Abstract
INTRODUCTION
Highly cross-linked polyethylene (XLPE) inserts have shown significant improvements in decreasing wear and osteolysis in total hip arthroplasty [1]. In contrast to that, XLPE has not shown to reduce wear or aseptic loosening in total knee arthroplasty [2,3,4].
One major limitation is that current wear testing in vitro is mainly focused on abrasive-adhesive wear due to level walking test conditions and does not reflect “delamination” as an essential clinical failure mode [5,6].
The objective of our study was to use a highly demanding daily activities wear simulation to evaluate the delamination risk of polyethylene materials with and without vitamin E stabilisation.
MATERIALS & METHODS
A cruciate retaining fixed bearing TKA design (Columbus® CR) with artificially aged polyethylene knee bearings (irradiation 30 & 50 kGy) blended with and without 0.1% vitamin E was used under medio-lateral load distribution and soft tissue restrain simulation. Daily patient activities measured by Bergmann et al. [7] in vivo, were applied for 5 million knee wear cycles in a combination of 40% stairs up, 40 % stairs down, 10% level walking, 8% chair raising and 2% deep squatting with up to 100° flexion [8] (Fig. 1).
The specimens were evaluated for gravimetric wear and analysed for abrasive-adhesive and delamination wear modes.
RESULTS
The total amount of gliding surface wear was 28.7±1.9 mg for the vitamin E stabilised polyethylene irradiated with 30 kGy and 26.5±5.7 mg with 50 kGy irradiation, compared to 355.9±119.8 mg for the standard material.
The combination of artificial ageing and high demanding knee wear simulation leads to visible signs of delamination in the articulating standard polyethylene bearing areas in vitro. Delamination began after 2 million test cycles for the standard polyethylene, indicated by the transition between linear and exponential slope in Fig. 2. Delamination was not found in the Vitamin E blended gliding surfaces.
CONCLUSION
To evaluate moderately or highly cross-linked polyethylenes in regard to ageing and wear behaviour in vitro, conditions are simulated to create clinical relevant failure modes given in total knee arthroplasty. With the applied test protocol it is possible to discriminate between the polyethylene bearing materials with and without Vitamin E stabilisation.
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