Are Locking Screws Advantageous with Plate Fixation of Humeral Shaft Fractures? A Biomechanical Analysis of Synthetic and Cadaveric Bone

OBJECTIVES:To investigate whether locking screws offer any advantage over nonlocking screws for plate fixation of humeral shaft fractures for weight-bearing applications. DESIGN:Mechanical evaluation of stiffness in torsion, bending, and axial loading and failure in axial loading in synthetic and cadaveric bone. SETTING:Biomechanical laboratory in an academic medical center. METHODS:We modeled a comminuted midshaft humeral fracture in both synthetic and cadaveric bone. Humeri were plated posteriorly. Two study groups each used identical 10-hole, 3.5-mm locking compression plates that can accept either locking or nonlocking screws. The first group used only nonlocking screws and the second only locking screws. Stiffness testing and failure testing were performed for both the synthetic bones (n = 6) and the cadaveric matched pairs (n = 12). Fatigue testing was set at 90,000 cycles of 440 N of axial loading. MAIN OUTCOME MEASURES:Torsion, bending, and axial stiffness and axial failure force after cyclic loading. RESULTS:With synthetic bones, no significant difference was observed in any of the 4 tested stiffness modes between the plates with locking screws and those with nonlocking screws (anteroposterior, P = 0.51; mediolateral, P = 0.50; axial, P = 0.15; torsional, P = 0.08). With initial failure testing of the constructs in axial loading, both plates failed above anticipated physiologic loads of 440 N (mean failure load for both constructs >4200 N), but no advantage to locking screws was shown. The cadaveric portion of the study also showed no biomechanical advantage of locking screws over nonlocking screws for stiffness of the construct in the 4 tested modes (P > 0.40). Fatigue and failure testing showed that both constructs were able to withstand strenuous fatigue and to fail above anticipated loads (mean failure >3400 N). No difference in failure force was shown between the 2 groups (P = 0.67). CONCLUSIONS:Synthetic and cadaveric bone testing showed that locking screws offer no obvious biomechanical benefit in this application.