Creep Of Polyethylene Research Articles

Low polyethylene creep and wear following mobile-bearing unicompartmental knee replacement

PurposeThe Oxford unicompartmental knee replacement (UKR) has a fully congruent mobile bearing to minimise wear. However, with younger higher demand patients, wear remains a concern. The aim of this study was to quantify the wear rate of Phase 3 Oxford UKR bearings over the course of 5 years and to identify the factors that influence it.Methods40 medial Oxford UKRs recruited for a randomised study of cemented and cementless fixation were studied with Radiostereometric analysis (RSA) at 1 week, 3 months, 6 months, 1 year, 2 years, and 5 years post-operatively and bearing thickness was calculated. Penetration, defined as the change in thickness compared to the 1-week measurement, was determined. Creep (early penetration) and wear (late penetration at a constant rate) were calculated. The influence of demographic factors, Oxford Knee Score (OKS), Tegner score, fixation and bearing overhang (determined by RSA) on wear was analysed.ResultsAfter 6 months the penetration rate was constant, indicating that wear alone was occurring. The wear rate was 0.07 mm/year (SD 0.03). The creep was 0.06 mm with about 95% occurring during the first 3 months. There was no significant relationship between fixation (cemented/cementless), age, component size, OKS and Tegner score with wear rate. Increasing BMI was associated with decreasing wear (p = 0.042). 37/40 bearings overhung the tibia to some extent and 23/40 overhung the tibia medially. An increase in the area of overhang (p = 0.036), amount of medial overhang (p = 0.028) and distance between the bearing and tibial wall (p = 0.019) were associated with increased wear. Bearings that did not overhang (0.06 mm/year) had less wear (p = 0.025) than those that did (0.08 mm/year). There was no relationship (p = 0.6) between the femoral contact area and wear.ConclusionDuring the first three to six months after implantation, the bearing becomes 0.06 mm thinner due to creep. The combined wear rate of the upper and lower surfaces of the bearing is constant (0.07 mm/year). The wear is lower if the bearing does not overhang the tibia so surgeons should aim for the bearing to be close to the tibial wall. The orientation of the femoral component does not influence wear.Level of evidenceRetrospective Study, Level III.

The effect of polyethylene creep on tibial insert locking screw loosening and back-out in prosthetic knee joints

A prosthetic knee joint typically comprises a cobalt–chromium femoral component that articulates with a polyethylene tibial insert. A locking screw may be used to prevent micromotion and dislodgement of the tibial insert from the tibial tray. Screw loosening and back-out have been reported, but the mechanism that causes screw loosening is currently not well understood. In this paper, we experimentally evaluate the effect of polyethylene creep on the preload of the locking screw. We find that the preload decreases significantly as a result of polyethylene creep, which reduces the torque required to loosen the locking screw. The torque applied to the tibial insert due to internal/external rotation within the knee joint during gait could thus drive locking screw loosening and back-out. The results are very similar for different types of polyethylene.

Effect of Molecular Structure on the Short-Term and Long-Term Mechanical Behavior of High-Density Polyethylene

Environmental stress cracking (ESC) of polyethylene (PE) in structural applications can result in catastrophic failure without any visible warning. Polyethylene with excellent short-term mechanical strength can have poor long-term performance because of ESC. Because PE structures can have an expected service life of 50 years or more, the lack of understanding regarding their long-term mechanical performance is a major concern. For practical reasons, it is not possible to test PE for years before use. Currently, most mechanical tests for PE only run for a few hours. Therefore, it is important to understand the connections between short-term and long-term mechanical properties. Short-term mechanical behavior, such as creep over 8 h, is studied in conjunction with ESC for six high-density PE resins. In addition, the molecular structure influences on creep of PE are extensively investigated. Complex creep behavior is explained from the point of combined effects of molecular properties. It was found that high-density polyethylene (HDPE) resins with large creep strain usually exhibit high environmental stress cracking resistance. However, the short-term creep behavior of HDPE was affected by molecular weight, molecular weight distribution, and short-chain branching content of the material.

Unicompartmental knee prosthetization: Which key-points to consider?

