Wear of Ultra High Molecular Weight Polyethylene
John Fisher, Martin H Stone, Eileen Ingham
Institute of Medical and Biological Engineering University of Leeds
Correspondence addresses: John Fisher Institute of Medical and Biological Engineering School of Mechanical Engineering University of Leeds Leeds, LS2 9JT, UK
[email protected] www.imbe.org.uk
1. Introduction Wear of ultra high molecular weight polyethylene and resulting wear debris induced osteolysis is a major cause of failure in both hip and knee prostheses ([1]). Ultra high molecular weight polyethylene (UHMWPE) wear particles generated at the articulating interfaces, enter the periprosthetic tissues, where they are phagocytosed by macrophages which release inflammatory cytokines such as TNF alpha and lead to bone resorption. Over the last forty years, different types of UHMWPE have been used clinically. Historically UHMWPE sterilised with gamma irradiation in air was used, and this was subsequently shown to oxidise and degrade with age resulting in accelerated wear ([2]).
More recently stabilised UHMWPE and
crosslinked UHMWPE have been introduced. The wear, wear debris, biological activity and osteolytic potential of different types of UHMWPE have been investigated in laboratory simulations, under a range of different conditions. Laboratory studies of historical materials have been compared to clinical retrievals. The importance of investigating the nature of the wear debris and its biological activity as well as the wear rate has been demonstrated, with various types of UHMWPE debris showing different levels of biological activity. Integration of the results of wear simulator studies and in vitro cell culture studies has allowed the prediction of the relative osteolytic potential of different UHMWPE materials.
2. Materials and Methods A range of different types of UHMWPE were studied. These are listed below: RCH 1000
Gamma irradiated in air-historical material
GUR 1020
Four million molecular weight – not irradiated
GUR 1050
Six million molecular weight – not irradiated
GUR 1020 GVF Gamma irradiated in a vacuum 4MRad moderately crosslinked GUR 1050
Medium crosslinked 5M Rad irradiation, annealed
GUR 1050
Highly crosslinked 10M Rad irradiation, annealed
These materials were studied in both hip joint simulators ([3]) and knee joint simulators ([4]) under a range of different conditions. Wear debris was isolated and analysed using SEM ([5]) and the biological activity of debris investigated in culture with primary human and murine macrophages ([6], [7]). Analyses of wear simulator data and cell culture studies were combined to predict functional osteolytic potential ([8]).
The following studies are summarised in this review paper. •
Effect of ageing in vitro and in vivo on the wear of gamma irradiated in air RCH 1000.
•
Influence of damaged metallic femoral heads on the wear of RCH 1000 in vitro and in
vivo. •
Analysis of wear particles and functional biological activity from RCH 1000 UHMWPE.
•
Comparison of stabilised and moderately crosslinked GUR 1020 UHMWPE.
•
Comparison of metallic and ceramic femoral heads on the wear of GUR 1020 GVF
UHMWPE. •
Influence of joint laxity on the wear of UHMWPE in the hip.
•
Influence of kinematic conditions and wear path on the wear of UHMWPE on the hip.
•
Influence of kinematics on the wear of UHMWPE in the knee.
•
Comparison of wear rate of non, medium highly crosslinked GUR 1050 UHMWPE.
•
Comparison of biological activity of wear debris from different molecular weights and
crosslinked UHMWPE.
3. Results •
Wear of historical RCH 1000 UHMWPE – Effect of ageing.
