Compressive creep characteristics of extruded ultrahigh-molecular-weight polyethylene

Effect of Ligament Properties on Nonlinear Dynamics and Wear Prediction of Knee Prostheses

Although wear is known as the primary cause of long-time failure of total knee arthroplasty (TKA), it can be vital in short- and midterm TKA failure due to laxity. One of the reasons leading to joint laxity and instability is ligamentous insufficiency. This study, therefore, aims to investigate the effects of insufficient ligaments-related knee laxity on both nonlinear dynamics and wear of TKA. The study hypothesizes (a) ligamentous insufficiency can increase TKA damage; (b) stiffness reduction of each of the posterior cruciate ligament (PCL) and medial-lateral collateral ligaments (MCL-LCL) can differently contribute to TKA damage. A forward dynamics methodology is developed and the ligament behavior is simulated employing an asymmetric nonlinear elastic model. External loads and moment, due to the presence of all soft tissues, e.g., muscles and hip joint reaction forces, applied to the femoral bone are determined using a musculoskeletal approach linked to the developed model. A mesh density analysis is performed and comparing outcomes with that available in the literature allows for the assessment of our approach. From the results acquired, reduced PCL stiffness leads to an increase in linear wear rates and results in the maximum damage in TKAs. However, the maximum linear wear rates on both condyles occur once the stiffness of all ligaments is reduced. Moreover, the worn area of the tibia surface increases with the reduction in MCL-LCL stiffness on the medial condyle. The joint with insufficient PCL also shows a considerable increase in ligament forces right after toe-off.

Stress and strain distribution in femoral heads for hip resurfacing arthroplasty with different materials: A finite element analysis

Femoral bone loss due to stress and strain shielding is a common problem in hip resurfacing arthroplasty (HRA), which arises from the different stiffness of implant materials and the adjacent bone. Usually, the implants used in HRA are made of cobalt-chromium alloy (CoCr). As a novel concept, implants may also be made of ceramics, whose stiffness exceeds that of the adjacent bone by a multiple. Therefore, this computational study aimed to evaluate whether poly (ether-ether-ketone) (PEEK) or a hybrid material with a PEEK body and ceramic surface made of alumina toughened zirconia (ATZ) might be more suitable implant alternatives for HRA, as they can avoid stress and strain shielding. A reconstructed model of a human femur with an HRA implant was simulated, whereby the material of the HRA was varied between CoCr, ATZ, zirconia toughened alumina (ZTA), PEEK, and a hybrid PEEK-ATZ material. The implant fixation method also varied (cemented or cementless). The simulated models were compared with an intact model to analyze stress and strain distribution in the femoral head and neck. The strain distribution was evaluated at a total of 30,344 (cemented HRA) and 63,531 (uncemented HRA) nodes in the femoral head and neck region and divided into different strain regions (<400 µm/m: atrophy; 400-3000 μm/m: bone preserving and building; 3000-20,000 μm/m: yielding and >20,000 μm/m fracture). In addition, the mechanical stability of the implants was evaluated. When the material of the HRA implant was simulated as metal or ceramic while evaluating the strains, it was seen that around 22-26% of the analyzed nodes in the femoral head and neck were in an atrophic region, 47-51% were in a preserving or building region, and 27-28% were in a yielding region. In the case of PEEK implant, less than 0.5% of the analyzed nodes were in an atrophic region, 66-69% in a preserving or building region, and 31-34% in a yielding region. The fixation technique also had a small influence. When a hybrid HRA was simulated, the strains at the analyzed nodes depended on the thickness of the ceramic material. In conclusion, the material of the HRA implant was crucial in terms of stress and strain distribution in the adjacent bone. HRA made of PEEK or a hybrid material leads to decisively reduced stress and strain alteration compared to stiffer materials such as CoCr, ATZ, and ZTA. This confirms the potential for reduction in stress and strain shielding in the femoral head with the use of a hybrid material with a PEEK body for HRA.

Long-term in vivo wear of different bearing types used for the Oxford Unicompartmental Knee Replacement

Objectives

The aim of this study was to determine the polyethylene wear rate of Phase 3 Oxford Unicompartmental Knee Replacement bearings and to investigate the effects of resin type and manufacturing process.

Methods

A total of 63 patients with at least ten years’ follow-up with three bearing types (1900 resin machined, 1050 resin machined, and 1050 resin moulded) were recruited. Patients underwent full weight-bearing model-based radiostereometric analysis to determine the bearing thickness. The linear wear rate was estimated from the change in thickness divided by the duration of implantation.

Results

The wear rate for 1900 resin machined (n = 19), 1050 machined (n = 21), and 1050 moulded bearings (n = 23) were 60 µm/year (sd 42), 76 µm/year (sd 32), and 57 µm/year (sd 30), respectively. There was no significant difference between 1900 machined and 1050 machined (p = 0.20), but 1050 moulded had significantly less wear than the 1050 machined (p = 0.05). Increasing femoral (p < 0.001) and tibial (p < 0.001) component size were associated with increasing wear.

