Molecular Weight Polyethylene Components Research Articles

Early monitoring of inlay wear after total knee arthroplasty on plain radiographs using model-based wear measurement

Wear of the ultra-high molecular-weight polyethylene (UHMWPE) component in total knee arthroplasty contributes to implant failure. It is often detected late, when patients experience pain or instability. Early monitoring could enable timely intervention, preventing implant failure and joint degeneration. This study investigates the accuracy and precision (repeatability) of model-based wear measurement (MBWM), a novel technique that can estimate inlay thickness and wear radiographically. Six inlays were milled from non-crosslinked UHMWPE and imaged via X-ray in anteroposterior view at flexion angles 0°, 30°, and 60° on a phantom knee model. MBWM measurements were compared with reference values from a coordinate measurement machine. Three inlays were subjected to accelerated wear generation and similarly evaluated. MBWM estimated inlay thickness with medial and lateral accuracies of 0.13 ± 0.09 and 0.14 ± 0.09 mm, respectively, and linear wear with an accuracy of 0.07 ± 0.06 mm. Thickness measurements revealed significant lateral differences at 0° and 30° (0.22 ± 0.08 mm vs. 0.06 ± 0.06 mm, respectively; t-test, p = 0.0002). Precision was high, with average medial and lateral differences of − 0.01 ± 0.04 mm between double experiments. MBWM using plain radiographs presents a practical and promising approach for the clinical detection of implant wear.

Experimental study on the performance of multi-layered bulletproof vest

A bulletproof vest should have high strength and durability to resist a bullet as vital war equipment. However, the cost of creating good quality bulletproof vest is expensive. Therefore, research about an alternative bulletproof vest with affordable cost and competitive quality is needed. This research aims to review and compare two alternative vest types with one commercial type IV vest. Both alternative vests have similar components, except the second one has an additional Ultra High Molecular Weight Polyethylene (UHMWPE) component. The ballistic test shows that alternative vests are still not enough to resist a 5.56 mm bullet, but it has the potency to handle a 9 mm bullet. Furthermore, the influence of UHMWPE to resist 9 mm bullets is shown in this research.

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.

Ultra-Strong Knits for Personal Protective Equipment

This work focused on the development of ultra-strong knitted fabrics for personal protective equipment used for protection against mechanical damages. Such knits have to have enhanced mechanical strength properties, which strongly depend on knitting pattern and structural characteristics. Six variants of weft knitted structures were developed and knitted from ultra-high molecular weight polyethylene and additional elastomeric component. The elastomeric component was used to increase the elasticity and toughness of knits; however, it had a high influence on mechanical properties, as well. The performed mechanical tests allowed us to identify dependence of mechanical properties, such as breaking force and elongation at break and resistance to abrasion, tearing, cutting and puncture, on architecture and structural parameters of the knits. Obtained results demonstrate that elastomeric component has high influence on mechanical properties knits and can change the principal mechanical behaviour of knits.

Longitudinal Model of Periprosthetic Joint Infection in the Rat.

The majority of periprosthetic joint infections occur shortly after primary joint replacement (<3 months) and require the removal of all implant components for the treatment period (~4 months). A clinically relevant animal model of periprosthetic infection should, therefore, establish an infection with implant components in place. Here, we describe a joint replacement model in the rat with ultrahigh molecular weight polyethylene (UHMWPE) and titanium components inoculated at the time of surgery by methicillin-sensitive Staphylococcus aureus (S. aureus), which is one of the main causative microorganisms of periprosthetic joint infections. We monitored the animals for 4 weeks by measuring gait, weight-bearing symmetry, von Frey testing, and micro-CT as our primary endpoint analyses. We also assessed the infection ex vivo using colony counts on the implant surfaces and histology of the surrounding tissues. The results confirmed the presence of a local infection for 4 weeks with osteolysis, loosening of the implants, and clinical infection indicators such as redness, swelling, and increased temperature. The utility of specific gait analysis parameters, especially temporal symmetry, hindlimb duty factor imbalance, and phase dispersion was identified in this model for assessing the longitudinal progression of the infection, and these metrics correlated with weight-bearing asymmetry. We propose to use this model to study the efficacy of using different local delivery regimens of antimicrobials on addressing periprosthetic joint infections. Statement of clinical significance: We have established a preclinical joint surgery model, in which postoperative recovery can be monitored over a multi-week course by assessing gait, weight-bearing, and allodynia. This model can be used to study the efficacy of different combinations of implant materials and medication regimens. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1101-1112, 2020.

