Effect of ultra-high molecular weight polyethylene thickness on contact mechanics in total knee replacement

One of the important design parameters in current knee joint replacements is the thickness of the ultra-high molecular weight polyethylene (UHMWPE) tibial insert, yet there is no clear definition of the upper limit of the 'thick' polyethylene insert. Using one design knee implant and subjecting it to the physiological loads encountered throughout the gait cycle, measurements of the lengths of contact imprints generated were compared with the corresponding theoretical predictions for different insert thicknesses under the same applied load. Multiple regression analysis was applied to test whether the dimensions of contact imprints are influenced by UHMWPE thickness. Good agreement was obtained between the theoretical predictions and the experimental measurements of the dimensions of contact imprints when the knee was at 60 degrees flexion. Therefore, it was possible to estimate the contact pressure at the articulating surface using the theoretical model. Contact imprint dimensions increased with increasing applied load. Statistical analysis of the experimental data revealed that, at 0 degree flexion, the overall imprint dimensions increased as the UHMWPE thickness increased from 8 to 20 mm. However, the increment was not significant when the thickness subinterval 10-15 mm was considered. Furthermore, at 60 degrees flexion, thickness was not a significant factor for the overall imprint dimensions. No evidence was found from the data to suggest that an increment in polyethylene thickness over 10 mm would significantly reduce the contact imprint dimensions. These findings suggest that thicker inserts can be avoided, as they require unnecessary bone resection.     

Increased total knee arthroplasty ultra-high molecular weight polyethylene wear using a clinically relevant hyaluronic acid simulator lubricant

In this study, osteoarthritic and periprosthetic synovial fluid samples were rheologically and biochemically compared to develop a hyaluronic acid (HA) supplemented bovine serum (BS) lubricant that mimicked the properties of human joint synovial fluid. The effect of this BS + HA lubricant (50 per cent bovine calf serum + 1.5 g/l HA) on the wear rate of ultra-high molecular weight polyethylene (UHMWPE) during a total knee replacement wear test was then investigated. In conjunction with biochemical similarities, the rheological analysis showed that the BS + HA lubricant viscosity was not statistically different to aspirated total knee arthroplasty (TKA) revision joint fluid viscosity over a range of physiologic shear rates. Gravimetric results at 5 million wear testing cycles showed that the BS + HA lubricant produced an average of 6.88 times more UHMWPE wear than 50 per cent bovine serum lubricant alone. The BS + HA lubricated CoCr femoral component surfaces revealed pitting and surface roughening that was not observed using standard bovine serum only lubricants, but that was similar to the metallic surface corrosion observed on in vivo CoCr femoral component retrievals. These findings support the hypothesis that the addition of HA to simulator lubricant is capable of producing CoCr femoral component surface damage similar to that observed in vivo.     

Reduction of total knee replacement wear with vitamin E blended highly cross-linked ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is a common bearing component in total knee replacement (TKR) implants, and its susceptibility to wear continues to be the long-term limiting factor in the life of these implants. This study hypothesized that in TKR systems, a highly cross-linked (HXL) UHMWPE blended with vitamin E will result in reduced wear as compared to a direct compression-moulded (DCM) UHMWPE. A wear simulation study was conducted using an asymmetric lateral pivoting '3D Knee' design to compare the two inserts. The highly cross-linked UHMWPE was aged prior to the testing and force-controlled wear testing was carried out for 5 million cycles using a load-controlled ISO-14243 standard at a frequency of 1 Hz on both groups. Gravimetric measurements of DCM UHMWPE (4.4 +/- 3.0 mg/million cycles) and HXL UHMWPE with vitamin E (1.9 +/- 1.9 mg/million cycles) showed significant statistical differences (p < 0.01) between the wear rates. Wear modes and surface roughness for both groups revealed no significant dissimilarities.     

Effect of contact stress on friction and wear of ultra-high molecular weight polyethylene in total hip replacement

This paper studies the effect of contact stress on friction and wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by means of friction and wear joint simulator testing under serum lubrication. For a given applied load, increasing the contact stress by increasing the ball/socket radial clearance decreased both the coefficient of friction and the wear rate. Friction and wear were highly correlated. The dependence of friction on contact stress for the UHMWPE socket under serum lubrication was similar to that of semi-crystalline polymers under dry sliding. This finding indicates the occurrence of partial dry contact at asperity levels for the metal-polyethylene ball-in-socket joint under serum lubrication.     