Conclusion

Wear rate is similar with 1050 and 1900 resin, but lower with moulded bearings than machined bearings. The currently used Phase 3 bearings wear rate is low (1050 moulded, 57 µm/year), but higher than the previously reported Phase 2 bearings (1900 moulded, 20 µm/year). This is unlikely to be due to the change in polyethylene but may relate to the minimally invasive approach used with the Phase 3. This approach, as well as improving function and thus increasing activity levels, may increase the risk of surgical errors, such as impingement or bearing overhang, which can increase wear. Surgeons should aim to use 4 mm thick bearings rather than 3 mm thick bearings in young patients, unless they are small and need conservative bone resections.

Cite this article: Bone Joint Res 2019;8:535–543.

Finite element analysis of polyethylene wear in total hip replacement: A literature review

Evaluation and prediction of wear play a key role in product design and material selection of total hip replacements, because wear debris is one of the main causes of loosening and failure. Multifactorial clinical or laboratory studies are high cost and require unfeasible timeframes for implant development. Simulation using finite element methods is an efficient and inexpensive alternative to predict wear and pre-screen various parameters. This article presents a comprehensive literature review of the state-of-the-art finite element modelling techniques that have been applied to evaluate wear in polyethylene hip replacement components. A number of knowledge gaps are identified including the need to develop appropriate wear coefficients and the analysis of daily living activities.

Load application for the contact mechanics analysis and wear prediction of total knee replacement

Tibiofemoral contact forces in total knee replacement have been measured at the medial and lateral sites respectively using an instrumented prosthesis, and predicted from musculoskeletal multibody dynamics models with a reasonable accuracy. However, it is uncommon that the medial and lateral forces are applied separately to replace a total axial load according to the ISO standard in the majority of current finite element analyses. In this study, we quantified the different effects of applying the medial and lateral loads separately versus the traditional total axial load application on contact mechanics and wear prediction of a patient-specific knee prosthesis. The load application position played an important role under the medial-lateral load application. The loading set which produced the closest load distribution to the multibody dynamics model was used to predict the contact mechanics and wear for the prosthesis and compared with the total axial load application. The medial-lateral load distribution using the present method was found to be closer to the multibody dynamics prediction than the traditional total axial load application, and the maximum contact pressure and contact area were consistent with the corresponding load variation. The predicted total volumetric wear rate and area were similar between the two load applications. However, the split of the predicted wear volumes on the medial and the lateral sides was different. The lateral volumetric wear rate was 31.46% smaller than the medial from the traditional load application prediction, while from the medial-lateral load application, the lateral side was only 11.8% smaller than the medial. The medial-lateral load application could provide a new and more accurate method of load application for patient-specific preclinical contact mechanics and wear prediction of knee implants.

Size and thickness effect on creep behavior in conventional and vitamin E-diffused highly crosslinked polyethylene for total hip arthroplasty

Since the early 2000s, the use of large femoral heads is becoming increasingly popular in total hip arthroplasty (THA), which provides an improved range of motion and joint stability. Large femoral heads commonly necessitate to be coupled with thinner acetabular liners than the conventionally used because of the limited sizes of outer shells (especially for patients with small pelvic size). However, the influence of the liner thinning on the mechanical performance is still not clearly understood. The objective of this study was to experimentally clarify the size and thickness effect on the rates of compressive creep strain in conventional (virgin low-crosslinked) and vitamin E-diffused highly crosslinked, ultra-high molecular weight polyethylene (UHMWPE) acetabular liners. We applied uniaxial compression to these liners of various internal diameters (28, 32 and 36mm) and thicknesses (4.8, 6.8 and 8.9mm) up to 4320min under the constant load of 3000N. Vitamin E-diffused highly crosslinked UHMWPE components showed significantly greater creep resistance than the conventional ones. In the both types of UHMWPE, the rates of creep strain significantly decreased by increasing the internal diameter and thickness. Varying the component thickness contributed more largely to the creep behavior rather than the internal diameter. Our results suggest the positive mechanical advantage of using large femoral heads, but at the same time, a considerable liner thinning is not recommended for minimizing creep strain. Therefore, the further in-vitro as well as in-vivo research are necessary to conclude the optimal balance of head diameter and liner thickness within the limited sizes of outer shells.

Quantification of clearance and creep in acetabular wear measurements

BACKGROUND

This study aimed to measure femoral head penetration before occurrence of real wear, and to quantify the portions attributable respectively to clearance and plastic deformations in various acetabular designs.

METHODS

We analyzed CT scans from 15 patients at 'day five' after total hip arthroplasty (THA). All patients received Exafit(®) femoral stems and 28 mm heads: 5 patients had cemented Durasul(®) all-PE cups, 5 patients had un-cemented Allofit(®) metal-backed cups, and 5 patients had un-cemented Stafit(®) dual-mobility cups. We also analyzed CT scans of samples of the three head-cup combinations to compare in vivo and in vitro measurements.

RESULTS

The mean femoral head penetration measured on 'day five' was lower for all-PE cups (0.196 mm) than for metal-backed cups (0.551 mm) and dual-mobility cups (0.634 mm).

