Total Shoulder Arthroplasty and Scapulohumeral Rhythm: The clinical impact of achieving native shoulder mechanics.

Abstract

Scapulohumeral rhythm (SHR) is a foundational metric of shoulder biomechanics and represents the ratio of humeral to scapular contributions during shoulder motion. SHR has been studied across physiologic, pathologic, and postoperative states, including following total shoulder arthroplasty (TSA). The clinical relevance of scapular contribution remains incompletely defined. Patient reported outcome measures (PROMs), including visual analogue scale (VAS), single assessment numeric evaluation (SANE), and American Shoulder and Elbow Surgeon (ASES) scores, provide valuable insight for clinical decision making, treatment comparisons, and optimization of patient care. We hypothesized that decreased SHR (increased scapular contribution) would be associated with worse PROMs. Eighty-five postoperative shoulders were included, consisting of 27 anatomic total shoulder arthroplasties (aTSAs) and 58 reverse shoulder arthroplasties (rTSAs), all with a minimum follow-up of 12 months postoperatively. PROMs were collected preoperatively when available and one year or greater postoperatively. Preoperative and postoperative (minimum 12 months postoperative) SHRs were calculated at rest, 30°, 60°, 90°, and 120° of shoulder elevation. A subgroup analysis was performed on 27 shoulders with both pre- and postoperative dynamic digital radiography. Patients were categorized into three cohorts: low (≤ 1 standard deviation below the mean), middle (± 1 standard deviation), and high SHR performers (≥ 1 standard deviation above the mean). The mean follow-up was 36.2 months. The mean postoperative total SHR was 2.05:1. The high SHR cohort (SHR >2.6:1) exhibited values approaching those of native shoulders. Patients in the low total SHR cohort demonstrated significantly higher postoperative VAS (p=0.02) and worse SANE (p=0.04) scores compared to those in the middle and high SHR cohorts. TSA improved SHR from 1.58:1 preoperatively to 1.92:1 postoperatively (p<0.001). Scapular contribution to overhead elevation improved after TSA across the entire range of elevation, particularly between 60-120° of elevation. rTSAs in the lower SHR group possessed significantly worse SANE (p=0.030) and VAS (0.045) scores compared to the middle and high SHR groups but no significant findings were found among the 27 aTSAs. Increasing scapular involvement during shoulder elevation following TSA (decreasing SHR) was associated with higher postoperative pain and worse shoulder function. Scapular-predominant motion may reflect maladaptive kinematics, potentially resulting from residual stiffness, muscle dysfunction, implant malposition, or other factors that may disrupt restoration of native physiological shoulder mechanics. Incorporating SHR and scapular motion assessment into clinical practice may facilitate targeted rehabilitation, postoperative surveillance, and continue to improve outcomes by reproducing native mechanics after TSA.