The Advent of Ceramics
Every few years some new procedure or material is offered as a major advancement that will solve orthodontic problems. In some instances, these advances have lived up to their expectations. Weldable stainless steel, bonding techniques, 3D brackets, and titanium wire alloys fall into this category. The latest addition to the list is the ceramic bracket.
The quest for an effective esthetic appliance has led to many innovative designs over the past 15 years. Metal band-bracket components were replaced by bondable metal attachments. Smaller bonding bases appeared as adhesives and bonding techniques improved. Brackets themselves were down-sized, in some cases to the point where function was impaired.
Clear brackets are not new. Unfilled polycarbonate brackets were available in the early '70s, but they had limited acceptance because the slots distorted under archwire loads and the bracket wings often broke under tipping forces. Today's translucent plastic brackets are both esthetic and functional; some have metal slot inserts, and others have ceramic fillers that give them improved durability. Bond strengths are quite adequate. The search for the ultimate clear but strong bracket has now progressed to the stage of both translucent and water-clear brackets made from ceramic materials.
Despite strong demand, spurred on by patients who have seen nationwide advertising, relatively few ceramic brackets are currently in use. Orthodontists' traditional resistance to change does not seem to be a factor in this case. The only reason the brackets are not already in wider use is the difficulty of manufacturing them. Even a price two to three times that of metal brackets does not seem to be a deterrent to the demand.
Metal brackets can be produced quickly and easily with conventional machining or molding techniques. Polycrystalline ceramic materials can be molded, which is how translucent ceramic brackets are produced. Single-crystal ceramic brackets, on the other hand, must be shaped by non-conventional machining techniques--including diamond saws, YAG lasers, and ultrasonics--due to the nature of the material, particularly its hardness. Such techniques have been used for years to produce jeweled watch bearings from synthetic sapphires and rubies. With these more exotic production methods, hardness is of little consequence and complex forms can be produced. But to do so with accuracy is time-consuming and costly.
In addition, there are a number of clinical drawbacks to be considered. One is the limited ability to adapt the base to the anatomy of a tooth, or to reshape a wing to avoid occlusal interference. Another problem is that the very transparency of the brackets makes accurate positioning more time-consuming. Also, because of the brittleness of ceramics, there is more likelihood of a bracket breaking during treatment or at debonding. At chairside, synthetic sapphire can only be ground with a diamond.
Whether debonding will present a problem depends on the strength of the bracket-adhesive interface. Currently there are two kinds of ceramic bracket bases. One type is formed with undercuts (like those of mesh-base metal brackets) that provide a mechanical interlock to the adhesive. The other configuration has a relatively smooth surface and relies on a chemical coating to enhance bond strength. Bond strength is more critical for brackets that use a chemical bond than for those with undercuts. If the bracket is the weakest link in the enamel-adhesive-bracket chain, it may shatter rather than break away in one piece. Ideally, the bracket-adhesive bond will be strong enough to withstand extended treatment forces, but weak enough to break free in one piece at debonding.
Cosmetic considerations must also include the parts of the appliance that are not ceramic--the archwire, O-rings, elastomeric chains, and adhesives. Clear elastomeric materials with optimum physical properties have not yet been developed, nor have functional tooth-colored archwires. The color of the adhesive is perhaps the easiest problem to solve because of the extensive use of tooth-colored composites in restorative dentistry. Light-cured adhesives will undoubtedly become widely used, and their relatively unlimited setting time will also assist in bracket placement.
Bracket designs can be developed that take into account the properties of ceramic materials. These brackets will probably have no sharp angles and will be manufactured to minimize lattice defects in the crystalline structure. This will produce less breakable brackets, but will not entirely eliminate the stresses that cause breakage.
Ceramic brackets are not transparent metal brackets. They have significantly different physical properties, and must be handled accordingly. Still, it is likely that ceramics will replace metal, at least on anterior teeth, in fewer years than it took for bonded attachments to replace bands. We are practicing in a consumer age. Few of us now have the luxury of unlimited numbers of patients seeking our services, and clear or nearly invisible appliances are what the public wants. Metal brackets bonded to anterior teeth will become a curiosity.