Unicompartmental knee arthroplasty (UKA) has evolved into a suitable option for diseased knees that cannot be managed with arthroscopic treatment and at the same time are not good candidates for total knee replacement. Since meticulous execution of the surgical technique is essential to optimizing UKA outcome, some procedural key-points are mandatory. Templates (phantoms) are then used to size the required prosthetic component (using these radiographs. Arthritic varus (or valgus) knees with an asymptomatic patello-femoral joint are typically ideal for UKA. Metal-backed tibial components should be favourite instead of all-polyethylene tibial components to avoid polyethylene creep that may occur in fixed bearings. Moreover, a proper thickness of the polyethylene layer is mandatory, in order to avoid early failure.

Computational modelling of polyethylene wear and creep in total hip joint replacements: Effect of the bearing clearance and diameter

Computational wear modelling has been improved using a contact area dependent wear law for the conventional ultra-high molecular weight polyethylene used in total hip joint bearings. The current designs of polyethylene bearings tend to use larger diameter heads to achieve improved motion function for patients in clinical practice, but the wear of the bearing may also increase due to the increase in contact area associated with the larger bearings. Additionally, bearing clearance, which is the difference in diameter or radius between the cup and head bearing surfaces, may also play an important role in affecting the wear. This study particularly investigates the effect of bearing clearance on polyethylene wear, for different femoral head diameters, through a parametric study using the computational modelling method. The effect of creep of polyethylene on the contact area and wear was considered, since polyethylene creep can account for a significantly large proportion of volumetric change, particularly during the initial loading stage, and an increase in the contact area. The predicted wear rate was found to be comparable to the experimental wear rate found in independent simulator tests. The polyethylene bearing with decreased bearing clearances led to a substantial increase in the wear rate, particularly when combined with large diameters.

Maleated amorphous ethylene propylene compatibilized polyethylene nanocomposites: Room temperature nonlinear creep response

AbstractNonlinear creep of polyethylene and its nanocomposites remains an area of significant interest. Maleated polyethylene is often used as a compatibilizer to ensure enhanced dispersion. This article investigates blown films of linear low‐density polyethylene and its nanocomposites with montmorillonite‐layered silicate (MLS). An amorphous ethylene propylene copolymer grafted maleic anhydride (amEP) was added to enhance the interaction between the PE and the MLS. Tensile results indicate that the addition of amEP and MLS separately and together produces a synergistic effect on the mechanical properties of the neat PE. Nonlinear creep was analyzed by examining creep and recovery of the films with a Burger model and the Kohlrausch‐Williams‐Watts relation. A consistent decrease in unrecoverable plastic strain was obtained in the nanocomposite samples. A decreased retardation time associated with MLS presence was determined. POLYM. ENG. SCI., 50:1620–1632, 2010. © 2010 Society of Plastics Engineers

Polyethylene in total hip arthroplasty: half a century in the limelight

Polyethylene in total hip arthroplasty: half a century in the limelight

Effect of an environmental stress cracking agent on the mechanism of fatigue and creep in polyethylene

It is of interest to determine whether the prediction of long-term creep failure from short-term fatigue experiments, as established for polyethylene in air, can be extended to environmental liquids. This article was undertaken to characterize the mechanism of creep crack growth in an environmental liquid at 50 °C and to determine whether the mechanism was conserved in fatigue as required for the fatigue-to-creep correlation. For this purpose, creep and fatigue tests at R-ratio (the ratio of minimum to maximum load in the fatigue cycle) of 1.0 (creep) and 0.1 were performed in air, water, and aqueous Igepal CO-630 (Igepal-630) solutions at various concentrations. It was found that fatigue and creep followed the same stepwise crack growth mechanism as in air in all the Igepal-630 concentrations studied. In air and water, fatigue substantially accelerated the crack growth kinetics compared to creep. A fatigue acceleration effect was also seen with the lower Igepal-630 concentrations. However, the acceleration effect lessened as the concentration increased to 0.05 vol.% due to the combined effects of the gradually decreasing creep lifetime and the gradually increasing fatigue lifetime. Above 0.05%, the lifetimes in creep and fatigue decreased in parallel with the fatigue lifetime only slightly lower than the creep lifetime. It appeared that Igepal-630 reduced the frictional resistance to chain slippage to the extent that any significant strain rate sensitivity was lost. Increasing the molecular weight had the equivalent effect of decreasing the Igepal-630 concentration. This was probably a kinetic effect related to the diffusion of the stress cracking liquid.

