Three cases of post-traumatic proximal radioulnar synostosis are presented. Each was treated with surgery with excision of the heterotrophic new bone and mobilization of the proximal radius. The anconeus muscle was mobilized as a vascularized pedicle graft and then interposed between the proximal radius and ulna. Two patients had postoperative radiotherapy. A forearm rotation assist splint was used to augment rehabilitation after surgery. This device consists of an inner sleeve that attaches to the hand and then rotates within a standard turnbuckle flexion/extension assist splint. The overall improvement in forearm rotation in each case was 100 degrees, 140 degrees, and 150 degrees, respectively.