An inlay is an indirect restoration that is custom made to fit exactly into a suitably prepared tooth cavity as one jigsaw piece would fit with another. Accuracy of fit is of paramount importance in order to prevent micro-leakage with the possibility of bacteria gaining entrance to the tooth and starting up secondary dental decay. The size and shape of an inlay will depend on the amount and location of decay in a tooth, and often involves the occlusal and one or more proximal areas of a tooth. Once all decay has been removed it is necessary for the dentist to consider the path of insertion for an inlay since it is all in one piece and has to be inserted as such. There can be no undercuts in the cavity that would prevent this. Undercuts can be blocked out sometimes with lining cement as is used on the floor of the cavity to protect the pulp.
An inlay can be constructed in gold, porcelain, or composite and thus the precise construction methods will vary slightly, but the traditional gold inlay is cast by means of a centrifuge which spins molten gold alloy into a plaster mould in what is termed the “lost wax technique” since the mould is made by incorporating a wax pattern of the desired inlay in plaster and then burning out the wax to leave the correctly shaped hollow.
At one time a dentist would have fabricated a wax pattern directly in the tooth in the mouth and then attach a sprue and gently withdraw it for casting. This was quite difficult in cases of compound cavities and required a good deal of patience to get a wax pattern from a tooth cavity in one piece without it breaking! The more commonly used method nowadays is to take an accurate impression of the tooth cavity to form a model and then the wax pattern is made on that in a laboratory. Modern rubber elastomeric impression materials are very accurate and dimensionally stable so this can produce excellent results provided all stages in the process are given meticulous care. Impressions of adjoining and opposing teeth are also required, plus a bite registration so that the inlay can be properly made to fit with all other teeth in the mouth, both at rest and in function.
A temporary filling or inlay is normally placed in order to protect the cavity of the tooth until the fit appointment, and also to maintain the contact points with adjacent teeth and opposing teeth. Any tilting of adjoining teeth could result in obstruction of the precisely fitting inlay at its proximal ends.
At the fitting appointment the temporary material or inlay is removed and then the permanent inlay is checked for fit all round and in the bite. Any slight premature contacts when the teeth are closed together means that a small amount of adjustment is needed by appropriate occlusal grinding. Once the inlay is seating properly it is cemented in place with one of a choice of permanent cements that are available.
The choice is between zinc phosphate, polycarboxylate, glass ionomer, and resin types and operators vary in their preference for one over the others.
Fine grain zinc phosphate cement is still used by many dentists although it has been mostly superseded by more modern alternatives. It is able to provide a thin film to allow for accurate seating of an inlay and is fairly strong. Glass ionomer, however, is also able to provide for a thin film thickness and has the advantage of slowly releasing fluoride over time which helps to inhibit the recurrence of tooth decay. It is also considered kinder to the pulp than the somewhat acidic zinc cement. Composite types of cements are generally used with composite and porcelain restorations as they provide an excellent chemical bond with these materials and a final result which is relatively insoluble in oral fluids and helps prevent micro-leakage as might otherwise occur in time if cement material leaches out. An auto-mixing dual-cure cement can be used for all indirect applications such as inlays, and the latest types are very colour stable and don’t tend to stain as previous types might have.