Biomedical Engineering Reference
In-Depth Information
calcium phosphate ceramics have been considered the most popular coating
materials on traditional implant metals as well as in pure form for bone graft
substitutes. For example, HA and TCP (
-crystalline) can serve as bone
tissue engineering scaffolds to facilitate new bone formation and as a coating
on femoral (metal) or socket (polymer) prostheses to inhibit complications
related to osteolysis [50]. HA and TCP have a relatively high rate of degradation
in vivo as well, making them promising in drug eluting biodegradable bone
graft therapies.
Furthermore, HA has been shown to have good osteoconductive properties
since their surfaces can undergo selective chemical reactivity with surrounding
tissues, resulting in a tight bond between bone and the implant [51]. Thus, new
bone formation will be promoted by such a bonded interface [52-53]. Moreover,
it has been demonstrated that osseoinductivity of calcium phosphate ceramics is
strongly related to their surface properties. Higher amounts of surface porosity,
increased surface roughness and improved wettability of nanophase ceramics
have positive effects on the osseointegration of calcium phosphate ceramics with
juxtaposed bone. The nanometer-sized grains and high volume fraction of grain
boundaries in nanostructured HA can increase osteoblast adhesion, proliferation,
and mineralization [54]. For example, Webster et al. showed that there was sig-
nifi cantly enhanced osteoblast adhesion and strikingly inhibited fi broblast adhe-
sion on nanophase HA (67 nm) compared to conventional HA (179 nm HA) after
four-hour cell culture studies [16].
Some in vivo studies also demonstrated that nanocrystalline HA can serve
as an osteoconductive coating on tantalum scaffolds to accelerate new bone for-
mation compared to uncoated or conventional micron size HA coated tantalum
[131]. As shown in Figure 7.4, nanocrystalline HA coatings promote more
new bone growth in the rat calvaria than that on uncoated and conventional HA
coated tantalum after a six-week implantation time [131]. Apparently, the special
surface properties of nanophase ceramics contribute to effi ciently enhancing
new bone growth regardless of whether the studies were conducted in vitro or
in vivo .
In addition, enhanced osteoclast-like cell functions (such as synthesizing
tartrate-resistant acid phosphatase (TRAP) and formation of resorption pits) on
naophase HA have also been observed compared to conventional HA [46]. In
contrast, decreased functions of fi broblasts have been observed on nanophase
compared to conventional HA [46]. Since excessive secretion of fi brous tissue
from fi broblasts causes the formation of a detrimental fi brous tissue capsule
which can contribute to loosening of implants, decreased fi broblast functions on
nanophase HA may contribute to a better bone-implant interface. Recently, a
bioresorbable nano-crystalline HA paste was used as a valuable addition to TCP-
HA ceramic granules for acetabular bone grafting in animal models. That study
showed that the nano-crystalline HA paste resulted in better acetabular cup sta-
bility than the pure allografts and at the same time didn't cause adverse biological
reactivity [55]. Histology slides of human cancellous bone revealed that the nano-
crystalline HA paste appeared to be a suitable bone substitute for bone defects
α
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