The wear rate of RCH 1000 gamma irradiated in air in the hip joint simulator was 40 ± 6 mm3/million cycles ([9]). This compared with a retrieval study with an average wear rate of 59 ± 20 mm3/year ([5]). The simulator study was carried out on UHMWPE that had not been aged and articulated against smooth femoral heads. RCH 1000 gamma in air UHMWPE which had been aged for five years in vitro, showed a threefold increase in wear rate compared to the non aged material. RCH 1000 gamma in air UHMWPE which had been implanted for fifteen years in the body and was then wear tested also showed a threefold increase in wear rate. This indicated that the rate of oxidation and acceleration of wear was greater in vitro prior to implantation, than in vivo once implanted. •
Wear with damaged femoral heads
The retrieval studies of RCH 1000 acetabular cups ([5]) showed a twofold increase in UHMWPE wear rates when the heads had been scratched or damaged compared to undamaged femoral heads ([5]). Simulation of scratched metallic femoral heads produced a threefold increase in the wear rate in laboratory tests ([9]). However it should be noted that in the 2
retrieval studies, it was not known at what time point, in the life of the prosthesis, the head had been scratched. The wear rate was therefore averaged over the lifetime of the device. •
Analysis of wear particles and biological activity
Analysis of the wear particles from RCH 1000 UHMWPE (gamma in air) showed that they were in the size range 0.1 µm to 100 µm, with the majority of the number of particles being less than one micron in size. However, the distribution of the volume of the wear particles was more evenly distributed throughout the size ranges ([5]). Cell studies of the biological activity of UHMWPE wear particles of different sizes, showed that an increased volumetric concentration produced elevated levels of osteolytic cytokines ([6], [7]). Particles in the size range 0.1 to 1 µm were most reactive and particles in the size range 1 to 10 µm were least reactive ([6], [7], [8]). Combining the volumetric wear rate with the specific biological activity of a unit volume of wear debris showed that the functional biological activity and osteolytic potential of aged RCH1000, and RCH1000 on damaged femoral heads was greater than for not aged RCH1000 articulating on smooth femoral heads. •
Comparisons of stabilised and moderately crosslinked GUR 1020 UHMWPE
GUR1020 UHMWPE that was not irradiated and sterilised by ethylene oxide had a higher wear rate 49 ± 8 mm3/million cycles, than GUR 1020 GVF moderately crosslinked UHMWPE 35 ± 9 mm3/million cycles ([10]). However, the wear debris from the moderately crosslinked UHMWPE, had a higher proportion of particles in the smallest size range studied (0.1 to 1 µm) which resulted in a higher level of specific biological activity. Combining the wear rates and specific biological activity resulted in no difference in the functional biological activity or osteolytic potential of the stabilised and moderately crosslinked material ([10]). Both materials showed lower osteolytic potential than the oxidised and aged RCH 1000 which had been gamma irradiated in air. •
Comparison of metallic and ceramic femoral heads on the wear of GUR 1020 GVF
UHMWPE The wear of UHMWPE was compared when articulating on polished cobalt chromium alloy and alumina ceramic femoral heads in the hip joint simulator. The wear rate with the cobalt chrome alloy was 35 ± 9 mm3/million cycles, whereas the wear rate with the alumina ceramic femoral head was significantly lower at 25 ± 6 mm3/million cycles ([10], [11]). The lower wear rate with the alumina ceramic may have been associated with its slightly lower surface 3
roughness or increased hydrophillicity. Both heads had substantially lower wear rates than the scratched or damaged metallic femoral head ([10]). •
Influence of joint laxity on the wear of UHMWPE
Fluoroscopic studies of patients with hip prostheses have shown separation of the femoral head and acetabular cup during the swing phase of gait in some patients. This microseparation can result in the femoral head contacting the rim of the acetabular cup.
This has been
investigated in laboratory hip joint simulator studies. The wear rate of the GUR1020 GVF polyethylene articulating on alumina ceramic femoral heads under standard conditions was 25 ± 6 mm3/million cycles. Introducing swing phase microseparation of 1 mm substantially reduced the wear rate of the UHMWPE to 5 ± 4 mm3/million cycles. During microseparation the femoral head contacted the superior rim of the UHMWPE cup, before reloading after heel strike. While there was deformation of the rim, the overall cup wear reduced ([11]). It is postulated that the separation of the bearing surface during swing phase encourages fluid to enter the contact, enhancing the squeeze film lubrication and reducing wear during the stance phase of the walking cycle. •
Influence of kinematic conditions and wear track on the wear in the hip
UHMWPE is a linear polymer which can orientate and strain harden during sliding, providing resistance to wear. However if the wear tracks are highly multidirectional, preferential strain hardening and wear reduction is less. A typical wear track in the hip can be approximated by an ellipse with an eccentricity of 4 to 1. That is the long axis of the ellipse is four times the depth of the short axis. The eccentricity of the elliptical wear tracks has been shown to vary from as low as 2:1 to as high as 9:1. The eccentricity of the elliptical wear track was altered in the hip joint simulator from standard ratio of 4:1 to a more eccentric or linear path of 8:1. This resulted in a reduction in wear of approximately 50% ([12]). Under the more eccentric or linear wear path, the UHMWPE was able to strain harden along the preferred long axis of sliding, so providing resistance to wear. Variations in the gait cycle of patients may lead to different wear rates. •
Influence of kinematic conditions on the wear of UHMWPE in the knee
Wear of RCH 1000 gamma or air UHMWPE has also been studied in a knee joint simulator, under high kinematic conditions, with ± 5° internal-external rotation and 0 – 10 mm anteriorposterior displacement. The wear rate was 41 ± 14 mm3/million cycles ([13]). When internal 4
