Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head

Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density was compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.     

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

Femoral resurfacing has become an increasingly popular procedure, especially for young, active patients. The procedure is known to alter load transfer through the proximal femur and this has been linked with the most commonly observed complication, neck fracture. An intriguing observation noted by registry data and clinical studies is an inverse relationship between implant size and revision rate. While computational analysis has become an established part of biomedical engineering, the majority of work uses a single or small set of bone models, with a single implant size, due to the constraints of time and data availability. Therefore, it has been infeasible to run a study incorporating natural inter-patient variability or the performance of smaller implants could not be meaningfully studied. In previous work a statistical model of the whole femur was used to generate large numbers of unique, realistic, FE-ready femur models describing both geometry and material properties. The current study demonstrates a methodology for virtually implanting and performing stress analysis of cemented femoral resurfacing components, with model specific sizing and orientation. Automated analysis of 400 generated femurs, in both implanted and intact configurations showed the strain changes induced by resurfacing. This produced a statistically meaningful number of results and allowed the examination of outliers. Results showed increased femoral neck strain changes potentially increasing the risk of neck fracture, associated with smaller, less dense femurs and smaller implant sizes; agreeing with clinical observations. The study demonstrates a methodology for more comprehensive analyses, based on populations rather than individuals.     

Numerical investigations on the strain-adaptive bone remodelling in the periprosthetic femur: Influence of the boundary conditions

Background  

There are several numerical investigations on bone remodelling after total hip arthroplasty (THA) on the basis of the finite element analysis (FEA). For such computations certain boundary conditions have to be defined. The authors chose a maximum of three static load situations, usually taken from the gait cycle because this is the most frequent dynamic activity of a patient after THA.     

The effect of muscle loading on the simulation of bone remodelling in the proximal femur

A large number of finite element analyses of the proximal femur rely on a simplified set of muscle and joint contact loads to represent the boundary conditions of the model. In the context of bone remodelling analysis around hip implants, muscle loading affects directly the spatial distribution of the remodelling signal. In the present study we performed a sensitivity analysis on the effect of different muscle loading configurations on the outcome of the bone remodelling simulation. An anatomical model of the femur with the implanted stem in place was constructed using the CT data of the Visible Human Project dataset of the National Institute of Health. The model was loaded with three muscle force configurations with increasing level of complexity. A strain adaptive remodelling rule was employed to simulate the post-operative bone changes around the implant stem and the results of the simulation were assessed quantitatively in terms of the bone mineral content changes in 18 periprosthetic regions of interest. The results showed considerable differences in the amount of bone loss predicted between the three cases. The simplified models generally predicted more pronounced bone loss. Although the overall remodelling patterns observed were similar, the bone conserving effect of additional muscle forces in the vicinity of their areas of attachment was clear. The results of this study suggest that the loading configuration of the FE model does play an important role in the outcome of the remodelling simulation.     

A multiscale mechanobiological model of bone remodelling predicts site-specific bone loss in the femur during osteoporosis and mechanical disuse

We propose a multiscale mechanobiological model of bone remodelling to investigate the site-specific evolution of bone volume fraction across the midshaft of a femur. The model includes hormonal regulation and biochemical coupling of bone cell populations, the influence of the microstructure on bone turnover rate, and mechanical adaptation of the tissue. Both microscopic and tissue-scale stress/strain states of the tissue are calculated from macroscopic loads by a combination of beam theory and micromechanical homogenisation. This model is applied to simulate the spatio-temporal evolution of a human midshaft femur scan subjected to two deregulating circumstances: (i) osteoporosis and (ii) mechanical disuse. Both simulated deregulations led to endocortical bone loss, cortical wall thinning and expansion of the medullary cavity, in accordance with experimental findings. Our model suggests that these observations are attributable to a large extent to the influence of the microstructure on bone turnover rate. Mechanical adaptation is found to help preserve intracortical bone matrix near the periosteum. Moreover, it leads to non-uniform cortical wall thickness due to the asymmetry of macroscopic loads introduced by the bending moment. The effect of mechanical adaptation near the endosteum can be greatly affected by whether the mechanical stimulus includes stress concentration effects or not.     

Bone remodelling analysis of a bovine femur for a veterinary implant design

The response of bovine bone to the presence of an implant is analysed with the aim of simulating bone remodelling in a developing model of a polymeric intramedullary interlocking nail for veterinary use. A 3-D finite element model of the femur diaphysis is built based on computed tomography images and using a CAD-based modelling pipeline. The bone remodelling process after the surgery is analysed and compared with the healthy bone. The remodelling law assumes that bone adapts to the mechanical environment. For the analyses a consistent set of loads is determined for the bovine walk cycle. The remodelling results reproduce the morphologic features of bone and provide evidence of the difference on the bone behaviour when comparing metallic and polymeric nails. Our findings indicate that an intramedullary polymeric nail has the advantage over the metallic one of improving long-term bone healing and possibly avoiding the need of the implant removal.     

