Abstract

One hundred and fifty-one cases of primary glenohumeral osteoarthritis were analyzed both clinically and radiographically with CT scanning. There were 76% females in this series, and the mean age at operation was 66.8 years. The diagnosis was made only with true anteroposterior views of the shoulder which show the narrowing of the glenohumeral joint and the presence of osteophytes. The arthrogram showed a supra-spinatus tear in 10% of the cases that extended to the infra-spinatus in 2.6%. Glenoid retroversion averaged 15.4 degrees. However, measurements of glenoid retroversion with CT-scan do not distinguish posterior wear from glenoid dysplasia. Posterior subluxation of the humeral head was observed in 45% of the cases. We describe a classification of the glenoid morphology in three types: Type A (53.5%) is characterized by an equal balance of forces acting on the glenoid and a centralized humeral head. Type B (39.5%) has asymmetrical posterior force distribution on the glenoid and a posterior subluxation of the humeral head. Type C (5%) is arbitrarily defined as glenoid retroversion greater than 25 degrees. Posterior subluxation of the proximal humeral head did not correlate with either glenoid retroversion or humeral retroversion. However, subluxation of the humeral head may be responsible for the posterior glenoid erosion and possibly for the biconcave appearance of the glenoid observed in primary glenohumeral osteoarthritis. The posterior subluxation of the humeral head may still be present after shoulder arthroplasty and may be responsible for glenoid loosening due to a "horizontal rocking-horse effect".