Glenoid morphology rather than version predicts humeral subluxation: a different perspective on the glenoid in total shoulder arthroplasty.

Glenoid retroversion is thought be important in shoulder stability before and after shoulder arthroplasty; thus, many authors recommend glenoid reaming to correct retroversion and improve stability. Genetic analysis has revealed that glenoid vault and scapular development are controlled by different genes and environmental factors, resulting in diverse glenoid morphologies. We therefore analyzed the relative contribution of glenoid morphology and version to humeral head position.We obtained 121 shoulder computed tomography scans preoperatively for shoulder arthroplasty. Humeral subluxation and glenoid version were measured on the axial image at the middle of each glenoid. Glenoid morphology was characterized as biconcave, worn, displaced, dysplastic, angled, or neutral. The strength of the correlation between humeral subluxation, glenoid version, and glenoid morphology was analyzed.Glenoid version did not correlate with humeral subluxation. The highest frequency of posterior subluxation was noted in biconcave glenoids. Shoulders with other glenoid morphologies were more likely to have anterior or central positioning of the humerus. The mean subluxation ratio for biconcave glenoids was 0.56 and was significantly different from all other morphologies (P < .02).Even in the arthritic shoulder, glenoid orientation does not appear to explain the complex biomechanics of shoulder stability. The causes of humeral head subluxation before and after total shoulder arthroplasty are likely multifactorial and may include static and dynamic soft-tissue forces. The biconcave glenoid deserves more attention at surgery because of the high association with posterior subluxation.