Interfacial conditions between a press-fit acetabular cup and bone during daily activities: implications for achieving bone in-growth

A macro-micro FE and ANN framework to assess site-specific bone ingrowth around the porous beaded-coated implant: an example with BOX® tibial implant for total ankle replacement

The use of mechanoregulatory schemes based on finite element (FE) analysis for the evaluation of bone ingrowth around porous surfaces is a viable approach but requires significant computational time and effort. The aim of this study is to develop a combined macro-micro FE and artificial neural network (ANN) framework for rapid and accurate prediction of the site-specific bone ingrowth around the porous beaded-coated tibial implant for total ankle replacement (TAR). A macroscale FE model of the implanted tibia was developed based on CT data. Subsequently, a microscale FE model of the implant-bone interface was created for performing bone ingrowth simulations using mechanoregulatory algorithms. An ANN was trained for rapid and accurate prediction of bone ingrowth. The results predicted by ANN are well comparable to FE-predicted results. Predicted site-specific bone ingrowth using ANN around the implant ranges from 43.04 to 98.24%, with a mean bone ingrowth of around 74.24%. Results suggested that the central region exhibited the highest bone ingrowth, which is also well corroborated with the recent explanted study on BOX®. The proposed methodology has the potential to simulate bone ingrowth rapidly and effectively at any given site over any implant surface.

Mechanobiological model to study the influence of screw design and surface treatment on osseointegration

This study aims at suggesting a new approach to peri-implant healing models, providing a set of taxis-diffusion-reaction equations under the combined influence of mechanical and biochemical factors. Early events of osseointegration were simulated for titanium screw implants inserted into a pre-drilled trabecular bone environment, up to 12 weeks of peri-implant bone healing. Simulations showed the ability of the model to reproduce biological events occurring at the implant interface through osteogenesis. Implants with shallow healing chamber showed higher proportions of lamellar bone, enhanced by the increase of mechanical stimulation. Osteoconduction was observed through the surface treatment model and similar bone healing patterns compared to in vivo studies.

Lower functioning patients demonstrate atypical hip joint loading before and following total hip arthroplasty for osteoarthritis

Previous studies have established that up to 1 year post total hip arthroplasty (THA), patients do not recover normal function and the magnitude of hip joint loading remains reduced compared to healthy individuals. However, the temporal nature of the loading profile has not been considered to identify individuals who are at a greater risk of poor functional outcomes following THA. This study aimed to determine changes to the profile and magnitude of the resultant hip joint reaction force before and up to 6 months post-primary THA, and factors associated with atypical loading profiles. Hip joint loading was computed using a personalized lower-limb musculoskeletal model in 43 participants awaiting primary THA for osteoarthritis (mean age: SD = 65, 14 years; body mass index: SD = 30, 5 kg/m² ) before and up to 6 months after THA. Atypical, single-peak loading profiles were observed for 11 patients before surgery, where four showed a single peak at 6 months. Patients displaying a single-peak profile walked slower (mean difference: -0.4 m/s) compared to individuals displaying double-peak profile (P = <.001) and had significantly reduced sagittal plane hip range of motion during gait (mean difference -9.6°, P = <.001). Self-reported pain, function, and stiffness did not differentiate between patients with a single or double-peak loading profile. Individuals with a single-peak force profile did not meet the minimal clinically important hip range of motion during gait and would be classified as low-functioning THA patients. Clinical Relevance: The temporal nature of the force profile may help to identify individuals who are at the greatest risk of poor functional outcomes after THA.

Effect of gap outside contact area on lubrication of metal-on-Metal total hip replacement

Ball-in-socket metal on metal (MOM) contacts were analysed using the Abaqus Finite Element package to simulate dry contact between the acetabular cup and the femoral head. Different cup thicknesses of 4, 6, 8, and 10 mm were considered using a polyurethane foam block support system. Elastohydrodynamic lubrication (EHL) analyses were developed for the contacts using three different approaches to specify the contact. These were (i) A simple model based on the radii of relative curvature, (ii) An equivalent contact model developed so that its dry contact area and maximum pressure replicated the values obtained from the FE analysis, and (iii) A modified version of (ii) that also ensured equivalence of the gap shape outside the contact area. Published in vivo information for the hip joint contact forces over the walking cycle was used to specify the operating conditions for the EHL analysis. The analysis method was found to be effective for all points of the walking cycle for cases where the cup thickness exceeded 5 mm and modelling approach (ii) was identified as satisfactory. For a cup thickness of 4 mm, membrane action began to emerge in the FE analyses so that such contacts behaved in a different way.

A comparative assessment of two designs of hip stem using rule-based simulation of combined osseointegration and remodelling

The stem–bone interface of cementless total hip arthroplasty undergoes an adaptive process of bone ingrowth until the two parts become osseointegrated. Another important phenomenon associated with aseptic loosening of hip stem is stress-shielding induced adverse bone remodelling. The objective of this study was to preclinically assess the relative performances of two distinct designs of hip stems by addressing the combined effect of bone remodelling and osseointegration, based on certain rule-based criteria obtained from the literature. Premised upon non-linear finite element analyses of patient-specific implanted femur models, the study attempts to ascertain in silico outcome of the hip stem designs based on an evolutionary interfacial condition, and to further comment on the efficacy of the rule-based technique on the prediction of peri-prosthetic osseointegration. One of the two hip stem models was a trade-off design obtained from an earlier shape optimization study, and the other was based on TriLock stem (DePuy). Both designs predicted similar long-term osseointegration (∼89% surface), although trade-off stem predicted higher post-operative osseointegration. Proximal bone resorption was found higher for TriLock (by ∼110%) as compared to trade-off model. The rule-based technique predicted clinically coherent osseointegration around both stems and appears to be an alternative to expensive mechanobiology-based schemes.

