Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 205

this value has been reported to be 2.5 MPa∙m 0.5 .) As subcritical cracks
grow, K I will increase and lead to sudden fracture in the case that K I > K Ic .
Wear properties are particularly important for ceramics in THA
applications, and generally it can be stated that they are directly depen-
dent on the grain size of the material. Some grain sizes are also reported
in Table 8.1.
Two composites are included in Table 8.1: carbon-silicon carbide
(50:50) and carbon-carbon fiber (30:70). These are examples of ceramic-
ceramic composites. They are discussed here since, unlike most compos-
ites, which are formed with the final part configuration in mind, these
are frequently made as bulk materials and machined to shape.
Carbons and graphites, unlike most ceramics, may be fairly easily
machined to close tolerances. Thus, both of these carbon matrix mate-
rials may be machined much like isotropic carbon, but possess superior
mechanical properties as shown. They also have fatigue endurance limits
that exceed 70% of their flexural strength and, in common with high-purity
aluminum oxide, show extremely low wear rates against UHMWPE.
Other carbon forms listed in Table 8.1 are more commonly used in the
cardiovascular arena, but may have potential applicability in certain
aspects of orthopaedics. Carbon materials are extremely tough compared
to alumina, have a much higher strain to failure, and a very good wear
resistance. The opportunities for incorporating carbon-based material into
orthopaedics could certainly expand with changing technologies.

PROBLEM 8.1
Aluminum oxide is an unsuitable material for fabrication of fracture
fixation hardware because

A. Its elastic modulus is too high

B. It displays no plastic deformation

C. It is difficult to machine

D. It will dissolve in vivo

E. It is a glassy material

ANSWER:
B and C . The modulus does not represent an intrinsic problem but does
limit design choices. Aluminum oxide (Al 2 O 3 , also called alumina ) may
possibly release ions at a very low rate, especially if impure, but it is
nonresorbable. It is a polycrystalline material.

reactive ceramics The presence of a ceramic phase in bone (calcium hydroxyapatite;

CaHAP or HA) accomodates attachment with collagen and external soft
tissues. Given this character, it has been suggested that ceramic implants
may be designed to bond with tissue. There have been a number of
approaches to this idea.
Titanium . Although thought to be a metal, titanium should be consid-
ered a ceramic insofar as local host response is concerned, since it has