A case of rotational restriction of the forearm due to abnormal configuration of pronator quadratus muscle.

We present a case of rotational restriction of the forearm due to abnormal configuration of the pronator quadratus muscle. A 20-year-old man developed right wrist joint pain on pitching of a baseball game and thereafter displayed rotational disorder of the forearm. Magnetic resonance imaging (MRI) revealed a space-occupying lesion from the volar side of the radius to the dorsal side of the ulna. The lesion was iso-hyperintense on T1-weighted (T1W) images and showed a mixed pattern of high intensities on T2-weighted (T2W) images. His symptoms were immediately reduced after removal of the mass. Histological examination showed that the mass contained much skeletal muscle and revealed myxoid degeneration of striated muscles. We assumed that his pronator quadratus muscle had been a divided form of deep layer and superficial layer tissue, possibly congenitally. We supposed that the deep layer had degenerated due to chronic stimulation and had extended around to the dorsal side of the ulna, which caused rotational restriction from the resulting impaired distal radioulnar joint. To our knowledge, there has been no similar case reported in the literature.