In-vivo analysis of sliding distance and cross-shear in Bi-cruciate retaining total knee arthroplasty

Polyethylene remains the most popular bearing material for total knee arthroplasty (TKA). Despite its widespread use, wear continue to be one of major factors implicated in revision surgery. Sliding distance, cross-shear, and contact stress are the major factors influencing polyethylene wear. As previous studies have either relied on wear simulations, computational modeling, or in vitro measurements to quantify sliding distance and cross-shear, in vivo subject-specific sliding distance and cross-shear after bi-cruciate retaining (BCR) TKA has not been previously reported. The objective of this study was to quantify the 6°-of-freedom (6DOF) in vivo kinematics, sliding distance, and cross-shear in BCR TKA patients during gait. Twenty-nine unilateral BCR TKA patients performed level walking on a treadmill under dual fluoroscopic imaging system (DFIS) surveillance. Cumulative normalized sliding distances between the lateral and medial compartments did not change significantly (p > 0.05) during the gait cycle. Although the total normalized sliding distance was similar between the lateral and medial compartments, the cross-shear at the lateral compartment differed significantly from that at the medial compartment (p < 0.001). Significant differences in the relative length positions of the peak sliding distance and cross-shear were found between the lateral and medial bearing components. The flexion-extension motion of the reconstructed knee was more associated with the linear displacements (anterior-posterior, R² = 0.6; lateral-medial, R² = 0.8, proximal-distal, R² = 0.7) than the angular displacement (varus-valgus, R² = 0.18; internal-external rotation, R² = 0.28). Despite some differences in peak sliding distance and cross-shear positons, our results suggest similar articular contact patterns between the lateral and medial compartments in BCR TKA patients during gait. The data could provide insights into understanding the potential wear patterns in BCR TKAs.     

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.     

A novel cross-shear metric for application in computer simulation of ultra-high molecular weight polyethylene wear

Wear testing of polyethylene in total joint replacements is common and required for any new device. Computational wear modelling has obvious utility in this context as it can be conducted with much greater economy than physical testing. Archard's law has become the accepted standard for wear simulation in total joints but it does not account for cross-shear, which is known to increase wear significantly relative to unidirectional sliding. The purpose of this study was to develop a robust cross-shear model applicable to any interface geometry under any kinematic conditions. The proposed metric, x ^*, is distinguished from existing cross-shear models by the fact that it measures cross-path motion incrementally throughout a motion cycle and quantifies cross-shear based on incremental changes in sliding direction. Validation showed strong support for the predictive capability of x ^* when applied to pin-on-disc test data.     

Quantification of the effect of cross-shear on the wear of conventional and highly cross-linked UHMWPE

A computational model has been developed to quantify the degree of cross-shear of a polyethylene pin articulating against a metallic plate, based on the direct simulation of a multidirectional pin-on-plate wear machine. The principal molecular orientation (PMO) was determined for each polymer site. The frictional work in the direction perpendicular to the PMO was assumed to produce the greatest orientation softening [Wang et al., 1997. Orientation softening in the deformation and wear of ultra-high molecular weight polyethylene. Wear 203-204, 230-241]. The cross-shear ratio (CS) was defined as the frictional work perpendicular to the PMO direction, divided by the total frictional work. Cross-shear on the pin contact surface was location specific, and of continuously changing magnitude because the direction of frictional force continuously changed due to pin rotation. The polymer pin motion was varied from a purely linear track (CS=0) up to a maximum rotation of +/-55 degrees (CS=0.254). The relationship between wear factors (K) measured experimentally and theoretically predicted CS was defined using logarithmic functions for both conventional and highly cross-linked ultra-high molecular weight polyethylene (UHMWPE). Cross-shear increased the apparent wear factor for both polyethylenes by more than fivefold compared to unidirectional wear.     

Effect of conformity and contact stress on wear in fixed-bearing total knee prostheses

Ultra high molecular weight polyethylene (PE) remains the primary bearing surface of choice in total knee replacements (TKR). Wear is controlled by levels of cross-shear motion and contact stress. The aim of this study was to compare the wear of fixed-bearing total knee replacements with curved and flat inserts and to test the hypothesis that the flat inserts which give higher contact stresses and smaller contact areas would lead to lower levels of surface wear. A low-conforming, high contact stress knee with a low-medium level of cross shear resulted in significantly lower wear rates in comparison to a standard cruciate sacrificing fixed-bearing knee. The low wear solution found in the knee simulator was supported by fundamental studies of wear as a function of pressure and cross shear in the pin on plate system. Current designs of fixed-bearing knees do not offer this low wear solution due to their medium cross shear, moderate conformity and medium contact stress.     

