In cemented total hip arthroplasty, the cement-bone interface can be considerably degraded in less than one year in-vivo service (Figure 1). This makes the interface much weaker relative to the direct post-operative situation. Retrieval studies show that patients do, to a certain extent, not suffer from the degraded cement-bone interface itself. It is, however, unknown whether the degraded cement-bone interface affects other failure mechanisms in the cemented hip reconstruction. A good understanding of the mechanics of the cement-bone interface is therefore essential. The aim of this study was to investigate the mechanics of the cement-bone interface in the direct post-operative and degraded situation by the utilization of finite element analysis (FEA) and laboratory experiments. It was subsequently analyzed how the mechanics of the cement-bone interface affect failure of the cement mantle in terms of crack formation. In order to investigate the mechanical response of the cement-bone interface, laboratory prepared (direct post-operative state) and postmortem (degraded state) specimens were loaded in various directions in the laboratory and FEA environment. From all specimens, multiple interface morphology parameters were documented, which were related to the interfacial response and subsequently converted to a numerical cohesive model. As a validation, this cohesive model was implemented into two FEA models of transverse sections of cemented hip reconstructions with distinct mechanical characteristics (Figure 2). Finally, the differences in fatigue crack formation in a complete hip reconstruction were determined by varying the cement-bone interface compliance (Figure 3). When loaded in multiple directions, the interface compliance could not be related to the cement interdigitation depth (r 2 =0.08). However, compliance did correlate to the gap thickness between the bone and cement (r 2 =0.81) and the amount of interfacial contact (r 2 =0.50). Surprisingly, for the same amount of contact, the interface was more compliant in degraded state than in the direct post-operative state. The mechanical response of the experimental and FEA cement-bone interface tests could, independent on the direct post-operative or degraded state, successfully be described by a cohesive model. The cohesive model was even more confirmed by the successful reproduction of the mechanics of the retrieved transverse sections. When the cohesive model was implemented in a complete reconstruction, we found that a compliant cement-bone interface resulted in considerably more fatigue cracks in the cement mantle than a very stiff interface. This study showed that an increased compliancy of the cement-bone interface results in an increase of cement cracks in the cement mantle. It is therefore crucial to minimize the interfacial gaps and, as a result, increase the amount of contact between the bone and cement to generate a stiff cement-bone interface. It is, unfortunately, unknown how this well fixed interface can be maintained. We finally conclude that the derived cohesive model of the cement-bone interface can be used for multiple applications in orthopaedics, including pre-clinical of implants and patient specific studies of failed cemented reconstructions.

A subject, who had undergone surgery to replace one hip joint and the proximal half of the femur with an instrumented titanium implant, performed brief exercises whilst simultaneous measurements were made of compressive axial force in the implant using short-range wireless telemetry, ground reactions using a Kistler force plate, and electromyographic activity of the vastus lateralis (VL) and erector spinae (ES) muscles using surface electrodes. Recordings were made barefoot and wearing ‘trainers’. The exercises (slow jumping in counter movement style, fast continuous jumping, and jogging on the spot) have been found effective in controlled interventions for increasing bone mineral density in women. The implant forces were 250–400% BW. The values were about twice the magnitude of the ground reaction forces and significantly correlated with them for both peak force and its rate of rise but their realtive magnitudes varied depending on mode of activity (jumping or jogging). Implant forces were significantly related to the muscle activity; in multiple regression analysis implant forces during take off from slow jumps VL contributed significantly in addition to the ground reaction (98% total explained variance). There was more activity in VL during jumping than jogging for the same implant force which may explain why jumping appears to be more osteogenic than jogging for the femur. For the same ground reaction, wearing trainers increased both the magnitude of the compressive loading of the femur and its rate of rise.

In finite element analysis (FEA) models of cemented hip reconstructions, it is crucial to include the cement-bone interface mechanics. Recently, a micromechanical cohesive model was generated which reproduces the behavior of the cement-bone interface. The goal was to investigate whether this cohesive model was directly applicable on a macro level. From transverse sections of retrieved cemented hip reconstructions, two FEA-models were generated. The cement-bone interface was modeled with cohesive elements. A torque was applied and the cement-bone interface micromotions, global stiffness and stem translation were monitored. A sensitivity analysis was performed to investigate whether the cohesive model could be improved. All results were compared with experimental findings. That the original cohesive model resulted in a too compliant macromechanical response; the motions were too large and the global stiffness too small. When the cohesive model was modified, the match with the experimental response improved considerably.

