A lame Alpaca A 2 year old Alpaca had muscle atrophy in the muscles of the right shoulder. Pain was present on extension of the joint and on medial rotation of the limb. Earlier clinical history was not known Radiographs were made of both shoulders Radiographs of the right shoulder Oblique views of right shoulderRight vs left lateral viewsRadiographic evaluation It is easiest to evaluate the normal left shoulder noting the smooth contour of the humeral head and opposing glenoid cavity plus the radiolucent joint space. The injured shoulder has none of these expected features - the humeral head, glenoid cavity, and interposed shoulder joint space are not identified - neither the intermediate tubercle or the caudal portion of the glenoid cavity are identified. In addition, increased opacification of the adjacent bone, plus a pattern of new reactive bone, plus mineralization of adjacent soft tissues suggests repair of a chronic lesion. Lateral displacement of the humeral head is noted on the caudocranial view suggests joint instability/luxation. Closure of the physeal plate in the proximal humeral head suggests chronic injury with premature physeal closure. Radiographic diagnosis Chronic trauma of the shoulder joint with extensive remodeling of adjacent bone with subluxation and possible fracture of the caudal aspect of the glenoid cavity Destruction of the articular surfaces of the shoulder joint result in a traumatic arthrosis Differential diagnosis
- Most likely etiology is a chronic traumatic injury with fracture/luxation of the shoulder joint, associated fracture and secondary arthrosis
- A superimposed secondary infection over the traumatic luxation is a possible etiology and would require evaluation of a joint tap to confirm - the infection could be active or quiescent
- An arthrosis without a traumatic component would not cause the development of this extensive periarticular secondary bony changes and is unlikely
- A primary joint infection could have been the primary lesion resulting in this degree of secondary bony repair but would have a different clinical history
- A chronic humeral head osteochondrosis causes arthrosis but with development of a much less prominent pattern of new bone
- Dysplasia of the scapulohumeral joint with and without subluxation has been described in Shetland ponies and Miniature Horses - the degree of secondary change is much less than noted in this patient
- Septic physitis in the physis of the proximal humerus does not cause this pattern of injury to the shoulder joint
