The purpose of this article was to review the laboratory and clinical performances since 1970 of a total hip prosthesis using alumina-alumina combination. The chemical and physical properties of dense alumina ceramic were studied in relation to biocompatibility, mechanical strength, and surface properties. Through the examination of 35 retrieved implants, it was found that the long-term success of alumina-alumina total hip replacement depends on both the ceramic microstructure (small grain size with uniform distribution, minimum porosity, absence of inclusions) and implant geometry (sphericity deviation +/- 1 micron, radius tolerance between components 7-10 microns). Alumina component wear and fractures have disappeared with the use of high-performance materials and severe manufacturing quality control. Examination of human biopsies from well-fixed prostheses showed that alumina particles deposits increase with time with only a low-grade macrophagic reaction. When loosening occurred, an inflammatory reaction appeared; this reaction was less striking than with loose metal-polyethylene prostheses, however. The long-term behavior of cementless alumina cup fixation depends upon initial positioning and stability; survivorship analysis of the cemented ceramic cups showed an 88% survival probability after 8 years with a 1.6% average annual probability of revision. The percentage of surviving was 100% after 8 years in patients who were less than 50 years old. Aseptic loosenings occurring at the cup-cement interface were assumed to be related to stress protection secondary to the high rigidity of the ceramic leading to a weakening of the spongious bone supporting the cement mantle. Good bone stock quality as well as high-quality ceramic appear to be the prerequisites for durable fixation of alumina sockets.
The wear behavior of hip prostheses alumina components was studied on 20 retrieved implants removed for socket loosening. The observed wear takes two forms: a running-in of the mating surfaces and a gross disruption of the surfaces. The running-in is very shallow, in the order of 0.2 micron after 8 years of implantation. This is observed on the load-bearing areas of the components. Gross wear is not material-related but had always been associated to a high contact pressure between the edge of the socket and the femoral head. Such a high contact pressure occurs after the socket had loosened and then tilted or if it was initially positioned too vertically. The long-term success of alumina-alumina THP components depends essentially upon biomechanical considerations and upon the quality of the material. The area of contact between the head and the socket should be large enough to minimize the contact stresses. This can only be achieved by a good positioning of the socket and by a close tolerance fit of the alumina components; any deviation from sphericity should also be avoided. The normal wear behavior of alumina is a running-in process leading to an average wear rate of 0.025 micron/year as compared to 100 microns/year for UHMWPE on metallic heads.
This paper describes the results of an investigation on the mechanical properties of canine anterior cruciate ligaments. A total of 38 ligaments were tested. It is shown that the completely reversible (elastic) range of strain is limited to 14 percent elongation, corresponding to an applied load of 200 N. Within this range each specimen was tested at different strain rates varying from 0.12 percent/s to 220 percent/s and it is demonstrated that the mechanical behavior of the ligaments is not sensitive to strain rate in the range investigated. After completion of tests in the reversible range, of strain ten ligaments were frozen and similar tests were performed after thawing. It is shown that freezing produces alterations of the mechanical properties. The ligaments become more rigid than when they are tested in fresh conditions. From room temperature up to 45C, the load-elongation relationship is not significantly dependent upon test temperature.
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