distribution in the as-synthesized powders and distribution of grains in heat-treated

powders confirmed that the latter acquires the structure of the as-synthesized

powders, which has a significant effect on its physicochemical characteristics.

23.2

Stoichiometric Hydroxyapatite and Associated Concerns

Hydroxyapatite (Ca10(PO4)6(OH)2) is identical to bone-like apatite structure and is a

vital inorganic ingredient of bone as it provides rigidity to bones and teeth. Pure HA

is stoichiometric apatite phase having molar ratio Ca/P as 1.67, hexagonal structure

with P63/m space group, and lattice parameters a ¼ b ¼ 9.418 Å and c ¼ 6.884 Å. It

is the most stable crystalline phase of apatites and has high biocompatibility with

natural bone (Kweh 1999; Kheradmandfard and Fathi 2013).

It possesses great biological properties like absence of immunological reactions,

non-toxicity, and lack of inflammatory response (Vallet-Regi 2000). Chemical

bonding of HA with the host tissue presents greater benefit in clinical applications.

When HA is implanted into a bone location, several physiochemical interactions

occur with the biological environment, causing the buildup of interfacial layers,

which helps in the adhesion of implant material to bone tissue (Jennifer et al. 2005),

resulting in implant stabilization and its superior fixation with adjoining tissues.

HA can stimulate new bone ingrowth via osteoconduction without causing any

localized toxicity and inflammation response. It also inhibits the growth of cancer

cells (Sadat-Shojai et al. 2013). Thus, HA has been widely recognized for repairing

damaged or diseased bone tissues (Ming-Fa et al. 2001; LeGeros 2008). It has been

effectively used as aesthetic restorative, bone filler, filler of inorganic/polymer

composites, and coating of orthopedic implants (Pramanik et al. 2009). It can also

be used as a carrier in drug delivery systems and catalysis (Constantin et al. 2012).

The application of stoichiometric HA in the form of powder, thin films, and

porous or dense blocks is in plenty at the microscale level (Prakasam et al. 2015).

But poor bioresorbability is an undesirable characteristic of microscale HA, as it

inhibits the rate of bone regeneration (Kivrak and Tas 1998). Micron size HA has

strong crystal-to-crystal bond and a low surface area as compared to bone mineral

crystals which are nanodimensional and have loose crystal-to-crystal bond and large

surface area. Stoichiometric HA also has poor thermal stability and mechanical

properties, restricting its use for medical applications (Chen and Miao 2004; Kim

et al. 2005).

There are considerable differences between stoichiometric and biological

apatites. Biological apatites are nonstoichiometric carbonated compounds and are

substituted with trace amounts of numerous ions (Combes et al. 2016; Supova 2015).

These ions have a considerable biological role, directly affecting host cell response

and/or exerting a therapeutic role; hence, their amount and presence in the peri-

implant environment are essential.

Current findings for ion-substituted hydroxyapatite (HA) could mark the path

towards its substantial growth in biomedicine, along with a prominence on a novel

generation of dentistry and orthopedic applications.

23

Unleashing Potential of Bone Mimicking Nanodimensional Hydroxyapatites and. . .

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