Posterior and inferior glenosphere position in reverse total shoulder arthroplasty supports deltoid efficiency for shoulder flexion and elevation

BACKGROUND For humeral flexion and elevation, most relevant for daily activities with reverse total shoulder arthroplasty, the anterior and lateral deltoid muscles are most important. However, how this direction of movement is best supported with the glenosphere position is not fully understood. We hypothesized that both inferior positioning and posterior positioning of the glenosphere may best support this direction of movement. METHODS A validated, anatomic biomechanical shoulder model was modified to host a reverse shoulder prosthesis. The glenoid baseplate was altered to allow inferior, lateral, and posterior center-of-rotation (COR) offsets. An optical tracking system was used to track the excursion of ropes simulating portions of various shoulder muscles during humeral abduction, elevation, and flexion. RESULTS The inferior COR offset resulted in a significant increase in the deltoid moment arm in all 3 planes of motion. The lateral COR offset showed a significantly lower posterior deltoid moment arm during humeral abduction and a significantly lower lateral deltoid moment arm during humeral elevation. The posterior offset showed significantly larger anterior and lateral deltoid moment arms during humeral flexion. DISCUSSION AND CONCLUSION Owing to the oblique direction of the deltoid muscle across the shoulder joint, an inferior offset of the COR in reverse total shoulder arthroplasty increases the deltoid moment arm during abduction, elevation, and flexion, whereas it mainly supports humeral flexion at a posterior offset. For humeral elevation and flexion, favorable positioning of the glenosphere may, therefore, be defined by a more inferior and posterior placement compared with the non-offset position