We read the paper by Tarala et al., which was recently published in this Journal. We believe the authors addressed a very important issue. In fact, we agree that artifacts are induced by bone deformation when implant–bone micromotion is measured at points that are far from each other, and more, in general we agree on the importance of assessing the precision of experimental measurements (as well as of numerical simulations). We would like to comment on some points. First of all, when quoting some of our previous studies they state ‘a single LVDT implant–bone relative motion measurement system is mounted transcortically by means of an anchorage set-up; the motion is measured between the pin connected to the stem and the linear variable differential transformers (LVDTs) support attached to the bone surface’. This is partly true for one of the two quoted papers (even in this paper three linear variable differential transformers (LVDTs) and one extensometer were used, not just ‘a single LVDT’). This statement is inaccurate for the other paper: in fact, as shown in that paper, the reference frame of each of the four LVDTs was connected to the bone close (500micron) to the stem–bone interface (as opposed to the bone surface) by means of a tiny sleeve inserted onto the transcortical holes (Figure 1). We used the same type of fixation for the LVDTs in a number of later studies on cemented and cementless stems, many of which are published in this Journal, which Tarala et al. did not mention. Therefore, the statement that ‘None of these experimental methods allow for micromotion measurement at the actual implant–bone interface’ seems unjustified. Second, referring to the errors induced by elastic deformation of bone, Tarala et al. state ‘It is not possible to assess the magnitude of these errors with the currently available experimental methods’. This is incorrect. In fact, in a study that they quoted we measured experimentally the relative motion of different points across the thickness of cortical wall. The anchorage of the LVDTs was fixed to the external surface of the bone, while the pin attached to the LVDT probe sensed the shear motions corresponding to the elastic strains at three controlled depths across the cortex (500micron below the external surface, 500micron above the bone–stem interface, and midway between these two levels). The LVDTs we used had an intrinsic precision of 1micron, and the entire measurement system had an overall precision of better than 2.3micron. The relative motions between a point on the external bone surface and the point close (500micron) to the stem–bone interface did not exceed 5micron in any of the bone areas where such artifact was assessed. Furthermore, based on the micromotions measured at three depths, it can be seen that such relative motion varied, more or less linearly, across the thickness of the cortical wall. Similarly, in a latter study a different group investigated this type of error combining in vitro measurements with a validated FE model. Third, Tarala et al. aim at assessing the error of experimental measurements. As a general rule, in order to measure the uncertainty of a measurement system it is necessary to use something that is more precise than the system under investigation. However, the precision of their finite element (FE) models is questionable:

New technologies, such as selective electron beam melting, allow to create complex interface structures to enhance bone ingrowth in cementless implants. The efficacy of such structures can be tested in animal experiments. Although animal studies provide insight into the biological response of new structures, it remains unclear how ingrowth depth is related to interface strength. Theoretically, there could be a threshold of ingrowth, above which the interface strength does not further increase. To test the relationship between depth and strength we performed a finite element study on micro models with simulated uncoated and hydroxyapatite (HA) coated surfaces. We examined whether complete ingrowth is necessary to obtain a maximal interface strength. An increase in bone ingrowth depth did not always enhance the bone–implant interface strength. For the uncoated specimens a plateau was reached at 1,500 μm of ingrowth depth. For the specimens with a simulated HA coating, a bone ingrowth depth of 500 μm already yielded a substantial interface strength, and deeper ingrowth did not enhance the interface strength considerably. These findings may assist in optimizing interface morphology (its depth) and in judging the effect of bone ingrowth depth on interface strength.

