Virtual trial to evaluate the robustness of cementless femoral stems to patient and surgical variation

Primary stability is essential for the success of cementless femoral stems. In this study, patient specific finite element (FE) models were used to assess changes in primary stability due to variability in patient anatomy, bone properties and stem alignment for two commonly used cementless femoral stems, Corail® and Summit® (DePuy Synthes, Warsaw, USA). Computed-tomography images of the femur were obtained for 8 males and 8 females. An automated algorithm was used to determine the stem position and size which minimized the endo-cortical space, and then span the plausible surgical envelope of implant positions constrained by the endo-cortical boundary. A total of 1952 models were generated and ran, each with a unique alignment scenario. Peak hip contact and muscle forces for stair climbing were scaled to the donor’s body weight and applied to the model. The primary stability was assessed by comparing the implant micromotion and peri-prosthetic strains to thresholds (150 μm and 7000 µε, respectively) above which fibrous tissue differentiation and bone damage are expected to prevail. Despite the wide range of implant positions included, FE prediction were mostly below the thresholds (medians: Corail®: 20–74 µm and 1150–2884 µε, Summit®: 25–111 µm and 860–3010 µε), but sensitivity of micromotion and interfacial strains varied across femora, with the majority being sensitive (p < 0.0029) to average bone mineral density, cranio-caudal angle, post-implantation anteversion angle and lateral offset of the femur. The results confirm the relationship between implant position and primary stability was highly dependent on the patient and the stem design used.     

Analysis of bone–prosthesis interface micromotion for cementless tibial prosthesis fixation and the influence of loading conditions

A lack of initial stability of the fixation is associated with aseptic loosening of the tibial components of cementless knee prostheses. With sufficient stability after surgery, minimal relative motion between the prosthesis and bone interfaces allows osseointegation to occur thereby providing a strong prosthesis-to-bone biological attachment. Finite element modelling was used to investigate the bone–prosthesis interface micromotion and the relative risk of aseptic loosening. It was anticipated that by prescribing different joint loads representing gait and other activities, and the consideration of varying tibial–femoral contact points during knee flexion, it would influence the computational prediction of the interface micromotion. In this study, three-dimensional finite element models were set up with applied loads representing walking and stair climbing, and the relative micromotions were predicted. These results were correlated to in-vitro measurements and to the results of prior retrieval studies. Two load conditions, (i) a generic vertical joint load of 3×body weight with 70%/30% M/L load share and antero-posterior/medial-lateral shear forces, acted at the centres of the medial and lateral compartments of the tibial tray, and (ii) a peak vertical joint load at 25% of the stair climbing cycle with corresponding antero-posterior shear force applied at the tibial–femoral contact points of the specific knee flexion angle, were found to generate interface micromotion responses which corresponded to in-vivo observations. The study also found that different loads altered the interface micromotion predicted, so caution is needed when comparing the fixation performance of various reported cementless tibial prosthetic designs if each design was evaluated with a different loading condition.     

Primary stability of cementless stem in THA improved with reduced interfacial gaps

Large interfacial gaps between the stem and the bone in cementless total hip arthroplasty may prevent successful bone ingrowth at the sites, and can also be a passage for wear particles. Furthermore, interfacial gaps between the stem and the bone are believed to compromise the primary stability of the implant. Thus, a broaching method that serves to reduce gaps is expected to give clinically preferable results. A modified broach system with a canal guide is introduced to enhance the accuracy of femoral canal shaping in comparison with the conventional broach system for a Versys fibermetal taper stem. The primary stability of the hip systems and the ratios of the stem surface in contact with the femur were measured in a composite femur model. With the conventional method, an average of 67% of the stem surface was shown to be in contact with the bone, and an average stem micromotion/migration of 35 microm 290 microm was observed under 1000 cycles of stair climbing loads. With the modified method, the stem-bone contact ratio significantly increased to 82% (p<0.05), and the average micromotion/migration reduced to 29 microm 49 microm, respectively (p<0.05 for migration). Our finite element models of the hip systems supported that the difference in micromotion could be attributed to the difference in interfacial contact. Interfacial gaps occurring with the conventional broach system were effectively reduced by the proposed method, resulting in improved primary stability.     

