Role of Surgical Position on Interface Stress and Initial Bone Remodeling Stimulus around Hip Resurfacing Arthroplasty

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

“…Patterns of bone remodelling based on the immediate post-operative remodelling stimulus were predicted to be in agreement with previous modelling studies and clinical observations. This included bone resorption in the superior femoral head, observed in loosened implants [16][17][18][20][21][22][23][24][25][26][27][28][29], and densification to form sclerotic 'pedestal lines' around the prosthesis stem tip and along its superior edge [14,48]. These were predicted to occur for both prosthesis sizes and all implant orientations.…”

“…A threshold level of remodelling stimulus of ±75% change was used in accordance with previous studies [23,28]. This value was an empirical fit of FE model results to radiographic data from older, total hip arthroplasty patients [44] and resulted in relatively small volumes of remodelling bone, so a second threshold value of ±50% change was used, to represent the younger patient with a more active metabolismthe target patient for hip resurfacing.…”

“…While these studies permit a preliminary understanding of the biomechanical behaviour of the resurfaced hip, there are still a number of factors that have yet to be addressed in detail in the literature to date; for example, the effect of clinically relevant implant positioning on the strains in the femur has been the subject of a limited number of studies [25,28]. Of increasing significance and relevance to the younger patient population, conventional studies have focussed on gait loading patterns, while the active patient is likely to subject the implant to more traumatic loading regimes such as stumbling and sideways falls.…”

“…Early, simplified finite element analysis (FEA) models demonstrated a reduction in stress within the resurfaced femoral head [16][17][18] and biomechanical theory [19] and analysis [20] showed that valgus orientation reduced the risk of femoral neck fracture. Later, more detailed, patient specific models with geometry and materials properties based on Computer Tomography (CT) scans were used to obtain more precise, absolute bone strain predictions [21][22][23][24][25][26][27][28][29][30]. These studies have considered the effects of different prosthesis-bone interface conditions, fixation methods including cementless and cemented fixation with different cement mantle thicknesses, and variations in prosthesis positioning including varus-valgus orientation and incomplete prosthesis seating.…”

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

“…Also, the type of bone represented by the element was taken into account, with cancellous or cortical yield strain values used [28] depending upon the element's density. The extent of bone remodelling resulting from implantation was quantified using a strain energy density (SED) stimulus [29,30], whereby the percentage change in SED was calculated from the pre-to post-operative conditions and the volume of remodelling bone was found using a threshold level of stimulus required for bone resorption or densification to occur (±75% for the elderly patient [13,29,31] and ±50% for a younger patient with a more active metabolism).…”