In cemented total hip arthroplasty, the cement-bone interface can be considerably degenerated after less than one year in vivo service; this makes the interface much weaker relative to the direct post-operative situation. It is, however, still unknown how these degenerated interfaces behave under mixed-mode loading and how this is related to the interface morphology. In this study, we used a finite element (FE) approach to analyze the mixed-mode response of the cement-bone interface taken from postmortem retrievals. We investigated whether it was feasible to generate a fully elastic and a failure cohesive model based on only morphological input parameters.Computed tomography-based FE-models of postmortem cement-bone interfaces were generated and the interface morphology was determined. The models were loaded until failure in multiple directions by allowing cracking of the bone and cement components and including periodic boundary conditions. The resulting stiffness was related to the interface morphology. A closed form mixed-mode cohesive model that included failure was determined and related to the interface morphology.The responses of the FE-simulations compare satisfactorily with experimental observations, albeit the magnitude of the strength and stiffness are somewhat overestimated. Surprisingly, the FE-simulations predict no failure under shear loading and a considerable normal compression is generated which prevents dilation of the interface. The obtained mixed-mode stiffness response could subsequently be related to the interface morphology and subsequently be formulated into an elastic cohesive zone model. Finally, the acquired data could be used as an input for a cohesive model that also includes interface failure. (C) 2011 Elsevier Ltd. All rights reserved.

There is continued interest in the maintenance and functional response of the cement-bone interface to loads that act across the interface. Recent experimental work has found that the interface is quite compliant (~10 microns/MPa) with sliding and opening occurring from shear and tensile loading conditions, respectively. In addition, this interface becomes more compliant following in vivo service (~50 microns/MPa). The question remains as to whether this micromotion is loading angle dependent. To address this question we performed multiloading angle experiments on cement-bone interface specimens. We asked three research questions: (1) does interface compliance depend on loading angle?, (2) are there appreciable coupled transverse motions?, and (3) can interface compliance be explained by contact fraction between cement and bone and source of bone (lab prepared or postmortem retrieval)?

The stability of cemented hip implants relies on the fixation of the cement mantle within the bone cavity. This fixation has been investigated in experiments with cement-bone interface specimens, which have shown that the cement-bone interface is much more compliant than is commonly assumed. Other studies demonstrated that the mechanical response of the interface is dependent on penetration of the cement into the bone. It is, however, unclear how cement penetration exactly affects the stiffness and strength of the cement-bone interface. We therefore used finite element (FE) models of cement-bone specimens to study the effect of cement penetration depth on the micromechanical behavior of the interface. The FE models were created based on micro computed tomography (micro CT) data of two small cement-bone interface specimens (8x8x4 mm). The specimens had distinct differences with respect to interface morphology. In these models we varied the penetration depth, with six different penetration levels for each model. We then incrementally deformed each model in tension and in shear, until failure of the models. Failure was simulated to occur in the bone and cement when the local ultimate tensile stress was exceeded, by locally reducing the material stiffness to near zero. From the resulting force-displacement curves we established the apparent tensile stiffness and strength for each of the models. Our results indicated that the strength and stiffness of the cement-bone interface increased with increasing cement penetration depth, both in tension and in shear. However, after reaching a certain penetration depth, both strength and stiffness did not further increase. This depth was dependent on the specific interface morphology. We furthermore found that the strength of the models was higher in shear than in tension. After failure of the models, damage was mainly found in the cement, rather than in the bone. The FE-based techniques developed for the current study are suitable for exploration of a variety of aspects that may affect the cement-bone interface micromechanics, such as biological changes to the bone and variations of cement material properties.

In total hip arthroplasty, micromotions at the implant—bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant—bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM). The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an ‘experimental’ method to measure micromotions. This ‘experimental’ method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone. No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface.

Maintaining adequate fixation between cement and bone is important for successful long term survival of cemented total joint replacements. Mixed-mode loading conditions (combination of tension/compression and shear) are present during in vivo loading, but the micromotion response of the interface to these conditions is not fully understood.Non-destructive, multi-axial loading experiments were conducted on laboratory prepared (n = 6) and postmortem (n = 6) human cement-bone interfaces. Specimens were mounted in custom loading discs and loaded at 0 degrees, 30 degrees, 60 degrees, and 90 degrees relative to the interface plane where 0 degrees represents normal loading to the interface, and 90 degrees represents shear loading along the longitudinal axis of the femur. Axial compliance did not depend on loading angle for laboratory prepared (p = 0.96) or postmortem specimens (p = 0.62). The cement-bone interface was more compliant under tensile than compressive loading at the 0(degrees) loading angle only (p = 0.024). The coupled transverse to axial compliance ratio, which is a measure of the coupled motion, was small for laboratory prepared (0.115 +/- 0.115) and postmortem specimens (0.142 +/- 0.101). There was a moderately strong inverse relationship between interface compliance and contact index (r(2) = 0.65).From a computational modeling perspective, the results of the current study support the concept that the cement-bone interface could be numerically implemented as a compliant layer with the same initial stiffness in tension and shear directions. The magnitude of the compliance could be modified to simulate immediate post-operative conditions (using laboratory prepared data set) or long-term remodeling (using postmortem data set). (C) 2010 Elsevier Ltd. All rights reserved.

