A Novel Tabletop Plane-Based Scapular Coordinate System Improves the Reproducibility of Preoperative Three-Dimensional Measurements in Reverse Total Shoulder Arthroplasty

Background/Objectives: Accurate three-dimensional (3D) assessment of glenoid morphology depends on the coordinate system used for measurement. Conventional landmark-based coordinate systems may be affected by observer-dependent variability in identifying the scapular landmarks. This study aimed to compare the reproducibility of a novel anterior scapular plane (ASP) coordinate system with that of the conventional surgical scapular plane (SSP). Methods: Twenty-five shoulders undergoing primary reverse total shoulder arthroplasty were evaluated using preoperative computed tomography scans. Glenoid version (GV), glenoid inclination (GI), glenoid height (GH), glenoid width (GW), and global offset (GO) were measured using both coordinate systems. Two fellowship-trained shoulder surgeons independently performed all measurements twice at a 1-month interval. Intra- and inter-observer reliability were assessed using intraclass correlation coefficients (ICCs). Results: ASP demonstrated almost perfect intra- and inter-observer reliability for all parameters, with ICCs ranging from 0.888 to 0.990 and from 0.864 to 0.932, respectively. In contrast, SSP demonstrated almost perfect intra-observer reliability (ICC range, 0.815–0.963) but only substantial inter-observer reliability for GV (ICC range, 0.660–0.705) and GI (ICC range, 0.660–0.738). Using ASP, inter-observer reliability improved to 0.887–0.924 for GV and 0.898–0.932 for GI. Conclusions: The ASP coordinate system demonstrated superior reproducibility compared with the conventional SSP coordinate system for preoperative 3D evaluation of glenoid morphology. Its anatomical accuracy and applicability to postoperative assessment remain to be validated.