect of the distal phalanx. In the left forelimb, stress remodeling of subchondral bone of the distal phalanx was diagnosed. Increased radiopharmaceutical uptake involving the lateral aspect of the left forelimb and medial aspect of the right forelimb distal phalanx is thought to be caused by counterclockwise training and racing, and stress-related bone injury can lead to fracture in these locations.

Carpal Lameness

The carpus is the most common articular location for lameness and conformational faults. In 2-year-olds, carpal synovitis is common. High prevalence of carpal lameness is caused in part by racing young, immature STBs around tight turns on firm surfaces. The medial aspect of the middle carpal joint and the right carpus are predisposed to injury. Lateral injury of the carpus is unusual. Heat, effusion, and pain on careful palpation of the dorsal surfaces are common. Heat (the perception of warmth) on the dorsal aspect of the carpus is the earliest clinical sign of subchondral bone injury of the dorsomedial aspect of the middle carpal joint. Lameness varies with severity of injury, and horses carry the limb in an abducted position during advancement (see Chapter 38). Response to flexion varies, and diagnosis should be confirmed using intraarticular or median and ulnar analgesia. Secondary shoulder muscle soreness often develops. Horses warm out of lameness, and weeks to months may pass before consistent lameness is seen. Radiographs are usually negative or equivocal initially, but increased radiopacity of the third carpal bone and mild marginal osteophyte production can predispose the horse to small osteochondral fragments or slab fractures of the third carpal bone. The mere presence of increased radiopacity of the third carpal bone, however, is not diagnostic for subchondral bone pain because mild sclerosis is an adaptive response to race training (Figure 108-6). Diagnosis of subchondral bone pain as a result of maladaptive or nonadaptive remodeling is made using a combination of abnormal clinical and imaging findings. In a radiological study evaluating degree of increased radiopacity of the third carpal bone, exercised horses had significantly more increased radiopacity than nonexercised control horses, and lameness referrable to the middle carpal joint developed more commonly in STBs with higher degrees of third carpal bone increased radiopacity.⁹ Thirty percent of STBs in the study developed lameness of the middle carpal joint.⁹ In a separate study the same investigators determined the importance of middle carpal joint pain in STBs and found carpal lameness to be present in 28% of STBs overall and in 56% of those with forelimb lameness.¹⁰ Carpal lameness was the most common reason for more than 1 month of rest in STBs in race training. Few clinical signs such as effusion were present. Risk factors were faulty conformation and intense speed training.¹⁰ Diagnostic analgesia remains a most important aspect of clinical diagnosis, given the lack of early clinical signs associated with subchondral bone injury. Scintigraphic examination often reveals focal areas of IRU, usually involving the third carpal bone or the subchondral bone of the dorsomedial aspect of the middle carpal joint collectively; bilateral lesions are common (see Figure 19-16). Stress-related bone injury results in a continuum of subchondral and overlying cartilage damage, eventually leading to OA or osteochondral fragments. Rest is the best form of management but is usually unacceptable to trainers and owners. Topical counterirritation with iodine paints is still popular and is used with apparent success because blistering usually is accompanied by a reduction in exercise intensity. The administration of NSAIDs for 3 to 5 days and a brief period of reduced exercise (2 weeks) may be an acceptable compromise between the veterinarian and the trainer, but keep in mind that although a reduction in training intensity is helpful, bone cannot heal in 2 weeks, and pain may not subside. Intraarticular injections of hyaluronan or polysulfated glycosaminoglycans (PSGAGs) may reduce inflammation and theoretically may be useful in cartilage healing, but these often are used in a palliative rather than therapeutic manner, given a lack of synovitis in most horses. Injections may be of little benefit when pain originates from subchondral bone. Ten 2-year-old STBs with mild carpitis were treated with three to five weekly intraarticular injections with PSGAGs (Adequan, Luitpold Pharmaceuticals, Inc., Shirley, New York, United States) and were compared with a control group receiving NSAIDs and topical counterirritation.¹¹ The treated group had substantially less carpal lameness during the ensuing racing season. All horses raced, but horses in the control group required intraarticular injections to maintain soundness. Early aggressive intraarticular therapy may forestall development of progressive OA in young STBs. Extracorporeal shock wave therapy has gained popularity as an adjunctive therapy in the management of STBs with subchondral bone injury and OA. With the exception of a palliative analgesic effect, the role of this modality in potentiating healing remains unknown. The possibility exists that as a result of the analgesic effect horses could be continued in training well past the ability of injured bone to adapt.

