Diagnosis

Osteoarthritis of the Distal Hock Joints and Distal Hock Joint Pain

Distal hock joint pain is common in horses from all disciplines and is often associated with OA. Distal hock joint pain is known colloquially as bone or jack spavin or occult or blind spavin in the absence of radiological abnormalities. The term juvenile spavin has been used to describe early OA that had a prevalence of 20% in a group of horses younger than 2 years of age.²⁶ Although it is usually seen in mature horses used for sport or pleasure, distal hock joint pain can occur in young Thoroughbred (TB) and STB racehorses and Western performance horses (see Chapter 120). Distal hock joint pain may be a sequela to incomplete ossification of the central and third tarsal bones (see page 517). Certain conformational abnormalities (such as sickle-hock, in-at-the-hock, or cow-hock conformation) or excessive straightness of the hindlimbs may predispose to distal hock joint pain, although this condition frequently occurs in normally conformed horses. Traditionally it has been proposed that OA of the distal hock joints is caused by excessive compression and rotation of the distal tarsal joints as the horse jumps or stops, which results in abnormal tension on the intertarsal ligaments. However, this theory is not consistent with the common recognition of distal hock joint pain in pleasure horses or its high incidence in Icelandic horses, a breed in which OA is thought to be a heritable condition.²⁷ Distal hock joint pain is classically thought to begin on the dorsomedial aspect of the joints and to progress dorsally. However, it is our experience that in OA, scintigraphic or radiological abnormalities are frequently first identified only on the dorsolateral aspect of the joints, a region of high compressive strain. Previous exercise history and thus loading of the joints may be influential.²⁸ Nuclear scintigraphic studies of clinically normal mature horses in active work has shown mild greater uptake of radiopharmaceutical on the lateral aspect, which is consistent with increased modeling, presumably the result of a relative increased loading laterally compared with medially.⁷

The centrodistal and tarsometatarsal joints are most commonly affected, either individually or together, but OA of the proximal intertarsal joint does occur, usually in association with OA of the more distal joints. The condition may be unilateral but is often bilateral. Occasionally OA of the talocalcaneal joint occurs either in isolation^29,30 or together with OA of the centrodistal and tarsometatarsal joints³¹ (see page 516).

Radiography and Radiology

Both hocks should be examined radiographically because the condition is often bilateral, although at the time of examination lameness may be unilateral. Radiological abnormalities consistent with osteoarthritis include periarticular osteophyte formation (Figure 44-2), subchondral lucent areas (Figure 44-3), increased opacity of subchondral and trabecular bone, periosteal new bone, and narrowing of joint spaces. The correlation between the degree of lameness and the extent of radiological abnormalities is poor.^33,34 Some horses show profound lameness initially, with no detectable radiological abnormality but with rapidly progressive lesions. Other horses respond to intraarticular analgesia but never develop radiological abnormalities. However, extensive changes may be present when lameness is first recognized. Some horses predominantly develop periarticular changes, whereas others have abnormalities confined to the central and third tarsal bones and the proximal aspect of the MtIII. The reason for these differences is currently unclear. Small osteophytes or enthesophytes on the dorsoproximal aspect of the MtIII can be incidental radiological abnormalities; they were found to occur in 25% of 455 horses, in 13% of which they were bilateral.³⁵ Osteophytes and enthesophytes could not be reliably distinguished. There was no significant difference in frequency of occurrence in lame and nonlame horses; nor was there any significant difference in frequency of occurrence among horses with distal hock joint pain, proximal suspensory desmitis, or other causes of lameness unassociated with the hock.³⁵ However, there was an association between the presence of a spur and the radiological grade of OA in the centrodistal and tarsometatarsal joints. Horses with severe subchondral radiolucency tend to be lamer and respond poorly to intraarticular analgesia and intraarticular medication. Involvement of the talocalcaneal-centroquartal (proximal intertarsal) joint merits a more guarded prognosis.

Fig. 44-2 A, Dorsolateral-plantaromedial oblique radiographic image of the tarsus of a horse with mild periarticular osteophyte formation (arrow) on the distal dorsomedial aspect of the central tarsal bone and increased radiopacity of the adjacent trabecular bone. B, Lateromedial radiographic image of the distal tarsal joints. There is a large periarticular osteophyte on the dorsoproximal aspect of the third metatarsal bone (arrow), traversing the tarsometatarsal joint.

Fig. 44-3 Dorsolateral-plantaromedial oblique radiographic image of the tarsus of a horse with osteoarthritis of the centrodistal joint. There is subchondral radiolucency involving the subchondral bone of the medial aspect of the centrodistal joint (arrow). There is also increased opacity in the region of the interosseous ligament between the central and third tarsal bones.

Osteoarthritis of the Talocalcaneal Joint

OA of the talocalcaneal joint is unusual and may occur in isolation^29,30 or together with OA of the centrodistal and/or tarsometatarsal joints.³¹ There are frequently no localizing signs, although in a retrospective study there was distention of the tarsocrural joint in six of 18 horses.³¹ Lameness is resolved by perineural analgesia of the fibular and tibial nerves and may be improved by intraarticular analgesia of the tarsocrural joint. Radiological abnormalities are best identified in lateromedial or dorsomedial-plantarolateral oblique images, or in some horses a flexed proximoplantar-distal plantar image, and include subchondral lucency or increased radiopacity and narrowing of the joint space (Figure 44-4). Scintigraphy reveals focal, moderate-to-intense IRU in the subchondral bone of the talocalcaneal joint and can be differentiated from enthesopathy of the attachment of the lateral CL and lesions involving the sustentaculum tali (see later) by acquiring lateral, plantar, and, if necessary, medial and flexed lateral scintigraphic images. Occasionally lesions that are not evident radiologically can be identified using MRI or CT. The prognosis with conservative management is guarded to poor. Surgical arthrodesis has resulted in improvement in, but not resolution of, lameness.³¹

Fig. 44-4 Lateromedial radiographic image of a tarsus of a horse with osteoarthritis of the talocalcaneal joint. Some loss of joint space and irregular subchondral radiolucency (arrows) are present.

