• Slab fractures (see Figure 25.16) extend right through a carpal bone from the proximal to the distal articular surface. They occur most commonly in the frontal plane and primarily affect the dorsomedial aspect (radial facet) of the third carpal bone. Because they involve larger bone fragments and two articular surfaces they are a more serious injury than a chip fracture, and affected horses have a correspondingly more guarded prognosis for return to athletic activity. Slab fractures in the sagittal plane are occasionally seen.

• Incomplete fissure fractures of the third carpal bone have been described and may cause lameness with few localizing clinical signs.

• Accessory carpal bone fractures are seen most commonly in jumping horses and often there is a history of a fall. The fracture usually occurs in the frontal plane in approximately the middle of the bone.

• Comminuted fractures of one or more carpal bones cause severe disruption of the articular surfaces and often result in instability of the carpus. They are usually due to severe trauma such as falls or road traffic accidents.

Clinical signs:

• Lameness localized to the carpus. There may be obvious unilateral lameness or a more insidious bilateral lameness manifested as shortening of the stride and abduction of the distal limb during protraction to reduce the necessity for carpal flexion.

• Swelling, especially carpal joint effusion in the early stages, although – as mentioned previously – synovial effusion may not be present in the early stages of subchondral bone disease. Fibrous thickening on the dorsal aspect of the joint occurs with time and obscures the clear contours of the dorsal carpal bone margins.

• Pain on joint flexion and sometimes on palpation of the joint margins (especially dorsomedial aspect of Cr and dorsolateral aspect of the distal radius).

• Positive response to carpal flexion test.

• Restricted range of carpal flexion.

• Severe fractures may cause crepitus and instability.

Radiology: The four standard weight-bearing projections of the carpus should be supplemented with flexed lateromedial views (to separate the dorsal margins of the radial and intermediate carpal bones) and flexed dorsoproximal–dorsodistal oblique views (to evaluate sclerosis, lysis and fracture of the individual carpal bones) (see Figures 17.1 and 25.55).

Soft-tissue swelling

Soft-tissue swelling may be a result of synovial effusion or thickening of the synovial membrane/joint capsule or periarticular tissues. It is useful to note on radiographs the level of maximal swelling in relation to the joints in the carpus (i.e. antebrachiocarpal joint, middle carpal joint or carpometacarpal joint) because this can help in identifying the specific carpal joint principally involved.

Entheseophytes

The carpal bones are arranged to allow for their slight displacement in a horizontal plane when the carpus is loaded. The carpal bones are linked together by ligaments which constrain this movement and absorb some of the imposed load. Overloading of this system may result in damage to the origins and insertions of the intercarpal ligaments on the dorsal aspect of the carpal bones, which results in non-articular periosteal new bone formation at the sites of these attachments, i.e. entheseophytes. Once formed, this bone tends to persist throughout the horse’s life, although it may remodel and become smoother in outline. Irregular, non-articular, periosteal new bone on the dorsal surface of the carpal bones in young racehorses in training may, therefore, be an indication that overloading of the carpal joints is occurring and that more serious articular surface damage may coexist. Such entheseophytes may, however, be an incidental observation without a great deal of clinical significance when seen later in life.

Osteophytes

Periarticular new bone is one of the cardinal radiological features of OA. Osteophytes may develop as a modelling response to joint overload and damage. They may reflect an attempt to increase joint surface area and thereby reduce loading per unit surface area, and also to increase joint stability. They may thus be viewed as part of the joint’s adaptive response to disease. Periarticular osteophytes tend to be seen primarily on the dorsomedial aspect of the carpal joints, and modelling of the dorsodistal margin of the radial carpal bone is particularly common.

Paramarginal subchondral bone lysis

Loss of subchondral cortex in a narrow strip approximately 5 mm palmar to the dorsal margin of the distal surface of the radial carpal bone is a common radiological finding in horses with OA of the midcarpal joint. It is seen as a small concave area of radiolucency in the subchondral cortical bone of the distal articular surface just palmar to the dorsodistal margin and is generally most apparent on the DLPMO projection. On the flexed dorsoproximal–dorsodistal oblique projection of the distal row (third carpal skyline) fissuring of the dorsal cortex of C3 may also be appreciated.

Subchondral bone densification

Densification (sclerosis) of the subchondral bone in the radial facet of the third carpal bone is recognized in young racehorses. This feature is best demonstrated on the dorsoproximal–dorsodistal oblique view of the third carpal bone and is seen as an increase in bone density and loss of the normal trabecular pattern. It is probably an adaptive change in response to increased loading with training, but, by stiffening the subchondral bone, it may increase the load on the overlying articular cartilage and thereby predispose it to damage. Densification and the resulting increase in stiffness, with transfer of loading, can in itself be painful and cause lameness: physiological imaging techniques such as scintigraphy and MRI often demonstrate increased uptake/high signal even with comparatively minor radiological change.

