Micromechanics of postmortem-retrieved cement-bone interfaces

Early Changes in Cement-Bone Fixation Using a Novel Rat Knee Replacement Model

Trabecular resorption from interdigitated regions between cement and bone has been found in postmortem-retrieved knee replacements, but the viability of interdigitated bone, and the mechanism responsible for this bone loss is not known. In this work, a Sprague-Dawley (age 12 weeks) rat knee replacement model with an interdigitated cement-bone interface was developed. Morphological and cellular changes in the interdigitated region of the knee replacement over time (0, 2, 6, or 12 weeks) were determined for ovariectomy (OVX) and Sham OVX treatment groups. Interdigitated bone volume fraction (BV/TV) increased with time for Sham OVX (0.022 BV/TV/wk) and OVX (0.015 BV/TV/wk) group, but the rate of increase was greater for the Sham OVX group (p = 0.0064). Tissue mineral density followed a similar increase with time in the interdigitated regions. Trabecular resorption, when it did occur, started at the cement border with medullary-adjacent bone in the presence of osteoclasts. There was substantial loss of viable bone (~80% empty osteocyte lacunae) in the interdigitated regions. Pre-surgical fluorochrome labels remained in the interdigitated regions, and did not diminish with time, indicating that the bone was not remodeling. There was also some evidence of continued surface mineralization in the interdigitated region after cementing of the knee, but this diminished over time. Statement of clinical significance: Interdigitated bone with cement provides mechanical stability for success of knee replacements. Improved understanding of the fate of the interdigitated bone over time could lead to a better understanding of the loosening process and interventions to prevent loss of fixation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2163-2171, 2019.

Periprosthetic bone quality affects the fixation of anatomic glenoids in total shoulder arthroplasty: in vitro study

BACKGROUND

Glenoid loosening, a common complication of shoulder arthroplasty, could relate to implant design and bone quality. However, the role of bone density has not been tested experimentally yet. In this study, tests on cadaveric specimens of varying bone density were performed to evaluate the effects of bone quality on loosening of typical anatomic glenoid implants.

METHODS

Cadaveric scapulae scanned with a quantitative computed tomography scanner to determine bone mineral density (BMD) were implanted with either pegged or keeled cemented glenoid components and tested under constant glenohumeral load while a humeral head component was moved cyclically in the inferior and superior directions. Implant superior and inferior edge lifting, defined as displacement from the underlying bone, was measured with linear variable differential transducers until we reached 23,000 test cycles, and statistical testing was performed for differences in edge lifting due to implant design and related to periprosthetic BMD.

RESULTS

Edge lifting was statistically significant at all time points, but on average, implant design had no effect. Lifting was highest in specimens in which BMD below the lifting edge was lower, with trends of increased displacement with decreased BMD.

CONCLUSIONS

Implant lifting was greater in glenoids of lower bone density for both implant designs. This finding suggests that fixation failure will most likely occur in bone of lower density and that the fixation design itself may play a secondary role.

The Influence of Cement Morphology Parameters on the Strength of the Cement-Bone Interface in Tibial Tray Fixation

BACKGROUND

The strength of the cement-bone interface in tibial component fixation depends on the morphology of the cement mantle. The purpose of this study was to identify thresholds of cement morphology parameters to maximize fixation strength using a minimum amount of cement.

METHODS

Twenty-three cadaveric tibiae were analyzed that had been implanted with tibial trays in previous studies and for which the pull-out strength of the tray had been measured. Specimens were separated into a group failing at the cement-bone interface (INTERFACE) and one failing in the bulk bone (BULK). Maximum pull-out strength corresponds to the ultimate strength of the bulk bone if the cement-bone interface is sufficiently strong. 3D models of the cement mantle in situ were reconstructed from computed tomography scans. The influences of bone mineral density and 6 cement morphology parameters (reflecting cement penetration, bone-cement interface, cement volume) on pull-out strength of the BULK group were determined using multiple regression analysis. The threshold of each parameter for classification of the specimens into either group was determined using receiver operating characteristic analysis.

RESULTS

Cement penetration exceeding a mean of 1.1 mm or with a maximum of 5.6 mm exclusively categorized all BULK bone failure specimens. Failure strength of BULK failure specimens increased with bone mineral density (R² = 0.67, P < .001) but was independent of the cement morphology parameters.

CONCLUSION

To maximize fixation strength, a mean cement penetration depth of at least 1.1 mm should be achieved during tibial tray cementing.

Biomechanical effects of morphological variations of the cortical wall at the bone-cement interface

BACKGROUND

The integrity of bone-cement interface is very important for the stabilization and long-term sustain of cemented prosthesis. Variations in the bone-cement interface morphology may affect the mechanical response of the shape-closed interlock.

