Every month, Dr. Gordon Christensen answers a reader question about everyday dentistry.
Q: Over the past few years, I’ve had many crowns come off, composites loosen, and build-up is released from the rest of the tooth structure. I have used the currently advertised adhesives exactly as the manufacturers recommend, but apparently the bonds still fail. What can I do to prevent these frustrating failures? They are upsetting for both me and my patients.
ONE: I hear this question often and have a few possible solutions for you. As you know, there are basic basic types of retention for dental restorations.
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Mechanical retention interface it requires abnormalities in the rest of the tooth structure into which the cement, composite, or build-up material can flow. The resulting entanglement of the tooth and the material placed on the irregular surface creates a physical barrier to the removal of the material and that the interlock is very restrained in the long term. Mechanically retaining dental materials is no different than laying bricks on the exterior of a building. Bricks placed on the surface of the building are held together by semi-liquid mortar that flows on the rough surface of the brick and the roughness of the interior wall of the building. The mortar sets into a rigid mass and the bricks are then held firmly in place.
Retention of chemicals is the molecular contact at the substrate surface—tooth structure. It is the strongest adhesion mechanism, with bonds occurring between functional groups in the adhesive and atoms/molecules on the surface of the substrate. This mechanism contributes to the structural strength of many adhesives, including dental adhesives. If the expansion-contraction coefficient of the two substrates is equal, there is minimal reduction in bond strength. That’s how it is not The case of some dental bonding techniques, including composite resin-based bonding to tooth structure. These materials placed over the tooth structure have expansion-contraction characteristics different from those of the tooth structure. Hot and cold foods cause retention stress and devaluing of this chemical bond.
Difference between enamel and dentin bond
The enamel is etched with acid, creating thousands of potentially interlocking irregularities into which the dental resin-based composite can flow. These anomalies create strong, long-lasting mechanical retention. This is not the case with dentin. Dental tubules are extremely small—just a few microns in diameter. The contact of the composite and the tooth with hot and cold foods together with the uneven expansion-contraction of dentin and composite resin soon degrade the original bond.
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When a significant proportion of the tooth preparation has enamel, the resin cannot be removed due to the mechanical retention provided by acid etching. If the preparation is mainly dentin, the chemical bond of the dentin composite soon diminishes and the resin restoration or cement may peel off.
A well-known clinical situation demonstrates this phenomenon. Enamel-mounted veneers must be removed regardless of how long they have been in the mouth, whereas dentin-mounted veneers can often be removed with light force applied after only a few months (figure 1).