The mechanical effects of varying the depth of cement penetration in the cement–bone interface were investigated using finite element analysis (FEA) and validated using companion experimental data. Two FEA models of the cement–bone interface were created from micro-computed tomography data and the penetration of cement into the bone was varied over six levels each. The FEA models, consisting of the interdigitated cement–bone constructs with friction between cement and bone, were loaded to failure in tension and in shear. The cement and bone elements had provision for crack formation due to excessive stress. The interfacial strength showed a strong relationship with the average interdigitation (r2=0.97 and r2=0.93 in tension and shear, respectively). Also, the interface strength was strongly related with the contact area (r2=0.98 and r2=0.95 in tension and shear, respectively). The FEA results compared favorably to the stiffness–strength relationships determined experimentally. Overall, the cement–bone interface was 2.5 times stronger in shear than in tension and 1.15 times stiffer in tension than in shear, independent of the average interdigitation. More cracks occurred in the cement than in the bone, independent of the average interdigitation, consistent with the experimental results. In addition, more cracks were generated in shear than in tension. In conclusion, achieving and maintaining maximal infiltration of cement into the bone to obtain large interdigitation and contact area is key to optimizing the interfacial strength.

While including the cement–bone interface of complete cemented hip reconstructions is crucial to correctly capture their response, its modelling is often overly simplified. In this study, the mechanical mixed-mode response of the cement–bone interface is investigated, taking into account the effects of the well-defined microstructure that characterises the interface. Computed tomography-based plain strain finite element analyses models of the cement–bone interface are built and loaded in multiple directions. Periodic boundaries are considered and the failure of the cement and bone fractions by cracking of the bulk components are included. The results compare favourably with experimental observations. Surprisingly, the analyses reveal that under shear loading no failure occurs and considerable normal compression is generated to prevent interface dilation. Reaction forces, crack patterns and stress fields provide more insight into the mixed-mode failure process. Moreover, the cement–bone interface analyses provide details which can serve as a basis for the development of a cohesive law.

In the current study, the effects of different ways to implement the complex micro-mechanical behavior of the cement–bone interface on the fatigue failure of the cement mantle were investigated. In an FEA-model of a cemented hip reconstruction the cement–bone interface was modeled and numerically implemented in four different ways: (I) as infinitely stiff, (II) as infinitely strong with a constant stiffness, (III) a mixed-mode failure response with failure in tension and shear, and (IV) realistic mixed mode behavior obtained from micro-FEA models. Case II, III, and IV were analyzed using data from a stiff and a compliant micro-FEA model and their effects on cement failure were analyzed. The data used for Case IV was derived from experimental specimens that were tested previously. Although the total number of cement cracks was low for all cases, the compliant Case II resulted in twice as many cracks as Case I. All cases caused similar stress distributions at the interface. In all cases, the interface did not display interfacial softening; all stayed the elastic zone. Fatigue failure of the cement mantle resulted in a more favorable stress distribution at the cement–bone interface in terms of less tension and lower shear tractions. We conclude that immediate cement–bone interface failure is not likely to occur, but its local compliancy does affect the formation of cement cracks. This means that at a macro-level the cement–bone interface should be modeled as a compliant layer. However, implementation of interfacial post-yield softening does seems to be necessary.

