Scapular notching after reverse total shoulder arthroplasty: Prediction using patient-specific osseous anatomy, implant location, and shoulder motion

Background: Scapular notching is frequently observed following reverse total shoulder arthroplasty (rTSA), although the etiology is not well understood. Methods: Twenty-nine patients with preoperative computed tomography (CT) scans who underwent rTSA with a Grammont design were evaluated after a minimum of 2 years of follow-up with video motion analysis (VMA), postoperative three-dimensional (3D) CT, and standard radiographs. The glenohumeral range of motion demonstrated by the VMA and the postoperative implant location on the CT were used in custom simulation software to determine areas of osseous impingement between the humeral implant and the scapula and their relationship to scapular notching on postoperative CT. Patients with and without notching were compared with one another by univariable and multivariable analyses to determine factors associated with notching. Results: Seventeen patients (59%) had scapular notching, which was along the posteroinferior aspect of the scapular neck in all of them and along the anteroinferior aspect of the neck in 3 of them. Osseous impingement occurred inexternal rotation with the arm at the side in 16 of the 17 patients, in internal rotation with the arm at the side in 3, and in adduction in 12. The remaining 12 patients did not have notching or osseous impingement. Placing the glenosphere in a position that was more inferior (by a mean of 3.4 ± 2.3 mm) or lateral (by a mean of 6.2 ± 1.4 mm) would have avoided most impingement in the patients' given range of motion. Notching was associated with glenosphere placement that was insufficiently inferior (mean inferior translation, 20.3 ± 3.4 mm in the notching group versus 3.0 ± 2.9 mm in the no-notching group; p = 0.01) or posterior (mean, 20.3 ± 3.5 mm versus 4.2 ± 2.2 mm; p < 0.001). Two-variable models showed inferior and posterior (area under the curve [AUC], 0.887; p <0.001), inferior and lateral (AUC, 0.892;p < 0.001), and posterior and lateral (AUC, 0.892; p < 0.001) glenosphere positions to be significant predictors of the ability to avoid scapular notching. Conclusions: Osseous impingement identified using patients' actual postoperative range of motion and implant position matched the location of scapular notching seen radiographically. Inferior, lateral, and posterior glenosphere positions are all important factors in the ability to avoid notching. Only small changes in implant position were needed to avoid impingement, suggesting that preoperative determination of the ideal implant position may be a helpful surgical planning tool to avoid notching when using this implant design. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.