Bioceramics are ceramic materials specially developed for use as medical and dental implants. They are usually used to replace hard tissue in the body like bone and teeth.
Common bioceramics are aluminazirconia and a form of calcium phosphate known as hydroxyapatite.
Bioactive and bioinert
Bioactive ceramics interact with the body so that tissue bonding and eventual incorporation into the body occurs after a time. Calcium phosphate-based bioceramics are bioactive.
Bioinert ceramics do not interact with the body’s environment apart from causing an initial ‘fibrous tissue’ reaction, which coats the ceramic. Alumina and zirconia-based ceramics are classed as bioinert.
Alumina (Al2O3) is a white powder. When shaped, compressed and heated to a high temperature (sintering), the ceramic that results has high density, high strength, excellent corrosion resistance, good biocompatibility and high wear resistance. In addition, it can be machined, ground and polished to a high-quality product.
Zirconia (ZrO2) is also a white powder. Like alumina, it can be compressed and sintered into a very strong ceramic. Unlike alumina, its wear-resistance properties are not as good. By adding yttrium oxide and small amounts of magnesium oxide, a better-wearing bioceramic called Y-TZP can be made.
Calcium phosphate or hydroxyapatite – Ca10(PO46(OH)2 – is the principal component of natural bone in the body. Ceramics made from synthetic calcium phosphate can also be used in medical applications. The problem is that these ceramics are not as strong as alumina or zirconia ceramics.
Bioceramic bones and teeth
Alumina bioceramics are used as replacement parts in hip and knee operations. The inertness of the ceramic, its high wear resistance and its excellent biocompatibility make it the ceramic of choice.
The high load-bearing properties of alumina also makes it an ideal ceramic for dental implants.
Zirconia bioceramics do not have the high wear resistance of alumina bioceramics and are not as widely used in hip joint applications. However, their fracture toughness and bending strength give them additional qualities.
Calcium phosphate ceramics can bond to bone and promote bone growth at their surfaces. A popular use of these ceramics is as coatings on dental and orthopaedic implants. For example, titanium tooth root pegs coated with hydroxyapatite (a form of calcium phosphate) give a longer lasting implant than pegs that have been glued or cemented in place. The hydroxyapatite binds chemically with living bone because it is a bioactive ceramic.
Often as a result of injuries sustained in road crash accidents, bone reconstruction operations are needed. A novel method involving the use of bovine hydroxyapatite has been recently successfully tested in sheep and dogs.
Spongy bone material from cow bones is cut out and subjected to chemical and physical processes that remove all of the fat and protein. The remaining material is then heated to 1,000°C. After this heating process, what remains is an open 3D mineral shell of hydroxyapatite.
Small cubes of this material can be grafted into the damaged living bone site. Over a period of time, new bone develops and grows in and around the implant. Eventually, successful repair of the damaged bone is achieved.
Although not yet tested on humans, the indications are that, in the future, this method of bone repair could be used, but only in non load-bearing settings.
Nature of Science
Scientists often work in collaborative teams. In the development of bovine hydroxyapatite as a bone substitute, the materials scientist and the medical researcher work closely together to develop and test their ideas.