Effect of impaction energy on dynamic bone strains, fixation strength, and seating of cementless acetabular cups

Seating a cementless acetabular cup via impaction is a balancing act; good cup fixation must be obtained to ensure adequate bone in-growth and cup apposition, while acetabular fracture must be avoided. Good impaction technique is essential to the success of hip arthroplasty. Yet little guidance exists in the literature to inform surgeons on "how hard" to hit. A drop rig and synthetic bone model were used to vary the energy of impaction strikes in low and high-density synthetic bone, while key parameters such as dynamic strain (quantifying fracture risk), implant fixation, and polar gap were measured. For high energy impaction (15 J) in low-density synthetic bone, a peak tensile strain was observed during impaction that was up to 3.4× as large as post-strike strain, indicating a high fracture risk. Diminishing returns were observed for pushout fixation with increasing energy. Eighty-five percent of the pushout fixation achieved using a 15 J impaction strike was attained by using a 7.5 J strike energy. Similarly, polar gap was only minimally reduced at high impaction energies. Therefore it is suggested that higher energy strikes increase fracture risk, but do not offer large improvements to fixation or implant-bone apposition. It may difficult be for surgeons to accurately deliver specific impaction energies, suggesting there is scope for operative tools to manage implant seating. © 2019 The Authors. Journal of Orthopaedic Research^® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2367-2375, 2019.

Changes in acetabular component alignment due to screw fixation in patients with hip dysplasia

INTRODUCTION

Adequate initial stability of the acetabular cup is essential for total hip arthroplasty (THA). However, changes in the alignment of the acetabular component caused by screw fixation are concerning in patients with inadequate bone stock. This study aimed to investigate the effects of screw fixation on the alignment of the acetabular component in THA patients with hip dysplasia.

METHODS

We retrospectively examined 256 hips (range 28-87 years) that underwent THA using a navigation system. Patients were divided into 2 groups based on the presence or absence of changes in the alignment of the intraoperative acetabular cup, and univariate and multivariate analyses were performed to identify factors that were predictive of changes in acetabular component alignment after screw fixation in 2 dimensions: inclination and anteversion.

RESULTS

Screw fixation led to a mean change in inclination of 1.6° (range 0-10°) and a mean change in anteversion of 1.4° (range 0-14°). The Crowe classification, the presence of bone cysts, and the use of an inferior quadrant screw were identified as factors that correlated with acetabular cup alignment changes in inclination (odds ratios, 6.01, 5.94 and 0.03, respectively). Only the Crowe classification was identified as a factor that correlated with intraoperative alignment changes in anteversion (odds ratio, 2.08).

CONCLUSIONS

Screw fixation altered the acetabular cup alignment. The inclination changes were related to the extent of the dysplasia, and the risk was reduced when the inferior quadrant screw was used. Surgeons should use caution during screw fixation in THAs performed on severely dysplastic hips.

Effect of bearing friction torques on the primary stability of press-fit acetabular cups: A novel in vitro method

Aseptic loosening is the main reason for revision of total hip arthroplasty, and relative micromotions between cementless acetabular cups and bone play an important role regarding their comparatively high loosening rate. Therefore, the aim of the present study was to analyze the influence of resulting frictional torques on the primary stability of press-fit acetabular cups subjected to two different bearing partners. A cementless press-fit cup was implanted in bone-like foam. Primary stability of the cup was analyzed by determining spatial total, translational, and rotational interface micromotions by means of an eddy current sensor measuring system. Torque transmission into the cup was realized by three synchronous servomotors considering resultant friction torques based on constant friction for ceramic-on-ceramic (CoC: μ = 0.044; max. resultant torque: 1.5 Nm) and for ceramic-on-polyethylene (CoP: μ = 0.063; max. resultant torque: 1.9 Nm) bearing partners. Rotational micromotion of CoC was 8.99 ± 0.85 µm and of CoP 13.39 ± 1.43 µm. Translational micromotion of CoC was 29.93 ± 1.44 μm and of CoP 39.91 ± 2.25 μm. Maximum total relative micromotions were 37.10 ± 1.07 μm for CoC and 51.64 ± 2.18 μm for CoP. Micromotions resulting from CoC were statistically lower than those resulting from CoP (p < 0.05). The described 3D-measuring set-up offers a novel in vitro method of measuring primary stability of acetabular cups. We can therefore conclude, that primary stability of acetabular cup systems can be observed using either the lower friction curve (CoC) or the higher friction curve (CoP). In future studies different cup designs or cup fixation mechanisms may be tested and compared in vitro and assessed prior to implantation. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2745-2753, 2018.

Primary stability of a cementless acetabular cup in a cohort of patient-specific finite element models

The primary stability achieved during total hip arthroplasty determines the long-term success of cementless acetabular cups. Pre-clinical finite element testing of cups typically use a model of a single patient and assume the results can be extrapolated to the general population. This study explored the variability in predicted primary stability of a Pinnacle^® cementless acetabular cup in 103 patient-specific finite element models of the hemipelvis and examined the association between patient-related factors and the observed variability. Cups were inserted by displacement-control into the FE models and then a loading configuration simulating a complete level gait cycle was applied. The cohort showed a range of polar gap of 284-1112 μm and 95th percentile composite peak micromotion (CPM) of 18-624 μm. Regression analysis was not conclusive on the relationship between patient-related factors and primary stability. No relationship was found between polar gap and micromotion. However, when the patient-related factors were categorised into quartile groups, trends suggested higher polar gaps occurred in subjects with small and shallow acetabular geometries and cup motion during gait was affected most by low elastic modulus and high bodyweight. The variation in primary stability in the cohort for an acetabular cup with a proven clinical track record may provide benchmark data when evaluating new cup designs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1012-1023, 2018.

