The most likely cause of the radial nerve palsy in a high energy open humerus fracture is laceration or complete disruption of the radial nerve (Neurotmesis).
Neurotmesis is complete disruption of nerve and is associated witn no spontaneous recovery without intervention. Axonotmesis constitutes axon disruption, but the surrounding neural connective tissue is intact and nerve regeneration can occur(Wallerian or antegrade degeneration). Neurapraxias occur often by compression and the axon maintains continuity but local demyelination and ischemia occur.
Ring et al. present a Level 4 study of 24 patients that had radial nerve palsy associated with a humerus fracture. All 6 patients with a transected radial nerve had an open humerus fracture also. The results of primary nerve repair in this circumstance found that there was no recovery in any of the patients.
Foster et al. authored a Level 4 review of 14 patients had a radial nerve palsy and an associated open humerus fracture. 64% of the 14 patients had a radial nerve that was either lacerated or interposed between the fracture fragments. They recommend exploration of the radial nerve in the setting of a radial nerve palsy and concomitant open humerus fracture in contrast to observation of a radial nerve palsy in closed humerus fractures.
Figure A demonstrates an open left humerus fracture.
Answer 1,3,5: The radial nerve provides distal motor activity to the ECRL, ECRB, EPL, EIP, and EPL.
Answer 2: Axonotmesis of the radial nerve is not the most common form of injury associated with closed or open humerus fractures.
Ring D, Chin K, Jupiter JB. Radial nerve palsy associated with high-energy humeral shaft fractures. J Hand Surg Am. 2004 Jan;29(1):144-7.
PMID:14751118 (Link to Abstract)
Foster RJ, Swiontkowski MF, Bach AW, Sack JT. Radial nerve palsy caused by open humeral shaft fractures. J Hand Surg Am. 1993 Jan;18(1):121-4.
PMID:8423295 (Link to Abstract)