Primary stability of uncemented femoral resurfacing implants for varying interface parameters and material formulations during walking and stair climbing

Primary stability of uncemented resurfacing prosthesis is provided by an interference fit between the undersized implant and the reamed bone. Dependent on the magnitude of interference, the implantation process causes high shear forces and large strains which can exceed the elastic limit of cancellous bone. Plastification of the bone causes reduced stiffness and could lead to bone damage and implant loosening. The purpose in this study was to determine press-fit conditions which allow implantation without excessive plastic bone deformation and sufficient primary stability to achieve bone ingrowth. In particular, the influence of interference, bone quality and friction on the micromotion during walking and stair-climbing was investigated. Therefore elastic and plastic finite element (FE) models of the proximal femur were developed. Implantation was realized by displacing the prosthesis onto the femur while monitoring the contact pressure, plastic bone deformation as well as implantation forces. Subsequently a physiologic gait and stair-climbing cycle was simulated calculating the micromotion at the bone-implant interface. Results indicate that plastic deformation starts at an interference of 30 μm and the amount of plastified bone at the interface increases up to 90% at 150 μm interference. This effect did not reduce the contact pressure if interference was below 80 μm. The micromotion during walking was similar for the elastic and plastic FE models. A stable situation allowing bony ingrowth was achieved for both constitutive laws (elastic, plastic) for walking and stair climbing with at least 60 μm press-fit, which is feasible with clinically used implantation forces of 4 kN.     

Primary stability of an anatomical cementless hip stem: a statistical analysis

The primary stability that the surgeon can achieve during surgery is a determinant of the clinical success of cementless implants. Thus, estimating what level of primary stability can be obtained with a new design is an important aspect of pre-clinical evaluation. The primary stability of a cementless hip stem is not only affected by the implant design, but also by other factors such as the mechanical quality of the host bone, the presence of gaps around the bone-implant interface, the body weight of the patient, and the size of the implant. Even the most extensive experimental study can only explore a small sub-set of all possible combinations found in vivo. To overcome this limitation, we propose a combination of experimental and numerical methods. The primary stability of a cementless anatomical stem is assessed in vitro. A finite element model is developed to accurately replicate the same experiment. The model is then parameterised over the various factors that affect the primary stability, and used in a Monte Carlo scheme to assess the primary stability over a simulated population. In this study, the method was used to investigate the mechanical stability of an anatomical cementless stem over more than 1000 simulated cases. Twenty cases were found macroscopically unstable, due to a combination of unfavourable conditions. The rest of the Monte Carlo sample showed on average a peak micromotion under stair climbing loading of 206 +/- 159 microm. The proposed method can be used to evaluate new designs in conditions more representative of the variability in clinical practice.     

The primary stability of a cementless stem varies between subjects as much as between activities

The rehabilitation program adopted immediately after a cementless total hip replacement is a very important factor, because of the known relationship between osseointegration and implant micromotion. The present study was aimed to evaluate which type of task is the most critical in terms of bone-implant relative micromotion. Both inter-task and inter-subject variability were taken into account to verify if the movement strategy could be determinant on this assessment. A previously validated finite element model was used to predict the peak total micromovements over the entire bone-implant contact surface in four different patients, performing nine different tasks, using published data on joint forces recorded by instrumented hip prostheses. The results predicted by the various simulations suggest that while stair climbing is surely a critical task for primary stability, for some subjects other tasks may be as critical as stair climbing. From a variance analysis for simple crossover design on the predicted peak micromotion, the inter-subject variability had much more influence on the primary stability of cementless implant than the inter-task variability. Even if the results of Patient IBL, who was reported to have difficulties to perform any activities in a normal way, were excluded from the statistical analysis, the inter-subject variability remained still higher than the inter-task variability. The results obtained from simulations suggest that the strategy the hip replacement patient adopts to perform a given motor task, may be, for the implant stability, equally or even more critical than the type of motor task performed.     