Fig. 108-6 Dorsoproximal-dorsodistal digital radiographic image of the right distal row of carpal bones in a 2-year-old Standardbred filly that was sound and racing successfully at the time of this radiological study conducted as part of an examination before purchase (medial is to the right). Mild-to-moderate increased radiopacity of the radial fossa (between arrows) is seen as an adaptive response to training and racing. This filly continued to race successfully for the remainder of the 2- and 3-year-old racing years.

Palmar foot pain may play a role in the development of carpal lameness in young STBs. Early carpal lameness is easier to treat if palmar foot pain is recognized early and managed aggressively. In horses with bruised bars and poor medial-to-lateral hoof balance, the application of wide web bar shoes reduces inflammation and signs of carpal lameness. In all ages of trotters, the flip-flop shoe appears to be effective in reducing clinical signs of carpal lameness. Small osteochondral fragments of the carpus are the most common fractures and involve the third and radial carpal bones and rarely other bones in the middle carpal and antebrachiocarpal joints. Arthroscopic surgical removal and PSGAG therapy after surgery are recommended. Frontal (dorsal) and sagittal slab fractures occur commonly, and repair using screws placed in lag fashion is recommended. Small, frontal slab fracture fragments can be removed. Although rest is an option in horses with nondisplaced fractures, internal fixation may shorten recovery period and reduce risk of recurrence (see Chapter 38).

Metatarsophalangeal Joint Lameness

The metatarsophalangeal joint is a common source of pain causing lameness in STBs, and diagnostic analgesia is usually essential for diagnosis.

Fractures

Acute, severe lameness is seen most commonly in STBs with fractures of the metatarsophalangeal joint, but some horses can race with incomplete fractures. Horses shorten the cranial phase of the stride, land on the toe, and rapidly compensate with other limbs. Often no effusion or pain on flexion is apparent unless a fracture is displaced. Diagnostic analgesia and examination in harness are avoided if a fracture is suspected because comminution or displacement can occur. Radiographs, including flexed dorsoplantar or 125° dorsoplantar and flexed lateromedial images, should be obtained.

Midsagittal fractures of the proximal phalanx are most common, causing severe intermittent or severe unrelenting lameness, depending on fracture length and configuration. Fractures can be short, incomplete with mild periosteal proliferation on the dorsal aspect of the proximal phalanx (Figure 108-7) or long, incomplete or complete midsagittal fractures, fractures that break out of the lateral cortex, and comminuted fractures (see Figure 36-10). Horses with short, midsagittal fractures often race or train because lameness is intermittent and often abates after several days. Clinical signs precede radiological evidence of fracture for 7 to 21 days, and scintigraphic examination reveals focal IRU in the proximal aspect of the proximal phalanx (see Figure 19-20, C). Horses with short, midsagittal fractures can be rested or have internal fixation and have a good prognosis with either treatment.¹³ Long incomplete, complete, displaced, or comminuted fractures should be repaired. Prognosis depends on degree of comminution and displacement and whether a strut of bone is intact to which to attach fragments. Horses with comminuted fractures rarely race again and are at risk of infection, fracture displacement, and OA of the metatarsophalangeal and proximal interphalangeal joints and contralateral limb laminitis (see Chapter 35).

Fig. 108-7 Dorsoplantar (A) and flexed lateromedial (B) xeroradiographic images showing a short, incomplete, midsagittal fracture of the proximal phalanx (arrow). There is slight periosteal new bone on the proximodorsal aspect of the proximal phalanx seen in the flexed lateromedial image (white arrow). Radiological changes often lag behind clinical signs, and follow-up radiological and scintigraphic examinations often are needed to establish a diagnosis.

Dorsal plane (frontal) fractures of the proximal phalanx can be unilateral or bilateral, are most common in the RH, and occur in trotters and pacers (see Figure 42-11 and accompanying text). Fractures of the PSBs are common and most often involve the lateral PSB. Small medial abaxial fragments can be difficult to identify. Apical PSB fractures are most common, but abaxial, midbody, and rarely basilar fractures occur, and the combination of suspensory desmitis, and splint bone and PSB fractures is not uncommon. Horses show acute lameness after training or racing, with grade 3 to 4 of 5 lameness, and prefer to bear weight only on the toe. Effusion and a positive response to flexion are usually present, and the diagnosis is confirmed radiologically. Ultrasonographic examination of the SL should be performed to establish baseline information and assess prognosis. Surgical removal of apical, abaxial, and basilar fragments and surgical repair of displaced midbody fractures using bone screws or wiring techniques are recommended. Prognosis is guarded in trotters and fair in pacers that have raced before fracture. Horses in which fractures occur early in training appear to have a poor prognosis. Of 43 STBs with apical PSB fractures no difference in prognosis was found between pacers and trotters; 86% of fractures involved the hindlimbs. Prognosis was better in STBs with hindlimb PSB fractures than with forelimb injuries. Eighty-eight percent of horses that raced before injury raced after injury, whereas only 56% of those previously unraced raced after injury. Neither fracture fragment dimensions nor associated suspensory desmitis negatively affected outcome.¹⁴ These latter two findings are surprising given our experience.

