The effects of prosthetic humeral head shape on glenohumeral joint kinematics during humeral axial rotation in total shoulder arthroplasty

Background: A non-spherical humeral head has been shown to influence kinematics and stability of the glenohumeral joint yet, most prosthetic humeral head components are designed to be a perfect sphere. The effect of humeral head shape on prosthetic joint kinematics after total shoulder arthroplasty is not well understood. We hypothesized that prosthetic joint kinematics during humeral axial rotation is dependent on humeral head shape, regardless of joint conformity. Methods: Four prosthetic configurations were investigated using a spherical and a non-spherical prosthetic humeral head articulated with a conforming and a non-conforming glenoid component. Testing was performed in the coronal, scapular, and forward flexion plane at 0°, 30°, and 60° of abduction. Prosthetic joint kinematics was measured in 10° intervals during a 100° arc of humeral axial rotation. Glenohumeral translation patterns, net glenohumeral translation, and averaged glenohumeral translation were compared for each of 4 configurations. Results: Non-spherical head configurations increased the net glenohumeral translation during humeral axial rotation in multiple test positions compared with spherical head configurations (P<.05). Spherical head configurations resulted in a relatively small amount of glenohumeral translation, less than 2mm. The radius of curvature of the glenoid component alone did not affect the net glenohumeral translation within each of the 2 head groups (P>.05). Conclusion: During humeral axial rotation, the non-spherical humeral head shape contributes to increased glenohumeral translation during humeral axial rotation. However, the spherical head shape does not show significant glenohumeral translation during humeral axial rotation, regardless of glenoid conformity.