The effect of interfacial parameters on cup-bone relative micromotions. A finite element investigation.

Achieving stability is a prerequisite for allowing bone to grow into the porous surface of non-cemented acetabular cups. The purpose of this study is to estimate the effects of interfacial characteristics on relative cyclical micromotion between cup and bone during gait in the immediate postoperative phase. The technique used is finite element analysis. Six models with different interfacial characteristics are created in order to study the effects of fixation technique. These include representation of a 1 mm press-fit, 2 mm press-fits (with and without an initial polar gap) and exact-fit conditions (with and without additional screw fixation). Although direct validation of the model has not been performed, the calculated micromotions under a static load of 1112 N are compared with appropriate experimental data. Generally, the model tends to underestimate micromotion and this underestimate is significant in the case of relative surface-normal micromotion in polar regions for models with low- and no-interference. The most likely cause of this significant underestimate is a failure of the model to accurately represent penetration of rough contacting surfaces under compression. Other types of micromotion, although low, are within standard deviations reported by Kwong et al. (1994 Journal of Arthroplasty 9, 163-170). Quasi-static joint contact and muscle forces, representative of the stance phase of gait are then applied and maximum micromotions are found to occur consistently prior to toe off: this being the point of maximum force. With regard to the press-fit simulations, good cup-bone contact in the superior region of the interface is required for stability and the greatest micromotions occur in the models with the larger interference and larger polar gaps. In contrast to the press-fit models, muscle activity in exact-fit models influences the calculations. Specifically, the early activity of m.semimembranosus modelled causes opening of the peripheral seal. Taken together it is found that polar gaps reduce the stability of the model and lack of pre-compresssion in the periphery allows this region of the interface to be opened up.     

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.     

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.     

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.     

Hip contact forces and gait patterns from routine activities.

In vivo loads acting at the hip joint have so far only been measured in few patients and without detailed documentation of gait data. Such information is required to test and improve wear, strength and fixation stability of hip implants. Measurements of hip contact forces with instrumented implants and synchronous analyses of gait patterns and ground reaction forces were performed in four patients during the most frequent activities of daily living. From the individual data sets an average was calculated. The paper focuses on the loading of the femoral implant component but complete data are additionally stored on an associated compact disc. It contains complete gait and hip contact force data as well as calculated muscle activities during walking and stair climbing and the frequencies of daily activities observed in hip patients. The mechanical loading and function of the hip joint and proximal femur is thereby completely documented. The average patient loaded his hip joint with 238% BW (percent of body weight) when walking at about 4 km/h and with slightly less when standing on one leg. This is below the levels previously reported for two other patients (Bergmann et al., Clinical Biomechanics 26 (1993) 969-990). When climbing upstairs the joint contact force is 251% BW which is less than 260% BW when going downstairs. Inwards torsion of the implant is probably critical for the stem fixation. On average it is 23% larger when going upstairs than during normal level walking. The inter- and intra-individual variations during stair climbing are large and the highest torque values are 83% larger than during normal walking. Because the hip joint loading during all other common activities of most hip patients are comparably small (except during stumbling), implants should mainly be tested with loading conditions that mimic walking and stair climbing.     

The influence of friction and interference on the seating of a hemispherical press-fit cup: a finite element investigation.

The formation of gaps in the polar region of acetabular cups is seen as a drawback of press-fit fixation of non-cemented acetabular cups. Recent findings indicate a link between long-term polar gaps and the gaps present directly after implantation. In this study the process of press-fitting is simulated with a linear-elastic two-dimensional axisymmetric finite-element model. The aim of this paper is to investigate the possible importance of friction and interference on the formation of these gaps. A range of cup-bone friction coefficients (mu = 0.1-0.5) is assigned to the cup-bone interface in order to represent the unknown amount of friction occurring during press-fitting. The cup is modeled with a radius of 27 mm, whereas the radius of the cavity is varied between 26.50 and 26.75 mm, thus, creating 0.50 and 0.25 mm radial interference fits. The difference in cavity radius represents the discrepancy between the radius of the last-reamer-used and radius of the cavity it creates. The subchondral plate is considered as being completely removed during reaming. The effects of impact blows via the surgeon's mallet during surgery are modeled as a series of four load pulses, in which peak force is gradually increased from 0.5 to 4.0 kN. The effects of load removal as well as those of load application are investigated. On load application, the cup penetrates into the cavity, and on load removal, the cup rebounds. Depending on the friction, interference and load applied, the position of the cup after the load pulse is somewhere between its position at peak force and its position at the beginning of the pulse. Although the simplifications and conditions involved in the creation of the model necessitate caution when interpreting the results for all clinical cases, it is found that the seating of hemispherical cups in trabecular bone could be more satisfactory for intermediate values of friction (mu = 0.2-0.3) and smaller interference fits (0.25 mm).     