A study of the nanotribological fatigue of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) provides a low friction, high toughness interface in artificial knees and hips. Micron-sized wear debris forms over time in these transplants leading to osteolysis and poor clinical outcomes. Using the atomic force microscope (AFM) as a model single asperity contact, tribological studies were performed on nanometer smooth samples of UHMWPE under dry conditions to elucidate the mechanisms of debris formation. Low loads produced no changes in friction or topography despite repeated scanning. Above a critical load, polymer accumulated at the perimeter of the scan and led to the formation of a wear debris particle. Plastically deformed material exhibited a surprisingly high friction compared to surrounding pristine areas, and may partially explain macroscale observations of adhesive wear. In contrast, the polymer in the interior of the scanned area exhibited a friction identical to pristine polymer. These data link strain-softening and delamination of the surface to the formation of wear debris.     

Nanoindentation properties of compression-moulded ultra-high molecular weight polyethylene

This paper investigates the elastic modulus and hardness of untreated and treated compression-moulded ultra-high molecular weight polyethylene (UHMWPE) tibial inserts of a total knee replacement (TKR) prosthesis. Investigations were carried out at a nanoscale using a Nanoindenter at penetration depths of 100,250 and 500 nm. The nanomechanical properties of surface and subsurface layers of the compression-moulded tibial inserts were studied using the untreated UHMWPE. The nanomechanical properties of intermediate and core layers of the compression-moulded tibial insert were studied using the cryoultrasectioned and etched UHMWPE treated samples. The cryoultrasectioning temperature (-150 degrees C) of the samples was below the glass transition temperature, Tg (-122 +/- 2 degrees C ), of UHMWPE. The measurement of the mechanical response of crystalline regions within the nanostructure of UHMWPE was accomplished by removing the amorphous regions using a time-varying permanganic-etching technique. The percentage crystallinity of UHMWPE was measured using differential scanning calorimetry (DSC) and the Tg of UHMWPE was determined by dynamic mechanical analysis (DMA). Atomic force microscopy (AFM) was used to assess the effect of surface preparation on the samples average surface roughness, Ra. In this study, it was demonstrated that the untreated UHMWPE samples had a significantly lower (p < 0.0001) elastic modulus and hardness relative to treated UHMWPE cryoultrasectioned and etched samples at all penetration depths. No significant difference (p > 0.05) in elastic modulus and hardness between the cryoultrasectioned and etched samples was observed. These results suggest that the surface nanomechanical response of an UHMWPE insert in a total joint replacement (TJR) prosthesis is significantly lower compared with the bulk of the material. Additionally, it was concluded that the nanomechanical response of material with higher percentage crystallinity (67 per cent) was predominantly determined by the crystalline regions within the semi-crystalline UHMWPE nanostructure.     

Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator

A loading protocol approximating forces, torques and motions at the knee during stair descent was developed from previously published data for input into a force-controlled knee simulator. A set of total knee replacements (TKRs) was subjected to standard walking cycles and stair descent cycles at a ratio of 70: 1 for 5 million cycles. Another set of implants with similar articular geometry and the same ultra-high molecular weight polyethylene (UHMWPE) resin (GUR 415), sterilization and packaging was tested with standard walking cycles only. Implant kinematics, gravimetric wear and surface roughness of the UHMWPE inserts were analysed for both sets of implants. Contact stresses were calculated for both loading protocols using a Hertzian line contact model. Significantly greater weight loss (p < 0.05) and more severe surface damage of UHMWPE inserts resulted with the walking + stair descent loading protocol compared to walking cycles only. Anterior-posterior (AP) tibiofemoral contact point displacements were lower during stair descent than walking, but not significantly different (p = 0.05). Contact stresses were significantly higher during stair descent than walking, owing to higher axial loads and the smaller radius of curvature of the femoral components at higher flexion angles. High contact stresses on UHMWPE components are likely to accelerate the fatigue of the material, resulting in more severe wear, similar to what is observed in retrieved implants. Thus the inclusion of loading protocols for activities of daily living in addition to walking is warranted for more realistic in vitro testing of TKRs.     

Wear of ultra-high molecular weight polyethylene (UHMWPE) in total knee prostheses: a review of key influences

The formation and development of wear is now widely accepted as one of the major concerns in the long-term survivorship of contemporary knee prostheses in vivo. This review examines the role of surface topography, third-body debris, load, contact mechanics and material quality in the wear process. Some of the kinematic and physiological issues that need to be modelled in the development of wear testing regimes for evaluation of material combinations and geometrical combinations in total knee implant designs are considered. Wear testing procedures and some of the results from wear tests are discussed and the need to consider the impact of rolling and sliding in the study of wear in total knee components is highlighted. The dominant wear mechanisms that occur in vivo are identified and the role of these mechanisms is currently being examined experimentally at the University of Limerick wear testing machine.     

