Background

Dental ceramics in restorations are essentially oxide based glass-ceramic systems. They have three essential features/requirements:

1. Ease of fabrication of complex shapes

2. Sufficient mechanical and corrosion resistance

3. Appropriate aesthetic appeal.

In the last few decades there has been tremendous advances in the mechanical properties and methods of fabrication of these materials. Whilst porcelain based materials are still a major component of the market there have been moves to replace metal cored systems with all ceramic systems. In this brief review the following topics will be addressed: natural teeth and their properties, alternate restorative materials and the various ceramic compositions and their usage by the dental practitioner. Examples of the microstructure property relationships for these ceramic materials will be addressed.
Introduction

The history of restorative dentistry can be traced back as far as ancient Egyptian times. Examples of tooth replacement prostheses made from gold wire, ox bone or wood have been found. More recent restoratives had a renaissance about two hundred years ago when air fired porcelains and cast gold restorations were made to restore and replace teeth. It seems that in ancient times the main requirement was to replace teeth lost as a result of gum disease, whereas in recent times it is to restore teeth damaged by decay.

Restorations today are largely required as a result of trauma, decay, gum disease and aesthetics. The latter being a more recent area of high demand and one in which ceramic materials play a large role. McLean (1979) provides a concise history of ceramic use in modern dentistry. The use of ceramics for the restoration of teeth has been a part of dentistry's modern period of evolution. This period started in the late seventeen hundreds but major advances have mainly come about this century. The demand for aesthetic restorations led to improvements in ceramic formulation and firing techniques. The types of ceramic systems have been largely summarised by McLean. See also a recent review by Kelly (1997).
Structure of a Tooth
Enamel

Teeth themselves are a complex hard tissue structure originally born from specialised cells called ameloblasts, odontoblasts and cementoblasts. The ameloblasts form the enamel, which is the hard outer coating seen as the clinical crown of the tooth. These cells occur in a layer on the outside of the tooth bud.

The enamel is laid down on the inside of the ameloblasts. When the tooth erupts, these cells are lost and enamel can no longer be formed. This has important implications because any wear or loss of enamel due to decay etc, cannot be repaired by the body.
Dentine

The dentine is formed by the odontoblasts. These cells are on the inner side of the tooth bud, between the enamel and the dental pulp. The dentine is formed by these cells as an inward growth. The dentine could be viewed as the main foundation of the tooth, supporting the enamel, providing protection to the pulp, and through its covering below the gums, giving rise to the attachment via a ligament to the surrounding bone. The dentine has an ability to continue laying down dentine internally at the expense of the pulp chamber size, throughout life. It cannot however, replace dentine that has been physically lost. Figure 1 depicts the lateral view of an incisor tooth.

AZoM - Metals, Ceramics, Polymers and Composites : Ceramics in Dental Restorations – A Review and Critical Issues : Incisors

Figure 1. Lateral view of an incisor.
Mechanical Differences

The mechanical properties and their interelationship of the three hard tissues mentioned above, enamel - dentine - bone, present an interesting method for dealing with stresses applied to the teeth as a result of chewing and also tooth grinding during periods of concentration or psychological stress. The enamel is relatively hard and brittle (E~ 65 - 70 GPa) the dentine much softer and more compliant (E~ 15 - 19 GPa) and bone even more compliant (E~ 12 GPa).
Dental Restorative Materials

As mentioned earlier man has been replacing lost tooth structure with gold and ceramic. The gold and ceramic materials were used because they could be custom fabricated to fit the needs of individual tooth requirements as far as form and aesthetics are concerned. They can be used independently or in combination with ceramic baked onto gold alloy subframes. Other metal alloys are also used in this metal-ceramic technique.
Silver Amalgam

Today as in the past these materials have proved to be relatively expensive for the population masses and alternatives have developed. The first material that was easily produced and relatively inexpensive was silver amalgam. This material, popularised by an American dentist, G. V. Black, in the 1890's has been used very widely throughout the world as a cheap and effective restorative to replace tooth structure lost through decay. The main disadvantages with this material are the concerns over its mercury content and its lack of aesthetic appeal.
Composite Resins and Glass Ionomers

The other two groups of restorative materials to be used widely are the composite resins and the glass ionomers. The composite resins developed in the 1950's when a breakthrough monomer was produced. Known as BisGMA, this resin monomer has become the backbone of most dental composite resins. These materials use various glass or ceramic particles as fillers to enhance their mechanical properties and give them tooth colouring and other aesthetic properties such as translucency etc.

The glass ionomers are a relatively recent development although their predecessor silicate cements have been in use for about the same time as the BisGMA resins. These materials are relatively weak, but are used to cement gold and ceramic crowns to teeth, with other versions being used as direct restoratives, depending on their filler content and the recommended application. They have a major advantage in that their have a reasonably constant fluoride release, acting as a strong decay inhibitor.
Issues Facing Ceramics as Dental Restorative Materials

The aim of this paper is therefore to review the role of ceramics in dentistry. The first consideration is why use ceramics. The reasons are as follows:

· Biocompatibility

· Aesthetics

· Durability

· Relative ease for customised units.