Wear Testing of Moderate Activities of Daily Living Using In Vivo Measured Knee Joint Loading

Resumption of daily living activities is a basic expectation for patients provided with total knee replacements. However, there is a lack of knowledge regarding the impact of different activities on the wear performance. In this study the wear performance under application of different daily activities has been analyzed. In vivo load data for walking, walking downstairs/upstairs, sitting down/standing up, and cycling (50 W & 120 W) has been standardized for wear testing. Wear testing of each activity was carried out on a knee wear simulator. Additionally, ISO walking was tested for reasons of comparison. Wear was assessed gravimetrically and wear particles were analyzed. In vivo walking produced the highest overall wear rates, which were determined to be three times higher than ISO walking. Moderate wear rates were determined for walking upstairs and downstairs. Low wear rates were determined for standing up/sitting down and cycling at power levels of 50 W and 120 W. The largest wear particles were observed for cycling. Walking based on in vivo data has been shown to be the most wear-relevant activity. Highly demanding activities (stair climbing) produced considerably less wear. Taking into account the expected number of loads, low-impact activities like cycling may have a greater impact on articular wear than highly demanding activities.     

Interfacial atom pair analysis

The relations of the binding free energies in a dataset of 69 protein complexes with the numbers of interfacial atom pairs, as well as with the atomic distances of the pairs, are analyzed. It is found that the interfacial main-chain atom pairs contribute more to the correlation than the interfacial side chain atom pairs do, and the polar atom pairs contribute more than the non-polar atom pairs do. Interfacial atom pairs with atomic distance in the range of 6-12 A are the most important to explain the differences in binding free energies in the datasets.     

Osteoclastogenesis can be mechanically-induced in the peri-implant bone

Total joint replacements are highly successful in relieving pain and restoring movement of damaged joints. However, the lifespan of the implants is limited. The implant's long-term stability depends largely on the preservation of periprosthetic bone. Debris-wear particulates were first identified as the factor inducing periprosthetic bone loss. However, it was later shown that the resorption process starts before the particulates reach the periprosthetic bone. Thus a mechanical factor, interface micromotions, has been suspected to be the initiator of the early bone loss. In this work, we then investigated the response of bone cells to micromotions. Using an ex vivo setup, we applied micromotions on fresh human bone cores and showed that micromotions could indirectly activate osteoclasts after only one hour of stimulation. Thus micromotion-related osteoclastic activity could be the initiator of periprosthetic bone loss.     

Experimental and computational analysis of micromotions of an uncemented femoral knee implant using elastic and plastic bone material models

It is essential to calculate micromotions at the bone-implant interface of an uncemented femoral total knee replacement (TKR) using a reliable computational model. In the current study, experimental measurements of micromotions were compared with predicted micromotions by Finite Element Analysis (FEA) using two bone material models: linear elastic and post-yield material behavior, while an actual range of interference fit was simulated. The primary aim was to investigate whether a plasticity model is essential in order to calculate realistic micromotions. Additionally, experimental bone damage at the interface was compared with the FEA simulated range.TKR surgical cuts were applied to five cadaveric femora and micro- and clinical CT- scans of these un-implanted specimens were made to extract geometrical and material properties, respectively. Micromotions at the interface were measured using digital image correlation. Cadaver-specific FEA models were created based on the experimental set-up. The average experimental micromotion of all specimens was 53.1 ± 42.3 µm (mean ± standard deviation (SD)), which was significantly higher than the micromotions predicted by both models, using either the plastic or elastic material model (26.5 ± 23.9 µm and 10.1 ± 10.1 µm, respectively; p-value < 0.001 for both material models). The difference between the two material models was also significant (p-value < 0.001). The predicted damage had a magnitude and distribution which was comparable to the experimental bone damage. We conclude that, although the plastic model could not fully predict the micro motions, it is more suitable for pre-clinical assessment of a press-fit TKR implant than using an elastic bone model.     

