Can finite element models detect clinically inferior cemented hip implants?

A statistical approach to explore cemented total hip reconstruction performance

This study was carried out to determine mechanical behavior and bone adaptation of total hip arthroplasty (THA) subject to concentrated and distributed muscle loads and hip contact forces during activities of walking and stair climbing. Finite element modeling of THA with different prostheses, activity and loading types was developed by adopting a statistical method. Two levels of prostheses, activity, and loading types were selected for the study. 2³ factorial method was then pursued to design input and output data of finite element analysis. Maximum von Mises stresses were chosen to be output data on which statistical investigation was performed to investigate contribution and interaction of main factors on mechanical failure of cemented THA reconstructions by utilizing analysis of variance method (ANOVA). This study illustrated that the maximum von Mises stresses of THA showed considerable variation for main factors and their two-factor interactions.

Migration and strains induced by different designs of force-closed stems for THA

Objectives

Subtle differences in stem design can result in different mechanical responses of the total hip arthroplasty. Tests measuring migration of the stem relative to the femur, as well as the strains in the cement mantle and on the femur can detect different mechanical behavior between stems.

Methods

In this article, conical, double and triple tapered stems were implanted in composite femurs and subjected to static and cyclic loads. Stems differed mainly on taper angle, calcar radius and proximal stiffness. Stem migration and strains on the femur and in the cement mantle were achieved.

Results

Significant differences (p < 0.05) were noted in the permanent rotation between double and triple tapers, in the strains on the proximal medial femur between triple and both conical and double tapers, and in the strains on the lateral proximal femur between double tapers and both conical and triple tapers.

Conclusion

The proposed mechanical tests were able to detect significant differences in the behavior of these resembling stems. Stem proximal stiffness and the calcar radius of the stem influence its rotational stability and the strain transmission to the femur.

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.

Effects of densitometry, material mapping and load estimation uncertainties on the accuracy of patient-specific finite-element models of the scapula

Patient-specific biomechanical models including patient-specific finite-element (FE) models are considered potentially important tools for providing personalized healthcare to patients with musculoskeletal diseases. A multi-step procedure is often needed to generate a patient-specific FE model. As all involved steps are associated with certain levels of uncertainty, it is important to study how the uncertainties of individual components propagate to final simulation results. In this study, we considered a specific case of this problem where the uncertainties of the involved steps were known and the aim was to determine the uncertainty of the predicted strain distribution. The effects of uncertainties of three important components of patient-specific models, including bone density, musculoskeletal loads and the parameters of the material mapping relationship on the predicted strain distributions, were studied. It was found that the number of uncertain components and the level of their uncertainty determine the uncertainty of simulation results. The 'average' uncertainty values were found to be relatively small even for high levels of uncertainty in the components of the model. The 'maximum' uncertainty values were, however, quite high and occurred in the areas of the scapula that are of the greatest clinical relevance. In addition, the uncertainty of the simulation result was found to be dependent on the type of movement analysed, with abduction movements presenting consistently lower uncertainty values than flexion movements.

Thermally induced strains and total shrinkage of the polymethyl-methacrylate cement in simplified models of total hip arthroplasty

An evaluation of transient and stabilized strains in the cement mantle during polymerization was carried out in simplified cemented total hip arthroplasty (THA) model. A mathematical approach combined with a simple finite element simulation was used to compare measured and calculated stabilized strain values and to provide the Von Mises stresses at the stem/cement interface due to shrinkage related to temperature decrease after exothermal reaction. A second similar model was carried out to measure stem/cement/mold interfacial shear strength and dimensional changes of the cement mantle to obtain total shrinkage due to temperature decrease plus cement polymerization. The results indicated that positive strain peaks found during the exothermic stage of polymerization have the potential to produce pre-loading cracking. After the initial expansion, it was observed a progressive strain decrease pattern down to stabilized values that takes place near 2h after the cementation. Even though there is a great deal of dispersion in the measured stabilized strain values, in average those values match quite well with the numerical simulations, indicating 4,7 MPa von Mises interfacial stress due to thermal shrinkage. The total cement shrinkage leads to a negative radial stress of 11 MPa and 14 MPa von Mises interfacial stress. Finally, total shrinkage has the potential to enhance gaps in the cement/mold interface.

