Technological Background Of Dental Ceramics Biology Essay

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Use of any natural or man-made materials like , glass, alloys, metals, natural or synthetic polymers, ceramic, and nanotechnology, comprising whole or part of a living structure or biomedical device that can be used for any period of time to perform augments, or replaces a tissue, organ, or body natural function that have been damaged by disease or injury.

Bioceramics

Ceramics are inorganic, non-metallic materials, formed out of high-temperature reactions comprising metal oxides and cement. Ceramics used as coatings to improve the biocompatibility of metal implants and function as resorbable lattices providing temporary structural function and framework and replaced as body parts are called as bioceramics. Coming to Structure-Property relationship the interatomic bonds in ceramics result in long-range three-dimensional crystalline structures in metals. Ceramics are hard and brittle because of strong ionic and covalent bonds influencing chemical behavior as the planes of atoms/ ions cannot slip past one another.

Ceramics should be bioinert (should be non-toxic, non-inflammation causing), bioactive (long lasting, corrosion resistant and can undergo interfacial interactions with surrounding tissues), and biodegradable (soluble and easily incorporated into tissue). Ceramics are naturally electrical and thermal insulators. Bioceramics additionally must have a low Young's modulus to prevent cracking of the material.

Bioceramics like polycrystalline aluminum oxide, hydroxyapatite, partially stabilized zirconium oxide, bioactive glass or glass-ceramics, and polyethylene-hydroxyapatite composites are used for the repair, reconstruction, and replacement of diseased or damaged parts of the body like hip implants, dental implants, middle ear implants, heart valves and synthetic bones.

Among biomedical ceramics, alumina has the highest mechanical properties and has low coefficient of friction and wear rate. Due to which, it has been used as a bearing surface in joint replacements.

Hydroxyapatite

Hydroxyapatite is a naturally occurring mineral form of calcium apatite [Ca10(PO4)6(OH)2 ] with two entities of crystal unit cell. Hydroxyapatite is the basic constituent of teeth and bones; and therefore it is called as "bone mineral". Dental enamel and dentin are mainly comprised of carbonated calcium-deficient hydroxyapatite.

Clinically, hydroxyapatite has been used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials and as an implant in load-free anatomic sites like nasal septal bone and middle ear.

Hydroxyapatite has been used by Noel Fitzpatrick to facilitate bionic development, by coating steel rods in hydroxyapatite to encourage natural growth of skin around it called as 'Osseointegration'. Fully dense Nano crystalline hydroxyapatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices and dental implants to aid in forming a strong bond to bone and other tissues in the body without rejection or inflammatory reactions. Because of which, they are of great interest for gene delivery and tissue engineering scaffolds. Porous Hydroxyapatite implants are applied in local drug delivery in bone.

Ultimately these ceramic materials are used as bone replacements or with the incorporation of protein collagens, synthetic bones.

Dental luting agents

The word 'luting agents' indicate viscous moldable material to seal the space between tooth structure and prosthesis. Dental cements are the hard and brittle substances formed from the mixture of powder and liquid.

Types of Dental Cements:

Type I - luting agents, that can hold cast restoration to a tooth.

Type II-are used to restore teeth.

Type III -also called liners and bases which are placed within a cavity preparation

Amalgam:

It is the mixture of approximately 50% mercury and an alloy, with silver, tin, copper, and other elements. The alloy powder settle at one end of the capsule separated from the liquid mercury by a breakable diaphragm. After trituration the amalgam is dispensed into the container, after which the amalgam carrier transfers the material into the cavity for condensing. There will be a hardening reaction between mercury and alloy followed by mixing which is termed as 'amalgamation'.

7 Ag3Sn + 28 Hg → Ag3Sn + 9 Ag2Hg3 + Sn7Hg

The main advantages of amalgam are its strength, durability, cheap and easy to handle. Its drawbacks are lack of adhesion, removal of lot of healthy tooth substance, high electrical and thermal conductivity, high thermal expansion coefficient, lack of biocompatibility and its toxicity for the environment because of mercury present in it.