Impingement Contributes to Backside Wear and Screw-Metallic Shell Fretting In Modular Acetabular Cups

Eighty-six polyethylene liners and 56 metallic shells of acetabular components were evaluated by visual and stereomicroscopic examination for impingement, backside polyethylene wear, creep, and fretting at the screw-metallic shell interface using a subjective scoring system. Medical records and radiological data were available in 65 and 46 patients, respectively. Impingement was found in 62 components (75%). Backside polyethylene wear and screw-metallic shell corrosion and fretting were significantly correlated (P < .001) with impingement. Polyethylene creep was significantly correlated to backside wear and tended to be higher for the cups that had impingement (P = .06). No correlation was found between backside polyethylene wear and implant design or cup position. Our results suggest that impingement in a modular acetabular component correlated with increased backside wear and screw-metallic shell corrosion and fretting.

Influence of polyethylene creep behavior on wear in total hip arthroplasty

After total hip arthroplasty (THA), polyethylene acetabular liner creep occurs quickly and serves to increases head-liner contact area and decrease contact pressures. What effect these early changes in contact mechanics will have on the wear behavior of the articulation remains unclear, and hence, selection or modification of polyethylene materials for optimal creep and wear performance is impossible. The objective of this study was to determine the influence of polyethylene creep behavior on volumetric wear and linear creep and wear penetration during simulated gait loading conditions. A finite element model of THA articulation was developed, and simultaneous numerical creep and wear simulation was performed to 10 million gait cycles with three levels of polyethylene creep behavior. Long-term volumetric wear and penetration were surprisingly unaffected by the polyethylene creep behavior due to the competing decrease in contact pressures coupled with increased contact area. In addition, variation in contact mechanics with the creep levels studied was only noteworthy in the initial postoperative period; after 1 million gait cycles, peak contact pressures and areas were within 13% regardless of the creep material behavior selected. Femoral head size had considerable impact on wear and penetration, while liner thickness primarily affected only early penetration. These results suggest that polyethylene creep behavior plays a major role in early penetration, but has little influence on the more important long-term volumetric wear.

Maleated and non-maleated polyethylene–montmorillonite layered silicate blown films: creep, dispersion and crystallinity

Abstract Non-linear time dependent creep of polyethylene (PE) montmorillonite layered silicate (MLS) nanocomposites was investigated. PE grafted maleic anhydride (PE- g -MA) was used, as a coupling agent to improve the miscibility between PE and organically modified MLS. The creep and tensile response of maleated and non-maleated PE nanocomposites were determined. Tensile properties of maleated PE nanocomposites were higher than the non-maleated nanocomposites. Non-linearity in the creep response was modeled using the Burger model. A drop in the retardation time was observed for maleated PE nanocomposites. XRD, polarized optical microscopy and a differential scanning calorimeter (DSC) were used to probe crystallinity and clay dispersion in the films. The tensile and creep properties were related to dispersion due to presence of MLS. The deformation response of PE blended with PE- g -MA and each of these separately modified by MLS showed synergistic contributions of the constituents. The response was attributed to dispersion effects with marginal effects of crystallinity.

Finite element simulation of early creep and wear in total hip arthroplasty

Polyethylene wear particulate has been implicated in osteolytic lesion development and may lead to implant loosening and revision surgery. Wear in total hip arthroplasty is frequently estimated from patient radiographs by measurement of penetration of the femoral head into the polyethylene liner. Penetration, however, is multi-factorial, and includes components of wear and deformation due to creep. From a clinical perspective, it is of great interest to separate these elements to better evaluate true wear rates in vivo. Thus, the aim of this study was to determine polyethylene creep and wear penetration and volumetric wear during simulated gait loading conditions for variables of head size, liner thickness, and head–liner clearance. A finite element model of hip replacement articulation was developed, and creep and wear simulation was performed to 1 million gait cycles. Creep of the liner occurred quickly and increased the predicted contact areas by up to 56%, subsequently reducing contact pressures by up to 41%. Greater creep penetration was found with smaller heads, thicker liners, and larger clearance. The least volumetric wear but the most linear penetration was found with the smallest head size. Although polyethylene thickness increases from 4 to 16 mm produced only slight increases in volumetric wear and modest effects on total penetration, the fraction of creep in total penetration varied with thickness from 10% to over 50%. With thicker liners and smaller heads, creep will comprise a significant fraction of early penetration. These results will aid an understanding of the complex interaction of creep and wear.