Computational wear prediction of artificial knee joints based on a new wear law and formulation

Laboratory joint wear simulator testing has become the standard means for preclinical evaluation of wear resistance of artificial knee joints. Recent simulator designs have been advanced and become successful at reproducing the wear patterns observed in clinical retrievals. However, a single simulator test can be very expensive and take a long time to run. On the other hand computational wear modelling is an alternative attractive solution to these limitations. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. However, all these models have adopted the classical Archard's wear law, which was developed for metallic materials, and have selected wear factors arbitrarily. It is known that such an approach is not generally true for polymeric bearing materials and is difficult to implement due to the high dependence of the wear factor on the contact pressure. Therefore, these studies are generally not independent and lack general predictability. The objective of the present study was to develop a new computational wear model for the knee implants, based on the contact area and an independent experimentally determined non-dimensional wear coefficient. The effects of cross-shear and creep on wear predictions were also considered. The predicted wear volume was compared with the laboratory simulation measurements. The model was run under two different kinematic inputs and two different insert designs with curved and custom designed flat bearing surfaces. The new wear model was shown to be capable of predicting the difference of the wear volume and wear pattern between the two kinematic inputs and the two tibial insert designs. Conversely, the wear factor based approach did not predict such differences. The good agreement found between the computational and experimental results, on both the wear scar areas and volumetric wear rates, suggests that the computational wear modelling based on the new wear law and the experimentally calculated non-dimensional wear coefficient should be more reliable and therefore provide a more robust virtual modelling platform.     

Problematic sites of third body embedment in polyethylene for total hip wear acceleration

A computational model was developed to identify the sites of third body particle embedment in a total hip acetabular component surface that are most problematic in terms of roughening the overpassing regions of the femoral head counterface, leading in turn to most severely accelerated polyethylene wear. The analytical approach used was to calculate loci of acetabular sites that, during the gait cycle, overpass previously documented regions of kinetically most critical femoral head roughening. Instantaneous local contact stress and sliding distance were postulated as factors contributing to the severity of the femoral head scratching/roughening which would be expected, due to otherwise-similar particles embedded along each such acetabular overpass locus. The computational results showed that the location of debris embedment was a potent determinant of the amount of polyethylene wear acceleration expected. The data also showed that the supero-lateral aspect of the acetabular cup is consistently and by far the most problematic area for third body particle embedment.     

Computational hip joint simulator for wear and heat generation

This paper presents a computational simulator for the hip to compute the wear and heat generation on artificial joints. The friction produced on artificial hip joints originates wear rates that can lead to failure of the implant. Furthermore, the frictional heating can increase the wear. The developed computational model calculates the wear in the joint and the temperature in the surrounding zone, allowing the use of different combinations of joint materials, daily activities and different individuals. The pressure distribution on the joint bearing surfaces is obtained with the solution of a contact model. The heat generation by friction and the volumetric wear is computed from the pressure distribution and the sliding distance. The temperature is obtained from the solution of a transient heat conduction problem that includes the time-dependent heat generated by friction. The contact and heat conduction problems are solved numerically with the Finite Element Method. The developed computational model performs a full simulation of the acetabular bearing surface behaviour, which is useful for acetabular cup design and material selection. The results obtained by the present model agree with experimental and clinical data, as well as other numerical studies.     

An improved method of computing the wear factor for total hip prostheses involving the variation of relative motion and contact pressure with location on the bearing surface

A new method of computing the wear factor for total hip prostheses is presented. In the conventional method, only the resultant contact force and the track drawn by the point of its application are considered so that the product of the instantaneous force and sliding increment is integrated over one motion cycle. In the present, improved, method the contact pressure distribution is discretized by a large number of smaller normal forces, and the contribution of each is summed. This is important because the relative motion and contact pressure vary strongly with location, and because the transverse pressure component is substantial. Hence, the present surface integral represents the large contact surface better than the conventional line integral. A prerequisite for the surface integral was the method of computing the relative motion correctly anywhere on the contact surface, developed and published earlier by the present authors. For the pressure discretization, the contact surface was divided into nearly equal-sized surface elements. The contact pressure was modelled with ellipsoidal, paraboloidal and sinusoidal distributions. Two load cases were studied, double-peak and static. When an ellipsoidal contact pressure distribution extending over a hemisphere was discretized by 1000 element forces, the computed wear factor for double-peak load in a biaxial hip wear simulator was 30% lower than in the conventional resultant force case. The present method can be later developed further to involve the temporal variation of size and location of the contact surface.     