OA of the carpometacarpal joint

The equine carpometacarpal joint is rarely overtly affected by OA, although its synovial cavity communicates with that of the frequently affected midcarpal joint. Old ponies are sometimes affected with OA of the carpometacarpal joint. The associated radiological changes bear similarities to ‘bone spavin’ in the hock with prominent periosteal new bone proliferation and subchondral bone lysis. Like ‘bone spavin’, these changes may rarely progress to fusion of the affected joints.

Treatment:

1. Traumatic arthritis – see Chapter 15.

2. Osteoarthritis – see Chapter 15.

3. Fractures: most carpal fractures can, in theory, be managed either surgically or conservatively. Surgical intervention is indicated particularly for horses with chip or slab fractures that are intended for athletic use. Surgery aims to restore articular congruency and stability so that the development of secondary OA is minimized. The limitations on this where the fractures are actually a product of existing OA have already been alluded to.

• Chip fractures are generally treated surgically by removing the fragments under arthroscopic visualization. Abnormal articular tissues – particularly soft, crumbly subchondral bone – may be debrided at the same time.

• Slab fractures are usually treated by lag-screw fixation. Initial debridement and reduction may be accomplished using arthroscopic visualization.

• Accessory carpal bone fractures commonly form fibrous rather than bony unions. Techniques for lag-screw repair have been described but are difficult due to the curved shape of the bone and the comminuted nature of many of the fractures.

• Comminuted fractures of one or more carpal bones usually end a horse’s athletic career. Salvage for breeding or as a pet may be feasible by combining internal fixation and cast support, with or without combined partial or pancarpal arthrodesis. Comminution of multiple carpal bones is usually an indication for euthanasia.

Osteochondritis dissecans (OCD) (see Chapter 15)

OCD is rare in the carpus but is occasionally seen in foals. It may be associated with disease in other joints.

Subchondral cystic lesions (SCLs) (see Chapter 15)

These have been reported in the carpus but may not be of clinical significance; early reports probably overstated their importance as a cause of lameness. They are seen frequently in the ulnar carpal bone and in association with the presence of first and fifth carpal bones; they are invariably of no significance in these situations. Occasionally, a large ‘cyst’ is identified in the radial carpal bone or distal radius, sometimes with obvious articular communication, which should be considered potentially more important and evaluated comprehensively to determine its significance.

Septic arthritis

See Chapter 15.

Angular limb deformities

The term ‘angular limb deformity’ refers to lateral (valgus) or medial (varus) deviation of the distal limb from the sagittal plane. Thus ‘carpal valgus’ refers to outward deviation of the distal limb originating at the level of the carpus (Figure 17.2). These deformities are usually seen in young foals and can be congenital or acquired.

Figure 17.2 Example of carpal valgus. The distal limb is deviated laterally from the level of the carpus.

Aetiology:

• Congenital deformities: cause often unknown, but suggestions include malpositioning of the foetus in utero, prematurity/dysmaturity, genetic factors, nutritional influences on the mare, and teratogens.

• Acquired: most commonly due to physeal injury or asymmetric loading due to excessive weight-bearing, e.g. as a result of overnutrition or contralateral limb injury, excessive exercise or improper foot trimming. In the case of carpal deformities, the distal radial physis is usually the site affected.

Pathogenesis:

• Congenital laxity of the ligamentous support of the carpus and/or hypoplasia (inadequate ossification) of the small carpal bones may result in angular deformity. If left unsupported, the inadequately ossified bones may become crushed and deformed as the foal loads the limbs after birth.

• Acquired deformities usually develop as a result of asymmetric growth from the distal radial physis, which may be due to excessive loading of one side of the physis.

History: Important facts to establish include:

• The age of the foal.

• If the foal was premature or smaller than normal at birth.

• If the deformity was present at birth, and if not, when it first appeared.

• How the deformity has developed since it was first seen (i.e. improving, deteriorating or static).

• If there has been any associated lameness or other signs.

Physical examination: The following points in particular should be established:

• The site or sites of deformity.

• The direction of the deformity: valgus or varus.

• The degree of deviation: mild, moderate or severe.

• Evidence of instability: can the deformity be manually reduced?

• Evidence of lameness.

• Signs of inflammation: pain, heat or swelling.

Radiography: It is important to obtain dorsopalmar views centred on the carpus but including as much as possible of the distal radius and proximal metacarpus in order to be able to evaluate the degree of deformity present.