METHODS

Self-developed new reamer was used to process fresh pig reamed femoral canal, creating cortical grooves in the canal wall of experimental group. The biomechanical effects of varying the morphology with grooves of the bone-cement interface were investigated using finite element analysis (FEA) and validated using companion experimental data. Micro-CT scans were used to document interlock morphology.

RESULTS

The contact area of the bone-cement interface was greater (P < 0.05) for the experimental group (5470 ± 265 mm(2)) when compared to the specimens of control group (5289 ± 299 mm(2)). The mechanical responses to tensile loading and anti-torsion showed that the specimens with grooves were stronger (P < 0.05) at the bone-cement interface than the specimens without grooves. There were positively significant correlation between the contact area and the tensile force (r (2) = 0.85) and the maximal torsion (r (2) = 0.77) at the bone-cement interface. The volume of cement of the experimental group (7688 ± 278 mm(3)) was greater (P < 0.05) than of the control group (5764 ± 186 mm(3)). There were positively significant correlations between the volume of cement and the tensile force (r (2) = 0.90) and the maximal torsion (r (2) = 0.97) at the bone-cement interface. The FEA results compared favorably to the tensile and torsion relationships determined experimentally. More cracks occurred in the cement than in the bone.

CONCLUSIONS

Converting the standard reaming process from a smooth bore cortical tube to the one with grooves permits the cement to interlock with the reamed bony wall. This would increase the strength of the bone-cement interface.

Construct damage and loosening around glenoid implants: A longitudinal micro-CT study of five cadaver specimens

The evolution of failure of bone and cement leading to loosening of glenoid components following shoulder arthroplasty is not well understood. The purpose of this study was to identify and visualize potential mechanisms of mechanical failure within cadavers, cemented with two types of components, and subject to cyclic loading. Five glenoid cadaver bones were implanted with either a three-pegged polyethylene component, or prototype posteriorly augmented component which addresses posterior bone loss. Specimens were loaded by constant glenohumeral compression combined with cyclic anterior-posterior displacement of the humeral head relative to the glenoid. At six time points across 100,000 cycles, implant loosening micromotions were optically measured, and specimens were imaged by micro-computed tomography. Scans were 3D registered and inspected for crack initiation and progression, and micro-CT based time-lapse movies were created. Cement cracking initiated at stress concentrations and progressed with additional cyclic loading. Failure planes within trabecular bone and the bone-cement interface were identified in four of the five specimens. Implant subsidence increased to greater than 1.0 mm in two specimens. Cemented glenoid structural failure can occur within the cement, along planes of trabecular bone, or at the bone cement interface. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1053-1060, 2016.

Loss of cement-bone interlock in retrieved tibial components from total knee arthroplasties

BACKGROUND

Aseptic loosening continues to be a short- and long-term complication for patients with cemented TKAs. Most studies to this point have evaluated tibial component fixation via radiographic changes at the implant-bone interface and quantification of component migration; direct assessment of morphologic features of the interface from functioning TKAs may provide new information regarding how TKAs function and are fixed to bone.

QUESTIONS/PURPOSES

In a postmortem retrieval study, we asked: (1) What are the morphologic features at the cement-trabecular bone interface in retrieved tibial components? (2) Do constructs with greater time in service have less cement-trabecular bone interlock? (3) Do constructs with more estimated initial interlock sustain more interlock with in vivo service?

METHODS

Fourteen postmortem retrieved tibial components with time in service from 0 to 20 years were sectioned and imaged at high resolution, and the current contact fraction, estimated initial interdigitation depth, current interdigitation depth, and loss of interdigitation depth were quantified at the cement-bone interface. Estimated initial interdigitation depth was calculated from the initial mold shape of the cement mantle that forms around the individual trabeculae at the time of surgery. Loss of interdigitation depth was the difference between the initial and current interdigitation depth.

RESULTS

There was resorption of trabeculae that initially interlocked with the cement in the postmortem retrievals as evidenced by the differences between current interdigitation and the estimated original interdigitation. The current contact fraction (r(2) = 0.54; p = 0.0027) and current interdigitation depth (r(2) = 0.33; p = 0.033) were less for constructs with longer time in service. The current contact fraction for implants with 10 or more years in service (6.2%; 95% CI, 4.7%-7.7%) was much less than implants with less than 10 years in service (22.9%; 95% CI, 8.9%-37%). Similarly, the current interdigitation depth for implants with 10 or more years in service (0.4 mm; 95% CI, 0.27-0.53 mm) was much less than implants with less than 10 years in service (1.13 mm; 95% CI, 0.48-1.78 mm). The loss of interdigitation depth had a strong positive relationship with time in service (r(2) = 0.74; p < 0.001). Using a two-parameter regression model, constructs with more initial interdigitation depth had greater current interdigitation depth (p = 0.011), but constructs with more time in service also had less current interdigitation depth (p = 0.008).

