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A Biomechanical Analysis of Two Constructs for Metacarpal Fixation: Is Bigger Better or Is There Strength in Numbers?
Andre Cheah, MBBS, MBA1; Anthony Behn, MS2; Garet Comer, MD2; Jeffrey Yao, MD2
1National University Hospital, Singapore, Singapore; 2Stanford University, Redwood City, CA

Introduction
Fixation of diaphyseal metacarpal fractures with cortical screws is a technique employed by many surgeons due to the implants low profile, biomechanical stability, and no need for removal. With each fracture a unique problem to solve, surgeons must decide the optimal construct to allow for stability and early range of motion. Frequently surgeons are confronted with a long oblique fracture that may either accommodate several small screws or fewer larger screws. We propose a biomechanical analysis of a metacarpal fracture model to determine whether three 1.5 mm cortical screws or two 2.0 mm cortical screws prove a more stable construct.

Materials and Methods
9 matched pairs of fresh frozen cadaveric upper limbs were used for this study. The second and third metacarpal was harvested from each of the pairs and a spiral fracture in each of the metacarpals. Specimens were then be divided into two groups. One group consisted of 18 specimens that underwent reduction and fixation with two 2.0 mm cortical lagged screws; the second group consisted of 18 specimens that underwent reduction and fixation with three 1.5 mm cortical lagged screws. Both groups were further divided into subgroups A and B with subgroup A undergoing torsional testing and subgroup B bending testing. Finally, all specimens were subject to torsional and bending load to failure, respectively. Statistical analysis with the paired Student's t-test were used to compare the results of each subgroups tests with a level of significance set a p <0.05.

Results
In cyclic torsional testing, the '2 screws' group exhibited significantly less rotational creep that the '3 screws' group. No other significant differences were found between the test groups. In the bending tests , there were no significant differences in any outcome parameters between the two groups, although the '2 screws' group exhibited slightly higher stiffness and less cyclic creep.

Conclusions
Based on this study, both constructs are biomechanically similar. In a clinical setting, it may be preferable to use two 2.0 mm cortical lagged screws when fixing spiral metacarpal fractures as there was significantly less loosening during torsional loading as well as a trend towards less loosening during bending loading. In addition, using two screws may be advantages due to the cost of an additional screw in the '3 screws' construct without an obvious biomechanical advantage.


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