Comparison of two tools for the measurement of interfragmentary movement in femoral neck fractures stabilised by cannulated screws

Abstract

Achieving stability at the site of femoral neck fracture is an important factor for callus formation in the post-operative period. However, measuring interfragmentary movement in vivo is not currently possible as telemetric screws have not been manufactured for surgical use. Understanding how the implantation of the screws can affect the stability of the fracture allows the surgeon to tailor the procedure to the patient and produce the best possible outcome. Two techniques have been developed that measure interfragmentary movement between fractured surfaces. The first was a FEA model of the proximal femur with screws represented by nodal links. Movement was quantified by the amount of relative motion occurring between paired nodes either side of the fracture. The second was a mechanical compression test of a composite femur that allowed the motion analysis of paired markers on the external surface of the femur. Movement was digitised with markers selected and displacements calculated by transforming the global coordinate system to a local system relative to the fracture plane. Validation of both techniques was achieved as inferior axial displacement of the femoral head was found to be similar to that recorded in the literature between 0.6 and 0.75 mm. The maximum interfragmentary movement recorded using the FEA model was 0.0446 and was 4.89 mm using the motion analysis technique. The finite element model and the motion capture analysis techniques were found to both valid and repeatable tools for the measurement of the performance of cannulated screws for the fixation of femoral neck fractures. The advantage of the finite element technique is the inclusion of realistic simulation of muscular and joint forces, which support the joint and result in the lower movements reported. However, the motion capture analysis technique utilises real screws and enables the examination of screw thread dependent factors such as screw tightening levels.