Slide track analysis of eight contemporary hip simulator designs

In an earlier paper, the authors presented a new method of computation of slide tracks in the relative motion between femoral head and acetabular cup of total hip prostheses. For the first time, computed tracks were verified experimentally and with an alternative method of computation. Besides being an efficient way to illustrate hip kinematics, the shapes of the slide tracks are known to be of fundamental importance regarding the wear behaviour of prostheses. The verified method was now applied to eight contemporary hip simulator designs. The use of correct motion waveforms and an Euler sequence of rotations in each case was again found to be essential. Considerable differences were found between the simulators. For instance, the shapes of the tracks drawn by the resultant contact force included a circle, ellipse, irregular oval, leaf, twig, and straight line. Computation of tracks correctly for the most widely used hip simulator, known as biaxial, was made possible by the insight that the device is actually three-axial. Slide track patterns have now been computed for virtually all contemporary hip simulators, and both for the heads and for the cups. This comparative analysis forms a valuable basis for studies on the relationship between the type of multidirectional motion and wear. These studies can produce useful information for the design of joint simulators, and improve the understanding of wear phenomena in prosthetic joints.     

Slide track analysis of the relative motion between femoral head and acetabular cup in walking and in hip simulators

Joint simulators are important tools in wear studies of prosthetic joint materials. The type of motion in a joint simulator is crucial with respect to the wear produced. It is widely accepted that only multidirectional motion yields realistic wear for polyethylene acetabular cups. Multidirectionality, however, is a wide concept. The type of multidirectional motion varies considerably between simulators, which may explain the large differences in observed wear rates. At present, little is known about the relationship between the type of multidirectional motion and wear. One illustrative way to compare the motions of various hip simulators is to compute tracks made on the counterface by selected points of the surface of the femoral head and acetabular cup due to the cyclic relative motion. A new computation method, based on Euler angles, was developed, and used to compute slide tracks for the three-axis motion of the hip joint in walking, and for two hip simulators, the HUT-3 and the biaxial rocking motion. The slide track patterns resulting from the gait waveforms were found to be similar to those produced by the HUT-3 simulator. This paper is the first to include a verification of the computed simulator tracks. The tracks were verified in the two simulators using sharp pins, embedded in acetabular cups, engraving distinct grooves onto the femoral heads. The engravings were identical to the computed tracks. The results clearly differed from earlier computations by another research group. This study is intended to start a thorough investigation of the relationship between the type of multidirectional motion and wear.     

Study of micromotion in modular acetabular components during gait and subluxation: a finite element investigation

Polyethylene wear after total hip arthroplasty may occur as a result of normal gait and as a result of subluxation and relocation with impact. Relocation of a subluxed hip may impart a moment to the cup creating sliding as well as compression at the cup liner interface. The purpose of the current study is to quantify, by a validated finite element model, the forces generated in a hip arthroplasty as a result of subluxation relocation and compare them to the forces generated during normal gait. The micromotion between the liner and acetabular shell was quantified by computing the sliding track and the deformation at several points of the interface. A finite element analysis of polyethylene liner stress and liner/cup micromotion in total hip arthroplasty was performed under two dynamic profiles. The first profile was a gait loading profile simulating the force vectors developed in the hip arthroplasty during normal gait. The second profile is generated during subluxation and subsequent relocation of the femoral head. The forces generated by subluxation relocation of a total hip arthroplasty can exceed those forces generated during normal gait. The induced micromotion at the cup polyethylene interface as a result of subluxation can exceed micromotion as a result of the normal gait cycle. This may play a significant role in the generation of backsided wear. Minimizing joint subluxation by restoring balance to the hip joint after arthroplasty should be explored as a strategy to minimize backsided wear.     

