10. Ceramics used in orthodontic devices
_____10.1. General characteristics
_____The word ceramic is derived from a Greek suburb of Athens where clay was fired. The modern usage of the term broadens the meaning to include all inorganic non-metallic solid materials that are processed or used in high temperatures. It also includes glass, enamel, glass-ceramic, and inorganic cementitious materials (cement, plaster and lime). Structurally, all belong to the class of inorganic polymers in which the carbon atoms in the main chain are replaced by silicon, boron, sulfur and phosphorus. While not fired, hydroxy apatite, the main component of teeth, is still considered a ceramic.
_____Non-ceramics components of this class include elastomers such as the silicones, the polyphosphazenes and some inorganic-organic hybrids that may have a bright future in medicine. During the last decades, the gap between organic and inorganic polymers has decreased, sustained efforts being made in this respect as shown in the books and journals dedicated to the field.
_____As plaster and similar products are commonly examined in dental biomaterials books, in what follows we will concentrate on the oxides of aluminum, silicon and zirconium (known also as alumina, silica and zirconia, respectively) used in orthodontics as fillers or raw materials for direct bonding brackets.
_____Ceramics, which are mostly the oxides, carbides, phosphates or silicates which can contain a large number of elements, as shown in Fig. 10.1, can be divided into two classes: crystalline and amorphous (non-crystalline). In crystalline materials, the lattice is occupied by atoms or ions (depending on the bonding mechanism) that are arranged in a regularly repeating pattern in three dimensions. In contrast, in amorphous materials, the atoms exhibit only short-range order. Some ceramic materials, like silicon dioxide or silica, can exist in either form. The type of bonding (ionic or covalent) and the internal structure (crystalline or amorphous) affects the properties of ceramic materials.
_____In orthodontics, the most used ceramic is aluminum oxide, the ore of which can be found as mountains (bauxite). If red tinted, this oxide is known as ruby, the only gem that can be more expensive than diamond. If green, is emerald, if light blue, aquamarin and if blue, sapphire. Its colorless variety used in orthodontics is known as leuco-sapphire (Greek: leuco, white).
_____The principal limitation of ceramics is their brittleness, i.e. the tendency to fail suddenly with little plastic deformation. This is of particular concern when the material is used in structural applications. In metals, the delocalized electrons allow the atoms to change neighbors without completely breaking the bond structure. In ceramics, due to the combined ionic and covalent bonding mechanism, the particles cannot shift easily.1 The ceramic breaks when too much force is applied, and the work done in breaking the bonds creates new surfaces upon cracking. Brittle fracture occurs by the formation and rapid propagation of cracks. In crystalline solids, cracks grow through the grains (transgranular) and along cleavage planes in the crystal.
_____The compressive strength of a ceramic is usually much greater than its tensile strength. To make up for this, ceramics are sometimes prestressed in a compressed state. Thus, when a ceramic object is subjected to a tensile force, the applied load has to overcome the compressive stresses (within the object) before additional tensile stresses can increase and break the object. Safety glass (thermal tempered glass) is one example of such a material. Ceramics are generally quite inelastic and exhibit low fracture toughness, the ability to resist fracture when a crack is present. It depends on the geometry of the object and the crack, the applied stress, and the length of the crack.
_____10.2 Impact of physico-chemical properties
_____While their thermal and electric properties are outstanding, in the case of orthodontics most important is the way that these materials respond to forces, loads, and impacts. Ceramics are attractive structural materials as these are aesthetic, strong and hard, while exhibiting low densities and high melting points. Being also resistant to corrosion (durable), ceramics are successfully used in dentistry.
_____Structural applications of advanced ceramics include components of automobile engines, armor for military vehicles, and aircraft structures. For example, titanium carbide, widely used in cutting devices inserts, has about four times the strength of steel.
_____Very hard materials such as alumina and zirconia are used to make brackets, act as fillers in composites or as abrasives in grinding wheels. For intricate shapes such as the orthodontic brac-kets, the resistance to fracture is deterred by their stress concentration points.