Micromotion-induced peri-prosthetic fluid flow around a cementless femoral stem

Micromotion-induced interstitial fluid flow at the bone-implant interface has been proposed to play an important role in aseptic loosening of cementless implants. High fluid velocities are thought to promote aseptic loosening through activation of osteoclasts, shear stress induced control of mesenchymal stem cells differentiation, or transport of molecules. In this study, our objectives were to characterize and quantify micromotion-induced fluid flow around a cementless femoral stem using finite element modeling. With a 2D model of the bone-implant interface and full-factorial design, we first evaluated the relative influence of material properties, and bone-implant micromotion and gap on fluid velocity. Transverse sections around a femoral stem were built from computed tomography images, while boundary conditions were obtained from experimental measurements on the same femur. In a second step, a 3D model was built from the same data-set to estimate the shear stress experienced by cells hosted in the peri-implant tissues. The full-factorial design analysis showed that local micromotion had the most influence on peak fluid velocity at the interface. Remarkable variations in fluid velocity were observed in the macrostructures at the surface of the implant in the 2D transverse sections of the stem. The 3D model predicted peak fluid velocities extending up to 2.2 mm/s in the granulation tissue and to 3.9 mm/s in the trabecular bone. Peak shear stresses on the cells hosted in these tissues ranged from 0.1 to 12.5 Pa. These results offer insight into mechanical stimuli encountered at the bone-implant interface.     

Loosening of cementless femoral stems: a biomechanical analysis of immediate fixation with loading vertical, femur horizontal

Large joint implants must have immediate fixation to be successful. Unfortunately, the magnitude and consistency of achieving this remains largely unknown. For cementless femoral components it is being increasingly appreciated that torsional loading as occurs during stair climbing or rising from a chair leads to loosening and thigh pain in some cases. A biomechanical test was developed to evaluate fixation in this position. Twelve pairs of human cadaveric femora were press-fit with an AML stem. Each femur was secured in a horizontal position, and the prosthetic head cyclically loaded in a vertically downward direction. The offset of the prosthetic head resulted in a combined torsional and compressive load being applied to the stem within the proximal femur. Loosening was found to consistently occur and rapidly accelerate when the head subsided more than 0.2 mm during 100 cycles. For the AML stem, loosening developed at loads from 62 to 171% of body weight and after as few as 800 cycles. This is within the physiologic range of normal daily activities as measured by others with instrumented prostheses. This poses a challenge to the ability of press-fit stems to tolerate torsional loads in vivo. Patients with a cementless prosthesis should be protected from torsional loading until porous ingrowth and/or bone remodelling have had time to occur. Testing the same stem in paired femora demonstrated no right vs left difference (p greater than 0.6).     

Finite element modelling of primary hip stem stability: the effect of interference fit

The most commonly reported complications related to cementless hip stems are loosening and thigh pain; both of these have been attributed to high levels of relative micromotion at the bone-implant interface due to insufficient primary fixation. Primary fixation is believed by many to rely on achieving a sufficient interference fit between the implant and the bone. However, attempting to achieve a high interference fit not infrequently leads to femoral canal fracture either intra-operatively or soon after. The appropriate range of diametrical interference fit that ensures primary stability without risking femoral fracture is not well understood. In this study, a finite element model was constructed to predict micromotion and, therefore, instability of femoral stems. The model was correlated with an in vitro micromotion experiment carried out on four cadaver femurs. It was confirmed that interference fit has a very significant effect on micromotion and ignoring this parameter in an analysis of primary stability is likely to underestimate the stability of the stem. Furthermore, it was predicted that the optimal level of interference fit is around 50 microm as this is sufficient to achieve good primary fixation while having a safety factor of 2 against femoral canal fracture. This result is of clinical relevance as it indicates a recommendation for the surgeon to err on the side of a low interference fit rather than risking femoral fracture.     

A new technique to measure micromotion distribution around a cementless femoral stem

The interfacial micromotion is closely associated to the long-term success of cementless hip prostheses. Various techniques have been proposed to measure them, but only a few number of points over the stem surface can be measured simultaneously. In this paper, we propose a new technique based on micro-Computer Tomography (μCT) to measure locally the relative interfacial micromotions between the metallic stem and the surrounding femoral bone. Tantalum beads were stuck at the stem surface and spread at the endosteal surface. Relative micromotions between the stem and the endosteal bone surfaces were measured at different loading amplitudes. The estimated error was 10 μm and the maximal micromotion was 60 μm, in the loading direction, at 1400 N. This pilot study provided a local measurement of the micromotions in the 3 direction and at 8 locations on the stem surface simultaneously. This technique could be easily extended to higher loads and a much larger number of points, covering the entire stem surface and providing a quasi-continuous distribution of the 3D interfacial micromotions around the stem. The new measurement method would be very useful to compare the induced micromotions of different stem designs and to optimize the primary stability of cementless total hip arthroplasty.     