external rotation was reduced to ± 2.5° and anterior posterior displacement reduced to 0-5 mm, the wear rate was reduced to 7.7 ± 2mm3/million cycles. Under high kinematic conditions the wear rate of the knee approached that of the hip, when the inputs were reduced and a more linear wear path was produced, the wear rate was significantly reduced. Similar results were found with GUR 1020 GVF polyethylene, which had a wear rate of
22 ± 6 under high kinematic
input conditions which was reduced to 4 ± 3 mm3/million cycles with reduced kinematic inputs ([14]). The activity levels and gait are likely to have a significant effect on wear in artificial knee joints. •
Comparison of the wear rate of crosslinked and non crosslinked polyethylene
The wear of three levels of crosslinked GUR 1050 UHMWPE have been studied with, 0, 5 MRad, 10 MRad irradiation crosslinking in a multidirectional wear simulation. Two different kinematic conditions were studied, medium levels of cross shear 25% reflecting conditions found in the hip, and low levels of cross shear < 10% reflecting conditions found in the knee ([15]). In both cases the non crosslinked material had the higher wear rate and the highly crosslinked 10 MRad material had the lowest wear rate. Under higher cross shear the wear of the 10 MRad material was 25% of the O MRad material wear rate, while under low cross shear the 10 MRad material wear was 66% of the O MRad material wear rate. For both the O and 5 MRad material, the higher cross shear produced higher wear rates. However with the 10 MRad material the kinematic conditions did not significantly affect the wear rate. The reduction in wear with crosslinking was consistent with other studies. This was the first reported study of less reduction on wear with the more linear kinematic conditions ([15]), which is relevant to the knee. •
Influence of molecular weight and level of crosslinking on the biological activity of
UHMWPE wear debris Recent developments in laboratory models have allowed the direct coupling of aseptic multidirectional wear simulations to cell culture systems to directly assess the biological activity and osteolytic potential of the wear debris generated ([16], [17]). The influence of molecular weight on the biological activity of UHMWPE wear debris was investigated by comparing GUR 1020 (4 million molecular weight) and GUR 1050 (6 million molecular weight) UHMWPE. There was no significant difference in the volumetric wear rate of the two materials. However the GUR 1050 was found to have a greater proportion of debris in the size range 0.1 to 1 µm, and a unit volume of GUR 1050 was found to produce significantly higher levels of osteolytic 5
cytokines in cell culture with murine macrophages ([16]). This study is particularly important as historically, GUR 1050 has been more extensively used in North America, where the prevalence of osteolysis has been more widely reported. Further studies have investigated the effect of crosslinking on the biological activity of GUR 1050 UHMWPE debris ([17]). Crosslinking with both 5 and 10 MRad levels of irradiation produced smaller and more biologically active wear debris compared to non irradiated GUR 1050. Although crosslinking reduced the wear volume by up to 75%, these studies have shown for the first time that crosslinking directly increased the osteolytic potential of the debris produced. Further work is needed to determine the functional osteolytic potential of a reduced wear volume and increased biological activity.
4. Discussion Since the realisation of the impact of wear debris induced osteolysis over ten years ago, the understanding of factors that influence the volumetric wear rate of UHMWPE has advanced considerably. The influence of femoral head damage, oxidative degradation of UHMWPE, kinematic conditions, separation and lubrication, as well as femoral head material are now well understood. During the last five years, new and improved materials have been developed, with the introduction of stabilised and moderately crosslinked UHMWPE and more recently highly crosslinked UHMWPE. The improvement in the wear performance of these materials under different conditions is being defined. As new material variables are being introduced into the system additional levels of complexity and uncertainty emerge. The recent discovery that increased molecular weight increased the biological reactivity of the debris ([16]) led to investigation of the biological activity of crosslinked UHMWPE debris. Direct cell culture studies of wear debris generated in aseptic multidirectional wear simulators have shown an increase in the biological activity and release of osteolytic cytokines with crosslinked debris compared to non crosslinked debris ([17]). Further work is needed to understand the impact of this debris from the different materials at functional concentrations. Early clinical studies are showing good results for highly crosslinked materials, as they are for medium crosslinked polyethylenes and moderately crosslinked and stabilised materials. Longer term clinical studies are awaited. Perhaps caution should be applied to the use of these new materials with large diameter heads where previously rapid failure has been seen with conventional UHMWPE. Laboratory studies now allow direct assessment of osteolytic potential, and it is clear that with differences in the reactivity of debris from different materials, measurement of wear volume alone is unlikely to be a satisfactory indicator of clinical outcome. The differences in the 6
reactivity of debris from different molecular weight materials may explain some of the historical trends in clinical performance.
Further work is needed to study the functional osteolytic
potential of crosslinked UHMWPE to allow the appropriate range of clinical applications to be defined.
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Acknowledgements The research described in this review has been supported by the Engineering and Physical Sciences Research Council and the Arthritis Research Campaign.
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