Using 'subcement' to simulate the long-term fatigue response of cemented femoral stems in a cadaver model: could a novel preclinical screening test have caught the Exeter matt problem?

Previously, cement was formulated with degraded fatigue properties (subcement) to simulate long-term fatigue in short-term cadaver tests. The present study determined the efficacy of subcement in a 'preclinical' test of a design change with known clinical consequences: the 'polished'-to-'matt' transition of the Exeter stem (revision rates for polished stems were twice those for matt stems). Contemporary stems were bead blasted to give Ra = 1 microm (matt finish). Matt and polished stems were compared in cadaver pairs under stair-climbing loads (three pairs of size 1; three pairs of size 3). Stem micromotion was monitored during loading. Post-test transverse sections were examined for cement damage. Cyclic retroversion decreased for polished stems but increased for matt stems (p < 0.0001). The implant size had a substantial effect; retroversion of (larger) size-3 stems was half that of size-1 stems, and polished size-3 stems subsided 2.5 times more than the others. Cement damage measures were similar and open through-cracks occurred around both stems of two pairs. Stem retroversion within the mantle resulted in stem-cement gaps of 50-150 microm. Combining information on cyclic motion, cracks, and gaps, it was concluded that this test 'predicted' higher revision rates for matt stems (it also implied that polished size-3 stems might be superior to size-1 stems).

Acetabular Component Deformation under Rim Loading Using Digital Image Correlation and Finite Element Methods

Total hip replacement is a highly successful operation; restoring function and reducing pain in arthritis patients. In recent years, thinner resurfacing acetabular cups have been introduced in order to preserve bone stock and reduce the risk of dislocation. However concerns have been raised that deformation of these cups could adversely affect the lubrication regime of the bearing; leading to equatorial and edge contact, possibly causing the implants to jam. This study aims to assess the amount of deformation which occurs due to the tight peripheral fit experienced during press-fit by applying rim loading to three different designs of acetabular cup: a clinically successful cobalt chrome resurfacing cup, a prototype composite resurfacing cup and a clinically successful polyethylene monobloc cup. Digital Image Correlation (DIC) was used to measure the deformation and to validate Finite Element (FE) models. DIC provided a non-contacting method to measure displacement; meaning the load could be increased continuously rather than in steps as in previous studies. The physical testing showed that the cobalt chrome cups were significantly stiffer than the composite prototype and polyethylene cups. The FE models were in good agreement with the experimental results for all three cups and were able to predict the deformation to within 10%. FE models were also created to investigate the effect of cup outside diameter and wall thickness on stiffness under rim loading. Increasing outside diameter resulted in a linear reduction in stiffness for all three materials. Increasing the wall thickness resulted in an exponential increase in cup stiffness. Rim loading an acetabular shell does not accurately simulate the in vivo conditions; however it does provide a simple method for comparing cups made of different materials.

Shear fatigue micromechanics of the cement-bone interface: An in vitro study using digital image correlation techniques

Loss of fixation at the cement-bone interface is known to contribute to aseptic loosening, but little is known about the mechanical damage response of this interface. An in vitro study using cement-bone specimens subjected to shear fatigue loading was performed, and the progression of stiffness changes and creep damage at the interface was measured using digital image correlation techniques. Stiffness changes and creep damage were localized to the contact interface between cement and bone. Interface creep damage followed a three-phase response with an initial rapid increase in creep, followed by a steady-state increase, concluding in a final rapid increase in creep. The initial creep phase was accompanied by an increase in interface stiffness, suggesting an initial locking-in effect at the interface. Interface stiffness decreased as creep damage progressed. Power law models were reasonably successful in describing the creep and stiffness damage response and were a function of loading magnitude, number of loading cycles, and contact area at the interface. More microcrack damage occurred to the cement when compared to the bone, and the damage was localized along the interface. These findings indicate that damage to the cement-bone interface could be minimized by improving cement-bone contact and by strengthening the fatigue resistance of the cement.

