THEORETICAL AND CLINICAL IMPACTION LOADS DURING HIP RESURFACING SURGERY

Abstract

Introduction: The process of impacting cemented hip resurfacing components may, in part, be associated with femoral neck fracture. The impaction process may introduce fractures due to the impact shock wave passing through the bone during the setting of the implant and achieving a completely seated position. The aim of this study was to measure the impaction loads during hip resurfacing surgery and correlate the measured loads to theoretical calculations.

Methods: Following ethical approval 3 patients have been enrolled out of 24 patients in a pilot study. A surgical mallet was manufactured and instrumented with a calibrated impact load cell. During the impaction procedure the impact loads are recorded to a laptop using Labview software. An Excel spreadsheet has been written using the finite difference method to calculate the impact loads based on a mass (hammer, impactor and implant) and spring system (compression only) defining each part of the surgical instrumentation used to impact the resurfacing component onto the femoral head.

Results: Clinically, upto 19 impacts are used to seat the resurfacing implant onto the femoral head. Loads upto 24kN were recorded. The finite difference model was calibrated to the clinical measurements. The Pearson’s R correlation coefficient for the net force on the mallet was 0.91 and for the impulse was 0.98

Discussion: This study has investigated the clinical impaction loads imparted onto an implant during resurfacing surgery and developed a finite difference model of the process. The finite difference approach can be used to better understand the loads applied to not only the implant, but the underlying bone. This may, in part, give the surgeon a better understanding as to whether the bone has been predisposed to fracture following the high impact loads and thereby affecting the long-term integrity of the joint replacement.