ABSTRACT
In artificial joints, biomechanical loading and load transfer over time are strongly affected by the choice of the bearing materials. Regarding such issues, the orthopedic literature contains a myriad of studies that consider material-related effects on joint performance and lifetime, such as surface chemistry and roughness, fretting, brittle or fatigue fracture, and so on. Particularly concerning ceramic biomaterials, it is well established that wear properties and structural reliability of these materials are drastically influenced by both material properties and manufacturing processes. However, multiple confounding conditions (e.g., patient-related factors, surgical techniques, differences in raw materials and processing) greatly limit the identification of simple cause-effect relationships [371-373]. Such a plethora of conditions necessarily makes the incorporation of any new material in the joint design a cumbersome and difficult issue. The most striking consequence of the above situation is that the design choices among available biomaterials are quite limited. Among commonly accepted biomaterials, alumina is by far the most popular ceramic employed in joint arthroplasty [374]. The use of alumina in ceramic-on-ceramic bearings is logical and appropriate if one just think that wear debris and debris-associated osteolysis are the most common (and severe) problems affecting
total hip arthroplastic surgery in the presence of polyethylene. Among various types of bearing material developed to reduce wear rate, alumina ceramic-on-ceramic bearing couples definitely have the greatest scientific and clinical support. In this context, it is interesting to note that, ironically, cross-linked polyethylene overcomes alumina in popularity, stemming as the most used material in hip arthroplasty. However, polyethylene possesses the shortest clinical experience (only since 1998) and the highest wear rate among biomaterials used in hip implants. So, why does such a mismatch between performance and popularity arise for alumina ceramics? This is the main question that we will try to answer in this chapter. The first use of alumina as a biomaterial can be traced back (from German patent activity) to 1930 [311]; however, Boutin was unequivocally the first to use alumina in orthopedic surgery, with successfully implanting alumina joints since 1970 [375]. During the following 40 years of clinical use, millions of alumina femoral heads have been implanted in patients worldwide and, nowadays, a very large family of orthopedic surgeons considers alumina femoral heads the best choice in hip arthroplasty [376-378]. The alumina ceramic used in biomedical applications is the so-called α-phase Al2O3, which possesses a hexagonal (corundum-type) crystallographic structure. Its strong ionic bond and oxygen-rich stoichiometry make alumina exceptionally stable in the human body, leaving its structure conspicuously unaffected by corrosive processes. Such stability is a key property in impeding Al-ion release from the bulk material and from wear debris. Alumina is quite strong under compressive load, but possesses limited tensile strength and is quite brittle under tension, showing no trace of plastic deformation at the onset for fracture at room temperature. Prosthesis components are made of polycrystalline alumina and, according to a general trend observed in ceramic polycrystals, also in alumina the tensile strength increases when fully dense microstructures are produced with an increasingly smaller grain size. Therefore, alumina manufacturers carefully select raw materials and strictly design the production process in order to minimize grain size, thus maximizing strength. However, the obtainment of dense alumina components with sub-micrometer grain size definitely does not represent an easy task and minimizing grain size in alumina has long been a main target for material
technologists. The fabrication of dense alumina bodies with a grain size in the order of the single micrometer has indeed become possible through years of technological improvements owing to two important findings: (i) The addition of a small fraction of magnesia (MgO) to the
raw alumina powder: Such addition enhances mass transport during solid-state sintering, thus allowing the ceramic body to reach full density at relatively low sintering temperatures, which in turn enables to control grain growth during sintering (i.e., with a beneficial effect on strength).