Subject-specific modeling of the scapula bone tissue adaptation

Adaptation of the scapula bone tissue to mechanical loading is simulated in the current study using a subject-specific three-dimensional finite element model of an intact cadaveric scapula. The loads experienced by the scapula during different types of movements are determined using a subject-specific large-scale musculoskeletal model of the shoulder joint. The obtained density distributions are compared with the CT-measured density distribution of the same scapula. Furthermore, it is assumed that the CT-measured density distribution can be estimated as a weighted linear combination of the density distributions calculated for different loads experienced during daily life. An optimization algorithm is used to determine the weighting factors of fourteen different loads such that the difference between the weighted linear combination of the calculated density distributions and the CT-measured density is minimal. It is shown that the weighted linear combination of the calculated densities matches the CT-measured density distribution better than every one of the density distributions calculated for individual movements. The weighting factors of nine out of fourteen loads were estimated to be zero or very close to zero. The five loads that had larger weighting factors were associated with either one of the following categories: (1) small-load small-angle abduction or flexion movements that occur frequently during our daily lives or (2) large-load large-angle abduction or flexion movements that occur infrequently during our daily lives.     

Tissue stresses and strain in trabeculae of a canine proximal femur can be quantified from computer reconstructions

A quantitative assessment of bone tissue stresses and strains is essential for the understanding of failure mechanisms associated with osteoporosis, osteoarthritis, loosening of implants and cell- mediated adaptive bone-remodeling processes. According to Wolff's trajectorial hypothesis, the trabecular architecture is such that minimal tissue stresses are paired with minimal weight. This paradigm at least suggests that, normally, stresses and strains should be distributed rather evenly over the trabecular architecture. Although bone stresses at the apparent level were determined with finite element analysis (FEA), by assuming it to be continuous, there is no data available on trabecular tissue stresses or strains of bones in situ under physiological loading conditions. The objectives of this project were to supply reasonable estimates of these quantities for the canine femur, to compare trabecular-tissue to apparent stresses, and to test Wolff's hypothesis in a quantitative sense. For that purpose, the newly developed method of large-scale micro-FEA was applied in conjunction with micro-CT structural measurements. A three-dimensional high-resolution computer reconstruction of a proximal canine femur was made using a micro-CT scanner. This was converted to a large-scale FE-model with 7.6 million elements, adequately refined to represent individual trabeculae. Using a special-purpose FE-solver, analyses were conducted for three different orthogonal hip-joint loading cases, one of which represented the stance-phase of walking. By superimposing the results, the tissue stress and strain distributions could also be calculated for other force directions. Further analyses of results were concentrated on a trabecular volume of interest (VOI) located in the center of the head. For the stance phase of walking an average tissue principal strain in the VOI of 279 strain was found, with a standard deviation of 212 microstrain. The standard deviation depended not only on the hip-force magnitude, but also on its direction. In more than 95% of the tissue volume the principal stresses and strains were in a range from zero to three times the averages, for all hip-force directions. This indicates that no single load creates even stress or strain distributions in the trabecular architecture. Nevertheless, excessive values occurred at few locations only, and the maximum tissue stress was approximately half the value reported for the tissue fatigue strength. These results thus indicate that trabecular bone tissue has a safety factor of approximately two for hip-joint loads that occur during normal activities.     

Tissue stresses and strain in trabeculae of a canine proximal femur can be quantified from computer reconstructions

A quantitative assessment of bone tissue stresses and strains is essential for the understanding of failure mechanisms associated with osteoporosis, osteoarthritis, loosening of implants and cell-mediated adaptive bone-remodeling processes. According to Wolff's trajectorial hypothesis, the trabecular architecture is such that minimal tissue stresses are paired with minimal weight. This paradigm at least suggests that, normally, stresses and strains should be distributed rather evenly over the trabecular architecture. Although bone stresses at the apparent level were determined with finite element analysis (FEA), by assuming it to be continuous, there is no data available on trabecular tissue stresses or strains of bones in situ under physiological loading conditions. The objectives of this project were to supply reasonable estimates of these quantities for the canine femur, to compare trabecular-tissue to apparent stresses, and to test Wolff's hypothesis in a quantitative sense. For that purpose, the newly developed method of large-scale micro-FEA was applied in conjunction with micro-CT structural measurements. A three-dimensional high-resolution computer reconstruction of a proximal canine femur was made using a micro-CT scanner. This was converted to a large-scale FE-model with 7.6 million elements, adequately refined to represent individual trabeculae. Using a special-purpose FE-solver, analyses were conducted for three different orthogonal hip-joint loading cases, one of which represented the stance-phase of walking. By superimposing the results, the tissue stress and strain distributions could also be calculated for other force directions. Further analyses of results were concentrated on a trabecular volume of interest (VOI) located in the center of the head. For the stance phase of walking an average tissue principal strain in the VOI of 279 strain was found, with a standard deviation of 212 microstrain. The standard deviation depended not only on the hip-force magnitude, but also on its direction. In more than 95% of the tissue volume the principal stresses and strains were in a range from zero to three times the averages, for all hip-force directions. This indicates that no single load creates even stress or strain distributions in the trabecular architecture. Nevertheless, excessive values occurred at few locations only, and the maximum tissue stress was approximately half the value reported for the tissue fatigue strength. These results thus indicate that trabecular bone tissue has a safety factor of approximately two for hip-joint loads that occur during normal activities.     

