Summary

After arthroscopic rotator cuff repair for rotator cuff tears, infraspinatus muscle contractility improved and was involved in postoperative acromio-humeral distance and outcomes. Infraspinatus muscle contractility was the most important factor in improving postoperative outcomes.

Abstract

Introduction

Postoperative fatty degeneration (FD) of the infraspinatus muscle (ISm) and narrowing of the acromio-humeral distance (AHD) have been reported as factors associated with poor outcomes after arthroscopic rotator cuff repair (ARCR). A correlation between the degree of FD of ISm and AHD has been reported. It implies that ISm dysfunction contributes to the narrowing of the AHD and poor outcomes. The purpose of this study is to clarify 1) whether ISm contractility improves postoperatively and 2) contributes to AHD and postoperative outcomes.

Methods

We conducted a prospective cohort study of patients who underwent ARCR from December 2021 to April 2023, with 1 year follow-up and no missing data. To clarify the relationship between ISm function within the transverse force couple and AHD, we excluded patients with subscapularis tendon tear. Of the 109 patients, 38 with subscapularis tendon tears, 3 with missing data, and 14 with dropouts were excluded. Finally, 54 patients were analyzed. Imaging and functional evaluations were performed preoperatively and 1 year postoperatively. To evaluate ISm function, the muscle elasticity of ISm was measured at 60° of shoulder abduction using real-time tissue elastography, and the difference in elasticity between resting and isometric contractions was defined as an index of muscle contractility (activity value: AV). True anteroposterior shoulder radiographs were obtained and AHD was measured. MRI was used to evaluate FD of ISm using the Goutallier classification and the integrity of the rotator cuff using the Sugaya classification. To avoid the impact of re-tear on outcomes, Type IV and V were excluded as re-tears. Tear size was measured intraoperatively and classified using the Cofield classification. Functional evaluations included range of motion (ROM), muscle strength, and Constant score.
Spearman's correlation coefficient and Wilcoxon test were used for statistical analysis. A p-value less than 0.05 was considered a significant difference. Multiple regression analysis was performed with Constant score as the objective variable and age, gender, AV of ISm, AHD, and each classification as explanatory variables.

Results

The mean age was 65.8 ± 7.8 (SD) years, and the male-to-female ratio was 29:25. Sugaya classification was IV and V in 1 patient each, and 2 cases of re-tear were excluded. The AV of ISm, AHD and Constant score improved significantly from 0.50 ± 0.24 to 0.75 ± 0.31, from 8.53 ± 1.93 to 9.84 ± 1.48, and from 70.2 ± 14.83 to 90.8 ± 9.38 respectively (P <.01 for all). ROM and muscle strength also improved. Significant positive correlations were found between AV of ISm and AHD (r = 0.594, P ＜.01), muscle strength in shoulder abduction (r = 0.43, P ＜.01), and Constant score (r = 0.412, P ＜.01) at 1 year postoperatively. Multiple regression analysis showed that AV of ISm, Cofield classification, and AHD were significantly associated with postoperative Constant score, with standard regression coefficients of 0.491, -0.351, -0.342, respectively (P <.01).

Conclusion

After ARCR, ISm contractility improved and was involved in postoperative AHD and Constant scores. ISm contractility was the most important for better postoperative outcomes.