RandomPOD--a new method and device for advanced wear simulation of orthopaedic biomaterials

A 16-station wear simulator of the pin-on-disc type, called RandomPOD, was designed, built, and validated. The primary area of application of the RandomPOD is wear studies of orthopaedic biomaterials. The type of relative motion between the bearing surfaces, generally illustrated as shapes of slide tracks, has been found to have a strong effect on the type and amount of wear produced. The computer-controlled RandomPOD can be programmed to produce virtually any slide track shape and load profile. In the present study, the focus is on the biomechanically realistic random variation in the track shape and load. In the reference test, the established combination of circular translation and static load was used. In addition, the combinations of random motion/static load, and circular translation/random load were included. The pins were conventional ultra-high molecular weight polyethylene (UHMWPE), the discs were polished CoCr, and the lubricant was diluted calf serum. The UHMWPE wear factor resulting from random motion was significantly higher than that resulting from circular translation. This was probably caused by the fact that in the random motion the direction of sliding changed more than in circular translation with the same sliding distance. The type of load, random vs. static, was unimportant with respect to the wear factor produced. The principal advantage of using the present random track is that possible unrealistic wear phenomena related to the use of fixed track shapes can be avoided.     

An energy dissipation and cross shear time dependent computational wear model for the analysis of polyethylene wear in total knee replacements

The cost and time efficiency of computational polyethylene wear simulations may enable the optimization of total knee replacements for the reduction of polyethylene wear. The present study proposes an energy dissipation wear model for polyethylene which considers the time dependent molecular behavior of polyethylene, aspects of tractive rolling and contact pressure. This time dependent – energy dissipation wear model was evaluated, along with several other wear models, by comparison to pin-on-disk results, knee simulator wear test results under various kinematic conditions and knee simulator wear test results that were performed following the ISO 14243-3 standard. The proposed time dependent – energy dissipation wear model resulted in improved accuracy for the prediction of pin-on-disk and knee simulator wear test results compared with several previously published wear models.     

Local head roughening as a factor contributing to variability of total hip wear: a finite element analysis

Large inter-patient variability in wear rate and wear direction have been a ubiquitous attribute of total hip arthroplasty (THA) cohorts. Since patients at the high end of the wear spectrum are of particular concern for osteolysis and loosening, it is important to understand why some individuals experience wear at a rate far in excess of their cohort average. An established computational model of polyethylene wear was used to test the hypothesis that, other factors being equal, clinically typical variability in regions of localized femoral head roughening could account for much of the variability observed clinically in both wear magnitude and wear direction. The model implemented the Archard abrasive/adhesive wear relationship, which incorporates contact stress, sliding distance, and (implicitly) bearing surface tribology. Systematic trials were conducted to explore the influences of head roughening severity, roughened area size, and roughened area location. The results showed that, given the postulated wear factor elevations, head roughening variability (conservatively) typical of retrieval specimens led to approximately a 30 degrees variation in wear direction, and approximately a 7-fold variation in volumetric wear rate. Since these data show that randomness in head scratching can account for otherwise-difficult-to-explain variations in wear direction and wear rate, third-body debris may be a key factor causing excessive wear in the most problematic subset of the THA population.     