The influence of lubricant on the morphology of ultra-high molecular weight polyethylene wear debris generated in laboratory tests.

Since the implication of polyethylene wear debris as a major cause of osteolysis in total joint replacements, there has been much interest in polyethylene wear studies and in cell culture studies using ultra-high molecular weight polyethylene (UHMWPE) wear debris. Studies have shown that particles in the 0.1-10 microns size range are particularly important in causing adverse cellular reactions resulting in osteolysis. The morphology, the mass and size distributions, and the number of wear particles produced at the joint surfaces are influenced by the tribological conditions at the joint. Laboratory wear tests are used to investigate the wear properties of prosthetic joint materials and different research groups have used different lubricants in these tests. This paper shows that the volumetric wear and morphology of UHMWPE particles generated in vitro are influenced by the type of lubricant used. This study compared, quantitatively, UHMWPE wear debris generated in deionized water to debris that was generated in a system lubricated by bovine serum which was diluted to 25 per cent. The wear factors of UHMWPE in water and serum lubricants were significantly different (p < 0.05). UHMWPE wore 14 times more in water than in serum. Quantitative analysis of the wear particles showed that the debris that was generated in serum was morphologically different from debris that was produced in a water-lubricated system. Furthermore, the particles produced in serum showed a closer similarity to those found in retrieved acetabular tissues.     

Dynamic compressive creep of extruded ultra-high molecular weight polyethylene

To estimate the true wear rate of polyethylene acetabular cups used in total hip arthroplasty, the dynamic compressive creep deformation of ultra-high molecular weight polyethylene (UHMWPE) was quantified as a function of time, load amplitude, and radial location of the specimen in the extruded rod stock. These data were also compared with the creep behavior of polyethylene observed under static loading. Total creep strains under dynamic loading were only 64%, 70%, and 61% of the total creep strains under static loading at the same maximum pressures of 2 MPa, 4 MPa, and 8 MPa, respectively. Specimens cut from the periphery of the rod stock demonstrated more creep than those cut from the center when they were compressed in a direction parallel to the extrusion direction (vertical loading), whereas the opposite was observed when specimens were compressed in a direction perpendicular to the extrusion direction (transverse loading). These findings show that creep deformation of UHMWPE depends upon the orientation of the crystalline lamellae.     

Effect of the compression ratio on wear properties of slightly cross-linked ultra-high molecular weight polyethylene, crystallized under uniaxial compression

To improve wear properties of artificial joints, cross-linked ultra-high molecular weight polyethylene (UHMWPE) was crystallized under compression in a molten state. Slight cross-linking was created by γ-ray irradiation at a 0.5 Mrad dose under reduced pressure at room temperature before the compression. Next, the UHMWPE was melted at 200°C and compressed using two metal plates. The compression ratio (CR) is defined as the ratio of the final thickness to the original thickness of the sample. The molecular chain of the UHMWPE was orientated to the direction of deformation and was crystallized by cooling to room temperature while maintaining the deformation. The (2 0 0) crystalline plane was only orientated parallel to the compression plane in the CR=2 sample; however, in the case of the CR=5 sample, both the (2 0 0) and the (1 1 0) crystalline planes were orientated parallel to the compressed surface. The density and melting point of the sample depended on the compression ratio. The physical and the mechanical properties were increased in accordance with their compression ratio. The _c-peak of the loss modulus was shifted to a higher temperature compared with the non-compressed sample. Dimensional stability of the compressed sample by heating near 135°C was not found. It was confirmed that the wear factor also depended on the compression ratio based on the findings of pin-on-disc and pin-on-flat wear tests. The wear factor of the (CR=2) sample was similar to the non-compressed sample (CR=1); in contrast, that of the CR=5 sample was significantly smaller.     