Simultaneous and multisite measure of micromotion, subsidence and gap to evaluate femoral stem stability

The initial stability of cementless femoral components is crucial for the long-term success of total hip arthroplasty. This has been reported in animal and clinical studies. Until now, the stability was evaluated by the measurement of relative micromotion on a few simultaneous locations around the stem in cadaveric experiments. This paper presents an extended experimental setup to measure simultaneously local micromotion, subsidence and gap on hundreds of points at the bone-stem interface. This technique we applied to anatomical and straight stems in three pairs of cadaveric femurs. Measurements were in agreement with typically reported values. Conversely to other methods, which measure micromotion between implant and bone anchoring points of the measuring device, our method provides local micromotion between stem surface and adjacent bone surface. The observed variation of micromotion at the peri-implant surface confirms the importance of this simultaneous measure on a lot of points around the implant.     

On the optimal shape of hip implants

The success of a total hip arthroplasty is strongly related to the initial stability of the femoral component and to the stress shielding effect. In fact, for cementless stems, initial stability is essential to promote bone ingrowth into the stem coating. An inefficient primary stability is also a cause of thigh pain. In addition, the bone adaptation after the surgery can lead to an excessive bone loss and, consequently, can compromise the success of the implant. These factors depend on prosthesis design, namely on material, interface conditions and shape. Although, surgeons use stems with very different geometries, new computational tools using structural optimization methods have been used to achieve a better design in order to improve initial stability and therefore, the implant durability. In this work, a multi-criteria shape optimization process is developed to study the relationship between implants performance and geometry. The multi-criteria objective function takes into account the initial stability of the femoral stem and the effect of stress shielding on bone adaptation after the surgery. Then, the optimized stems are tested using a concurrent model for bone remodeling and osseointegration to evaluate long-term performance. Additionally, the sensitivity to misalignments is analyzed, since femoral stems are often placed in varus or valgus position. Results show that the different criteria are contradictory resulting in different characteristics for the hip stem. However, the multi-criteria formulation leads to compromise solutions, with a combination of the geometric characteristics obtained for each criterion separately.     

Validation of model-predicted tibial tray-synthetic bone relative motion in cementless total knee replacement during activities of daily living

As fixation of cementless total knee replacement components during the first 4–6 weeks after surgery is crucial to establish bony ingrowth into the porous surface, several studies have quantified implant-bone micromotion. Relative motion between the tray and bone can be measured in vitro, but the full micromotion contour map cannot typically be accessed experimentally. Finite element models have been employed to estimate the full micromotion map, but have not been directly validated over a range of loading conditions. The goal of this study was to develop and validate computational models for the prediction of tray-bone micromotion under simulated activities of daily living. Gait, stair descent and deep knee bend were experimentally evaluated on four samples of a cementless tibial tray implanted into proximal tibial Sawbones™ constructs. Measurements of the relative motion between the tray and the anterior cortical shell were collected with digital image correlation and used to validate a finite element model that replicated the experiment. Additionally, a probabilistic analysis was performed to account for experimental uncertainty and determine model sensitivity to alignment and frictional parameters. The finite element models were able to distinguish between activities and capture the experimental trends. Best-matching simulations from the probabilistic analysis matched measured displacement with an average root mean square (RMS) difference of 14.3 µm and Pearson-product correlation of 0.93, while the mean model presented an average RMS difference of 27.1 µm and a correlation of 0.8. Maximum deviations from average experimental measurements were 40.5 and 87.1 µm for the best-matching and average simulations, respectively. The computational pipeline developed in this study can facilitate and enhance pre-clinical assessment of novel implant components.     

