Calcium Phosphate Bioceramics: An Alternative to Autograft and Allograft? - Engineered Bone: Tissue Engineering Intelligence Unit - page 62

Figure 1b. Physicochemical process of biological precipitation
Biodegradation and Biodissolution
The biodegradation of BCP included the dissolution of the individual HA or β -TCP crys-
tals. 11,22 The proportion of HA to β -TCP crystals in BCP appeared greater after implantation
because of the higher reactivity or solubility of β -TCP compared to HA. The propensity to the
resorption (reflecting in vivo dissolution) of BCP ceramics depends on their β -TCP/HA ratios
the higher the ratio, the greater the resorption. Formation of microcrystals (which are able to
diffract X-rays) with Ca/P ratios similar to those of bone apatite crystals were also observed
after implantation (Fig. 1a and 1b). The abundance of these crystals were directly related to the
initial β -TCP/HA ratio in the BCP: the higher the ratio is, the greater the abundance of the
microcrystals associated with the BCP crystals are. According to these data it is possible to
control the kinetic of dissolution and precipitation, and subsequently the bioactivity.
Physicochemical Events
The Bonding Zone
Using high resolution TEM Daculsi et al 12 demonstrated for the first time that the formation
of these microcrystals after implantation were non-specific, i.e., not related to implantation
site, subjects of implantation, and types of CaP ceramics. The coalescing interfacial zone of
biological apatite and residual crystals provides a scaffold for bone-cell adhesion and further
bone ingrowth. 21 The restoring process involves a dissolution of calcium phosphate crystals
and then a precipitation of CHA needle-like crystallites in micropores close to the dissolving
crystals. The coalescing zone constitute the new biomaterial/bone interface which includes the
participation of proteins and CHA crystals originating from the CaP materials but does not
include the biomaterial surface. The following events of bone ingrowth and the newly formed
bone progressively replaces the initially formed CHA from the CaP biomaterials.
The process of cell colonization, adhesion, phagocytosis and osteoclastic resorption, ECM
elaboration and mineralization, bone in growth and bone remodelling associated with the bio-
logical apatite precipitation during CaP ceramics dissolution, are continuously in progress.
Consequently the interface is not static but dynamic, in constant evolution, taking into ac-
count bone physiopathology, biomechanical factors and bone maturation (Fig. 2).