Performance of the resurfaced hip. Part 1: The influence of the prosthesis size and positioning on the remodelling and fracture of the femoral neck

Abstract

Abstract:Hip Resurfacing is an established treatment for osteoarthritis in young, active patients. Failure modes include femoral neck fracture and prosthesis loosening, which may be associated with medium term bone adaptation, including femoral neck narrowing and densification around the prosthesis stem.Finite Element modelling was used to indicate the effects of prosthesis sizing and positioning on bone remodelling and fracture strength under a range of normal and traumatic loads, aiming to understand these failure modes better.The simulations predicted increased superior femoral neck stress shielding in young patients with small prostheses, which required shortening of the femoral neck to give an acceptable implant-bone interface.However with a larger prosthesis, natural femoral head centre recreation in the implanted state was possible, so stress shielding was restricted to the prosthesis interior, and its extent was less sensitive to prosthesis orientation. With valgus orientation, the implanted neck strength was, at worst, within 3% of its intact strength.The study suggests that femoral neck narrowing may be linked to a reduction in horizontal femoral offset, occurring if the prosthesis is excessively undersized. As such, hip resurfacing should aim to reproduce the natural femoral head centre, and for valgus prosthesis orientation to avoid femoral neck fracture.

Implant–bone interface healing and adaptation in resurfacing hip replacement

“…This study also produced predictions of the full set of radiographic changes which are observed clinically around resurfacing implants, including femoral neck narrowing. In previous work (Dickinson et al 2010a), neck narrowing was predicted to be linked to a shortened horizontal femoral offset in resurfaced hips compared with healthy joints (Silva et al 2004), and the present study aimed to achieve such clinically representative prosthesis positioning and biomechanical measurements. The present study predicted a BMD reduction at the superior head -neck junction with an internal-remodelling algorithm, which would be indicative of additional external remodelling of the neck cortex, which is thin in this region.…”

“…An FE model of the geometry and materials properties of a proximal femur was generated as described in a previous article (Dickinson et al 2010a), using a CT scan from a 63-year-old male (height 1.77 m, mass 85 kg) with no known orthopaedic disease. The CT scan resolution was 0.781 mm, with 2-mm slice thickness.…”

“…Most commonly, these studies produced a static FE model of the intact and resurfaced joints and used the comparative mechanical strain or strain energy density (SED) in the two situations to predict the immediately post-operative remodelling stimulus (Watanabe et al 2000;Ong et al 2006Ong et al , 2009Taylor 2006;Taylor 2007a, 2007b;Dickinson et al 2010aDickinson et al , 2010b. These studies have presented predictions of stress shielded bone within the femoral head and the superior femoral neck, with bone densification around the prosthesis stem.…”

“…These studies have presented predictions of stress shielded bone within the femoral head and the superior femoral neck, with bone densification around the prosthesis stem. They allow the comparative performance of different surgical and implant design variables to be predicted, such as prosthesis position (Radcliffe and Taylor 2007a;Ong et al 2009;Dickinson et al 2010a) and sizing (Dickinson et al 2010a), fixation methods (Ong et al 2006;Radcliffe and Taylor 2007b), stem diameter (Taylor 2006), stem length (Dickinson et al 2010b) and the extent of stem -bone contact (Taylor 2006;Ong et al 2009). However, these results only represent the driving force for bone remodelling at one instant and do not predict the progression or end point of the nonlinear process.…”

Pre-clinical evaluation of ceramic femoral head resurfacing prostheses using computational models and mechanical testing

“…Second, the cement pockets within the head in which bone cement cures, aiding torsional stability, were replaced with less stress concentrating, smooth, circular section scallops. The long stem in traditional designs is intended to act primarily as a surgical alignment aid [2], although its load bearing is thought to contribute to femoral component loosening failures, possibly through proximal stress shielding [22][23][24]. Therefore, a second perceived biomechanical advantage to the short stem was to reduce the risk of prosthesis loosening compared to traditional designs as a result of proximal femoral head and neck strain shielding, resulting from undesirable stem load-bearing [25].…”