Ceramics – Shadow of Enamel : an Overview

6Dental Surgeon, Jaipur, Rajasthan Abstract The demand for esthetics in dentistry is increasing day by day. Ceramics have been introduced to fulfil the requiremnets of the patient. With years, various modifications and improvements in ceramic in relation to properties and esthetics have been made. This review will put a light on the basic properties and esthetic ability of the ceramics so as to understand the basics of it.


INTRODUCTION
F irst ever man made materials are believed to be ceramics. They are among the earliest group of inorganic materials to be structurally modified by man. (1) The desire for the use of durable and esthetic material was always preferred and ceramics are the recent addition to it. (2) These are the most natural appearing replacement material for missing tooth and is available in different translucencies and shades so as to achieve good results. (3) However, certain drawbacks have been seen in ceramics in relation to manufacturing technique, mechanical properties and physical properties i.e. crack propagation, sintering shrinkage,

Supplementary information
The online version of this article (https://doi.org/10.15520/jcmro.v3i08. 31 9) contains supplementary material, which is available to authorized users.
In dentistry, three different types of porcelain compositions are used (depending on their applications) Table 1 (7,8) Structure (7,9) Ceramics can appear as either crystalline or amorphous solids (also called glasses). Thus, ceramics can be broadly classified as: Composition (7,8) Ingredients used for various formulations of ceramics are:

13.Alternative Additives
Classification of ceramic materials I. According to application (10) • For porcelain teeth • For Ceramo-metal restorations (Metal-Ceramic Systems) • For All-ceramic restorations (All-Ceramic System)
• By processing methods: sintering, casting or machining.
Various methods for fabricating ceramic restorations are : pressure molding and sintering, condensation and sintering, casting and ceraming, slip casting, sintering and glass-infiltration, computer controlled mining.
III. Dental porcelains are classified according to the firing temperatures (6,11) High fusing 1300 • C (2072 • F) After the water of crystallization is lost, the flux reacts with the outer layers of the grains of silica (filler), kaolin (binder) and feldspar (basic glass former) and partly combines them together Fritting is the process of blending, melting and quenching the glass components.
Dispensing of dental porcelain (14) The conventional dental porcelain material may be generally supplied as a kit containing • Fine ceramic powders in different shades of enamel, dentin, core/opaque • Special liquid or distilled water vehicle/medium for ceramic powder (binder) • Stains and color modifiers • Glazes and Add-on porcelains Steps in fabrication (6) Mixing ceramic powders of selected shades with distilled water or a special liquid (binder)

Condensation (Compaction)
Porcelain powder is built into shape using a liquid binder to hold the particles together. The process of packing the particles together and removing the liquid binder is known as condensation. It is a 2part process -Agitation of the particles & Removal of excess moisture. It is repetitious and the two components are carried out alternatively until no further moisture comes to the surface. The movement of the particles is generated by a number of standard methods such as • Vibration • Spatulation • Whipping A working model; die of the prepared tooth is used for condensation of porcelain. A matrix is used to support the unfired porcelain both during condensation and firing.

Binder
It helps to hold the particles together, as the porcelain material is extremely fragile in the 'green' state.
Types of binder used: • Distilled water -most commonly used, especially for dentin / enamel porcelain

Sintering or Firing of Dental Porcelain
It is defined as a process of heating closely packed particles to achieve inter particle bonding and sufficient diffusion to decrease the surface area or increase density of the structure. The partial fusion or compaction of glass is often referred to as sintering.
During the process of sintering, the individual particles are in contact (grain bound-aries) soften and fuse at sufficiently high temper-atures. This process relies on diffusion, which is greatly accelerated by elevated temperatures.

Glaze
Surface porcelain would undergo pyroplastic flow i.e. the matter surface would disappear and a smooth shiny surface would result (self-glaze) if the porcelain was held in the furnance for a greater length of time at the end of high bisque stage.
Stages of maturity of porcelain has been described in (19)Table 2 Bonding to porcelain (20, 21) The bonding of resins and ceramics introduced new restorative techniques and arouse considerable interest. Bonding composite resins (organic substance) to a porcelain surface (inorganic substrate) requires the modification of the porcelain surface to enhance the compatibility of resin and achieve high bond strength.
For the bonding between porcelain (inorganic substrate) and composite resin (organic substrate) the silane primer is essential.

Silanization (20, 21)
This refers to silane coating of an etched glass surface to increase its surface affinity to polymers. The silanes, often called Coupling agents react and bond to the silica crystals in the glass matrix through the ethoxy-, chloro-or amino-groups leaving the vinyl group to react and form a bond with the resin. When applied and subsequently dried, the resultant condensation forms a strong chemical bond.