CONCLUSIONS

The present study indicates that isolated measurements of femoral head penetration include 0.15-0.46 mm of radial clearance and 0.05-0.27 mm of creep, and confirms that the majority of so-called bedding-in observed in the first post-operative months is not entirely due to wear.

Friction measurement in a hip wear simulator

A torque measurement system was added to a widely used hip wear simulator, the biaxial rocking motion device. With the rotary transducer, the frictional torque about the drive axis of the biaxial rocking motion mechanism was measured. The principle of measuring the torque about the vertical axis above the prosthetic joint, used earlier in commercial biaxial rocking motion simulators, was shown to sense only a minor part of the total frictional torque. With the present method, the total frictional torque of the prosthetic hip was measured. This was shown to consist of the torques about the vertical axis above the joint and about the leaning axis. Femoral heads made from different materials were run against conventional and crosslinked polyethylene acetabular cups in serum lubrication. Regarding the femoral head material and the type of polyethylene, there were no categorical differences in frictional torque with the exception of zirconia heads, with which the lowest values were obtained. Diamond-like carbon coating of the CoCr femoral head did not reduce friction. The friction factor was found to always decrease with increasing load. High wear could increase the frictional torque by 75%. With the present system, friction can be continuously recorded during long wear tests, so the effect of wear on friction with different prosthetic hips can be evaluated.

Surface Damage Is Not Reduced With Highly Crosslinked Polyethylene Tibial Inserts at Short-term

BACKGROUND

Highly crosslinked ultrahigh-molecular-weight polyethylene (XLPE) has been shown to reduce wear in hip arthroplasty, but the advantages over conventional polyethylene (PE) in total knee arthroplasty (TKA), if any, remain unclear.

QUESTIONS/PURPOSES

Do differences exist in (1) surface damage as measured by damage score and percent area affected; and (2) extent and location of dimensional changes between XLPE and conventional PE observed on retrieved TKA tibial inserts?

METHODS

In this study of components retrieved at the time of revision surgery, we matched 44 XLPE to 44 conventional PE inserts from four manufacturers; the matching approach considered implant design (exact match), insert size (exact match), and length of implantation (matched ± 6 months). Surface damage on the articular surfaces was subjectively graded and digitally mapped to determine the percent damaged area of each damage mode. Three-dimensional changes that had occurred as a result of implantation were determined by comparing laser scans of the retrieved inserts with size-matched pristine inserts.

RESULTS

The differences of damage scores and percent damaged areas between the matched XLPE and conventional PE inserts were not large enough to be clinically significant with low corresponding levels of statistical significance (scores: 42 ± 13; 95% confidence interval [CI], 38-46 versus 45 ± 13; 95% CI, 41-49; p = 0.4; percent areas: 54% ± 38%; 95% CI, 44%-64% versus 54% ± 32%; 95% CI, 42%-65%; p = 0.9). However, XLPE inserts showed greater articular surface dimensional changes with high significance (root mean square of the distance: 0.16 ± 0.06 mm; 95% CI, 0.13-0.18 mm versus 0.14 ± 0.05 mm; 95% CI, 0.11-0.16 mm; p = 0.03). Within the same design, deviation patterns were consistent between the two materials; however, as expected, the location of the dimensional changes differed among designs: the negative deviations on the plateaus were centrally located in Zimmer PS inserts, were located on the perimeter in Smith & Nephew PS inserts, and were across the entire surface in DePuy PS inserts.

CONCLUSIONS

We found no difference in surface damage between matched XLPE and conventional PE inserts of the same designs. However, increased dimensional changes in TKAs with XLPE may reflect larger contact areas and potentially explain improved performance of XLPE in published simulator studies.

CLINICAL RELEVANCE

The lack of meaningful differences between the two polyethylene materials suggests caution in adopting a new, more expensive bearing material over another material that has a long track record of excellent behavior. A possible advantage is the greater dimensional changes, which could be the result of the lower creep resistance of XLPE, but this advantage awaits long-term results.

The effect of polyethylene creep on tibial insert locking screw loosening and back-out in prosthetic knee joints

A prosthetic knee joint typically comprises a cobalt-chromium femoral component that articulates with a polyethylene tibial insert. A locking screw may be used to prevent micromotion and dislodgement of the tibial insert from the tibial tray. Screw loosening and back-out have been reported, but the mechanism that causes screw loosening is currently not well understood. In this paper, we experimentally evaluate the effect of polyethylene creep on the preload of the locking screw. We find that the preload decreases significantly as a result of polyethylene creep, which reduces the torque required to loosen the locking screw. The torque applied to the tibial insert due to internal/external rotation within the knee joint during gait could thus drive locking screw loosening and back-out. The results are very similar for different types of polyethylene.