Osteoarthritis of the Tarsocrural Joint

OA of the tarsocrural joint may occur as a primary disease, may develop secondarily to osteochondrosis (see Chapter 56) or OA of the distal hock joints (see page 512), or may be a sequela to trauma. There may be distention of the tarsocrural joint capsule, but this is a variable feature. The joint capsule may feel thickened, more so than observed in horses with idiopathic effusion or a bog spavin as the result of osteochondritis dissecans (OCD). Initially horses respond favorably to intraarticular injections into the tarsocrural joint. There is controversy regarding the association of corticosteroid injections and the subsequent development of OA of this joint. Although in most horses with end-stage OA there is a history of numerous injections including corticosteroids into the tarsocrural joint, this is not always the case. We both believe that there is an association between the presence of OCD and the subsequent development of severe OA in older horses (6- to 8-year-old racehorses and sports horses), whether or not the affected horse had arthroscopic surgery to remove the fragments at a young age.

Lameness varies from mild to severe. Horses with longitudinal wear lines on the trochleas of the talus secondary to OCD of the cranial intermediate ridge of the distal aspect of the tibia often have only mild lameness. However, horses with focal partial- or full-thickness cartilage erosions on weight-bearing parts of the trochleas of the talus may have severe lameness (see Figure 56-11). The response to proximal limb flexion is variable. Lameness usually is partially improved by intraarticular analgesia, but occasionally there are false-negative results. A large volume (up to 50 mL) of local anesthetic solution may be necessary, and often it takes 45 to 60 minutes for lameness to improve in horses with severe OA of the tarsocrural joint. Perineural analgesia of the fibular and tibial nerves results in substantial improvement in lameness. In horses with primary OA of the tarsocrural joint, usually there are no detectable radiological abnormalities, or only subtle evidence of subchondral sclerosis and narrowing of the tarsocrural joint space, and scintigraphic examination is often inconclusive. Diffuse mild IRU may be all that is seen, sometimes localized to the cranial and caudal distal aspect of the tibia. In racehorses, scintigraphic examination often reveals IRU involving the distal aspect of the tibia and proximal aspect of the talus and must be differentiated from subchondral trauma or fracture of the distal tibia and talus.⁵⁸ A flexed lateral scintigraphic image reveals that IRU involves both sides of the tarsocrural joint, unlike focal uptake, which remains associated with either the talus or the distal tibia. Ultrasonographic examination may reveal synovial proliferation even in the absence of detectable joint effusion. Definitive diagnosis is dependent on arthroscopic examination of the joint. The prognosis for resolution of lameness and return to full athletic function is generally poor to grave. Response to intraarticular medication is often disappointing.

Incomplete Ossification of the Central and Third Tarsal Bones

Incomplete ossification of the central and third tarsal bones is most common in premature or twin foals, but it can occur in full-term singles and is characterized by a sickle-hocked appearance. It may occur unilaterally or bilaterally. Radiologically the bones are smaller and more rounded than usual. Early recognition and treatment are essential to avoid crushing of these bones with resultant progressive OA. Foals with this condition must be rested in a stall or turned out in a small paddock with only their mare. However, many owners or farm managers are loath to give foals rest, and the condition becomes progressive, often leading to collapse and OA and fracture. Cylinder tube casts can be used effectively with good results. In some horses the condition goes unrecognized until a young horse increases work intensity, although the horse may have a preexisting curblike appearance of the hock. Lameness may be sudden in onset despite the existence of advanced radiological changes. Radiological examination reveals a wedge shape of the central and third tarsal bones, which are narrower dorsally (see Figure 128-4). One or more bones may be fractured. There is often advanced OA. Although affected horses may be used for pleasure riding, the prognosis for competition use is guarded.

Distention of the Tarsocrural Joint Capsule

There are many causes of acute or chronic synovitis of the tarsocrural joint (also known as bog spavin) including osteochondrosis, OA, trauma, poor conformation, hemarthrosis, infection, and idiopathic causes. There are case reports of eosinophilic synovitis⁵⁹ and ectopic cartilage of unknown origin.⁶⁰ Signalment varies and depends on origin. Clinical signs are excessive tarsocrural joint fluid, which is most easily recognized on the dorsomedial and plantarolateral aspects of the joint. Lameness may or may not be present and is dependent on the cause. Excessive distention of the joint capsule may result in mechanical lameness. Diagnosis is made on the basis of clinical appearance and the response to intraarticular analgesia if the horse is lame. Arthrocentesis may reveal hemorrhage. Oddly, the tarsocrural and antebrachiocarpal joints are the most common joints in which we have diagnosed intermittent severe lameness as a result of hemarthrosis.²¹ Large vessels that traverse the dorsal aspects of the joint capsules in both joints may be at risk for injury and cause subsequent bleeding into the joint. Radiographic examination should include four standard images, but if no radiological abnormality is detected, flexed lateromedial and flexed dorsoplantar (skyline) images may be helpful. Slightly oblique dorsoplantar images may be necessary to identify OCD of the medial malleolus of the tibia (see Chapter 56). The radiographs should be inspected carefully for evidence of previous surgery for removal of an osteochondral fragment from the cranial intermediate ridge of the distal aspect of the tibia or other common locations associated with OCD. Occasionally, small fragments remain at previous sites from which OCD lesions were removed, and it is possible that these small fragments or repair tissue at the site may be loose, causing persistent effusion. Lameness in these horses is usually minimal, and the decision to reoperate should be made on a case-by-case basis and reserved for those horses with pain that can be abolished with diagnostic analgesia. Effusion often persists even after removal of OCD fragments from the tarsocrural joint, and persistent effusion is usually not a cause of lameness but may be of concern cosmetically.

