Review Question - QID 217867

Figure A demonstrates a mid-diaphyseal humeral shaft fracture, and Figure B is a graphical representation of an antegrade humeral intramedullary nail (IMN), with the distal anterior-to-posterior interlocking screw hole highlighted. Placement of this screw is associated with iatrogenic injury to the musculocutaneous nerve, which arises from Branch E (Answer E) of the brachial plexus shown in Figure C.

Humeral IMN represents an attractive treatment option for certain acute humerus fractures. Furthermore, they have an established track record in the stabilization of impending humeral shaft pathologic fractures. Recent meta-analyses have demonstrated equivalent union rates between properly indicated fractures treated with either open reduction and internal fixation (ORIF) and IMN. While IMNs are more frequently associated with the development of shoulder pain when compared to ORIF, functional shoulder outcomes do not differ. Although sound surgical technique can lessen the risk of intraoperative complications, the placement of distal interlocking screws does place certain neurovascular structures at risk. Specifically, the radial nerve faces risk of injury with lateral-to-medial interlocking screws and the musculocutaneous nerve with anterior-to-posterior interlocks.

Rupp et al. provide a seminal article on the risk of neurovascular injury during humeral IMN fixation. This cadaveric study reported on the proximity of various neurovascular structures to interlocking holes for two different humeral IMNs. The authors report that the radial nerve is at direct risk with lateral to medial screw placement and that the ulnar nerve and median nerve/brachial artery bundle are at risk if significant over-penetration of the medial cortex occurrs during the passage of this screw. Important for this question, the authors note that the musculocutaneous nerve is at direct risk with the placement of a distal screw from anterior to posterior.

Steinmetz et al. built upon the work of Rupp et al. with a cadaveric study of 10 cadavers. The authors utilized a single nail with three distal interlocking trajectories. The authors conclude that the musculocutaneous nerve is at risk with passage of an anterior-to-posterior interlocking screw, and that oblique anteromedial-to-posterolateral screws risk injury to the median nerve and brachial artery, and recommend against use of this trajectory.

Figure A demonstrates a mid-diaphyseal fracture of a left humerus in a skeletally mature patient. Figure B is a graphical representation of a humerus IMN, with the anterior-to-posterior interlocking hole highlighted. Figure C is a graphical representation of the brachial plexus, in which branch A is the axillary nerve, branch B is the ulnar nerve, branch C is the median nerve, branch D is the radial nerve and branch E is the musculocutaneous nerve.

Incorrect Answers:
Answer 1: A is the axillary nerve, the first terminal branch off of the posterior cord, and it is not at risk with anterior-to-posterior distal interlocking screws.
Answer 2: B is the ulnar nerve, the terminal branch off of the medial cord, and it is not at risk with anterior-to-posterior distal interlocking screws.
Answer 3: C is the median nerve, the terminal branch of contributions from both the medial and lateral cords. The median nerve is at risk with anteromedial-to-posterolateral oblique screws, not direct anterior-to-posterior screws.
Answer 4: D is the radial nerve, the second terminal branch off of the posterior cord, and it is not at risk with anterior-to-posterior distal interlocking screws.