Effects of glenoid inclination and acromion index on humeral head translation and glenoid articular cartilage strain.

BACKGROUND Previous clinical studies have reported associations between glenoid inclination (GI), the acromion index (AI), and the critical shoulder angle (CSA) on the one hand and the occurrence of glenohumeral osteoarthritis and supraspinatus tendon tears on the other hand. The objective of this work was to analyze the correlations and relative importance of these different anatomic parameters. METHODS Using a musculoskeletal shoulder model developed from magnetic resonance imaging scans of 1 healthy volunteer, we varied independently GI from 0° to 15° and AI from 0.5 to 0.8. The corresponding CSA varied from 20.9° to 44.1°. We then evaluated humeral head translation and critical strain volume in the glenoid articular cartilage at 60° of abduction in the scapular plane. These values were correlated with GI, AI, and CSA. RESULTS Humeral head translation was positively correlated with GI (R = 0.828, P < .0001), AI (R = 0.539, P < .0001), and CSA (R = 0.964, P < .0001). Glenoid articular cartilage strain was also positively correlated with GI (R = 0.489, P = .0004) but negatively with AI (R = -0.860, P < .0001) and CSA (R = -0.285, P < .0473). CONCLUSIONS The biomechanical shoulder model is consistent with clinical observations. The prediction strength of CSA is confirmed for humeral head translation and thus presumably for rotator cuff tendon tears, whereas the AI seems more appropriate to evaluate the risk of glenohumeral osteoarthritis caused by excessive articular cartilage strain. As a next step, we should corroborate these theoretical findings with clinical data.