Ultrasonography is useful for characterizing the appearance of the fluid within the joint; synovial fluid is anechogenic, whereas blood is echogenic. Ultrasonography is also useful for assessment of the thickness of the joint capsule, the presence of synovial hyperplasia, and the thickness of the articular cartilage and may be more accurate than radiography for identification of osteochondral fragments.⁶¹ In the absence of radiological or ultrasonographic abnormalities, a diagnosis of idiopathic synovitis is made.

Treatment of horses with idiopathic synovitis involves fluid drainage and intraarticular injection of either 80 mg of methylprednisolone acetate or 12 mg of triamcinolone acetonide. Combining hyaluronan with the corticosteroids may provide joint protection, but the strong antiinflammatory effect of the corticosteroids seems most beneficial. Pressure bandages should be applied to help maintain joint decompression. Phenylbutazone (2.2 mg/kg) once daily for 7 days and confinement to a small area for 2 weeks is recommended. Approximately 50% of horses have resolution or decrease in effusion, although some horses may require retreatment. Intraarticular injection of atropine (8 mg) is sometimes successful. If joint effusion returns and the horse is not lame, the owner is advised that the horse has a cosmetic blemish that will probably not resolve completely. Horses that are lame and block to the tarsocrural joint but have no radiological lesions are candidates for scintigraphic examination and diagnostic arthroscopy (see discussion of OA of the tarsocrural joint, page 516). Hemarthrosis is discussed in Chapter 66.

Osteochondrosis of the Tarsocrural Joint

See Chapter 56 for a discussion of osteochondrosis of the tarsocrural joint (page 635).

Subchondral Bone Trauma

With the advent of MRI and CT there is increasing recognition of the role of subchondral bone trauma and pain in causing lameness.⁶² There are anecdotal reports of lesions involving the talus, calcaneus, and central and third tarsal bones, but as yet these lesions have been rather poorly defined, and risk factors and response to treatment have not been documented.

Osseous Cystlike Lesions

Occult osseous cystlike lesions, detected using CT but usually not by radiography, have been recorded in the medial malleolus of the tibia (five horses), intertrochlear groove of the talus (four horses), lateral malleolus (two horses), and cranial intermediate ridge of the distal aspect of the tibia (one horse).⁶³ We have also seen single or multiple lesions in the distal subchondral bone plate of the tibia and the trochleas of the talus that were detectable scintigraphically and radiologically.⁶⁴ There are usually no localizing clinical signs. Lameness is moderate to severe. In some but not all horses lameness is substantially improved by intraarticular analgesia of the tarsocrural joint. These lesions are associated with focal intense IRU. These lesions are believed to be of infectious or traumatic origin. Conservative management has resulted in persistent lameness in most but not all horses. Surgical curettage may offer a more favorable prognosis, but lesions involving the subchondral bone of the distal aspect of the tibia in the trochlear grooves are not accessible to arthroscopic evaluation and curettage. To access these lesions an extraarticular approach may be necessary. Four of six horses (67%) managed surgically returned to athletic function.⁶³

Fragments in the Talocalcaneal-Centroquartal (Proximal Intertarsal) Joint

The tarsocrural joint communicates directly with the proximal intertarsal joint in immature horses through a slitlike opening and in adult horses through a broad opening in the dorsal reflection of the synovium that is located at the distal dorsal aspect of the talus (see Chapter 56). The proximal intertarsal joint is approached through this synovial fenestration during arthroscopic exploration of the tarsocrural joint. A report describes 17 horses with either a free-floating fragment, which was suspected to have originated and been dislodged from an associated OCD lesion in the tarsocrural joint, or a fragment located at the distal end of the medial trochlear ridge with attachment to the synovial reflection that separates the tarsocrural and proximal intertarsal joints.⁶⁵ However, such fragments can be seen as incidental abnormalities and unassociated with lameness.¹⁸ Fragments associated with the distal aspect of the medial trochlear ridge of the talus can be seen in only a lateromedial radiographic image, whereas loose fragments within the proximal intertarsal joint most often can be seen on numerous radiographic images. Careful examination of the cranial intermediate ridge of the distal aspect of the tibia is necessary, because loose fragments within the proximal intertarsal joint most often dislodge from this location, and a depression will often be seen from which original fragment originated. Fragments associated with the distal aspect of the medial trochlear ridge are often enveloped in the joint capsule separating the tarsocrural and proximal intertarsal joints, medial to the fenestration between the joints. Sharp dissection is necessary to incise capsular attachments, and care must be taken to avoid damage to the articular surface of the central tarsal bone. Fourteen of 17 horses returned to racing or intended performance activity after arthroscopic removal of the fragment.⁶⁵

Fractures of the Distal Tarsal Bones

Fractures of the central and third tarsal bones occur most frequently in STBs, cutting horses, or TB racehorses,^66-70 but they occasionally occur in other horses, sometimes secondary to previous fusion of the centrodistal and tarsometatarsal joints. These fractures cause an acute onset of a moderate-to-severe hindlimb lameness that is usually most noticeable when the affected limb is on the inside of a circle.³² Despite acute onset of lameness, these fractures in young racing TBs may be the result of stress-related bone injury.⁶⁹ The lameness is exacerbated by upper limb flexion. The degree of lameness diminishes after 1 to 2 weeks of rest, but lameness returns if the horse returns to work. Some horses with incomplete fractures of the central tarsal and third tarsal bones can race a number of times before lameness becomes pronounced. On initial diagnosis, some fractures appear chronic in both the TB and STB racehorse. Horses with bilateral third tarsal bone fractures may be examined for poor performance rather than overt unilateral hindlimb lameness. Within STB racehorses in North America, central and third tarsal bone fractures are more common than expected in pacers compared with trotters, even if the normal 3 : 1 pacer/trotter ratio is considered. Heat, soft tissue swelling, and pain on digital palpation of the distal tarsal bones usually accompany the fracture. Synovial effusion of the tarsocrural joint may occur in horses with central tarsal bone fractures but is not usually seen in horses with third tarsal bone fractures. Intraarticular analgesia is used to diagnose central and third tarsal bone slab fractures, but in some horses, particularly those with incomplete or complete, nondisplaced fractures, fibular (peroneal) and tibial nerve blocks are needed to abolish pain.