Enhanced computational prediction of polyethylene wear in hip joints by incorporating cross-shear and contact pressure in additional to load and sliding distance: Effect of head diameter

A new definition of the experimental wear factor was established and reported as a function of cross-shear motion and contact pressure using a multi-directional pin-on-plate wear testing machine for conventional polyethylene in the present study. An independent computational wear model was developed by incorporating the cross-shear motion and contact pressure-dependent wear factor into the Archard's law, in additional to load and sliding distance. The computational prediction of wear volume was directly compared with a simulator testing of a polyethylene hip joint with a 28 mm diameter. The effect of increasing the femoral head size was subsequently considered and was shown to increase wear, as a result of increased sliding distance and reduced contact pressure.     

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.     

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.     

A new ISO/FDIS 14242-1 compatible hip prosthesis simulator: E-SIM]

The continuing development of new, highly sophisticated materials for the articulating surfaces of total hip endoprostheses involves the need for testing, not only of biocompatibility and dynamic loadability, but also of tribological properties (friction, wear, lubrication). For decades, the wear resistance of these materials has been tested in wear simulators. In consequence of the currently often widely differing test methods, the technical committee (TC 150) of the ISO (International Organization for Standardization) has been concerned to develop an International Standard (ISO/FDIS 14242 1 and 2: Implants for Surgery--wear of total hip joint prostheses--on the basis of kinetic and kinematic data from gait analysis. This new standard will be the basis for ensuring the comparability of scientific data obtained from tribological testing of total hip endoprothesis. The new hip simulator, E-SIM, presented in this paper, complies with the currently published FDIS (Final Draft International Standard), and enables testing in accordance with these specifications.     

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.     

Analysis of the Biotribological Behavior for the Stainless Steel/Polyethylene Contact Using a Knee Prosthesis Simulator

In this work,a friction and wear simulator was used to reproduce the Anterior-Posterior (AP) sliding and the Flexion-Extension (FE) rotation generated in the knee joint during human gait cycle.We chose to simplify the contact geometry between the Total Knee Arthroplasty (TKA) femoral component and tibial insert.A 304L stainless steel cylinder which replaces the femoral component was loaded onto a flat High Density Polyethylene (HDPE) block which replaces the tibial insert.The tribological behavior of the considered contact was analyzed by tracking the number of cycles,the friction coefficient,the roughness of the wear track on HDPE,the HDPE weight loss and the damage mechanisms.The friction coefficient shows a gradual increase with the number of cycles for both AP and FE kinematics.The evolution of friction coefficient with the number of cycles is not affected by the value of the imposed normal load in the case of AP sliding.For the FE rotation,decreased friction coefficient is obtained when the imposed normal load increases.For both considered AP and FE kinematics,the roughness of the wear track on the HDPE is not affected by the imposed normal load.It shows a progressive decrease when the number of cycles increases.The wear of HDPE obeys the Archard law and the wear coefficient increases with the normal force.For a given value of normal load,the obtained wear coefficient for the AP sliding is larger than that obtained for FE rotation.A predominant adhesive wear mechanism was identified for both AP and FE kinematics.Under the same normal load,damage development in terms of plastic deformation,micro-cracking and debonding is more pronounced for the AP sliding if compared with the FE rotation.For a given kinematics,the damage severity increases with the normal load.This finding is in good agreement with the predicted values of the wear coefficient according to the Archard law.     

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.     

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.     

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.     

Sliding distance per ATP molecule hydrolyzed by myosin heads during isotonic shortening of skinned muscle fibers.

We measured isotonic sliding distance of single skinned fibers from rabbit psoas muscle when known and limited amounts of ATP were made available to the contractile apparatus. The fibers were immersed in paraffin oil at 20 degrees C, and laser pulse photolysis of caged ATP within the fiber initiated the contraction. The amount of ATP released was measured by photolyzing 3H-ATP within fibers, separating the reaction products by high-pressure liquid chromatography, and then counting the effluent peaks by liquid scintillation. The fiber stiffness was monitored to estimate the proportion of thick and thin filament sites interacting during filament sliding. The interaction distance, Di, defined as the sliding distance while a myosin head interacts with actin in the thin filament per ATP molecule hydrolyzed, was estimated from the shortening distance, the number of ATP molecules hydrolyzed by the myosin heads, and the stiffness. Di increased from 11 to 60 nm as the isotonic tension was reduced from 80% to 6% of the isometric tension. Velocity and Di increased with the concentration of ATP available. As isotonic load was increased, the interaction distance decreased linearly with decrease of the shortening velocity and extrapolated to 8 nm at zero velocity. Extrapolation of the relationship between Di and velocity to saturating ATP concentration suggests that Di reaches 100-190 nm at high shortening velocity. The interaction distance corresponds to the sliding distance while cross-bridges are producing positive (working) force plus the distance while they are dragging (producing negative forces). The results indicate that the working and drag distances increase as the velocity increases. Because Di is larger than the size of either the myosin head or the actin monomer, the results suggest that for each ATPase cycle, a myosin head interacts mechanically with several actin monomers either while working or while producing drag.     