Repair of ceramic restorations (22-26)
Fracture is totally in porcelain (Simplest repair) • Fractured porcelain fragment • When missing : Fabricate a piece of porcelain or porcelain veneer to replace missing portion / fractured area or direct composite bonding with shade matched composite to repair defect.
• When available -The fractured fragment and surface to be repaired are etched, silanated, coated with suitable bonding agent luted together with resin based luting agent.
2. Application of Silane coupling agent (e.g. Scotchprime) and allow to dry for 1 min.

It involves exposed metal
Remaining porcelain: • If adequate to retain composite: Exposed metal and remaining porcelain is veneered with composite opaquer and subsequently with layers of shade matched composite (after preparation of both porcelain and metal surface for repair by bonding). • If inadequate to retain composite: Exposed metal surface is used as an adhesive substrate after preparation, for bonding with composite opaquer layer followed by shade matched composite.
Preparation of metal surface for repair by bonding:

Metal repair (Most difficult)
It involves exposed metal with minimal or no porcelain Two methods : • Veneering exposed metal surface with direct bonding of shade matched composite after preparation of exposed metal surface for bonding.
•. Fabrication of an overcasting: Small areas of remaining porcelain are removed if present. Crown /Pontic is reduced circumferentially (incisally, facially, and lingually) to provide room for both porcelain and metal, and provide margin for the laboratory technician. A thin metal overcasting with a fused porcelain veneer is fabricated. The metal surface of the repair area (substrate) and inner surface of the overcasting are air abraded (50µ A12O3) and bonded with suitable adhesive resin.

Sandblasting (Grit blasting/ Microetching)
Gritblasting refers to the process of air abrading a material with alumina particles (25-250µm) or glass beads at a specific pneumatic pressure. Microetching also refers to the use of sandblasting to prepare all types of surfaces for bonding restorations.This technique is used for : • Removal of investment material and cleaning the ceramic copings by abrading away surface contamination, which hinders good contact and bonding.
• To increase the surface area by the creation of numerous ridges and crevices, thus providing more surface area for bonding.
• Pretreatment and roughening of metal, resin and porcelain surfaces increases microscopic roughness for providing mechanically retentive surface for resins adhesive (to lock into crevices) to enhance bond strengths and repairs with various materials.

Welding of ceramic materials
Laser induced surface homogenization of dental ceramics can be used to remove local surface defects and polishing marks without the need to reconstruct a complete construction firing.
The surface characteristics of Dental Ceramics following firing: • Main vacuum firing: Irregular wave-like structures with dendrite crystal features.
• Furnance Gloss firing: Dendrite crystal structures are leveled off, but the crystal features are not removed.
• Laser Gloss firing: A full leveling of the surface is achieved.
Laser-induced modification of ceramic materials can be done by a process of heat induction using a suitable laser such as CO 2 laser (its emission wavelength is almost totally absorbed by ceramics).During focused CO 2 laser beam, a local glaze firing occurs on the ceramic surface. Table 3 Color stability

Properties of dental ceramics
Ceramics are the most stable tooth colored materials. The metallic oxides used as colorants do not undergo any change in shade after firing is complete. Adherence of exogenous stains is resisted by the smooth glossy surface. In fact over a period of years, a porcelain restoration may develop a mismatch with adjacent teeth caused by changes in color of the adjacent natural teeth with age. (27)

Brittleness
It is the relative inability of a material to sustain plastic deformation before fracture of the material occurs. Ceramics are brittle at oral temperatures (5 0 to 55 0 C). In other words it fractures at or near its proportional limit. (28) Strength (

Dimensional Stability
Porcelain has a coefficient of thermal expansion, slightly less than that of the tooth structure. It does not exhibit microleakage and is comparable to a cemented metal restoration. It also does not imbibe or synergize water.

Abrasion resistance and wear
The hardest dental material which is commonly used is fused porcelain. Wear resistance by opposing restoration or natural teeth is better in fused porcelain than other dental materials. On the other hand, it will cause metal restorations and tooth structure to wear more rapidly; particularly when not adequately glazed or when glaze is removed during occlusal adjustment (should be smoothened by polishing). (27)

Coefficient of thermal expansion (CTE)
Coefficient of expansion of porcelain is slightly less than that of tooth structure. When different porcelain formulations are veneered together (all-ceramic) and over metal copings (metal -ceramics) ,coefficient of thermal expansion should be matched to prevent development of interfacial stresses leading to separation or fracture. (32) CURRENT MEDICAL RESEARCH AND OPINION CMRO 03 (08), 549−562 (2020) Strength properties of dental ceramics (33) Dental ceramics are inherently fragile in tension. While the theoretical strength of porcelain is dependent upon the silicon -oxygen bond, the practical strength is 10 to 1000 times less than the nominal strengths.
In ceramics, microcracks are caused by: • The condensation, melting and sintering process Minimizing tensile stress Reducing stress raisers.