Diagnosis is made by radiography or scintigraphy, but a fracture may not be radiologically apparent until up to 10 days after injury, when demineralization of the fracture line occurs. Scintigraphy is useful in horses with mild or bilateral hindlimb lameness. Incomplete or bilateral fractures may be seen. Many fractures can often be identified on a lateromedial radiographic image but most consistently can be seen on a DM-PlLO image. If no radiological abnormality is detectable in standard images, additional oblique images, including a plantar 25° lateral-dorsomedial oblique image, should be obtained (or D65° M-PlLO image) (Figure 44-5). However, the precise location and orientation of fractures seem to vary between disciplines and between the central and third tarsal bones. Thus if a fracture is suspected, additional oblique images may be required. CT can be useful to determine the exact location and configuration of a fracture and to pinpoint screw location if surgical repair is elected. In STBs, two types of central tarsal bone fractures are seen. Authentic frontal (dorsal) plane slab fractures extending from the proximal-to-distal articular surfaces of the central tarsal bone are most common and are best seen in either a DM-PlLO or lateromedial image (see Figure 44-1). Another type of central tarsal bone fracture courses distodorsally from the proximal aspect of the central tarsal bone in the dorsal plane and breaks out of the dorsal cortex of the bone just proximal to the distal articular surface of the central tarsal bone (Figure 44-6). This second type of fracture can best be seen in a lateromedial radiographic image, and because the fracture line does not appear to extend to the distal articular surface, this fracture may not be an authentic slab fracture. Lameness is pronounced, and horses exhibit identical clinical signs to those with larger fragments. In Western performance horses the fracture line is generally located in a more plantar region of the third and central tarsal bones than is seen in racehorses. Third tarsal bone fractures are most commonly dorsal or dorsolateral. In TB racehorses a wedge-shaped conformation of the dorsolateral aspect of the third tarsal bone may be a risk factor for fracture.⁷¹ Comminuted fractures are more common in the central tarsal bone and may be difficult to identify radiologically.⁶⁷ If comminution occurs, it is usually at the proximal aspect of the fracture, and fragments can be seen and removed using arthroscopic surgical techniques through the tarsocrural joint. Nuclear scintigraphy is invaluable for identifying the likely presence of a fracture if it cannot be determined radiologically.

Fig. 44-5 Plantar 25° lateral-dorsomedial oblique radiographic image of a tarsus. There is a complete slab fracture of the third tarsal bone. This radiographic image is the same projection as a dorsomedial-plantarolateral oblique image showing that the fracture is located on the dorsolateral aspect of the third tarsal bone.

Fig. 44-6 Lateromedial digital radiographic image of the right tarsus of a 3-year-old Standardbred filly with pronounced lameness abolished by perineural analgesia of the fibular (peroneal) and tibial nerves. Lameness did not change after intraarticular analgesia of the tarsocrural joint. There is an oblique dorsal plane fracture of the central tarsal bone (arrows) that courses distodorsally from the proximal articular surface of the central tarsal bone and to the dorsal cortex, just proximal to the centrodistal joint. Mild proliferative changes can be seen, indicating that at the time of diagnosis the fracture was chronic.

Horses may be treated conservatively^69,70,72,73 or surgically.^67,68,70,71 Seven horses with fractures of the third tarsal bone that were treated conservatively remained lame for 12 months after injury,^67,68 whereas three horses treated by lag-screw compression returned to racing within 6 months.⁶⁷ Six racehorses with central tarsal bone fractures remained lame, whereas six Quarter Horses treated surgically made a complete recovery.⁶⁸ However, conservative management resulted in return to training within 8 months for 73% of 45 TB racehorses with fractures of the central or third tarsal bone,⁶⁹ and this corresponds with our experience. In a retrospective study 20 of 24 horses with slab fractures of the central tarsal bone were STBs, and of these 82% raced after injury with conservative management.⁷⁰ Of 28 horses with third tarsal bone slab fractures managed conservatively, 25 had unilateral and three had bilateral fractures, and there were 12 TBs, 15 STBs and one Quarter Horse.⁷⁰ After conservative management, 87% of the STBs and 71% of the TBs raced after injury.⁷⁰ During the study period five STBs with slab fractures of the central (one horse) and third (four horses) tarsal bones underwent surgical management with interfragmentary compression, and three raced after surgry.⁷⁰ Ten of 14 STBs (71%) and two of six TBs (33%) returned to racing and started at least five races after injury, and four of five Quarter Horses (80%) returned to previous athletic function with conservative management.⁷² The prognosis in STBs appears satisfactory when conservative management is used, but there appears to be room for improvement in the TB racehorse. We do not know the long-term prognosis of a large group of TB racehorses managed surgically, however. If surgical treatment is performed, case selection is important, because many horses with central tarsal bone fractures have hidden fracture lines that could be inadvertently displaced during screw placement. The large head of a 4.5-mm cortex bone screw may cause soft tissue inflammation and a periosteal reaction. For third and central tarsal bone fractures, 3.5-mm cortex bone screws are preferred. One or two screws should be placed through stab incisions by using radiographic or fluoroscopic guidance, or by using CT. The use of a Herbert cannulated compression screw has been reported.⁶⁸ Horses with fragments that are too small to allow lag-screw fixation, or those with chronic fractures of the third tarsal bone causing OA of the centrodistal or tarsometatarsal joints, can be managed by surgical drilling to facilitate arthrodesis of the joints.

