Ultra-high Molecular Weight Polyethylene Components Research Articles

Is Heterotopic Ossification Removal From Alloplastic Temporomandibular Joint Prosthesis Using Er,Cr:YSGG Laser Superior to Conventional Methods?

Heterotopic ossification (HO) formed over the major components and fixation screw heads of an alloplastic temporomandibular joint replacement (TMJR) prosthesis can result in decreased quality of life, limited function, prosthesis failure, and hinder prosthesis revision, replacement, or removal. This study simulated HO removal from the major components and fixation screw heads of alloplastic TMJR prostheses using an erbium, chromium-doped yttrium, scandium, gallium, and garnet (Er,Cr:YSGG) laser and compared the results to conventional methods of HO removal. The surface morphology and chemical structure of the exposed components were analyzed. The investigators hypothesize that HO removal with an Er,Cr:YSGG laser causes less damage to TMJR prosthesis components compared to conventional HO removal methods. This multiple test descriptive analysis simulated HO removal from TMJR prostheses mounted to stereolithic models. Simulated HO removal was completed using a novel Er,Cr:YSGG laser method and conventional methods which utilized a fissure carbide bur in a high-speed rotary instrument, a standard osteotome, and an ultrasonic aspirator. Surfaces exposed on the TMJR prostheses were analyzed for morphological or chemical change using scanning electron microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. The Er,Cr:YSGG laser did not adversely affect the titanium screws or titanium components of the TMJR prostheses, while conventional methods of HO removal did. HO removal using the Er,Cr:YSGG laser and conventional methods both inflicted surface damage to the fossa ultrahigh molecular weight polyethylene component of the TMJR prostheses. Damage inflicted to titanium alloy or commercially pure titanium components of TMJR prostheses by conventional HO removal methods can be avoided by instead removing HO with an Er,Cr:YSGG laser. However, long exposure of the Er,Cr:YSGG laser to ultrahigh molecular weight polyethylene surfaces should be avoided. Additional research to expand on applications to other procedures and in other surgical fields is encouraged.

Bioceramics are Not Bioinert: The Role of Oxide and Non-Oxide Bioceramics on the Oxidation of UHMWPE Components in Artificial Joints

The following research is aimed at understanding the influence of Zirconia-Toughened Alumina (ZTA) and Silicon Nitride (Si3N4) on Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular liners. Bioceramic femoral heads were systematically tested against UHMWPE in controlled environment according to static/load-free coupling in hydrothermal environment, pin-on-ball wear testing, and hip-simulator wear testing. In addition, a retrieved ZTA femoral head has been analyzed and results have been compared to the simulations. Experimental results from X-ray photoelectron (XPS), cathodoluminescence (CL), Raman and Fourier-Transformed Infrared spectroscopy suggest that, despite conventional notions imply that bioceramics are inert, the surface chemistry of bioceramics was relevant to the oxidation rate of polyethylene liners. Non-biointertness could either be advantageous or disadvantageous toward polyethylene oxidation. The main reason resides in the peculiar chemical interactions between polyethylene and different ceramics, and, more specifically, depends on the direction of oxygen flow at the interface between the ceramic and the polymer. ZTA femoral heads were found to release a non-negligible amount of oxygen moieties from their surfaces, thus accelerating oxidative degradation of polyethylene. Conversely, Si3N4 ceramics exerted a protective role towards the polyethylene liner by scavenging oxygen from the tribolayer. The results of this work provide new insights into the interaction between bioceramics and polymers, which should also be considered when designing the next generation artificial hip joints with significantly elongated lifetimes.

Evaluating the durability of UHMWPE biomaterials used for articulating surfaces of joint arthroplasty using delamination tests.

Ultra-high molecular weight polyethylene (UHMWPE) is the most popular material used for the articulating surface of joint replacements. Delamination is a common fatigue-related failure mode in UHMWPE components; however, the relationship between delamination resistance and fatigue crack growth has not been reported. Here, the delamination resistance of contemporary UHMWPE materials, including highly cross-linked UHMWPE (HXLPE), vitamin E blended UHMWPE (VEPE), and vitamin E blended HXLPE (VEXLPE), was measured to verify a previously proposed accelerated test method using a U-shaped sliding motion; the results were compared with those of fatigue crack growth tests. The oxidative stability of each material was estimated using Fourier transform infrared analysis. UHMWPE sterilized by gamma irradiation in an inert atmosphere and annealed HXLPE had lower delamination resistance than virgin UHMWPE after artificial aging. This was consistent with previous findings from retrieval studies, and in vitro knee simulator and ball-on-flat unidirectional reciprocation wear studies. In contrast, remelted HXLPE, VEPE, and VEXLPE showed excellent delamination resistance after artificial aging. The results of the delamination tests were not consistent with those of fatigue crack growth tests, indicating the complex delamination mechanism and importance of evaluating these factors separately. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 65-72, 2019.

