The longevity of highly cross-linked polyethylene (XLPE) bearings is primarily determined by its resistance to long-term oxidative degradation. Addition of vitamin E to XLPE is designed to extend in vivo life, although it has unintended consequences of inducing higher frictional torque and increased wear when articulating against metallic femoral heads.1–3 Conversely, lower friction was observed when oxide ceramic heads were utilized.3 Previous studies suggest that oxide ceramics may contribute to XLPE oxidation, whereas a non-oxide ceramic, silicon nitride (Si3N4), might limit XLPE's degradation.4 To corroborate this observation, an accelerated hydrothermal ageing experiment was conducted using static hydrothermal contact between XLPE and commercially-available ceramic femoral heads.
Materials and Methods
Two sets of four types of ceramic femoral heads, consisting of three oxides (Al2O3 BIOLOX®forte, and ZTA BIOLOX®delta, CeramTec, GmbH, Plochingen, Germany; and m-ZrO2 OXINIUMTM, Smith & Nephew, Memphis, TN, USA) and one non-oxide (MC2®Si3N4, Amedica Corp., Salt Lake City, UT, USA) were cut into hemispherical sections. Six highly crosslinked polyethylene liners (X3TM Stryker Orthopedics, Inc., Mahwah, New Jersey, USA) were also sectioned, gamma irradiated (32 kGy), and mechanically clamped (25 kN) to the convex surfaces of the ceramic heads (Figure 1(a)). All surfaces were dipped in water and placed into an autoclave at 121°C under adiabatic conditions for 24 hr. The test was repeated three times using two couples for each material along with XLPE-on-XLPE controls. Each XLPE sample was characterized before and after ageing using Raman spectroscopy for variations in their crystalline phase and oxidation indices using the intensities of unpolarized vibrational bands at 1296, 1305, and 1418 cm−1. Significance (p<0.05) was determined using Student's t-test with a sample size of n=18.
Results are provided in Figure 1(b) for changes in crystallinity. Detectable crystallinity values were significantly lower for XLPE/XLPE (5.47%) and XLPE/Si3N4 (6.74%) pairs when compared with average increases of 9.37, 9.43, and 10.52% for XLPE/ZTA, XLPE/Al2O3, and XLPE/m-ZrO2, respectively.
It is evident that crystallinity and oxidation changes occur in XLPE even under simple static hydrothermal test conditions. As expected, the XLPE control couple showed the lowest overall change because oxygen molecules could similarly diffuse and react with either of the identical counterparts. However, the oxide ceramics were not as effective as Si3N4 in preventing dissolved oxygen from reaching the polyethylene surface.
Coupling oxide ceramics to XLPE in a simple static hydrothermal test increased XLPE's crystallinity and oxidation, while the converse was apparent for Si3N4. These experiments revealed differing roles for oxide and non-oxide ceramics in either promoting or preventing XLPE degradation, respectively. They offer a new paradigm of an “integrated joint space” where biomaterial surfaces affect each other's properties as well as their in vivo tribological behavior.