Case Report An acute subchondral cystic lesion of the equine shoulder causing lameness

Subchondral bone cysts (subchondral cystic lesions or osseous cyst-like lesions) are relatively uncommon in the horse (Baxter 1996). They may be articular or extra-articular, and may, or may not, result in lameness (Baxter 1996), although most cystic lesions that contribute to lameness are articular in nature (Bramlage 1993). Horses from all breeds are susceptible, and whilst the disease has been more commonly reported in individuals less than 3 years of age, any age can be affected (McIlwraith 1982, 1987). Multiple theories as to the pathogenesis of subchondral bone cysts (SCBs) have been proposed, with 2 currently being the most accepted. The first is linked to osteochondrosis, where a disruption to the normal pattern of endochondral ossification occurs at the metaphysis or epiphysis, resulting in a failure of normal cell differentiation and calcification and retention of a thickened and structurally inferior cartilage matrix. Mineralised new bone forms around the margins, but articular involvement and therefore clinical signs only become evident following further physical insult to the weakened site (Rooney 1975; McIlwraith 1982, 1993a). The second theory supports articular trauma as an initiating factor. Microfracture of subchondral bone within the epiphysis could result in bone resorption (Yovich and Stashak 1989). Synovial fluid then gains access through the surface defect, and the repeated hydraulic forces created during normal weightbearing result in the formation of a subchondral cyst-like lesion (Jeffcott et al. 1983; Verschooten 1982; Kold 1986). Subchondral cystic lesions can appear at single or multiple sites (McIlwraith and Trotter 1996) in the same animal, those affecting immature horses often occurring bilaterally. The earliest clinical account in the literature was in 1968 by Petterson and Sevelius, where lesions involving the phalanges and radiocarpal bone were discovered. The most frequently recorded and investigated site in the horse is the medial femoral condyle in the stifle (Bramlage 1993). Other less frequently affected sites pertaining to the shoulder include the glenoid of the scapula (McIlwraith and Trotter 1996), intermediate tubercle (Ramzan 2004) and lateral intertubercular groove (McDiarmid 1999) of the proximal humerus. Although most cases of shoulder OCD present as a unilateral lameness, both forelimbs may be affected (McIlwraith 1993b). Osteochondrosis with, or without, subchondral cystic lesions located at the glenoid, humeral head, or both, often have little palpable synovial effusion due to the degree of muscular and tendinous coverage (McIlwraith 1993b). This is similar clinically to subchondral bone cysts of the medial femoral condyle and metacarpophalangeal/metatarso-phalangeal joints, whereby varying degrees of unilateral or bilateral lameness are evident with, or without, the presence of synovial effusion (Petterson and Sevelius 1968; Verschooten and DeMoor 1982; Jeffcott and Kold 1982a,b; McIlwraith 1990). This case report details the clinical, radiographic and scintigraphic investigation and interpretation of an active subchondral bone cyst of the glenoid of the scapula, causing lameness in a mature horse. Interestingly, the horse became sound enough to be used for light work after 3 months of conservative treatment with no clinical evidence of lameness up to 12 months later.