Longitudinal joint loading in patients before and up to one year after unilateral total hip arthroplasty

Abnormal kinematics and kinetics have been reported in hip osteoarthritis (OA) patients before and after total hip arthroplasty (THA). These changes can affect the loading of the ipsilateral hip, as well as the contralateral hip and knee joint. As it is not clear how hip and knee loading evolves in THA patients during the first year after surgery, the goal of this study is to define how joint loading changes in patients before and at three evaluation times after THA surgery. Musculoskeletal modelling in combination with gait analysis data was used to calculate hip and knee contact forces in 14 patients before and 3-, 6- and 12-months after unilateral THA, as well as in 18 healthy controls. Results showed that bilateral hip and knee loading were decreased compared to controls, both before and after THA surgery. Loading symmetry was altered compared to controls at 3-months post-surgery for the hip and at all evaluation times, except for 6-months post-surgery, for the knee, with ipsilateral joint loading decreased compared to the contralateral side. To conclude, 12-months after THA joint loading was not normalized, with both hip and knee loading in patients decreased compared to controls. Therefore, no overloading of the ipsi- or contralateral hip and knee joint was found before and up to one year after unilateral THA.

Influence of anisotropic bone properties on the biomechanical behavior of the acetabular cup implant: a multiscale finite element study

Although the biomechanical behavior of the acetabular cup (AC) implant is determinant for the surgical success, it remains difficult to be assessed due to the multiscale and anisotropic nature of bone tissue. The aim of the present study was to investigate the influence of the anisotropic properties of peri-implant trabecular bone tissue on the biomechanical behavior of the AC implant at the macroscopic scale. Thirteen bovine trabecular bone samples were imaged using micro-computed tomography (μCT) with a resolution of 18 μm. The anisotropic biomechanical properties of each sample were determined at the scale of the centimeter based on a dedicated method using asymptotic homogenization. The material properties obtained with this multiscale approach were used as input data in a 3D finite element model to simulate the macroscopic mechanical behavior of the AC implant under different loading conditions. The largest stress and strain magnitudes were found around the equatorial rim and in the polar area of the AC implant. All macroscopic stiffness quantities were significantly correlated (R² > 0.85, p < 6.5 e-6) with BV/TV (bone volume/total volume). Moreover, the maximum value of the von Mises stress field was significantly correlated with BV/TV (R² > 0.61, p < 1.6 e-3) and was always found at the bone-implant interface. However, the mean value of the microscopic stress (at the scale of the trabeculae) decrease as a function of BV/TV for vertical and torsional loading and do not depend on BV/TV for horizontal loading. These results highlight the importance of the anisotropic properties of bone tissue.

Effects of various anchoring components and loading conditions on primary stability of acetabular revision implant

PURPOSE

In revision total hip arthroplasty, until today, orthopaedic surgeons are missing evidence-based guidelines on cementless acetabular cup fixation.

METHODS

5 finite element models were generated featuring the following anchorage strategies: 1 short peg, 1 long peg, 2 long screws, 3 short screws and zero anchoring components for reference. The micromotions at the implant-bone interface were analyzed for 3 different loadcases, "Seated leg-crossing" (joint force 940 N, impingement force 750 N), "Normal gait" (joint force 1820 N), and "Stumbling" (joint force 4520 N).

RESULTS

Within the same loadcase, percentages of interface area below 28 µm are nearly identical in all anchorage strategies. The average percentage of interface area below 28 µm is 31% for "Seated leg-crossing", 17% for "Normal gait", and 11% for "Stumbling". Maximal von Mises stresses in "Normal gait", for example, reach 12 MPa in the short peg, 48 MPa in the long peg, 15 MPa in 1 of the 2 long screws, and 85 MPa in 1 of the 3 short screws.

CONCLUSIONS

Common orthopaedic practice, to use peg or screw fixation alternatively according to bone availability or other clinical aspects, can be confirmed. The short peg may be a good alternative to the long peg with regard to the preservation of bone stock. However, the current study implies that the extent of potential osseointegration depends less on the chosen anchorage strategy but strongly on postoperative loading conditions. Total hip patients should be instructed on adequate postoperative activities.

Gait alterations to effectively reduce hip contact forces

Patients with hip pathology present alterations in gait which have an effect on joint moments and loading. In knee osteoarthritic patients, the relation between medial knee contact forces and the knee adduction moment are currently being exploited to define gait retraining strategies to effectively reduce pain and disease progression. However, the relation between hip contact forces and joint moments has not been clearly established. Therefore, this study aims to investigate the effect of changes in hip and pelvis kinematics during gait on internal hip moments and contact forces which is calculated using muscle driven simulations. The results showed that frontal plane kinetics have the largest effect on hip contact forces. Given the high correlation between the change in hip adduction moment and contact force at initial stance (R(2)  = 0.87), this parameter can be used to alter kinematics and predict changes in contact force. At terminal stance the hip adduction and flexion moment can be used to predict changes in contact force (R(2)  = 0.76). Therefore, gait training that focuses on decreasing hip adduction moments, a wide base gait pattern, has the largest potential to reduce hip contact forces.

