A 30-year-old plastic surgery resident fell from a height of 12' and sustained a right both-bone forearm fracture. The patient underwent open reduction and internal fixation with 3.5mm Limited Contact Dynamic Compression Plating. Three months after fixation the patient has no forearm tenderness and has full active range of motion of his fingers, hand, and wrist. However, pronation and supination are severely limited. This affects his ability to suture during surgery and knit, although he has returned to his other recreational activities. His most recent radiographs are shown in Figures A and B. What is the most likely cause for his lack motion?