Radiographic outcomes of patients undergoing reverse shoulder arthroplasty using inlay versus onlay components: is there really a difference?

Abstract

IntroductionAchieving soft tissue tension in RSA occurs by displacement of the humerus from the glenoid. We compared the lateral and inferior humeral displacement of two RSA systems radiographically. Each system utilized a humeral implant with a 135-degree neck-shaft angle and offered lateralized glenospheres. One had an onlay component and the other an inlay. Our primary hypothesis was that an alteration of surgical technique would negate the differences in their geometries radiographically. Secondarily, we sought to determine if a difference in complications or revisions occurred with these different designs. MethodsTwo hundred and eleven patients underwent RSA by a single surgeon with either an inlay or onlay prosthesis over a 2-year period. A true AP Grashey radiograph was utilized to measure: 1) Glenohumeral offset (GHO); 2) Acromiohumeral distance (AHD); 3) Pivot point (PP); 4) Humeral head cut surrogate (HHC) and 5) Humeral Socket Depth (HSD). Complications recorded included postoperative acromial fractures, revision for any reason, instability, and infections. ResultsThere was no significant difference in GHO or AHD between the two groups. There was a difference in PP and HHC between the groups (P < .001). The onlay group had an HHC 1.8 mm larger than inlay. Of the inlay group patients, 66% had their humeral tray placed above the level of the humeral osteotomy. DiscussionA larger HHC in the onlay group and implanting the inlay above the humeral osteotomy negates differences in AHD and GHO. Level of evidenceLevel III; Retrospective Comparative Study