Abstract
Introduction
In total hip arthroplasty (THA), polyethylene (PE) liner oxidation leads to material degradation and increased wear, with many strategies targeting its delay or prevention. However, the effect of femoral head material composition on PE degradation for ceramic-PE articulation is yet unknown. Therefore, using two different ceramic materials, we compared PE surface alterations occurring during a series of standard ceramic-PE articulation tests.
Materials and Method
Ceramic-PE THA bearings were tested in a simulator, using ASTM F2003-02, ASTM F1714-96 (2013) and ISO 14242:1–3 standards. Acetabular liners (Apex-Link PolyTM, OMNI Life Science, East Taunton, MA, USA) were articulated against Ø28 mm Si3N4 femoral heads (Amedica Corp., Salt Lake City, UT, USA). For comparison, ArCom® PE liners (Biomet Inc. Warsaw, IN, USA) were also tested against Ø28 mm zirconia-toughened alumina (ZTA) femoral heads (BIOLOX®delta, CeramTec GmbH, Plochingen, Germany), under the same conditions. After 5 million cycles of wear, all specimens were examined using nano-spectroscopy tools. Evaluations were performed on six couples per group, plus 3 untested control couples; n= 6 (+3). Spectrographic examinations generated 8 maps of 400 points each randomly selected on the wear zones of each liner, with each map area being 20 µm2 at an in-plane spatial resolution of 1 µm.
Results
Volumetric wear loss for the ArCom®-BIOLOX®delta bearing was twofold greater than for Apex-Link PETM-Amedica Si3N4 (i.e., ∼220 mm3 vs. ∼100 mm3). Crystallization is a consequence of the PE chain-scission induced by oxidation. When compared to new Si3N4 heads, the crystallinity increase in the worn area of the PE tested against Si3N4 was consistently 3%, with negligible oxidation (i.e., oxidation index; OI < 0.25). In comparison, the amount of PE crystallization in ArCom®-PE worn against ZTA was ∼21% higher when compared to new liners of the same type (Fig. 1). Also, higher oxidation was consistently detected in PE liners articulated against ZTA (average OI increase = 0.42 at a sub-surface depth of 5 µm in the wear zone with hot spots up to OI = 2.1). Surface inspection of the worn ZTA and Si3N4 heads unequivocally revealed the occurrence of oxygen-release and oxygen-trapping mechanisms, respectively (i.e., formation of oxygen vacancies in ZTA vs. amorphous silica on the surface of Si3N4 (Fig. 2)).
Discussion
Despite different brands of PE liners in this study, their underlying chemistry was identical. Our data showed that, when compared to Al2O3, non-oxide ceramics like Si3N4 may discourage PE oxidation in ceramic-PE articulations. One explanation for these observations is that Al2O3 is known to release dehydroxylated and ionized oxygen under wear conditions due to a frictional triboplasma within the contact region, and this phenomenon contributes to oxidative degradation. In contrast, Si3N4 (a non-oxide ceramic) scavenges oxygen under identical conditions, thus “protecting” the polyethylene from oxidation. These observations may have relevance to the anticipated longevity of PE liners in ceramic-PE THA.
Conclusions
Consistent with the unique surface characteristics of these bioceramics at the molecular level, Al2O3 and Si3N4 exert different effects on the oxidation, and therefore the projected lifespan of PE in vivo.
