Clinical Appearance of Curb

The convex profile typical of curb is best seen from the side (Figure 78-1). Careful evaluation of swelling from all perspectives, palpation, and thorough lameness examination are critical. Curb must be differentiated from other swellings of the hock, including capped hock, effusion, edema and fibrosis of the calcaneal bursa (see Chapter 79), tarsal tenosynovitis (see Chapter 76), thoroughpin (with or without involvement of the tarsal sheath), and bony enlargements of the distal hock region (see Chapter 44). Injuries of the deep digital flexor tendon (DDFT) as it courses along the plantaromedial aspect of the hock within the tarsal sheath can produce typical signs of curb, but they account for only a small percentage of injuries in horses with curb.

Horses with sickle-hocked conformation are said to be curby (see Chapter 4). Sickle-hocked and in-at-the-hock conformation lead directly to curb, a finding most common in Standardbred (STB) and Thoroughbred (TB) racehorses. Prognosis for STB racehorses with sickle-hocked conformation and curb is worse in a trotter than in a pacer. Trotters with sickle-hocked conformation are usually fast early in training and racing, but this conformation is often career limiting. Sickle-hocked conformation is also undesirable in TB racehorses. Horses with sickle-hocked conformation often develop curb first, but they then independently or concomitantly develop other lameness associated with the distal hock joints. Tarsal region lameness begins with curb in 2- and 3-year-olds and progresses to osteoarthritis of the centrodistal and tarsometatarsal joints or slab fractures of the central tarsal bone, or more commonly the third tarsal bone.

Horses can have curby conformation without developing curb, and horses with normal hindlimb conformation can develop curb. The proximal aspect of the fourth metatarsal bone (MtIV) is often prominent in horses with sickle-hocked conformation. The most dramatic example of altered joint morphology occurs in young foals with tarsal crush syndrome, the result of delayed or incomplete ossification of the tarsal cuboidal bones (see Chapter 44).

Firm, fibrous soft tissue swelling can develop just proximal to the MtIV as a sequela to injection or local analgesia of the tarsometatarsal joint, presumably from local trauma, hemorrhage, or leakage into the subcutaneous tissues. Mild bony proliferation or fragmentation of the proximal aspect of the MtIV, or of the fourth tarsal bone, can cause focal swelling easily mistaken for curb.

Considerable variation in clinical signs occurs, and the injury cannot be categorized, or a management program and prognosis established, without thorough clinical and ultrasonographic examinations. Historically, owners and trainers consider curb to be an annoying, self-limiting problem that rarely causes lameness or poor performance, that responds to a single treatment that is uniformly effective, and that is cured by treatment. Most racehorse trainers are opposed to resting a horse with curb unless lameness is performance limiting, so veterinarians often are faced with management decisions without an option for even short-term rest or a reduction in training intensity. Many traditional therapies have no data to support efficacy. Thus, progress in understanding and management of this complex soft tissue injury has been limited.

Lameness ranges from none, mild, or severe depending on the structure involved and extent of injury. Lameness tends to be worse if the soft tissue structure involved is located dorsal to the superficial digital flexor tendon (SDFT) in the plantar aspect of the tarsus (i.e., the DDFT and/or the long plantar ligament [LPL]), if SDFT injury is diffuse, or if a mixed injury involves more than one structure. Diagnosis is straightforward in horses with obvious lameness seen at a trot in hand and painful swelling, but lameness may be evident only as a slight loss of performance or unlevelness when the horse is performing maximally and may be perceived only by trainers, drivers, or riders. A horse with chronic curb may not exhibit signs of pain during palpation, or lameness at a trot in hand, but can show lameness at speed, and convincing a trainer that the long-term swelling is a source of pain may be difficult. The area should be palpated carefully with the limb bearing weight and flexed. Swelling may be firm and fibrous, with few signs of active inflammation, or may be warm, painful, and edematous. Acute, compliant, or mushy swelling is associated with hemorrhage or other subcutaneous fluid accumulation and sometimes deeper soft tissue injury. Horses with this form of curb usually have acute, moderate-to-severe lameness. Horses with distal hock joint pain often exhibit a painful response when direct pressure is placed on plantar hock structures, including the SDFT, proximal aspect of the MtIV, second metatarsal bone, and proximal aspect of the suspensory ligament. Often swelling is not detected in these horses. Response to upper limb flexion varies and is nonspecific. Direct digital palpation followed by trotting is useful, because horses with active inflammation show increased lameness.