The effect of insert conformity and material on total knee replacement wear

The mean average life is increasing; therefore, there is a need to increase the lifetime of the prostheses. To fulfil this requirement, new prosthetic designs and materials are being introduced. Two of the design parameters that may affect wear of total knee replacements, and hence the expected lifetime, are the insert conformity and material. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. The objective of the present study was to use a previously validated non-dimensional wear coefficient-based computational wear model to investigate the effect of insert conformity and material on the predicted wear in total knee replacements. Four different inserts (curved, lipped, partial flat and custom flat), with different conformity levels, were tested against the same femoral and under two different kinematic inputs (intermediate and high), with different levels of cross-shear. The insert bearing materials were either conventional or moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE). Wear predictions were validated against the experimental data from Leeds knee simulation tests. The predicted wear rates for the curved insert (most conformed) were more than three times those for the flat insert (least conformed). In addition, the computationally predicted average volumetric wear rates for moderately cross-linked UHMWPE bearings were less than half of their corresponding conventional UHMWPE bearings. Moreover, the wear of the moderately cross-linked UHMWPE was shown to be less dependent on the degree of cross-shear, compared to conventional UHMWPE. These results along with supporting experimental studies provide insight into the design variables, which may reduce wear in knee replacements.

Effect of motion inputs on the wear prediction of artificial hip joints

Hip joint simulators have been largely used to assess the wear performance of joint implants. Due to the complexity of joint movement, the motion mechanism adopted in simulators varies. The motion condition is particularly important for ultra-high molecular weight polyethylene (UHMWPE) since polyethylene wear can be substantially increased by the bearing cross-shear motion. Computational wear modelling has been improved recently for the conventional UHMWPE used in total hip joint replacements. A new polyethylene wear law is an explicit function of the contact area of the bearing and the sliding distance, and the effect of multidirectional motion on wear has been quantified by a factor, cross-shear ratio. In this study, the full simulated walking cycle condition based on a walking measurement and two simplified motions, including the ISO standard motion and a simplified ProSim hip simulator motion, were considered as the inputs for wear modelling based on the improved wear model. Both the full simulation and simplified motions generated the comparable multidirectional motion required to reproduce the physiological wear of the bearing in vivo. The predicted volumetric wear of the ProSim simulator motion and the ISO motion conditions for the walking cycle were 13% and 4% lower, respectively, than that of the measured walking condition. The maximum linear wear depths were almost the same, and the areas of the wear depth distribution were 13% and 7% lower for the ProSim simulator and the ISO condition, respectively, compared with that of the measured walking cycle motion condition.

The robustness and accuracy of in vivo linear wear measurements for knee prostheses based on model-based RSA

Accurate in vivo measurements methods of wear in total knee arthroplasty are required for a timely detection of excessive wear and to assess new implant designs. Component separation measurements based on model-based Roentgen stereophotogrammetric analysis (RSA), in which 3-dimensional reconstruction methods are used, have shown promising results, yet the robustness of these measurements is unknown. In this study, the accuracy and robustness of this measurement for clinical usage was assessed. The validation experiments were conducted in an RSA setup with a phantom setup of a knee in a vertical orientation. 72 RSA images were created using different variables for knee orientations, two prosthesis types (fixed-bearing Duracon knee and fixed-bearing Triathlon knee) and accuracies of the reconstruction models. The measurement error was determined for absolute and relative measurements and the effect of knee positioning and true seperation distance was determined. The measurement method overestimated the separation distance with 0.1mm on average. The precision of the method was 0.10mm (2*SD) for the Duracon prosthesis and 0.20mm for the Triathlon prosthesis. A slight difference in error was found between the measurements with 0° and 10° anterior tilt. (difference=0.08mm, p=0.04). The accuracy of 0.1mm and precision of 0.2mm can be achieved for linear wear measurements based on model-based RSA, which is more than adequate for clinical applications. The measurement is robust in clinical settings. Although anterior tilt seems to influence the measurement, the size of this influence is low and clinically irrelevant.

Cross-shear implementation in sliding-distance-coupled finite element analysis of wear in metal-on-polyethylene total joint arthroplasty: intervertebral total disc replacement as an illustrative application

Computational simulations of wear of orthopaedic total joint replacement implants have proven to valuably complement laboratory physical simulators, for pre-clinical estimation of abrasive/adhesive wear propensity. This class of numerical formulations has primarily involved implementation of the Archard/Lancaster relationship, with local wear computed as the product of (finite element) contact stress, sliding speed, and a bearing-couple-dependent wear factor. The present study introduces an augmentation, whereby the influence of interface cross-shearing motion transverse to the prevailing molecular orientation of the polyethylene articular surface is taken into account in assigning the instantaneous local wear factor. The formulation augment is implemented within a widely utilized commercial finite element software environment (ABAQUS). Using a contemporary metal-on-polyethylene total disc replacement (ProDisc-L) as an illustrative implant, physically validated computational results are presented to document the role of cross-shearing effects in alternative laboratory consensus testing protocols. Going forward, this formulation permits systematically accounting for cross-shear effects in parametric computational wear studies of metal-on-polyethylene joint replacements, heretofore a substantial limitation of such analyses.

