Chapter 13 Bioceramics

The clinical goal when using ceramic biomaterials, as is the case with any biomaterial, is to replace lost tissue or organ structure and/or function. The rationale for using ceramics in medicine and dentistry was initially based on the relative biological inertness of ceramic materials compared with metals. However, in the past two decades, this emphasis has shifted more toward the use of bioactive ceramics, materials that not only elicit normal tissue formation but may also form an intimate bond with bone tissue. Most recently, bioceramics have been utilized in conjuction with more biological therapies. In other words, the ceramic, usually resorbable (i.e., a greater degree of bioactivity than surface-reactive materials), facilitates the delivery and function of a biological agent (i.e., cells, proteins, and/or genes) with an end goal of eventually regenerating a full volume of functional tissue. Ceramic biomaterials are processed to yield one of four general types of surfaces and associated mechanisms of tissue attachment (Kohn and Ducheyne, 1992): (1) fully dense, relatively inert crystalline ceramics that attach to tissue by either a press fit, tissue ongrowth onto a roughened surface, or a grouting agent, (2) porous, relatively inert ceramics into which tissue ingrowth occurs, creating a mechanical attachment, (3) fully dense, surface-active ceramics that attach to tissue via a chemical bond, and (4) resorbable ceramics that integrate with tissue and eventually are replaced by host tissue. Ceramics may therefore be classified by their macroscopic surface characteristics (smooth, fully dense, roughened, or porous) or their chemical stability (inert, surface reactive, or bulk reactive/ resorbable). The integration of biological (i.e., inductive) agents with ceramics further expands the clinical potential of these materials. Relatively inert ceramics elicit minimal tissue response and lead to a thin layer of fibrous tissue immediately adjacent to the surface. Surface-active ceramics are partially soluble, resulting in ionexchange and the potential to lead to a direct chemical bond with bone. Bulk bioactive ceramics are fully resorbable, have much greater solubility than surface-active ceramics, and may ultimately be replaced by an equivalent volume of regenerated tissue. The relative level of bioactivity mediates the thickness of the interfacial zone between the biomaterial surface and host tissue (Fig. 13.1). There are, however, no standardized measures of reactivity, but the most common are pH changes, ion solubility, tissue reaction, and any number of assays that assess some parameter of cell function. 13.5 BIOMIMETIC CERAMICS 13.17 REFERENCES 13.22