In vitro wear simulation on the RandomPOD wear testing system as a screening method for bearing materials intended for total knee arthroplasty

The 16-station RandomPOD wear test system, previously validated for prosthetic hip wear, was used in the simulation of knee wear mechanisms with a ball-on-flat test configuration. This consisted of a CoCr pin with a ground and polished spherical bearing surface (radius 28 mm) against a conventional, gamma-sterilized UHMWPE disk in serum lubrication. The biaxial motion, consisting of x and y translations, and the load was non-cyclic. Relative to the disk, the center of contact wandered within a circle of 10 mm diameter, and the average sliding velocity was 15.5 mm/s (ranging from 0 to 31 mm/s). The load varied non-cyclically between 0 and 142 N (average 73 N). In the 60-day test with 16 similar wear couples, moderate adhesive wear, the principal wear mechanism of a well-functioning prosthetic knee, dominated. This showed as a burnished, circular wear mark (diameter 13.2 mm, area 137 mm²). The wear factor was 2.04±0.03×10⁻⁶ mm³/N m (mean±95 percent confidence limit). For the first time a truly multidirectional, realistic and uniform, large capacity pin-on-disk simulation of knee wear mechanisms was implemented.     

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.     

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.     

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.     

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.     

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.     

The influence of design, materials and kinematics on the in vitro wear of total knee replacements

Debris-induced osteolysis due to surface wear of ultra high molecular weight polyethylene (UHMWPE) bearings is a potential long-term failure mechanism of total knee replacements (TKR). This study investigated the effect of prosthesis design, kinematics and bearing material on the wear of UHMWPE bearings using a physiological knee simulator. The use of a curved fixed bearing design with stabilised polyethylene bearings reduced wear in comparison to more flat-on-flat components which were sterilised by gamma irradiation in air. Medium levels of crosslinking further improved the wear resistance of fixed bearing TKR due to resistance to strain softening when subjected to multidirectional motion at the femoral-insert articulating interface. Backside motion was shown to be a contributing factor to the overall rate of UHMWPE wear in fixed bearing components. Wear of fixed bearing prostheses was reduced significantly when anterior-posterior displacement and internal-external rotation kinematics were reduced due to decreased cross shear on the articulating surface and a reduction in AP displacement. Rotating platform mobile bearing prostheses exhibited reduced wear rates in comparison to fixed bearing components in these simulator studies due to redistribution of knee motion to two articulating interfaces with more linear motions at each interface. This was observed in two rotating platform designs with different UHMWPE bearing materials. In knee simulator studies, wear of TKR bearings was dependent on kinematics at the articulating surfaces and the prosthesis design, as well as the type of material.     

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.     

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.     

Preparation of UHMWPE particles and establishment of inverted macrophage cell model to investigate wear particles induced bioactivites

Total joint replacement surgery has been widely applied to patients with severe osteoarthritis. Aseptic loosening induced by wear particles generated during joint movement is the major reason causing the failure of joint implants. Interaction of ultra-high molecular weight polyethylene (UHMWPE) wear particles with macrophages stimulates the release of inflammatory cytokines and leads to bone resorption and osteolysis. Effect of UHMWPE particle size and shape on the bioactivities remains unclear due to the lack of particles with controlled morphology as well as adequate in-vitro cell culture models for further investigations. We have developed a micro-cutting procedure to generate UHMWPE particles with desired sizes and shapes by rubbing UHMWPE with microfabricated surfaces. A narrow distribution and sterility of the generated particles was achieved. An inverted cell culturing apparatus and procedures were created and the contact between particles and macrophage cells was observed. No significant difference of the cell proliferations under normal and inverted positions further demonstrates the feasibility of the system. This newly developed platform can assist in the further understanding of the mechanism and therapy strategies of osteolysis induced by polyethylene particles.     

In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3°) than is prescribed by the displacement standard (5.7°). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.     

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.     

Perception of shape-from-motion in macaque monkeys and humans

Motion is one of the most efficient cues for shape perception. We conducted behavioral experiments to examine how monkeys perceive shapes defined by motion cues and whether they perceive them as humans do. We trained monkeys to perform a shape discrimination task in which shapes were defined by the motion of random dots. Effects of dot density and dot speed on the shape perception of monkeys were examined. Human subjects were also tested using the same paradigm and the test results were compared with those of monkeys. In both monkeys and humans, correct performance rates declined when density or speed of random dots was reduced. Both of them tended to confuse the same combinations of shapes frequently. These results suggest that monkeys and humans perceive shapes defined by motion cues in a similar manner and probably have common neural mechanisms to perceive them. Electronic Publication     

The Contractile Mechanism in Cilia

A detailed analysis is made of the motion and the forces in the cilium of Sabellaria over the complete cycle. The results indicate that the stiffness of the cilium is directly related to the moments produced by the internal contractile elements. A sliding filament model is developed to generate the complete cycle of motion. The activation of the force-producing elements, the peripheral fibers, occurs over their entire length at once during the effective stroke. In the recovery stroke the sliding of peripheral fibers relative to each other produces activation. The peripheral fibers contribute to the stiffness of the cilium in the sliding filament model only when they are not free to slide because of cross-linkage. The model describes successfully the motion of a variety of types of cilia.     