The cement–bone interface provides fixation for the cement mantle within the bone. The cement–bone interface is affected by fatigue loading in terms of fatigue damage or microcracks and creep, both mostly in the cement. This study investigates how fatigue damage and cement creep separately affect the mechanical response of the cement–bone interface at various load levels in terms of plastic displacement and crack formation. Two FEA models were created, which were based on micro-computed tomography data of two physical cement–bone interface specimens. These models were subjected to tensile fatigue loads with four different magnitudes. Three deformation modes of the cement were considered: ‘only creep’, ‘only damage’ or ‘creep and damage’. The interfacial plastic deformation, the crack reduction as a result of creep and the interfacial stresses in the bone were monitored. The results demonstrate that, although some models failed early, the majority of plastic displacement was caused by fatigue damage, rather than cement creep. However, cement creep does decrease the crack formation in the cement up to 20%. Finally, while cement creep hardly influences the stress levels in the bone, fatigue damage of the cement considerably increases the stress levels in the bone. We conclude that at low load levels the plastic displacement is mainly caused by creep. At moderate to high load levels, however, the plastic displacement is dominated by fatigue damage and is hardly affected by creep, although creep reduced the number of cracks in moderate to high load region.

The goal of this study was to quantify the micromechanics of the cement-bone interface under tensile fatigue loading using finite element analysis (FEA) and to understand the underlying mechanisms that play a role in the fatigue behavior of this interface. Laboratory cement-bone specimens were subjected to a tensile fatigue load, while local displacements and crack growth on the specimen's surface were monitored. FEA models were created from these specimens based upon micro-computed tomography data. To accurately model interfacial gaps at the interface between the bone and cement, a custom-written erosion algorithm was applied to the bone model. A fatigue load was simulated in the FEA models while monitoring the local displacements and crack propagation. The results showed the FEA models were able to capture the general experimental creep damage behavior and creep stages of the interface. Consistent with the experiments, the majority of the deformation took place at the contact interface. Additionally, the FEA models predicted fatigue crack patterns similar to experimental findings. Experimental surface cracks correlated moderately with FEA surface cracks (r(2) = 0.43), but did not correlate with the simulated crack volume fraction (r(2) = 0.06). Although there was no relationship between experimental surface cracks and experimental creep damage displacement (r(2) = 0.07), there was a strong relationship between the FEA crack volume fraction and the FEA creep damage displacement (r(2) = 0.76). This study shows the additional value of FEA of the cement-bone interface relative to experimental studies and can therefore be used to optimize its mechanical properties. (C) 2009 Elsevier Ltd. All rights reserved.

In cemented total hip arthroplasty, the implant needs a mechanically stable cement-bone interface for its survival. This can be achieved by adequate cement penetration into the bone lacunar and trabecular spaces. Many studies have been conducted to investigate the strength of the cement-bone interface in relation to the amount of cement penetration, but mainly on a macro-scale [1]. However, to gain a more detailed insight into the mechanical aspects of this geometrically complex interface, the cement-bone interface should be studied on a micro-scale. We developed micro Finite Element (FE) models of the cement-bone interface and varied the cement penetration depth. Subsequently, we loaded the FE models until failure and asked the following questions: (1) Is there a relationship between penetration depth, contact area and strength?; (2) Is the interface stronger in shear than in tension?; (3) How valid are the FE models compared to experimental findings?

Fatigue failure of the cement mantle in terms of cement cracking is one of the failure mechanisms that leads to aseptic loosening in cemented hip reconstructions. Recently, experiments have demonstrated that there is substantial motion at the cement-bone interface, which may compromise cemented reconstructions [1]. In Finite Element (FE) models of cemented hip reconstructions, the cement-bone interface has been implemented as (I) an infinitely stiff interface, (II) a soft tissue layer with constant stiffness, or (III) a layer of cohesive elements with a mixed-mode behavior based on experimental data on failure in tension and shear. Recently μFE-models have been utilized to study the mixed-mode behavior of the cement-bone interface in more detail [2]. It is, however, unknown whether it is necessary to include the complex mixed-mode response of the cement-bone interface in FE models of cemented hip reconstructions. The following research questions were stated: (1) What is the evaluation of cement crack formation as a result of different cement-bone interface characteristics? (2) Is cement-bone interface failure likely? (3) Does fatigue failure of the cement mantle increase or decrease the probability of failure of the cement-bone interface?