Short-term creep and recovery behavior of medical grade ultra-high molecular weight polyethylene (UHMWPE)

Purpose: In this study, short-term tensile creep and recovery behaviors of medical gradeultra-high molecular weight polyethylene (UHMWPE) were investigated to contributedeformation behaviour of UHMWPE components in knee and hip prosthesis during daily lifeactivities of patients.Design/methodology/approach: Tensile test specimens were machined fromcompression molded UHMWPE sheets having commercial brand name: Chirulen 1020 andthey were prepared according to ASTM 527-2. The tensile creep tests were performed atconstant stress levels of 5, 9, 13, 18 and 21 MPa as long as 1 hour for each test. Then, thespecimens were allowed to recover unloaded for 1 hour. Automatic extensometer was usedto measure the deformations precisely for each test.Findings: Results show that creep rate linearly increased with increasing the stress levels.Permanent deformations were observed after recovery. Recovery of the material becamedifficult with increasing the applied load at intended time interval.Research limitations/implications: UHMWPE components used in prosthesis havebeen subjected to complex loading conditions during service life. Polymeric materials showthe viscoelastic material properties like strain rate sensitivity, relaxation, creep and recoveryat room temperature. Because of the viscoelastic material properties of the UHMWPE,it makes difficult to predict the failure of the UHMWPE components in hip and knee prosthesis.Therefore, deformation behavior of medical grade UHMWPE should be investigated in manydifferent loading conditions.Practical implications: Medical grade ultra-high molecular weight polyethylene(UHMWPE) have been used commonly in total hip replacements as acetabular cup and intotal knee replacements as tibial insert since early 1960s.

CERAMIC THIN FILMS REALIZED BY MEANS OF PULSED PLASMA DEPOSITION TECHNIQUE: APPLICATIONS FOR ORTHOPEDICS

Joint prosthesis are usually subjected to several failing mechanisms, including wear of the ultra-high molecular weight polyethylene (UHMWPE) insert. The main goal of this study was to assess the possibility to improve the tribological properties of titanium component by depositing zirconia thin films on its surface by pulsed plasma deposition (PPD) method. Zirconia-coated titanium spheres were tested against UHMWPE disks, both in dry and wet conditions. Zirconia films exhibited a homogenous sub-micrometric grain size distribution and low roughness. Interestingly, zirconia-coated titanium spheres showed lower wear rate of the UHMWPE component, compared to uncoated titanium spheres, supporting the feasibility of the proposed approach.

Improving the integration of HDPE/UHMWPE blends by high temperature melting and subsequent shear

Blends of high density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) were prepared by melt blending, and then the high temperature melting (HTM) and subsequent shear were used to improve the integration of HDPE with UHMWPE. Based on accelerating diffusion of polymer chains at high temperature, our hypothesis was that the HTM could enhance chains diffusion across the phase interface, thus the integration of HDPE with UHMWPE components could be improved. An important finding of this study was that melting HDPE/UHMWPE blends at 280°C in inert atmosphere improved the swelling of UHMWPE by HDPE and enhanced the phase interaction. And then a subsequent shear resulted in a better integration of HDPE with UHMWPE components than that by a conventional process.

Long-Term Outcomes of Proximal Interphalangeal Joint Surface Replacement Arthroplasty

