Structural and Functional Alterations of the Deltoid Muscle After Arthroscopic Rotator Cuff Repair: A 2-Year Longitudinal Observation Study.

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Deltoid muscle detachment and atrophy have been reported to occur after shoulder surgery. To investigate the 2-year changes in deltoid muscle structure and function after arthroscopic rotator cuff repair (ARCR) using magnetic resonance imaging (MRI) and electrophysical examination. Case series; Level of evidence, 4. A total of 72 patients (72 shoulders) who underwent ARCR between 2015 and 2020 were enrolled. Whole deltoid muscle volume and regional (anterior, lateral, and posterior) muscle thicknesses were determined on T2-weighted MRI scans of both shoulders taken preoperatively and at 1, 3, 6, 12, and 24 months postoperatively, and their correlations with compound muscle action potentials (CMAPs), shoulder abduction muscle strength, and Constant scores were investigated. Comparison between groups was performed using paired or Student t tests, and the relationship between deltoid muscle volume and various factors was determined using Pearson correlation analysis. The volume of the deltoid muscle on the affected side decreased from 44,369 ± 12,371 mm3 preoperatively to 38,139 ± 10,615 mm3 at 1 month postoperatively (P < .05), representing a 14% decrease. The deltoid muscle volume of the contralateral side also significantly decreased during the same time frame, from 43,278 ± 12,248 to 40,273 ± 11,464 mm3 (P < .05), representing a 7% decrease at 1 month postoperatively. Subsequently, the deltoid muscle volume on both sides recovered to preoperative levels at 12 months and was maintained at 24 months. Only the thickness of the anterior part of the deltoid was markedly decreased, from 13.9 ± 3.7 mm preoperatively to 12.0 ± 3.2 mm at 1 month postoperatively (P < .05), representing a 14% reduction. The CMAP amplitude showed a significant decrease at 1 month postoperatively; however, no significant difference was observed after 12 months when compared with the preoperative values or the values on the contralateral side. Positive correlations were found between deltoid muscle volume and CMAP amplitude at 24 months as well as between deltoid muscle volume and shoulder abduction muscle strength (R 2 = 0.698; P < .05) and Constant score (R 2 = 0.133; P < .05). Our study demonstrated that the early structural and functional decline of the deltoid muscle after ARCR was fully recovered within 1 year, confirming that this procedure does not negatively affect the deltoid muscle.