Comparison of Clinical Performance of Inlay versus Onlay Humerus Implants in Reverse Total Shoulder Arthroplasty

Reverse total shoulder arthroplasty (RTSA) has become the treatment of choice for the management of massive rotator cuff tears combined with cuff tear arthropathy, and many novel designs have been proposed to overcome the shortcomings of classic RTSA. This study sought to evaluate and compare RTSA outcomes among patients with cuff tear arthropathy treated by a medialized inlay humerus implant with a neck shaft angle of 155° or a lateralized onlay implant with a neck shaft angle of 145°.

A retrospective review of 32 inlay implants and 32 onlay implants was performed. The active range of motion (ROM), visual analog scale (VAS) for pain, motor power for elevation and external rotation, and functional scores including the American Shoulder and Elbow Surgeons score, Constant score, and Korean Shoulder Scoring system were assessed before surgery, at 3, 6, and 12 months after surgery, and at the last follow-up at least 24 months after surgery. Scapular notching, lateral humeral offset, and deltoid wrapping offset were assessed for radiographic evaluation.

The preoperative demographic data of both groups showed no significant differences (p > 0.05). The mean follow-up period was 24.9 months. Significant improvements in forward flexion, functional scores, and pain VAS score were observed in both groups at the last follow-up. No significant differences in ROM or functional scores were found between two groups at each time point, except that the onlay implant group exhibited a significantly greater range of external rotation at 3 and 12 months after surgery and at the last follow-up. The rate of scapular notching and the final power improvement did not show significant differences between the groups.

Primary RTSA using inlay or onlay humerus implants was associated with recovery from pseudoparalysis and good clinical outcomes. However, RTSA with onlay humerus implantation led to clinically superior results in terms of external rotation.