Ceramics and Glasses

The advantages of the class of materials known as ceramics, which includes glasses, are that they are very biocompatible (particularly with bone), are inert, have low wear rates, are resistant to microbial attack, and are strong in compression. Some disadvantages include brittleness, the potential to fail catastrophically, and being difficult to machine. These properties arise from the atomic structure of ceramics. Unlike metal, in which atoms are loosely bound and able to move, ceramics are composed of typically two different types of atoms that are ionically and/or covalently bound into compound forms. This atomic immobility means that most ceramics do not conduct heat or electricity. A ceramic that does not have a crystalline structure and is amorphous is referred to as a glass. Glasses are often silica-based. Silica is a network-forming oxide that can be heated to its melting point and, unlike most ceramics, is more easily manufactured. Two very obvious properties of ceramics that are different from metals are melting point and brittleness. Ceramics have very high melting points, generally above 1,000C, and are brittle. Examples of ceramics used in medical devices are shown in Table 5.1. A photograph of a ceramic femoral head of a hip implant is shown in Figure 5.6, and an example of a granular calcium phosphate bone graft substitute is shown in Figure 5.7.

Certain compositions of ceramics, glasses, glass-ceramics, and composites have been shown to stimulate direct bone bonding, which is important in securing orthopaedic medical devices such as replacement hips and knees and spinal fusion devices. These types of materials are known as bioactive ceramics. Studies on retrieved implants have shown that a biologically active calcium phosphate forms on the biomaterial surface upon implantation in the body. Since the calcium phosphate that forms are much like that found in our bones

FIGURE 5.6 In this artificial hip joint, the polymer-bearing surface and some of the metallic components have been replaced by ceramics to improve the durability of the joint replacement. This design features a ceramic femoral head and an acetabular cup. Photograph of the LINEAGEW ceramic-ceramic acetabular cup system is courtesy of Wright Medical Technology, Inc.

bone cells are able to form an intimate attachment to the biomaterial surface after this bone mineral-like layer has formed. The same results are attained by implanting a material that already has a bone-like calcium phosphate surface, such as the granules shown in Figure 5.7. Further studies have shown that the bone-like calcium phosphate actually provides a direct cue to the cells because of the way the cell attaches to the calcium phosphate.

Several different atomic structures (or phases) of calcium phosphate have been used in medical applications, including hydroxyapatite, carbonated apatite, di-calcium phosphate

FIGURE 5.7 If there is an insufficient amount of the patient’s own bone or donor bone available to fill a bone defect, synthetic bone graft substitutes made of calcium phosphate or calcium sulphate may be used. (a) These are biphasic calcium phosphate granules made of hydroxyapatite and tri-calcium phosphate with an optimized porosity and particle shape used for repair of mandibular bone defects (StraumannW Bone Ceramic, Institute Straumann AG). (b) The high porosity visible in this scanning electron micrograph allows maximum space for new blood vessel ingrowth and bone cell influx.

or brushite, beta-tricalcium phosphate or tetra calcium phosphate, and amorphous calcium phosphate. The stability of a given calcium phosphate medical device depends on the crystal phase, the crystal size and perfection, the temperature used during processing, the density, and the in-use environment. At physiological temperature and pH, hydroxyapatite is the stable phase, and it generally takes a long time to resorb via physiochemical dissolution. However, bone cells and other cells called macrophages can initiate cell-mediated resorption of calcium phosphates by changing the local pH to acidic. Nonhydroxyapatite phases of calcium phosphate or other calcium-based biomaterials such as calcium carbonate or calcium sulphate can simply dissolve in the body and do not require cell-mediated resorption.