Improved single- and multi-contact life-time testing of dental restorative materials using key characteristics of the human masticatory system and a force/position-controlled robotic dental wear simulator

This paper presents a new in vitro wear simulator based on spatial parallel kinematics and a biologically inspired implicit force/position hybrid controller to replicate chewing movements and dental wear formations on dental components, such as crowns, bridges or a full set of teeth. The human mandible, guided by passive structures such as posterior teeth and the two temporomandibular joints, moves with up to 6 degrees of freedom (DOF) in Cartesian space. The currently available wear simulators lack the ability to perform these chewing movements. In many cases, their lack of sufficient DOF enables them only to replicate the sliding motion of a single occlusal contact point by neglecting rotational movements and the motion along one Cartesian axis. The motion and forces of more than one occlusal contact points cannot accurately be replicated by these instruments. Furthermore, the majority of wear simulators are unable to control simultaneously the main wear-affecting parameters, considering abrasive mechanical wear, which are the occlusal sliding motion and bite forces in the constraint contact phase of the human chewing cycle. It has been shown that such discrepancies between the true in vivo and the simulated in vitro condition influence the outcome and the quality of wear studies. This can be improved by implementing biological features of the human masticatory system such as tooth compliance realized through the passive action of the periodontal ligament and active bite force control realized though the central nervous system using feedback from periodontal preceptors. The simulator described in this paper can be used for single- and multi-occlusal contact testing due to its kinematics and ability to exactly replicate human translational and rotational mandibular movements with up to 6 DOF without neglecting movements along or around the three Cartesian axes. Recorded human mandibular motion and occlusal force data are the reference inputs of the simulator. Experimental studies of wear using this simulator demonstrate that integrating the biological feature of combined force/position hybrid control in dental material testing improves the linearity and reduces the variability of results. In addition, it has been shown that present biaxially operated dental wear simulators are likely to provide misleading results in comparative in vitro/in vivo one-contact studies due to neglecting the occlusal sliding motion in one plane which could introduce an error of up to 49% since occlusal sliding motion D and volumetric wear loss V(loss) are proportional.     

Force track analysis of contemporary hip simulators

In an earlier paper, the authors presented the first verified method of computation of slide tracks in the relative motion between femoral head and acetabular cup of total hip prostheses. The method was applied for gait and for two hip simulator designs, and in a subsequent paper, for another eight designs. In the present paper, the track drawn by the resultant contact force, the so-called force track, was studied in depth. The variations of sliding distance, sliding velocity and direction of sliding during a cycle, all of which are important with respect to wear, were computed for gait and for 11 hip simulator designs. Moreover, the product of the instantaneous load and increment of sliding distance was numerically integrated over a cycle. This integral makes it possible to compare clinical wear rates with those produced by hip simulators in terms of a wear factor. For the majority of contemporary hip simulators, the integral has so far been unknown. The computations revealed considerable differences, which are likely to explain the substantial differences in wear produced by the simulators. With the most common head diameter, 28 mm, the ranges for sliding distance per cycle, mean sliding velocity, total change of direction of sliding and integral were: 19.7-34.3 mm, 19.7-49.0 mm/s, 360-1513 degrees, and 17.4-43.5 Nm, respectively.     

Comparison of long-term numerical and experimental total knee replacement wear during simulated gait loading

Pre-clinical experimental wear testing of total knee replacement (TKR) components is an invaluable tool for evaluating new implant designs and materials. However, wear testing can be a lengthy and expensive process, and hence parametric studies evaluating the effects of geometric, loading, or alignment perturbations may at times be cost-prohibitive. The objectives of this study were to develop an adaptive FE method capable of simulating wear of a polyethylene tibial insert and to compare predicted kinematics, weight loss due to wear, and wear depth contours to results from a force-controlled experimental knee simulator. Finite element-based computational wear predictions were performed to 5 million gait cycles using both force- and displacement-controlled inputs. The displacement-controlled inputs, by accurately matching the experimental tibiofemoral motion, provided an evaluation of the simple wear theory. The force-controlled inputs provided an evaluation of the overall numerical method by simultaneously predicting both kinematics and wear. Analysis of the predicted wear convergence behavior indicated that 10 iterations, each representing 500,000 gait cycles, were required to achieve numerical accuracy. Using a wear factor estimated from the literature, the predicted kinematics, polyethylene wear contours, and weight loss were in reasonable agreement with the experimental data, particularly for the stance phase of gait. Although further development of the simplified wear theory is important, the initial predictions are encouraging for future use in design phase implant evaluation. In contrast to the experimental testing which occurred over approximately 2 months, computational wear predictions required only 2h.     

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.     

