The influence of elbow position on the range of motion of the wrist following transfer of the brachioradialis to the extensor carpi radialis brevis tendon

Background: In patients who have an injury of the cervical spinal cord, the brachioradialis tendon may be transferred to the extensor carpi radialis brevis tendon to restore voluntary wrist extension. We hypothesized that the active range of motion of the wrist depends on the position of the elbow after this transfer because the brachioradialis changes length substantially during elbow flexion, which implies the maximum force that the muscle can produce varies with elbow position. The objectives of this study were to determine whether the position of the elbow influences the range of motion of the wrist following transfer of the brachioradialis to the extensor carpi radialis brevis tendon and to evaluate the effect of surgical tensioning. Methods: The range of motion of eight wrists was assessed after brachioradialis transfer. Two positions of the elbow were tested, the passive limit of elbow extension and 120° of flexion. The range of motion of the wrist was also simulated with use of a biomechanical model. Using the model, we compared the active range of motion of the wrist, with the elbow at 0° and 120° of flexion, following three different approaches to surgical tensioning. The simulations were also repeated to evaluate how muscle strength influences outcomes. Results: Wrist extension decreased and passive flexion increased when the elbow was flexed. Maximum wrist extension was significantly correlated with passive flexion in all subjects (r = 0.95 and p < 0.001 when the elbow was extended and r = 0.82 and p < 0.03 when the elbow was flexed). The biomechanical model suggested that tensioning the tendon transfer so that the fibers of the brachioradialis do not become excessively short when the elbow is flexed may improve outcomes. The simulations also revealed that it is more difficult to maintain a consistent wrist position with the elbow in different postures when a weaker muscle is transferred. Conclusions: The model suggests that altering the surgical tension could improve wrist extension when the elbow is flexed. However, the ultimate result is sensitive to the strength of the brachioradialis. Clinical Relevance: The brachioradialis is the donor muscle most commonly used to restore the function of the wrist and hand in tetraplegia. Because many self-care activities involve acquiring an object at a distance from the body and then bringing the object close to the body, it is important to consider the influence of the position of the elbow on the function of the transferred brachioradialis.