Morphological changes of the supraspinatus muscle were evaluated by magnetic resonance imaging (MRI) to assess recovery after a rotator cuff repair. Ninety-eight shoulders in ninety-seven patients who underwent surgery for rotator cuff tears were included in this study. The shoulders were divided into three groups according to the size of the cuff tear: group A consisted of thirty-three shoulders with small to moderate sized tears, group B consisted of thirty-five shoulders with large tears, and group C consisted of thirty shoulders with massive tears. The width of the supraspinatus muscle on the MRI oblique sagittal view was assumed to reflect a rotator cuff atrophy and was measured preoperatively and at six months, one, one point five and two years postoperatively. In group A and group B, no significant changes were found over time. In group C, significant recovery of the supraspinatus muscle atrophy was seen between six months and two years postoperatively (p<0.05). In group C the ratio of the width of the supraspinatus muscle against the depth of the supraspinatus fossa was less than that in group A or B preoperatively and at six months postoperatively (p<0.01). The ratio in group C was less than that in group A at one, one point five and two years postoperatively (p<0.05). The width of the supraspinatus muscles at six months postoperatively appeared to serve as the standard to evaluate time-course changes of supraspinatus muscle atrophy. The width of the supraspinatus muscles would change six months postoperatively as a result of stretch of the muscle during a rotator cuff repair. Recovery in the supraspinatus muscle atrophy was seen two years postoperatively in the group with massive tears. Measuring a supraspinatus muscle atrophy by MRI appears to be useful to evaluate the postoperative recovery of the rotator cuff in patients with massive tears.