Surface replacement arthroplasty is a reconstructive alternative for the treatment of pain and deformity due to osteoarthritis and rheumatoid arthritis of the proximal interphalangeal joint of the finger. This retrospective study was performed to examine long-term outcomes of proximal interphalangeal joint prosthetic surface replacement with a proximal cobalt-chromium (CoCr) and distal ultra-high molecular-weight polyethylene component over thirty years at a single institution. Sixty-seven prostheses were implanted in forty-seven patients between 1974 and 2007. The mean duration of follow-up was 8.8 years. There were fifty joints (75%) with osteoarthritis and seventeen (25%) with rheumatoid arthritis. Fifty-six prostheses (84%) were implanted via a dorsal approach, forty-eight (72%) were cemented, and nineteen (28%) were press-fit. Postoperative evaluation, consisting of a clinical history and examination, radiographs, the Short Form-36 (SF-36) and Disabilities of the Arm, Shoulder and Hand (DASH) questionnaires, and a visual analog scale (VAS) pain score, was performed for thirty-six patients. Demographic, surgical, and implant failure data were obtained from the medical charts of eleven patients (sixteen implants) who had died prior to the time of postoperative follow-up. At the time of follow-up, the median total active proximal interphalangeal joint motion was 40°. Eight prostheses had failed, yielding a cumulative incidence of implant failure of 3% at one year, 8% at three years, 11% at five years, and 16% at fifteen through twenty-five years. Prostheses implanted via a volar approach failed more often than those implanted via a dorsal approach (relative risk: 6.59, p = 0.004). The failure rate did not differ significantly between patients with rheumatoid arthritis and those with osteoarthritis (p = 0.17). The median VAS pain score at the time of follow-up was 3 (of a maximum of 100). There were twenty-two complications in fourteen patients, resulting in four interphalangeal fusions and two amputations. There were no infections. Proximal interphalangeal surface replacement arthroplasty is a reliable treatment alternative for pain and deformity due to proximal interphalangeal joint osteoarthritis and rheumatoid arthritis. At the time of long-term follow-up, pain was minimal and joint motion was similar to preoperative levels.

Friction and wear characteristics of UHMWPE against Co-Cr alloy under the wide range of contact pressures in lumbar total disc replacement

Ultra-high molecular weight polyethylene (UHMWPE) and Co-Cr alloy are still commonly used bearing materials for total disc replacement (TDR) as well as total joint replacement (TJR). Compared to TJR, in TDR, there are different articulating conditions such as higher contact pressure, shorter sliding distance, and less lubricant, unlike hip and knee joints. In this study, friction and wear phenomena between UHMWPE and Co-Cr alloy were characterized under the wide range of contact pressures including the extremely high contact pressure with limited amount of lubricant in lumbar TDR. All friction and wear tests were conducted by using a Pin-on-Disk type tribo-tester under normal contact pressures of 5, 10, 20, 40, 60 and 80MPa in the repeat pass rotational and the linear reciprocal sliding motions, respectively. UHMWPE cylindrical pins slid against Co-Cr alloy disks in three different kinds of dry, rarely and fully immersed lubricated conditions for friction tests and in a fully immersed lubricated condition for wear tests. For all lubricated conditions, mean coefficients of friction decreased as contact pressure increased. There were statistically significant differences in the mean coefficients of friction among six levels of contact pressures and among three kinds of lubricated conditions. The amount of wear linearly increased as number of sliding cycles increased for all contact pressures, and also it increased as the contact pressure increased. There were statistically significant differences in the wear rates of UHMWPE among six levels of contact pressures. Coefficients of friction of UHMWPE against Co-Cr alloy under very high contact pressure such as 40–80MPa in the lubricated condition were so low as to induce no clinical problem at the bearing surface. For reducing wear of UHMWPE component in TDR, it is necessary to decrease the contact pressure in design as much as possible.

Concave polyethylene component improves biomechanical performance in lumbar total disc replacement—Modified compressive-shearing test by finite element analysis

Failure of ultra-high molecular weight polyethylene components after total disc replacements in the lumbar spine has been reported in several retrieval studies, but immediate biomechanical evidence for those mechanical failures remained unclear. Current study aimed to investigate the failure mechanisms of commercial lumbar disc prostheses and to enhance the biomechanical performances of polyethylene components by modifying the articulating surface into a convex geometry. Modified compressive-shearing tests were utilized in finite element analyses for comparing the contact, tensile, and shearing stresses on two commercial disc prostheses and on a concave polyethylene design. The influence of radial clearance on stress distributions and prosthetic stability were considered. The modified compressive-shearing test revealed the possible mechanisms for transverse and radial cracks of polyethylene components, and would be helpful in observing the mechanical risks in the early design stage. Additionally, the concave polyethylene component exhibited lower contact and shearing stresses and more acceptable implant stability when compared with the convex polyethylene design through all radial clearances. Use of a concave polyethylene component in lumbar disc replacements decreased the risk of transverse and radial cracks, and also helped to maintain adequate stability. This design concept should be considered in lumbar disc implant designs in the future.

