Stability of Distal Ulnar Neck Fractures During Cyclic Loading Following Intramedullary Threaded Nail Fixation or Volar-Locking Plate and Screw Fixation: A Cadaveric Biomechanical Comparison.

Abstract

This study compared cadaveric distal ulnar neck fractures following fixation with either retrograde intramedullary threaded nail (IMTN) fixation or volar-locked plate and screw (VLPS) fixation. We hypothesized that there would be no differences in fatigue resistance between the two methods. Twelve matched pairs of cadaveric forearms were randomized into either wrist flexion-extension or forearm pronation-supination groups. In each group, six forearm pairs were cut at the distal ulnar neck. One was fixed with IMTN fixation, and their matched pair was fixed with VLPS fixation. Forearms were cycled 2,000 times with a servohydraulic testing system under displacement control with a physiologic force necessary to move the forearm. Fracture displacement was measured with a linear variable differential transformer. The primary outcome was mean cycles completed, which represented fatigue resistance. Secondary outcomes, after 2,000 cycles, included net displacement and absolute net displacement, which represented damage accumulation. During wrist flexion-extension cyclic loading, one IMTN specimen failed, and no significant differences were found between the VLPS and IMTN groups in cycles completed net displacement or absolute net displacement. During forearm pronation-supination cyclic loading, four IMTN specimens failed and the IMTN group completed significantly fewer cycles than the VLPS group, but net displacement and absolute net displacement were greater in the VLPS groups than the IMTN groups. During cyclic loading of a distal ulnar neck fracture model, IMTN fixation demonstrated statistically similar fatigue resistance compared with VLPS fixation during wrist flexion-extension. However, during forearm pronation-supination, IMTN fixation demonstrated statistically lower fatigue resistance compared with VLPS fixation. Both IMTN fixation and VLPS fixation stabilize distal ulnar neck fractures sufficiently to allow for immediate wrist flexion-extension. VLPS fixation stabilizes distal ulnar neck fractures sufficiently to allow for immediate pronation-supination, while this should be considered with caution in IMTN fixation.