Polyethylene wear in Oxford unicompartmental knee replacement

The Oxford Unicompartmental Knee replacement (UKR) was introduced as a design to reduce polyethylene wear. There has been one previous retrieval study involving this implant, which reported very low rates of wear in some specimens but abnormal patterns of wear in others. There has been no further investigation of these abnormal patterns. The bearings were retrieved from 47 patients who had received a medial Oxford UKR for anteromedial osteoarthritis of the knee. None had been studied previously. The mean time to revision was 8.4 years (sd 4.1), with 20 having been implanted for over ten years. The macroscopic pattern of polyethylene wear and the linear penetration were recorded for each bearing. The mean rate of linear penetration was 0.07 mm/year. The patterns of wear fell into three categories, each with a different rate of linear penetration; 1) no abnormal macroscopic wear and a normal articular surface, n = 16 (linear penetration rate = 0.01 mm/year); 2) abnormal macroscopic wear and normal articular surfaces with extra-articular impingement, n = 16 (linear penetration rate = 0.05 mm/year); 3) abnormal macroscopic wear and abnormal articular surfaces with intra-articular impingement +/− signs of non-congruous articulation, n = 15 (linear penetration rate = 0.12 mm/year). The differences in linear penetration rate were statistically significant (p < 0.001).

These results show that very low rates of polyethylene wear are possible if the device functions normally. However, if the bearing displays suboptimal function (extra-articular, intra-articular impingement or incongruous articulation) the rates of wear increase significantly.

Design Optimization of a Total Hip Prosthesis for Wear Reduction

Aseptic loosening from polyethylene debris is the leading cause of failure for metal-on-polyethylene hip implants. The accumulation of wear debris can lead to osteolysis, the degradation of bone surrounding the implant components. In the present study, a parametric three-dimensional finite element model of an uncemented total hip replacement prosthesis was constructed and implanted into a femur model constructed from computed tomography (CT) scan data. Design optimization was performed considering volumetric wear as an objective function using a computational model validated in a previous study through in vitro wear assessment. Constraints were used to maintain the physiological range of motion of wear-optimum designs. Loading conditions for both walking and stair climbing were considered in the analysis. In addition, modification of the acetabular liner surface nodes was performed in discrete intervals to reflect the actual wear and creep damage occurring on the liner surface. Stair climbing was found to produce 49% higher volumetric wear than walking. Using a sensitivity analysis, it was found that the objective function sensitivity to the chosen design variables was identical for both walking and stair climbing. The greatest reduction in volumetric wear achieved while maintaining a physiological range of motion was 16%. It was found that including nodal modification in the sensitivity analysis produced little or no difference in the sensitivity analysis results due to the linear nature of volumetric wear progression. Thus, nodal modification was not used in optimization. An increase in the maximum contact pressure was observed for all wear-optimized designs, and an increase in head-liner penetration was found to be related to a reduction in volumetric wear.

Total Hip Wear Assessment: A Comparison Between Computational and In Vitro Wear Assessment Techniques Using ISO 14242 Loading and Kinematics

In the present study a direct comparison was made between in vitro total hip wear testing and a computational analysis considering the effects of time and a nonlinear stress-strain relationship for ultrahigh molecular weight polyethylene (UHMWPE) at 37°C. The computational simulation was made correct through calibration to experimental volumetric wear results, and the predicted damage layout on the acetabular liner surface was compared with results estimated from laser scanning of the actual worn specimens. The wear rates for the testing specimens were found to be 17.14±1.23 mg/106 cycles and 19.39±0.79 mg/106 cycles, and the cumulative volumetric wear values after 3×106 cycles were 63.70 mm3 and 64.02 mm3 for specimens 1 and 2, respectively. The value of the calibrated wear coefficient was found to be 5.32(10−10) mm3/N mm for both specimens. The major difference between the computational and experimental wear results was the existence of two damage vectors in the experimental case. The actual location of damage was virtually the same in both cases, and the maximum damage depth of the computational model agreed well with the experiment. The existence of multiple wear vectors may indicate the need for computational approaches to account for multidirectional sliding or strain hardening of UHMWPE. Despite the limitation in terms of describing the overall damage layout, the present computational model shows that simulation can mimic some of the behavior of in vitro wear.

Does highly cross-linked polyethylene wear less than conventional polyethylene in total hip arthroplasty? A double-blind, randomized, and controlled trial using roentgen stereophotogrammetric analysis

A prospective double-blind, randomized, and controlled trial was conducted using roentgen stereophotogrammetric analysis; 54 total hip arthroplasty patients were randomized to receive either highly cross-linked polyethylene (HXLPE) or standard ultra-high-molecular-weight polyethylene (UHMWPE) liners. The 3-dimensional penetration of the liner was determined over 2 years. For the first 3 months, both polyethylene types had a rapid penetration rate (HXLPE: 0.22 mm, SD = 0.17 mm; UHMWPE: 0.21 mm, SD = 0.15 mm; P = .78). After 3 months, the HXLPE penetration rate (0.06 mm/y, SD = 0.06 mm/y) was significantly lower than the UHMWPE penetration rate (0.10 mm/y, SD = 0.07 mm/y; P = .04). The penetration in the first 3 months was probably caused by creep or bedding in; from 3 months onward, much of the penetration was probably caused by wear. We conclude that HXLPE has a 40% lower wear rate as compared with UHMWPE, suggesting that it will perform better in the long term.