Interfacial tension of phosphatidylcholine-cholesterol system in monolayers at the air/water interface

Interfacial tension of an egg lecithin-cholesterol system was measured across the whole concentration range. Surface pressure-area isotherm measurements were carried out in a Langmuir trough at the air/water interface at room temperature (22 degrees C). The interfacial tension of the air/water interface was divided into contributions of components. The interfacial tension of a 1:1 complex between phosphatidylcholine and cholesterol was calculated. Its value equals 18 mN/m. The difference between the stability constant of 1:1 complex in the bilayer and the monolayer at the air/water interface is discussed.     

Dependence of interfacial properties of normal and transformed 3T3 cell membranes on treatment with factors modifying proliferation

Interfacial properties of the outer cell membrane of normal and transformed in vitro cultures of mouse 3T3 cells have been investigated. The contact angles of sessile drops on dried cell preparations were measured and the interfacial tensions derived using the thermodynamic approach introduced by Neumann. Interfacial tensions were found to be within an order of magnitude of those determined for other cell and model membranes. Treatment of cells with calf serum, a stimulant to proliferation, resulted in a decrease in the interfacial tension of normal and transformed cells, whereas use of concanavalin A and its succinylated derivative lead to an increase of interfacial tensions of both cell types. These and further results show a detailed correlation between the growth-regulating effects and the effects on interfacial properties of these proliferation-modifying factors. An interpretation of the results of serum depression of the interfacial tension in terms of a binding equilibrium dependent on the concentration of humoral growth factors in the medium is attempted.     

Dependence of interfacial properties of normal and transformed 3T3 cell membranes on treatment with factors modifying proliferation

Interfacial properties of the outer cell membrane of normal and transformed in vitro cultures of mouse 3T3 cells have been investigated. The contact angles of sessile drops on dried cell preparations were measured and the interfacial tensions derived using the thermodynamic approach introduced by Neumann. Interfacial tensions were found to be within an order of magnitude of those determined for other cell and model membranes. Treatment of cells with calf serum, a stimulant to proliferation, resulted in a decrease in the interfacial tension of normal and transformed cells, whereas use of concanavalin A and its succinylated derivative lead to an increase of interfacial tensions of both cell types. These and further results show a detailed correlation between the growth-regulating effects and the effects on interfacial properties of these proliferation-modifying factors. An inter-pretation of the results of serum depression of the interfacial tension in terms of a binding equilibrium dependent on the concentration of humoral growth factors in the medium is attempted.     

Introduction of a C-terminal aromatic sequence from snake venom phospholipases A2 into the porcine pancreatic isozyme dramatically changes the interfacial kinetics.

Porcine pancreatic phospholipase A2 (PLA2) was modified by single and multiple site-directed mutations at sites thought to be involved in interfacial binding. Charged and polar residues in the C-terminal region were replaced by aromatic residues on the basis of an analogy with snake venom PLA2s, which display high affinity for a zwitterionic interface. The PLA2 variants constructed were N117W, N117W/D119Y and K116Y/N117W/D119Y. Titration with micelles of a zwitterionic substrate suggests that the variants N117W and K116Y/N117W/D119Y possess improved ability to bind to the micellar substrate interface, relative to the wild-type enzyme. Improved interfacial binding was confirmed by direct binding studies with micelles of a zwitterionic substrate analogue, indicating up to five times higher affinity for both variants. Interfacial binding is not improved for the variant N117W/D119Y. Maximal enzyme velocities (Vapp./max) with the zwitterionic substrate were between 25 and 75% of that of the wild-type enzyme. However, competitive inhibition and direct binding studies with a strong inhibitor revealed that the affinity for substrate present at the interface (Km*) is perturbed by the mutations made. For the variant N117W, the slight decrease observed in Vapp./max is most likely made up of a 24-fold reduction in catalytic turnover (kcat) and 18-fold improved substrate binding (Km*).     

