Materials and Methods

Medical grade UHMWPE (GUR1050) was blended with vitamin E and the peroxide (2,5-Dimethyl-2,5-di(t-butylperoxy)hexyne-3 or P130) before compression molding. Various vitamin E (0.1, 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0 wt%) and peroxide concentrations (0.5, 1 and 1.5 wt%) were studied. The cross-link density was calculated as previously described (Oral 2010). The wear rate was determined using a custom-designed pin-on-disc wear tester against CoCr polished discs at 2 Hz and a rectangular path of 5 × 10 mm in undiluted bovine serum (Bragdon 2001). Tensile mechanical properties were determined using Type V dogbones according to ASTM D638. Oxidative stability was determined using oxidation induction testing (Braithwaite 2010). Double-notching and IZOD impact testing was performed according to ASTM D256. Samples prepared with vitamin E concentrations of 0.3 wt% and above and P130 concentrations of 0.5 and 1 wt% were also terminally gamma sterilized. Controls were 150-kGy irradiated vitamin E blends of UHMWPE.

Materials and Methods

Medical grade slab compression molded UHMWPE (GUR1050) was irradiated using 10, 50, 75, 100, 120 or 150 kGy. The irradiated and melted UHMWPEs were accelerated aged at 70°C for 2, 3, 4, 6 and 8 weeks at 5 atm of oxygen.

Oxidation profiles were determined by first microtoming 150 μm cross sections; these were then extracted by boiling hexane for 16 hours and vacuum dried for 24 hours. They were then analyzed on an infrared microscope as a function of depth away from the surface. An oxidation index was calculated per ASTM 2102 as the ratio of the area under the carbonyl peak at 1740 cm-1 to the area under the crystalline polyethylene 1895 cm-1 peak. The cross-link density was calculated as previously described (Oral 2010). The wear rate was determined using a custom-designed pin-on-disc wear tester against CoCr polished discs at 2 Hz and a rectangular path of 5 × 10 mm in undiluted bovine serum (Bragdon 2001). Tensile mechanical properties were determined using Type V dogbones according to ASTM D638.

Introduction

Irradiated ultra-high molecular weight polyethylene (UHMWPE), used in the fabrication of joint implants, has increased wear resistance. But, increased crosslinking decreases the mechanical strength of the polymer, thus limiting the crosslinking to the surface is desirable. Here, we used electron beam irradiation with low energy electrons to limit the penetration of the radiation exposure and achieve surface cross-linking.

Methods:

Medical grade 0.1 wt% vitamin E blended UHMWPE (GUR1050) was consolidated and irradiated using an electron beam at 0.8 and 3 MeV to 150 kGy. Fourier Transform Infrared Spectroscopy (FTIR) was used from the surface along the depth at an average of 32 scans and a resolution of 4 cm⁻¹. A transvinylene index (TVI) was calculated by normalizing the absorbance at 965 cm⁻¹ (950–980 cm⁻¹) against 1895 cm⁻¹ (1850–1985 cm⁻¹). TVI in irradiated UHMWPE is linearly correlated with the radiation received [3]. Vitamin E indices were calculated as the ratio of the area under 1265 cm⁻¹ (1245–1275 cm⁻¹) normalized by the same. Pin-on-disc (POD) wear testing was conducted on cylindrical pins (9 mm dia., 13 mm length, n = 3) as previously described at 2 Hz [4] for 1.2 million cycles (MC). Wear rate was measured as the linear regression of gravimetric weight change vs. number of cycles from 0.5 to 1.2 MC. Double notched IZOD impact testing was performed (63.5 × 12.7 × 6.35 mm) in accordance with ASTM F648. Cubes (1 cm) from 0.1 wt% blended and 150 kGy irradiated pucks (0.8 MeV) were soaked in vitamin E at 110°C for 1 hour followed by homogenization at 130°C for 48 hours.

Methods

Medical grade GUR1050 UHMWPE with vitamin E (0.1 wt%) was irradiated to 150 kGy. Tibial knee insert preforms were irradiated to 100 kGy, diffused with vitamin E using a doping and homogenization procedure. This UHMWPE was used either before or after gamma sterilization. One set of machined blocks (10 × 10 × 6 mm; n = 6) were extracted in boiling hexane for 4 days, then dried. The extracted blocks were doped with squalene at 120°C for 2 hours. One block each was analyzed after doping. The rest were accelerated aged at 70°C and 5 atm. of oxygen for 6 (n = 2) and 14 days (n = 3). Thin sections (150 micron thick) were microtomed and analyzed by Fourier Transform Infrared Spectroscopy to determine a vitamin E index (1245–1275 cm⁻¹ normalized to 1850–1985 cm⁻¹) and an oxidation index (1700 cm⁻¹ normalized to 1370 cm⁻¹) after extraction with boiling hexane for 16 hours and drying.