Biomechanical Comparison of Fully Threaded Solid Cortical Versus Partially Threaded Cannulated Cancellous Screw Fixation for Lisfranc Injuries.

ABSTRACT

There currently exists an array of operative strategies to manage Lisfranc injuries. Modular fixation systems present surgeons with a choice between fully threaded solid cortical (FSC) and partially threaded cannulated cancellous (PCC) bone screws when using a transarticular screw approach. It is currently unknown how screw design influences fixation strength in Lisfranc reconstructions. The purpose of this study was to evaluate the biomechanical differences of FSC and PCC screws using a cadaveric model of a simulated Lisfranc injury and controlled benchtop experiments. Ten matched pairs of cadaveric feet received an acute Lisfranc injury and were repaired with FSC or PCC screws. Diastasis was measured between the medial and intermediate cuneiforms and the first and second metatarsals during simulations of partial weight bearing. Three-point bending and axial pull-out tests were performed to characterize screw mechanics that could not be measured within the cadaveric model. Screw design did not affect cuneiform or metatarsal diastasis. Neither screw loosening nor deformation was observed following cadaveric testing. Bending tests indicated FSC screws had higher ultimate strength, but there was no significant difference in yield load. Partially threaded cannulated cancellous bone screws exhibited superior axial pull-out strength. Fully threaded solid cortical and PCC screws provide equal amounts of fixation strength during partial weight bearing and similar resistance to deformation under bending loads. Partially threaded cannulated cancellous screws may simplify the operative procedure and minimize nonoptimal screw placement. If a clinician so desires, PCC screws may be used in lieu of FSC screws without sacrificing fixation strength. [Orthopedics. 2018; 41(2)e222-e227.].