Reverse Shoulder Arthroplasty Implant Design and Configuration has a Significant Effect on Conjoint Tendon Impingement.

Background: Anterior shoulder pain after reverse shoulder arthroplasty (RSA) is not uncommon and may be due to humeral impingement against the conjoint tendon during internal rotation (IR). It is unknown what effect different implant designs and configurations have on conjoint tendon impingement. The purpose of this study was to investigate the influence of several RSA implant design parameters on conjoint tendon impingement during IR.

Methods: Twelve upper extremity cadavers were dissected to visualize and digitize the path of the coracobrachialis using a tracking system. This data was transformed onto the corresponding computer tomography derived bone models, while previous literature was used to approximate the muscle attachment locations for the conjoint tendon origin and short head of the biceps insertion. Each model then underwent three-dimensional virtual RSA implantation using a generic implant design. A baseline configuration was first implanted which utilized a 25mm glenoid baseplate placed in 0° inclination and version and positioned flush to the inferior glenoid rim with a 36mm glenosphere. The humeral baseline configuration consisted of an implant placed in 20° retroversion at a neck-shaft-angle (NSA) of 135°, centered on the humeral cut plane with a zero-thickness polyethylene cup. Additional implant designs were then configured by independently changing various design parameters including glenoid lateralization, glenosphere diameter, glenoid baseplate position, humeral polyethylene thickness, humeral component position, humeral NSA, and humeral version. Each implant configuration was then examined using a custom motion software which modeled the muscle path of both the coracobrachialis and short head of the biceps. IR was performed until conjoint tendon impingement was detected. All implant configurations were compared using a repeated measures analysis of variance (P<0.05).

Results: Glenosphere size, glenoid baseplate anterior-posterior and superior-inferior position, humeral polyethylene insert thickness, humeral anterior-posterior and medial-lateral position, and humeral version significantly influenced conjoint tendon impingement (P<0.001). Glenoid lateralization and humeral NSA did not have a significant effect on conjoint tendon impingement (P>0.293). Overall, earlier conjoint tendon impingement occurred with larger glenosphere sizes, anteriorly and superiorly positioned glenoid baseplates, greater polyethylene thickness, medially and posteriorly positioned humeral implants, and greater humeral retroversion.

Conclusion: RSA implant parameters and positions have been identified that can significantly increase the risk of conjoint tendon impingement, such as larger glenospheres, anterosuperior baseplate & glenosphere positioning, humeral implants or trays positioned medial and posterior, and increased humeral component retroversion. These results may be considered by surgeons preoperatively to limit conjoint tendon impingement, or intra-operatively when conjoint tendon impingement is identified.