•A Low Wear, High Fatigue Strength UHMWPE with a Gradient Vitamin E Concentration

INTRODUCTION: Radiation cross-linking is used to decrease wear in ultra high molecular weight polyethylene (UHMWPE) as a total joint arthroplasty bearing surface [1,2] but it also reduces its fatigue strength [1]. Current research focus is to improve the fatigue strength of wear and oxidation resistant cross-linked UHMWPEs in order to improve their use in high stress applications and young and active patients. It has been shown that the residual free radicals in radiation cross-linked UHMWPE can be stabilized by vitamin E [1]. In addition, vitamin E hinders radiation cross-linking in UHMWPE [3]. We proposed to exploit this phenomenon to obtain a spatially controlled distribution of cross-linking in UHMWPE with high cross-link density on the surface for wear resistance and low cross link density in the bulk for fatigue resistance while maintaining oxidative stability. For this purpose, we radiation cross-linked Vitamin E blended UHMWPE with 0.05 wt% Vitamin E on the surface and 0.5 wt% Vitamin E in the bulk. We performed pin-ondisc (POD) wear testing to determine wear resistance, and fatigue crack propagation testing to determine fatigue strength. We also determined the delamination resistance of the gradient cross-linking interface by tensile testing and articulation under unidirectional motion. METHODS: Vitamin-E blended UHMWPE (62 mm dia., 39 mm length) with 0.05 wt% Vitamin E in one half (region 1, Fig. 1) and 0.5 wt% Vitamin E in the other half (region 3, Fig. 1) were gamma irradiated to 150 kGy. Blocks with uniform 0.5 wt% and 0.05 wt% Vitamin E were used as controls. Cubes (3 mm) were cut from the low and high vitamin E regions (n=3) and the gradient region (n=6). Specimens were swollen in 25 ml of hot xylene (130°C) for 2 hours, then blot-dried and weighed. The gravimetric swelling ratio was converted to cross link density per reference [2]. Pins were machined for POD wear testing such that the articulating surfaces were at either at region 1, 2 or 3 (Fig 1). We tested the pins against CoCr, under 163N load per pin, in bovine serum at 2 Hz for up to 2 million cycles (MC) [4]. Wear was determined gravimetrically every 0.5 MC. Wear rate was calculated as the linear regression of weight loss versus number of cycles from 0.5 MC to the end of the test. Tensile tests were performed on ASTM D-638 type V dog-bone specimens (n≥5) punched from 3.2 mm sections machined from the blocks at 10 mm/min on an electromechanical testing system using a laser extensometer. Fatigue crack propagation tests were on A1 C(T) specimens per ASTM E 647 at 40°C in distilled water with a stress ratio of 0.1 at 5Hz. Cracks were measured optically every 20000 cycles. The stress intensity factor range at crack inception (∆Ki) at 10 mm/cycle was calculated. Cylindrical pucks (50 mm dia., 12.7 mm thickness, n=2) were machined from irradiated blocks with gradient vitamin E concentration such that the articulating surface was located 2 mm into the highly crosslinked region. The pucks were accelerated aged at 80°C in air for 5 weeks, then were articulated by CoCr right-lateral uni-compartmental femoral components (Zimmer Inc., Warsaw, IN) in undiluted preserved bovine serum under a constant load of 375 lbf per puck at 2Hz for 5 MC. Photographs of the top surface were taken every 1 MC. After the test, oxidation profiles were measured by Fourier Transform Infrared Spectroscopy (FTIR). The oxidation index was the ratio of the area under the carbonyl absorbance at 1740 cm to the area under the methylene absorbance at 1370 cm after boiling in hexane for 16 hours. One puck was melted in vacuum at 170°C. RESULTS AND DISCUSSION: By varying Vitamin E concentration we tailored a gradient of crosslinking in vitamin E-blended, cross-linked UHMWPE. Crosslink density had an inverse relationship with vitamin E concentration (Table 1). The improved wear resistance due to high cross-link density at the surface provided the benefits of 1st and 2nd generation highly cross-linked UHMWPEs (Table 1). Fatigue strength had an inverse relationship with vitamin E concentration (Table 1) with high bulk fatigue strength compared to irradiated, melted UHMWPE and to irradiated, vitamin E doped UHMWPE (Table 2).