Predicting Knee Replacement Damage in a Simulator Machine Using a Computational Model With a Consistent Wear Factor

Wear of ultrahigh molecular weight polyethylene remains a primary factor limiting the longevity of total knee replacements (TKRs). However, wear testing on a simulator machine is time consuming and expensive, making it impractical for iterative design purposes. The objectives of this paper were first, to evaluate whether a computational model using a wear factor consistent with the TKR material pair can predict accurate TKR damage measured in a simulator machine, and second, to investigate how choice of surface evolution method (fixed or variable step) and material model (linear or nonlinear) affect the prediction. An iterative computational damage model was constructed for a commercial knee implant in an AMTI simulator machine. The damage model combined a dynamic contact model with a surface evolution model to predict how wear plus creep progressively alter tibial insert geometry over multiple simulations. The computational framework was validated by predicting wear in a cylinder-on-plate system for which an analytical solution was derived. The implant damage model was evaluated for 5 million cycles of simulated gait using damage measurements made on the same implant in an AMTI machine. Using a pin-on-plate wear factor for the same material pair as the implant, the model predicted tibial insert wear volume to within 2% error and damage depths and areas to within 18% and 10% error, respectively. Choice of material model had little influence, while inclusion of surface evolution affected damage depth and area but not wear volume predictions. Surface evolution method was important only during the initial cycles, where variable step was needed to capture rapid geometry changes due to the creep. Overall, our results indicate that accurate TKR damage predictions can be made with a computational model using a constant wear factor obtained from pin-on-plate tests for the same material pair, and furthermore, that surface evolution method matters only during the initial “break in” period of the simulation.

No increased migration in cups with ceramic-on-ceramic bearing: an RSA study

Ceramic-on-ceramic hip replacements might stress the bone interface more than a metal-polyethylene because of material stiffness, microseparation, and sensitivity to impingement. To ascertain whether this potentially increased stress caused an increased cup migration we compared a ceramic-on-ceramic with a metal-on-polyethylene implant for cup migration. Sixty one patients (61 hips) undergoing THA for osteoarthritis were randomized to ceramic on ceramic (Ce/Ce) or cobalt-chromium on cross-linked polyethylene bearings (PE) in the same uncemented cup shell. Migration was followed with RSA. At 2 years we observed similar mean cup translations in the 3 directions (0.07-0.40 mm vs. 0.05-0.31 mm, Ce/Ce vs. PE), as well as similar rotations around the 3 axes (0.31-0.92 degrees vs. 0.57-1.40 degrees). WOMAC and SF-36 scores were also similar and no radiolucent lines or osteolysis found. The large migration seen in some cups in both implant groups will require close monitoring to ascertain the reasons. Mean proximal wear of the polyethylene liners measured 0.016 mm between 2 and 24 months. Our data suggest there is no increased cup migration in the ceramic-on-ceramic implant compared with the metal-on-polyethylene, and they seem an equally safe choice. However, the low wear measured with the more versatile and less expensive cross-linked polyethylene makes it a strong contender.

LEVELS OF EVIDENCE

Therapeutic Level I. See the Guidelines for Authors for a complete description of levels of evidence.

Influence of polyethylene creep behavior on wear in total hip arthroplasty

After total hip arthroplasty (THA), polyethylene acetabular liner creep occurs quickly and serves to increases head-liner contact area and decrease contact pressures. What effect these early changes in contact mechanics will have on the wear behavior of the articulation remains unclear, and hence, selection or modification of polyethylene materials for optimal creep and wear performance is impossible. The objective of this study was to determine the influence of polyethylene creep behavior on volumetric wear and linear creep and wear penetration during simulated gait loading conditions. A finite element model of THA articulation was developed, and simultaneous numerical creep and wear simulation was performed to 10 million gait cycles with three levels of polyethylene creep behavior. Long-term volumetric wear and penetration were surprisingly unaffected by the polyethylene creep behavior due to the competing decrease in contact pressures coupled with increased contact area. In addition, variation in contact mechanics with the creep levels studied was only noteworthy in the initial postoperative period; after 1 million gait cycles, peak contact pressures and areas were within 13% regardless of the creep material behavior selected. Femoral head size had considerable impact on wear and penetration, while liner thickness primarily affected only early penetration. These results suggest that polyethylene creep behavior plays a major role in early penetration, but has little influence on the more important long-term volumetric wear.

Ten-year in vivo wear measurement of a fully congruent mobile bearing unicompartmental knee arthroplasty

Polyethylene particulate wear debris continues to be implicated in the aetiology of aseptic loosening following knee arthroplasty. The Oxford unicompartmental knee arthroplasty employs a spherical femoral component and a fully congruous meniscal bearing to increase contact area and theoretically reduce the potential for polyethylene wear. This study measures the in vivo ten-year linear wear of the device, using a roentgenstereophotogrammetric technique. In this in vivo study, seven medial Oxford unicompartmental prostheses, which had been implanted ten years previously were studied. Stereo pairs of radiographs were acquired for each patient and the films were analysed using a roentgen stereophotogrammetric analysis calibration and a computer-aided design model silhouette-fitting technique. Penetration of the femoral component into the original volume of the bearing was our estimate of linear wear. In addition, eight control patients were examined less than three weeks post-insertion of an Oxford prosthesis, where no wear would be expected. The control group showed no measured wear and suggested a system accuracy of 0.1 mm. At ten years, the mean linear wear rate was 0.02 mm/year. The results from this in vivo study confirm that the device has low ten-year linear wear in clinical practice. This may offer the device a survival advantage in the long term.