The importance of a thick cement mantle depends on stem geometry and stem—cement interfacial bonding

The thickness of the cement mantle around the femoral component of total hip replacements is a contributing factor to aseptic loosening and revision. Nevertheless, various designs of stems and surgical tooling lead to cement mantles of different thicknesses. Opinion is divided on whether a thick mantle enhances implant longevity. This study investigates the effect of cement mantle thickness on accumulated damage in the cement, and how this is influenced by the presence or absence of a proximal collar and on whether the stem—cement interface remains bonded. Three-dimensional finite element simulations incorporating creep and non-linear damage accumulation were performed to investigate cracking in the cement mantles around Stanmore Hips under physiologically informed stair-climbing and gait loads. Cement mantle thickness, stem—cement interfacial bonding, and collar design were varied to assess the interactive effects of these parameters. In all cases, damage levels were three to six times higher when the stem—cement interface remained bonded. Cement mantle thickness had little effect on cement damage accumulation around debonded collared stems but was critical in both bonded and collarless cases, where a thicker mantle reduced cement cracking. Damage around a smooth debonded stem with a collar is thus much less sensitive to cement thickness than around bonded or collarless stems.

A modified PMMA cement (Sub-cement) for accelerated fatigue testing of cemented implant constructs using cadaveric bone

Pre-clinical screening of cemented implant systems could be improved by modeling the longer-term response of the implant/cement/bone construct to cyclic loading. We formulated bone cement with degraded fatigue fracture properties (Sub-cement) such that long-term fatigue could be simulated in short-term cadaver tests. Sub-cement was made by adding a chain-transfer agent to standard polymethylmethacrylate (PMMA) cement. This reduced the molecular weight of the inter-bead matrix without changing reaction-rate or handling characteristics. Static mechanical properties were approximately equivalent to normal cement. Over a physiologically reasonable range of stress-intensity factor, fatigue crack propagation rates for Sub-cement were higher by a factor of 25+/-19. When tested in a simplified 2 1/2-D physical model of a stem-cement-bone system, crack growth from the stem was accelerated by a factor of 100. Sub-cement accelerated both crack initiation and growth rate. Sub-cement is now being evaluated in full stem/cement/femur models.

Small stem total hip arthroplasty in hypoplasia of the femur

Total hip arthroplasty in hypoplastic femurs is technically difficult and the incidence of complications and aseptic loosening is relatively high. Cemented, uncemented, off-the-shelf, and custom-made stems all have been advocated in these cases. From 1978 to 1997, we performed 86 total hip arthroplasties in 77 patients with a hypoplastic femur using a cemented, off-the-shelf, small, curved, cobalt-chromium stem. We hypothesized results equaled those of the identical but larger-sized stems in normal-sized femora which were used as comparisons. Clinical and radiographic evaluations were performed. Minimum followup was 4.2 years (mean, 12 years; range, 4.2-20.3 years); mean Harris hip score was 88, and mean hip flexion was 104 degrees . Six stems were revised: four because of aseptic loosening, one after a femoral fracture, and one because of malpositioning. Complications included one perforation and one fracture of the femur, one fracture, one nonunion of the greater trochanter, and one deep infection. Implant survivorship for all hips at 15 years with aseptic revision of the stem as the end point was 90% (confidence interval, 82-99) which equaled results of the larger stems. The small off-the-shelf cemented Weber stem has a high long-term survival and a low complication rate. Survival compares favorably with other small-sized total hip systems.

LEVEL OF EVIDENCE

Level III, therapeutic study, case-control.