Sagittal Fracture of the Talus

Sagittal fractures of the talus are rare, usually nondisplaced, can be difficult to diagnose, and have been recorded most frequently in STB and TB racehorses,^74,75 but they can occur in any type of horse. Twelve racehorses had a history of chronic, mild hindlimb lameness that became acutely severe during a race.^74,75 One of us (SJD) has also seen two endurance horses with acute-onset severe lameness after an endurance race.⁶⁴ Lameness in other types of horses may be associated with trauma. Lameness is moderate to severe and is sometimes associated with distention of the tarsocrural joint capsule. The horse may stand with the limb somewhat protracted and show accentuation of lameness when turning. Intraarticular analgesia may improve lameness but rarely removes it completely, and negative results may also occur. Horses show marked improvement after fibular (peroneal) and tibial perineural analgesia. Fractures in racehorses usually originate at the proximal aspect of the sagittal groove of the talus and are often incomplete. Nondisplaced fractures can be difficult to identify radiologically; extra images, including a dorsal 10- to 20° lateral-plantaromedial oblique and a flexed dorsoplantar (skyline) image, can be helpful. Fractures caused by trauma are frequently comminuted, and the degree of damage may not be apparent radiologically. Nuclear scintigraphy can be helpful in confirming the presence of a fracture; there is usually intense focal IRU in the proximal aspect of the talus.⁷⁴

Eleven racehorses with incomplete sagittal fractures of the talus were managed conservatively with a minimum of 1 month of stall rest followed by small paddock turnout. Seven returned to racing performance within 7 to 8 months after injury; approximately 50% had the same or improved performance.⁷⁴ Both endurance horses were managed conservatively and returned to full athletic function.⁵⁹ Horses with complete sagittal fractures have been managed by lag-screw compression using two 4.5-mm cortex bone screws.⁷⁰ Prognosis may be favorable in horses with simple acute fractures, but the prognosis for horses with comminuted fractures is poor.

One of us (MWR) has seen two foals that had complete, nondisplaced slab fractures of the talus that healed well with conservative management.⁶⁴ Acute lameness and moderate tarsocrural effusion were found in both foals; the fracture line could best be seen coursing from the proximal-to-distal articular surface of the talus in a dorsal 15° lateral-plantaromedial oblique radiographic image (Figure 44-7).

Fig. 44-7 Initial (A) and 6-week follow-up (B) digital dorsal 15° lateral-plantaromedial oblique radiographic images of a suckling Standardbred foal with acute hindlimb lameness and moderate tarsocrural joint effusion. A, A nondisplaced sagittal fracture of the talus can be seen (arrows) coursing from the proximal-to-distal articular surfaces. B, Six weeks later the fracture line appears blurred and cannot be seen at the proximal aspect of the talus but can still be seen at the distal aspect (arrows). This foal made a complete recovery using conservative management consisting of 10 weeks of box stall rest.

(Courtesy Dr. Jan Henriksen, Walnridge Equine Clinic, Creamridge, New Jersey, United States.)

Fractures of the Fibular Tarsal Bone

Fractures of the fibular tarsal bone (or calcaneus) are uncommon and usually the result of trauma. Physeal fractures in foals and fractures through the body of the bone are easily diagnosed because of the obvious loss of gastrocnemius muscle function, which results in a dropped-hock appearance. Chip fractures involving the plantar aspect of the calcaneus can be difficult to diagnose unless soft tissue swelling is present or a draining tract secondary to sequestra formation exists. Surgical removal of small fragments may be necessary, depending on size and location of the fragment (intraarticular or extraarticular). Diagnosis is confirmed radiologically. Flexed lateromedial and skyline images of the calcaneus are recommended in addition to standard views (Figure 44-8). Complete body and physeal fractures can be difficult to reduce and stabilize, but horses can be managed successfully with bone plates, screws, and wires by using the tension-band principle. It is not possible to place a bone plate directly plantad because of the presence of the superficial digital flexor tendon, but the plantarolateral or lateral surface of the calcaneus has been used successfully. One of us (MWR) has used a combination of interfragmentary compression and figure-eight tension band wiring to successfully repair fractures of the calcaneus in two miniature horses.⁶⁴ Conservative therapy using casting methods alone has been unrewarding and is not recommended. Horses with open, comminuted fractures have a grave prognosis, and humane destruction should be advised. Prognosis for future performance activity in horses with physeal or full-body fractures is considered poor.

Fig. 44-8 Flexed lateromedial radiographic image of a tarsus. There is a complete, comminuted articular fracture through the calcaneus.

Fracture of the Lateral Malleolus of the Tibia

The lateral malleolus is considered to be the distal end of the fibula and develops as a separate center of ossification that fuses to the distal tibial epiphysis by 1 year of age. The long lateral and deeper short CLs originate on the lateral malleolus of the tibia, just plantar to the groove for the tendon of the lateral digital extensor muscle. Only a small portion of the lateral malleolus is intraarticular; therefore most fracture fragments are located within the joint capsule and CLs. The lateral malleolus is mostly covered by soft tissue attachments; therefore careful dissection and the use of a motorized synovial resector are necessary for successful removal of fracture fragments using arthroscopic techniques.

Lateral malleolar fractures are usually traumatic in origin. Small, well-rounded fragments, probably a manifestation of osteochondrosis, occur occasionally and should be differentiated because horses are often asymptomatic. Clinical and radiological signs and results of surgical removal of the fragments have been reported in 16 horses.⁷⁶ All were TBs with injuries incurred either during a fall in a race over fences or from being kicked. All horses had a moderate degree of lameness and tarsocrural effusion. Approximately 50% of the horses had periarticular swelling, thickening of the CL, and pain on digital palpation of the lateral malleolus. All fractures were visible on a dorsoplantar radiographic image (Figure 44-9). Fourteen fractures were unilateral and two bilateral, with nine simple and nine comminuted. Thirteen of 16 fragments were displaced distally and rotated 90 degrees.

Fig. 44-9 Dorsoplantar radiographic image of a tarsus. There is a fracture of the lateral malleolus (arrow).