Design issues in clinical studies of the in vivo volumetric wear rate of polyethylene bearing components.

The issue of wear of ultra-high molecular weight polyethylene bearing surfaces in total hip and knee replacements is, without doubt, one of the dominant themes in contemporary orthopaedics. Thus, an array of relevant facets of this topic have been extensively and intensively researched, spawning, in the process, a very large volume of literature. The main topics that have been studied are the place of wear in the hierarchy of degradation modes of polyethylene bearing surfaces, the role of polyethylene wear particles in the in vivo longevity of the prosthesis, the methods of determining the extent of wear of polyethylene bearing components (for example, the acetabular cup, acetabular cup liner, or tibial insert), the magnitude of the volumetric wear rate, the relationship between the results of in vitro wear-testing of polyethylene specimens or components and the clinical performance of polyethylene components, and the key factors influencing the clinical wear of polyethylene components. It is widely acknowledged that wear is one of many degradation modes that affect polyethylene bearing surfaces in vivo (other modes include creep, delamination, and burnishing) and that, under certain circumstances, it is not the predominant one. For example, polyethylene creep of the liner predominates during the initial stages of penetration of the femoral head into the acetabular cup, but additional penetration after the first twelve to eighteen months in situ is due to wear24. A consensus has emerged that wear of polyethylene bearing components is implicated in the aseptic loosening of hip and knee implants, and in the development of osteolysis around those implants, through resorption of the periprosthetic bone initiated by a foreign-body-giant-cell granulomatous tissue reaction. This biological reaction arises in response to the polyethylene and other wear particles (such as acrylic cement and metal) that are released into the periprosthetic bone. Furthermore, in the …

Multidirectional deformation in fully congruent acetabular components

Most clinical studies have used femoral head migration as an index of acetabular wear, but a previous study showed multiple wear vectors in 30% of retrieved acetabular components with noncongruent liners. The origin of multiple wear vectors is unclear, and it has been suggested that polyethylene creep in a noncongruent shell might influence deformation on the articular surface. We used shadowgraph and volumetric methods to evaluate the extent and direction of surface deformation of 37 retrieved polyethylene liners that were fully congruent to a single design of metal backing. The results show that multiple deformation vectors are relatively common in retrieved acetabular cups (27% in this study) and are independent of congruency between liner and metal backing, rim impingement, and backside creep. Polyethylene liners with multiple wear vectors were significantly thinner than those of cups with a single vector. The origin of multiple vectors is still unclear, but clinical and laboratory studies measuring linear wear alone without recognizing multiple vectors underestimate total in vivo volumetric wear.

Regularities in the variation of the rate of stepped creep in polyethylene with different supermolecular structure

The study of stepped creep, previously discovered with micron-size deformation increments (ɛ) of polymers, in the form of a variation of the rate near the average value is continued. A scheme based on a laser interferometer was used to record the creep; this made it possible to perform precise measurements. Attention was focused on the degree of scatter of the rate h in the process of deformation of polyethylene fibers. It is shown that the creep rate of textured fibers is extremely nonuniform and pulsates continuously, forming beats of different periods, i.e., deformation jumps of different height. The ratio of h of the highest to the lowest rate for arbitrarily chosen small increments of the deformation has a maximum near the start of the “flow” stage and prior to fracture. The h-ɛ curve shifts along the deformation scale as the polymer structure changes, but the form of the curve and the overall level of h change very little. It is also established that the value of h for identical deformations is higher in more highly oriented polymers, and the value of h is higher in cross-linked structures than in unmodified structures. It is proposed that h reflects not only the deformation heterogeneity, but also influences crack formation during the creep process.