Wear behaviour of polyethylene glenoid inserts against PyroCarbon humeral heads in shoulder arthroplasties

The generation of polyethylene wear debris, and the subsequent tissue reaction to such debris is considered to be a limitation in the long-term survival of shoulder arthroplasties. The purpose of this study was to investigate, for the first time, the wear of a novel PyroCarbon-on-Polyethylene (PyCoP) shoulder arthroplasty system. A 5 million cycle wear test was performed on PyroCarbon humeral heads, which were articulated against commercially available polyethylene glenoid insert components to form an anatomic total shoulder arthroplasty (aTSA). A “Repeat-motion-load” physiological combined cycle was applied using the unique Newcastle Shoulder Wear Simulator. Wear was assessed gravimetrically, and the change of the surface roughness was measured with a non-contacting profilometer. The mean wear rate of the ultra-high molecular weight polyethylene (UHMWPE) components was 19.3 ± 9.5 mm3/million cycles after 5 million cycles of testing. The roughness value, Sa, of the UHMWPE glenoid inserts, reduced, changing from 296 ± 28 nm Sa to 32 ± 8 nm Sa. In contrast, the mean roughness of the PyroCarbon humeral heads remained in the same range (21 ± 2 nm Sa to 20 ± 10 nm Sa). There was no reduction in weight (no measurable wear) of the PyroCarbon humeral heads over the duration of testing. This study is the first to describe the wear performance of UHMWPE glenoid inserts against PyroCarbon humeral heads. No significant difference in the wear of UHMWPE was found in comparison with published studies.

The Influence of SBF on Surface Properties of Irradiated GO/UHMWPE Nanocomposites

In this study, the irradiated ultra-high molecular weight polyethylene (UHMWPE) components doping with graphene oxide (GO) were immersed in simulated body fluid (SBF) environment at 37°C for 6 months, and their surface properties were studied by ball indentation, scratch, and wetting tests. The results show that both the irradiation cross-linking treatment and the addition of 0.5 wt % GO can increase the indentation hardness and scratching coefficient of the surface of UHMWPE composites, and wettability of it becomes better. After soaking in SBF, the ball indentation and scratch coefficient of irradiated GO/UHMWPE nanocomposites decreased by 12.4 and 10.0%, respectively. Which may due to a swelling action and the oxidative degradation occurred on the surface of GO/UHMWPE nanocomposites. However, irradiation cross-linking and filling with GO can prevent the macromolecules from penetrating into UHMWPE base material and materials, thereby reducing the rate of swelling and oxidative degradation.

Effect of energy input on the UHMWPE fabricating process by selective laser sintering

PurposeThis study aims to achieve customized prosthesis for total joint arthroplasty and total hip arthroplasty. Selective laser sintering (SLS) as additive manufacturing could enable small-scale fabrication of customized Ultra High Molecular Weight Polyethylene (UHMWPE) components; however, the processes for SLS of UHMWPE need to be improved.Design/methodology/approachThis paper begins by improving the preheating system of the SLS fabricating equipment and then fabricating cuboids with the same size and cuboids with same volume and different size to study the warpage, demonstrating the effect of the value and uniformity of the preheating temperature on component fabrication. Warpage, density and tensile properties are investigated from the perspective of energy input density. Finally, complicated industrial parts are produced effectively by using optimized technological parameters.FindingsThe results show that components can be fabricated effectively after the optimization of the SLS technological parameters i.e. the preheating temperature the laser power the scanning interval and the scanning speed. The resulting warpage was found to be less than 0.1 mm along with the density as 83.25 and the tensile strength up to 14.1 Mpa. UHMWPE sample parts with good appearance and strength are obtained after ascertaining the effect of each factor on the fabrication of the sample parts.Originality/valueIt is very challenging to fabricate UHMWPE sample parts by SLS. This is a new step in the fabrication of customized UHMWPE sample parts.

