Function of supraspinatus muscle with torn cuff evaluated by magnetic resonance imaging.

An estimation of the residual function of the supraspinatus muscle after rotator cuff tear is useful to select the best operative method for repair of the torn cuff and to evaluate prognosis after surgery. The residual function of the supraspinatus muscle was studied by magnetic resonance imaging with motor unit potential amplitudes on electromyograms. Forty-three normal shoulders and 41 shoulders in patients with cuff tears were scanned by magnetic resonance imaging parallel to the long axis of the supraspinatus muscle. The ratio of the largest width of the supraspinatus muscle belly to the distance from the greater tubercle to the proximal end of the supraspinatus muscle was termed the supraspinatus muscle belly ratio. The linear bands of the supraspinatus muscle and the variation in the muscular signal intensity were classified into 3 grades. With a reduction in the motor unit potential amplitude, the supraspinatus muscle belly ratio became lower than 0.15. Additionally, the degree of appearance of the linear bands in the supraspinatus muscle belly increased, and the signal intensity became less uniform. The residual function of the supraspinatus muscle decreased when such morphologic changes were shown by magnetic resonance images. By analyzing these images, the best operative method for repairing a rotator cuff tear can be selected and the prognosis can be evaluated.