The influence of screw length on predicted cut-out failures for proximal humeral fracture fixations predicted by finite element simulations.

Abstract

The aim of this study was to identify the effect of screw length on predictions of fixation failure in three-part proximal humeral fractures using a finite element-based osteosynthesis modelling toolkit. A mal-reduced unstable three-part AO/OTA 11-B3.2 fracture with medial comminution was simulated in forty-two digitally processed proximal humeri covering a spectrum of bone densities and fixed with the PHILOS plate using three distal and six proximal locking screws. Four test groups were generated based on the screw tip to joint surface distance (TJD), with all proximal screws being shortened from 4mm TJD to be 8, 12 or 16mm TJD. Average bone strains around the screw tips, correlating with biomechanical cyclic cut-out-type failure, were evaluated in three physiological loading protocols representing simple shoulder motions. Six further groups were tested, where five of the proximal screws were inserted to 4mm TJD and the sixth screw to 8mm TJD. Exponential increases in the predicted risk of fixation failure were seen with increased tip-to-joint distances (p < 0.001). When one of the proximal screws was placed 8mm from the joint, with the remaining five at 4mm distance, significant increases (p < 0.001) were registered in the strains around the screw tips in all except the two superior screws. This effect was maximal around the calcar screws (p < 0.001) and for lower density samples (p < 0.001). These results suggest that longer screws provide reduced risk of cut-out failure, i.e. distalisation and/or varisation of the head fragment, and thus may decrease failure rates in proximal humeral fractures treated with angular stable plates. These findings require clinical corroboration and further studies to investigate the risk of screw perforation.