Effects of working gas pressure on zirconium dioxide thin film prepared by pulsed plasma deposition: roughness, wettability, friction and wear characteristics

In joint arthroplasty one of the main issues related to the failure of prosthetic implants is due to the wear of the ultra-high molecular weight polyethylene (UHMWPE) component. Surface treatments and coatings have been recognized as enhancing methods, able to improve the tribological properties of the implants. Therefore, the main objective of this work was to investigate the possibility to fabricate yttria-stabilized zirconia (YSZ) coatings on a metal (AISI 316-L) substrate by means of Pulsed Electron Deposition, in order to improve the tribological behavior of the polymer-metal coupling, by reducing the initial wear of the UHMWPE component. In order to optimize the coating characteristics, the effects of working gas pressure on both its morphological and tribological properties were analyzed. Morphological characterization of the films was evaluated by Atomic Force Microscopy (AFM). Coating wettability was also estimated by contact angle (CA) measurement. Tribological performance (coupling friction and wear of UHMWPE) was evaluated by using a ball-on-disc tribometer during highly-stressing tests in dry and lubricated (i.e. NaCl and serum) conditions; friction and wear were specifically evaluated at the initial sliding distances - to highlight the main effect of coating morphology - and after 100m - where the influence of the intrinsic materials properties prevails. AFM analysis highlighted that the working pressure heavily affected the morphological characteristics of the realized films. The wettability of the coating at the highest and lowest deposition pressures (CA ~ 60°, closed to substrate value) decreased for intermediate pressures, reaching a maximum CA of ~ 90°. Regarding tribological tests, a strong correlation was found in the initial steps between friction coefficient and wettability, which decreased as the distance increased. Concerning UHMWPE wear associated to coated counterpart, at 100m a reduction rate of about 7% in dry, 12% in NaCl and 5% in presence of serum was obtained compared to the uncoated counterpart. Differently from what highlighted for friction, no correlation was found between wear rate and morphological parameters. These findings, in agreement with literature, underlined the effect of the deposition pressure on the morphological properties, but suggested that physical characteristics are influenced too. Further research on the deposition process will be required in order to improve the tribological performance of the coating at long distances, addressing – above all - orthopedic applications.

Investigation of surgically retrieved, vitamin E-stabilized, crosslinked UHMWPE implants after short-term in vivo service.

Antioxidant stabilized highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) components have been in clinical use since 2008. In vitro testing has shown excellent oxidation resistance, wear resistance, mechanical properties, and fatigue strength. In this study, we analyzed surgically retrieved components to investigate in vivo behavior and changes in the material. Fifteen surgically retrieved, vitamin E-stabilized, and radiation crosslinked UHMWPE components were analyzed to determine their oxidative stability, extent of lipid absorption in vivo, free radical content, hydroperoxide index, and extent of visible wear damage after in vivo service (0.1-36.6 months). Retrievals showed no significant carbonyls at the time of surgical removal, while free radical content was observed to decay with increasing in vivo duration. There was no increase in hydroperoxide index. Lipid penetration increased with time. Ex vivo oxidation was not observed after 18 months of aging in air at room temperature. The free-radical scavenging activity of the vitamin E appears to successfully prevent both in vivo and ex vivo oxidation for short durations, while reducing free radical content overall. Without an increase in hydroperoxides, the oxidation cascade initiated by radiation-induced and lipid-derived free radicals appears to have been inhibited. Further investigation is required with longer duration implants. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1132-1140, 2016.