Predicting revision risk for aseptic loosening of femoral components in total hip arthroplasty in individual patients--a finite element study

Advances in surgical procedure, prosthesis design, and biomaterials performance have considerably increased the longevity of total joint replacements. Preoperative planning is another step in joint replacement that may have the potential to improve clinical outcome for the individual patient, but has remained relatively consistent for a long time. One means of advancing this aspect of joint replacement surgery may be to include predictive computer simulation into the planning process. In this article, the potential of patient-specific finite element analysis in preoperative assessment is investigated. Seventeen patient-specific finite element models of cemented Charnley reconstructions were created, of which six were early (<10 years) revisions. Creep was simulated using a Maxwell model, and fatigue damage was simulated using an anisotropic continuum damage formulation. Account was taken of the relationship between annual loading cycles and age, and stair-climbing loads were included using a walking to stair-climbing cycle ratio of 9:1. Simulations for the equivalent of 10 years of loading were performed. Accumulated damage, inducible displacement, and migration were computed. Five of the six early revisions had the highest migration indicating that migration could have been used to identify early failures of these prostheses. Resultant migration showed the most significant difference between the early revised and unrevised groups (p = 0.0024). Furthermore, this trend was apparent from 1 year postimplantation (p = 0.0052). This ability to differentiate early revisions shows that computational simulation of aseptic loosening in cemented prostheses could prove clinically useful in helping surgeons optimize the preoperative plan for individual patients.

Stem-cement porosity may explain early loosening of cemented femoral hip components: experimental-computational in vitro study

A combination of laboratory experiment and computational simulation was performed to assess the role of interface porosity on stem migration. The early motion of in vitro prepared cemented femoral components was measured during application of cyclic stair climbing loads. Following testing, transverse sections were obtained and the distribution of pores at the stem-cement interface was determined. Finite element models of cemented stem constructs were developed and a scheme was implemented to randomly assign pores to the stem-cement interface. For a series of 14 in vitro prepared components, pore fractions at the stem-cement interface ranged from 23% to 67%. The majority of pores at the stem-cement interface were less than 1 mm in length with a mean length of 1.27 +/- 2.7 mm and thickness of 0.12 +/- 0.11 mm. For stems with large pore fractions, pores tended to coalesce in longer extended gaps over the stem surface. Finite element and experimental models both revealed strong positive correlations (r(2) = 0.55-0.72; p < 0.0001) between stem-cement pore fraction and stem internal rotation, suggesting that the presence and extent of pores could explain the early motion of the stems. There was an increased volume of cement at risk of fatigue failure with increasing stem migration. Pore fractions greater than 30% resulted in large increases in stem internal rotation, suggesting that attempts to maintain surface porosity at or below this level may be desirable to minimize the risk of clinical loosening.

Finite element-based preclinical testing of cemented total hip implants

We developed a finite element model to preclinically test cemented hip implants for damage accumulation, including cement crack formation, creep, and stem migration. Using this model, we simulated the mechanical failure processes of four cemented total hip arthroplasty implants (Lubinus SPII, Mueller Curved, Exeter and Charnley, all with known clinical results) during cyclic normal walking and stair-climbing loads. These four implants were selected to ascertain whether the simulation predicted greater damage development around clinically inferior stems, whether clinically inferior designs could be identified by an initial stress analysis without the prediction of cement damage, and whether the simulation could predict high implant subsidence rates in combination with minimal cement damage. Based on the predicted cement crack patterns and crack formation rates, the simulation correctly identified the clinically inferior implant designs. Based only on the initial stress analysis under a stair-climbing load, it was not possible to identify clinically inferior designs. High subsidence values and minimal cement damage were predicted for the Exeter implant, similar to clinical findings. Our findings suggest the simulation may be effective in differentiating between a range of implants and design features.

Cement mantle fatigue failure in total hip replacement: Experimental and computational testing

One possible loosening mechanism of the femoral component in total hip replacement is fatigue cracking of the cement mantle. A computational method capable of simulating this process may therefore be a useful tool in the preclinical evaluation of prospective implants. In this study, we investigated the ability of a computational method to predict fatigue cracking in experimental models of the implanted femur construct. Experimental specimens were fabricated such that cement mantle visualisation was possible throughout the test. Two different implant surface finishes were considered: grit blasted and polished. Loading was applied to represent level gait for two million cycles. Computational (finite element) models were generated to the same geometry as the experimental specimens, with residual stress and porosity simulated in the cement mantle. Cement fatigue and creep were modelled over a simulated two million cycles. For the polished stem surface finish, the predicted fracture locations in the finite element models closely matched those on the experimental specimens, and the recorded stem displacements were also comparable. For the grit blasted stem surface finish, no cement mantle fractures were predicted by the computational method, which was again in agreement with the experimental results. It was concluded that the computational method was capable of predicting cement mantle fracture and subsequent stem displacement for the structure considered.

