VITELENE®, A VITAMIN E-STABILISED HIGHLY CROSSLINKED POLYETHYLENE: WEAR, MECHANICAL AND OXIDATION PROPERTIES

INTRODUCTION

Highly cross linked polyethylenes (HXPE) have to be treated thermally after irradiation to eliminate residual free radicals. By adding vitamin E in the polyethylene powder a post-irradiation thermal treatment is not necessary.

In this review the correlation between the intrinsic properties and the long-term stability of Vitelene® as a high performance material for artificial hip articulation will be displayed.

MATERIALS & METHODS

Three different types of polyethylene (UHMWPE; GUR1020) were analyzed to compare mechanical properties as well as oxidative stability: PE_STD (γ, 30 kGy, N₂), HXPE_REM (γ, 75 kGy, remelted, EO), Vitelene® (β, 80 kGy, 0.1% Vitamin E, EO).

Artificial aging (ASTM F2003 − 70 °C, O₂ at 5 bar) was used to simulate environmental damage. To evaluate the oxidation stability the Oxidation-Induction-Time (OIT) was measured by Differential Scanning Calorimetry (DSC - ASTM D3895) and the Oxidation Index (OI) was determined by Fourier-Transformation-Infrared-Spectroscopy (FTIR - ASTM F2102). The mechanical properties were analyzed by tensile- and impact investigations (ASTM D638 and ISO 11542-2) as well as by Small Punch Testing (SPT - ASTM F2183). The amount of wear was measured gravimetrically (ISO 14242-2).

RESULTS

OIT [minutes], after 0, 14, 28, 42, 56 and 70 days aging, respectively 0.47, 0.41, 0.45, 0.42, 0.42 and not determined (nd) for PE_STD, 0.46, 0.46, 0.47, 0.41, 0.41 and nd for HXPE_REM, 12.09±0.50, 11.67±0.54, 10.78±0.25, 10.42±0.36, nd and 9.25±0.19 for Vitelene®.

Cristallinity [%], after 0, 14, 28 and 42 days aging, respectively 55±2, 63±2, 79±4, 88±3 for PE_STD, 47±2, 48±0, 50±1, 57±1 for HXPE_REM, 52±1, 51±1, 53±14, 53±2 for Vitelene®.

OI, after 0, 14, 28, 35 and 42 days aging, respectively 0.11±0.03, 0.67±0.15, 4.48±1.17 for PE_STD, 0.07±0.05, 0.06±0.02, 0.09±0.02, 0.24±0.05, 0.69±0.36 for HXPE_REM, 0.06±0.01, 0.08±0.01, 0.08±0.01, 0.09±0.01, 0.09±0.01 for Vitelene®.

Tensile Strength [MPa], after 0 and 42 days aging, respectively 47.9±10, 0.7±0 for PE_STD, 56.0±4.0, 25.0±2.0 for HXPE_REM, 53.1±1.0, 52.0±4.3 for Vitelene®.

Elongation [%], after 0 and 42 days aging, respectively 469±69 and 0 for PE_STD, 343±14, 7±3 for HXPE_REM, 372±11, 380±15 for Vitelene®.

Impact Strength [kJ/m²], after 0 and 42 days aging, respectively 149±6, 4±1 for PE_STD, 95±1, 5±1 for HXPE_REM, 86±10, 91±7 for Vitelene®.

SPT - Average Ultimate Load [N], after 0, 14 and 28 days aging, respectively 61.5±4.0, 56.3±5.3, 8.2±0.2 for PE_STD, 71.4±2.2, 68.0±9.9, 64.4±8.2 for Vitelene®.

Wear [mg/Mio cycles] (Ø36 mm Biolox® delta), after 0, 14 and 42 days aging, respectively 19.0±0.6, 30.3±3.1 and 365.8±37,2 for PE_STD, 2.0±0.3, nd and 52.0±16.4 for HXPE_REM, 2.5±0.5, nd and 2.3±0.7 for Vitelene®.

CONCLUSION

The mechanical properties of Vitelene® are unchanged even after 42 days of artificial aging which is correlated to low wear in total hip arthroplasty. Vitamin E stabilization is effective in preventing oxidation and aging of the polyethylene after irradiation cross linking.