Analysis of UHMWPE wear particles produced in the simulation of hip and knee wear mechanisms with the RandomPOD system

If the wear rate of ultrahigh molecular weight polyethylene (UHMWPE) components of prosthetic joints is high, the microscopic UHMWPE wear particles that are produced in large numbers are known to cause osteolysis. This may lead to the loosening of fixation of the implant. Conventional UHMWPE GUR 1020 wear particles produced with the novel RandomPOD wear test system were analyzed by scanning electron microscopy. Worn UHMWPE surfaces were analyzed as well. The wear tests included the simulation of both hip (flat-on-flat) and knee (ball-on-flat) wear mechanisms against polished CoCr in serum. The same non-cyclic motion and load input were used in both cases. The diameter of the hip wear particles was 0.30μm±0.15μm. The knee wear particles were on the average five-fold larger, 1.5μm±0.9μm in diameter. The principal wear mechanism was moderate adhesive wear, which was macroscopically manifested as burnishing. The sizes of the particles and the burnishing were in agreement with clinical findings. The RandomPOD was shown to be the first pin-on-disc wear test device to meet these principal validation criteria regarding simulation of wear mechanisms for both the prosthetic hip and the prosthetic knee.

High frequency circular translation pin-on-disk method for accelerated wear testing of ultrahigh molecular weight polyethylene as a bearing material in total hip arthroplasty

The temporal change of the direction of sliding relative to the ultrahigh molecular weight polyethylene (UHMWPE) component of prosthetic joints is known to be of crucial importance with respect to wear. One complete revolution of the resultant friction vector is commonly called a wear cycle. It was hypothesized that in order to accelerate the wear test, the cycle frequency may be substantially increased if the circumference of the slide track is reduced in proportion, and still the wear mechanisms remain realistic and no overheating takes place. This requires an additional slow motion mechanism with which the lubrication of the contact is maintained and wear particles are conveyed away from the contact. A three-station, dual motion high frequency circular translation pin-on-disk (HF-CTPOD) device with a relative cycle frequency of 25.3Hz and an average sliding velocity of 27.4mm/s was designed. The pins circularly translated at high frequency (1.0mm per cycle, 24.8Hz, clockwise), and the disks at low frequency (31.4mm per cycle, 0.5Hz, counter-clockwise). In a 22 million cycle (10 day) test, the wear rate of conventional gamma-sterilized UHMWPE pins against polished CoCr disks in diluted serum was 1.8mg per 24h, which was six times higher than that in the established 1Hz CTPOD device. The wear mechanisms were similar. Burnishing of the pin was the predominant feature. No overheating took place. With the dual motion HF-CTPOD method, the wear testing of UHMWPE as a bearing material in total hip arthroplasty can be substantially accelerated without concerns of the validity of the wear simulation.

Quantification of structural changes of UHMWPE components in total joint replacements

BackgroundAt present time the number of implantations of joint replacements as well as their revisions increases. Higher demands are required on the quality and longevity of implants. The aim of this work was to determine the degree of oxidative degradation and the amount of free/residual radicals in selected ultra-high molecular weight polyethylene (UHMWPE) components of the joint replacements and demonstrate that the measured values are closely connected with quality and lifetime of the polymer components.MethodsWe tested both new (4 samples) and explanted (4 samples) UHMWPE polymers for total joint replacements. The samples were characterized by infrared spectroscopy (IR), electron spin resonance (ESR) and microhardness (MH) test. The IR measurements yielded the values of oxidation index and trans-vinylene index. The ESR measurements gave the free radicals concentration.ResultsIn the group of new polyethylene components, we found oxidation index values ranging from 0.00-0.03 to 0.24. The trans-vinylene index values ranged from 0.044 to 0.080. The value of free radical concentration was zero in virgin and also in sample of Beznoska Company and non-zero in the other samples. In the group of explanted components, the measured values were associated with their history, micromechanical properties and performance in vivo.ConclusionsWe demonstrated that measuring of oxidative damage may help the orthopaedic surgeon in estimating the quality of UHMWPE replacement component and thus radically to avoid early joint replacement failure due to worse polyethylene quality.