CONCLUSIONS

The cement-trabecular bone interlock obtained initially appears to diminish with time with in vivo service by resorption of the trabeculae in the cement interlock region.

CLINICAL RELEVANCE

Our study supports the surgical concept of obtaining sufficient initial cement interlock (approximately 3 mm), with the acknowledgment that there will be loss of interlock with time with in vivo service.

Characterisation of a metallic foam-cement composite under selected loading conditions

An open-cell metallic foam was employed as an analogue material for human trabecular bone to interface with polymethyl methacrylate (PMMA) bone cement to produce composite foam-cement interface specimens. The stress-displacement curves of the specimens were obtained experimentally under tension, shear, mixed tension and shear (mixed-mode), and step-wise compression loadings. In addition, under step-wise compression, an image-guided failure assessment (IGFA) was used to monitor the evolution of micro-damage of the interface. Microcomputed tomography (µCT) images were used to build a subject-specific model, which was then used to perform finite element (FE) analysis under tension, shear and compression. For tension-shear loading conditions, the strengths of the interface specimens were found to increase with the increase of the loading angle reaching the maximum under shear loading condition, and the results compare reasonably well with those from bone-cement interface. Under compression, however, the mechanical strength measured from the foam-cement interface is much lower than that from bone-cement interface. Furthermore, load transfer between the foam and the cement appears to be poor under both tension and compression, hence the use of the foam should be discouraged as a bone analogue material for cement fixation studies in joint replacements.

A new interface element with progressive damage and osseointegration for modeling of interfaces in hip resurfacing

Finite element models of orthopedic implants such as hip resurfacing femoral components usually rely on contact elements to model the load-bearing interfaces that connect bone, cement and implant. However, contact elements cannot simulate progressive degradation of bone–cement interfaces or osseointegration. A new interface element is developed to alleviate these shortcomings. This element is capable of simulating the nonlinear progression of bone–cement interface debonding or bone–implant interface osseointegration, based on mechanical stimuli in normal and tangential directions. The new element is applied to a hip resurfacing femoral component with a stem made of a novel biomimetic composite material. Three load cases are applied sequentially to simulate the 6-month period required for osseointegration of the stem. The effect of interdigitation depth of the bone–cement interface is found to be negligible, with only minor variations of micromotions. Numerical results show that the biomimetic stem progressively osseointegrates (α averages 0.7 on the stem surface, with spot-welds) and that bone–stem micromotions decrease below 10 µm. Osseointegration also changes the load path within the femoral bone: a decrease of 300 µε was observed in the femoral head, and the inferomedial part of the femoral neck showed a slight increase of 165 µε. There was also increased stress in the implant stem (from 7 to 11 MPa after osseointegration), indicating that part of the load is supported through the stem. The use of the new osseointegratable interface element has shown the osseointegration potential of the biomimetic stem. Its ability to model partially osseointegrated interfaces based on the mechanical conditions at the interface means that the new element could be used to study load transfer and osseointegration patterns on other models of uncemented hip resurfacing femoral components.

Interface micromechanics of transverse sections from retrieved cemented hip reconstructions: an experimental and finite element comparison

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.

A new approach to quantify trabecular resorption adjacent to cemented knee arthroplasty

A new micro-computed tomography (μCT) image processing approach to estimate the loss of cement-bone interlock was developed using the concept that PMMA cement flows and cures around trabeculae during the total knee arthroplasty procedure. The initial mold shape of PMMA cement was used to estimate the amount of interdigitated bone at the time of implantation and following in vivo service using enbloc human postmortem retrievals. Laboratory prepared specimens, where there would be no biological bone resorption, were used as controls to validate the approach and estimate errors. The image processing technique consisted of identifying bone and cement from the μCT scan set, dilation of the cement to identify the cement cavity space, and Boolean operations to identify the different components of the interdigitated cement-bone regions. For laboratory prepared specimens, there were small errors in the estimated resorbed bone volume fraction (reBVfr=0.11 ± 0.09) and loss in contact area fraction (CAfr=0.06 ± 0.15). These values would be zero if there were no error in the method. For the postmortem specimens, the resorbed volume fraction (reBVfr=0.85 ± 0.16) was large, meaning that only 15% of the cement mold shape was still filled with bone. The loss of contact area fraction (CAfr=0.84 ± 0.17) was similarly large. This new approach provides a convenient method to visualize and quantify trabecular bone loss from interdigitated regions from postmortem retrievals. The technique also illustrates for the first time that there are dramatic changes in how bone is fixed to cement following in vivo service.

