Background: Shoulder function after rotator cuff repair is associated with patient satisfaction after surgery. Several studies have demonstrated that the muscle strength ratio (external rotators/internal rotators) is an important factor to evaluate shoulder function, but little is known about the relationship between the preoperative muscle strength ratio and postoperative shoulder function. Purpose: To evaluate the effect of the preoperative muscle strength ratio of the shoulder rotators on function after rotator cuff repair. Study Design: Cohort study; Level of evidence, 3. Methods: The study participants were patients with small- to medium-sized rotator cuff tears diagnosed by magnetic resonance imaging; 77 patients were included in the analysis. Preoperative muscle strength was assessed through use of isokinetic equipment. Patients were classified into 2 groups (normal and abnormal) according to a normal strength ratio range of 55% to 75%, with “abnormal” meaning a deviation of more than 15% from the normal range. The American Shoulder and Elbow Surgeons (ASES) score and the Constant score were used to evaluate shoulder function preoperatively and postoperatively at 6 months, 1 year, and 2 years. Results: There were 30 patients in the normal group and 47 in the abnormal group, with a preoperative muscle strength ratio of 63.5% ± 5.5% and 42.6% ± 6.1%, respectively. The ASES score was 88.6 ± 9.1 in the normal group and 77.5 ± 13.6 in the abnormal group at 2 years postoperatively, and the Constant score was 82.7 ± 8.4 in the normal group and 69.5 ± 13.4 in the abnormal group at 2 years postoperatively. A significant difference was found in postoperative shoulder function between the normal and abnormal groups. Conclusion: This study demonstrated that the preoperative muscle strength ratio was associated with postoperative shoulder function. The preoperative muscle strength ratio should be considered an important predictor of shoulder function after rotator cuff repair.
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