Predictors of External Rotation Recovery following Reverse Shoulder Arthroplasty in Patients with Preoperative Lag Signs.

BACKGROUND: Reverse shoulder arthroplasty (rTSA) restores forward elevation reliably but produces variable gains in external rotation (ER), particularly in patients with posterior rotator cuff deficiency. The relative contributions of external rotation in adduction (ER1) and abduction-plane (ER2) motions, and their impact on functional recovery, remain poorly defined. This study sought to identify predictors of ER recovery following rTSA in a group of patients with preoperative ER lag signs.

METHODS: A retrospective cohort of 55 primary rTSA patients (mean age = 69 years; 56% male) with ≥2-year follow-up was analyzed. Active and passive ER1 and ER2 were recorded with ER lag defined as passive minus active ER. Radiographs measured the lateralization and distalization shoulder angle (LSA/DSA) and pre-operative Hamada grading. Functional ER was defined as ER1 ≥30° and ER2 ≥60°. ROC analysis identified preoperative motion thresholds predictive of functional recovery.

RESULTS: rTSA significantly improved forward elevation (112° to 142°) and active ER1 (20° to 44°), ER2 (53° to 69°) (p< 0.001). Mean ER lag decreased from 23° to 8° (ER1) and 31° to 15° (ER2). Functional ER1 increased from 38% to 81% and functional ER2 from 60% to 86% of subjects. Failure to achieve both functional ER1 (p=0.03) or ER2 (p=0.045) negatively affected ASES scores. Preoperative active ER1 >25°, ER2 > 40°, and ER2 lag < 25° predicted functional recovery (AUC 0.74 to 0.76). No independent correlation was found between LSA/DSA and ER function.

CONCLUSION: rTSA reliably improves ER function in patients with preexisting ER signs, but recovery depends primarily on preoperative motion and lag magnitude rather than radiographic measurements of implant geometry. Patients with active ER1 > 25°, ER2 > 40° and ER2 lag <25° can expect functional ER following rTSA alone. Patients with ER lag signs >50% of passive ER are less likely to achieve functional ER following rTSA alone.

LEVEL OF EVIDENCE: Level IV; Case series; Treatment study.