MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 60

indicate the formation of mineralized collagenous extracellular matrix as well as
active osteoblast in HAp/BMO composite at two weeks after implantation. It was
concluded that the application of low-pressure system to subculture of bone cells
to porous HAp blocks is benefi cial to increase bone tissue formation in vivo .
Gatti et al. 17 compared the bone in growth on implantation of HAp and TCP
granules. They claimed that the TCP granules induce total bone growth after four
months of implantation, whereas HAp granules crumbled and no new bone in-
duction was seen even 12 months after implantation. Such observation with HAp
granules in the biological fl uid was attributed to the scant cohesive strength
among the particles.
Blacik et al. 18 conducted the weight bearing study of porous HAp/TCP
(60/40) ceramics, implanted as intramedullary fi xation in segmental bone defects
in rabbit bone. They found that these ceramics are limited in their ability to treat
load-bearing segmental bone defects, but no failure was observed at the early
stages of implantation. However, additional internal fi xation should be used when
immediate mobilization and load bearing is required.
Fini et al. 19 studied the effect of pulse electromagnetic fi elds (PEMF) on the
osteointegration of hydroxyapatite implants in cancellous bone. They found that
PEMFs enhanced early HAp osteointegration in the trabecular bone of healthy
rabbits and were associated with a higher degree of bone mineralization and
maturation, which was also maintained for six week and three week periods of
non - stimulation.
It is known that microstructure plays an important role in mechanical proper-
ties, and therefore a study was conducted to assess such effect in the implanted
material in vivo . Okuda et al. 20 observed the effect of the microstructure of

β -tricalcium phosphate on the metabolism of subsequently formed bone tissue.

Comparing rod shaped and globular shaped particles, it was argued that rod
shaped

-TCP possibly helped to stimulate osteoblastic activity. There have been
some reports, mostly from the research group of Webster, that the presence of
increased grain boundaries enhances osteoblast cell adhesion. However, the au-
thors' recent study on thermally etched surfaces of sintered HAp shows that L929
fi broblast cells spread across the grains in a signifi cant manner (see Figure 3.4).
Such aspect requires further study in this direction to confi rm the microstructural
aspect on cell adhesion.
From the preceding discussion, it is clear that HAp is a highly biocompatible
and bioactive material the dissociation of HAp to TCP has been a major process-
ing challenge. Nevertheless, an optimal presence of TCP, in particular 60 : 40 ratio
of HAp :TCP is an ideal example of biphasic calcium phosphate (BCP) ceramics,
which can exhibit good combination of controlled biodegradation and bioactivity.
A subsequent chapter by Daculsi and co-workers introduces the concept of the
development of BCP ceramics as well as their processing and properties.
Despite good biocompatibility property, pure HAp has very poor mechani-
cal properties (Table 3.1) and therefore cannot be used in load bearing applica-
tions. The use of HAp is limited by bone fi ller application, mostly as porous
HAp block. Consequently, hydroxyapatite needs to be used in combination with

β