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Plating of Metacarpal Fractures with Locked or Nonlocked Screws: A Biomechanical Evaluation of the Number of Cortices Necessary for Stable Fixation
Jeffrey Yao, MD; Cameron Barr, MD; Anthony Behn
Stanford University Medical Center, Palo Alto, CA
Introduction: With the advent of locking technology, fewer screws and shorter plates may provide acceptable clinical and biomechanical results while minimizing soft tissue stripping and preserving blood supply to the metacarpal when plating fractures. This study aims to help elucidate the optimal number of cortices per segment necessary for stable metacarpal fracture fixation using locking or nonlocking plate-screw constructs.
Materials and Methods: With biomechanical testing-grade composite Sawbones, a comminuted metacarpal fracture model was used to test two fixation constructs consisting of a standard dorsal seven-hole 2.0 mm LCP stainless steel straight plate and either six bicortical nonlocking screws (three screws per segment) or four bicortical locking screws (two screws per segment). Twenty-six specimens were tested to failure in cantilever bending and 30 specimens were tested to failure in torsion. Means and standard deviations for bending stiffness, maximum bending load, torsional stiffness, and maximum torque were calculated for the groups. Comparisons between the two constructs were carried out with a two-tailed t-test with significance set at alpha = 0.05. If no statistical difference was found, we tested the hypothesis that the groups were equivalent using a 10% difference as the threshold for equivalence. The equivalence hypotheses were evaluated with a two one-sided test (TOST) procedure and by constructing confidence intervals for the differences between the means.
Results: There was no statistical difference (p>0.05) between the two constructs in any of the mechanical testing. Equivalence testing was carried forward for each comparison. There was equivalence between the locking and nonlocking constructs in cantilever bending stiffness (locking 5.5±0.7 N/mm; nonlocking 5.6±0.5 N/mm; p=0.01), torsional stiffness (locking 69.6±9.3 Nmm/deg; nonlocking 68.6±7.9 Nmm/deg; p=0.04), and maximum torque (locking 1858±161 Nmm; nonlocking 1862±196 Nmm; p=0.004). Equivalence was not reached in maximum bending load (locking 184±29 N; nonlocking 185±24 N; p=0.06).
Conclusions: The tested metacarpal fracture model had equivalent bending stiffness, torsional stiffness, and maximum torque when fixed with a standard dorsal plate and either six bicortical nonlocking screws or four bicortical locking screws. By utilizing fewer cortices of fixation there is the opportunity for smaller dissections and less soft tissue stripping during open reduction and internal fixation. This may also be of benefit to very proximal or distal fractures as multiple cortices of fixation are often difficult to obtain during stabilization. Further biomechanical and clinical studies are needed to identify the optimal construct.
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