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

Finite element has been used for more than four decades to study and evaluate the mechanical behaviour total joint replacements. In Huiskes seminal paper "Failed innovation in total hip replacement: diagnosis and proposals for a cure", finite element modelling was one of the potential cures to avoid poorly performing designs reaching the market place. The size and sophistication of models has increased significantly since that paper and a range of techniques are available from predicting the initial mechanical environment through to advanced adaptive simulations including bone adaptation, tissue differentiation, damage accumulation and wear. However, are we any closer to FE becoming an effective screening tool for new devices? This review contains a critical analysis of currently available finite element modelling techniques including (i) development of the basic model, the application of appropriate material properties, loading and boundary conditions, (ii) describing the initial mechanical environment of the bone-implant system, (iii) capturing the time dependent behaviour in adaptive simulations, (iv) the design and implementation of computer based experiments and (v) determining suitable performance metrics. The development of the underlying tools and techniques appears to have plateaued and further advances appear to be limited either by a lack of data to populate the models or the need to better understand the fundamentals of the mechanical and biological processes. There has been progress in the design of computer based experiments. Historically, FE has been used in a similar way to in vitro tests, by running only a limited set of analyses, typically of a single bone segment or joint under idealised conditions. The power of finite element is the ability to run multiple simulations and explore the performance of a device under a variety of conditions. There has been increasing usage of design of experiments, probabilistic techniques and more recently population based modelling to account for patient and surgical variability. In order to have effective screening methods, we need to continue to develop these approaches to examine the behaviour and performance of total joint replacements and benchmark them for devices with known clinical performance. Finite element will increasingly be used in the design, development and pre-clinical testing of total joint replacements. However, simulations must include holistic, closely corroborated, multi-domain analyses which account for real world variability.

ACETABULAR LOAD-TRANSFER AND MECHANICAL STABILITY: A FINITE ELEMENT ANALYSIS COMPARING DIFFERENT CEMENTLESS SOCKETS

Acetabular stress shielding may be a failure mechanism of acetabular constructs promoting osteolysis, aseptic loosening and failure. We used three-dimensional finite element analysis (FEA) to evaluate the effect of flexible sockets on acetabular stress shielding. The sockets were made of (1) full polyethylene (PE), (2) PE with a metal bearing and (3) a PE insert with a metal backing was used as a traditional stiff implant. We compared the strain energy density and interfacial micro-motions between bone and cementless sockets during walking. In our FEA model, the most elastic socket (case 1) showed the highest levels of micro-motion during walking (400 μm). The most rigid socket (case 3) showed smaller areas of high micro-motions. Assuming a threshold for ingrowth of 50 microns, the flexible cup showed an ingrowth area of almost 40%, whereas the other two cases showed stable areas covering 60% of the total bone–component interface. Furthermore, we found that the introduction of an implant generates a very different strain pattern directly around the implant as compared with the intact case, which has a horse-shoe shaped cartilage layer in the acetabulum. This difference was not affected much by the stiffness of the implant; a more flexible implant resulted in only slightly higher strain levels. Bone strains over 1.5 mm from the cup showed physiological values and were not affected by the stiffness of the implant. Hence, this study shows that the physiological strain patterns are not obtained in the direct periprosthetic bone, regardless of the stiffness of the material.

The influence of acetabular bone cracks in the press-fit hip replacement: Numerical and experimental analysis

BACKGROUND

The press-fit hip acetabular prosthesis implantation can cause crack formation in the thin regions surrounding the acetabular. As a consequence the presence of cracks in this region can lead to poor fixation and fibrous tissue formation.

METHODS

Numerical and experimental models of commercial press-fit hip replacements were developed to compare the behavior between the intact and implanted joints. Numerical models with an artificial crack and without crack were considered. The iliac and the femur were created through 3D geometry acquisition based on composite human replicas and 3D-Finite Element models were generated.

FINDINGS

The mechanical behavior was assessed numerically and experimentally considering the principal strains. The comparison between Finite Element model predictions and experimental measurements revealed a maximum difference of 9%. Similar distribution of the principal strains around the acetabular cavity was obtained for the intact and implanted models. When comparing the Von Mises stresses, it is possible to observe that the intact model is the one that presents the highest stress values in the entire acetabular cavity surface.

INTERPRETATION

The crack in the posterior side changes significantly the principal strain distribution, suggesting bone loss after hip replacement. Relatively to micromotions, these were higher on the superior side of the acetabular cavity and can change the implant stability and bone ingrowth.

Experimental validation of numerically predicted strain and micromotion in intact and implanted composite hemi-pelvises

The failure mechanisms of acetabular prostheses may be investigated by understanding the changes in load transfer due to implantation and using the analysis of the implant–bone micromotion. Computational finite element (FE) models allow detailed mechanical analysis of the implant–bone structure, but their validity must be assessed as a first step, before they can be employed in preclinical investigations. In this study, FE models of composite hemi-pelvises, intact and implanted with an acetabular cup, were experimentally validated. Strains and implant–bone micromotions in the hemi-pelvises were compared with those predicted by the equivalent FE models. Regression analysis indicated close agreement between the measured and FE strains, with a high correlation coefficient (0.95–0.98), a low standard error (SE) (36–53 µε) and a low error in regression slope (7%–11%). Measured micromotions along three orthogonal directions were small, less than 30 µm, whereas the FE-predicted values were found to be less than 85 µm. Although the trends were similar, the deviations are due to artefacts in experimental measurement and additional imperfections in recreating experimental loading and boundary conditions in the FE model. This supports the FE model as a valid predictor of the measured strain in the composite pelvis models, confirming its suitability for further computational investigations on acetabular prostheses.

The effect of cup orientation and coverage on contact mechanics and range of motion of metal-on-metal hip resurfacing arthroplasty

Implant malpositioning has been identified as a factor associated with clinical failures of metal-on-metal hip resurfacings (MoMHRs). This study investigated the effect of cup orientation and cup coverage on the contact mechanics (incidence of edge-loading) and range of motion (ROM) of MoMHR. Three generic MoMHRs with differing amounts of cup coverage were considered at various orientations. Contact area and contact pressure at the bearing surface were predicted for each design using finite element (FE) method. The ROM was determined based on the geometry overlap. Edge contact was found at lower angles of inclination (65°) for lower coverage cup designs; however, they also provided the greatest ROM. Conversely, cups with greater coverage did not exhibit edge contact until the cup was more steeply positioned (75°), however ROM was reduced. This study enables both sets of variable to be considered in the design of metal-on-metal bearings in hip.