Radiology:

1. Morphological changes: various combinations of the following features may be seen:

• Irregularity of the line of the physis; often appears particularly disturbed on the convex side of the deformity.

• Metaphyseal and epiphyseal flaring.

• Wedge-shaped epiphysis.

• Diaphyseal remodelling with asymmetry in cortical thickness.

• Hypoplastic, rounded, collapsed, or subluxated carpal bones.

2. The radiographs may also be used to gain an estimate of the degree of angulation present. Traditionally, a piece of clear X-ray film is placed over the dorsopalmar radiograph and lines drawn down the centre of the radius and the centre of the third metacarpal bone. For single-site angulations, the intersection of these lines gives some indication of the level within the carpus at which the deformity is occurring and the degree of angulation (see Figure 25.56). The use of digital radiography and computer software now enables these measurements to be made more easily.

Treatment: Some degree of angular limb deformity is not uncommon in neonatal foals and often improves dramatically during the first 2 weeks of life without specific treatment.

Certain management principles are helpful in dealing with angular limb deformities regardless of what other techniques are employed.

• Foals should generally be confined to a large box or small yard while the deformity is present, as excessive exercise on an angulated limb tends to exacerbate asymmetric overloading of the physis, carpal bones and other joint structures.

• Limb angulation leads to asymmetric wear of the hoof. Foals with carpal valgus tend to wear the medial aspect of the hoof wall excessively. The hoof should be rasped regularly to bring it back into balance. This may be combined with the use of glue-on plastic shoes with medial (in the case of carpal valgus) or lateral (for carpal varus) extensions.

• Affected foals should not be allowed to become overweight. This may require restricting the mare’s diet to control milk production, or it may require early weaning.

• Neonatal foals with deformity due to ligamentous laxity or carpal bone hypoplasia benefit from external support of the limb with either a well-padded splint or tube cast for 2–4 weeks. Splints should be reset daily and casts changed after 2 weeks to reduce the chance of development of pressure sores. In foals, there is usually little to be gained by external support where there is no evidence of instability, the deformity cannot be reduced manually, and the carpal bones do not appear hypoplastic radiographically.

• Foals with persistent deformities due to asymmetric growth from the distal radial physis are candidates for surgical intervention if they fail to respond to the measures outlined above, or if the age of the foal indicates that conservative measures will not have time to be effective. Surgery aims to manipulate growth at the distal radial physis to cause limb straightening. This may involve either:

• temporary transphyseal bridging with either staples, screws and wire, or a transphyseal screw, or

• hemicircumferential periosteal transection.

Both techniques depend on continued growth of the bone to straighten the limb, and there are therefore time limits after which they are likely to be much less effective. In the case of angular limb deformities of the carpus, the surgery should be undertaken before the foal is 12 weeks of age. The earlier surgery is performed, however, the more rapid and complete will be the response.

Temporary transphyseal bridging aims to slow the rate of growth on the more rapidly growing (i.e. convex) side of the physis. It is therefore performed medially in the case of carpal valgus deformity. The use of two screws (usually fully threaded, 6.5-mm diameter AO cancellous screws) with one or more figure-of-eight wires (18G) joining them, has certain advantages over the use of staples. The two screws can be placed independently, unlike the tines of a staple, and immediate compression across the physis is achieved as the wire(s) is tightened. There is also less risk of extrusion of the implants as the foal continues to grow. Alternatively, a single 4.4-mm cortical screw can be inserted diagonally across the physis on the medial side. The implants must be removed as soon as the limb is straight, or overcorrection and deviation in the opposite direction may occur.

Hemicircumferential periosteal transection: the periosteum is postulated to exert a restraining influence on longitudinal growth of long bones. By releasing this restraint on one side of the bone, it is possible to induce asymmetric growth and thereby correct existing deformities. The transection is thus done on the side of the bone that is growing less quickly, i.e. the concave side (lateral side in the case of carpal valgus). The periosteal incision is made about 2.5 cm proximal to the physis. A simple horizontal incision is probably sufficient though many surgeons create an inverted ‘T’ incision and then elevate the flaps of periosteum created from the bone. In the case of carpal valgus deformities the rudimentary distal ulna should also be transected at the caudal end of the horizontal periosteal incision.

Hemicircumferential periosteal transection has certain advantages over temporary transphyseal bridging:

• There is no need for a second surgery to remove implants.

• There is less risk of postoperative infection because there are no implants.

• There appears to be no risk of overcorrection.

Hemicircumferential periosteal transection and temporary transphyseal bridging may be employed simultaneously in severely affected foals. However, more recently, the effectiveness of periosteal transection has been challenged.

