Bioactive ceramic in clinical use

Bioactive ceramics have already played an important role in bone repair in the clinical fields because of their ability to bond to living bone. However, the use of these bioactive ceramics is limited because of their brittleness and higher Young s modulus than bone. The combination of ceramic and organic components provides us with a design for novel tissue-repairing materials. The establishment of these techniques promises a novel bone repairing material. 

No convincing conclusion has been reached as to the biodegradation mechanisms of bioactive ceramics. Many researchers have reported different results, as described above. These discrepancies are considered to be caused by the fact that materials used for the experiments were different, and that experimental methods and analytical methods were also different. Therefore, when these reported results are compared, it is important to consider the characteristics of the material used (chemical compositions, impurity, crystallinity, dense or porous, micro- or macro-porous, porosity), experimental methods used in vivo or in vitro, animal species, implanted duration, implanted sites, load bearing or not), and analytical methods used (radiographic, optical microscopic, electron microscopic). Futhermore, a good understanding of the characteristics of the materials to be used becomes important when bioactive ceramics are used clinically. 

Hydroxyapatite (Hench, 1993), /3-tricalcium phosphate (Wilson, 1993), Bioglass (Hench, 1991) and Glass Ceramics A-W (Yamamuro, 1993 Kokubo, 1991) are typical inorganic materials that can directly bond to bone tissues when embedded in human bodies. They have already been used as bone-substitutes in clinics. Such tissue-bonding property is denoted as bioactivity and is exceptional because it is only found for a limited kind of materials (Ohtsuki, 1991, 1992) and the rest of all materials, i.e., metals, ceramics, and polymers are not bioactive. However, those ceramic materials are far from ideal tissue substitutes since their fracture toughness is lower than that ofhuman cortical bone, and too hard to be applied to soft tissue replacement. Hence, they find a limited range of use. 

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