Shoulder arthrodesis

Shoulder arthrodesis is a surgical treatment for osteoarthritis (OA) of the scapulohumeral (shoulder) joint which is not common but Shetland ponies, Miniature Shetland ponies and American Miniature horses suffer this condition with greater frequency than other breeds [1-4]. Dysplasia of the scapulohumeral joint in these breeds results in joint instability and early onset OA. Lameness is often severe and medical therapy provides only temporary and, in many cases, minimal relief from discomfort. Arthrodesis of the scapulohumeral joint is therefore indicated in these animals to provide a functional limb.

Presentation

Affected ponies present with severe lameness, most often in one forelimb but, on occasion, lameness is bilateral [2,3]. Lameness is often reported as occurring suddenly but is often longstanding at the time of surgery. Lameness is obvious at walk and severe at trot with these small ponies choosing to carry the limb at times. Muscle atrophy of the affected limb is common, as is swelling of the joint and discomfort on palpation or manipulation of the shoulder region. Diagnostic analgesia of the shoulder joint should lead to a significant improvement in lameness [4].

Diagnostic imaging

Radiography is used most frequently to image the shoulder region. Marked signs of OA, joint incongruity and flattening of the glenoid such that it appears shallow are the most common radiographic findings in affected ponies [1,4]. Radiographic abnormalities may be bilateral despite unilateral lameness, with future lameness in the asymptomatic limb surprisingly rare [4].

Surgery

Surgery is performed under general anaesthesia with the pony in lateral recumbency with the affected forelimb uppermost. A 20–30 cm curvilinear incision is made through the skin from the midpoint of the scapula to the midpoint of the humerus. That incision is continued deep to access the joint. In that dissection, the biceps is transected via a tenotomy and the supraspinous muscle is transected close to the point of insertion on the greater tubercle of the humerus. The joint capsule is incised to allow complete removal of the joint cartilage with a curette or motorised bur. The intermediate tubercle of the humerus is removed with an osteotome or oscillating saw to provide a flat surface for the plate. A long (often 11-hole), narrow, locking compression plate (4.5/5.0 mm LCP) is contoured to the cranial aspect of the scapula and humerus at an angle of approximately 120 degrees. The plate is held to the bone surface with 4.5 mm cortical bone screws before two 4.5 mm cortical bone screws are placed in lag fashion through the plate and across the joint to provide transarticular compression. The remaining plate holes are filled with 5.0 mm locking screws. The surgical incision is closed routinely, and a gauze stent is sutured over the incision [4,5].

Post-operative care

Ponies are manually assisted to stand. They are kept either on cross-ties or in a stall for the first 2 weeks and kept in a stall in total for 12 weeks. Ponies are sore after surgery and require analgesia, often incorporating NSAIDs and opioids. They also require post-operative antimicrobial therapy [2-4]. The small ponies affected by this disease are also prone to laminitis and hyperlipaemia and we have seen both conditions post- operatively in our hospital. Intestinal impactions are common, and ponies benefit from daily paraffin oil. Surgical site seromas and surgical site infections can occur, most likely in the first 2 weeks after surgery and scapula fracture has been reported after this surgery [4].

Outcome

In a recent study, 13 out of 15 equids that underwent shoulder arthrodesis survived long term [4]. Of the two ponies that did not survive, one was euthanased as a result of subluxation of the contralateral shoulder, and the other due to implant infection and contralateral shoulder instability and contralateral limb laminitis. Of the 13 ponies that survived long term, most had an excellent or good outcome but in four cases the outcome was considered moderate. Follow-up radiographs were available for nine ponies and revealed complete bony fusion in five of them, and partial fusion in four.

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