An elasto-plastic finite element model for polyethylene wear in total hip arthroplasty

A new finite element model (FEM) based on an elasto-plastic behavior of ultra high molecular weight polyethylene (UHMWPE) was used to study the wear behavior of UHMWPE acetabular cup, which has a 32 mm diameter femoral head. The model imposed a plastic yield stress of 8 MPa on the UHMWPE so that any stresses beyond this would automatically be redistributed to its neighbor. The FEM model adopted a unique mesh design based on an open cube concept which eliminated the problems of singularities. Wear prediction combined the influences of contact stress, sliding distance and a surface wear coefficient. The new model predicted significantly higher volumetric wear rate (57 mm(3)/yr) well within the average reported clinical values. The model was also used to study the effect of friction and clearance between the acetabular cup and the femoral head. Increase in friction increased the volumetric wear rate but did not appear to affect the linear wear rate, which remained at 0.12 +/- 0.02 mm/yr. The predicted wear was sensitive to clearance. It was found that when the clearance was close to 0 and >0.5mm, severe wear occurred. The best clearance range was between 0.1 and 0.15 mm where the average linear wear rate was 0.1mm/yr and the volumetric wear was 55 mm(3)/yr. The present work indicates the importance of avoiding too tight or too loose a diametrical clearance.     

A model of temporomandibular joint function in anthropoid primates based on condylar movements during mastication

The hypothesis that the shape of the bony temporomandibular joint (TMJ) is functionally related to sagittal sliding of the condyle during mastication is tested, and a model of the relation of sagittal sliding to mandibular size, TMJ shape, and diet is developed. Sagittal sliding is defined as fore-aft motion of the condyle during mandibular translation and/or angular rotation. Ascending ramus height is used as a structural correlate of the distance between the condyle and the mandibular axis of rotation (CR). Cineradiographic data on sagittal sliding and gape during mastication in Ateles spp., Macaca fascicularis, Papio anubis, and Pan troglodytes in conjunction with comparative data on mandibular size and TMJ shape are used to evaluate the hypothesis. The results show that 1) linear and angular gape are highly positively correlated with sagittal sliding, 2) pure mandibular translation is rare during mastication, 3) the CR is rarely if ever located at the condyle during mastication, 4) angular gape should be standardized in interindividual comparisons of sagittal sliding, and 5) the height of the ascending ramus (and by inference the CR-to-condyle distance) is highly positively correlated with absolute sagittal sliding. Sagittal sliding relative to the length of the articular eminence was the variable used to explore the relation between TMJ shape and sliding. This variable standardized absolute sagittal sliding relative to joint size. The relative depth and orientation of the articular eminence were not correlated with relative sagittal sliding. The anteroposterior curvature of the condyle was highly negatively correlated with relative sagittal sliding. Flat condyles are associated with large amounts of relative sagittal sliding. A flat condyle increases joint contact area, which reduces joint stress. A flat condyle also increases joint congruence, and this may facilitate the combined sliding and rolling motion of the condyle when the sliding motion is relatively large. The shape of the entoglenoid process was also positively correlated with relative sagittal sliding. A relatively large entoglenoid process may help to guide sagittal sliding and prevent excessive mediolateral sliding of the condyle. The functional model makes a number of predictions about the correlations between food consistency and food object size, mandibular size, TMJ shape, and sagittal sliding of the condyle during mastication and incision. Am J Phys Anthropol 109:67–88, 1999. © 1999 Wiley-Liss, Inc.     

Shape discrimination by wasps (<Emphasis Type="Italic">Paravespula germanica</Emphasis>) at the food source: generalization among various types of contrast

Wasps (Paravespula germanica) were trained and tested at an artificial feeding site, using convex shapes that produced colour contrast, luminance contrast, or motion contrast against the background. With each of the three types of contrast, we tested the wasps capacity to discriminate the learned shape from novel shapes. In addition, in each experiment we tested the wasps capability to recognize the learned shape when it offered a different type of contrast than that it had during the training. With the coloured shapes, a side-glance at the colour discrimination performance of the wasps was possible in addition. Wasps are found to discriminate between a variety of convex shapes regardless of the type of contrast that they produce against the background. Mainly, they discriminate the learned shape from novel shapes even if the colour of the shapes or the type of contrast they produce against the background is altered in the test. Thus, wasps generalize the learned shape from one colour to another, as well as between colour contrast, luminance contrast, and motion contrast.     

