Handbook of Ionic Substituted Hydroxyapatites

Abdul Samad Khan,Aqif Anwar Chaudhry

• Publisher: Woodhead Publishing
• Year: 2019
• ISBN:
  9780081028353
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Handbook of Ionic Substituted Hydroxyapatites provides scientists and researchers with comprehensive information on the synthesis processes of hydroxyapatite, also explaining the application of substituted hydroxyapatite. The book's content is very structured and explanatory, starting with a detailed overview of biological apatite in bones and teeth, as well as a presentation of the analytical tools for hydroxyapatite. Bioceramics and the relative modern and emerging processing techniques are covered, as is 3-D printing, which has gained increasing importance within biomedical materials and in the use of hydroxyapatite in tissue engineering. Finally, the advantages and disadvantages of using ionic substitutions in clinical application are presented.

Students and researchers in disciplines, such as Material Science, Ceramics, and Bioengineering will find this book to be very helpful in their work. It will also be a valuable resource for practitioners and surgeons in orthopedics, perio/implantology and maxillo-facial disciplines, and professionals working in R&D in ceramics and pharmaceuticals.

• Provides responses to the lack of scientific information about hydroxyapatites for biomedical applications
• Solves researchers' issues regarding phase changes with respect to substituted ions and how these substitutions can alter/improve the properties of stoichiometric hydroxyapatite
• Explains modern clinical applications and the effects of apatites within biomedical applications
• Includes both the advantages and disadvantages of using ionic substitutions in clinical application

1

Structure of biological apatite

bone and tooth

Ahmed Talal ¹ , Shorouq Khalid Hamid ¹ , Maria Khan ² , and Abdul Samad Khan ^{1 1} Department of Restorative Dental Sciences, College of Dentistry, Imam Abdulrahman Bin Faisal University, Dammam, Eastern Province, Saudi Arabia ² Department of Oral Biology, University of Health Sciences, Lahore, Pakistan

Abstract

Biological apatite is an inorganic calcium phosphate and is considered as a main constituent of bone and tooth structure. The nanosized biological apatite has wide biomedical and dental applications. Over the period, researchers have been trying to develop synthetic apatites, which simulate to biological apatites. Human body contains hard and soft tissue, where hard tissues are mainly composed of mineral or inorganic components. Given this, it is of great significance to obtain a complete knowledge of structure, anatomy, composition, and properties of these hard tissues, i.e., bones and teeth. This chapter provides some insights rather than a thorough review of biological apatite, bone, and tooth structures.

Keywords

Biological apatite; Bone; Composition; Structure; Tooth; Properties of bone; Properties of tooth

1.1. Introduction

Human body is composed of soft and hard tissue structure that works in dynamic condition. Soft tissues are composed of collagen, elastin, and ground substance. The function of hard tissues is to connect, support, and surround body organs and structures, and hard tissue includes bone and mineralized tooth structures, such as are enamel, dentin, and cementum.

The hard tissue is composed of organic and inorganic constituents. Apatite or apatite calcium phosphates are the principal inorganic constituents of bone and teeth. De Jong in 1926 was the first to identify the structure of the apatite Ca–P solid phase in the bone by chemical analysis as a crystalline calcium phosphate resembling geological apatite (De Jong, 1926), however, not similar to stoichiometric hydroxyapatite (HA). The bone crystals are thin plates having approximately 500 Å length, 250 Å width, and 100 Å thickness (Finean and Engstrom, 1953). The studies showed that there is an existence of bone mineral ions in nonapatitic arrays, and these ions (bivalent ions, i.e., CO₃ ²⁻ and HPO₄ ²⁻ ) are mainly present in a hydrated layer on the crystal surfaces (Rey et al., 1990; Eichert et al., 2007).

