Unit 10 Bioceramics: Medical Applications of Ceramics

  The materials that are used in medicine, i.e. as surgical implants, include metals, plastics, textiles, rubber and ceramics. Biomaterials, in general can be defined as: (1) materials for long term implantation in human tissue, such as arterial and dental prostheses, prostheses of organs (heart, blood vessels), and artificial joints (hip, knee); (2) products for prolonged contact with vessels and tissue or bone; (3) products for short term contact with tissue or bone such as probes or instruments for tests and inspection; (4) materials for obtaining and storing blood and blood plasmas; (5) products used as instruments and tools during surgery.

  Ceramic materials to be used in this context have therefore to fulfill some or all of the following requirements: high chemical inertness; absence of adverse effects on surrounding tissue; long term life expectancy; strength/fatigue strength; absence of effects on free metabolic processes.

  These requisites can be summarized under the term 'biocompatibility'. Biocompatible ceramics, also termed bioceramics, include pure oxides (alumina, zirconia); complex oxides (hydroxy apatite, calcium phosphates); carbon; fiber reinforced carbon composites; and glasses (so called bioglasses). They can be used in load bearing and non load bearing functions, as resorbable or nonresorbable biomaterials; as structured parts or coating on surgical implants made of other materials.

  Load bearing applications which are in general non resorbable include, among others, knee and hip implants; dental implants, bone plates; artificial heart valves; artificial tendons and, to some extent, coatings on metal prostheses.

  Non loading bearing applications are primarily temporary space fillers which can be penetrated and resorbed by the reconstructed natural tissue. They are used to treat maxillofacial defects; or as composite bone plates. The ceramic materials will, in the course of the healing process, dissolve in the surrounding matrix and display ingrowth into the supporting tissue or bone. Materials can be fast or slow degrading, depending on the required functions.

  Biomaterials can be principally classified into three groups: bioinert, resorbable and bioactive. Examples of bioinert materials are oxide ceramics such as Al2O3 and ZrO2-TZP (tetragonal zirconia polycrystals) and biocarbons. Calcium phosphstes are examples of resorbable materials, and hydroxy apatite and bioglasses are instances of bioactive materials. Bioinert materials are used mostly in loadbearing applications. Resorbable materials are conceptually favored, as the host tissue will replace them, but these will also gradually deteriorate as they are chemically dissolved or disintegrated. Therefore they cannot be applied under loadbearing conditions. Bioactive materials are those which tend to form an interfacial chemical bond between the tissue and the implant. These materials can be used in either loadbearing or nonloadingbearing applications (e.g. coatings on metallic implants).

  Bioactive glass and glass ceramic implants have been used for more than 10 years to replace the small bones of the middle ear damaged by chronic infection. Survivability of the bioactive glass implants for middle ear replacements is considerably longer than occurs when bioinert implants are used for the same purpose. Bioinert implants do not bond to the eardrum and, therefore, gradually erode through the tissue and are extruded through the eardrum within 2~3 years. In contrast, highly bioactive glasses from a bond with the collagen of the eardrum and also bond firmly to the remaining bone of the stapes footplate and, thereby, are anchored on both ends, which prevents extrusion. Sound conduction is excellent, and there is no fibrous tissue growth to impair sound transmission.

  The clinical application of bioactive glasses that is most important is in the form of a particulate that is placed around teeth that have had periodontal (gum) disease, a clinical problem that affects tens of millions of people. The 45S5 bioactive glass material rapidly leads to new bone formation around the bioactive glass particles. Because of the speed of formation of the new bone, the epithelial tissues are stopped from migrating down the tooth, a common problem if nothing is used to fill the space between the tooth and repairing bone. The junction between the tooth and the periodontal membrane is stabilized by use of bioactive glass particulate, and the tooth is saved.

  An especially important clinical application of bioactive implants is the use of A/W bioactive glass ceramic with high strength and fracture toughness in the repair of the spine. The material is made by densifying 5-μm sized glass powders into the desired shape, then precipitating oxyfluorapatite (Ca10(PO4)6(O,F2)) and wollastonite (CaO-SiO2) phases to yield a crack and porefree, dense, homogenous glass ceramic. The high compressive and bend strengths, 1080 MPa and 215 MPa, respectively; high fracture toughness, 2.0 MPa·m[1/2]; high interfacial bond strength to bone; and excellent resistance to degradation of properties when exposed to physiological loading conditions provide confidence in the use of this material to replace surgically removed vertebrae.

Selected from: " Medical High Tech Ceramics: Viewpoints and Perspectives", Gernot Kostorz, Academic Press Ltd., London, 1989

Words and Expressions

1. implant 植入物

2. arterial 动脉的

3. prosthesis 假肢

4. inertness 惰性

5. expectancy 期待

6. metabolic 代谢的

7. biocompatibility 生物相容性

8. hydroxy apatite 羟基磷灰石

9. resorbable 重吸收的

10. tendon 腱

11. degrade 降解

12. disintegrate 分解

13. deteriorate 退化

14. erode 腐蚀

15. collagen 骨胶原

16. periodontal 牙周的

17. epithelial 上皮的

18. membrane 薄膜

19. fracture toughness 断裂韧性

20. vertebra 脊椎