Review Question - QID 217049

This patient who presents after a high-energy fall on her hand and is found to have volar dislocation of her lunate on radiography most likely has impairment of median nerve function. This nerve normally innervates the opponens pollicis.

Lunate dislocations are high-energy injuries to the wrist that are associated with neurological injury and poor functional outcomes. They occur due to the traumatic disruption of the perilunate ligaments that connect the lunate to the surrounding wrist bones. When the lunate dislocates volarly, it can acutely compress the median nerve and cause carpal tunnel syndrome. This would present with paresthesias over the palmar first 3 digits and dysfunction of the thenar muscles. Diagnosis requires careful evaluation of plain radiographs to determine whether the lunate is aligned with the remainder of the carpus.

Goodman et al. present the evaluation and outcomes of patients with lunate dislocations. They discuss how this injury can cause median nerve dysfunction, carpal instability, posttraumatic arthritis, reduced functionality, and avascular necrosis.

Figure/Illustration A is an anteroposterior and lateral radiograph of the wrist showing volar dislocation of the lunate (red circle). The complete volar dissociation of the lunate on the lateral radiograph places this patient at high risk of acute carpal tunnel syndrome.

Incorrect Answers:
Answer 1: The coracobrachialis is innervated by the musculocutaneous nerve but this nerve does not cross the wrist. It can be injured due to excessive retraction in the deltopectoral approach to the humerus.

Answer 2: The deltoid muscle is innervated by the axillary nerve but this nerve does not cross the elbow. It can be injured due to excessive division of the deltoid in the deltoid splitting approach to the shoulder.

Answer 3: The extensor carpi radialis longus muscle is innervated by the radial nerve, but this nerve is on the dorsal rather than the volar aspect of the forearm. It can be injured due to humerus fractures as it crosses from medial to lateral in the posterior spiral groove of the humerus.

Answer 4: The flexor carpi ulnaris is innervated by the ulnar nerve but this nerve lies between the pisiform and the hamate bones as it crosses the wrist. It can be injured in flexion-type supracondylar humerus fractures.

Bullet Summary:
Volar lunate dislocation can result in acute carpal tunnel syndrome with dysfunction of the median nerve.