The composition, crystal size, morphology, and stoichiometry of biological apatite are different from the pure HA. The Ca/P molar ratio is 1.67 for pure HA, whereas for enamel and dentin, it is 1.62 and 1.64, respectively. Generally, biological apatites are calcium deficient or nonstoichiometric. The other minors, e.g., magnesium (Mg), carbonate (CO₃), sodium (Na), chloride (Cl), acid phosphate (HPO₄), etc., and trace elements such as strontium (Sr) and lead (Pb) are associated with these apatites. The biological apatites can be classified as follow:

• Carbonate-apatite (CO₃)-AP

• Fluor–carbonate apatite (F, CO₃)-AP

It can be represented by the chemical formula given below:

A₁₀(BO₄)₆X₂

where

• A₁₀ is Ca, Na, Sr, Pb, Cd, Mg, and K

• BO₄ is (PO₄, CO₃, VO₄, SiO₄, AsO₄, and HPO₄)₆

• X is (OH, Cl, CO₃F)₂

where A represents trace cations with concentration less than 0.1 wt.%. The minor elements, carbonates, magnesium, and fluoride are responsible for the biological apatites stability or instability. Water has also been found in the apatite structure in several forms. The term “apatite” applies to a broad category of structures comprising different constituents.

Hydroxyapatite is one such constituent, another being carbonate hydroxyl apatite, where carbonate ions substitute for some of the hydroxyl ions (A type) or the carbonate ions may be present on the phosphate sites (B type) (Bano et al., 2019). Synthetic HA is a representative material for bone substance because of its chemical similarities with the inorganic phase of bone and capability of undergoing bonding osteogenesis. Moreover, it is chemically stable for long period of time in vivo (Khalid et al., 2018a; Cai et al., 2019).

In addition to calcium and phosphate, the mineral phase in enamel and dentin contains considerable amounts of sodium, carbonate, and magnesium ions and smaller amounts of potassium, chlorine, and fluorine, and in addition to these, dentin contains citrate ions. Enamel apatite crystals are much larger than those present in both bone and dentin. Human enamel apatite has larger a-axis dimension than pure HA, i.e., 0.944 nm compared with 0.942 nm. The crystallographic properties of enamel and dentin are shown in Table 1.1, and the composition of enamel and dentin is given in Table 1.2.

These ions may be incorporated in the enamel and dentin in the form of a magnesium whitlockite and a sodium and carbonate containing apatite. The formula given for the sodium and carbonate containing apatite could not account for the total amount of carbonate found in enamel, and it may contain excess carbonate and also chloride and fluoride ions:

Table 1.1

Crystallographic properties of enamel and dentin.

Crystallographic properties: lattice parameters (±0.0003 nm)	Enamel	Dentin
a-axis c-axis	0.9441 0.6880	0.9421 0.6887
Crystallinity index	70–75	33–37
Crystallite size (nm)	0.13 × 0.03	0.0200 × 0.0040
Ca/P molar	1.63	1.61

Table 1.2

Constituents of enamel and dentin.

Components	Enamel %	Dentin %
Ca	36.4	36.8
P	17.1	18.0
CO	3.4	6.55
Na	0.64	0.38
Mg	0.43	1.24
F	0.01	0.03

Ca₁₀ (PO₄)₆ (OH)_1.59 (CO)_0.15 (Cl)_0.1 (F)_0.01

The relative proportion of constituents in both enamel and dentin are shown in Table 1.3. Bone and dentin are similar in their mineral contents and chemical composition. Enamel contains more minerals than bone and dentin and contains a slightly carbonated apatite instead o...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• About the editors
• 1. Structure of biological apatite: bone and tooth
• 2. Analytical tools for substituted hydroxyapatite
• 3. Bioceramics: types and clinical applications
• 4. Basics of hydroxyapatite—structure, synthesis, properties, and clinical applications
• 5. Role of substitution in bioceramics
• 6. Carbonate substituted hydroxyapatite
• 7. Fluoride-substituted hydroxyapatite
• 8. Magnesium-substituted hydroxyapatite
• 9. Zinc-substituted hydroxyapatite
• 10. Silver-substituted hydroxyapatite
• 11. Iron-substituted hydroxyapatite
• 12. Silicon-substituted hydroxyapatite
• 13. Effects of strontium substitution in synthetic apatites for biomedical applications
• 14. Coating of hydroxyapatite and substituted apatite on dental and orthopedic implants
• 15. Three-dimensional printing of hydroxyapatite
• 16. Hydroxyapatite and tissue engineering
• Index