The long-term mechanical integrity of non-reinforced PEEK-OPTIMA polymer for demanding spinal applications: experimental and finite-element analysis

Polyetheretherketone (PEEK) is a novel polymer with potential advantages for its use in demanding orthopaedic applications (e.g. intervertebral cages). However, the influence of a physiological environment on the mechanical stability of PEEK has not been reported. Furthermore, the suitability of the polymer for use in highly stressed spinal implants such as intervertebral cages has not been investigated. Therefore, a combined experimental and analytical study was performed to address these open questions. A quasi-static mechanical compression test was performed to compare the initial mechanical properties of PEEK-OPTIMA polymer in a dry, room-temperature and in an aqueous, 37 degrees C environment (n=10 per group). The creep behaviour of cylindrical PEEK polymer specimens (n=6) was measured in a simulated physiological environment at an applied stress level of 10 MPa for a loading duration of 2000 hours (12 weeks). To compare the biomechanical performance of different intervertebral cage types made from PEEK and titanium under complex loading conditions, a three-dimensional finite element model of a functional spinal unit was created. The elastic modulus of PEEK polymer specimens in a physiological environment was 1.8% lower than that of specimens tested at dry, room temperature conditions (P<0.001). The results from the creep test showed an average creep strain of less than 0.1% after 2000 hours of loading. The finite element analysis demonstrated high strain and stress concentrations at the bone/implant interface, emphasizing the importance of cage geometry for load distribution. The stress and strain maxima in the implants were well below the material strength limits of PEEK. In summary, the experimental results verified the mechanical stability of the PEEK-OPTIMA polymer in a simulated physiological environment, and over extended loading periods. Finite element analysis supported the use of PEEK-OPTIMA for load-bearing intervertebral implants.

Computational wear prediction of a total knee replacement from in vivo kinematics

Wear of ultra-high molecular weight polyethylene bearings in total knee replacements remains a major limitation to the longevity of these clinically successful devices. Few design tools are currently available to predict mild wear in implants based on varying kinematics, loads, and material properties. This paper reports the implementation of a computer modeling approach that uses fluoroscopically measured motions as inputs and predicts patient-specific implant damage using computationally efficient dynamic contact and tribological analyses. Multibody dynamic simulations of two activities (gait and stair) with two loading conditions (70-30 and 50-50 medial-lateral load splits) were generated from fluoroscopic data to predict contact pressure and slip velocity time histories for individual elements on the tibial insert surface. These time histories were used in a computational wear analysis to predict the depth of damage due to wear and creep experienced by each element. Predicted damage areas, volumes, and maximum depths were evaluated against a tibial insert retrieved from the same patient who provided the in vivo motions. Overall, the predicted damage was in close agreement with damage observed on the retrieval. The gait and stair simulations separately predicted the correct location of maximum damage on the lateral side, whereas a combination of gait and stair was required to predict the correct location on the medial side. Predicted maximum damage depths were consistent with the retrieval as well. Total computation time for each damage prediction was less than 30 min. Continuing refinement of this approach will provide a robust tool for accurately predicting clinically relevant wear in total knee replacements.

A novel method for in vivo knee prosthesis wear measurement

Wear remains an important cause of failure in knee replacement. Of the current methods of early performance assessment or prediction, simulators have been un-physiological, single X-ray film analyses remain limited by accuracy and retrieval and survival methods have a prohibitive time scale. An accurate method is needed to allow a timely assessment of polyethylene component wear in vivo, when a new design is introduced, in order to predict likely outcome. We present a new method for measuring wear in vivo that we believe will allow this prediction of long-term wear. X-ray film pairs were taken of implanted prosthetic metal components. When the X-ray system was calibrated, projections of the appropriate Computer Aided Design (CAD) model could be matched to the shapes on the scanned X-ray films to find component positions. Interpenetration of the metal femoral component into the polyethylene component could then be established and represents our estimate of "wear". This method was used to measure in vivo prosthesis wear to an accuracy of 0.11 mm.

Finite element simulation of early creep and wear in total hip arthroplasty

Polyethylene wear particulate has been implicated in osteolytic lesion development and may lead to implant loosening and revision surgery. Wear in total hip arthroplasty is frequently estimated from patient radiographs by measurement of penetration of the femoral head into the polyethylene liner. Penetration, however, is multi-factorial, and includes components of wear and deformation due to creep. From a clinical perspective, it is of great interest to separate these elements to better evaluate true wear rates in vivo. Thus, the aim of this study was to determine polyethylene creep and wear penetration and volumetric wear during simulated gait loading conditions for variables of head size, liner thickness, and head-liner clearance. A finite element model of hip replacement articulation was developed, and creep and wear simulation was performed to 1 million gait cycles. Creep of the liner occurred quickly and increased the predicted contact areas by up to 56%, subsequently reducing contact pressures by up to 41%. Greater creep penetration was found with smaller heads, thicker liners, and larger clearance. The least volumetric wear but the most linear penetration was found with the smallest head size. Although polyethylene thickness increases from 4 to 16 mm produced only slight increases in volumetric wear and modest effects on total penetration, the fraction of creep in total penetration varied with thickness from 10% to over 50%. With thicker liners and smaller heads, creep will comprise a significant fraction of early penetration. These results will aid an understanding of the complex interaction of creep and wear.