The design features of cemented femoral hip implants

We undertook a review of the literature relating to the two basic stem designs in use in cemented hip replacement, namely loaded tapers or force-closed femoral stems, and the composite beam or shape-closed designs. The associated stem fixation theory as understood from in vitro studies and finite element modelling were examined with reference to the survivorship results for each of the concepts of fixation.

It is clear that both design principles are capable of producing successful long-term results, providing that their specific requirements of stem metallurgy, shape and surface finish, preparation of the bone and handling of the cement are observed.

Probabilistic analysis of the influence of the bonding degree of the stem–cement interface in the performance of cemented hip prostheses

The long-term behavior of the stem-cement interface is one of the most frequent topics of discussion in the design of cemented total hip replacements, especially with regards to the process of damage accumulation in the cement layer. This effect is analyzed here comparing two different situations of the interface: completely bonded and debonded with friction. This comparative analysis is performed using a probabilistic computational approach that considers the variability and uncertainty of determinant factors that directly compromise the damage accumulation in the cement mantle. This stochastic technique is based on the combination of probabilistic finite elements (PFEM) and a cumulative damage approach known as B-model. Three random variables were considered: muscle and joint contact forces at the hip (both for walking and stair climbing), cement damage and fatigue properties of the cement. The results predicted that the regions with higher failure probability in the bulk cement are completely different depending on the stem-cement interface characteristics. In a bonded interface, critical sites appeared at the distal and medial parts of the cement, while for debonded interfaces, the critical regions were found distally and proximally. In bonded interfaces, the failure probability was higher than in debonded ones. The same conclusion may be established for stair climbing in comparison with walking activity.

Finite-element analysis of failure of the Capital Hip designs

The Capital Hip implant was a Charnley-based system which included a flanged and a roundback stem, both of which were available in stainless steel and titanium. The system was withdrawn from the market because of its inferior performance. However, all four of the designs did not produce poor rates of survival. Using a simulated-based, finite-element analysis, we have analysed the Capital Hip system. Our aim was to investigate whether our simulation was able to detect differences which could account for the varying survival between the Capital Hip designs, thereby further validating the simulation.

We created finite-element models of reconstructions with the flanged and roundback Capital Hips. A loading history was applied representing normal walking and stair-climbing, while we monitored the formation of fatigue cracks in the cement.

Corresponding to the clinical findings, our simulation was able to detect the negative effects of the titanium material and the flanged design in the Capital Hip system. Although improvements could be made by including the effect of the roughness of the surface of the stem, our study increased the value of the model as a predictive tool for determining failure of an implant.

A fatigue damage model for the cement-bone interface

Loss of fixation at the cement-bone interface can contribute to clinical loosening of cemented total hip replacements. In this study, the fatigue damage response was determined for cement-bone constructs subjected to shear fatigue loading. A typical three-phase fatigue response was observed with substantial early damage, followed by a long constant damage rate region and a final abrupt increase in damage to fracture. All of the damage resulted from creep (permanent) deformation during fatigue loading and there was no loss in cyclic stiffness. Using a Von Mises equivalent stress/strain concept, a general damage model was developed to describe the fatigue creep response of the cement-bone interface under either shear or tensile fatigue loading. Time to failure was highly correlated (r2=0.971) with equivalent creep strain rate and moderately related (r2=0.428) with equivalent initial strain for the two loading regimes. The equivalent creep strain at failure (0.052+/-0.018) was found to be independent of the applied equivalent stress. A combination of the creep damage model (to describe the damage process) with a constant final equivalent strain (as a failure criteria) could be used to assess the cement-bone failure response of cemented implant systems.

