Biomedical Engineering Reference
In-Depth Information
osteointegration (the close apposition of bone to implant surface) examinations during
in
vivo
studies are essential for further analysis about the effect of the implant on the expres-
sion of osteoblast phenotype and even for future clinical application.
Histological analysis of HA-coated implants has shown that bone ingrowth with osseous
integration occurs as early as 10 days following implantation (Furlong and Osborn 1991).
Thereby, HA coating could provide the crucial initial fixation requirement for the success
of an implant. Furthermore, a histomorphometric study by Moroni et al. (1994) compar-
ing the percentage of bone bonding to plasma sprayed HA-coated and uncoated implants
in dogs revealed a significant increase in bone amount in the HA-coated implants. They
also showed enhancement of bone-to-pin osseointegration and interfacial strength in
HA-coated pins as compared to uncoated pins in a sheep study (Moroni et al. 1998). Figure
1.14 shows the typical histological micrograph on the enhancing effect of HA coating on
bone-implant fixation (Hirai et al. 2004): after 4 weeks' implantation in the transfemoral
drill hole on a rat, the area of the new bone of the cross-sectional implants was 0.108 mm
2
for sol-gel HA-coated titanium and only 0.087 mm
2
for uncoated samples. That is to say,
the implant with HA coating had comparatively higher bone apposition ability than the
uncoated implant. Similar results have been reported in other studies (Ramires et al. 2003;
Gerber et al. 2003; You, Yeo et al. 2005).
Along with the histological analysis, extensive investigations have been conducted on
the bonding strength of HA-coated implants during
in vivo
animal trials. For instance, in
the studies of Oonishi et al. (1989), the adhesion strength between the implant and bone
was ~0.53 and ~1.35 MPa for uncoated and HA-coated implants, respectively, at 2 weeks
after implantation, whereas those bonding strengths increased to ~7.5 and ~14.15 MPa at
6 weeks, correspondingly. Other short-term
in vivo
trials also elucidated the similar trend
regarding the bone-implant bonding property (Yang, Lin et al. 1997; Chang et al. 1997;
Dalton et al. 1995). Essentially, it is revealed that those rapid and positive responses of HA
coatings
in vivo
benefit significantly from the fast dissolution of related impure phases
(e.g., amorphous phase, TCP), which results in an increase in Ca
2+
and
PO
3
−
ion concentra-
tions in local areas around the implant-bone interface. Nevertheless, from the viewpoint
of long-term stability, whether the loss of HA coating could maintain such a desired bond-
ing strength between the implant and surrounding tissue is questioned.
(a)
(b)
0.2 mm
0.2 mm
FIGURE 1.14
Histological sections of the nondecalcified sections after 4 weeks' implantation in a transfemoral drill hole on
a male Wister rat: (a) sol-gel coated titanium, (b) uncoated titanium. The area of the bone of the cross-sectional
implants was 0.108 mm
2
for sol-gel HA-coated titanium and 0.087 mm
2
for uncoated titanium. (From Hirai
et al.,
Journal of the American Ceramic Society
,
87, 1, 29-34, 2004. With permission.)