In Vivo Biological Response to Vitamin E and Vitamin-E-Doped Polyethylene

Cross-linking has decreased the wear of ultra-high molecular weight polyethylene, a cause of osteolysis leading to total joint replacement failure. Compared with melting or annealing, doping cross-linked ultra-high molecular weight polyethylene with vitamin E stabilizes free radicals from irradiation while maintaining mechanical properties and wear resistance. This study was done to determine the local tissue effects of free vitamin E and vitamin E eluted from ultra-high molecular weight polyethylene implants in the joint space. Three studies were performed. First, pure vitamin E and solubilized vitamin E were injected into rabbit knees to simulate vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; second, vitamin-E-doped, irradiated ultra-high molecular weight polyethylene plugs were implanted into dorsal subcutaneous pouches of rabbits to determine the local effects of vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; and, third, two groups of vitamin-E-doped, irradiated acetabular liners (high surface and uniform vitamin-E concentration profiles) were compared with undoped, control ultra-high molecular weight polyethylene liners in a canine model of total hip replacement to determine the effect of possible vitamin-E elution on bone ingrowth and the local tissue response to it in a load-bearing environment. Injection of solubilized vitamin E resulted in histologically normal surrounding soft tissue at both two and twelve-week follow-up intervals, while injection of pure vitamin E resulted in acute and chronic inflammation at the time of the two-week follow-up. Both control and vitamin-E-doped subcutaneous plugs showed inflammation associated with surgery at two weeks of follow-up, but showed stable fibrous encapsulation without inflammation at twelve weeks of follow-up. In the canine total hip replacement model, there was no qualitative difference in local tissue appearance and no significant difference in the percent bone ingrowth and the percent bone density between the control and vitamin-E groups. These investigations showed that vitamin-E-doped ultra-high molecular weight polyethylene plugs and total hip replacement components are well tolerated in both a small and a large-animal model with no observed adverse effects on the surrounding tissues at twelve weeks of follow-up.

Wear testing of a DJOA finger prosthesis in vitro

Although the market for replacement of diseased metacarpophalangeal (MCP) joints is dominated by single-piece silicone prostheses, several two-piece designs have been implanted. One such is the Digital Joint Operative Arthroplasty (DJOA) which consists of a part-spherical stainless steel metacarpal component which articulates within a matching concave phalangeal component made of ultra high molecular weight polyethylene (UHMWPE). A DJOA MCP prosthesis was tested using a clinically-validated finger simulator while a second DJOA prosthesis acted as a statically-loaded soak-control. Testing ran to 7.1 million cycles of flexion-extension. It was found that the UHMWPE components, both test and control, gained in weight by a similar amount. Therefore apparently there was no wear of the test components. However, the initial and final surface finish values of the test stainless steel metacarpal head were relatively high. Calculations based on this roughness data, plus recent dynamically-loaded soak data, may explain the apparent lack of wear.

Direct evaluation of fatigue property of ultra-high molecular weight polyethylene components of retrieved knee implants using small specimens

Ultra-high molecular weight polyethylene (UHMWPE) has been widely used as a bearing surface of joint implants. Since wear of UHMWPE component has been the primal cause of implant failure, crosslinking of UHMWPE is now widely used to improve its wear property. On the other hand, crosslinking degrades its fatigue property and increase of fatigue related failure, such as delamination, is possible. However, the relationship between degraded fatigue property and implant failure is not clear, primarily because the fatigue properties of failed and retrieved UHMWPE components are not known. In this study, we propose a new test method to evaluate the fatigue property of retrieved UHMWPE components or final products using small specimens. First, the effects of test conditions were investigated using virgin UHMWPE. Then, tests were repeated using specimens from UHMWPE components of retrieved knee implants. The fatigue properties of virgin and retrieved UHMWPE were suc- cessfully evaluated by the new test method and a relationship was found among the extent of oxidation, the fatigue property, and occurrence of delamination. This result suggested that evaluation of fatigue property of UHMWPE component can predict the occurrence of fatigue related implant failure during operation in vivo.

Structural, mechanical and wear resistance assessment of UHMWPE orthopedic components.

The long-term survival of total joint prostheses is influenced by many factors. Among these factors, the most critical one is the presence of wear debris, particularly from the joint articulating surfaces. While ultra high molecular weight polyethylene (UHMWPE) has shown to be extremely promising in vitro and short-term clinical results, shelf-aged, oxidized components have been extremely unsuccessful clinically. Although shelf-aged components have been frequently tested in the laboratory, few studies have compared the properties of clinical retrievals to shelf-aged UHMWPE components. In this study, a thorough analysis method was developed and applied to both UHMWPE components retrieved at the time of revision, and non-implanted, shelf-aged UHMWPE components with the aim to understand better the significance of the in vivo factors on the material properties of the retrievals. The UHMWPE components were analyzed and characterized by visual observation of the surfaces, mechanical properties were assessed by small punch tests, wear resistance was analyzed by a multidirectional pin-on-disk test, and the oxidation level was detected by the Fourier transformed infrared (FT-IR) technique. The results indicated a strong correlation between the UHMWPE oxidation level, mechanical properties and wear resistance.