Femoral component loosening in high-flexion total knee replacement

High-flexion total knee replacement (TKR) designs have been introduced to improve flexion after TKR. Although the early results of such designs were promising, recent literature has raised concerns about the incidence of early loosening of the femoral component. We compared the minimum force required to cause femoral component loosening for six high-flexion and six conventional TKR designs in a laboratory experiment.

Each TKR design was implanted in a femoral bone model and placed in a loading frame in 135° of flexion. Loosening of the femoral component was induced by moving the tibial component at a constant rate of displacement while maintaining the same angle of flexion. A stereophotogrammetric system registered the relative movement between the femoral component and the underlying bone until loosening occurred.

Compared with high-flexion designs, conventional TKR designs required a significantly higher force before loosening occurred (p < 0.001). High-flexion designs with closed box geometry required significantly higher loosening forces than high-flexion designs with open box geometry (p = 0.0478). The presence of pegs further contributed to the fixation strength of components.

We conclude that high-flexion designs have a greater risk for femoral component loosening than conventional TKR designs. We believe this is attributable to the absence of femoral load sharing between the prosthetic component and the condylar bone during flexion.

Morphology based cohesive zone modeling of the cement-bone interface from postmortem retrievals

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.

Mixed-mode loading of the cement-bone interface: a finite element study

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.

Multi-axial loading micromechanics of the cement-bone interface in postmortem retrievals and lab-prepared specimens

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°, 30°, 60°, and 90° relative to the interface plane where 0° represents normal loading to the interface, and 90° 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° 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).

The effect of cement creep and cement fatigue damage on the micromechanics of the cement-bone interface

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 mechanical effects of different levels of cement penetration at the cement-bone interface

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 (r(2)=0.97 and r(2)=0.93 in tension and shear, respectively). Also, the interface strength was strongly related with the contact area (r(2)=0.98 and r(2)=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.

Functional interface micromechanics of 11 en-bloc retrieved cemented femoral hip replacements

BACKGROUND AND PURPOSE

Despite the longstanding use of micromotion as a measure of implant stability, direct measurement of the micromechanics of implant/bone interfaces from en bloc human retrievals has not been performed. The purpose of this study was to determine the stem-cement and cement-bone micromechanics of functionally loaded, en-bloc retrieved, cemented femoral hip components.

METHODS

11 fresh frozen proximal femurs with cemented implants were retrieved at autopsy. Specimens were sectioned transversely into 10-mm slabs and fixed to a loading device where functional torsional loads were applied to the stem. A digital image correlation technique was used to document micromotions at stem-cement and cement-bone interfaces during loading.

RESULTS

There was a wide range of responses with stem-cement micromotions ranging from 0.0006 mm to 0.83 mm (mean 0.17 mm, SD 0.29) and cement-bone micromotions ranging from 0.0022 mm to 0.73 mm (mean 0.092 mm, SD 0.22). There was a strong (linear-log) inverse correlation between apposition fraction and micromotion at the stem-cement interface (r(2) = 0.71, p < 0.001). There was a strong inverse log-log correlation between apposition fraction at the cement-bone interface and micromotion (r(2) = 0.85, p < 0.001). Components that were radiographically well-fixed had a relatively narrow range of micromotions at the stem-cement (0.0006-0.057 mm) and cement-bone (0.0022-0.029 mm) interfaces.

INTERPRETATION

Minimizing gaps at the stem-cement interface and encouraging bony apposition at the cement-bone interface would be clinically desirable. The cement-bone interface does not act as a bonded interface in actual use, even in radiographically well-fixed components. Rather, the interface is quite compliant, with sliding and opening motions between the cement and bone surfaces.

Novel methods to study functional loading micromechanics at the stem-cement and cement-bone interface in cemented femoral hip replacements

We have developed a technique to directly observe the micromechanics of the stem-cement and cement-bone interfaces of cemented femoral stems under physiologically relevant loading conditions. Thick transverse sections of a stem-cement-femur construct were fixed to the base of a test frame. Ante- and retro-verting torques were applied to the femoral stem by screwing the stem (via a pair of through holes) to an axle, which was turned using a lever arm actuated by the test frame cross-head. The surface of each transverse section was serially digitally imaged during loading. The displacements of the stem, cement and bone were determined using digital image correlation. These data were then used to calculate the relative displacements across the interfaces. This method provides a path to more thorough understanding of load-transfer from femoral stem to femur.