Experimental Validation of Finite Element Models of Intact and Implanted Composite Hemipelvises Using Digital Image Correlation

A detailed understanding of the changes in load transfer due to implantation is necessary to identify potential failure mechanisms of orthopedic implants. Computational finite element (FE) models provide full field data on intact and implanted bone structures, but their validity must be assessed for clinical relevance. The aim of this study was to test the validity of FE predicted strain distributions for the intact and implanted pelvis using the digital image correlation (DIC) strain measurement technique. FE models of an in vitro hemipelvis test setup were produced, both intact and implanted with an acetabular cup. Strain predictions were compared to DIC and strain rosette measurements. Regression analysis indicated a strong linear relationship between the measured and predicted strains, with a high correlation coefficient (R = 0.956 intact, 0.938 implanted) and a low standard error of the estimate (SE = 69.53 με, 75.09 με). Moreover, close agreement between the strain rosette and DIC measurements improved confidence in the validity of the DIC technique. The FE model therefore was supported as a valid predictor of the measured strain distribution in the intact and implanted composite pelvis models, confirming its suitability for further computational investigations.

Validation of FE micromotions and strains around a press-fit cup: introducing a new micromotion measuring technique

Finite element (FE) analysis provides an useful tool with which to analyze the potential performance of implantations in a variety of surgical, patient and design scenarios. To enable the use of FE analysis in the investigation of such implants, models must be experimentally validated. Validation of a pelvic model with an implanted press-fit cup in terms of micromotion and strain is presented here. A new method of micromotion has been introduced to better describe the overall movement of the cup within the pelvis. The method uses a digitizing arm to monitor the relative movement between markers on the cup and the surrounding acetabulum. FE analysis was used to replicate an experimental set up using a synthetic hemi-pelvis with a press-fitted all-metal cup, subject to the maximum loading observed during normal walking. The work presented here has confirmed the ability of FE models to accurately describe the mechanical performance of the press-fitted acetabulum and surrounding bone under typical loading conditions in terms of micromotion and strain distribution, but has demonstrated limitations in its ability to predict numerical micromotion values. A promising digitizing technique for measuring acetabular micromotions has also been introduced.

Combined multi-body and finite element investigation of the effect of the seat height on acetabular implant stability during the activity of getting up

An important question in assessing the stability of a total hip arthroplasty is the effect of daily physical activities of patients. The aim of this study is to examine these effects when standing up from three different seat heights. A musculoskeletal body model has been modified to simulate the three different seat heights. The calculated muscle forces have been transferred to a finite element model of a pelvis. The pelvis model was created from a hemipelvis CT dataset. As an implant component, a metal socket with a polyethylene insert was used. A primary implantation situation was modelled. For the analysed patient activities the highest hip contact forces and the highest micromotions occur at the beginning of the motion. The results of this study show that standing up from a certain seat height can have a significant influence on the micromotions in the implant-bone interface.

Computational assessment of press-fit acetabular implant fixation: The effect of implant design, interference fit, bone quality, and frictional properties

Patients suffering from rheumatoid arthritis typically have a poor subchondral bone quality, endangering implant fixation. Using finite element analysis (FEA) an investigation was made to find whether a press-fit acetabular implant with a polar clearance would reduce interfacial micromotions and improve fixation compared with a standard hemispherical design. In addition, the effects of interference fit, friction, and implant material were analysed. Cups were introduced into an FEA model of a human pelvis with simulated subchondral bone plasticity. The models were loaded with a loading configuration simulating two cycles of normal walking, during which contact stresses and interfacial micromotions were monitored. Subsequently, a lever-out simulation was performed to assess the fixation strength of the various cases. A flattened cup with good bone quality produced the lowest interfacial micromotions. Poor bone decreased the fixation strength regardless of the geometry of the cup. Increasing the interference fit of the flattened cup compensated for the loss of fixation strength caused by poor bone quality. In conclusion, a flattened cup did not significantly improve implant fixation over a hemispherical cup in the case of poor bone quality. However, implant fixation can be optimized by increasing interference fit and avoiding inferior frictional properties and low-stiffness implants.

Primary total hip arthroplasty with a flattened press-fit acetabular component in osteoarthritis and inflammatory arthritis: a prospective study on 416 hips with 6-10 years follow-up

INTRODUCTION

A flattened cup was designed to create a more physiological load transfer to the pelvic bone compared to hemispherical cups, and to allow more bone contact compared to low-profile' spherical cups. To investigate these theoretical advantages and the potential influence of the quality of the acetabular bone, a clinical study was performed in patients with osteoarthritis (OA) and inflammatory arthritis (IA). The aims of the study were (1) to evaluate the fixation of the cup, postoperatively and later when osseous integration should have taken place, (2) to assess perioperative complications such as acetabular fractures and (3) to monitor the polar gap, a potential risk factor for osteolysis.

PATIENTS AND METHODS

A prospective study was performed on all consecutive OA and IA patients with an indication for primary total hip arthroplasty (THA). Three hundred and nine OA patients (340 hips) and 65 IA patients (76 hips) were included. The acetabular component was the flattened press-fit EPF-PLUS cup, the femoral component the tapered cementless Zweymueller SL-PLUS stem. All revisions and complications were recorded. Clinical and radiographical evaluation was performed on regular basis during 6-10 years.

RESULTS

The incidence of early loosening of the cup was 0 out of 340 in the OA group and 1 out of 76 in the IA group. The incidence of acetabular fractures was 7 out of 340 in the OA group and 3 out of 76 in the IA group. Failure rate for the acetabular component due to aseptic loosening or osteolysis after 6-10 years was 0% in the OA group and 4.8% in the IA group. In all cases available for follow-up the polar gap had disappeared and full osseous integration had taken place in both the groups.

