Biomedical Engineering Reference
In-Depth Information
Hydroxyapatite and other calcium phosphates bioceramics are important for hard tissue re‐
pair because of their similarity to the minerals in natural bone, and their excellent biocom‐
patibility and bioactivity [81-86]. When implanted in an osseous site, bone bioactive
materials such as HAp and other CaP implants and coatings provide an ideal environment
for cellular reaction and colonization by osteoblasts. This leads to a tissue response termed
osteoconduction in which bone grows on and bonds to the implant, promoting a functional
interface [81, 84, 87]. Extensive efforts have significantly improved the properties and per‐
formance of HAp and other CaP based implants [88-92]. Calcium phosphate cements can be
molded or injected to form a scaffold in situ, which can be resorbed and replaced by new
bone [93, 65-67]. Chemically, the vast majority of calcium phosphate bioceramics is based on
HAp, β-TCP, α-TCP and/or biphasic calcium phosphate (BCP), which is an intimate mixture
of either β-TCP - HAp [94-100] or α-TCP - HAp [101-111]. The preparation technique of
these calcium phosphates has been extensively reviewed in literature [1, 4, 37, 102-104].
When compared to both β- and α-TCP, HAp is a more stable phase under physiological con‐
ditions, as it has a lower solubility (Table 1) [37, 109-110]. Therefore, the BCP concept is de‐
termined by the optimum balance of a more stable phase of HAp and a more soluble TCP.
Due to a higher biodegradability of the β - or α -TCP component, the reactivity of BCP in‐
creases with the TCP-HAp the increase in ratio. Thus, in vivo bioresorbability of BCP can be
controlled through the phase composition [95]. As implants made of calcined HAp are
found in bone defects for many years after implantation, bioceramics made of more soluble
calcium phosphates is preferable for the biomedical purposes [94-110]. HAp has been clini‐
cally used to repair bone defects for many years [3]. However, Hap has poor mechanical
properties [3]. Their use at high load bearing conditions has been restricted due to their brit‐
tleness, poor fatigue resistance and strength.
The main reason behind the use of β-TCP as bone substitute materials is their chemical
similarity to the mineral component of mammalian bone and teeth [1-3]. The application
of tricalcium phosphate as a bone substitute has received considerable attention, because
it is remarkably biocompatible with living bodies when replacing hard tissues and be‐
cause it has biodegradable properties [1-29]. Consequently, β-TCP has been used as bone
graft substitutes in many surgical fields such as orthopedic and dental surgeries [3, 11-12,
16-17]. This use leads to an ultimate physicochemical bond between the implants and
bone-termed osteointegration. Even so, the major limitation to the use of β-TCP as load-
bearing biomaterial is their mechanical properties which make it brittle, with poor fati‐
gue resistance [3, 10, 21-29]. Moreover, the mechanical properties of tricalcium phosphate
are generally inadequate for many load-carrying applications (3 MPa - 5 MPa) [3, 10,
20-29]. Its poor mechanical behaviour is even more evident when used to make highly
porous ceramics and scaffolds. Hence, metal oxides ceramics, such as alumina (Al
2
O
3
), ti‐
tania (TiO
2
) and some oxides (e.g. ZrO
2
, SiO
2
) have been widely studied due to their bioi‐
nertness, excellent tribological properties, high wear resistance, fracture toughness and
strength as well as relatively low friction [19, 21-22, 29-31]. However, bioinert ceramic ox‐
ides having high strength are used to enhance the densification and the mechanical prop‐
erties of β-TCP. In this chapter, we will try to improve the strength of β-TCP by
introducing a bioinert oxide like alumina. This is because there are few articles reporting
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