3D Computer Modeling Analysis of the Surface Area and Thread Volume of the Common Sliding Hip Screw

Abstract

Introduction: The Sliding Hip Screw (SHS) is a tried and tested method used to treat intertrochanteric hip fractures. Lag screw cut-out is a major complication of SHS fixation and accepted risk factors are often re-evaluated. We aim to demonstrate a new hypothesis of cut-out using 3D reconstructive modelling. We propose that the risk of cut-out is much greater in lag screws with a smaller thread surface area and larger thread volume.

Method: The four commonly used SHS lag screw systems (Biomet, Synthes, Stryker and Smith & Nephew) were CT scanned and the images processed and turned into 3D computer models for further analysis.

Results: There were significan differences between the surface area and thread volumes of the lag screws between manufacturers ranging from 67.0mm2 - 347.4 mm2 and 166.376mm3 - 225.687mm3 respectively.

Discussion: Assuming the consistency of bone is equal, we propose that the risk of cut-out is much greater in lag screws with a smaller surface area to thread volume ratio (SA:TV). The reamer design of lag screws is also discussed.

Conclusion: Although there are many non-modifiable risk factors that contribute to screw cut-out, selecting the right raw materials for SHS fixation will have a favorable impact on the overall risk.