ORIGINAL ARTICLE Are Locking Screws Advantageous with Plate Fixation of Humeral Shaft Fractures? A Biomechanical Analysis of Synthetic and Cadaveric Bone Robert V. O’Toole, MD,* Romney C. Andersen, MD,*† Oleg Vesnovsky, PhD,‡ Melvin Alexander, MS,§ L.D. Timmie Topoleski, PhD,‡ Jason W. Nascone, MD,* Marcus F. Sciadini, MD,* Clifford Turen, MD,* and W. Andrew Eglseder, Jr., MD* 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 (anteropos- terior, 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. Key Words: humeral shaft fractures, locking screws, plate fixation, axial loading, biomechanics (J Orthop Trauma 2008;22:709–715) INTRODUCTION When lower extremity weight bearing is restricted, fractures of the humeral shaft often are treated operatively to allow full upper extremity weight bearing on crutches. 1 Little biomechanical evidence is presented in the literature to guide clinicians who are plating humeral shaft fractures for weight bearing. To our knowledge, the few biomechanical analyses for humeral fixation have focused on various 2-plate constructs for humeral nonunions 2 or other comparisons of fixation devices for treating humeri in a non-weight-bearing clinical application. 3–7 Similarly, although locking plates have become in- creasingly popular for a myriad of fractures, 8–12 little biomechanical evidence has been presented to validate the use of locking screws in the humeral shaft. 7 Although studies have shown potential advantages of locking plates over other constructs, the focus has been on metaphyseal bone. 13–25 To our knowledge, no biomechanical studies have been reported Accepted for publication September 3, 2008. From the *Department of Orthopaedics, R Adams Cowley Shock Trauma Center, University of Maryland School of Medicine, Baltimore, MD; the †Walter Reed National Military Medical Center, Washington DC, Bethesda, MD; the ‡Mechanical Engineering Department, University of Maryland, Baltimore County, Baltimore, MD; and the §National Study Center for Trauma and Emergency Medical Systems, University of Maryland Medical Center, Baltimore, MD. Research support for this project, in the form of implants and materials, was received from Synthes Inc. Institutional support was received from Synthes Inc., Stryker, and Wyeth. Presented as a poster at the 2005 Annual Meeting of the Orthopaedic Trauma Association, October 20–22 in Ottawa, Ontario, Canada as a podium presentation at the 2005 Basic Science Focus Forum of the Orthopaedic Trauma Association, and as a podium presentation at the 2005 Annual Meeting of the Eastern Orthopaedic Association October 5–8 in Cambridge, Maryland. This work was awarded first place in Basic Science Research at the 2005 meeting of the Maryland Orthopaedic Association. The devices that are the subject of this article are approved by the Food and Drug Administration. Reprints: Robert V. O’Toole, MD, Department of Orthopaedics, R Adams Cowley Shock Trauma Center, University of Maryland School of Medicine, 22 South Greene Street, T3R62, Baltimore, MD 21201 (e-mail: rvo3@yahoo.com). Copyright Ó 2008 by Lippincott Williams & Wilkins J Orthop Trauma  Volume 22, Number 10, November/December 2008 709