A nonlinear homogenized finite element analysis of the primary stability of the bone–implant interface

Stability of an implant is defined by its ability to undergo physiological loading–unloading cycles without showing excessive tissue damage and micromotions at the interface. Distinction is usually made between the immediate primary stability and the long-term, secondary stability resulting from the biological healing process. The aim of this research is to numerically investigate the effect of initial implantation press-fit, bone yielding, densification and friction at the interface on the primary stability of a simple bone–implant system subjected to loading–unloading cycles. In order to achieve this goal, human trabecular bone was modeled as a continuous, elasto-plastic tissue with damage and densification, which material constants depend on bone volume fraction and fabric. Implantation press-fit related damage in the bone was simulated by expanding the drilled hole to the outer contour of the implant. The bone–implant interface was then modeled with unilateral contact with friction. The implant was modeled as a rigid body and was subjected to increasing off-axis loading cycles. This modeling approach is able to capture the experimentally observed primary stability in terms of initial stiffness, ultimate force and progression of damage. In addition, it is able to quantify the micromotions around the implant relevant for bone healing and osseointegration. In conclusion, the computationally efficient modeling approach used in this study provides a realistic structural response of the bone–implant interface and represents a powerful tool to explore implant design, implantation press-fit and the resulting risk of implant failure under physiological loading.     

Modeling the tensile mechanical behavior of bone along the longitudinal direction

The tensile stress-strain behavior of bone along its longitudinal axis is modeled by using a simple shear-lag theory, wherein, stresses and strains in a unit cell consisting of an organic matrix reinforced by overlapped mineral platelets are computed. It is assumed that loads are transferred between overlapped mineral-platelets by shear in the organic matrix. The mechanical behavior of bone in which the matrix partially or completely debonds from the sides of the overlapped mineral platelets (after an ultimate interfacial shear stress value is exceeded) is modeled. It is shown that the tensile mechanical behavior of bone can be modeled only by assuming little or no debonding of the organic from the mineral. A physical phenomenon that explains the tensile behavior of bone is, after the interfacial shear stress has reached a constant value over the length of the mineral platelets, the collagen molecules/microfibrils (with the associated mineral platelets) move relative to one another. The tensile stress-strain curve of bovine bone is modeled using this model. The theory predicts the mechanical behavior of the tissue in the elastic, yield and post-yield region. The ultimate strain and strengths are not predicted in the present model.     

Influence of material coupling and assembly condition on the magnitude of micromotion at the stem-neck interface of a modular hip endoprosthesis

Hip prostheses with a modular neck exhibit, compared to monobloc prostheses, an additional interface which bears the risk of fretting as well as corrosion. Failures at the neck adapter of modular prostheses have been observed for a number of different designs. It has been speculated that micromotions at the stem-neck interface were responsible for these implant failures. The purpose of this study was to investigate the influence of material combinations and assembly conditions on the magnitude of micromotions at the stem-neck interface during cyclic loading. Modular (n = 24) and monobloc (n = 3) hip prostheses of a similar design (Metha, Aesculap AG, Tuttlingen, Germany) were subjected to mechanical testing according to ISO 7206-4 (F(min) = 230N, F(max) = 2300N, f = 1Hz, n = 10,000 cycles). The neck adapters (Ti-6Al-4V or Co-Cr29-Mo alloy) were assembled with a clean or contaminated interface. The micromotion between stem and neck adapter was calculated at five reference points based on the measurements of the three eddy current sensors. The largest micromotions were observed at the lateral edge of the stem-neck taper connection, which is in accordance with the crack location of clinically failed prostheses. Titanium neck adapters showed significantly larger micromotions than cobalt-chromium neck adapters (p = 0.005). Contaminated interfaces also exhibited significantly larger micromotions (p < 0.001). Since excessive micromotions at the stem-neck interface might be involved in the process of implant failure, special care should be taken to clean the interface prior to assembly and titanium neck adapters with titanium stems should generally be used with caution.