Orthopaedics 3. The proximal limbs

Chapter 17


Orthopaedics 3. The proximal limbs




Contents



17.1 Carpus



17.2 Elbow



17.3 Shoulder



17.4 Hock



17.5 Stifle



17.6 Hip



Further reading



17.1 Carpus



Anatomy



Bones


The carpus normally contains seven small carpal bones in two rows (see Figures 25.5125.54).


Proximal row:



Distal row:



A first carpal bone is found in about 50% of horses and may be unilateral. It is usually about the size and shape of a pea and is found embedded in the medial ligament of the carpus, behind the second carpal bone. A fifth carpal bone is also occasionally seen on the lateral aspect of the carpometacarpal joint.


The carpal bones articulate proximally with the distal radius, which has three articular facets relating to the proximal surfaces of the radial, intermediate and ulnar carpal bones. The lateral aspect of the distal radius is phylogenetically the distal ulnar epiphysis and is seen as a separate ossification centre in foals. The line of fusion to the rest of the distal radial epiphysis closes radiographically before 12 months of age but often persists into adulthood as a groove on the distal radial articular surface. The epiphysis fuses with the metaphysis at 20–24 months of age. A linear, mineralized density is sometimes seen on the caudolateral aspect of the distal antebrachium, representing the vestigial distal ulna. The distal row of carpal bones articulate distally with the second, third and fourth metacarpal bones.



Joints


The articulations between the carpal bones are complex, and some consist of two or three separate facets. Altogether, there are approximately 26 separate articulations. These are divisible into three principal joint levels:



There are usually two separate synovial cavities:



The antebrachiocarpal joint moves with a rotating and gliding action. The midcarpal joint has a hinge-like action while the carpometacarpal joint is capable of very little movement. During flexion, the articular surfaces of the antebrachiocarpal and midcarpal joints become widely separated dorsally but remain apposed at their palmar margins. The intermediate and ulnar carpal bones tend to move as one unit, but the radial carpal bone is not displaced as far from the distal row of carpal bones on flexion. Its dorsodistal margin is therefore separated from the intermediate carpal bone on a flexed lateromedial radiograph.


The fibrous palmar carpal joint capsule is dense and closely attached to the palmar aspect of the carpal bones. Palmarly, it forms the smooth dorsal wall of the carpal canal. It is continued distally to form the accessory head of the deep digital flexor tendon (inferior check ligament). The carpal fascia on the palmar aspect of the carpal region forms the flexor retinaculum (transverse carpal ligament) between the free palmar edge of the accessory carpal bone and the medial aspect of the carpus. This completes the carpal canal, which contains the superficial and deep digital flexor tendons, blood vessels, and a portion of the accessory ligament of the superficial digital flexor tendon (ALSDFT/superior check ligament).



Traumatic arthritis, osteoarthritis (degenerative joint disease)



Aetiology and pathogenesis


Equine carpal joint disease is seen most commonly as an ‘occupational disease’ of racehorses and occurs less commonly in horses used for other purposes. A form of traumatic arthritis/osteoarthritis (OA) appears to be the usual underlying disease entity. The vast majority of cases are a consequence of the chronic repetitive trauma suffered by the joint as a result of training and racing. Adaptive remodelling of bone in response to training may have deleterious consequences for the integrity of the joint as a whole. Initial osteoclastic resorption may weaken the bone, predisposing to microscopic or gross fracture. Subsequent densification increases the stiffness of the bone, increasing the peak load on the overlying articular cartilage. Chronically accumulating damage of OA may give rise to clinical signs of uni- or bilateral forelimb lameness in its own right, as well as acting as a precursor to more acute injuries such as carpal chip and slab fractures.


Carpal OA affects the middle carpal joint more commonly than the antebrachiocarpal joint in Thoroughbred racehorses. The articular surface damage is concentrated on the medial aspect of the joint and particularly involves the radial and third carpal bones. The articular margins and surfaces on the lateral aspect of the joint often appear relatively normal both radiologically and arthroscopically. This distribution of lesions is hypothesized to be a consequence of biomechanical influences; the ability of carpal bones to dissipate energy through lateral movement is thought to be more limited on the medial aspect, predisposing to fracture on this side of the joints.


Carpal OA is seen in horses used for other disciplines, although it is an uncommon cause of lameness in non-racehorses. Diagnosis and management is the same for other joints.



Carpal fractures


Carpal ‘chip’ fractures are frequently seen in association with OA and have been commonly considered as a potential cause of ‘secondary’ OA. Although this sequence of events undoubtedly occurs, the alternative scenario – that the arthrosis causes or predisposes to the fractures – may well be more common and more important, particularly with respect to distal radial carpal chip fractures. A similar role of carpal OA/subchondral disease in predisposing to third carpal bone slab fractures has also been suggested, and clinical evidence supports this as almost all third carpal slab fractures are pathological and occur through obviously diseased bone. Such a viewpoint puts the acute injury into perspective as part of a chronic disease process and thereby establishes the limitations on the success that might be expected with surgical treatment (e.g. fracture fragment stabilization or removal). The prodromal changes may be present in a large number of racehorses, not all of which go on to sustain a fracture. These changes can be present with minimal lameness or mild bilateral lameness, confounding detection clinically. Additionally, these changes often cause no synovial effusion, giving the impression that the fracture occurs instantaneously.



• Chip fractures occur at the dorsal margins of the bones forming the antebrachiocarpal and middle carpal joints. Each individual chip fracture affects only one articular surface of the parent bone, although there may be multiple chips affecting different bones, different joints and different limbs in the same horse. The most common sites are the distal margin of the radial carpal bone (Figure 17.1) and the proximal margin of the third carpal bone, i.e. on the dorsomedial margin of the middle carpal joint. The distolateral radius, within the antebrachiocarpal joint is another predilection site.



• 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.





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.





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.






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.











Radiology:



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



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:



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








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’.










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




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:



Jun 18, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Orthopaedics 3. The proximal limbs

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