Intramedullary Threaded Nail versus Headless Compression Screws for Metacarpal Fractures: A Biomechanical Comparison
Nicole M Sgromolo, MD1; Casey Imbergamo, MD2; Sean Sequeira, MD2; Gabriel Yohe, MS2; Pooyan Abbasi, MS3; Aviram M Giladi, MD, MS3
1Walter Reed National Military Medical Center, Bethesda, MD; 2The Curtis National Hand Center, MedStar Union Memorial Hospital, Baltimore, MD; 3The Curtis National Hand Center, Baltimore, MD
Introduction
Intramedullary fixation for metacarpal shaft fractures is desirable for various reasons, including allowing for early range of motion (ROM). Intramedullary options include threaded nails (IMN) and headless compression screws (HCS). These implants depend on friction fit with the cortex for stability, and for early ROM the implant must withstand physiologic forces. There are no interlocking screws to maintain rotation, increasing potential for malrotation. The purpose of our study was to compare torsional stability between IMN and HCS in transverse metacarpal fractures and to determine the optimal canal fit needed to withstand the force needed for grasp (64N).
Materials & Methods We used the index through small finger metacarpals from 15 fresh cadaveric specimens and created a transverse metacarpal fracture at the isthmus of each. Intramedullary fixation of was performed using 4.0 HCS, 5.0 HCS, and 4.5 mm threaded IMN. Radiographs were taken and used to measure canal fit at the isthmus. Following fixation, metacarpals were mounted, and torque was applied at a rate of 2 degrees/second. Torque was applied to 15 degrees of malrotation. Using a linear regression, the impact of variables on torque were examined. We separated the specimens into 6 quantiles based on percent canal fill and performed a Moody’s non-parametric Median Test to analyze the relationship between canal fill and torsional stability.
Results Torsional stability didn’t significantly differ between nails and screws (p = 0.08). There was a positive moderate correlation between percent canal fill and torque (correlation coefficient 0.46, p < 0.001). When nails and screws were separately analyzed, this correlation was still present, but there was no significant difference between the two groups. As canal fill increased the percent of specimens able to withstand the 64N needed for grasp increased. The difference in medians for each grouping of torques was statistically significant (p = 0.017). (Table 1) Above 80% canal fill, force tolerance significantly increased (p < 0.001).
Conclusions Canal fit, not type of implant, is critical in improving torsional stability. As canal fill increases, the ability to withstand physiologic forces increases. One should aim to optimize canal fill to allow early range of motion by achieving fill of greater than 80%.
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