Influence of scapular neck length on the extent of impingement-free adduction after reverse total shoulder arthroplasty.

Abstract

Following reverse total shoulder arthroplasty, a short scapular neck length (SNL) decreases postoperative impingement-free adduction, and impingement between the neck of the scapula and the humeral polyethylene cup may cause scapular notching. However, no reports have evaluated the influence of SNL on impingement-free adduction. The purposes of this study were to evaluate the influence of SNL on impingement-free adduction and to examine the effect of glenoid component lateralization and inferiorization on impingement-free adduction. By use of 3-dimensional templating software, a virtual reverse total shoulder arthroplasty model was created in 15 patients who had no osteoarthritic change or any other bony deformity. We measured SNLs separately before implant placement (preoperative SNL) and after implant placement (postoperative SNL). The implant used was the Comprehensive Reverse Shoulder System (Zimmer Biomet, Warsaw, IN, USA), and baseplate bony lateralization of 0, 5, and 10 mm, with inferior eccentricity of 0.5 or 4.5 mm, was tested for impingement-free adduction. Correlations between the preoperative and postoperative SNLs and impingement-free adduction were analyzed. The mean preoperative SNL was 8.2 ± 1.9 mm (range, 5.0-11.7 mm), and the mean postoperative SNL was 6.0 ± 2.0 mm (range, 2.1-9.8 mm). There was a moderate correlation between the preoperative SNL and impingement-free adduction (r = 0.628, P = .12) and a strong correlation between the postoperative SNL and impingement-free adduction (r = 0.771, P = .001). Use of the model with 10 mm of bony lateralization and 4.5 mm of inferior eccentricity provided the best results in terms of impingement-free adduction. There were correlations between both the preoperative and postoperative SNLs and impingement-free adduction. Although the lateralized and inferiorized center of rotation may increase the risk of loosening of the glenoid component, this offset significantly increased impingement-free adduction.