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Condylar fractures of the McIII or the MtIII occur, and fractures of the lateral condyles are more common than those of the medial condyles. Medial condylar fractures of the MtIII outnumber those occurring in the McIII. Clinical signs are similar to those caused by midsagittal fracture of the proximal phalanx, but diagnosis can be difficult unless good-quality radiographs with appropriate views are obtained. A flexed dorsoplantar or 125° dorsoplantar image should be obtained. Medial condylar fractures often spiral proximally and should be repaired, although even with repair there is a risk of catastrophic failure of the bone. In general, lateral condylar fractures of the McIII or the MtIII are best managed using surgical fixation, although STBs with short, incomplete fractures can be successfully managed conservatively. Regardless of management method, prognosis is favorable; 12 of 13 STBs with incomplete condylar fractures of the MtIII and 7 of 9 with incomplete condylar fractures of the McIII raced after injury.¹⁵ Four of six surviving horses with medial spiral fractures of the MtIII raced after injury.¹⁵ In that study, TBs were overrepresented and STBs were underrepresented when compared with the hospital population; TBs had significantly more lateral condylar fractures and forelimb condylar fractures than did STBs.¹⁵

Distal Hock Joint Pain and Other Tarsal Lameness

The hock joint is under considerable stress and strain, especially going into turns and with quick acceleration. Distal hock joint pain and progressive OA of the TMT and CD joints are common. However, distal hock joint pain should not be a default diagnosis, and careful lameness examination, including diagnostic analgesia, should always be performed. Injection of the “knees and hocks” is commonplace in the STB sport, despite our experience highlighting the importance of the metatarsophalangeal joint as a cause of hindlimb pain. Horses with distal hock joint pain warm out of lameness, often dramatically, because they may come out of the barn or paddock on the toe and eventually be able to train or race successfully. Horses show a mildly shortened cranial phase of the stride and a stabbing type of gait, with medial deviation of the limb during protraction and landing first on the outside toe (stabbing laterally), but this gait is not pathognomonic for distal hock joint pain. Palpation in a flexed position often reveals pain of periarticular soft tissue structures, and the Churchill test is often positive. Upper limb flexion is positive, but diagnostic analgesia is essential for diagnosis. Radiographs may show evidence of OA in older horses, but radiographs are negative or equivocal in most 2- and 3-year-olds. Changes are seen most often in the DM-PlLO image, but dorsoplantar, lateromedial, and slightly off dorsoplantar images can be helpful. Scintigraphic examination confirms that OA of the TMT joint is most common, and changes are lateral and dorsolateral (Figure 108-8). Focal IRU is sometimes seen involving only the CD joint in horses that failed to respond to intraarticular analgesia of the TMT joint, emphasizing the need to block and treat each joint separately.

Fig. 108-8 A, Plantar delayed (bone) phase scintigraphic image of the hocks. There is focal, nucleate (right) and intense (left) increased radiopharmaceutical uptake in the dorsolateral aspect of both tarsometatarsal joints (left hindlimb is on the left) in this 2-year-old Standardbred filly. B, Dorsomedial-plantarolateral oblique xeroradiographic image of the left hindlimb showing minimal changes (arrow). Lameness and scintigraphic evidence of distal tarsitis precede radiological changes by many months.

Treatment comprises intraarticular injections, NSAIDs, shoeing changes, and rest. Early, intraarticular hyaluronan is useful to decrease inflammation, and concomitant use of NSAIDs such as phenylbutazone (4.4 mg/kg PO daily for 5 days and 2.2 mg/kg PO daily for 5 days) is recommended initially. Later, as OA progresses, methylprednisolone acetate (80 mg/joint every 6 to 8 weeks) is most effective to manage inflammation. A series of intraarticular and intramuscular injections of PSGAGs also can be given. In horses with severe OA, 4 to 6 months of rest is successful, but recurrent lameness often mandates a limited racing schedule (10 to 15 starts/year). The toe is shortened, and a flat aluminum or steel squared-toe shoe is applied to allow easy breakover. All shoe additives are removed to decrease shear stress.¹¹

Slab fractures of the third tarsal bone and fractures of the proximal, dorsomedial aspect of the MtIII result from stress-related bone injury or from pathological fractures in horses with OA (see Chapter 44). Third tarsal bone fractures are the most common and are best seen on a DM-PlLO image. Fractures of the central tarsal bone are usually oblique, involve the dorsal aspect of the bone, occur without concomitant OA, and are best seen in lateromedial images. Conservative management is recommended in horses with nondisplaced fractures, but displaced fractures should be repaired. Ninety-two percent of horses with slab fractures of the third tarsal bone and 88% with slab fractures of the central tarsal bone raced after conservative management.¹⁶