Influence of ingrowth regions on bone remodelling around a cementless hip resurfacing femoral implant

Hip resurfacing arthroplasty is an alternative to traditional hip replacement that can conserve proximal bone stock and has gained popularity but bone resorption may limit implant survival and remains a clinical concern. The goal of this study was to analyze bone remodelling patterns around an uncemented resurfacing implant and the influence of ingrowth regions on resorption. A computed tomography-derived finite element model of a proximal femur with a virtually implanted resurfacing component was simulated under peak walking loads. Bone ingrowth was simulated by six interface conditions: fully bonded; fully friction; bonded cap with friction stem; a small bonded region at the stem-cup intersection with the remaining surface friction; fully frictional, except for a bonded band along the distal end of the cap and superior half of the cap bonded with the rest frictional. Interface condition had a large influence on remodelling patterns. Bone resorption was minimized when no ingrowth occurred at the bone-implant interface. Bonding only the superior half of the cap increased bone resorption slightly but allowed for a large ingrowth region to improve secondary stability.     

Consideration of multiple load cases is critical in modelling orthotropic bone adaptation in the femur

Functional adaptation of the femur has been investigated in several studies by embedding bone remodelling algorithms in finite element (FE) models, with simplifications often made to the representation of bone’s material symmetry and mechanical environment. An orthotropic strain-driven adaptation algorithm is proposed in order to predict the femur’s volumetric material property distribution and directionality of its internal structures within a continuum. The algorithm was applied to a FE model of the femur, with muscles, ligaments and joints included explicitly. Multiple load cases representing distinct frames of two activities of daily living (walking and stair climbing) were considered. It is hypothesised that low shear moduli occur in areas of bone that are simply loaded and high shear moduli in areas subjected to complex loading conditions. In addition, it is investigated whether material properties of different femoral regions are stimulated by different activities. The loading and boundary conditions were considered to provide a physiological mechanical environment. The resulting volumetric material property distribution and directionalities agreed with ex vivo imaging data for the whole femur. Regions where non-orthogonal trabecular crossing has been documented coincided with higher values of predicted shear moduli. The topological influence of the different activities modelled was analysed. The influence of stair climbing on the properties of the femoral neck region is highlighted. It is recommended that multiple load cases should be considered when modelling bone adaptation. The orthotropic model of the complete femur is released with this study.     

Computational simulation of simultaneous cortical and trabecular bone change in human proximal femur during bone remodeling

In this study, we developed a numerical framework that computationally determines simultaneous and interactive structural changes of cortical and trabecular bone types during bone remodeling, and we investigated the structural correlation between the two bone types in human proximal femur. We implemented a surface remodeling technique that performs bone remodeling in the exterior layer of the cortical bone while keeping its interior area unchanged. A micro-finite element (μFE) model was constructed that represents the entire cortical bone and full trabecular architecture in human proximal femur. This study simulated and compared the bone adaptation processes of two different structures: (1) femoral bone that has normal cortical bone shape and (2) perturbed femoral bone that has an artificial bone lump in the inferomedial cortex. Using the proposed numerical method in conjunction with design space optimization, we successfully obtained numerical results that resemble actual human proximal femur. The results revealed that actual cortical bone, as well as the trabecular bone, in human proximal femur has structurally optimal shapes, and it was also shown that a bone abnormality that has little contribution to bone structural integrity tends to disappear. This study also quantitatively determined the structural contribution of each bone: when the trabecular adaptation was complete, the trabecular bone supported 54% of the total load in the human proximal femur while the cortical bone carried 46%.     