"In vivo" determination of hip joint separation and the forces generated due to impact loading conditions

Numerous supporting structures assist in the retention of the femoral head within the acetabulum of the normal hip joint including the capsule, labrum, and ligament of the femoral head (LHF). During total hip arthroplasty (THA), the LHF is often disrupted or degenerative and is surgically removed. In addition, a portion of the remaining supporting structures is transected or resected to facilitate surgical exposure. The present study analyzes the effects of LHF absence and surgical dissection in THA patients. Twenty subjects (5 normal hip joints, 10 nonconstrained THA, and 5 constrained THA) were evaluated using fluoroscopy while performing active hip abduction. All THA subjects were considered clinically successful. Fluoroscopic videos of the normal hips were analyzed using digitization, while those with THA were assessed using a computerized interactive model-fitting technique. The distance between the femoral head and acetabulum was measured to determine if femoral head separation occurred. Error analysis revealed measurements to be accurate within 0.75mm. No separation was observed in normal hips or those subjects implanted with constrained THA, while all 10 (100%) with unconstrained THA demonstrated femoral head separation, averaging 3.3mm (range 1.9-5.2mm). This study has shown that separation of the prosthetic femoral head from the acetabular component can occur. The normal hip joint has surrounding capsuloligamentous structures and a ligament attaching the femoral head to the acetabulum. We hypothesize that these soft tissue supports create a passive, resistant force at the hip, preventing femoral head separation. The absence of these supporting structures after THA may allow increased hip joint forces, which may play a role in premature polyethylene wear or prosthetic loosening.     

Development of computational wear simulation of metal-on-metal hip resurfacing replacements

As one of the alternatives to traditional metal-on-polyethylene total hip replacements, metal-on-metal hip resurfacing prostheses demonstrating lower wear have been introduced for younger and more active patients during the past decade. However, in vitro hip simulator testing for the predicted increased lifetime of these surface replacements is time-consuming and costly. Computational wear modelling based on the Archard wear equation and finite element contact analysis was developed in this study for artificial hip joints and particularly applied to metal-on-metal resurfacing bearings under simulator testing conditions to address this issue. Wear factors associated with the Archard wear equation were experimentally determined and based on the short-term hip simulator wear results. The computational wear simulation was further extended to a long-term evaluation up to 50 million cycles assuming that the wear rate stays constant. The prediction from the computational model shows good agreement with the corresponding simulator study in terms of volumetric wear and the wear geometry. The simulation shows the progression of linear wear penetrations, and the complexity of contact stress distribution on the worn bearing surfaces. After 50 million cycles, the maximum linear wear was predicted to be approximately 6 and 8 microm for the cup and head, respectively, and no edge contact was found.     