Surface slide track mapping of implants for total disc arthroplasty

Total disc arthroplasty has recently become a potential alternative to spinal arthrodesis. Until recently, there has been no standardized method for evaluating the wear of an artificial disc and myriad testing conditions have been used. The American Society for Testing and Materials (ASTM) and International Organization of Standardization (ISO) recently published guidance documents for the wear assessment of intervertebral spinal disc prostheses; however, various kinematic profiles are suggested, leading to different wear paths between the articulating surfaces of the implants. Since the wear between materials is influenced by the type of relative motion, it is important to select test conditions that lead to clinically realistic results. The purpose of this study was to characterize the slide tracks generated by 7 test conditions allowed for by the ISO and ASTM guidance documents and in Euler sequences consistent with 4 commercially available spine wear simulators. The analysis was performed for a ball-in-socket articulation under both lumbar and cervical motion test conditions. Results were generated analytically using a mathematical algorithm and then validated experimentally. Four tests resulted in elliptical sliding tracks of similar geometries for both the lumbar and cervical conditions. Curvilinear and ribbon-shaped wear paths were generated for 3 tests. With the data normalized for implant diameter, the sliding distance was similar between the lumbar and cervical conditions allowed for in the ASTM guidance. This distance differed compared with the results for the ISO guidance document where the lengths of cervical slide tracks were twice those for the lumbar conditions. Slide tracks were also found to be insensitive to the type of simulator under all testing conditions.     

Simulation of initial frontside and backside wear rates in a modular acetabular component with multiple screw holes.

A sliding distance-based finite element formulation was implemented to predict initial wear rates at the front and back surfaces of a commercially available modular polyethylene component during in vitro loading conditions. We found that contact area, contact stress, and wear at the back surface were more sensitive to the liner/shell conformity than the presence of multiple screw holes. Furthermore, backside linear and volumetric wear rates were at least three orders of magnitude less than respective wear estimates at the articulating surface. This discrepancy was primarily attributed to the difference in maximum sliding distances at the articulating surfaces (measured in mm) versus the back surface (measured in microm). This is the first study in which backside wear has been quantified and explicitly compared with frontside wear using clinically relevant metrics established for the articulating surface. The results of this study suggest that with a polished metal shell, the presence of screw holes does not substantially increase abrasive backside wear when compared with the effects of backside nonconformity.     

A direct comparison of patient and force-controlled simulator total knee replacement kinematics

The need to critically evaluate the efficacy of current total knee replacement (TKR) wear testing methodologies is great. Proposed international standards for TKR wear simulation have been drafted, yet their methods continue to be debated. The "gold standard" to which all TKR wear testing methodologies should be compared is measured in vivo TKR performance in patients. The current study compared patient TKR kinematics from fluoroscopic analysis and simulator TKR kinematics from force-controlled wear testing to quantify similarities in clinical ranges of motion and contact bearing kinematics and to evaluate the proposed ISO force-controlled wear testing methodology. The treadmill walking kinematics from eight well-functioning, 13 month average post-op patients were compared to the 2 million cycle interval walking cycle kinematics from a force-controlled (Instron/Stanmore Knee Joint Simulator, Instron, Canton, MA) knee simulator using identical implant designs (Natural Knee II, Standard Congruent, Zimmer, Warsaw, IN). The in vivo and simulator data showed good agreement in kinematic patterns and ranges of clinical motion. Tribologically the data sets showed similar contact pathway ranges of motion and wear travel distances per cycle. Surgical and simulator alignments of the implant systems were determined to be a contributing factor in observed kinematic differences. This study's statistical findings offer supporting evidence that the simulation of in vivo walking cycle wear kinematics can be accurately reproduced with a force controlled testing methodology.     

The transport of wear particles in the prosthetic hip joint: a computational fluid dynamics investigation

The joint fluid mechanics and transport of wear particles in the prosthetic hip joint were analyzed for subluxation and flexion motion using computational fluid dynamics (CFD). The entire joint space including a moving capsule boundary was considered. It was found that particles suspended in the joint space are drawn into the joint gap between prosthesis cup and head during subluxation, which was also documented by Lundberg et al. (2007; Journal of Biomechanics 40, 1676-1685), however, wear particles remain in the joint gap. Wear particles leave the joint gap during flexion and can finally migrate to the proximal boundaries including the acetabular bone, where the particle deposition can cause osteolysis according to the established literature. Thus, the present study supports the theory of polyethylene wear particle induced osteolysis of the acetabular bone as a major factor in the loosening of hip prosthesis cups.