Glenohumeral translation after total shoulder arthroplasty.

To define the amount and direction of anteroposterior translation of the glenohumeral joint after total shoulder arthroplasty, 13 patients who had previously undergone nonconstrained total shoulder arthroplasty were studied roentgenographically. Patients were placed on the x-ray table in a supine position, and axillary roentgenograms of the shoulder were made with the arm at 90° abduction and in four positions within the horizontal plane of motion: - 30° (horizontal extension), 0° (neutral), 30°, and 60° (horizontal flexion). For all positions the shoulder was in neutral rotation. The center of the prosthetic humeral head was identified, and the distance between it and a perpendicular line bisecting the glenoid component was measured. At follow-up the patients had statistically significant improvements in pain, motion, and function. No patients had any clinical evidence of instability. The mean total translation was 4 mm posteriorly (range 0 to 12 mm). Most translation occurred between - 30° and 30°. Anteroposterior translation appears to occur in a posterior direction. For this to occur the prosthetic humeral head slides onto the polyethylene rim of the glenoid component, subjecting it to eccentric loading and the possibilities of wear, polyethylene debris formation, induction of osteolysis, and subsequent component loosening.