INTERPRETATION

This study shows that the flattened press-fit acetabular component creates adequate initial mechanical stability to allow osseous integration and that the cup can be safely used in both OA and IA patients. However, after 6-10 years, in the IA group failure of the cup due to aseptic loosening occurred once and failure due to osteolysis occurred three times, while these type of failures did not occur in the OA group.

Development and Validation of Patient-Specific Finite Element Models of the Hemipelvis Generated From a Sparse CT Data Set

To produce a patient-specific finite element (FE) model of a bone such as the pelvis, a complete computer tomographic (CT) or magnetic resonance imaging (MRI) geometric data set is desirable. However, most patient data are limited to a specific region of interest such as the acetabulum. We have overcome this problem by providing a hybrid method that is capable of generating accurate FE models from sparse patient data sets. In this paper, we have validated our technique with mechanical experiments. Three cadaveric embalmed pelves were strain gauged and used in mechanical experiments. FE models were generated from the CT scans of the pelves. Material properties for cancellous bone were obtained from the CT scans and assigned to the FE mesh using a spatially varying field embedded inside the mesh while other materials used in the model were obtained from the literature. Although our FE meshes have large elements, the spatially varying field allowed them to have location dependent inhomogeneous material properties. For each pelvis, five different FE meshes with a varying number of patient CT slices (8–12) were generated to determine how many patient CT slices are needed for good accuracy. All five mesh types showed good agreement between the model and experimental strains. Meshes generated with incomplete data sets showed very similar stress distributions to those obtained from the FE mesh generated with complete data sets. Our modeling approach provides an important step in advancing the application of FE models from the research environment to the clinical setting.

Bone ingrowth into a porous coated implant predicted by a mechano-regulatory tissue differentiation algorithm

Bone ingrowth into a porous surface is one of the primary methods for fixation of orthopaedic implants. Improved understanding of bone formation and fixation of these devices should improve their performance and longevity. In this study predictions of bone ingrowth into an implant porous coating were investigated using mechano-reculatory models. The mechano-regulatory tissue differentiation algorithm proposed by Lacroix et al., and a modified version that enforces a tissue differentiation pathway by transitioning from differentiation to bone adaptation were investigated. The modified algorithm resulted in nearly the same behavior as the original algorithm when applied to a fracture-healing model. The algorithms were further compared using micromechanical finite element model of a beaded porous scaffold. Predictions of bone and fibrous tissue formation were compared between the two algorithms and to clinically observed phenomena. Under loading conditions corresponding to a press-fit hip stem, the modified algorithm predicted bone ingrowth into approximately 25% of the pore space, which is similar to that reported in experimental studies, while the original algorithm was unstable. When micromotion at the bone-implant interface was simulated, 20 microm of transverse displacement resulted in soft tissue formation at the bone-implant interface and minimal bone ingrowth. In contrast, 10 and 5 microm of micromotion resulted in bone filling 40% of the pore space and a stable interface, again consistent with clinical and experimental observations.

The initial stability and contact mechanics of a press-fit resurfacing arthroplasty of the hip

Finite element analysis was used to examine the initial stability after hip resurfacing and the effect of the procedure on the contact mechanics at the articulating surfaces. Models were created with the components positioned anatomically and loaded physiologically through major muscle forces. Total micromovement of less than 10 μm was predicted for the press-fit acetabular components models, much below the 50 μm limit required to encourage osseointegration. Relatively high compressive acetabular and contact stresses were observed in these models. The press-fit procedure showed a moderate influence on the contact mechanics at the bearing surfaces, but produced marked deformation of the acetabular components. No edge contact was predicted for the acetabular components studied.

It is concluded that the frictional compressive stresses generated by the 1 mm to 2 mm interference-fit acetabular components, together with the minimal micromovement, would provide adequate stability for the implant, at least in the immediate post-operative situation.

Loss in mechanical contact of cementless acetabular prostheses due to post-operative weight bearing: A biomechanical model

The primary stability of cementless acetabular components is a prerequisite for their clinical success. The target of the present study was to analyse possible effects of post-operative joint loading on the initial mechanical stability of a press-fitted acetabular prosthesis. For this purpose, a three-dimensional finite element model of the pelvic bone with acetabular reconstruction was set-up. The analysis included two steps: (1) simulation of the prosthesis press-fit implantation and (2) simulation of the instant of peak resultant hip loading during the one-legged stance. The difference between the contact pressures at the bone/implant interface, at the end of the second step and those at the end of the first step was calculated and assumed as an index of variation in mechanical contact due to post-operative weight bearing. The results show that, due to hip loading, contact pressures given by press-fit increase in the postero-superior acetabular region but decrease in the antero-inferior acetabular region. The calculated area in which the contact pressures decrease extend to about 30% of the total contact surface. These results imply that post-operative joint loading significantly reduces the mechanical stability given by press-fit. The decrease in contact pressures at the bone/implant interface may result in a lack of osteointegration, possibly hindering the implant secondary stability. It may also create a route for wear debris, possibly favouring periprosthetic osteolysis, which may lead to further loss in contact and clinical failure of the implant due to loosening.

Development rationale for an articular surface replacement: a science-based evolution

Hip resurfacing has an enduring appeal because of the advantages of bone conservation and maximal joint stability. However, a far from satisfactory experience with earlier resurfacing designs led to its virtual disappearance in the 1980s. The concept was reintroduced in the late 1990s. The current generation of resurfacing devices generally consisted of a large-diameter metal-on-metal articulation, the femoral components being cemented and the acetabular components utilizing various forms of cementless fixation. The encouraging medium-term results, with a follow-up of up to 8 years using the current generation of surface replacement joints, combined with favourable reports related to long-term performance of some metal bearings have led to a rapid increase in the use of such components with these devices. This trend is most marked in younger, more active patients who have expectations of restoration of lifestyle in addition to improved mobility and pain relief and in whom failure with conventional total hip replacement is much higher than previously reported with more sedentary patients. The aim of this paper is, firstly, to highlight a number of areas of improvement and, secondly, to explain how these may be addressed by making modifications to the design of both implants and instrumentation and to the surgical technique. The areas identified for improvement were tissue preservation (thinner components, and reduced steps between sizes), acetabular cup issues (fixation, insertion, and positioning), femoral component issues (design, loading, and cementation), improved bearing surface characteristics, and simplified precise instrumentation with a low-trauma surgical technique.