Why would cement porosity reduction be clinically irrelevant, while experimental data show the contrary

Laboratory bench tests have shown that porosity reduction increases the fatigue life of bone cement specimens. Clinically, however, the effect porosity reduction is subject to debate. We hypothesized that the discrepancy between clinical and experimental findings is related to differences in the stress distribution, which is typically uniform in experimental test specimens, while stress concentrations exist in cement around hip implants. We simulated fatigue failure of cement in a finite element model of an experimental test specimen and of a transverse slice of a total hip arthroplasty with a sharp-cornered stem. Four levels of porosity were introduced. In the fatigue test specimen model, the fatigue life clearly was dependent on the level of porosity, while in the transverse slice model, the level of porosity had virtually no effect on failure of the cement mantle. The results of the simulations confirmed our hypothesis. In simulations of laboratory tests, pores clearly acted as crack initiators, while in the simulation of a real total hip reconstruction, crack formation was governed by local stress singularities. This explains why the beneficial effect of cement porosity reduction on the lifetime of total hip reconstructions may be hard to detect clinically.

The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups--finite element simulations and experimental tests

BACKGROUND

Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups.

METHODS

Acetabular cups with different geometries (Link: IP and Lubinus eccentric) were tested cyclically in a simulator at 45 degrees and 60 degrees abduction angles. Finite element (FE) meshes were generated and two additional designs were reconstructed to test the effects of the cup clearance and thickness. Contact pressures at cup-head and cup-cement interfaces were calculated as a function of loading force at 45 degrees, 60 degrees and 80 degrees abduction angles.

RESULTS

At the cup-head interface, IP experienced lower contact pressures than the Lubinus eccentric at low loading forces. However, at higher loading forces, much higher contact pressures were produced on the surface of IP cup. An increase in the abduction angle increased contact pressure in the IP model, but this did not occur to any major extent with the Lubinus eccentric model. At the cup-cement interface, IP experienced lower contact pressures. Increased clearance between cup and head increased contact pressure both at cup-head and cup-cement interfaces, whereas a decreased thickness of polyethylene layer increased contact pressure only at the cup-cement interface. FE results were consistent with experimental tests and acetabular cup deformations.

CONCLUSION

FE analyses showed that geometrical design, thickness and abduction angle of the acetabular cup, as well as the clearance between the cup and head do change significantly the mechanical stresses experienced by a cemented UHMWPE acetabular cup. These factors should be taken into account in future development of THR prostheses. FE technique is a useful tool with which to address these issues.

Determination of muscle loading at the hip joint for use in pre-clinical testing

The stability of joint endoprostheses depends on the loading conditions to which the implant-bone complex is exposed. Due to a lack of appropriate muscle force data, less complex loading conditions tend to be considered in vitro. The goal of this study was to develop a load profile that better simulates the in vivo loading conditions of a "typical" total hip replacement patient and considers the interdependence of muscle and joint forces. The development of the load profile was based on a computer model of the lower extremities that has been validated against in vivo data. This model was simplified by grouping functionally similar hip muscles. Muscle and joint contact forces were computed for an average data set of up to four patients throughout walking and stair climbing. The calculated hip contact forces were compared to the average of the in vivo measured forces. The final derived load profile included the forces of up to four muscles at the instances of maximum in vivo hip joint loading during both walking and stair climbing. The hip contact forces differed by less than 10% from the peak in vivo value for a "typical" patient. The derived load profile presented here is the first that is based on validated musculoskeletal analyses and seems achievable in an in vitro test set-up. It should therefore form the basis for further standardisation of pre-clinical testing by providing a more realistic approximation of physiological loading conditions.