Ultrasonographic evaluation is useful to determine the location of the fracture in a dorsoplantar plane and to identify a concurrent collateral desmitis.²² Horses with small or minimally displaced fractures have been successfully managed conservatively. Surgical removal may result in a quicker recovery. The tarsocrural joint should be inspected arthroscopically to remove any debris, and surgical removal of most fragments is possible; use of a motorized synovial resector facilitates debridement of the joint capsule and CL attachments to the fracture fragment. In some horses the fragment can be easily removed via an incision through the lateral CL of the tarsocrural joint, depending on fragment orientation. Thirteen of 16 horses treated by fragment removal returned to full athletic function.⁷⁶ If the fracture fragment is of adequate size or if there is instability of the tarsocrural joint, lag-screw fixation and tension band wiring should be considered. One of us (MWR) examined a horse with lateral and medial malleolar fractures; repaired the larger, comminuted, displaced lateral malleolar fracture using a single 4.5-mm cortex bone screw placed in lag fashion augmented by a figure-eight tension band wire; and repaired the nondisplaced medial fracture using interfragmentary compression.⁶⁴ The repair was performed using conventional surgical techniques, and small fragments originating from the lateral malleolus were removed. With conservative or surgical management the prognosis is good.

Fractures of the Medial Malleolus of the Tibia

Fractures of the medial malleolus of the distal tibia are rare. Similar to fractures of the lateral malleolus (see earlier), occasionally, small rounded fragments are displaced distal to the medial malleolus, intercalated in the medial CL. These fragments may be a manifestation of osteochondrosis and are distinct from the more common OCD fragments originating from the cranial, articular component of the medial malleolus (usually axially), in the tarsocrural joint. Alternatively, these rounded fragments may be the result of an old fracture. Authentic fracture of the medial malleolus occurs rarely alone, or in combination with lateral malleolar fractures (see earlier).^64,77,78

Tarsal Joint Luxation

Complete luxation or subluxation of the tarsocrural, talocalcaneal-centroquartal (proximal intertarsal), and tarsometatarsal joints may occur, with or without concurrent tarsal bone fractures.^79,80 Tarsal luxations are the result of severe trauma from kicks from other horses or limb entrapment in fixed objects, such as a fence or cattle guards. Proximal intertarsal and tarsometatarsal luxations are most common. Luxation causes severe, non–weight-bearing lameness, and there may be abnormal deviation of the limb at the tarsus. Crepitus may be present with a fracture, and palpation reveals tarsal joint instability. Diagnosis is confirmed by radiological examination. Dorsoplantar radiographic images obtained with medial or lateral limb traction are recommended to determine the extent of CL damage (Figures 44-10, 44-11, and 44-12). Treatment consists of reduction of the luxation with the horse under general anesthesia and stabilization with the application of a full hindlimb cast extending from the foot to a point level with the tibial tuberosity. Alternatively, external coaptation consisting of a Robert Jones bandage with splints (custom-made) or external cast material (cast, bandage) can be applied to a standing, sedated horse. Although reduction will be less than ideal and invariably an angular deformity will persist, a successful outcome can be achieved.⁶⁴ Internal fixation is rarely indicated but may be necessary with concurrent tarsal bone fracture. Double plating techniques using conventional or locking compression plates applied proximal to the fracture or luxation using the plantarolateral aspect of the calcaneus and the medial aspect of the hock and ending distally in the metatarsal bones, in combination with external coaptation, could be used. Tarsocrural or tarsal luxation with functionally intact CLs can be difficult to reduce, but manipulation of the tarsus with the limb in a flexed position may facilitate reduction. The limb should be maintained in a cast or other external coaptation constructed to be as rigid as possible for 4 to 6 weeks if the CLs are intact and 6 to 8 weeks if the CLs are ruptured.^79,80 After the cast has been removed, the hindlimb is placed in a heavy cotton bandage with rigid splints for an additional 4 to 6 weeks before turnout in a small area is allowed. Of the seven reported horses with tarsal joint luxation (three proximal intertarsal, three tarsometatarsal, and one tarsocrural), all were treated with closed reduction and external coaptation, and all returned to either light riding, pasture soundness, or breeding activity. The presence of tarsal bone fractures did not affect outcome. All horses had a reduction in range of motion of the tarsus and OA that prevented the return to athletic soundness. Prognosis for horses with proximal intertarsal and tarsometatarsal joint luxations is reasonably good for pasture soundness.⁷⁹ A guarded prognosis is warranted for horses with tarsocrural joint luxation because of the difficulty in reducing the luxation.

Fig. 44-10 Flexed lateromedial image of a hock. There is widening of the plantar aspect of the talocalcaneal-centroquartal joint (arrows), which is indicative of luxation.

Fig. 44-11 Stressed dorsoplantar image of a hock. There is luxation of the tarsometatarsal joint, which is associated with fracture of the third and fourth tarsal bones.

Fig. 44-12 Stressed dorsoplantar image of a hock. There is luxation of the tarsocrural joint.

Collateral Ligament Damage

Complete extension of the tarsocrural joint is prevented by tension in the CLs. CL injury is usually the result of trauma or a fall; however, it is not a common injury. CL injury was identified in only 23% of 133 horses that underwent ultrasonographic evaluation of the tarsus because of suspected soft tissue injury.²² Lameness varies from mild to severe, depending on the degree of damage. There is often distention of the tarsocrural joint capsule and periarticular soft tissue swelling. Flexion is usually resented. Analgesic techniques are usually not required, but lameness may be improved in some horses by intraarticular analgesia of the tarsocrural joint. In an acute injury there often are no radiological abnormalities unless there is an avulsion fracture⁸¹; however, 4 to 6 weeks after injury, enthesophytes develop at the origin or insertions of the injured CLs. Nuclear scintigraphy is helpful in horses with mild injuries because it demonstrates intense, focal IRU at the ligament attachment sites.⁸² Ultrasonographic evaluation is used to identify the injured structure(s) and assess the severity of the injury (see Chapter 17).²² In a series of 23 horses, 18 sustained medial CL injury, including three with both medial and lateral CL injury, and five horses had lateral CL injury alone, with injury to the superficial components being most common either alone or together with injury to the deep component. Initial treatment involves stall rest, cold water therapy, topical dimethyl sulfoxide with or without corticosteroids, and NSAIDs. Long periods of rest (e.g., 6 months) followed by a controlled exercise program have been recommended.⁸¹ Prognosis depends on the severity of injury, but horses with very extensive periosteal new bone have been able to return to full athletic function, although some residual enlargement of the hock has persisted.^21,64

Old avulsion fractures at the insertion of the lateral and less commonly the medial short CL are sometimes detected in horses with pain causing lameness localized to the hock with no other explanation for the cause of lameness. There is no associated IRU, and the clinical significance of such fractures is difficult to determine accurately. It is conceivable that low grade motion may cause pain.