The tarsocrural joint is often injected, but tarsocrural joint lameness is uncommon, except in young horses with osteochondrosis. Trainers often request injection of the upper and lower hock joints (tarsocrural and TMT joints), but lameness most likely is caused by pain associated with the distal hock joints. Bog spavin occurs most commonly in weanlings and yearlings, and radiological identification of osteochondrosis fragments prompts surgical removal. Most yearlings begin training without fragments. Although debate exists about the importance of tarsocrural osteochondrosis and lameness, we recommend surgical removal to resolve current inflammation, if present, and to prevent future lameness. However, osteochondrosis fragments are seen in some older racehorses in which neither lameness nor effusion is present. Tarsocrural lameness may occur without effusion, and diagnostic analgesia is necessary to differentiate the site of hock pain. Good-quality radiographs are essential to discover occult osteochondrosis fragments and fractures. The DM-PlLO image is best for evaluating the cranial aspect of the intermediate ridge of the tibia and a slightly oblique dorsoplantar image (dorsal 5° to 7° lateral-plantaromedial oblique) is best to evaluate the medial malleolus. The most common sites for osteochondrosis are the cranial aspect of the intermediate ridge of the tibia, lateral trochlear ridge of the talus, medial malleolus of the tibia, medial trochlear ridge of the talus, and lateral malleolus of the tibia. Medial malleolar fragments appear to be increasing in importance, particularly in trotters, although a heightened awareness and improved imaging may be uncovering fragments in this location that may have previously been missed. Osteochondrosis fragments can be large, numerous, loose, and free floating, and cartilage damage can be prominent even in yearlings; however, prognosis is good with removal. If distention of the tarsocrural joint is unexplained, osteochondrosis of the medial malleolus of the tibia should be suspected, and diagnostic arthroscopy is recommended. Small osteochondrosis fragments (dewdrop lesions) of the distal aspect of the medial trochlear ridge of the talus may be incidental but can cause lameness if displaced or if impinging on the central tarsal bone. Surgical removal is recommended.

Tarsocrural OA occurs in older horses (usually >5 years) and may be associated with osteochondrosis because fragments often are seen concomitantly. OA causes progressive lameness and effusion, but intraarticular analgesia should be performed to validate the source of pain. Large volumes of local anesthetic solution (30 to 50 mL) are necessary because pain is severe; false-negative responses occur if 10 to 15 mL is used. Radiographs may reveal prominent increased radiopacity and narrowing of the tarsocrural joint space, but end-stage OA can be present with only subtle radiological abnormalities (Figure 108-9, A). Scintigraphic evaluation reveals diffuse IRU on both sides of the tarsocrural joint and is useful to differentiate OA from sagittal fracture of the talus (Figure 108-9, B) or subchondral injury of the distal aspect of the tibia. Prognosis is grave.

Fig. 108-9 A, In this dorsomedial-plantarolateral oblique xeroradiographic image, the tarsocrural joint space is narrow, indicating end-stage OA of the tarsocrural joint, but this radiological sign is difficult to recognize because proliferative changes are minimal. B, Flexed lateral delayed (bone) phase scintigraphic image of a different Standardbred with a sagittal fracture of the talus. With flexion, the area of moderate to intense increased radiopharmaceutical uptake moves with the talus and can be differentiated clearly from end-stage osteoarthritis (OA) or distal tibial subchondral trauma.

Sagittal fracture of the talus is unusual. Acute, grade 3 to 4 of 5 lameness is localized to the tarsocrural joint by diagnostic analgesia, but often subtle tarsal lameness precedes fracture. Focal IRU is seen involving the talus (see Figure 108-9, B), and slightly oblique dorsoplantar images reveal subchondral lucency or fracture. Eight of 11 horses with sagittal fracture of the talus were STBs, and 7 raced after being managed conservatively.¹⁷

Enthesitis of the long, lateral collateral ligament of the tarsocrural joint, on the lateral aspect of the calcaneus, results in mild to moderate lameness and mild tarsocrural effusion (Figure 108-10). Soft tissue swelling is easy to miss initially, and radiological changes lag behind clinical signs for at least 14 to 21 days; however, lameness abates or is improved considerably with tarsocrural analgesia. Scintigraphic examination reveals focal IRU in the lateral aspect of the calcaneus. Local injections and radiation therapy may allow continued racing, but rest to allow enthesitis to subside is often necessary.