Reciprocating sliding of uniaxially-stretched ultra-high molecular weight polyethylene for medical device applications

Solid-state deformation of UHMWPE used in total joint prosthesis could be a relevant treatment prior to cross-linking to obtain high strength. However, little is known about the influence of chain alignment (texture) on sliding wear behavior. In this work, we analyzed i) the deformation mechanisms of UHMWPE resulting from a uniaxial tension (elongation of about 300%) and ii) the influence of the texture on the friction behavior resulting from ball-on-flat sliding tests (reciprocating sliding mode). Microstructural investigations show that tension induces a transformation of the initial lamellar morphology into specific microfibrillar morphology. White light interferometer (WLI) studies show that texturing causes an increase of the roughness by a factor of about two. After 50,000 sliding cycles, the cumulative dissipated energy decreases from 109,300-103,600 μJ for untreated UHMWPE to 64,600 and 43,150 μJ for textured UHMWPE, parallel and perpendicular to the texture direction, respectively. Considering that wear resistance increases with decreasing dissipated energy, textured UHMWPE may have anti-wear properties. However, cracks are noted at microscopic scale for textured UHMWPE. Regarding wear resistance, such defects are not suitable and can be avoided by reducing the roughness of textured UHMWPE prior to wear tests.     

Effect of contact pressure on wear and friction of ultra-high molecular weight polyethylene in multidirectional sliding

Computational wear models need input data from valid tribological tests. For the wear model of a total hip prosthesis, the contact pressure dependence of wear and friction of ultra-high molecular weight polyethylene (UHMWPE) against polished CoCr in diluted calf serum lubricant was studied, and useful input data produced. Two test devices were designed and built: a heavy load circularly translating pin-on-disc (HL-CTPOD) wear test device and an HL-CTPOD friction measurement device. Both can be used with a wide range of loads. The wear surface diameter of the test pin was kept constant at 9 mm, whereas the load was varied so that the nominal contact pressure ranged from 0.1 to 20 MPa. The wear factor decreased with increasing contact pressure, whereas the coefficient of friction first increased with increasing contact pressure with low pressure values and then decreased. Up to the pressure of 2.0 MPa, the wear mechanisms and wear factors were in good agreement with clinical findings. In the critical range of 2.0-3.5 MPa, the wear mechanisms and wear factors started to differ from clinical ones, and the decrease of the wear factor steepened. The discrepancy became more and more evident as the pressure was gradually increased beyond 3.5 MPa. It appears that the pressure value of 2.0 MPa should not be exceeded in pin-on-disc wear tests that are to reproduce the clinical wear of UHMWPE acetabular cups.     

Fluid composition impacts standardized testing protocols in ultrahigh molecular weight polyethylene knee wear testing

Wear of total knee replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Weight gain due to fluid absorption during testing is corrected using a load soak station. In this study, three sets of ultrahigh molecular weight polyethylene tibial plateau were tested against highly polished titanium condyles. Test 1 was performed in two different institutions on the same simulator according to the standard ISO 14243-1, using two testing lubricants. Test 2 and test 3 repeated both previous test sections. The wear and load soak rates changed significantly with the lubricant. The wear rate decreased from 16.9 to 7.9 mg weight loss per million cycles when switching from fluid A to fluid B. The weight gain of the load soak specimen submersed in fluid A was 6.1 mg after 5 x 10(6) cycles, compared with 31.6 mg for the implant in fluid B after the same time period. Both lubricants were mixed in accordance with ISO 14243 (Implants for surgery - wear of total knee-joint prostheses), suggesting that calf serum should be diluted to 25 +/- 2 per cent with deionized water and a protein mass concentration of not less than 17 g/l. The main differences were the type and amount of additives that chemically stabilize the lubricant throughout the test. The results suggest that wear rates can only be compared if exactly the same testing conditions are applied. An agreement on detailed lubricant specifications is desirable.     

The effect of protein adsorption on the friction behavior of ultra-high molecular weight polyethylene

Medical-grade UHMWPE samples with two different surface finishing treatments, milling and melting/reforming were exposed to 10% bovine serum albumin solution and their friction responses were quantified using atomic force microscopy. The observed friction increase upon exposure to proteins was attributed to the formation of a layer of denatured proteins on the surface. Changing the crystallinity and surface energy of UHMWPE affected the protein adsorption mechanism and the resulting increase in friction behavior.     

Tribology of ultra-high molecular weight polyethylene disks molded at different temperatures

Tribological characteristics of ultra-high molecular weight polyethylene (UHMW-PE) disks molded at 130–190 °C were studied. The highest crystallinity was obtained for the sheet molded at 130 °C, but crystallinity decreased with increasing molding temperature. Beyond 150 °C, the resultant crystallinity reached a constant level. The dynamic friction coefficients of these UHMW-PE disks were measured using a ball-on-disk friction tester. The friction coefficient decreased with increasing number of rotations in the early stage of the measurement, and achieved at an equilibrium level, independent of the molding temperature. The steady-state friction coefficient was 0.04 for the disk molded at 130 °C and increased with increasing molding temperature. The disks molded at 150–190 °C always had a steady-state friction coefficient of 0.065. The surface deformation of each disk was evaluated from the observation of the resultant wear track. Analyzing the relationship between the above friction coefficient and width of the wear track enabled us to interpret the tribological mechanism generated in this study.     

