Scapular and humeral elevation coordination patterns used before vs. after Reverse Total Shoulder Arthroplasty.
The purpose of this study was to compare scapulohumeral coordination used before and after Reverse Total Shoulder Arthroplasty (RTSA) during the ascent phase of scapular plane arm elevation tasks performed with varied shoulder rotations (neutral, external rotation, and internal rotation). We expected that after RTSA, participants would decrease scapulothoracic upward rotation angular displacement and increase the scapulohumeral rhythm (SHR) vs. before RTSA. 11 RTSA patients (12 shoulders) participated in this study before and after RTSA while optical motion capture measured kinematics of the humerus and scapula relative to the thorax. Angular kinematics were compared pre vs. post-RTSA within-participant using One Dimensional Statistical Parametric Mapping (SPM) t-tests (α = 0.05) and across-participants, using paired t-tests (α = 0.05) adjusted for multiple comparisons. As a group, during arm elevation with neutral rotation, the mean (SD) SHR pre-RTSA was 1.5 (0.5) and increased to 1.7 (0.3) post-RTSA, though, not significantly (p = 0.182). In contrast, during arm elevation with external rotation, the mean (SD) SHR pre-RTSA was 1.3 (0.4) and significantly increased (p = 0.018) post-RTSA to 1.7 (0.3). Likewise, during arm elevation with internal rotation, the mean (SD) SHR pre-RTSA was 1.2 (0.3) and significantly increased (p < 0.001) post-RTSA to 1.7 (0.2). In addition to these and other group trends, participant-specific patterns were uncovered through SPM analyses - with some participants significantly increasing and others significantly decreasing scapulothoracic angular displacements across humerothoracic elevation ranges. Both before and after RTSA, scapulohumeral rhythm ratios were within the range of those previously reported in post-RTSA patients and were smaller than those used by healthy populations.