Large-sliding contact elements accurately predict levels of bone-implant micromotion relevant to osseointegration

Primary stability is recognised as an important determinant in the aseptic loosening failure process of cementless implants. An accurate evaluation of the bone–implant relative micromotion is becoming important both in pre-clinical and clinical studies. If the biological threshold for micro-movements is in the range 100–200 μm then, in order to be discriminative, any method used to evaluate the primary stability should have an accuracy of 10–20 μm or better. Additionally, such method should also be able to report the relative micromotion at each point of the interface. None of the available experimental methods satisfies both requirements. Aim of the present study is to verify if any of the current finite element modelling techniques is sufficiently accurate in predicting the primary stability of a cementless prosthesis to be used to decide whether the micromotion may or may not jeopardise the implant osseointegration. The primary stability of an anatomic cementless stem, as measured in vitro, was used as a benchmark problem to comparatively evaluate different contact modelling techniques. Frictionless contact, frictional contact and press-fitted frictional contact conditions were modelled using alternatively node-to-node, node-to-face and face-to-face contact elements. The model based on face-to-face contact elements accounting for frictional contact and initial press-fit was able to predict the micromotion measured experimentally with an average (RMS) error of 10 μm and a peak error of 14 μm. All the other models presented errors higher than 20 μm assumed in the present study as an accuracy threshold.     

Stair climbing is more critical than walking in pre-clinical assessment of primary stability in cementless THA in vitro

Pre-clinical testing of hip endoprostheses is a mandatory requirement before clinical release. Inadequate loading conditions may lead to lower elastic and plastic interface movements than those occurring post-operatively in vivo. This study investigated the influence of patient activity on the primary stability of cementless prostheses with a special emphasis on active simulation of muscle forces. A loading setup, based on validated musculo-skeletal analyses, was used to generate the hip contact force during walking and stair climbing by transmitting muscle forces through the femur. In addition, a loading configuration which only generated the hip contact force occurring during stair climbing at the prosthesis head was simulated. CLS prostheses were implanted in 18 composite femora and subjected to cyclical loading. The relative micro-movements at the bone-prosthesis interface were determined and appeared to be extremely sensitive to the specific patient activity. Compared to walking, stair climbing generated higher micro-movements, with pronounced axial and rotational components. Stair climbing with the femur loaded by the resultant hip contact force only exhibited a characteristic valgus tilt of the stem with significantly lower interface micro-movements than under active simulation of muscle forces. The analyses suggest that stair climbing induced the highest mechanical instability at the bone-prosthesis interface, a level which may compromise the necessary osseointegration process. Active simulation of muscle forces considerably affects the primary stability of cementless hip endoprostheses. Pre-clinical in vitro tests should therefore simulate stair climbing and include muscle activity in the assessment of initial implant stability, otherwise micro-movements may be underestimated and the primary stability overestimated.     

A contact model with ingrowth control for bone remodelling around cementless stems.

This work presents a computational model for bone remodelling around cementless stems. The problem is formulated as a material optimisation problem considering the bone and stem surfaces to be in contact. To emphasise the behaviour of the bone/stem interface, the computer model detects the existence of bone ingrowth during the remodelling; consequently, the contact conditions are changed for a better interface simulation. The trabecular bone is modelled as a strictly orthotropic material with equivalent properties computed by homogenisation. The distribution of bone relative density is obtained by the minimisation of a function that considers both the bone structural stiffness and the biological cost associated with metabolic maintenance of bone tissue. The situation of multiple load conditions is considered. The remodelling law, obtained from the necessary conditions for an optimum, is derived analytically from the optimisation problem and solved numerically using a suitable finite element mesh. The formulation is applied to an implanted femur. Results of bone density and ingrowth distribution are obtained for different coating conditions. Bone ingrowth does not occur over the entire coated surfaces. Indeed, we observed regions where separation or high relative displacement occurs that preclude bone ingrowth attachment. This prediction of the model is consistent with clinical observations of bone ingrowth. Thus, this model, which detect bone ingrowth and allow modification of the interface conditions, are useful for analysis of existing stems as well as design optimisation of coating extent and location on such stems.     

