A numerical study of failure mechanisms in the cemented resurfaced femur: Effects of interface characteristics and bone remodelling

Abstract

Failure mechanisms of the resurfaced femoral head include femoral neck fracture in the short-term and stress shielding and implant loosening in the long-term. In this study, finite element simulations of the resurfaced femur considering a debonded implant-cement interface, variable stem-bone interface conditions, and bone remodelling were used to study load transfer within the resurfaced femur and to investigate its relationship with known failure mechanisms. Realistic three-dimensional finite element models of an intact and resurfaced femur were used. Various conditions at the interface between the stem of the prosthesis and the bone were considered. Loading conditions included normal walking and stair climbing. For all stem-bone contact conditions, the tensile stresses in the cement mantle varied between 1 MPa and 5.4 MPa, except near the distal rim of the resurfacing component where they reached 5.4-7MPa. In the case of full stem-bone contact, high von Mises stresses (114-121MPa) were generated in the implant at the stem-cup junction. These stresses were considerably reduced (maximum von Mises stress, 76 MPa) where a gap was present at the stem-bone interface. Resurfacing led to strain shielding of the bone of the femoral head (20-75 per cent strain reductions) and periprosthetic bone resorption (50-80 per cent bone density reductions) for all interface stem-bone contact conditions. In the lateral femoral head and the proximal femoral shaft around the trochantric region, bone density reductions varied between 10 per cent and 50 per cent. Bone apposition was observed in the inferior-medial part of the femoral head and proximal femoral neck region. For full stem-bone contact, more load was transferred through the stem to the surrounding bone, exacerbating strain shielding. Although femoral hip resurfacing conserves bone stock at the primary operation, strain shielding and periprosthetic bone resorption might lead to eventual loosening over time. Post-operatively, the resurfacing procedure generated elevated strains (0.50-0.75 per cent strain) in the proximal femoral neck-component junction irrespective of the variation in interface conditions, indicating an initial risk of femoral neck fracture. Subsequent to bone remodelling, this strain concentration was considerably reduced (0.35-0.50 per cent strain), lowering the risk of neck fracture. In order to reduce the potential risk of neck fracture, patients should avoid activities which might induce high loading of the hip during the early post-operative period to allow the bone around the proximal femoral neck to remodel and heal.

Are There Long-term Benefits to Cementing the Metaphyseal Stem in Hip Resurfacing?

“…However, there is a concern that this technique might have long-term detrimental effects related to stress shielding [24,29] or thermal necrosis associated with the increase in overall cement needed for component fixation [15,18,20]. Our study compared survivorship, hip scores, and other radiographic findings between hips resurfaced with a cemented metaphyseal stem and hips resurfaced with a press-fit metaphyseal stem.…”

“…Cementing this metaphyseal stem increases the area for fixation between the bone and the prosthesis, but this increases the total amount of bone cement needed for fixation, and several studies suggest that the heat generated from the cementing process could lead to thermal necrosis of the surrounding cancellous bone [15,18,20], which could in turn cause femoral neck fracture or femoral loosening. Additionally, cementing the stem may alter the load transfer between the femoral component and the femoral neck, leading to adverse modifications of the proximal femur [24,29].…”

Activity intensity, assistive devices and joint replacement influence predicted remodelling in the proximal femur

“…In addition to remodelling, the cementless implant scenario involves the healing of surgical damage to the bone and surrounding soft tissues, transient responses to primary implant fixation, potentially external remodelling, and often the progressive osseointegration of implants through bioactive surface coatings for secondary fixation. Osseointegration has been simulated alone and combined with remodelling (Tarala et al 2011), and the effects of progressive implant-cement and implant-bone interface fixation and failure have been considered in hip resurfacing (Pal et al 2009;Caouette et al 2013). A simplification of the present study is the neglect of these effects.…”

“…A mechanostat principle was applied, based upon the strain history stimulus required to produce a bone volume change (Carter 1984), and implemented using Euler forward integration to calculate iterative changes in the thickness of cortex elements (external remodelling), or the heterogeneous density-and hence the Young's modulus-of trabecular elements (internal remodelling). This method has been applied to femoral implants in THR (Kerner et al 1999;Turner et al 2005), RHR (Gupta et al 2006;Pal et al 2009;Rothstock et al 2011;Dickinson et al 2012;Perez et al 2014), to acetabular cups (Ghosh et al 2013), and in other joints (van Lenthe et al 1997). Advanced approaches have combined strain adaptive bone remodelling with other associated mechanobiological processes, including cementless implant ingrowth (Tarala et al 2011) and periprosthetic defect healing (Dickinson et al 2012).…”

Four decades of finite element analysis of orthopaedic devices: Where are we now and what are the opportunities?

“…A high proportion of all FE studies only examine representations of the early post-operative mechanical environment and these predictions also act as the basis for time based, adaptive simulations and errors made in the first iteration will propagate through these time based solutions. Some of the early FE studies of cemented hip stems (Huiskes, 1990;Crowninshield et al, 1980;Prendergast et al, 1989) assumed idealised cement mantle geometry, no interdigitation of the cement into cancellous bone and elastic properties for the cement and these assumptions are still routinely used today (Taddei et al, 2010;Galloway et al, 2013;Pal et al, 2009;Ramos et al, 2013). One of the main challenges is that the mechanical behaviour of the cement (Whitehouse and Evans, 2010) and of the stem-cement and cementbone interfaces are still poorly understood, largely due to a lack of experimental data.…”