**1. Introduction**

Titanium (Ti) and its alloys are used as artificial joints and teeth roots in orthopedic and dental settings because they have the advantage that their mechanical properties are closer to those of bone than are those of stainless steel or cobalt–chromium alloys. However, the difference in mechanical properties between Ti and natural bone leads to negative effects, such as stress shielding. To mitigate these effects, many new Ti alloys have been developed for hard tissue implants, with a focus on controlling the alloy element and its content, phase, and other characteristics.

When implants do not undergo surface modification to enhance the osteoconductivity, it takes a relatively long time to fix the metallic implant to bone such that it is stable. There are many approaches for improving the osteoconductivity of Ti and its alloys. These approaches can be classified into the following two techniques: (1) bioactive compounds that accelerate bone formation are coated on metallic implants, and (2) a rough surface at the macro‐level is formed on the metallic implants, and the ingrowth of bone results in anchorage of the implants. These techniques have achieved a certain level of success, and the surface‐modified implants have been used clinically. However, there are still weaknesses with the coating that need resolution, as well as unclear points regarding the effect of the surface properties on the osteoconductivity. Since hydroprocessing can be used to prepare the coating on complex‐shaped substrates with complex topography, which many implants have, we focus on the use of hydroprocessing in many techniques for coating the bioactive compound, especially hydroxyapatite (HAp), and expound on the characteristics of the techniques and issues. Moreover, we describe in detail the evaluation of the osteoconductivity of implants coated with HAp, using *in vivo* testing in rat tibiae.

© 2013 Kuroda and Okido; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Kuroda and Okido; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

> HAp can be easily obtained in a solution where pH > 5 and where the ion content and temperature are controlled. However, HAp cannot precipitate in the pH < 5 solution, and hydroprocessing using the precipitation phenomenon in the aqueous solution cannot give β‐

Hydroxyapatite Coating on Titanium Implants Using Hydroprocessing and Evaluation of their Osteoconductivity 289

**H2PO4 H - 3PO4 HPO4**

**2- PO4 3-**

**DCPD**

**DCPA**

**OCP**

**‐TCP HAp**

0 2 4 6 8 10 12 14 pH

> pH 2 4 6 8 10

**Figure 2.** Solubility curves of calcium orthophosphoric compounds at 37 oC, depending on pH in aqueous solution. HAp: hydroxyapatite (Ca10(PO4)6(OH)2), TCP: calcium phosphate (Ca3(PO4)2), OCP: octacalcium phosphate (Ca8H2(PO4)6 5H2O), DCPA: dicalcium phospate anhydrous (CaHPO4), DCPD: dicalcium phospate dihydrate (CaHPO4 2H2O).

Ca3(PO4)2 (β‐TCP), a bioactive compound.


**Figure 1.** Logarithmic concentration diagram for orthophosphoric acid.

0






Content, log (*C*

> Calcium

content, log (*C*Ca / mol

L‐1)

H*x*PO4(3‐*x*)‐/mol

L‐1)





0
