Abstract
Summary Statement
In the present hip simulator studies, bearings with the newest generation of HXLPE, stabilised with vitamin E, did not show increased wear under severe conditions, such as accelerated ageing, component mal-orientation and third body wear.
Introduction
Unfortunately, acetabular hip components cannot always be implanted in optimal condition. Therefore, we performed hip simulator studies with cups made from highly cross-linked, vitamin E stabilised UHMWPE in i) artificially aged condition, ii) with an inclination angle corresponding to 80 ° in vivo and iii) with third bodies coming from the Ti coating of the acetabular cup.
Methods
For these hip simulator studies, seleXys cup inlays, size 28/EE, and RM Pressfit samples 50/28 (Mathys Ltd Bettlach, Switzerland) were used. Standard PE parts and vitamys® inlays (highly cross-linked, vitamin E stabilised UHMWPE) were tested in the same series. PE cups were machined out of sintered GUR 1020 slabs, packaged and gamma-sterilised in inert atmosphere at 30 kGy. The vitamys® material was made in-house by adding 0.1 wt.-% of vitamin E (Merck KGaA, Darmstadt/Germany) to GUR 1020 powder from Ticona GmbH, Kelsterbach/Germany. Cross-linking used 100 kGy gamma-irradiation and the final sterilisation was gas plasma. Artificial ageing was done under pressurised oxygen at 70 °C according to ASTM F2003 for 14 days (standard PE) and 60 days (vitamys®), respectively. The hip simulator test protocol of ISO 14242 was kept for the artificially aged cups, but the inclination angle altered to 80 ° for the test with the steep cup position. In the third test, the test fluid (diluted bovine serum stabilised with sodium azide and EDTA) was altered by adding about 10 Ti particles to the bearing for the first million cycles. This test condition imitates third body wear by particles shed from the coating of the RM cups. All testing was conducted at the RMS Foundation (Bettlach / Switzerland) on a servo-hydraulic six-station hip simulator (Endolab, Thansau/Rosenheim, Germany) at a temperature of 37±1°C. At lubricant change interval of 500’000 cycles, the inlays were measured gravimetrically with an accuracy of 0.01 mg.
Results
The wear rate of the standard UHMWPE in the condition aged for 14 days reached 45 mg/Mcycle, corresponding to a 57 % increase over non-aged cups. For the vitamys® cups, the wear rate was virtually unchanged even after 60 days ageing (5.8 mg/Mcycle vs. 5.9 mg/Mcycle). For standard UHMWPE tested with an inclination of 80°, wear was 16% lower than those of the inlays with 45° inclination. Whereas for the vitamys® inlays, the wear rate was about the same for both inclination angles (5.4 mg/Mcycle vs. 5.9 mg/Mcycle,). The addition of Ti particles increased the wear rate of standard UHMWPE to 35 mg/Mcycle. However, vitamys® was hardly affected by the third bodies: the wear rate stood at 7.8 mg/Mcycle.
Conclusions
Based on the present simulator study, it seems that hip bearings with the newest generation of HXLPE, stabilised with vitamin E, are exempt from increased wear rate when subjected to severe conditions, such as accelerated ageing, component mal-orientation and third body wear.