_____Make-up and manufacture. While still a century ago all ceramics were made by from natural sources, process that couldn’t go to higher purities, today’s advanced engineered ceramic parts are synthesized.
_____To achieve high strength (which increases with decreasing grain size), most starting powders are milled (or ground) to produce a fine powder (diameter < 1 µm). Advanced ceramics can be formed by single axis pressing, isostatic pressing, slip casting, extrusion, and especially by injection molding. Special grades of alumina can be metallized and/or brazed to metal parts.
_____For orthodontic purposes, alumina made by attacking electrolytic aluminum with nitric acid and thermally decomposing the aluminum nitrate formed may require further purification. Thus, clear, “single crystal” brackets such as Starfire® demand a supplementary purification by the Freeze-Thaw or Zone Melting process. The purified bar resulted is stepwise cut in sections to lead to as shown in Fig. 10.2.
_____After the “green” has been consolidated, it is subjected to sintering at temperatures up to 1800°C). During this process, the individual ceramic particles coalesce to form a continuous solid network and pores are eliminated. Typically, the microstructure of the sintered product contains dense grains, where each individual grain is composed of many starting particles.
_____10.3 Potential health hazards
_____Received with enthusiasm years ago, the sales of ceramic brackets are now sagging. The reasons are their extreme hardness, their high coefficient of friction against metals, and especially their low fracture toughness.
_____When ceramic brackets were first marketed, few realized that the high strength of an alumina mono-crystal has little to do with the fracture resistance of an agglomeration of such crystals, or with the intricate processing required to make orthodontic brackets. Thus, synthetic sapphire was heralded by manufacturers as “the strongest material ever used for orthodontic brackets”, and it was suggested that clinicians “use the full range of orthodontic forces without fear of bracket failure.”2
_____At debonding time, in alumina brackets the difference in hardness between the three substrates involved favors the enamel’s breakage. Indeed, according to the Mohs scale of hardness, the latter is even softer than the adhesive, a composite containing usually up to 80% silica, as shown in Fig. 10.3. While a plain enamel fracture may not take place, ceramic debonding lead to internal fissures that later could develop in cavities.
_____Enamel fracture depends not only on the attachment, but also on the adhesive and the condition of the tooth. If the first is made intentionally soft, the bracket will detach rather than break; if the enamel is weak, an unscathed bracket may leave behind a fractured tooth.
_____A related problem is the abrasion of the latter when the two substrates are in close and repeated contact, as shown in Fig. 10.4. Indeed, being both softer than alumina, enamel and the dentine underneath are eroded phenomenon that is not only unsightly but also leads to pain and sensitivity. In addition, alumina brackets are brittle: their fracture toughness is some twenty times less than that of stainless steel they replace in orthodontics. As a difference from metals that give in under strain, ceramics are easily broken.
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_____The brackets’ complex shape, along with the trends toward miniaturization and less bulk, renders them highly vulnerable to breakage. While in common gems the size and number of pores as well as surface imperfections impair only the esthetics, in the synthetic leuco-sapphire brackets such imperfections constitute a major liability when it comes to perform a difficult task. The undesirable breakage of the ceramics or of the contiguous enamel can occur during clinical activations,3-6 debonding,7-15 and accidental impact.16 While enamel fracture is the most serious problem, ceramic breakage ranks not far behind, since the failure rate reached in vitro during debonding, i.e. in better controlled conditions, can be up to 80%.11
_____While less addressed, ceramic bracket breakage has been considered a major clinical problem,3 since it prolongs the treatment and leads to compromises in its final outcome.4 In addition, broken ceramic brackets are uncomfortable for the patient and difficult to remove,5 requiring an extensive finishing and polishing.7 Broken ceramic brackets can draw blood from both clinician and patient, and the use of diamond stones to remove ceramic fragments from the enamel becomes necessary,8 with the potential of harming the thin layer of fluorapatite that protects the enamel.