Wear ranking of hard on hard bearings for prosthetic hip joints

With the increasingly ageing population across the developed world the numbers of people requiring surgical procedures to replace damaged, worn and arthritic joints is hugely increasing. In fact these operations are now the most common surgical procedure carried out in hospitals with 250,000 total hip replacement (THR) procedures carried out in the EU annually. The standard design of the THR comprises of a metal or sometimes ceramic head running against an ultra high molecular weight polyethylene (UHMWPE) acetabular cup (Fig. 1). THR can be successful for in excess of 20 years however up to 10% of prostheses fail prematurely. The 75% early failure results from the process of aseptic loosening. This is classified as mechanical failure of the joint through loosening of one or more of the prosthetic components. General loosening of the prosthesis has been widely attributed to generation of wear debris resulting from the relative movement of the femoral head and acetabular cup. The debris generated at the tribological interface accumulates in the surrounding tissue. The debris initiates the body's immune system into producing macrophages which aggressively attacks the debris and also the immediate surrounding bone tissue. This causes bone resorption and consequent loosening (Fig. 2). For standard THR systems the primary wear debris generated form the UHMWPE component and takes the form of micro- and submicro-scale polymeric particles [1].

A study of the nanostructure and tensile properties of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) has gained worldwide acceptance as a bearing material used in orthopaedic implants. Despite its widespread use, inherent properties of the polymer continue to limit the wear resistance and the clinical lifespan of implanted knee and hip prosthetics containing UHMWPE components. The degree of crystallinity of UHMWPE is known to strongly influence several of its tensile mechanical properties such as Young's modulus, yield stress, strain-hardening rates, work of fracture and ultimate tensile properties. In this study, medical grade UHMWPE was subjected to four different crystallization conditions resulting in UHMWPE with a range of crystalline morphologies. Thereafter, the crystalline nanostructure was quantitatively characterized using a combination of ultra-small angle X-ray scattering and differential scanning calorimetry. Low-voltage scanning electron microscopy was employed as a supplementary technique to compare the crystalline morphology resulting from each crystallization condition. In addition, uniaxial tensile tests were performed to assess the effects of crystallization conditions on the mechanical properties of UHMWPE. This study showed that while crystallization conditions strongly influenced the morphology of UHMWPE, in most cases the mechanical properties of the material were not significantly affected.

New polys for old: contribution or caveat?

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 1999 more than 500,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to polyethylene debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for twenty to thirty years (Figs. 1-A and 1-B). Fig. 1-A: Radiographs of a Charnley cemented hip replacement, made immediately postoperatively (Fig. 1-A) and twenty-five years postoperatively (Fig. 1-B). Fig. 1-B: Radiographs of a Charnley cemented hip replacement, made immediately postoperatively (Fig. 1-A) and twenty-five years postoperatively (Fig. 1-B). Until recently, gamma irradiation in air has been the predominant method of sterilization of ultra-high molecular weight polyethylene components and, despite current concerns, it represents the only gold standard against which contemporary material improvements will be measured over time. The ultra-high molecular weight polyethylene used in hip and knee arthroplasty components results from polymerization of ethylene gas into a fine resin powder of submicrometer and micrometer size distribution. It is consolidated with use of ram-extrusion or compression-molding techniques. Structurally, the polymer is made up of repeating carbon-hydrogen (-CH2-) units that are arranged in ordered (crystalline) and disordered (amorphous) regions. Irradiation to sterilize components breaks the polymer chains, creating free radicals, which in an air environment combine with oxygen, facilitating ongoing oxidative degradation of the polymer (Fig. 2). Fig. 2: Depiction of polymer chain breakage following irradiation in air and combination with oxygen, facilitating oxidative degradation of ultra-high molecular weight polyethylene. Continued exposure of the component to oxygen through prolonged shelf storage in air before clinical use results in a progressive stiffening and embrittlement of the polymer, reducing wear resistance and fatigue strength. This is thought to …

Effect of cross-linking on the wear beha vior of ultra high molecular weight polyethylene (UHMWPE) used in total joint replacements

