Page 386 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOCERAMICS 363

Propagating
crack

Tetragonal grain

Transformed monoclinic grain

Compressive stress ahead of the crack

FIGURE 15.3 Schematic of microstructure in yttria partially stabilized zirconia (YPSZ) bioceramic
undergoing transformation toughening at a crack tip. [From Miller et al. (1981), with permission.]

usually initiates at a critical defect, at a stress level that depends on the geometry of the defect. To
account for these variables and minimize the probability of failure, fracture mechanics and statis-
tical distributions are used to predict failure probability at different load levels (Soltesz and
Richter, 1984).

15.3 BIOACTIVE CERAMICS

The concept of bioactivity originated with bioactive glasses via the hypothesis that the biocompati-
bility of an implant is optimal if it elicits the formation of normal tissues at its surface, and if it estab-
lishes a contiguous interface capable of supporting the loads which occur at the site of implantation
(Hench et al., 1972). Under appropriate conditions, three classes of ceramics may fulfill these
requirements: bioactive glasses and glass ceramics, calcium phosphate ceramics, and composites of
glasses and ceramics. Incorporation of inductive factors into each of these ceramics may enhance
bioactivity. These different classes of ceramics (and biological constituents) are used in a variety of
applications, including bulk implants (surface active), coatings on metal or ceramic implants (sur-
face active), permanent bone augmentation devices/scaffold materials (surface active), temporary
scaffolds for tissue engineering (surface or bulk active), fillers in cements or scaffolds (surface or
bulk active), and drug delivery vehicles (bulk active).
The nature of the biomaterial/tissue interface and reactions (e.g., ion exchange) at the ceramic
surface and adjacent tissues dictate the resultant mechanical, chemical, physical, and biological
properties. Four factors determine the long-term effect of bioactive ceramic implants: (1) the site of
implantation, (2) tissue trauma, (3) the bulk and surface properties of the material, and (4) the relative