Glass-ceramics are used in medical applications due to their unique interaction, or lack thereof, with human body tissue. Bioceramics are typically placed into the following groups based on their biocompatibility: biopassive (bioinert), bioactive, or resorbable ceramics.
Biopassive (bioinert) ceramics are, as the name suggests, characterized by the limited interaction the material has with the surrounding biological tissue. Historically, these were the "first generation" biomaterials used as replacements for missing or damaged tissues. One problem resulting from using inert biomaterials was the body's reaction to the foreign object; it was found that a phenomenon known as "fibrous encapsulation" would occur, where tissues would grow around the implant in an attempt to isolate the object from the rest of the body. This occasionally caused a variety of problems such as necrosis or sequestration of the implant. Two commonly used bioinert materials are alumina (Al2O3) and zirconia (ZrO2).
Bioactive materials have the ability to form bonds and interfaces with natural tissues. In the case of bone implants, two properties known as osteoconduction and osteoinduction play an important role in the success and longevity of the implant. Osteoconduction refers to a material's ability to permit bone growth on the surface and into the pores and channels of the material. Osteoinduction is a term used when a material stimulates existing cells to proliferate, causing new bone to grow independently of the implant. In general, the bioactivity of a material is a result of a chemical reaction, typically dissolution of the implanted material. Calcium phosphate ceramics and bioactive glasses are commonly used as bioactive materials as they exhibit this dissolution behavior when introduced to living body tissue. One engineering goal relating to these materials is that the dissolution rate of the implant be closely matched to the growth rate of new tissue, leading to a state of dynamic equilibrium.
Resorbable ceramics are similar to bioactive ceramics in their interaction with the body, but the main difference lies in the extent to which the dissolution occurs. Resorbable ceramics are intended to gradually dissolve entirely, all the while new tissue grows in its stead. The architecture of these materials has become quite complex, with foam-like scaffolds being introduced to maximize the interfacial area between the implant and body tissue. One issue that arises from using highly porous materials for bioactive/resorbable implants is the low mechanical strength, especially in load-bearing areas such as the bones in the legs. An example of a resorbable material that has seen some success is tricalcium phosphate (TCP), however, it too falls short in terms of mechanical strength when used in high-stress areas.