Prosthetic radial head replacement is a well-documented procedure; however, loss of elbow flexion after radial head arthroplasty has only recently been reported. This study reviews 6 patients who received modular prosthetic radial heads and had a clinically significant decrease in elbow flexion. The implant was removed in 4 of these patients because of reduced elbow motion and pain. This clinical finding is correlated with biomechanical data obtained by use of matched-pair, fresh-frozen upper extremity specimens loaded to 330 N with the forearm in positions of neutral, pronation, and supination for each of three elbow positions (0 degrees, 90 degrees, and 135 degrees). The radiocapitellar gap was monitored and was significantly smaller under load during elbow flexion compared with extension. This study indicates the need for verification of the radiocapitellar gap throughout elbow range of motion in order to prevent these complications.