Preclinical Testing of Femoral Hip Components: An Experimental Investigation With Four Prostheses

Existing standards for the preclinical testing of femoral hip implants have been successful in the objective of guaranteeing the implant’s fatigue strength. There is a need for an experimental test which could ensure prostheses were not susceptible to aseptic loosening. In this study we measure the relative movement between the prosthesis and the bone of four different cemented femoral component designs in in vitro tests. The aim is to determine if differences can be distinguished and whether the differences correlate with clinical performance. The four designs are the Charnley (DePuy International Ltd., UK), the Exeter (Stryker Osteonics Howmedica Corp., USA), the Lubinus SPII (Waldemar-Link GmbH, Germany), and the Müller Curved (JRI Ltd, UK). Five tests were carried out for each femoral component type, giving a total of 20 tests, and their permanent relative displacement (termed migration) and recoverable (i.e., elastic) relative displacement (termed inducible displacement) monitored over one million loading cycles. Considerable variation occurred in the tests. Nonetheless, most femoral components migrated medially, posteriorly, and distally. Most also rotated into varus. Translations of the Charnley (64microns) and Lubinus (67microns) implants were less than the Müller (72microns) and Exeter (94microns) implants, but this difference is not statistically significant. Most of the femoral components had rapid early migration followed by slower steady-state migration. With regard to the steady state inducible displacements of the prostheses, those of the Charnley, Exeter, and Lubinus decreased or were stable with respect to time, whilst those of the Müller typically increased with respect to time. It is concluded that migration is not a suitable basis for in vitro comparison of prosthesis designs. However, inducible displacement trends provide a clinically comparable performance ranking.

On the Importance of Considering Porosity When Simulating the Fatigue of Bone Cement

Fatigue cracking in the cement mantle of total hip replacement has been identified as a possible cause of implant loosening. Retrieval studies and in vitro tests have found porosity in the cement may facilitate fatigue cracking of the mantle. The fatigue process has been simulated computationally using a finite element/continuum damage mechanics (FE/CDM) method and used as a preclinical testing tool, but has not considered the effects of porosity. In this study, experimental tensile and four-point bend fatigue tests were performed. The tensile fatigue S-N data were used to drive the computational simulation (FE/CDM) of fatigue in finite element models of the tensile and four-point bend specimens. Porosity was simulated in the finite element models according to the theory of elasticity and using Monte Carlo methods. The computational fatigue simulations generated variability in the fatigue life at any given stress level, due to each model having a unique porosity distribution. The fracture site also varied between specimens. Experimental validation was achieved for four-point bend loading, but only when porosity was included. This demonstrates that the computational simulation of fatigue, driven by uniaxial S-N data can be used to simulate nonuniaxial loadcases. Further simulations of bone cement fatigue should include porosity to better represent the realities of experimental models.

The contradictory effects of pores on fatigue cracking of bone cement

The beneficial effect of porosity reduction on the fatigue life of bone cement has been demonstrated in numerous experimental studies. Clinically, however, it seems that the beneficial effect of porosity reduction of cement around total hip replacement components can only be found in large follow-up studies. Little is known about the actual mechanical effect of a pore on fatigue crack formation in cement mantles. We studied the effect of pores on the crack formation process in a finite element model of a transverse slice of a total hip reconstruction. We created models with a single large pore and models with multiple pores at levels of 2, 4, and 9%. The models were cyclically torque-loaded, causing macrocracks to appear in the cement mantle. In all models, we found that pores acted as microcrack initiators. However, pores could have both a detrimental and a beneficial effect on the macrocrack propagation in the cement mantle. Both effects were seen in the models with a single large pore and in the models with multiple pores. Pores would either accelerate, deviate, or decelerate the macrocrack propagation in the cement mantle. The effect of the pores depended on the location of the pores with respect to the stress intensities in the model, but was independent of the pore size or the level of porosity. The results may explain why the beneficial effect of vacuum mixing is difficult to demonstrate clinically. Stress intensities that are present in a cement mantle in an in vivo situation may overshadow the detrimental effect of a pore, while the beneficial effect may become more pronounced.

Finite element analysis of three designs of an implant-supported molar crown

STATEMENT OF PROBLEM

The optimal method of implant support for a single mandibular molar crown is controversial because commonly used, threaded, root-form implants developed by Branemark were not originally designed to support individual crowns.

