Page 385 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

362 BIOMATERIALS

3000
Liquid (L) L + F

Cubic (F)
2500

2000
T + F

Temperature (°C) 1500 Transformable
tetragonal
(T)

1000
M + T

500
Mono-
clinic
(M)

0
Monoclinic Cubic
Nontransformable
tetragonal (T')
0 5 10 15 20
Mole % YO 1.5
FIGURE 15.2 Schematic phase diagram of the ZrO −Y O system. [From
2 2 3
Cales and Stefani (1995), with permission.]

15.2.4 Critical Properties of Bioinert Ceramics

Properties of bioinert ceramics important for their long-term clinical function include stiffness,
strength, toughness, wear resistance, and biological response. Stiffness represents one gauge of the
mechanical interaction between an implant and its surrounding tissue; it is one determinant of the
magnitude and distribution of stresses in a biomaterial and tissue, and dictates, in part, the potential
for stress shielding (Kohn and Ducheyne, 1992; Ko et al., 1995). Load-bearing biomaterials must
also be designed to ensure that they maintain their structural integrity, that is, designed to be fail-safe
at stresses above peak in-service stresses for a lifetime greater than the expected service life of the
prosthesis. Thus, the static (tensile, compressive, and flexural strength), dynamic (high-cycle fatigue),
and toughness properties of ceramics, in physiological media, under a multitude of loading condi-
tions and rates must be well-characterized.
Although knowledge of these properties is an important aspect of bioceramic design, the mechani-
cal integrity of a bioceramic is also dependent on its processing, size, and shape. Failure of ceramics