Effect of implant geometry on range of motion in reverse shoulder arthroplasty assessed using glenohumeral separation distance.

BACKGROUND

Whereas reverse shoulder arthroplasty has been successful in treating patients with cuff tear arthropathy, implant impingement after the procedure often causes complications, including reduced range of motion, bone loss, and instability. Attempts to simulate this problem in vitro typically rely on subjective visual methods to detect impingement. The purpose of this study was to determine the effect of humeral neck-shaft angle, implant diameter, humeral cup depth, and glenoid component eccentricity on minimum abduction angle and range of motion using an implant-tracking method for impingement detection.

METHODS

Tests were performed in a kinematic shoulder simulator with actuated cables representing the deltoid. The humerus was manually adducted and abducted past the point of impingement in either direction. Centers of the implant components were tracked with optical motion capture and processed with a thresholding algorithm to determine the minimum abduction angle where impingement occurred.

RESULTS

Humeral cup depth had the largest effect on minimum abduction angle and range of motion, with a retentive cup reducing range of motion by 26°. A decreased neck-shaft angle reduced minimum abduction angle by 10° but had little effect on overall range of motion. Diameter and eccentricity had little effect.

CONCLUSION

A reduced neck-shaft angle reduces minimum abduction angle but does not improve overall range of motion. A more retentive humeral cup increases minimum abduction angle significantly. Although retentive cups are intended to improve joint stability, the reduced range of motion that they impart to the joint may partly counteract the benefits of increased constraint.