PURPOSE

The purpose of this study was to develop a finite element model of a single mandibular first molar crown supported by (1) a standard 3.75-mm-diameter implant, (2) a 5-mm, wide-diameter implant, and (3) double standard-diameter implants, and to compare the induced displacements as a result of various loading conditions.

MATERIAL AND METHODS

Three-dimensional finite element models were made to simulate the 3 single-molar implant designs. Each model was analyzed with 2 force magnitudes (35 N and 70 N) and with 2 force directions (vertical and 15 degrees to the vertical axis). Displacements in 3-dimensional space as a result of the simulated loading conditions were evaluated along 3 primary axes, mesiodistal, faciolingual, and superior-inferior.

RESULTS

Mesiodistal and buccolingual displacements for the crown supported by the 5-mm-diameter implant were reduced by approximately 50% compared with the crown supported by the 3.75-mm implant when the crowns were loaded at the distobuccal cusp tip or the distal marginal ridge. The double-implant design recorded the least mesiodistal displacement with off-center loading of the crown.

CONCLUSION

When the crown was loaded off-center, the double-implant design produced substantially less displacement when compared with either of the single-implant designs.

A conical-collared intramedullary stem can improve stress transfer and limit micromotion

OBJECTIVE

The objective of this study was to quantify the effect of collar geometry on stress transfer and micromotion in idealized models of a cementless implant having an intramedullary stem.

BACKGROUND

Intramedullary stems exist on several types of orthopaedic implants, including the femoral component of hip arthroplasties and segmental replacements used in the surgical treatment of a tumor or trauma in the diaphysis of a long bone.

METHODS

Using three-dimensional finite element analysis, we compared four idealized, straight-stemmed, axisymmetric prostheses: flat-collared (0 degrees), conical-collared (30 degrees and 60 degrees), and collarless tapered (80 degrees). We simulated axial and non-axial (20 degrees oblique) loads as well as non-ingrown and ingrown interface conditions.

RESULTS

Without bone ingrowth, stress transfer to bone adjacent to the collar increased with collar angle. Micromotion at the distal stem increased moderately with collar angle from 0 degrees through 60 degrees, then increased markedly from 60 degrees to 80 degrees. With simulated bony ingrowth, the effect of the collar was greatly reduced.

CONCLUSIONS

The results of this study suggest that the selection of collar angle represents a tradeoff between initial stress transfer and micromotion. Stems with conical collar angles in the range of 30-60 degrees can provide increased stress transfer compared to a flat collar design and reduced micromotion compared to a collarless tapered design.

Prevention of mesh-dependent damage growth in finite element simulations of crack formation in acrylic bone cement

Peak stress levels predicted in finite element analysis (FEA) usually depend on mesh density, due to singular points in the model. In an earlier study, an FEA algorithm was developed to simulate the damage accumulation process in the cement mantle around total hip replacement (THR) implants. It allows cement crack formation to be predicted, as a function of the local cement stress levels. As the simulation is driven by mesh-dependent peak stresses, predicted crack formation rates are also likely to be mesh dependent. The aim of this study was to evaluate the mesh dependence of the predicted crack formation process, and to present a method to reduce the mesh dependence. Crack-propagation experiments were simulated. Experimental specimens, representing transverse slices of cemented THR reconstructions, were subjected to cyclic torsional loading. Crack development around the corners of the stem was monitored. The experiments were simulated using three meshes with increasing levels of mesh refinement. Crack locations and orientations were accurately predicted, and were virtually independent of the level of mesh refinement. However, the experimental crack propagation rates were overestimated considerably, increasing with mesh refinement. To eliminate the effect of stress singularities around the corners of the stem, a stress averaging algorithm was applied in the simulation. This algorithm redistributed the stresses by weighted spatial averaging. When damage accumulation was computed based on averaged stresses, the crack propagation rates predicted were independent of the level of mesh refinement. The critical distance, a parameter governing the effect of the averaging algorithm, was optimized such that the predicted crack propagation rates accurately corresponded to the experimental ones. These results are important for the validity and standardization of pre-clinical testing methods for orthopaedic implants.