One of serious issues in total hip arthroplasty (THA) is the osteolysis which results in aseptic loosening caused by the wear particles from a polyethylene (PE) acetabular cup. In addition, oxidation degradation of PE cup resulting in the fracture or the severe wear caused by the reduction of mechanical properties in vivo is also the issue. The oxidation degradation is considered to be induced by residual free radicals generated by gamma-ray irradiation for cross-linking to reduce wear or for sterilization. In this study, (1) wear property, (2) oxidation degradation of retrieved PE and highly cross-linked PE (CLPE) cups against alumina ceramic femoral heads, and (3) the correlation between those properties were evaluated.

The radiographic wear of six conventional PE cups with the mean follow-up of 19.1–23.3 years and 60 CLPE cups with the mean follow-up of 3.1–9.1 years were measured by a non-radiostereometric analysis method (Vectorworks^® 10.5 software package). As a retrieval analysis, 26 retrieved acetabular cups were evaluated; 16 cups were ethylene oxide gas-sterilized conventional PE cups with clinical use for 16.0–24.9 years and 10 cups were gamma-ray-sterilized CLPE cups with clinical use for 0.9–6.7 years. The linear and the volumetric wear were measured using a three-dimensional (3D) coordinate measurement machine. The shapes of unworn and worn surfaces with 15- and 30-point intervals, respectively, were measured. Oxidation degradation of the surface, sub-surface and inner for both worn and unworn parts of the retrieved cups was measured using a Fourier-transform infrared (FT-IR) spectroscopy. Oxidation indices were calculated using the peak at 1740 cm⁻¹ and 1370 cm⁻¹ according to ASTM F2012.

In the radiographic analysis, the linear wear rate of CLPE cups was significantly lower than that of conventional PE cups [Fig. 1]. In the retrieval analysis, the linear wear rate of CLPE cups (mean: 0.07 mm/year) showed a 51% reduction (p = 0.002) compared to conventional PE cups (mean: 0.14 mm/year) [Fig. 2]. The retrieval and the radiographic analysis for both conventional PE and CLPE cups showed similar results (p = 0.7 and 0.1, respectively). Maximum oxidation indices for CLPE cups were similar to those of conventional PE cups regardless of the difference of clinical duration [Fig. 3]. This result is different from in vivo wear, which increases as the clinical duration. For both conventional PE and CLPE cups, the oxidation indices of subsurface were higher than those for surface. The worn parts showed higher oxidation indices than those for unworn parts. From the results, even when the free radicals were so few or absent, the oxidation degradation would be induced in vivo.

In conclusion, the wear resistance for CLPE cups was greater than that for conventional PE cups from both radiographic and retrieval analyses. The in vivo oxidation degradation might not be caused by only residual free radicals. It was found that oxidation degradation of PE cups when used with alumina ceramic femoral heads is not correlated to their wear properties.

In total hip arthroplasty (THA), aseptic loosening induced by polyethylene (PE) wear debris is the most important cause that limits the longevity of implants. Abrasive wear generated through the mechanism such that micrometer-roughened regions and small asperities on the metallic femoral heads surface locally plow through the PE cup surface. Abrasive wear results in the PE material being removed from the track traced by the asperity during the motion of the metallic femoral heads surface. For the purpose of reducing wear, alumina ceramics was introduced in Europe and Japan in 1970s. The clinical results of ceramic-on-PE bearings regarding the wear resistance have been superior to that of the metal-on-PE bearings. Compared with Co–Cr–Mo alloys, alumina ceramics is advantageous for precision machining because of its higher hardness, enable to form spherical and smooth surface. The fracture resistance of the alumina ceramics itself is related to grain size; the grain size reduction leads to the improvement of its resistance. In this study, we evaluated the roundness and the roughness of retrieved two distinct alumina ceramics having different grain size, and Co–Cr–Mo alloy heads.

Fourteen retrieved alumina ceramic femoral heads; ten heads with a diameter of 28 mm made of small grain size alumina (SG-alumina; mean grain size is 3.4 μm) with clinical use for 16–28 years and four heads with a diameter of 26 mm made of extra-small grain size alumina (XSG-alumina; mean grain size is 1.3 μm) with clinical use for 14–19 years, were examined. Six retrieved Co–Cr–Mo alloy femoral heads with a diameter of from 22 to 32 mm with average clinical use for 12–28 years were examined.

SG-alumina and XSG-alumina heads showed significantly lower roundness compared with Co–Cr–Mo alloy heads, due to higher precision machining [Fig. 1]. The surface roughness for the contact area of the heads increased in order of XSG-alumina, SG-alumina and Co–Cr–Mo alloy. The surface roughness of the non-contact area for all kinds of heads was lower than that for the contact area [Fig. 2]. Surface profiles of the SG-alumina and XSG-alumina showed the reentrant surface while Co–Cr–Mo alloy heads showed the protrusion surface. The roundness and roughness of the Co–Cr–Mo alloy or ceramic surface and the presence or absence of hard third-body particles correlate to the amount of abrasive PE wear. When the third-body was entrapped during the clinical use, a reentrant surface might be formed on the ceramic while protrusion surface formed on the Co–Cr–Mo alloy. The differences in clinical results may be due in part to the influence of third-body particles. The ceramic becomes more resistant than Co–Cr–Mo alloy against the scratching by the entrapped abrasive contaminants because of its harder surface. From the good clinical results of more than 20 years using SG-alumina, the greater long term clinical results using XSG-alumina will be expected.