Primary rotational stability of cylindrical and conical revision hip stems as a function of femoral bone defects: an in vitro comparison

Bone stock losses in cementless femoral stem revisions compromise a stable fixation. The surgeon has to rely on his wealth of experience in deciding which stem shape to use. The aim of our study was to compare the primary rotational stability of cylindrical and conical revision hip stems subjected to femoral defects. Four current prostheses (two cylindrical, two conical) were implanted into four synthetic femora. Micro-motion was measured under torque application and femoral neck osteotomy and segmental AAOS Type I and III defects were simulated. The relative movements of all prostheses were significantly influenced by the extent of bone loss (p<0.01). Major differences were seen in fixation behavior (p<0.01). The main fixation area of conical stems is within the distal femoral isthmus, whereas cylindrical implants are dependent on proximal bone stock. In our study, cylindrical stems are advantageous for minor defects because they provide a proximal fixation. In cases of extensive substance loss, the conical implants showed lesser relative movements. These findings should be taken into account for clinical decisions.     

Predicting the subject-specific primary stability of cementless implants during pre-operative planning: preliminary validation of subject-specific finite-element models

Pre-operative planning help the surgeon in taking the proper clinical decision. The ultimate goal of this work is to develop numerical models that allow the surgeon to estimate the primary stability during the pre-operative planning session. The present study was aimed to validate finite-element (FE) models accounting for patient and prosthetic size and position as planned by the surgeon. For this purpose, the FE model of a cadaveric femur was generated starting from the CT scan and the anatomical position of a cementless stem derived by a skilled surgeon using a pre-operative CT-based planning simulation software. In-vitro experimental measurements were used as benchmark problem to validate the bone-implant relative micromotions predicted by the patient-specific FE model. A maximum torque in internal rotation of 11.4 Nm was applied to the proximal part of the hip stem. The error on the maximum predicted micromotion was 12% of the peak micromotion measured experimentally. The average error over the entire range of applied torques was only 7% of peak measurement. Hence, the present study confirms that it is possible to accurately predict the level of primary stability achieved for cementless stems using numerical models that account for patient specificity and surgical variability.     

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

The effect of a short-stem femoral resurfacing component on load transfer and potential failure mechanisms has rarely been studied. The stem length has been reduced by approximately 50% as compared to the current long-stem design. Using 3-D FE models of natural and resurfaced femurs, the study is aimed at investigating the influence of a short-stem resurfacing component on load transfer and bone remodelling. Applied loading conditions include normal walking and stair climbing. The mechanical role of the stem along with implant–cement and stem–bone contact conditions was observed to be crucial. Shortening the stem length to half of the current length (long-stem) led to several favourable effects, even though the stress distributions in the implant and the cement were similar in both the cases. The short-stem implant led not only to a more physiological stress distribution but also to bone apposition (increase of 20–70% bone density) in the superior resurfaced head, when the stem–bone contact prevailed. This also led to a reduction in strain concentration in the cancellous bone around the femoral neck–component junction. The normalised peak strain in this region was lower for the short-stem design as compared to that of the long-stem one, thereby reducing the initial risk of neck fracture. The effect of strain shielding (50–75% reduction) was restricted to a small bone volume underlying the cement, which was approximately half of that of the long-stem design. Consequently, bone resorption was considerably less for the short-stem design. The short-stem design offers better prospects than the long-stem resurfacing component.     

Shape optimization of a cementless hip stem for a minimum of interface stress and displacement

The primary stem stability is an essential factor for success of cementless hip stems. A correct choice of the stem geometry can improve the stem stability and, consequently, increase the life time of a hip implant. In this work, it is proposed a computational model for shape optimization of cementless hip stems. The optimization problem is formulated by the minimization of relative displacement and stress on bone/stem interface using a multi-criteria objective function. Also multiple loads are considered to incorporate several daily life activities. Design variables are parameters that characterize the geometry of selected cross sections, which are subject to geometric constraints to ensure a clinically admissible shape. The stem/bone set is considered a structure in equilibrium with contact conditions on interface. The contact formulation allows us to analyze different lengths of porous coating. The optimization problem is solved numerically by a steepest descent method. The interface stress and relative displacement are obtained solving the contact problem by the finite element method. Numerical examples are presented for a two-dimensional model of a hip stem, however, the formulation is general and can be applied to the three-dimensional case. The model gives indications about the relation between shape, porous coating and prosthesis stability.