Bending vibrations of the femur and the oscillatory behavior of a cemented femoral hip endoprosthesis

The paper presents a novel method for recording amplitude and phase of 6D-vibrations of a spatial pendulum over a wide frequency range (10 Hz up to 20 kHz). The six degrees of freedom of the pendulum mass were monitored by three electrodynamic stereo pickups. At rest, the tips of the needles and the pendulum's center of mass defined the reference system with respect to which the oscillations of the mass were recorded in terms of their amplitudes and phases. Its small dimensions, constant transfer characteristics, linearity, high dynamics, and virtual lack of reaction onto the moving system over the entire frequency range provided the advantages of the measuring system. This method was used to analyze the spatial 6D-vibrations of the head of a cemented femoral hip endoprosthesis when the femur was stimulated to bending vibrations. The head of the prosthesis carried out axial rotational vibrations at every frequency used to stimulate the femur. The amplitudes of the axial rotations of the cortical bone were small in comparison to the ones of the prosthesis head, indicating that axial rotational vibrations following femur bending vibrations mainly stressed the spongiosa and the cement layer. This was observed over the entire frequency range, including at the low frequencies relevant for gait. Over the low-frequency range, as well as at some of the higher resonance frequencies, stationary instantaneous helical axes characterized the vibrations. The measurements suggest the mechanism that the interface "implant-bone" may already be stressed by axial torsional loads when the femur is loaded by bending impacts that are known to occur during walking.     

Quantitative analysis of change in active muscle length with varying external load

A steady rate of efferent stimulation was applied to anesthetized cats during acute experiments. Changes in the length of m.soleus, m. gastrocnemius, and m. plantaris of the hindlimb were studied during linear changes in the external load (ramp-and-hold alterations). A servocontrolled linear motor was used to provide mechanical stimulation. It was found that muscular contraction produced by load may be extremely accurately approximated by reaction of a dynanic system consisting of a sequence with nonlinear statistics and linear dynamics. Nonlinear statistics of muscle properties are determined according to the nonlinear statistical pattern of the power itself on the one hand and by hysteresis effects on the other. A hypothesis is presented regarding connections between the latter and function of the troponin-tropomyosin regulatory complex. A first order linear differential equation is used to describe the dynamic sequence corresponding to the familiar three component viscoelastic muscle model. The proposed model could be used to help analyze processes of muscle stretch in a yield situation, although satisfactory indication was only obtained for fairly slow load increments.A. A. Bogomolets Institute of Physiology, Academy of Scences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 5, pp. 694–702, September–October, 1988.     

Long-term study of bone remodelling after femoral stem: a comparison between dexa and finite element simulation

A hip replacement with a cemented or cementless femoral stem produces an effect on the bone called adaptive remodelling, attributable to mechanical and biological factors. The objective of all of cementless prostheses designs has been to achieve a perfect transfer of loads in order to avoid stress-shielding, which produces an osteopenia. In order to quantify this, the long term and mass-produced study with dual energy X-ray absorptiometry (DEXA) is necessary. Finite element (FE) simulation makes possible the explanation of the biomechanical changes which are produced in the femur after stem implantation. The good correlation obtained between the results of the FE simulation and the densitometric study allow, on one hand, to explain from the point of view of biomechanical performance the changes observed in bone density in the long-term, where it is clear that these are due to a different transfer of load in the implanted model compared to the healthy femur; on the other hand, it validates the simulation model, in a way that it can be used in different conditions and at different time periods, to carry out a sufficiently precise prediction of the evolution of the bone density from the biomechanical behaviour in the interaction between the prosthesis and femur.     

Improving the damage accumulation in a biomechanical bone remodelling model

We extend, reformulate and analyse a phenomenological model for bone remodelling. The original macrobiomechanical model (MBM), proposed by Hazelwood et al. [J Biomech 2001; 34:299–308], couples a population equation for the cellular activities of the basic multicellular units (BMUs) in the bone and a rate equation to account for microdamage and repair. We propose to account for bone failure under severe overstressing by incorporating a Paris-like power-law damage accumulation term. The extended model agrees with the Hazelwood et al. predictions when the bone is under-stressed, and allows for suitably loaded bones to fail, in agreement with other MBM and experimental data regarding damage by fatigue. We numerically solve the extended model using a convergent algorithm and show that for unchanging loads, the stationary solution captures fully the model behaviour. We compute and analyse the stationary solutions. Our analysis helps guide additional extensions to this and other BMU activity based models.     

The influence of proximal stem geometry and surface finish on the fixation of a double-tapered cemented femoral stem

In this study, the in vitro fixation of four otherwise identical double-tapered stem-types, varying only in surface finish (polished or matte) and proximal stem geometry (with or without flanges) were compared under two conditions. First, four specimens of each stem type were tested with initially bonded stem–cement interfaces, representing early post-operative conditions. Then, simulating conditions a few weeks to months later, stems were implanted in unused synthetic femurs, with a thin layer coating the stem to prevent stem–cement adhesion. Per-cycle motions were measured at both cement interfaces throughout loading. Overall, surface finish had the smallest relative effect on fixation compared to flanges. Flanges increased axial fixation by 22 μm per-cycle, regardless of surface finish (P=0.01). Further, all stems moved under dynamic load at the stem–cement interface during the first few cycles of loading, even without a thin film. The results indicate that flanges have a greater effect on fixation than surface finish, and therefore adverse findings about matte surfaces should not necessarily apply to all double-tapered stems. Specifically, dorsal flanges enhance the stability of a tapered cemented femoral stem, regardless of surface finish.     