A novel low wearing differential hardness, ceramic-on-metal hip joint prosthesis

Osteolysis and loosening of artificial joints caused by polyethylene wear debris has prompted renewed interest in alternative bearing materials for hip prosthesis designs. Lower wearing metal-on-metal (MOM) and ceramic-on-ceramic prostheses are being used more extensively, and there is considerable interest in further improving on their performance. This study investigated the wear properties and debris morphology of a novel differential hardness ceramic-on-metal (COM) prosthesis, in comparison with MOM articulations in a physiological anatomical hip joint simulator.The COM pairings were found to have wear rates approximately 100-fold lower than the MOM pairings. The MOM pairings showed a higher "bedding in" wear rate (3.09+/-0.46mm(3)/10(6) cycles) in the first million cycles, which then reduced to a steady state wear rate of 1.23+/-0.5mm(3)/10(6) cycles. The wear rate of the COM pairings over the duration of the test was approximately 0.01mm(3)/10(6) cycles with very little wear detected on the surface of the prosthesis components.The wear particles from both articulations were oval to round in shape and in the nanometer size range. After one million cycles the mean maximum diameter of the MOM and COM wear particles were 30+/-2.25 and 17.57+/-1.37nm, respectively. After five million cycles the wear particles were statistically significantly smaller than at one million cycles, 13.9+/-0.72nm for the MOM pairings and 6.11+/-0.40nm for the COM pairings.The wear rates of the MOM prostheses were representative of clinical values. The use of differential hardness COM pairings dramatically reduced the wear rate compared to MOM hip prostheses. The wear particles from the MOM articulation were similar to particles found in retrieved tissues from around MOM prostheses. The extremely low wearing differential hardness COM bearings presented in this study produced far smaller volumetric particle loads compared to MOM prostheses currently used clinically.     

In vitro fatigue failure of cemented acetabular replacements: a hip simulator study

Although hip simulators for in vitro wear testing of prosthetic materials used in total hip arthroplasty (THA) have been available for a number of years, similar equipment has yet to appear for endurance testing of fixation in cemented THA, despite considerable evidence of late aseptic loosening as one of the most significant failure mechanisms in this type of replacements. An in vitro study of fatigue behavior in cemented acetabular replacements has been carried out, utilizing a newly developed hip simulator. The machine was designed to simulate the direction and the magnitude of the hip contact force under typical physiological loading conditions, including normal walking and stair climbing, as reported by Bergmann et al. (2001, Hip 98, Freie Universitaet, Berlin). A 3D finite element analysis has been carried out to validate the function of the hip simulator and to evaluate the effects of boundary conditions and geometry of the specimen on the stress distribution in the cement mantle. Bovine pelvic bones were implanted with a Charnley cup, using standard manual cementing techniques. Experiments were carried out under normal walking and descending stairs loading conditions with selected load levels from a body weight of 75-125 kg. Periodically, the samples were removed from the test rigs to allow CT scanning for the purpose of monitoring damage development in the cement fixation. The hip simulator was found to be satisfactory in reproducing the hip contact force during normal walking and stair climbing, as reported by Bergmann et al. Finite element analysis shows that the stress distributions in the cement mantle and at the bone-cement interface are largely unaffected by the geometry and the boundary conditions of the model. Three samples were tested up to 17 x 10(6) cycles and sectioned post-testing for microscopic studies. Debonding at the bone-cement interface of various degrees in the posterior-superior quadrant was revealed in these samples, and the location of the failures corresponds to the highest stressed region from the finite-element analysis. Preliminary experimental results from a newly developed hip simulator seem to suggest that debonding at the bone-cement interface is the main failure mechanism in cemented acetabular replacements, and descending stairs seem to be more detrimental than normal walking or ascending stairs with regard to fatigue integrity of cement fixation.     

A New Simulator for Bio-Tribological Study

A new bio-tribological simulator system was designed and built for basic wear tests on artificial hip joint materials and for common tribological studies applications. The module of hip joint simulator is consisted of Flexion and Extension (FE), Abduction and Adduction (AA) and Internal and External Rotation (IER). Preliminary tests were done with a 28 mm CoCrMo femoral head and a Ultra High Molecular Weight Polyethylene (UHMWPE) cup. Results showed that the wear rate was close to the clinical one. A frequency control system and a heating system were developed to offer a wide range of rotation frequency and temperature so as to provide proper experiments. In the Pin-on-Disk (POD) part, eight precision-made pins were generated and an advanced computer system was built up to measure the friction coefficient of the tests samples.     

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.     

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.