Better Scapulohumeral Rhythm is Associated with Superior Patient-Reported Outcome Measures in Shoulders with Semi-Inlay Type Reverse Shoulder Arthroplasty.

Abstract

Background: Shoulders capable of achieving active abduction greater than 90 degrees following reverse total shoulder arthroplasty (rTSA) have been reported to exhibit better scapulohumeral rhythm (SHR) compared to those limited to less than 90 degrees of active abduction. This study aimed to calculate SHR in shoulders following semi-inlay rTSA and to investigate whether improved SHR is associated with better postoperative patient-reported outcome measures (PROMs) in shoulders achieving active abduction greater than 90 degrees.

Methods: Twenty shoulders of 19 patients who underwent semi-inlay rTSA were analyzed. Each shoulder underwent computed tomography (CT) and fluoroscopy. Fluoroscopic images were acquired during scapular plane abduction. Using model-image registration techniques, the poses of 3-dimensional (3D) models were iteratively adjusted to match the silhouettes in the fluoroscopic images. SHR was defined as (ΔH-ΔS)/ΔS, where ΔH is the increment in humeral elevation angle and ΔS is the increment in scapular upward rotation angle. The mean postoperative SHR assessed from 20° to 90° of humeral abduction was used to divide the shoulders into two groups: SHR < 2 or SHR ≥ 2. American Shoulder and Elbow Surgeons (ASES) Standardized Shoulder Assessment Form score was evaluated preoperatively and 1 year after surgery.

Results: The mean postoperative SHR was 1.7. Fourteen shoulders had SHR < 2, while six shoulders had SHR ≥ 2. There were no significant differences in demographic data, preoperative active range of motion (ROM), or preoperative ASES scores between the groups. Although no significant differences in postoperative ROM were observed, shoulders with SHR ≥ 2 had significantly higher postoperative ASES scores (94.3 ± 4.6) compared to those with SHR < 2 (82.1 ± 9.4, p = 0.007). A significant positive correlation was observed between the postoperative ASES scores and both the mean SHR (r = 0.452, P = 0.045) and the overall SHR measured from arm at side to maximum abduction (r = 0.478, P = 0.033) across all shoulders.

Conclusions: Shoulders following semi-inlay rTSA with SHR ≥ 2 exhibited significantly higher postoperative ASES scores compared to those with SHR < 2. A significant positive correlation was also observed between the mean SHR and postoperative ASES scores across all shoulders, highlighting the positive impact of improved SHR on achieving superior postoperative PROMs.