Hip resurfacing increases bone strains associated with short‐term femoral neck fracture

Abstract

Short-term femoral neck fracture is a primary complication associated with contemporary hip resurfacing. Some fractures are associated with neck notching, while others occur in the absence of notching. These unexplained fractures may be due to large magnitude strains near the implant rim, which could cause bone damage accumulation and eventual neck fracture. We used statistically augmented finite element analysis to identify design and environmental variables that increase bone strains near the implant rim after resurfacing, and lead to strain magnitudes sufficient for rapid damage accumulation. After resurfacing, the compressive strains in the inferior, peripheral neck increased by approximately 25%, particularly when the implant shell was bonded. While the tensile strains in the peripheral neck were low in magnitude in the immediate postoperative models, they increased substantially following compressive damage accumulation. Low bone modulus, within the range of normal bone, and high head load contributed the most to large magnitude strains. Therefore, in some cases, hip resurfacing may cause a region of compressive bone damage to develop rapidly, which in turn leads to large tensile strains and potential neck fracture. Our study suggests that indications for surgery should account for bone material quality, and that rehabilitation protocols should avoid high-load activities. ß

Influence of femur size and morphology on load transfer in the resurfaced femoral head: A large scale, multi-subject finite element study

“…The main strain alterations observed were strain shielding in the femoral head, particularly in the superior zones, and increased strain in the femoral neck, again most notable superiorly. These patterns of load transfer change have been reported in previous FE and experimental work (Huiskes et al, 1985;Watanabe et al, 2000;Ong et al, 2006;Taylor, 2006;Cristofolini et al, 2009;Long et al, 2009).…”

“…Previous FE and experimental work have shown increased strain in the superior femoral neck although not over a volume sufficient to cause fracture but the addition of a notch could be enough to change this (Watanabe et al, 2000;Ong et al, 2006;Taylor, 2006;Cristofolini et al, 2009). Although Long et al (Long et al, 2009) argued that short term neck fracture is not always accompanied by evidence of notching, they suggested that bones with lower modulus and where higher loads were applied, the areas around the implant rim would experience high strain concentrations and may suffer damage accumulation, leading to fatigue failure before the bone adaption cycle can complete in response. The average strain changes predicted in the neck in this study are of a similar magnitude to earlier predictions, supporting this possibility.…”

“…Often damage done during surgery or poor surgical accuracy are blamed for weakening the femoral neck; cutting blood supply leading to necrosis, notching the neck producing a stress concentration, poorly seating an implant leaving uncovered cancellous bone or malalignment of the implant (varus) resulting in unfavourable loading (McMinn et al, 1996;Shimmin and Back, 2005;Morlock et al, 2006;Steffen et al, 2009). Clinically, neck fractures are often seen within the first few months of surgery (Long et al, 2009;Steffen et al, 2009). Previous FE and experimental work have shown increased strain in the superior femoral neck although not over a volume sufficient to cause fracture but the addition of a notch could be enough to change this (Watanabe et al, 2000;Ong et al, 2006;Taylor, 2006;Cristofolini et al, 2009).…”

Performance of the resurfaced hip. Part 2: The influence of prosthesis stem design on remodelling and fracture of the femoral neck

“…In other cohorts, femoral neck fracture has had an incidence of approximately 1-3% [19,35,36], and in several cases could be explained by surgical technique. Ritter et al [21] reported an 18% femoral neck fracture rate, but [7,13,17,22,31,[37][38][39][40][41][42][43][44][45][46]. The results of this study suggested that the femoral head resorption was caused largely by stress shielding from the thick metal shell of the prosthesis head, and was only slightly improved in the immediate postoperative situation by the stem design.…”

Influence of the change in stem length on the load transfer and bone remodelling for a cemented resurfaced femur

“…A number of studies using finite element (FE) analysis have reported the generation of elevated strains in the proximal femoral neck-component junction and strain shielding in the resurfaced femoral head, indicating the risk of neck fracture and adverse bone remodelling (Gupta et al, 2006;Long et al, 2009;Pal et al, 2009;Watanabe et al, 2000). The causes of failure seem to depend on several biomechanical factors related to the implant (design, positioning, orientation, fixation method) and the patient (bone quality, joint loading).…”