Onlay versus inlay reverse total shoulder arthroplasty: a retrospective comparison of radiographic and clinical outcomes

Abstract

IntroductionRecent innovations in reverse shoulder arthroplasty (RSA) have presented 2 distinct humeral stem designs: an onlay system that rests above the anatomic neck and an inlay component that rests within the metaphysis. The purpose of this study is to compare clinical and radiographic outcomes between inlay and onlay-designed humeral stems in lateral center of rotation RSA implant systems. MethodsA retrospective cohort study was performed on primary RSA patients treated by 2 surgeons at 2 separate hospitals with a minimum 2-year follow-up. Patients were categorized based on treatment with an onlay or inlay humeral design and matched 1:1 by indication and age. Patient-reported outcome measures (PROMs), including the Simple Shoulder Test, American Shoulder and Elbow Surgeons, and Visual Analog Score for pain, as well as active motion (forward elevation, internal rotation) were recorded at pre- and postoperative intervals. An Inlay-Onlay index assessed the degree of inset or offset of each particular implant referencing the anatomic neck. Radiographic analysis focused on scapular notching, bone resorption around the humeral stem, and acromion stress fractures. ResultsA total of 92 patients participated in the 1:1 matched analysis (46 each group). Cohorts were similar in age, gender, indication, follow-up length, and preoperative PROMs, with the exception of Simple Shoulder Test. At the most recent follow-up, there were no differences in all PROMs between groups. There were no differences in active internal rotation, but patients with an onlay-configuration demonstrated greater external rotation (P< .001) and forward flexion (P< .001). Greater tuberosity and calcar resorption occurred in 34 (74%) and 18 (39%) patients with an onlay-designed prosthesis, compared to 13 (28%) and 1 (2%) in the inlay group, respectively (P< .0001). Both groups had low rates of scapular notching (P= 1.0), while acromial fractures occurred in 6 patients with an onlay stem and in 4 patients with the inlay stem (P= .73). ConclusionThere were no differences in clinical outcomes or incidence of acromial fractures following RSA with an onlay- or inlay-style humeral stem prosthesis. Bone resorption of the proximal humerus occurred more frequently in patients with an onlay prosthesis, suggesting that an inlay prosthesis may afford better prevention of humeral stress shielding. Level Of EvidenceLevel III; Retrospective Comparative Study