Characterisation of Vitamin E–Blended UHMWPE for Higher In Vivo Performance in Orthopaedic Arthroplasty

Abstract

State-of-the-art cross-linked UHMWPE and recent findings about UHMWPE oxidation from in vivo stresses lead to the need of a better understanding of why anti-oxidant additives have to be added in orthopaedics and which anti-oxidant additivation method is the best option for the use in joint replacements. A GUR 1020 cross-linked vitamin E–blended UHMWPE has been investigated to provide an accurate outline of its properties.

DSC and FTIR measurements, together with ageing and tensile tests, were performed on compression-moulded blocks, as well as biocompatibility tests, including implantation on rabbits. Moreover, wear simulations on finished components (Delta acetabular liners, LimaCorporate, Italy) have been completed.

All the test procedures have been repeated for a reference material, a GUR 1050 cross-linked and remelted standard UHMWPE (UHMWPE X-Lima, LimaCorporate, Italy), and the outcomes have been compared to the cross-linked vitamin E–blended UHMWPE ones.

On the additivated UHMWPE, we found an ultimate tensile strength of 43 MPa, a yield strength value of 25 MPa and an elongation to breakage equal to 320%. The degree of crystallinity was 45 ± 2%, and no signal of creation of oxidation products was detected up to 2,000 h of permanence in oxidant ambient after the ageing test. The reference material showed lower mechanical resistance values (σ* = 33 MPa, σ _y = 19 MPa, 305% elongation), a crystallinity of 46 ± 2% and the creation of oxidation products starting from 700 h in O₂ ambient. The biocompatibility tests indicate that the additivated material is biocompatible, as the reference X-Lima UHMWPE.

Wear tests gave a wear rate of 5.12 mg/million cycles against 6.13 mg/million cycles of the reference material and no sign of run-in wear rate.

Our results indicate that there is a loss in mechanical properties in the reference material due to the post-irradiation thermal treatment. DSC measurements instead show no change in crystallinity between the additivated UHMWPE and the reference material. The blend between polymer and additive assures a uniform concentration of vitamin E across the whole thickness of the moulded block, and ageing test results on additivated UHMWPE have shown that the material possesses a superior resistance to degradation phenomena.

Biocompatibility assesses that the presence of vitamin E is not detrimental for the in vivo use of the material, and wear results indicate a better wear resistance of the material, especially in the first stages of the wear process.

From these considerations, it can be concluded that the choice of a vitamin E–blended material, in respect to the standard cross-linked UHMWPE, is highly resistant to oxidation phenomena and retains the mechanical properties thanks to the absence of a post-irradiation thermal treatment; therefore, it is expected to have superior in vivo endurance performance.