Der acetabuläre Pressfit bei äquatorialer Beschichtung der zementfreien Hüftpfanne – eine finite-Elemente-Analyse Pressfit of equatorially roughened cementless acetabular components – a finite elements analysis

AIM

Does the pressfit anchorage of cementless acetabular cups depend on the roughness of the pole? To answer this question the primary pressfit of two cementless acetabular cups which differ only with regard to the roughness of their poles were compared by means of finite elements analysis.

MATERIALS AND METHODS

It was assumed that the material properties of bone are homogeneous, isotropic and linearly elastic. Material-specific values of cancellous bone with three different bone densities were used. Assumption of isotropy represents an approximation.

RESULTS

Comparison of the two prosthesis designs revealed that both designs/shapes cause similar patterns of bone deformation and tension.

CONCLUSIONS

It can therefore be concluded that with regard to pressfit anchorage the prosthesis with milled polar surface is according to FEA mechanically equivalent to the prosthesis with non-milled polar surface.

Deformation of press-fitted metallic resurfacing cups. Part 2: Finite element simulation

The deformation of metallic acetabular cups employed for metal-on-metal hip resurfacing procedures was considered theoretically using the finite element method in the present study, following on the experimental investigation reported in Part 1. Three representative cups, characterized by the cup wall thickness as thin, intermediate, and thick, were considered. For the intermediate cup, the effects of both the size and the diametral interference on the cup deformation were investigated. Both two-dimensional axisymmetric and three-dimensional finite element models were developed to examine the important parameters during and after the press-fit procedure, and in particular the deformation of the metallic cup. The theoretical prediction of the cup deformation was in reasonable agreement with the corresponding experimental measurement reported in Part 1. The most significant factor influencing the cup deformation was the cup wall thickness. Both the size and the diametral interference were also shown to influence the cup deformation. It is important to ensure that the cup deformation does not significantly affect the clearance designed and optimized for tribological performances of metal-on-metal hip resurfacing prostheses. Furthermore the contact parameters at the cup and bone interface associated with the press fit were also discussed.

Bone ingrowth simulation for a concept glenoid component design

Glenoid component loosening is the major problem of total shoulder arthroplasty. It is possible that uncemented component may be able to achieve superior fixation relative to cemented component. One option for uncemented glenoid is to use porous tantalum backing. Bone ingrowth into the porous backing requires a degree of stability to be achieved directly post-operatively. This paper investigates the feasibility of bone ingrowth with respect to the influence of primary fixation, elastic properties of the backing and friction at the bone prosthesis interface. Finite element models of three glenoid components with different primary fixation configurations are created. Bone ingrowth into the porous backing is modelled based on the magnitude of the relative interface micromotions and mechanoregulation of the mesenchymal stem cells that migrated via the bonded part of the interface. Primary fixation had the most influence on bone ingrowth. The simulation showed that its major role was not to firmly interlock the prosthesis, but rather provide such a distribution of load, that would result in reduction of the peak interface micromotions. Should primary fixation be provided, friction has a secondary importance with respect to bone ingrowth while the influence of stiffness was counter intuitive: a less stiff backing material inhibits bone ingrowth by higher interface micromotions and stimulation of fibrous tissue formation within the backing.

Numerical model to predict the longterm mechanical stability of cementless orthopaedic implants

The objective of this research was to develop a purely biomechanical model, intended to predict the long-term secondary stability of the implant starting from the biomechanical stability immediately after the operation. A continuous rule-based adaptation scheme was formulated as a dynamic system, and the work verified if such a model produced unique and clinically meaningful solutions. It also investigated whether this continuous model provided results comparable with those of a simpler, discrete-states model used in a previous study. The proposed model showed stable convergence behaviour with all investigated initial conditions, with oscillatory behaviour limited to the first steps of the simulation. The results obtained with the wide range of initial conditions support the hypothesis of the existence and uniqueness of the solution for all initial conditions. The differences between the continuous model and the simpler and more efficient finite-states model were found to be extremely modest (less than 4% over the predicted bonded area). Because of these minimal differences, the use of the much faster finite-states model is recommended to investigate asymptotic conditions, and the continuous model described should be used to investigate the evolution over time of the adaptive process.

Effect of the initial implant fitting on the predicted secondary stability of a cementless stem

A numerical model able to investigate the influence of biomechanical factors on the long-term secondary stability of implants would be extremely useful for the design of new cementless prosthetic devices. A purely biomechanical model of osseo-integration has been developed, formulated as a rule-based adaptation scheme. Due to its complexity, the problem was divided into three steps: preliminary implementation of the model (proof of concept); implementation of the complete model and investigation of the model solution; and model validation. The paper describes the first of these three steps. The model was implemented as a discrete-states machine, and the few parameters required were derived from the literature. It was then applied to a real clinical case. The study was conducted using the frictional contact finite element model of a human femur implanted with a cementless anatomical stem. A stable solution was achieved after between three and 15 iterations for all initial positions considered. Similar initial conditions yielded similar final configurations. The model predicted all initial configurations, with the exception of a partial osseo-integration, ranging between 62% (distal fit) and 78% (proximal fit) of the viable interface. This is in good agreement with the values reported in the literature that never exceed 75%, even in the best conditions, and report better clinical results for proximal fit. For the varus configuration, which lacks cortical support, the algorithm predicted a completed loosening.

