Background
Traditional Bioceramics

Many of the more “traditional” ceramics have been used for bioceramics applications. Alumina and zirconia, for example, have been used as inert materials for a range of applications from the 1960’s. Their high hardness, low friction coefficient and excellent corrosion resistance offers a very low wear rate at the articulating surfaces in orthopaedic applications. Microstructures are controlled to inhibit static fatigue and slow crack growth while the ceramic is under load.
Alumina Ceramics in Implant Applications

Alumina is currently used for orthopaedic and dental implants. It has been utilised in wear bearing environments such as the total hip arthroplasties (THA) as the femoral head generating reductions in wear particles from ultra-high molecular weight polyethylene (UHMWPE). Other applications for alumina encompass porous coatings for femoral stems, porous alumina spacers (specifically in revision surgery) and in the past as polycrystalline and single crystal forms in dental applications as tooth implants.
Partially Stabilized Zirconia in Implant Applications

Compared to alumina, PSZ has higher flexural strength, fracture toughness and high Weibull modulus (better reliability), as well as lower Young's modulus and the ability to be polished to a superior surface finish. The higher fracture toughness is of importance in femoral heads due to the tensile stresses induced by the taper fit onto the femoral stem.
Zirconia in Clinical Applications

Partially stabilized zirconia femoral heads make up about 25% of the total number of operations per year in Europe, and 8% of the hip implant procedures in USA. It has been reported that over 400,000 zirconia hip joint femoral heads have been implanted since 1985 until 2001. Most of the zirconia femoral heads (tetragonal zirconia polycrystal, TZP) consists of 97 mol% ZrO2 and 3 mol %Y2O3. Although not quite as hard as alumina, PSZ still possesses excellent wear resistance and has been used for similar orthopaedic applications as alumina. Wear rates of UHMWPE against partially stabilised zirconia have been found to be low enough such that tribological debris would not be a problem in clinical applications.
Hydroxyapatite

The first x-ray diffraction study of bone was published16 by De Jong in 1926, in which apatite was identified as the only recognizable mineral phase. He also reported marked broadening of the diffraction lines of bone apatite, which he attributed to small crystal size. It was not until the 1970’s that synthetic hydroxyapatite [Ca10(PO4)6 (OH)2] was accepted as a potential biomaterial that forms a strong chemical bond with bone in vivo, while remaining stable, under the harsh conditions encountered in the physiologic environment.
Bioglasses and Bioactive Glass Ceramics

Since discovery of the bioglasses, which bond to living tissue (Bioglass®) by Hench and Wilson, various kinds of bioactive glasses and glass-ceramics with different functions such as high mechanical strength, high machinability and fast setting ability have been developed. The glasses that have been investigated for implantation are primarily based on silica (SiO2), which may contain small amounts of other crystalline phases. The most prominent and successful application of this is Bioglass® which can be found in detail in various comprehensive reviews. Bioactive glass compositions lie in the system CaO-P2O5-SiO2. The first development of such a bioglass began in 1971 when 45S5 Bioglass® was proposed with a composition of 45% SiO2, 24.5% CaO, 24.5% NaO2, and 6% P2O5 by weight. Hench, and Vrouwenvelder et al., suggested that bioglass® 45S5 has greater osteoblastic activity as compared to hydroxyapatite. Li et al., prepared glass ceramics with differing degrees of crystallinity and found that the amount of glassy phase remaining directly influences the formation of an apatitic layer, with total inhibition when the glassy phase constituted less than about 5 wt.%.
Clinical Applications of Bioglasses and Bioactive Glass Ceramics

Due to the surface-active response of these types of materials, they have been accepted as bioactive (or surface-active) biomaterials and have found applications in middle ear, alveolar ridge maintenance implants and other non-load bearing conditions. Kokubo et al. in 1982 produced a glass-ceramic containing oxyfluorapatite Ca10(PO4)6(OH,F)2 and wollastonite (CaO.SiO2) in a MgO-CaO-SiO2 glassy matrix, which was named A-W glass-ceramic. It was reported that this glass-ceramic A-W, spontaneously bond to living bone without forming the fibrous tissue around them. A bioactive and machinable glass ceramic named Bioverit® has also been developed, which contains apatite and phlogophite (Na,K)Mg3(AlSi3O10)(F)2. It is used in clinical applications as artificial vertebra.
Innovative Ceramics
New Modified Zirconia Implants

Zirconia ceramic implants somehow have had a controversial history regarding their phase metastability, degradation in water lubricants in simulation studies and influence on friction and wear phenomena.

There have been some concerns regarding this degradation phenomenon associated with the tetragonal-to-monoclinic phase transformation under the long-term aqueous condition such as in vivo. One of the current manufacturers of zirconia femoral heads has improved the conventional zirconia, leading to the increased strength and the high resistance to the phase transformation. In addition, it was reported that in hip simulator testing demonstrated that the polyethylene wear against the improved zirconia head is lower than that against the Co-Cr head. When articulated with highly cross linked polyethylene, not only zirconia heads but also Co-Cr heads showed very low wear rates. However, because zirconia is more scratch-resistant than Co-Cr, it would be more suitable implant for the long-term clinical use.
Yttrium Stabilized Tetragonal Polycrystalline Zirconia

Yttrium stabilised tetragonal polycrystalline zirconia (Y-TZP) has a fine grain size and offers the best mechanical properties. Low temperature degradation of TZP is known to occur as a result of the spontaneous phase transformation of the tetragonal zirconia to monoclinic phase during ageing at 130-300°C possibly within water environment. It has been reported that this degradation leads to a decrease in strength due to the formation of microcracks and accompanying phase transformation.
Zirconia/Alumina Composite Biomaterials

Recently degradation free new zirconia-alumina composites have been reported; TZP/alumina composite (80% TZP of [90 mol% ZrO2-6 mol%Y2O3-4 mol%Nb2O5 composition] and 20% Al2O3). Another potential composite comprised of 70 vol% TZP (stabilized with 10mol% CeO2), and 30vol% Al2O3 and 0.05mol% TiO2 is currently being investigated in Japan.

A complete set of references can be found by referring to the original paper.