Inlays and onlays are types of indirect dental restorations for teeth, i.e. they are constructed in a dental laboratory, and usually on a model made from impressions of the teeth after the cavity preparation is complete, and then subsequently fitted to the tooth. They are generally indicated where an ordinary direct filling is judged to be inadequate because the cavity is too extensive.
As the names imply an inlay fits into a cavity of a tooth whilst an onlay goes over the top, or at least partially, often to restore a tooth that has been worn down or to protect the cusps of teeth in large complex restorations where the cusps might otherwise be subject to fracture. Inlays and onlays can be constructed from gold, porcelain or composite resin. Inlays used to be made exclusively of gold because of its durability, its ability to be accurately cast, and its resistance to corrosion in the mouth. New inlay alloys of palladium, nickel, or chromium are now sometimes used, but gold is preferred because of its softer nature allowing burnishing at the margins to give a good seal.
Although metals have been the best choice for inlays in molars, there is an increasing demand for more aesthetic restorations in the mouth, even in the posterior teeth, and so tooth-coloured porcelain inlays and composites are becoming more widely employed .
Reinforced porcelain and Lucite porcelain are fairly durable but still may not be suitable for patients who grind their teeth since they can be prone to fracture under a heavy load.
An inlay or onlay could be thought of as a half-way house between a filling and a crown since it is more extensive than the former but does not incorporate all the tooth structure all the way round, right down to the gum margin like a crown. An inlay is confined within the cusps of a back tooth whilst an onlay can cover one or more cusps.
Inlays or onlays are indicated where dental decay has ravaged a large portion of a tooth and so the cavity preparation required in order to remove all decay results in a large cavity of more than one surface, or the loss of a cusp due to fracture or significant undermining.
If there is cuspal involvement or a severely undermined cusp, then an onlay may be a better long term proposition than an inlay. If the damage to the tooth is too extensive, involving more than two cusps for instance, then it may be better to prescribe a full crown in order to adequately restore the tooth and provide a good long term prognosis. The judgement here depends on balancing the need to conserve as much natural tooth substance as possible whilst providing protection for the remaining tooth and the best long term prognosis.
The inlay or onlay is custom made to fit the tooth exactly and gold inlays may be cemented with zinc phosphate or carboxylate type cements, whilst porcelain and composites restorations are more usually cemented with a thin layer of resin cement.
When a patient undergoes treatment for a decayed back tooth the first requirement is of course the removal, under local anaesthesia of all decayed tooth substance which is often present in more than one part of the tooth, typically in both occlusal and one or more proximal areas. This leaves a severely compromised tooth with a large cavity and the dentist has to decide whether an inlay, onlay, or crown is the best option. If there is adequate supporting tissue for the cusps of a tooth then an inlay may suffice and the cavity is shaped accordingly with slightly diverging walls from the pulp floor towards the occlusal so that it is possible to make an inlay which can be inserted like the plug in a sink. There must be no undercuts to obstruct the possibility of insertion as one. Slight undercuts can be blocked out with a suitable cement as used for lining the cavity to protect the pulp before impressions are taken.
From this point on it is a matter of gaining sufficient information to enable the dental technician in a laboratory to construct an accurately fitting inlay or onlay, and traditionally this is done by taking impressions of the teeth to make accurate cast models on which the restoration is made. Modern computer technology has afforded a different method by using three dimensional measurements accurately recorded y scanning the cavity and then using a computer controlled milling machine to carve out an inlay or onlay from porcelain. This process gives good results but is not yet widely available because of the considerable investment required for all the hardware.
More commonly the dentist uses accurate impressions of the teeth made using a rubber base material which may be polysulphide, polyether, or silicone in nature. Modern impression materials are fairly stable but still it is wise to get the impressions to the laboratory as quickly as possible so that they can be cast as plaster models with the least likelihood of dimensional change.
The dentist also has to take note of the patient’s bite or occlusion, meaning the precise manner in which the teeth meet together. An inlay that is made without due regard to the opposing tooth or teeth in the mouth could well be too high and stand proud when fitted and this can cause interference when the patient closes the teeth together leading to possible deviation of the lower jaw or pain on biting. This can be one of the most problematic parts of providing indirect restorations since not only do they have to fit to the tooth underneath, but must also align correctly on their top surface with the teeth that bite against them. Accurate records of how the teeth meet are essential to minimize this problem, but even then it is sometimes necessary for the dentist to carry out slight adjustments by judicious grinding when the inlay or onlay is fitted.
Between the preparation and fitting appointments some form of temporary restoration is necessary and this can vary in type from a simple bulk placement of cement, to an acrylic restoration which better takes up the shape required and helps to maintain the occlusal relationship between opposing sets of teeth, and also the contact points between adjacent teeth. Any slight movement of a prepared tooth or the adjacent teeth between appointments can cause problems when fitting the final restoration, either from interference in the bite, or from tilting of adjacent teeth obstructing the insertion path of the restoration. Teeth are not fixed solidly in the jaw but can tilt or over-erupt if not kept in their proper place, and even the slightest movement can affect the fitting of a cast restoration such is the degree of accuracy required.
Porcelain inlays and onlays are increasingly used to replace old amalgam fillings in order to achieve a much more aesthetic result as people become more demanding in the desire to have as attractive a smile as possible.