Variation in surface texture measurements

Surface texture influences cellular response to implants, implant wear, and fixation, yet measurement and reporting of surface texture can be confusing and ambiguous. Seven specimens of widely different surface textures were submitted to three internationally renowned laboratories for surface texture characterization. The specimens were from dental implants, orthopedic implants, and femoral heads. Areas to be measured were clearly marked; simplified instructions were supplied but specific measurement parameters were not requested. Techniques used included contact profilometry, two- and three-dimensional laser profilometry, and atomic force microscopy. Four to thirteen parameters were reported, 2D or 3D, including R(a) or S(a); only three were common to all centers. The results varied by as much as +/-300-1000%, depending on technique and surface type. Some surfaces were not measurable by some techniques. One dental implant surface was reported with R(a) of 0.17, 0.85, 1.9, and 4.4 microm. The CoCr femoral head ranged from an R(a) of 0.011 to 0.10 microm; the zirconia head from 0.006 to 0.05 microm. Similar variability was reported for the other parameters. Useful surface texture characterization requires reporting of all measurement parameters. Comparisons between studies may be compromised if differences in technique are not considered.

The effect of saddle design on stresses in the perineum during cycling

PURPOSE

Repetitive internal stress in the perineum has been associated with soft-tissue trauma in bicyclists. Using an engineering approach, the purpose of this study was to quantify the amount of compression exerted in the perineum for a range of saddle widths and orientations.

METHODS

Computer tomography was used to create a three-dimensional voxel-based finite element model of the right side of the male perineum-pelvis. For the creation of the saddle model, a commercially available saddle was digitized and the surface manipulated to represent a variety of saddle widths and orientations. The two models were merged, and a static downward load of 189 N was applied to the model at the region representing the sacroiliac joint. For validation purposes, external stresses along the perineum-saddle interface were compared with the results of pressure sensitive film. Good agreement was found for these external stresses. The saddles were then stretched and rotated, and the magnitude and location of maximum stresses within the perineum were both recorded. In all cases, the model of the pelvis-perineum was held in an upright position.

RESULTS

Stresses within the perineum were reduced when the saddle was sufficiently wide to support both ischial tuberosities. This supporting mechanism was best achieved when the saddle was at least two times wider than the bi-ischial width of the cyclist. Stresses in the anterior of the perineum were reduced when the saddle was tilted downward, whereas stresses in the posterior were reduced when the saddle was tilted upward.

CONCLUSIONS

Recommendations that saddles should be sufficiently wide to support the ischial tuberosities appear to be well founded. Recommendations that saddles be tilted downward (i.e., nose down) are supported by the model, but with caution, given the limitations of the model.

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.

Finite element analysis of the initial stability of ankle arthrodesis with internal fixation: flat cut versus intact joint contours

OBJECTIVE

Qualitative comparison of the initial stability provided by two joint preparation techniques and various screw configurations in ankle arthrodesis, using the finite element method.Design. A three-dimensional model of a healthy ankle was developed from computed tomography images. Two groups of models were built, one with the joint contours resected to produce flat surfaces, and the second with the joint contours preserved. In each case, a variety of screw orientations were examined.

BACKGROUND

Despite the improved results of ankle arthrodesis, failure rates due to non-union are still reported. The initial stability of the arthrodesis construct seems important in the final outcome of the fusion.

METHODS

Non-linear contact finite element analyses were performed in the arthrodesis constructs subjected to internal/external torsion and dorsiflexion. Micromotions at the bone-to-bone interface were calculated for frictionless and Coulomb friction contact, and compared for the two joint preparation techniques and screw configurations.

RESULTS

Overall lower peak micromotions were predicted when preserving the joint contours both in torsion and dorsiflexion. For both preparation techniques, the lowest micromotions tended to occur with the screws inserted at 30 degrees with respect to the long axis of the tibia, crossing above the fusion site. Inclusion of friction in the models caused a general decrease on the magnitude of the micromotions as compared to the frictionless case, but did not affect the ranking of the models.

CONCLUSIONS

The finite element method can be used as a qualitative tool to study the initial stability of ankle arthrodesis, overcoming the difficulties of measuring bone-to-bone interface micromotions experimentally. Better initial stability was predicted for ankle arthrodesis when the joint contours were preserved rather than resected. Crossing the screws above the fusion site at a steeper angle also tended to increase the stability at the fusion site.

RELEVANCE

Finite element analyses can help during the pre-operative planning of ankle arthrodesis. When bone density is not compromised, preserving the joint contour and inserting the screws at less than 45 degrees to the long axis of the tibia, crossing over the arthrodesis site, may offer better initial stability.

Effects of acetabular resurfacing component material and fixation on the strain distribution in the pelvis

A 3D finite element (FE) model of an implanted pelvis was developed as part of a project investigating an all-polymer hip resurfacing design. The model was used to compare this novel design with a metal-on-metal design in current use and a metal-on-polymer design typical of early resurfacing implants. The model included forces representing the actions of 22 muscles as well as variable cancellous bone stiffness and variable cortical shell thickness. The hip joint reaction force was applied via contact modelled between the femoral and acetabular components of the resurfacing prosthesis. Five load cases representing time points through the gait cycle were analysed. The effect of varying fixation conditions was also investigated. The highest cancellous bone strain levels were found at mid-stance, not heel-strike. Remote from the acetabulum there was little effect of prosthesis material and fixation upon the von Mises stresses and maximum principal strains. Implant material appeared to have little effect upon cancellous bone strain failure with both bended and unbonded bone-implant interfaces. The unbonded implants increased stresses in the subchondral bone at the centre of the acetabulum and increased cancellous bone loading, resembling behaviour obtained previously for the intact acetabulum.