(643d) Interparticle Cohesion and Wear Performance of Ultra-High Molecular Weight Polyethylene

Since the 1970s, orthopaedic grade ultra-high molecular weight polyethylene (UHMWPE) has served as the only widely used bearing material for articulation with metallic components in total knee arthroplasty (TKA). However, polyethylene-related total knee failures especially Type 2 fusion defects have limited the lifetime of these joints [1, 2, 3].

The work to be presented has incorporated wear data together with numerical modelling and aims to improve the understanding of the effect of interparticle cohesion on wear and fatigue of UHMWPE clinical TKA components. The hypothesis examined was that the cause of material failure of UHMWPE knee joint components lies in the precise time-temperature history employed during manufacture of the UHMWPE, due to their strong effect on interparticle cohesion[1]. Reptation and thermal modelling allows quantitative prediction and optimisation of the extent of interparticle cohesion to ensure that interparticle boundaries are of high integrity during moulding [2]. This enables one to define the upper bound of Mw of those molecular chains which have fully reptated across grain boundaries, labelled as maximum reptated molecular weight (MRMW) [2].  Temperatures in the range 145 to 175°C were considered together with moulding times of 15 and 30 minutes.

The material combination of flat-ended metallic indentors loaded against UHMWPE plates was constructed to mimic conformal contact conditions in knee prostheses.  UHMWPE plates, direct compression moulded in the same way as ArCom UHMWPE (Biomet UK Ltd) at a variety of processing conditions were evaluated. Wear tests were carried out using a Durham four-station multi-directional pin-on-plate machine, which generated both reciprocating and rotating motions simultaneously. The articulating surfaces were lubricated using 25% diluted bovine serum (proteins: 17.5g/L). The load applied in the present study was 40N giving a pressure of 2.0MPa. The frequencies were pre-set at 1Hz for both reciprocation and rotation.

Wear tests (complying with BS ISO 14243-2: 2000 for cleaning and measurements) were conducted up to three million cycles. The wear rates averaged over four stations will be presented. Low wear was found for UHMWPE moulded at 175°C with wear rate of 0.93×10^-4±0.11×10^-4mg/m. It corresponds to MRMW of 2.19×10⁶g/mol. An user-defined thermal model with diffusive heat transfer 8-node linear elements was analyzed in ABAQUS/Standard v6.8 to simulate the processing of the UHMWPE plates used in the wear study. The material properties of the polymer were the same as our previous work [2].

In summary, the fusion of UHMWPE is of great importance in the manufacture of bearing surfaces in orthopaedic implants especially as Type 2 fusion defects have been implicated many times in premature failures of UHMWPE orthopaedic joint components [1,3]. The present work has further examined the relationship between wear and the extent of interparticle diffusion. A clear trend has been found between lower wear and increased MRMW; an increase of 45% in MRMW halves the wear rate. 

References

1. Wu et al., 2002. Biomaterials 23: 3773-3783.

2. Wu et al., 2002. Transactions of the Institution of Chemical Engineers Part A 80: 423-431.

3. Muratoglu et al., 2010. Polymer 51:2721-2731.