Fig. 108-10 A, Lateral delayed (bone) phase scintigraphic image of a hock showing focal, moderate to intense increased radiopharmaceutical uptake in the calcaneus. B, Dorsoplantar xeroradiographic image showing proliferative changes associated with the collateral ligament attachment on the lateral aspect of the calcaneus (arrows).

Injury of the fibularis tertius tendon anywhere from the midcrus to the distal tarsus can occur but is rare, and injury usually occurs at the insertion on the MtIII and the third tarsal bone. Soft tissue swelling is present, and the hock can be pushed forward without concomitant extension of the stifle with the limb in standing position. Horses require 4 to 6 months of rest.

Inflammatory synovitis or infection of the tarsocrural joint is not uncommon. Prognosis depends on the extent of existing cartilage damage and the financial ability of owners to manage infection aggressively (see Chapter 65).

Suspensory Desmitis

Suspensory desmitis is the most common soft tissue injury and can be classified as unilateral branch desmitis, bilateral branch desmitis, body desmitis, and PSD. Suspensory desmitis is often an overload compensatory lameness condition because fatigue and improper landing occur in lame horses. Suspensory desmitis is seldom a single-event injury but is caused by accumulated damage over weeks to months (see Chapter 72). Diagnosis is made by careful palpation from the origin to the insertion of the SL with the limb in standing and flexed positions. The splint bones and PSBs are examined for associated injuries. One of us (JBM) has found that horses with core lesions are lamer, but exhibit fewer outward signs of inflammation, than those with peripheral lesions. Horses with suspensory branch desmitis are common and heal quicker (6 to 8 weeks) with lower recurrence than do those with body desmitis and PSD (4 to 6 months). Core lesions recur frequently. Because suspensory desmitis is often a compensatory injury, the primary lameness needs to be resolved. For example, OA and osteochondral fragments of the left carpus frequently cause suspensory desmitis of the RF in horses of both gaits, RH suspensory desmitis in trotters, and LH suspensory desmitis in pacers.

Ultrasonographic examination is critical in assessing the SL, and its entire length should be examined systematically. In hindlimbs, enlargement of the origin and body is more common than core lesions, and cross-sectional area measurements of the affected and contralateral limb are mandatory. Branches are evaluated for change in shape, loss of margin definition, central or marginal hypoechoic lesions, subcutaneous hemorrhage and edema, fibrosis and calcification, irregular contour (step line defects), and avulsion fragments of the PSBs. The intersesamoidean ligament rarely is involved.

Forelimb unilateral branch desmitis results from toed-in or toed-out conformation, hoof imbalance, and uneven landing. To correct interference injury with knee knocking resulting from toed-out conformation, the hoof is commonly lowered on the inside, causing uneven loading and medial unilateral branch desmitis, whereas in horses with toed-in conformation, lateral unilateral branch desmitis occurs. Improper impact can occur from fatigue or if a horse is stopped suddenly. Lameness varies with severity of injury and presence of associated splint and PSB disorders.

Forelimb bilateral branch desmitis is caused by acute or chronic hyperextension of the metacarpophalangeal joint. Prognosis depends on the degree of fetlock drop seen when the contralateral limb is held in flexion. Concomitant PSB fractures can occur. Dorsal subluxation (flexion) of the proximal interphalangeal joint is seen in horses with bilateral branch desmitis and body desmitis, whereas palmar subluxation (extension) is seen in horses with distal sesamoidean desmitis and loss of palmar support. If the axis of the proximal interphalangeal joint is altered, the prognosis for racing is poor.

Forelimb body desmitis often begins just proximal to the bifurcation of the SL, and careful palpation reveals pain and mild enlargement. The first sign may be mild swelling, without lameness, and training and racing are often continued, leading to further suspensory desmitis. Horses often race successfully with chronic suspensory branch desmitis, but if desmitis progresses into the body, lameness becomes pronounced. There is a considerable difference in the potential for catastrophic breakdown injury with branch desmitis between TBs and STBs. Rarely, a STB develops catastrophic injury of the suspensory apparatus as a result of chronic suspensory desmitis, whereas continued racing with any suspensory branch desmitis is dangerous in TBs. Chronic enlargement and thickening of the SL often cause bowing of the splint bones, and concussion leads to fracture. Horses with recalcitrant swelling of the SL should be examined with ultrasonography because exuberant exostosis from fractures may encroach on the SL and cause swelling and pain.

