Changes in Appearance of Fatty Infiltration and Muscle Atrophy of Rotator Cuff Muscles on Magnetic Resonance Imaging After Rotator Cuff Repair: Establishing New Time-Zero Traits

PurposeTo investigate whether and how arthroscopic rotator cuff repair changes the appearance of fatty infiltration and muscle atrophy as shown on magnetic resonance imaging (MRI) by comparing measurements taken before and immediately after surgery.MethodsThe first study determined appropriate measurements of fatty infiltration and muscle atrophy, and the second study assessed immediate postoperative changes caused by surgery per se. Forty-two patients who underwent arthroscopic shoulder surgery for reasons other than rotator cuff repair were included in the first study, and 101 patients who underwent arthroscopic rotator cuff repair were included in the second study. MRI was undertaken preoperatively and 3 days after surgery. Fatty infiltration was evaluated with the Goutallier grade and by measuring signal intensities of rotator cuff muscles, and muscle atrophy was assessed with the tangent sign, occupation ratio, and cross-sectional areas of muscles.ResultsIn the first study, only the cross-sectional area of the subscapularis significantly changed after surgery and was excluded in the second study. In the second study, fatty infiltration seemingly improved by at least 1 grade in 50.0%, 75.0%, and 95.8% of patients with Goutallier grade 2, 3, and 4, respectively, in the supraspinatus (P < .001) and in 28.6%, 57.1%, and 66.7%, respectively, in the infraspinatus (P < .001). Muscle atrophy of the supraspinatus changed by at least 1 grade in 93.6% of patients with a grade 2 tangent sign and 100.0% with a grade 3 tangent sign, as well as 84.0% of patients with a grade 3 occupation ratio; cross-sectional areas of the supraspinatus and infraspinatus significantly increased by 21.6% and 7.0%, respectively (all P < .001).ConclusionsThis study showed that arthroscopic rotator cuff repair changed the appearance of both fatty infiltration and muscle atrophy of the supraspinatus and infraspinatus on MRI. We suggest that these changes be considered when one is assessing rotator cuff muscle changes by comparing the appearance on MRI before surgery with that at a certain time after surgery.Level of EvidenceLevel IV, therapeutic case series.