Vitamin E stabilised polyethylene for total knee arthroplasty evaluated under highly demanding activities wear simulation

Abstract

As total knee arthroplasty (TKA) patients are getting more active, heavier and younger and structural material fatigue and delamination of tibial inserts becomes more likely in the second decade of good clinical performance it appears desirable to establish advanced pre-clinical test methods better characterizing the longterm clinical material behaviour. The questions of our study were 1) Is it possible to induce subsurface delamination and striated pattern wear on standard polyethylene TKA gliding surfaces? 2) Can we distinguish between γ-inert standard polyethylene (PEstand.30kGy) as clinical reference and vitamin E stabilised materials (PEVit.E30kGy & PEVit.E50kGy)? 3) Is there an influence of the irradiation dose (30vs 50kGy) on oxidation and wear behaviour? Clinical relevant artificial ageing (ASTM F2003; 2weeks) of polyethylene CR fixed TKA inserts and oxidation index measurements were performed by Fourier transform infrared spectroscopy prior testing. The oxidation index was calculated in accordance with ISO 5834-4:2005 from the area ratio of the carbonyl peak (between 1650 and 1850cm^-1) to the reference peak for polyethylene (1370cm^-1). Highly demanding patient activities (HDA) measured in vivo were applied for 5million 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. After 3.0mc all standard polyethylene gliding surfaces developed noticeable areas of progressive delamination. Cumulative gravimetric wear was 355.9mg for PEstand.30kGy, 28.7mg for PEVit.E30kGy and 26.5mg for PEVit.E50kGy in HDA knee wear simulation. Wear rates were 12.4mg/mc for PEstand.30kGy in the linear portion (0-2mc), 5.6mg/mc for PEVit.E30kGy and 5.3mg/mc for PEVit.E50kGy. In conclusion, artificial ageing of standard polyethylene to an oxidation index of 0.7-0.95 in combination with HDA knee wear simulation, is able to create subsurface delamination, structural material fatigue in vitro, whereas for the vitamin-E-blended materials no evidence of progressive wear, fatigue or delamination was found.

Statement of significance: As total knee arthroplasty patients are getting more active, heavier and younger and structural material fatigue and delamination of polyethylene tibial inserts becomes more likely in the second decade of good clinical performance, it appears desirable to establish advanced pre-clinical test methods better characterizing the longterm clinical material behaviour. Various studies reported in literature attempted to artificially create delamination during in vitro knee wear simulation. We combined artificial ageing to clinically observed oxidation of gamma inert and vitamin E stabilised polyethylene inserts and highly demanding patient activities knee wear simulation based on in vivo load data. With this new method we were able to create clinically relevant subsurface delamination and structural material fatigue on standard polyethylene inserts in vitro.

Keywords: Delamination; High demanding activities; Total knee arthroplasty; Vitamin E stabilised polyethylene; Wear simulation.