Because horses with curb can have concomitant osteoarthritis or other problems of the distal hock joints, differentiation of the source of pain is important but difficult. Diagnostic analgesia is useful but not foolproof. If horses are lame at a trot in hand, local infiltration of local anesthetic solution subcutaneously over the curb is effective. A minimum of 20 to 30 mL of local anesthetic solution should be infiltrated along the lateral, plantar, and medial aspects of the curb. Small-gauge needles should be avoided (to avoid needle breakage), and the injection should be performed with the limb in flexion, because horses may object to several injections. If horses are not visibly lame at a trot in hand, examination at the track or under saddle should be performed. Selective intraarticular analgesia of the distal hock joints and sequential perineural analgesia to rule out the lower limb are essential. A tibial nerve block alleviates pain with curb, but it is seldom done because other common sources of pain are abolished similarly.

Radiography and scintigraphy help differentiate other sources of pain, but ultrasonography is the imaging method of choice to determine which structures are involved and the extent of damage.

Applied Anatomy and Normal Ultrasonographic Examination of the Plantar Aspect of the Tarsus

Plantar to the calcaneus are skin, subcutaneous tissues, a thin fibrous tissue layer, the SDFT, and the LPL (Figure 78-2). Medially the DDFT courses distally over the sustentaculum tali, within the tarsal sheath. Normally the tarsal sheath has a small amount of fluid that can be seen during ultrasonographic examination, but it is not felt. The LPL originates from the calcaneus, closely adheres to this bone, and inserts distally on the plantar surface of the fourth tarsal bone and the MtIV. The plantar aspect of the tarsus can be divided into zones to classify findings (Figure 78-3) or the distance measured from the proximal aspect of the calcaneus (point of hock). Swelling comprising curb occurs in zone 1 of the tarsal and metatarsal regions but can extend into zone 2 if SDF tendonitis occurs. Transverse and longitudinal images of both limbs should be obtained from the plantar midline, plantaromedial (to evaluate the DDFT), and slightly plantarolateral (to evaluate the distal aspect of the LPL). Measurement of cross-sectional area (CSA) is important to confirm lesions in which enlargement has occurred but with no overt fiber damage. Precise placement of the ultrasound transducer is important because the LPL changes size and shape as it courses distally. Knowledge of normal ultrasonographic anatomy is crucial (Figures 78-4 to 78-7).^3,4

Curb: A Collection of Soft Tissue Injuries³

One hundred and ten horses with curb were examined using ultrasonography, including 87 (79%) racehorses (72 STB and 15 TB) and 23 nonracehorse sports horses (six field or show hunters, six Western performance horses, four show jumpers, two event horses, two gaited horses, two horses of unknown use, and one dressage horse).³ Mean and median ages (range 1 to 13 years) were 3.4 and 3.0, respectively. There were 32 late yearlings or 2-year-old horses, of which all were STBs; 24 of these STBs were examined before or near the time of a qualifying race. There were 24 intact males, 35 females, and 51 geldings. No sex predilection was found. Curb occurred in the left hindlimb (LH) in 60 horses, right hindlimb (RH) in 37 horses, and bilaterally in 13 horses. Of the 13 horses with bilateral curb, 11 were STBs. Within STBs there were 61 pacers and 11 trotters, compared with an expected population (ratio of pacers:trotters is 3:1 in North America) of 54 pacers:18 trotters. Curb was thought to result from excessive stress or strain from race training (80 horses), direct trauma (11 horses), previous injection (three horses), associated injuries (three horses), and an apparent “bad step” or unknown cause (13 horses). The most common reported form of direct trauma was stall or wall kicking. Curb was the primary reason for examination in 94 horses, was a compensatory lameness in 10 horses, and was seen in association with other tarsal injury or ipsilateral hindlimb lameness in six horses. Sixty-three horses were lame, and although lameness score was not uniformly recorded in the medical record, recorded lameness scores ranged from 1 to 2 out of 5. Horses with LP desmitis, those with SDF tendonitis extending distally from zone 1 into zone 2 of the proximal metatarsal region, those with deep digital flexor (DDF) tendonitis, and those with combined soft tissue injuries exhibited most pronounced lameness. Swelling (prominence of curb) was pronounced in horses with recent or combined soft tissue injury and in those with hematoma or abscess formation in the peritendonous and periligamentous (PT-PL) tissue. Swelling was described as fibrous in nature in many horses, particularly in STBs in which curb was chronic and in horses in which previous management included cryotherapy, topical counterirritation, or thermocautery. However, in acute injuries the swelling was often edematous, warm, and painful on palpation.