Elasto-plastic contact analysis of an ultra-high molecular weight polyethylene tibial component based on geometrical measurement from a retrieved knee prosthesis

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of these implants. Especially in retrieved knee prostheses with anatomical design, the predominant types of wear on UHMWPE tibial components are delamination and pitting. These fatigue wear patterns of UHMWPE are believed to result from repeated plastic deformation owing to high contact stresses. In this study, the elasto-plastic contact analysis of the UHWMPE tibial insert, based on geometrical measurement for retrieved knee prosthesis, was performed using the finite element method (FEM) to investigate the plastic deformation behaviour in the UHMWPE tibial component. The results suggest that the maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial component. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at macroscopic level.     

A simple fully integrated contact-coupled wear prediction for ultra-high molecular weight polyethylene hip implants

A fully coupled contact and wear model was developed in the present study for hip implants employing an ultra-high molecular weight polyethylene (UHMWPE) cup in combination with a metallic or ceramic femoral head. A simple elasticity equation based on the concept of constrained column model was employed to solve the contact mechanics between the acetabular cup and the femoral head under the three-dimensional physiological loading condition. The wear model was based on the classical Archard-Lancaster equation in common with all other studies reported in the literature. The fully coupled contact and wear model was applied to both conventional and cross-linked UHMWPE cups under a wide range of design parameters such as the clearance and the femoral head radius. The predicted linear and volumetric wear as well as their rates for conventional UHMWPE cups were found to be in good agreement with those obtained from a similar analysis by Maxian but using the finite element method for the contact mechanics analysis. The predicted maximum contact pressure was found to decrease rapidly within the first 10(6) cycles, and below the limit to cause plastic deformation within the UHMWPE cup with a nominal radial clearance of 0.2 mm. The effect of the clearance between the head and the cup on the predicted wear was found to be negligible. For the cross-linked UHMWPE cup with relatively large diameters up to 48 mm and a fixed outside diameter of 50 mm, the predicted wear, which was found to increase with increasing femoral head radius, remained small owing to the small wear factor associated with these materials. Furthermore, if the head diameter increases beyond 42 mm, a rapid increase in the contact pressure was predicted, owing to the decrease in the wall thickness of the cross-linked UHMWPE cup.     

Pattern abrasion of ultra-high molecular weight polyethylene: Microstructure reconstruction of worn surface

The friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against bearing steel (AISI 52100) in a ring-on-block contact mode under the lubrication of aqueous solution of 3.5% NaCl was evaluated. The worn polymer surfaces were analyzed by means of three dimensional profiling, atomic force microscopy, Polarized Raman microanalysis, field emission scanning electron microscopy, and nanoindentation testing. It was found that unusual wavelike abrasion patterns were formed on the worn surface of UHMWPE under properly selected sliding conditions. In the presence of plowing effect, the molecular chains of UHMWPE and short-rod like microcrystalline grains of abrasion pattern were both further oriented along the plowing direction and became tiny and dense owing to microstructure reconstruction. Resultant microstructurally reconstructed worn surface of UHMWPE had a higher nanoindentation hardness and modulus as well as increased wear resistance.     

Comparison of gas plasma and gamma irradiation in air sterilization on the delamination wear of the ultra-high molecular weight polyethylene used in knee replacements

Early failure of knee replacements is thought to be due to the combination of sterilization by gamma irradiation in air and the high cyclic stresses that they endure during use. Such failures are shown through delamination and permanent deformation of the ultra-high molecular weight polyethylene (UHMWPE) component. This study investigated whether gas plasma sterilization, as an alternative to gamma irradiation in air, would give better performance after ageing in a knee replacement using a metal pin on polymer plate wear test. Fourier transform infrared (FTIR) analysis was performed on the components to assess oxidation levels and a finite element stress analysis model is presented to estimate strain at failure in the UHMWPE. Delamination occurred in the majority of the gamma-irradiated plates but did not occur in any of the gas-plasma-sterilized plates. The FTIR analysis showed that the plates gamma irradiated in air were highly oxidized when compared with the gas-plasma-sterilized plates. Plastic strain at failure was determined for the gamma-irradiated plates and found to be less than 2.4-14 per cent.