INTRODUCTION The major problem in total joint replacements remains to bc the wear debris induced peri-prosthetic osteolysis (mostly from UHMWPE components) that leads to aseptic loosening. Recently, cross-linking was shown to reduce the adhesive wear of UHMWPE under bovine serum lubrication. In this study, we investigated the prop&es of radiation crosslinked IJHMWPE as a function of cross-link density using the following two methods of irradiation: (i) irradiation at room temperature with subsequent melt-annealing (Method A) and (ii) irradiation with in-situ phase transformation (Method B). METHODS Bi-Directional pin-on-disk (POD) testing up to 2 million cycles with a Paul physiologic loading (peak load = 750 lb.) curve was used to determine the wear behavior as a function of cross-link density. The mechanical properties, thermal properties, density and surface oxidation was also determined. The cross-link density and molecular weight between cross-links was determined using equilibrium volume swelling. RESULTS The physical properties of the cross-linked UHMWPE varied significantly between the two methods used. At equal dose levels. the Method B always yielded far better mechanical properties than Method A. While the yield strength, ultimate tensile strength, density, and crystallinity decreased with increasing radiation dose, the wear resistance of the cross-linked polymer increased dramatically following both methods. DISCUSSION The inserted figure shows the wear behavior of UHMWPE as a function of radiation dose for both methods. The wear rate of the cross-linked polymer followed a transition from high wear to negligibly low wear. The transition to undetectable wear occurted at around absorbed radiation dose of 150 kGy for both methods, above which dose the pins tested on the POD did not show any sign of wear. In fact the original machining marks were still present after 2 million cycles of bi-directional loading. The method B which leads to minimum changes in material properties with dramatic improvement in the wear resistance of UHMWPE promises to be a preferable method of cross-linking IJHMWPE for applications in total joint arthroplasty as bearing surfaces.

Postirradiation Aging Affects Stress and Strain in Polyethylene Components

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging in air. The effects of shelf aging on the stresses and strains associated with surface damage in tibial and acetabular components were examined. A material model was developed to predict the stress and strain relationship of oxidatively degraded polyethylene as a function of density using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of shelf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components with different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the conforming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% in both tibial models and by 15% to 17% in the acetabular model. Postirradiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses and decreased strains predicted to occur as a result of aging.

In vitro simulation of contact fatigue damage found in ultra-high molecular weight polyethylene components of knee prostheses.

An in vitro simulation of fatigue loading of ultra-high molecular weight polyethylene (UHMWPE) knee components was carried out on a knee simulator and on a rolling and sliding wear tester. Tibial components for the knee simulator were gamma-sterilized, implantable components taken from manufacturing inventory. The rolling/sliding UHMWPE discs were machined from bar stock and either gamma sterilized in air and accelerated aged, or left as non-sterilized (controls). Cracking and delamination of samples that had been gamma sterilized in air and aged were observed in both types of tests. Contact fatigue damage was visible in as few as 150,000 cycles using the knee simulator at loads of 122 N (275 1b). The rolling/sliding samples showed signs of damage in as few as 130,000 cycles with an estimated stress of 15 MPa and 25 per cent sliding. However, cracking and delamination were not generated in the never-sterilized or recently sterilized controls. UHMWPE that has been gamma sterilized in air and aged is shown to be susceptible to contact fatigue damage. These results are important to the interpretation of in vitro total knee replacement simulations used to assess the performance of tibial bearings.

Compressive creep characteristics of extruded ultrahigh-molecular-weight polyethylene.

The static compressive creep behavior of ultrahigh-molecular-weight polyethylene (UHMWPE) was studied under physiologic conditions. Specimens were machined from the center and periphery of extruded GUR 4150HP rod stock and were subjected to constant pressures of 2, 4, or 8 MPa for intervals as long as 10(4) min. The creep strain (creep divided by initial thickness) was compared to the pressure and duration of loading by using analysis of variance and linear regression analysis. The amount of creep strain increased rapidly in the early period of testing and was followed by a reduced rate of creep, which reached a steady state after approximately 4000 min. The amount and rate of creep strain increased linearly with pressure. Surprisingly, the rate of creep strain varied with the radial position in the rod stock: specimens obtained from the periphery had 8-19% larger creep strain rates than did specimens obtained from the center (p = 0.1 to p < 0.001). These results advance the characterization of creep's contribution to the in vivo penetration of the metallic component into the UHMWPE component, thereby facilitating the measurement of true in vivo wear. These data also help explain the azimuthally nonuniform deformation observed in retrieved acetabular cups.