Enthesopathy of the Lateral Collateral Ligaments of the Tarsocrural Joint

Enthesopathy of the long, and less commonly, the short lateral CLs is a rare condition that is recognized most often in STB racehorses, especially pacers, but does rarely occur in TBs. It results in lameness that is improved by analgesia of the tarsocrural joint. There may be distention of the tarsocrural joint capsule, and subtle, localized soft tissue swelling may be present. Diagnosis usually depends on scintigraphic findings⁸³ with focal, intense IRU on the lateral aspect of the calcaneus. Entheseous new bone is usually detectable radiologically. Local injections with methylprednisolone acetate and Sarapin, an extract of the pitcher plant, may result in improvement, but rest is the treatment of choice.

Rupture of the Fibularis Tertius

See Chapter 80 for a discussion of rupture of the fibularis (peroneus) tertius.

Stringhalt

See Chapter 48 for a discussion of stringhalt.

Osseous Cystlike Lesions in the Tuber Calcanei

Osseous cystlike lesions in the tuber calcanei have been identified as a cause of moderate to severe lameness either with no localizing clinical signs (Figure 44-13)⁶⁴ or in association with distention of the calcaneal bursa.⁸³ It has been suggested that such lesions may be a reaction at the enthesis of the gastrocnemius tendon, but no associated lesions of the gastrocnemius tendon have been seen.⁸⁴ In two horses lameness developed after known trauma (a kick and a fall),⁸⁴ but in other horses there was no known potential cause. In the latter horses lameness was abolished by fibular and tibial nerve blocks. Lateromedial, dorsolateral-plantaromedial oblique and skyline images of the calcaneus are required for accurate localization of the lesion. Treatment of two horses with associated distention of the calcaneal bursa by either conservative management or surgical debridement resulted in persistent lameness.⁸⁴ The horses with no associated soft tissue swelling did not respond to rest, but intralesional injection of corticosteroids has resulted in resolution of lameness in two horses.⁶⁴ Calcaneal bursitis is rarely caused by primary injury of the gastrocnemius tendon at the attachment to the calcaneus or a combined injury of the gastrocnemius and superficial digital flexor tendons (see Chapters 79 and 80).

Fig. 44-13 Flexed plantaroproximal-plantarodistal radiographic view of the calcaneus. There is a well-defined focal osteolytic lesion in the plantarolateral aspect of the tuber calcanei (arrow). Compare with Figure 44-14. There were no localizing clinical signs.

Infectious (Septic) Osteitis of the Calcaneus

Osteitis of the tarsal bones is uncommon. However, the enlarged end of the tuber calcanei has minimal soft tissue protection and is at risk for traumatic injury and subsequent infection. Management of these injuries can be complicated if the tarsocrural joint, calcaneal bursa, or tarsal sheath is involved. Most horses sustain a traumatic injury from kicking a fixed object or being kicked by another horse.^85,86 Osteitis usually causes moderate-to-severe lameness, cellulitis, and tarsal tenosynovitis. The severity of lameness often decreases once drainage has been established. Twenty-two of 28 horses had a wound or draining tract over the plantar aspect of the calcaneus.⁸⁵ Lateromedial, flexed lateromedial, and plantaroproximal-plantarodistal (skyline) radiographic images are the most helpful (Figure 44-14). Radiological signs of calcaneal osteitis or sequestra formation may take several days or weeks to appear, and thus sequential radiological evaluation is recommended. Rarely is the medullary cavity of the calcaneus involved; hence the term osteitis rather than osteomyelitis is preferred. Ultrasonography is useful to assess calcanean bursa or tarsal sheath involvement. If tarsal sheath involvement is suspected, cytological evaluation of the tendon sheath fluid with aerobic and anaerobic microbial culture and susceptibility testing is recommended (see Chapter 76). Treatment is based on resolving the infection and debriding any bony lesions. Debridement can often be done using bursoscopic techniques, but if wound revision is needed a conventional surgical approach to the infected bone can be used. One of us (MWR) has had success in managing horses with infectious calcaneal bursitis and calcaneal osteitis, using liberal debridement and primary wound revision and closure, closed suction drainage, and external coaptation using a full-limb or sleeve cast or bandage to immobilize the tarsus after surgery. External coaptation appears important in overall management because limiting motion improves the horse’s comfort and allows early healing of revised wounds in this region. Although debridement of all infected bone and soft tissue is an important principle in management of horses with infectious osteitis of the calcaneus, aggressive debridement of the calcaneus to remove large amounts of bone should be avoided. Broad-spectrum antimicrobial drugs such as penicillin (22,000 IU/kg q12h IM) and gentamicin sulfate (6.6 mg/kg q24h IV) are recommended until culture results are known. Pure bacterial cultures were isolated in 14 of 28 horses in one report,⁸⁵ but mixed bacterial isolates do occur. The most common pure bacteria colonies isolated were Escherichia coli, Streptococcus zooepidemicus, Staphylococcus aureus, and Enterobacter cloacae. All isolates were susceptible to penicillin, gentamicin, or trimethoprim sulfamethoxazole. With acute infection, antimicrobial medication alone may be satisfactory, but surgical intervention to remove infected soft tissue and damaged bone or sequestra is usually required. Resection of the infected soft tissues and curettage of the bony lesion can resolve calcaneal lesions that do not involve the tarsal sheath. Infections involving the tarsal sheath require flushing the sheath and establishing drainage (see Chapters 24 and 76). Duration of antimicrobial therapy depends on clinical response, but therapy should continue for at least 7 days after resolution of the lameness and wound drainage. Phenylbutazone (2.2 mg/kg orally [PO]) should be used as needed for pain relief.