Esthetic Properties of Dental Ceramics
The principal reason for the choice of porcelain as a restorative material is its aesthetic qualities in matching the adjacent tooth structure in translucency, colour and chroma.

Color reproduction
Perfect color matching is extremely difficult, if not impossible. Correct color matching of natural teeth by the observer (clinician / ceramic technician) is dependent upon his subjective assessment and even with the use of the most modern types of shade guide and colour corrected lighting, he will experience difficulty in producing consistent shade matchings. (6) Color production in natural teeth (27) The structure of tooth influences its color. The bulk of tooth structure is comprised of 2 layers of calcified tissues; enamel and dentin, surrounding a central core or pulp chamber.
Variation of tooth color is also apparent in different regions of the tooth such as the incisal, middle and cervical/gingival third Table 4 Specimens of each shade (collectively called a shade guide) are provided for the dentist, who in turn, attempts to match the tooth colour as nearly as possible. Shade guides made of solid porcelain are used most often by dentists to describe a desired appearance of a natural tooth or ceramic prosthesis.
Colour reproductivity (6) Without intrinsic and extrinsic colorants, dental porcelain match the color of its respective shade tab.
To produce an acceptable match with corresponding shade guides, several factors plays important role which include porcelain type, batch, underlying metal, manufacturers, thickness and perceptible differences in color imparted by extrinsic colorants before and after firing whereas varied firing temperatures condensation techniques, repeated firings and firing cycles do not affect the color of dental porcelain.
Problems in reproducing natural teeth colors in ceramics restorations: • Duplication of enamel color and translucency is difficult as no material available as yet can produce the optical effect created by the closely packed prismatic nature of natural enamel.
• No biologically inert cements are available, that are transparent and which match the refractive indices of the tooth structure.
• Color assessment is a complex psychophysiologic process, which is subject to numerous variables.
Side effect of dental ceramics (34)(35)(36) Ceramics are inert; nevertheless they do pose certain problems: 1 Side effects to Laboratory technician and dentist • Prolonged exposure to finely divided inorganic dust in the atmosphere.
• Silicosis affect workers exposed to silica dust.
• Inhalations and prolonged exposures to silica are associated with malignancies especially lung cancer.
2 Side effect to patients • Wear of opposing teeth due to abrasive nature of porcelain (especially rough, unglazed surface).
•. Localized tissue changes -silica granulomas, which occurs as a result of introduction of dental ceramic particles into tissue (Schmidt and Joachimi, 1987) -possibly a delayed hyper sensitivity reaction due to fluorescing agents (Radioactive uranium salts used earlier).
• Systemic effects -due to leaching of silica or fluorescing agents 3 Effects of material deficiencies • Substantial tooth reduction for bulk, translucency and esthetics.
• Fracture of ceramic material -due to inherent brittleness of ceramics.

SUMMARY
Ceramic materials have been used in dentistry for well over 200 years. They are the most biocompatible dental restorative materials, because they are chemically very stable. A desirable feature of ceramics is that their appearance can be customized to simulate the colour, translucency and flourescence of natural teeth. A major problem with the use of ceramics as tooth replacement materials is that they exhibits a very low flexibility before fracture and also they exhibit large firing shrinkage.

CONCLUSION
Presently there is no restorative system which can ideally replace the natural tooth structure. In the last few years, ceramic research has gained attention for restorative use. Ceramics will play an important role in restorative dentistry. Improvements in fracture resistance and wear properties enhance their restorative use. Used for ceramo-metal restora ons; begins as a mixture of powders of potassium feldspar and glass. It can also be used for fabrica ng porcelain veneers and inlays.
Used in PJS's. It is composed of mixture similar to that of feldspathic porcelain with increased amounts of aluminium oxide   Middle third Cervical/gingival third Enamel covering with li le or no den n underneath produces a wrap around effect which results in increased translucency in the incisal third and approximal areas.
This region consists predominantly of den n, hence the overlying enamel takes on some of the den nal hue (yellow-orange) which is modified by the translucent blue grey enamel resul ng in a composite colour.
Enamel thins down towards the cervical line, hence the underlying den nal hue results in a deep hue ranging from orange-yellow to o en a dis nct brown depending on the degree of calcifica on of den n.