Development of a testing methodology to quantify bone load transfer patterns for multiple stemmed implants in a single bone with an application in the distal ulna

Optimal parameters for many orthopaedic implants, such as stem length and material, are unknown. Geometry and mechanical properties of bone can vary greatly amongst cadaveric specimens, requiring a large number of specimens to test design variations. This study aimed to develop an experimental methodology to measure bone strains as a function of multiple implant stem designs in a single specimen, and evaluate its efficacy in the distal ulna. Eight fresh-frozen cadaveric ulnae were each instrumented with 12 uniaxial strain gauges on the medial and lateral surfaces of the bone. The proximal portion of each ulna was cemented in a custom-designed jig that allowed a medially directed force to be applied to the distal articular surface. An implant with a finely threaded stem was cemented into the canal by an experienced upper extremity orthopaedic surgeon. Six loads (5-30 N) were applied sequentially to the lateral surface of the prosthetic head using a materials testing machine. Testing was repeated after breaking the stem-cement bond, and after removing and reinserting the stem several times into the threaded cement mantle. Near the end of the testing period, the initial stem was reinserted and data were collected to determine if there was any change in bone properties or testing setup over time. Finally, a smooth stem was inserted for comparison to the threaded stem. Strain varied linearly with load (R(2)> or =0.99) for all testing scenarios. Bending strains were not affected by breaking the stem-cement bond (P=0.7), testing durations up to 18 h (P=0.7), nor the presence of threads when compared to a smooth stem (P>0.4). Furthermore, for all gauges, there was no interaction between the effect of the threads and level of applied load (P>0.1). This methodology should prove to be useful to compare stem designs of varying lengths and materials in the same bone, allowing for a direct comparison between implant designs for the ulna and other bones subjected primarily to bending loads. Furthermore, it will minimize the need for large numbers of specimens to test multiple implant designs. The ultimate goal of using this protocol is to optimize implant stem properties, such as length and material, with respect to load transfer.     

Bone remodelling of a proximal femur with the thrust plate prosthesis: an in vitro case

The key to the development of a successful implant is an understanding of the effect of bone remodelling on its long-term fixation. In this study, clinically observed patterns of bone remodelling have been compared with computer-based predictions for one particular design of prosthesis, the Thrust Plate Prosthesis (Centerpulse Orthopedics, Winterthur, Switzerland). Three-dimensional finite-element models were created using geometrical and bone density data obtained from CT scanning. Results from the bone remodelling simulation indicated that varying the relative rate of bone deposition/resorption and the interfacial conditions between the bone and the implant could produce the trend towards the two clinically observed patterns of remodelling.     

Influence of microcurrent on the modulation of remodelling genes in a wound healing assay

Molecular Biology Reports - The literature has shown the beneficial effects of microcurrent (MC) therapy on tissue repair. We investigated if the application of MC at...     

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

Bone morphology and density changes are commonly observed following joint replacement, may contribute to the risks of implant loosening and periprosthetic fracture and reduce the available bone stock for revision surgery. This study was presented in the ‘Bone and Cartilage Mechanobiology across the scales’ WCCM symposium to review the development of remodelling prediction methods and to demonstrate simulation of adaptive bone remodelling around hip replacement femoral components, incorporating intrinsic (prosthesis) and extrinsic (activity and loading) factors. An iterative bone remodelling process was applied to finite element models of a femur implanted with a cementless total hip replacement (THR) and a hip resurfacing implant. Previously developed for a cemented THR implant, this modified process enabled the influence of pre- to post-operative changes in patient activity and joint loading to be evaluated. A control algorithm used identical pre- and post-operative conditions, and the predicted extents and temporal trends of remodelling were measured by generating virtual X-rays and DXA scans. The modified process improved qualitative and quantitative remodelling predictions for both the cementless THR and resurfacing implants, but demonstrated the sensitivity to DXA scan region definition and appropriate implant–bone position and sizing. Predicted remodelling in the intact femur in response to changed activity and loading demonstrated that in this simplified model, although the influence of the extrinsic effects were important, the mechanics of implantation were dominant. This study supports the application of predictive bone remodelling as one element in the range of physical and computational studies, which should be conducted in the preclinical evaluation of new prostheses.