Zinc Phosphate Cement:

It is an inorganic chemical compound which is corrosion resistant. It is the mixture of zinc and magnesium oxide powders and liquids containing phosphoric acid, water and buffers. When the powder reacts with liquid a small exothermic reaction occurs raising the pH to 3.5.The less is the powder and liquid ratio, the longer time the cement takes to harden. Setting time varies from 5-9 minutes. It is used as both intermediate base and cementing medium.

Polycarboxylate Cement:

It is a long-lasting provisional cement used for cementing restorations and as a cavity lining. The powder consists of zinc oxide, 1-5% magnesium oxide, and 10-40% aluminum oxide. The liquid comprises of 40% aqueous solution of polyacrylic acid copolymer along with other organic acids like itaconic acid. But, it has less compressive strength.

Glass Ionomer Cement:

It is classified into three types based on its use:

Type I : Luting crowns, bridges and orthodontic brackets.

Type II a : Aesthetic restorative cements

Type II b : Reinforced restorative cements

Type III : Lining cements, Base

It is chemically bound to dentine and enamel by setting due to chelation releasing fluoride. The powder consists of acid soluble calcium fluoroalumino silicate glass and also fusion of quartz, alumina, cryolite, fluorite, aluminum trifluoride and aluminum phosphate at ambient temperatures of 1100° or 13000° C. The liquid is made of poly acrylic, itaconic, tartaric and maleic acid to accelerate setting reaction. There is an acid -base reaction between polyelectrolyte and aluminosilicate glass as the polyacid attacks glass particles releasing cations and fluoride ions which react with polyanions to form salt gel matrix followed by hardening and slow maturation. GIC have favorable adhesive properties, exquisite thermal properties, biocompatibility and fluoride release but it has even drawbacks of handling sensitivity, poor mechanical and abrasive properties, susceptibility to acid corrosion, lack of radio-opacity, and lack high chemical resistance and mechanical strength.

MO•SiO2 + H2A → MA + SiO2 + H2

Glass polyacid salt silica gel

(MO - metal oxide and A - acid)

Resin cements:

They are of polymer based for which filler and fluoride are added. Resin cement along with dentin bonding agent are used in cementing porcelain laminate veneers.

Resin modified glass ionomer cements:

These are hybrid formulations of resin and glass ionomer components. It is known that HEMA (Hydroxyethylmethacrylate) is released from this, which has tendency to damage various biological properties, ranging from pulpal inflammation to allergic contact dermatitis. Adhesion is not so strong and also excess water sorption causing swelling and frequent ceramic fractures, as well as degradation through hydrolysis leading to potential hazards.

Adhesive resin cements:

There is no need of pretreatment like etching, priming and bonding agents. Polymerization takes place in deeper areas of teeth cavity.

Calcium aluminate

Use of amalgam is discontinued due to its potential hazard because of which use of resin has been increased. But, the polymerization shrinkage of it is very high and handling of uncured resin may cause contact dermatitis. Therefore, a new alternative restorative material is found known as calcium aluminate cement. CAC consists of 70% alumina made by sintering calcined alumina with quicklime(Cao) and small amounts of zirconium, ferric, and silicone oxides. An acid base reaction is initiated when calcium aluminate tablets are brought in contact with liquid containing water and lithium which acts as accelerator. Water acts as a weak acid and calcium aluminate dissolves to form Ca2+, Al (OH)4- and OH-. The solute precipitates to form amorphous gel which gradually changes into crystalline phase [(CaO)3(Al2O3)(H2O)6].

3 CaO•Al2O3 + 12 H2O → Ca3[Al(OH)4]2(OH)4 + 4Al(OH)3

Calcium Aluminate Water Katoite Gibbsite

The main reason behind the use of Calcium Aluminate is explained in Tab. 1.

Table 1: The main reason behind the use of Ca-Al in dental restorative materials

Chemistry

Composed of common elements like oxides, hydrophilic nature, similarities with apatite

Biology

Biocompatible and eco-friendly

Thermal

Expansion and conductivity analogous to tooth tissue

Mechanics

Hardness and stiffness is same as that of hard tissues and bone, and strong in compression

Processing

Providing In-situ room-temperature, flexible rheology and curing time

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