Abstract
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.