Fig. 44-14 Flexed plantaroproximal-plantarodistal radiographic view of the calcaneus. There is an osteolytic lesion in the tuber calcanei (arrows), the result of infection. There is also new bone on the lateral aspect (to the left) of the calcaneus.

The prognosis for horses with calcaneal osteitis depends on the structures involved, duration of infection before treatment, type and susceptibility of isolated bacteria, and the ability to get the original wound to heal. Long-term management (several weeks to months) is often needed, and an important but often limiting factor is the owner’s financial commitment. Chronic persistent drainage, persistent severe lameness, or contralateral laminitis can negatively influence prognosis, and aggressive, expensive therapy is often needed. Quick and aggressive medical and surgical treatment improves the chances of horses returning to athletic function. In one study, nine of 18 horses were used as broodmares and nine returned to athletic function.^85,86

Osteitis of the Sustentaculum Tali

The sustentaculum tali is on the plantaromedial aspect of the calcaneus. Injuries involving the medial aspect of the hock often involve the sustentaculum tali and adjacent tarsal sheath. Injury usually occurs from direct trauma, most commonly from a kick wound. Lameness is often severe. Diagnosis can be difficult because of the severe soft tissue swelling that accompanies this injury, especially when acute. Although unusual, there can be communication of the site of infection or fragmentation of the sustentaculum tali with the tarsocrural joint. Radiographic examination should include dorsomedial-plantarolateral oblique, dorsoplantar, and plantaroproximal-plantarodistal images. Ultrasonography is useful to assess the deep digital flexor tendon (DDFT) and tarsal sheath. Infection of the tarsal sheath as a result of sustentaculum tali fragmentation (see Figure 76-3) or lysis requires surgical curettage of the sustentaculum tali and lavage and flushing of the tarsal sheath. Horses with bony irregularities in the tarsal groove of the sustentaculum tali can be severely lame, even without infection, because of motion of the DDFT against the roughened bone, which results in severe tendonitis. The tarsal retinaculum can become constrictive if the tendon is inflamed and swollen, which causes further tendon damage and pain. Surgery with the horse under general anesthesia is recommended to transect the tarsal retinaculum, explore and curette necrotic bone involving the sustentaculum tali, and drain and flush the tarsal sheath.⁸⁷ The tarsal groove should be smoothed with a curette to avoid abrasion to the DDFT after surgery. Endoscopic examination of the tarsal sheath may be difficult in horses with severe swelling from concomitant infection. In horses with mild swelling, with drainage localized to the region of the sustentaculum tali, and without substantial tarsal tenosynovitis, a small incision should be made directly over the fragments. The fragments should be removed and the site curetted; the incision is closed primarily. In these horses, prognosis for future soundness is favorable. Horses should be treated with systemic antibiotics based on culture and susceptibility results. Antimicrobial therapy is continued for at least 7 days after resolution of clinical signs and may require 4 to 6 weeks of treatment. After surgery, horses should be confined to a stall or small run for 8 weeks with daily handwalking, followed by 6 more weeks of pasture turnout before returning to normal activity. Prognosis is good for returning to athletic activity. Seven of nine horses returned to previous use (three barrel racing, one cutting, and three pleasure riding) after surgery.⁸⁷

Lesions of the Sustentaculum Tali

Lesions of the sustentaculum tali of the fibular tarsal bone may occur as a result of known direct trauma, such as a kick injury to the medial aspect of the hock,⁸⁸ or in association with chronic injuries of the DDFT. Eight of eight horses with proliferative new bone on the plantar and/or medial aspect of the sustentaculum tali had marked distention of the tarsal sheath and lameness. Four had a history of a kick injury. Conservative or surgical management resulted in persistent lameness. Postmortem examination revealed marked fibrillation of the DDFT and adhesions to the tarsal sheath and/or the sustentaculum tali.

Chronic lesions of the DDFT may be associated with the development of dystrophic mineralization within the tarsal sheath and osseous irregularity of the sustentaculum tali. Distention of the tarsal sheath and lameness are present. The prognosis is poor.

Congenital Deformations of the Tarsus

Congenital dysplasia of the sustentaculum tali is rare and has been documented in TBs⁸⁹ and Saddlebreds characterized clinically by a tarsal valgus conformation and a broader plantar profile than usual. The DDFT is positioned more dorsally and medially than usual because of flattening of the proximal aspect of the sustentaculum tali.

A complete fibula may be present in Shetland ponies.^90,91 This results in an abnormal swelling on the distal lateral aspect of the tibia and outward splaying of the limbs distal to the hocks.

Infectious Arthritis

See Chapter 65 for a discussion of infectious arthritis.

Periarticular Cellulitis (see also Chapter 107)

Peritarsal infection results in acute-onset, very severe non–weight-bearing lameness associated with localized swelling that is usually but not invariably on the dorsal aspects of the hock. The swelling is hot and exquisitely painful to palpation. These signs are associated with an elevated rectal temperature, neutrophilia, and hyperfibrinogenemia. In some horses, skin abrasions can be identified somewhere in the more distal part of the limb.

Prompt, aggressive treatment with oxytetracycline (5 mg/kg IV sid) or other antimicrobial agents and phenylbutazone (4.4 mg/kg) for at least 5 days, combined with forced walking exercise for as much as possible daily, usually results in rapid resolution of lameness. Horses can often start trotting within 3 days of the onset of clinical signs and should be trotted for 10- to 15-minute periods repeatedly through the day. The prognosis is usually favorable; 1 of 10 horses developed osteomyelitis of the third tarsal and fourth tarsal bones, although this horse ultimately made a complete recovery.⁹²