Proximal palmar metacarpal pain is common and often called a check ligament problem, although genuine desmitis of the accessory ligament of the DDFT is rare. Palmar metacarpal pain can be caused by avulsion or longitudinal fracture of the McIII and stress reaction/stress fracture of the McIII, but PSD is most common. STBs often have pain on palpation of the proximal palmar metacarpal region and may have compensatory PSD, with primary lameness elsewhere. Lameness is noticed in the first half of the support phase of stride. An exaggerated head nod is often noted (JSM). A carpal type of gait may be seen. Lameness is most prominent when the affected limb is on the outside of a circle. Local analgesia and ultrasonographic examination should be performed. Radiographs are necessary to identify bone injury. Scintigraphic examination is useful to differentiate proximal metacarpal pain from carpal pain.

In hindlimbs, unilateral branch desmitis, bilateral branch desmitis, body desmitis, and PSD occur but less frequently than in forelimbs. Degree of lameness varies, and similar concerns exist about the MtIII, a splint bone, and a PSB involvement. Horses often land on the toe and are reluctant to put the heel down, especially if a concurrent pathological condition of the PSB exists. Suspensory branch desmitis often occurs first and can be managed successfully with continued work in some horses, but proximal progression toward the bifurcation and body causes substantial increase in lameness and may involve the splint bones, and often horses lose support of the metatarsophalangeal joint. Racing can no longer continue. Hindlimb suspensory desmitis is particularly important in young trotters. Many top 2- and 3-year-old trotters have been retired because of hindlimb suspensory desmitis. Second to carpal lameness, hindlimb suspensory desmitis is the most dangerous, career-limiting cause of lameness in trotters.

Early PSD can go unrecognized because of lack of obvious swelling. Local analgesia is critical for diagnosis. Once desmitis involves the body, swelling may be recognized, but damage is now extensive.

Inflammation can be suppressed, but rest is essential for healing. Rest is not well received by owners and trainers, who often exert pressure to keep a horse in work. Cryotherapy is used to manage horses with suspensory branch desmitis and distal body desmitis. Horses with marginal lesions respond well but not those with core lesions. Cryotherapy decreases pain and seems to promote more or better-organized scar tissue. Periligamentous injections of sclerosing agents such as sodium oleate promote fibrosis and can be used to cause a swollen SL to “set up.” Periligamentous injections of corticosteroid mixtures reduce inflammation and lameness, but if training and racing continue, further damage occurs. Many naive owners lack understanding that the latest popular injection does not promote healing but simply masks inflammatory signs. Although horses may continue to race a few more starts, further damage inevitably occurs. Horses with PSD often can be managed using injections of corticosteroids, internal blisters, and, if available, radiation therapy, and if lameness is mild and support is not lost, racing may continue. Shock wave therapy has gained prominence and shows promise.

Rest is the best approach, and there is no substitute. Between 3 to 6 months of rest may be necessary to allow healing. A combination of stall rest, handwalking, walking in the jog cart, and swimming is recommended. We do not recommend turnout, but many horses are blistered and turned out, although this has been shown to be detrimental to healing. Healing progress is assessed by ultrasonography. Surgical management using a modified Asheim approach (ligament splitting, or desmoplasty) and ostectomy of fractured splint bones and PSB fragments is recommended. Desmoplasty and injection of fresh liquid bone marrow have been used successfully (MWR). Other intraligamentous injections with products such as platelet-rich plasma and stem cells have shown modest success. One of us (MWR) has performed bone marrow injection and fasciotomy on a limited number of top trotters with hindlimb PSD and body desmitis with modest success.

Prognosis depends on level of injury, gait (trotters worse than pacers), severity of injury (poor prognosis with fetlock drop), and associated bony pathological conditions. Unless the SL is totally disrupted, prognosis for racing is good, but racing class is an important consideration because few STBs return to the same or improved race class after sustaining suspensory desmitis (Figure 108-11).

Fig. 108-11 Images of a 3-year-old Standardbred colt pacer in May after being trained and qualified for racing; the colt had won divisional honors the previous year but developed substantial (3 of 5) left hindlimb lameness as a result of complex injury in the proximal plantar metatarsal region. A, Delayed phase lateral (left) and plantar scintigraphic images showing focal, intense increased radiopharmaceutical uptake in the proximal, plantar aspect of the third metatarsal bone (MtIII) indicative of bony injury at the origin of the suspensory ligament (double-headed arrow). B, Dorsoplantar digital radiograph (lateral is to the left) of the left proximal metatarsal region showing increased radiopacity (arrow) and mild radiolucency (arrowhead) of the MtIII; neither avulsion nor longitudinal fracture was seen, but changes of this magnitude in bone indicate the lesion is chronic. C, Transverse (left) and longitudinal ultrasonographic images acquired with the transducer medial to the plantar midline showing enlargement (double-headed arrow) of the proximal aspect of the suspensory ligament (cross-section area measured 2.4 cm²) and a large hypoechogenic region (arrowheads). This colt was retired from racing.