Extracorporeal shockwave therapy has been used for the treatment of ALD. However, although subjectively the results may appear impressive, no controlled studies have been undertaken, to date. Whenever therapy for ALD is being considered, it must be remembered that many cases resolve spontaneously, without the need for treatment.

Carpal canal syndrome

Aetiology and pathogenesis: This syndrome is caused by trauma or a space-occupying lesion within the carpal canal leading to compression of the soft tissues within the canal (e.g. accessory carpal bone fracture, osteochondroma of the distal radius, tendinitis of the flexor tendons). It is questionable that this syndrome ever exists as a primary condition and, in the vast majority of cases, is NOT analogous to the human condition, carpal tunnel syndrome, which is associated with nerve compression without a primary lesion. Therefore it is strongly recommended to comprehensively evaluate the horse (using diagnostic imaging and, if necessary, tenoscopy) for an inciting cause.

Clinical signs:

• Distension of the carpal sheath with synovial fluid.

• Reduced range of carpal flexion.

• Pain on carpal flexion.

• Reduced pulse volume in the digital vessels distal to the carpus.

Treatment:

• Treat the primary lesion if possible (frequently requires tenoscopic examination of the carpal sheath).

• Resection of the carpal flexor retinaculum (rarely indicated).

Osteochondroma of the distal radius

This is a bony exostosis that develops on the caudal aspect of the distal radius. Its cortex is continuous with that of the distal radius and it has a cartilage ‘cap’.

Aetiology and pathogenesis: There is localized abnormal growth of cartilage from an area of metaphyseal periosteum that subsequently undergoes endochondral ossification. It appears to be distinct from the condition of ‘multiple exostoses’, which is thought to be inherited.

Clinical signs: Signs are as for carpal canal syndrome plus the bony protruberance on the caudomedial aspect of the distal radius. Osteochondromas are not necessarily a cause of lameness and may thus be an incidental radiological finding. When they do cause lameness, it is often because they impinge on the flexor tendons within the sheath, initially often only at maximal exercise, and hence signs can be intermittent, and therefore diagnosis difficult.

Radiology: Radiology shows an irregular exostosis on the caudomedial aspect of the distal radius proximal to the physis; this should not be confused with the normal distal radial physis, which can, in some horses, appear pronounced.

Ultrasonography: Inspection of the flexor tendons and the surface of the caudal radius can provide further diagnostically useful information.

Treatment: Treatment comprises excision of the lesion: a tenoscopic approach minimizes postoperative morbidity. The prognosis is usually very good for simple cases with minimal soft-tissue damage.

Soft-tissue injuries of the carpus

Collateral ligament desmitis is invariably the result of significant trauma and may be seen in association with bone damage. Ultrasonography is the technique of choice for imaging the collateral ligaments. Prognosis depends on the extent of damage and instability, plus other lesions that have been caused by the trauma.

Tendonitis can affect the tendon of the extensor carpi radialis (ECR), the common digital extensor, or lateral digital extensor muscle. Injuries can be traumatic (common) or strains (rare). There may be associated synovial sepsis if the injury involves the enclosing sheaths. If present, infection requires aggressive therapy, although the prognosis is considerably better than for infection of joints or flexor tendon sheaths. Radical ‘stripping’ of the sheath of ECR has been described to treat chronic infection, often secondary to foreign-body penetration, with success. Extensor tendons are considerably more forgiving than flexor tendons when injured.

The accessory ligament of the SDFT (superior check ligament) arises from the caudodistal radius and joins the SDFT within the carpal sheath. Injury is rare but an important differential diagnosis when considering lameness related to the carpal sheath. Diagnosis is by careful ultrasonographic assessment. Rest is the recommended treatment, although surgical transection can be performed tenoscopically in refractory cases.

17.2 Elbow

Anatomy

The elbow is a ginglymus or hinge joint formed between the distal humerus and the proximal end of the radius and ulna and is restrained by collateral ligaments medially and laterally.

In the foal, the distal humerus comprises separate ossification centres for the distal epiphysis and the medial epicondyle. The distal humeral epiphysis fuses to the metaphysis at 11–24 months. Both the radius and ulna each have a single proximal epiphysis. The proximal radial epiphysis fuses to the metaphysis at 11–24 months. The proximal epiphysis of the ulna initially appears quite small radiographically and well separated from the rest of the ulna. It fuses to the rest of the ulna at 24–36 months.

Ulnar fractures

These usually involve the olecranon process (see Figure 25.8) and commonly the articular surface of the semilunar notch. They compromise the ability of the horse to maintain the elbow in extension during weight-bearing because the fracture interferes with the triceps mechanism of which the olecranon is the insertion. They are subdivided into types according to configuration:

Type 1 – physeal fractures occurring in the young horse.