A Prosim Knee Simulator Study of UHMWPE Wear: The
Effect of Molecular Architecture and New Generation
Radiation Crosslinking

Abstract

Arthritis is a painful and degenerative condition that affects synovial joints within the human body. Total knee replacement can reduce the pain and increase mobility. Metal on Ultra-high molecular weight polyethylene (UHMWPE) implants have been used successfully for the last 40 years. However, the increasing incidence of implantation in younger patients and the resultant increase in activity has led to higher mechanical demands on the implants. The work in this thesis aims to improve the understanding of UHMWPE with the clinical goal of improved resistance to wear by reducing type 2 fusion defects through improving processing methods and post-processing treatments.

The wear properties of four variants of direct compression moulded (DCM) UHMWPE plates were assessed using statically loaded, multidirectional, pin-on-plate machines. Moulding temperatures and durations were varied between 145°C - 195°C, and 15 min - 30 min, respectively. Mean wear factors for the specific processing conditions were as follows 145°C 30 min - 4.362±0.535, 155°C 30 min - 2.903±0.443, 165°C 15 min - 3.026±0.379, 195°C 15 min - 2.631±0.295 (All wear factors x10^(-6)mm^(3)N^(-1)m^(-1)). Wear properties improved with increased temperature. For cases where moulding temperatures were within 10°C, an increase in moulding duration had a positive effect on wear properties. Surface analysis revealed a link between increased number of type 2 fusion defects and an increase in wear factor. Tests using roughened pins increased wear factors, but the differences between processing conditions became smaller. Molecular weight was also found to affect the wear factor with best performance exhibited by that with a molecular weight of 5,000,000 g/mol, compared to 600,000 g/mol and 9,000,000 g/mol specimens.

A state-of-the-art Prosim knee simulator was used to assess the wear properties of both the current `gold standard' ArCom DCM UHMWPE and a new generation of vitamin E infused, highly cross-linked E1 UHMWPE inserts. Inserts were of geometrically identical designs and were assessed using a high kinematic test profile. A new load soak control (LSC) station was developed and incorporated into the simulator. Volumetric wear results were 6.46±1.80 mm^(3)/million cycles and 0.853±0.131 mm^(3)/million cycles for the ArCom and E1 inserts respectively. Radiation cross-linking was found to reduce wear by 87%. Both wear rates compared favourably to those obtained in similar tests at other research centres, however care must be taken when comparing results as material, experimental, and geometric design differences may all contribute to variations in wear rates. Identical testing using extremely high femoral surface roughness (Ra=0.295µm) increased wear rates to 100±38.5 mm^(3)/million cycles and 50.7±17.3 mm^(3)/million cycles for the ArCom and E1 inserts respectively, suggesting that with highly roughened counterfaces, some of the cross-linking advantage is lost. Tests on E1 inserts using smooth femoral components but increased axial force (4.0kN) revealed no detrimental effects on wear. Accelerated ageing and subsequent wear testing of the E1 inserts showed no change in wear rate, indicating excellent resistance to oxidative degradation afforded by the infused Vitamin E. Temperature monitoring revealed 6.5°C rises in test cell temperature due to frictional heating between the bearing surfaces. Lubricant uptake for both types of insert was found to increase under dynamic loading, indicating the importance of a LSC.

In conclusion: it is possible to improve the wear properties of DCM UHMWPE with small changes in processing conditions. Cross-linking resulted in substantial decreases in wear, however this was less evident when tested using femoral components with increased surface roughness. Vitamin E proved successful in preventing oxidation of the E1 inserts.