Ultrasonographic diagnosis included peritendonous-periligamentous (PT-PL) swelling without injury of the SDFT, DDFT, and LPL (29 [26%] horses) (Figures 78-8 to 78-10), PT-PL swelling primarily consisting of hematoma (six horses), and PT-PL swelling primarily consisting of abscess formation (five horses). Overall, 40 (36%) horses had swelling confined to the PT-PL tissues without other underlying soft tissue injury. There were 32 (29%) horses with PT-PL swelling and SDF tendonitis (Figure 78-11); 25 horses with PT-PL and LP desmitis (Figure 78-12); five horses with LP desmitis alone; five horses with DDF tendonitis; complex tarsal soft tissue injury of the LPL, PT-PL tissues, and gastrocnemius tendon in two horses; and tarsocrural collateral desmitis in one horse. Overall, only 32 (29%) horses had involvement of the LPL.

Management and outcome in these 110 horses with curb were difficult to assess, and information in medical records was lacking. There was a tendency for clinicians to manage horses with curb that were not lame or those in which curb was a compensatory issue with subcutaneous injections of a corticosteroid-containing preparation in combination with a reduction in exercise intensity or a short period of rest. Most of these horses had PT-PL swelling. Horses in which curb caused actual lameness were generally managed with rest and nonsteroidal antiinflammatory drug (NSAID) administration, but cryotherapy and topical counterirritation were often performed as well. Horses with severe SDF tendonitis extending into zone 2, LP desmitis, DDF tendonitis, or combination soft tissue injury were given long-term rest (>3 months) and controlled return to exercise, but they were the group most likely to do poorly or to remain lame. Horses (five) with PT-PL swelling and abscessation had surgical drainage and appropriate antimicrobial therapy and successfully returned to exercise. Associated ipsilateral lameness conditions included osteoarthritis of the centrodistal and tarsometatarsal joints and metatarsophalangeal joint pain.

Curb was previously described as simply LP desmitis,^1,2,5 but is a collection of plantar tarsal soft tissue injuries. In fact, in 68% of horses with curb in this study, the LPL was normal ultrasonographically, and horses with curb were more likely to have injury of the PT-PL tissue alone or in combination with SDF tendonitis than injury of the LPL.³ Curb should not be used synonymously with LP desmitis. Although not directly studied, CSA measurement (see Figure 78-7) of all soft tissue structures should routinely be performed and compared with measurements from the contralateral limb (some horses are bilaterally affected), because common forms of SDF tendonitis and LP desmitis are often associated with increased CSAs rather than frank fiber tearing.

Curb is primarily an injury of racehorses, especially STBs. Gait, speed, and training methods differ between racing breeds, and STB racehorses have a higher prevalence of conformational abnormalities, such as sickle-hocked and in-at-the-hock conformation. These conformational abnormalities have not been directly studied, and a cause-and-effect relationship is therefore difficult to establish. However, sickle-hocked conformation may increase load on the distal plantar soft tissue structures, predisposing to curb. Weight and load distribution are considerably different in STBs and TBs. Curb develops frequently in STB racehorses that train and race on a thin, near-hard surface, contrary to many soft tissue injuries that result from work on deep surfaces. Many curbs develop in young STB racehorses early in training when they are jogging and not formally performing speed work. In some instances track surfaces are inconsistent and perhaps slippery, because early training is done in the winter months. In many horses clinical signs are recurrent and progressive, and the development of clinical signs early in training supports a theory that curb develops primarily as a result of overload of plantar soft tissue structures, in a roughly plantar-to-dorsal direction, because the most common tissue involved is the PT-PL tissues. For instance, in our study SDF tendonitis was not seen in horses without PT-PL tissue enlargement, suggesting that PT-PL injury may have preceded injury of the SDFT.³ Pronounced lameness in horses with SDF tendonitis was seen only when injury progressed distally into zone 2 in the proximal metatarsal region; in some horses sickle-hocked conformation and alleged loss of support of the hock produced what appeared as a dropped hock. Horses with this form of curb, LP desmitis, or combination injury are most likely to be lame. Curb was more common in the LH limb, and although it is tempting to associate this finding with counterclockwise training, many horses developed curb before speed work commenced in that direction. Perhaps in STBs the conventional practice of giving horses many slow miles of jogging in a clockwise direction may place additional load on the outside LH, predisposing to curb. Curb appears more common in pacers than trotters. Nonracehorse sports horses develop curb, but only sporadically, and lameness is often moderate to severe and most commonly is associated with PT-PL tissue swelling, although other structures are sometimes involved.