Anti-oxidation Treatment of Ultra High Molecular Weight Polyethylene Components to Decrease Periprosthetic Osteolysis: Evaluation of Osteolytic and Osteogenic Properties of Wear Debris Particles in a Murine Calvaria Model

Wear debris-induced osteolysis remains the greatest limitation of long-term success for total joint replacements with ultra-high molecular weight polyethylene (UHMWPE) bearings. To address oxidative degradation post-gamma irradiation, manufacturers are investigating the incorporation of antioxidants into PE resins. Similarly, larger molecular weight monomers have been developed to increase crosslinking and decrease wear debris, and ultimately osteolysis. However, the effects of modifying monomer size, crosslink density, and antioxidant incorporation on UHMWPE particle-induced osteoclastic bone resorption and coupled osteoblastic bone formation have never been tested. Here, we review the field of antioxidant-containing UHMWPE, and present an illustrative pilot study evaluating the osteolytic and osteogenic potential of wear debris generated from three chemically distinct particles (MARATHON®, XLK, and AOX™) as determined by a novel 3D micro-CT algorithm designed for the murine calvaria model. The results demonstrate an approach by which the potential osteoprotective effects of antioxidants in UHMWPE can be evaluated.

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.

Bacterial adherence to separated modular components in joint prosthesis: A clinical study

Bacterial adherence on total joint replacement implants may lead to biofilm formation and implant-related osteoarticular infection. It is unclear if different biomaterials in the prosthetic components are more prone to facilitate this bacterial adherence, although ultrahigh molecular weight polyethylene (UHMWPE) component exchange in modular systems has been clinically utilized in the early management of these infections. To clarify if the amount of clinically adhered microorganisms was related to the material or the component, we investigated retrieved implants from infected joint replacements. Thirty-two patients were revised after confirmed implant-related infection through positive cultures. Eighty-seven total joint components (hip and knee) were obtained and separately sonicated following a previously published protocol. Cultures were quantified, and detected colony forming units (CFU) were adjusted according to the component surface and compared based on the component material and location. Variable adherence of bacteria to chrome cobalt alloys, UHMWPE, hydroxyapatite coated components, and titanium alloys. The commonest isolated organisms were Staphylococcus epidermidis (23 of 87 components) and Staphylococcus aureus (10 of 87). Twelve components did not show any microorganism adhered despite location in an infected joint, with positive cultures in other components. A mixed linear model adjusted for random effects (the random effect being the infected patient) obtained convergence for the CFU/mm(2) variable, but could not confirm a significantly higher adherence to a particular component or to a particular biomaterial. Therefore, the bacterial adherence primarily depends on the infective microorganism and the response of each individual patient, rather than materials or components.

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.

The ceramic-on-metal coupling in total hip replacements for young patients: a review study

In the last two decades the performance of total hip replacements (THR) has constantly improved, thanks largely to advances in the field of material science and technology. Although contemporary devices are clinically very reliable, some problems still need to be addressed: the wear of ultra high molecular weight polyethylene components, the release of metal ions from metal-on-metal joints, the toughness of the ceramic cups and the possibility of squeaking for ceramic-on-ceramic couplings. All these drawbacks become particularly relevant in case of THR for young and active patients, when the life expectancy of the orthopedic device is very high and the possibility of mechanical shocks is not negligible.In the last few years an innovative ceramic-on-metal tribologic coupling has been introduced in clinical use. This novel configuration represents a very promising solution to all the aforementioned issues. Ceramic-on-metal offers both the strength points of ceramic and metal surfaces, and overcomes the limits of metal-on-metal and ceramic-on-ceramic bearings. For this reason, the ceramic-on-metal coupling has become the object of investigation for several laboratory studies and has been tested in clinical trials. This review study compares the scientific outcomes of the research on ceramic-on-metal bearings, confirming that this innovative articulation has the potential to be a suitable and attractive solution for young and active patients.

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.

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.