Metacarpophalangeal Joint Lameness

Fatigue from underconditioning and other factors causing overload may lead to hyperextension of the metacarpophalangeal joint. Hyperextension leads to injury of the dorsal aspect of the joint, such as small osteochondral fragments of the proximal phalanx and proliferative synovitis, and to injury of the palmar aspect of the joint, such as fractures of the PSBs and suspensory branch insertional injury. Traumatic disruption of the suspensory apparatus is rare but usually involves rupture of the SL. Stress-related bone injury of subchondral bone leading to OA and fracture occurs in the metacarpophalangeal joint, as described for the metatarsophalangeal joint. In a STB with normal conformation, metacarpophalangeal effusion should be a red flag (a barometer), often a sign of compensatory lameness. Young horses often have bilateral metacarpophalangeal synovitis secondary to palmar foot pain because toe-first landing overloads the joint. Corrective trimming and shoeing often helps. In older horses unilateral carpal lameness often causes contralateral metacarpophalangeal synovitis. Primary lameness of the metacarpophalangeal can begin when horses break stride, stumble, or stop quickly.

OA and stress-related bone injury are the most common problems of the metacarpophalangeal joint. Early radiographs are usually negative, but radiolucency and increased radiopacity of the distal medial aspect of the McIII develop later. Scintigraphic evidence of IRU is more prominent medially, unlike in the metatarsophalangeal joint, in which IRU is more common laterally. In older horses (about 4 years of age) with progressive severe OA, narrowing of the medial joint space indicates severe cartilage damage. Owners are incredulous that the horse has end-stage OA because lameness can be acute and severe, and fracture often is suspected.

Early OA is treated using intraarticular injection of hyaluronan alone or a combination of hyaluronan and PSGAGs. The latter is preferred, and three to four weekly injections are combined with a reduction in exercise. Horses that do not respond may have proliferative synovitis, usually seen in older horses with chronic OA. Injecting the joint in the dorsal pouch may be difficult, and little synovial fluid may be present. Surgical resection of synovial pads and postoperative intraarticular injections of PSGAGs are recommended.

Fractures and osteochondritis dissecans occur, but palmar fragments are rare (see Chapter 36).

Splint Bone Disease

Stifle Joint Lameness

The stifle joint often is incriminated as a source of pain, but diagnostic analgesia identifies a problem in the more distal part of the limb. OA of the medial femorotibial joint is the most important stifle lameness, but osteochondrosis should be considered in young horses. OA is a progressive disease beginning in late 2-year-olds, and degree of lameness varies from grade 1 to 4 of 5. Distention of the medial femorotibial joint capsule is an important finding, but in some STBs this is a subtle finding. The limb may be carried forward slightly lateral or outward during the cranial phase of the stride. The cranial phase is shortened, and lameness is worse in the turns. Radiographs are usually negative or equivocal in 2-year-olds, and arthroscopic evaluation reveals generalized cartilage thinning and fissure formation. Later, in caudocranial radiographic images, narrowing of the medial femorotibial joint space and mild osteophyte formation on the proximal medial aspect of the tibia and distal medial aspect of the femur are visible (Figure 108-12, A). Scintigraphic examination can confirm extensive OA. Diffuse IRU involves both sides of the medial femorotibial joint and is best seen in a caudal image, although if extensive IRU can be seen in a lateral image as well. Arthroscopic evaluation at this time reveals exposed subchondral bone and severe synovitis, but therapeutic value of arthroscopy is negligible. If the diagnosis of OA is made in a 2-year-old, a period of 3 to 4 months of rest is recommended. The stifle joint often is injected in horses without stifle lameness, and few untoward effects are seen. Treatment of STBs with OA of the medial femorotibial joint with methylprednisolone acetate is contraindicated early in the course of the disease. Although the drug is effective and allows continued racing, OA can progress rapidly, resulting in severe lameness. Hyaluronan (40 mg) and isoflupredone acetate (4 to 6 mg) or triamcinolone acetonide (4 mg), interleukin-1 receptor antagonist protein, or PSGAGs (750 to 1000 mg) are preferred, and although they are less effective than methylprednisolone acetate, they decrease inflammation and prolong the racing career. Systemic administration of PSGAGs or hyaluronan is helpful. Methylprednisolone acetate is necessary for older horses with advanced OA, but if no improvement is seen, the racing career is usually over (see Figure 108-12, B). Once severe lameness occurs, rest may improve comfort slightly, but horses will not tolerate training again. Trotters can tolerate racing with more advanced OA than can pacers.