In vivo measurement of total knee replacement wear

Polyethylene wear is one of the most important causes of failure of total knee replacements (TKRs). Currently, wear can only be accurately measured by retrieval studies. There is a need for a method to measure wear accurately in vivo. We have developed a Roentgen stereophotogrammetric analysis (RSA) system that can measure penetration of the metallic femoral component into the polyethylene of the tibia. We have used this system to study six AGC TKRs at 6 years postoperatively and six control AGC TKRs at 2 weeks postoperatively. The mean difference between the RSA measured bearing thickness and the manufacturer's quoted values for the control group was -0.03 mm (S.D. 0.17). The average linear penetration in the study group was 0.8 mm (S.D. 0.46). This was significantly (P<0.0001) different from the control group. The average linear penetration rate was 0.13 mm per year (S.D. 0.08). We would expect the penetration to deepen with time. In young active patients, this could be a cause for concern, particularly with a thin bearing. The current system is accurate enough to measure wear at 5 years post TKR. It has the potential for predicting long-term wear problems with new designs of TKR and new materials within 2 years.

Contact stresses in the glenoid component in total shoulder arthroplasty

Several studies of retrieved glenoid components from total shoulder arthroplasty show an erosion of the rim, surface irregularities, component fracture and wear resulting from polyethylene deformation in vivo. Particles resulting from polyethylene wear might be one of the reasons for the very high rate of glenoid component loosening found clinically. Because wear can be the result of high contact stresses, the aim of this study is to find out whether or not contact stresses are high enough to cause wear of the glenoid component and what influence the component type and geometry have on polyethylene contact stresses for different humerus abduction angles. Elasticity theory is used in a parametric study of contact stresses in several glenoid component designs. A finite element method is used to confirm the accuracy of the analytical solution. The analysis shows that the peak stress generated in glenoid components under conditions of normal living can be as high as 25 MPa; since this exceeds the polyethylene yield strength, wear and also cold flow of the components can be expected. It is predicted that more conforming components have lower contact stresses, which might result in lower wear rate and less cold flow. It is also found that a metal-backed component promotes higher contact stresses than an all-polyethylene component with the same total thickness, therefore it can be expected that metal-backed components have inferior wear properties.

Coventry Award paper. Backsurface wear and deformation in polyethylene tibial inserts retrieved postmortem

Wear and deformation were characterized at the backsurface of 25 posterior cruciate-retaining total knee arthroplasty polyethylene inserts retrieved postmortem from 20 subjects. The mean implantation time was 64.1 months (range, 4-156 months). The backsurface of the inserts was inspected using a stereomicroscope with a digital optical system. Coronal histologic sections of 13 proximal tibias were inspected for the presence and extent of penetration of granuloma. Damage to the backsurface was limited. Polishing was recorded on 21 (84%) of the inserts and abrasive wear on five (20%) inserts. Pitting was present in 21 (84%) components, but involved less than 1% of the area in all but one of these components. Delamination and cracking were not observed. Extrusions were seen in all 10 of the components that had screw holes in the tibial tray. A correlation was found between the depth of penetration of the granuloma along the posteromedial screw and the height of the corresponding extrusion. The anteroposterior profiles showed a concave deformation of the backsurface in 24 (96 %) of the cases. The concave deformation of tibial inserts may facilitate accumulation and transportation of wear debris to the tibial bone-implant interface through the screw holes in implants designed for cementless fixation.

Serial measurement of polyethylene wear of well-fixed cementless metal-backed acetabular component in total hip arthroplasty: an over 10 year follow-up study

Serial radiographic measurements of polyethylene wear were performed in 38 hips (33 patients) with primary cementless total hip arthroplasty (THA). The average follow-up period was 131.8 months. All prostheses were assessed as radiographically stable at the latest follow-up. A two-dimensional method was used to calculate the relative migration of the femoral head center to the cup center. The average total linear wear and wear rate were 1.22 mm and 0.11 mm/year, respectively. The degree of wear in the first 2 postoperative years accounted for nearly 40% of the total wear at the end of the study (average follow up: 131.8 +/- 10.0 months, +/-SD). The migration of the femoral head at an average period of 3. 4 months after operation accounted for 56% of the amount of wear in the first 2 years. Wear rate decreased gradually with time and stabilized after the fourth year. However, in 2 patients, a progressive increase in the wear rate was associated with severe osteolysis and failure of THA. Both creep and wear contributed to the femoral penetration into the polyethylene liner. The influence of creep cannot be ruled out, especially in the early period after operation. Polyethylene wear is a multifactorial process, and the study of individual wear patterns might be useful in identifying patients who are at risk of late failure of THA.