Prospective analysis of total shoulder arthroplasty biomechanics.

A prospective clinical trial was undertaken to evaluate preoperative and postoperative shoulder motion clinically and roentgenographically in 9 patients with severe arthritis who underwent a total shoulder arthroplasty. The relative contributions of glenohumeral and scapulothoracic motion to scapular plane abduction were determined by obtaining roentgenograms of the shoulder region with the arm in five positions before surgery and at follow-up. Before surgery, the ratio of glenohumeral to scapulothoracic motion was 0.32:0.68, or 1:2, compared with a normal ratio of 2:1, meaning that for every degree of glenohumeral movement there were 2 degrees of scapulothoracic motion. At follow-up, statistically significant improvements in pain, motion, and function were found. However, regression analysis determined the ratio of glenohumeral to scapulothoracic motion to be 1:1.3. An abnormal ratio before surgery was associated with significant pain and decreased motion in the shoulder and represented the patients' attempt to immobilize the glenohumeral joint for pain relief and maximize shoulder movement with scapulothoracic motion. The abnormal glenohumeral-to-scapulothoracic ratio after total shoulder arthroplasty indicates that for a given amount of arm abduction, less motion occurs between the prosthetic components than in a normal glenohumeral joint. Abnormal shoulder biomechanics appear to be a function of the underlying disease process and were not restored after total shoulder arthroplasty.