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Determining the Optimal Intramedullary Screw Canal Fill Ratio in Length Unstable Metacarpal Fractures: A Biomechanical Investigation.
Spencer B Chambers, MD
1; Daniel Thompson, MS
2; Christine V Schaeffer, MD
3; Alexander R Graf, MD
4; Eric R. Wagner, MD
5; Michael B. Gottschalk, MD
5; Nina Suh, MD
51Western University, Roth | McFarlane Hand and Upper Limb Centre, London, ON, Canada; 2Emory Univeristy, Atlanta, GA; 3Emory University, Atlanta, GA; 4Medical College of Wisconsin, Milwaukee, WI; 5Emory University School of Medicine, Atlanta, GA
IntroductionIntramedullary (IM) screw fixation is gaining clinical popularity in the treatment of metacarpal fractures. Despite its rapid adoption, there is paucity of evidence regarding parameters to optimize effectiveness. This study aimed to quantify the relationship between stability, IM screw size, and canal fill using a cadaveric model.
Materials & Methods Thirty cadaveric metacarpals (14 index, 13 long, 3 ring) [Mean age :58.3 years (range: 48-70)] were selected to allow for canal fill ratios of 0.7-1.1 for screws sized 3.0-, 3.5-, and 4.5mm. Metacarpals underwent a 45° volar-dorsal osteotomy at the midpoint before fixation with an IM screw. Specimens were subjected to 100 cycles of loading at 10N, 20N, and 30N before load-to-failure testing. Correlation coefficients for angular displacement on the final cycle at each load, peak load-to-failure, and average stiffness were assessed.
ResultsCorrelation coefficients for the angular displacement on the 100
th cycle was as follows: 10N - R=0.62, 20N - R=0.57, and 30N - R=0.58. Correlation values for peak load-to-failure as a function of canal fit was as follows: 3.0mm - R=0.5, 3.5mm - R=0.17, and 4.5mm - R=0.44. The canal fill ratio that intersected of the line of best fit at an angular deformity of 10° was 0.74. Average peak forces for 3.0-, 3.5-, and 4.5-mm screws were 79.5N, 136.5N and 179.6N respectively. Average stiffness for each calibre was 14.8 N/mm, 33.4 N/mm, and 52.3 N/mm.
ConclusionsIncreasing screw diameter and intramedullary fill resulted in more stable fixation, but marginal gains were seen in ratios >0.9. A minimum fill ratio of 0.74 was sufficient to withstand forces of early active motion with angular deformity <10°. An understanding of the biomechanical relationship of intramedullary fill ratio of metacarpal screws on fracture stability provides a framework for clinicians to optimally size these implants.
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