Prediction of lubricating film thickness in UHMWPE hip joint replacements

An elastohydrodynamic lubrication model developed for a ball-in-socket configuration in a previous studies by the present authors (Jalali-Vahid et al., Thinning films and tribological interfaces, 26th Leeds-Lyon Symposium on Tribology, 2000, pp. 329-339) was applied to analyse the lubrication problem of a typical artificial hip joint replacement, consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup against a metallic or ceramic femoral head. The cup was assumed to be stationary whilst the ball was assumed to rotate at a steady angular velocity and under a constant load. A wide range of main design parameters were considered. It has been found that the predicted lubricating film thickness increases with a decrease in the radial clearance, an increase in the femoral head radius, an increase in UHMWPE thickness and a decrease in UHMWPE modulus. However, the predicted lubricating film thicknesses are not found to be sufficiently large in relation to the surface roughness of the cup and head to indicate separation of the two articulating surfaces. It should also be noted that if the design features are unable to secure full fluid film lubrication, it may be preferable to select them for minimum wear rather than maximum film thickness. For example, an increase in head radius will enhance the film thickness, but it will also increase the sliding distance and hence wear in mixed or boundary lubrication conditions. Furthermore, it is pointed out that an increase in the predicted lubricant film thickness is usually associated with an increase in the contact area, and this may cause lubricant starvation and stress concentration at the edge of the cup, and adversely affect the tribological performance of the implant. The effect of running-in process on the lubrication in UHMWPE hip joint replacements is also discussed.     

Experimental and numerical modeling of variable friction between nanoregions in coventional and crosslinked UHMWPE

Recently, highly crosslinked UHMWPE components have been promoted for their high abrasive wear resistance over conventional UHMWPE (PE) in total joint replacement (TJR) prostheses to minimize osteolysis and consequent implant loosening. This study was aimed at investigating the role of friction gradients induced by localized coefficients of friction at both crystalline and amorphous nanoregions in PE, and crystalline and crosslinked nanoregions in crosslinked UHMWPE (XPE), in submicron wear debris generation. An abrasive wear study performed on both XPE and PE using atomic force microscopy (AFM) illustrated that the onset of plastic deformation for XPE occurred at a normal load that was approximately 3 times higher when compared to PE. Coefficients of friction (mu d) of 0.2, 0.35, and 0.61, experimentally derived using AFM, were used as representative mu d for crystalline, amorphous, and crosslinked nanoregions, respectively, in a numerical Hertzian model. An increase in mu (0.2 +/- 0.02, 0.35 +/- 0.01 and 0.6 +/- 0.04) was observed with a decrease in crystallinity and storage modulus at 22 degrees C. Using the Hertzian contact model, it was observed that variability in friction between nanoregions contributed to higher magnitude stresses for XPE (0.2 to 0.61; maximum sigma eff = 2.8) compared to PE (0.2 to 0.35; maximum sigma eff = 1.1) over a negligible thickness of the interfacial zone (IZ) between nanoregions. The experimentally observed increase in abrasive wear resistance of XPE could be attributed to an increase in the thickness of the interfacial zone between nanoregions with mu changing gradually from crystalline to crosslinked nanoregions, a situation that may not be observed with PE. This would cause a decrease in the friction gradient and resulting stresses thereby agreeing with the observed experimental higher abrasive wear resistance for XPE. However, in both PE and XPE, the presence of stress concentrations over a period of time could lead to irreversible damage of the material eventually generating submicron wear debris. Hence, semicrystalline, inhomogenous UHMWPE with several nanoregions (amorphous and crystalline) would be at a disadvantage for bearing application in terms of abrasive wear resistance compared to UHMWPE with relatively lower number of nanoregions and crosslinked nanoregions.     

Study on Biotribological Behavior of the Combined Joint of CoCrMo and UHMWPE/BHA Composite in a Hip Joint Simulator

UHMWPE composites reinforced with Bovine Bone Hydroxyapatite(BHA)in different contents were prepared by heatpressing formation method.A hip joint wear simulator was used to investigate the biotribological behavior of UHMWPE/BHAcomposite acetabular cups against CoCrMo alloy femoral heads in bovine synovia lubrication at 37±1 ℃.It was found that theaddition of BHA powder to UHMWPE can improve the hardness and creep modulus of UHMWPE/BHA composites,anddecrease their wear rates under bovine synovia lubrication.When the content of BHA filler particles was up to 30 wt%,UHMWPE/BHA composites demonstrated the well design performances of the surface and biotribological properties.Fatigue,ploughing and slight adhesive wear were the main wear mechanisms for UHMWPE and its composites.In addition,the sizes ofwear particles became larger with an increase in BHA powder addition.These results suggest that BHA filler is a desirablecomponent to increase the wear resistance of UHMWPE/BHA composites for biomedical applications.