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Intramedullary Threaded Nail Versus Dorsal Plate and Screw Fixation of Comminuted Metacarpal Shaft Fractures: A Biomechanical Study
Andrew Allen, MD
1,2, Alex Jeffs, MD
1, Yukun Zhang, MS
3, Zohair Zaidi, MD
1, Nathaniel Adams, BA
4, Bradley J Lauck, BA
4, Matthew Fisher, PhD
3, J. Megan M. Patterson, MD
5; Reid W Draeger, M.D.
6(1)University of North Carolina, Chapel Hill, NC, (2)University of Alabama at Birmingham, Birmingham, AL, (3)North Carolina State University, Raleigh, NC, (4)UNC School of Medicine, Chapel Hill, NC, (5)Department of Orthopaedics, University of North Carolina, Chapel Hill, NC, (6)Department of Orthopaedics, The University of North Carolina School of Medicine, Chapel Hill, NC
IntroductionThe noncompressive design of metacarpal intramedullary threaded nail (IMTN) implants theoretically prevents unwanted shortening in length-unstable fracture patterns, though the stability of these implants in comminuted metacarpal fractures remains unknown. The purpose of this study was to compare the stability of IMTN to plate and screw fixation in comminuted metacarpal shaft fractures using a cadaveric model.
Materials & MethodsIndex, long, ring, and small finger metacarpals were harvested from matched pair cadaveric hands (n=18). In the midshaft of each metacarpal, a 1.75 mm transverse fracture gap was created to simulate a comminuted fracture. Specimens were divided into two groups for fixation with either a retrograde IMTN or dorsal plate and bicortical locking screws. Specimens underwent three-point bend testing with a materials testing system. Each specimen underwent cyclic loading at 70 N for 2000 cycles to stimulate composite grasp, followed by cyclic loading at 120 N for 2000 cycles to simulate tip pinch. Specimens were then loaded to failure. Outcomes included cycles to failure, bending stiffness, peak load to failure, and location of failure.
ResultsBending stiffness was significantly greater in the IMTN group compared to the plate group (136.71 ± 24.19 N/mm vs 39.89 ± 14.08 N/mm, p <0.05). Peak load to failure was greater in the plate group, but this difference was not statistically significant (365.33 ± 79.85 N vs 598.11 ± 372.76 N, p >0.05). During the cyclic loading phases, all specimens in the IMTN group completed the 70 N and 120 N loading phases without failure. In the plate group, 2 of 9 specimens progressed to catastrophic failure during the 120 N cyclic loading phase. When loaded to failure, specimens in the IMTN group failed by implant fracture (n=8) or fracture at the bone-implant interface (n=1). Plates all failed by the fracture gap bone ends touching followed by propagation of a fracture from the osteotomy site and proximal screw failure (n=6), propagation of a fracture from the osteotomy site and distal screw failure (n=2), and fracture at the distal plate-bone interface (n=1).
ConclusionsIMTN fixation had significantly greater bending stiffness and more consistent load to failure characteristics compared to dorsal plate and screw fixation. IMTN fixation provided sufficient stability in comminuted metacarpal shaft fractures to withstand cyclic forces consistent with early ROM. Plate fixation demonstrated more variable results, with 22% of specimens failing during the 120 N cyclic loading phase.
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