Medial shoulder instability: the abduction angle
5-year-old Border Collie competing in agility, with a 3-month intermittent right forelimb lameness. Under sedation, with shoulder and elbow extended, the abduction angle of the right shoulder is clearly greater than that of the left. The biceps test is equivocal.
What is the most likely diagnosis?
chevron_right Show answer and explanation
A. Osteochondritis dissecans of the shoulder
It is a disease of the growing dog (usually under one year old) and does not, by itself, increase abduction.
B. Infraspinatus muscle contracture
Contracture restricts motion: it causes external rotation and circumduction with a swing-phase lameness, not increased abduction.
C. Primary bicipital tenosynovitis
It is painful when the biceps is tensioned, but it does not increase abduction, which depends on the medial ligamentous complex and the subscapularis.
D. Medial shoulder instability
Correct. Injury to the medial glenohumeral ligament and subscapularis tendon increases abduction. In normal dogs the angle is around 30°, and in medial instability it is usually clearly greater.
lightbulb Key takeaway
Assess abduction under sedation, with shoulder and elbow extended and the scapula stabilised, and compare both sides. Beware: the disease may be bilateral, in which case the comparison misleads. Confirm by arthroscopy, which shows the medial glenohumeral ligament and subscapularis.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.