Fig. 108-12 A, Caudocranial digital radiographic image of the left stifle of a 4-year-old Standardbred (STB) trotter with marked narrowing of the medial femorotibial joint (MFTJ, arrow). Radiological evidence of narrowing on a well-positioned caudocranial image is the earliest sign of osteoarthritis (OA) of the MFTJ. B, Advanced OA of the MFTJ is seen in this caudocranial image of a 9-year-old STB pacer. Absence of an obvious MFTJ space (arrow), marginal osteophyte formation (arrowheads) of the distal, medial aspect of the femur and proximal medial aspect of the tibia, and radiolucency of the distal aspect of the medial femoral condyle can be seen. This horse was 4 of 5 lame in the left hindlimb, and the prognosis for racing is grave.

In young horses osteochondrosis can cause lameness and effusion in early training, prompting radiographic examination, but signs of osteochondrosis often are seen in weanlings and yearlings, and surgery is performed before training begins. Osteochondritis dissecans of the lateral trochlear ridge of the femur is most common; it causes considerable effusion of the femoropatellar joint and if bilateral may cause a bunny-hopping gait. Surgical treatment, 4 to 6 months of rest, and intraarticular therapy are recommended. Prognosis for racing is good unless lesions are severe or cartilage damage of the patella is advanced. Subchondral bone cysts most commonly involve the medial femoral condyle but can occur in the proximal aspect of the tibia. Lameness is often progressive, with horses first being on a line. Surgical debridement of the cyst is recommended, although injection of corticosteroids beneath the lining of the cyst and into the cyst cavity under arthroscopic guidance can be used. After debridement, lameness persists for several months, but this approach may be a better long-term solution. Horses need a minimum of 6 to 8 months of rest to allow healing and for lameness to abate. Prognosis is fair (70%) for successful racing.

Rhabdomyolysis and Muscle Soreness

We divide muscle lameness into metabolic disease (recurrent exertional rhabdomyolysis [RER]) and nonmetabolic disease (muscle soreness). Signs vary, but horses appear stiff and hindlimbs are not fully engaged in the stride unilaterally or bilaterally. Palpation often reveals muscle pain but must be done carefully because aggressive, sudden, and deep palpation elicits false-positive responses.

RER occurs more commonly in females and can cause swollen muscles. Overt clinical signs of tying up after racing and training can be seen, but subclinical RER can be a cause of poor performance. Myoglobinuria and elevation of serum creatine kinase (CK) and aspartate transaminase (AST) levels are diagnostic. Horses are managed initially with intravenous fluids, NSAIDs, tranquilization, and methocarbamol (22 mg/kg intravenously [IV] or 33 mg/kg PO twice daily). Care must be taken when using acetylpromazine and methocarbamol simultaneously because effects may be additive, and horses may act severely tranquilized (JSM). Walking is recommended in horses with mild RER but is contraindicated in those with severe RER. Dantrolene sodium (2 mg/kg IV or via nasogastric tube) and intravenous dimethyl sulfoxide solution are recommended in horses with severe RER. Large quantities of intravenous or oral fluids should be administered. After an acute episode, signs may recur or subclinical myositis may persist, reflected by elevations in CK and AST concentrations. Enzyme levels should return to baseline before intense training or racing resumes.

In horses with subclinical RER or those with recurrent acute episodes, we recommend a management protocol that includes a reduction in carbohydrate intake, especially before exercise, a high-fat diet, vitamin E and selenium supplementation, supplementation with oral potassium, and a regular 7-day exercise program combining jogging, brief periods of intensive training, and paddock exercise, if feasible. Chronic administration of dantrolene sodium (2 mg/kg) or phenytoin (8 to 10 mg/kg PO twice daily) is recommended, but racing jurisdiction rules must be followed.

Nonmetabolic myositis is common, and gait is similar to that described for stifle lameness. Signs are similar to RER, but serum CK and AST levels are usually within or just above normal limits. The middle gluteal muscle and associated tendons commonly are affected unilaterally or bilaterally. Myositis may be a secondary problem or a primary source of pain after traumatic injuries such as accidents during transport, paddock injuries, slipping while racing on muddy or other undesirable surfaces, or by making a break. Horses are managed initially with muscle relaxants, NSAIDs, and rest (5 to 7 days). Horses with chronic soreness are treated with an internal counterirritant or corticosteroids injected strategically in the sore areas (see Chapter 88).

Curb and Superficial Digital Flexor Tendonitis

Other soft tissue injuries such as curb and superficial digital flexor tendonitis occur commonly. Curb in the STB is discussed in detail in Chapter 78. Superficial digital flexor tendonitis is discussed in detail in Chapter 69. Surgical management using desmotomy of the accessory ligament of the superficial digital flexor muscle (superior check desmotomy) and annular desmotomy if tendonitis involves the distal metacarpal region are recommended.