Results of total knee arthroplasty with a metal-backed patellar component: A 6-year follow-up study

This study reviews the clinical outcome of knee artroplasty (Microloc, Johnson & Johnson Orthopedics, Norderstedt, Germany) with a resurfacing metal-backed patellar component. Thirty patients were available for clinical and radiographic reexamination with follow-up periods of 36 and 72 months. There were 21 women and 9 men, with an average age of 71.1 years. At the follow-up examination, seven patients demonstrated a metallic friction noise, and three had only mild crepitation at the patellofemoral joint level. In the group complaining of knee pain, the average knee score dropped from 76.9 ± 9.9 at 36 months to 44.7 ± 15.3 ( P < .05) prior to revision surgery on average at 66 months. The function score dropped during the same time from 72.0 ± 19.7 to 53.5 ± 14.9. In contrast, the pain-free group did not demonstrate any significant changes in either knee or function score. Radiographic evaluation showed a patellar tilt of 8.1° ± 4.0° in the group complaining of pain, compared with 2.5° ± 2.3° in the pain-free patient collective. All 10 patients complaining of pain and increasing disability underwent revision surgery. At the time of surgery, an obvious polyethylene particle-induced synovitis was found, and in 7 patients it was associated with a metallosis. One of the main reasons for failure of metal-backed polyethylene components is thought to be the undue high stresses forced onto the insert. High loading of only a small fraction of the surface will lead to increased polyethylene creep and particle formation, resulting in synovitis and finally metallosis. Early revision seems to be the best solution to prevent progressive destruction of the entire joint. Previously reported results on metal-backed patellar components show a failure rate of 8.4% after 12 to 24 months. In this study, this rate had already increased to 33.3% after 6 years. One might therefore speculate that at 10 years, revision surgery might become necessary in more than 50% of the surviving patients with this type of implant.

Creep in polyethylene and hydroxyapatite reinforced polyethylene composites

Isochronous stress-strain relationships and long term creep performance for unfilled and hydroxyapatite filled polyethylene composites have been studied. The tests were carried out in a buffered (pH=7.5) Ringer's solution at 37°C. It was observed that the inclusion of hydroxyapatite does not remove the non-linear viscoelasticity of polyethylene. The creep resistance is found to increase with increase in volume fraction of hydroxyapatite. The creep failure of composites at long times can occur due to debonding of the interface.

Crack initiation and fibre creep in polyethylene

A critical appraisal has been carried out of the creep performance of the fibrils in the pre-crack damage zone (craze) of a series of polyethylene homo- and copolymers. It has been shown that by assuming that the fibrils are drawn to the natural draw ratio of the material, a correlation can be established between the creep rate of oriented specimens and the stress crack performance of the material. The existence of such a correlation paves the way to simpler and faster testing routines and to a better understanding of the factors controlling stress crack performance.

Volume strains and structural changes in polyethylene with small creep flows

1. The decrease in the volumeθ (t) at the onset of creep of polyethylene (after the realization of positive elastic volume deformations) is accompanied by an increase in the number of twinned crystallites, which could be one of the structural mechanisms which causes condensation of the material on extension. 2. In the period of the intense decrease in the volume,θ (t) is linearly dependent on the longitudinal and transverse deformations; the proportionality factor is a function of the value of the load and the type of deformation. 3. Repeated loading causes a new decrease in the volume. The dependences ofθ (t) for initial and repeated loading do not coincide.

Thermally stimulated creep of polyethylene, polypropylene, copolymers, and blends

AbstractThe technique of Thermo Stimulated Creep (TSC) has been applied to the study of anelastic properties of polyethylene, polypropylene, their copolymers and blends. In the temperature range −200 to 100°C, complex TSC peaks were observed in all samples, namely around 0°C, about the same temperature as for the homopolypolymer polypropylene. By applying “fractional stresses”, with a convenient choice or the loading program, these peaks have been experimentally resolved. Two components can be distinguished: 1. The “low temperature” component is characterized by mechanical retardation times following a compensation law. It has been attributed to microbrownian motions of polypropylene sequences liberated at the glass transition of the “true” amorphous regions. 2. The “high temperature” component which is influenced by thermal treatment has been assigned to microbrownian motions of polypropylene sequences liberated at the glass transition of the “constrained” amorphous regions. In block polymers, an additional TSC peak is observed around −50°C: it has been associated with the glass transition of ethylene‐propylene‐rubber (EPR) interphase. The coupling of this interphase with polyethylene and polypropylene phases is insured by diffusion of some ethylene and propylene sequences in‐EPR. At about −140°C, a TSC peak associated with the low temperature component of the glass transition of polyethylene can be distinguished in all the materials studied.