Developmental orthopedic disease manifests in the scapulohumeral (shoulder) joint principally as subchondral cyst-like lesions (bone cysts) affecting the glenoid of the scapula or as osteochondritis dissecans of the humeral head. Also, a condition almost exclusive to miniature ponies, caused by dysplasia of the joint and attributable to hypoplasia of the joint surfaces, results in instability and secondary arthritis.
Subchondral Cyst-like Lesions:
Bone cysts may develop in the glenoid, or socket, of the shoulder joint. They may or may not communicate with the shoulder joint and respond variably to intra-articular anesthesia. Although part of the developmental orthopedic disease complex, signs may not be apparent until the animal is mature. In common with other manifestations of this syndrome, lameness may not become a feature until the animal begins work (typically breaking in or early ridden exercise). Occasionally, bone cysts may be a cause of lameness in an older horse, having remained quiescent for most of the animal’s life; the reason for these later onset cases is not clear.
Diagnosis is made by localization with intra-articular anesthesia, by exclusion of lower limb disease, or occasionally by gamma scintigraphy. Radiographs should document the lesion, although some cysts are too small to be seen.
Treatment in young horses consists of rest in the hope the cyst will remodel to become nonpainful; however, this happens only rarely. Intra-articular medication can provide relief from lameness but usually only transiently. Some disease-modifying preparations show promise (eg, autologous conditioned serum), and some clinicians favor the use of systemic glycosaminoglycans. Surgical debridement is difficult in most cases, because the cyst location and articular cartilage damage that causes secondary osteoathritis may limit its effectiveness; however, it can be very successful. Injection of corticosteroids directly into the cyst via an extra-articular approach could be appropriate, but the lack of 3-dimensional imaging of this region in horses makes the approach hard to plan.
Derangement of cartilage and bone development on the humeral head can result in weakness within the articular cartilage that may lead to erosion or formation of a free flap of cartilage. Typically, the caudal part of the head is affected, or at least it is the part most evident on radiographs. In other joints, osteochondritis dissecans often can be treated successfully with arthroscopic debridement. Unfortunately, access to the shoulder joint is severely restricted, and in most cases the full extent of the lesion cannot be seen or treated. Clinical resolution in all but the mildest cases in young horses is rare. Rest and various medications have been tried, with little documented success.
Seemingly unique to the miniature breeds, this condition arises from a mismatch between the size of the glenoid and humeral head. This causes instability of the joint and secondary arthritis. Although undoubtedly a developmental problem, probably with a significant degree of heritability, many cases do not present until the animal is an adult, and the history is often of sudden onset lameness. On physical examination, proximal limb muscle atrophy is often profound and lameness considerable. These factors, along with the breed disposition and the often-present sign of resentment of proximal limb manipulation, make localization straightforward. Radiographs reveal the presence of osteoarthritis and variable subluxation of the scapulohumeral articulation. Oblique views may demonstrate deep erosion of the humeral head in severe cases. The generalized destruction of the joint produces a “hot spot” on a bone scan, if performed. There is no simple treatment. Most cases present at such an advanced stage that even palliative care is impossible; euthanasia on humane grounds should be considered in such situations. Surgical arthrodesis has been described but is rarely performed.