Evaluation of the accuracy and interobserver agreement of the reverse shoulder angle in the preoperative planning of reverse total shoulder arthroplasties.

Abstract

Background: Differences in implant positioning between anatomical and reverse shoulder arthroplasties have raised concerns about the adequacy of assessing the global glenoid inclination (GGI) using the method described by Maurer to define the position of the metallic base in reverse shoulder arthroplasty. The reverse shoulder angle (RSA) has been proposed to measure the inclination of the lower half of the glenoid. This study aims to evaluate the interobserver agreement of manual measurements of the RSA using two-dimensional (2D) computed tomography (CT) images and its relationship with the automated measurement of the GGI.

Methods: This cross-sectional study evaluated 2D CT images of 38 CT scans of patients with degenerative shoulder diseases. Manual measurements of the RSA were conducted by five independent shoulder surgeons. GGI measured by automated software was determined.

Results: The interclass correlation coefficient was 0.72 for RSA. Mean RSA was 25.7° ± 7.1°, significantly higher than automated GGI measurements (11.2 ± 9.0; p < .0001). On average, RSA was 14.6 ± 6.3 greater than GGI, irrespective, diagnosis, Favard's glenoid subtype, and version angle.

Conclusion: Our study demonstrated that the RSA angle can be reproducibly used to assess glenoid inclination on 2D CT images presenting high interobserver agreement. RSA differs significantly from the GGI, indicating that measuring glenoid inclination for reverse or anatomical arthroplasty requires distinct methodologies that account for the inherent differences in these angles.