Dental Inlay and Onlay Restorations

Posterior dental caries (cavities located in the back of the mouth) are most often treated using composite (white) fillings and to a lesser extent amalgam or metal fillings. In more extreme cases, dental crowns are used. However, there are many instances where the extent or location of the decay makes treatment impractical with composite resin but at the same time contraindicates the invasive preparation required for a dental crown. In these circumstances, a dental inlay or onlay is indicated.

How Do They Work?

Dental inlays are designed to restore the intracoronal tooth structure that is bounded by a tooth’s cusps. Onlays can be used to restore both occlusal (biting) surfaces and exterior portions of the tooth. The extensive decay that indicates the use of inlays or onlays often emerges from old fillings, such as amalgam fillings. These restorations are often subject to leakage around the margins, which means that bacteria can penetrate inside the tooth triggering decay. Composite resin and amalgam fillings are both more thermally reactive than natural tooth structure, placing undue stress on the tooth and increasing the risk for leakage.

Restorative materials like composite resin and amalgam fillings also have low radiopacity, which makes it difficult for the dentist to detect decay within the tooth. As a result, the decay is allowed to go unabated compromising healthy tooth structure. It’s not until these caried teeth become symptomatic that the decay is uncovered. The patient usually presents with complaints of tooth sensitivity.

Dental inlays and onlays are ideal for the application described above as they strengthen the remaining tooth structure once cemented into place. This reduces the risk of the tooth fracturing. Furthermore, they require a much more conservative preparation technique, which leaves healthy tooth structure intact. Thus, inlays present a very good alternative to conventional fillings for even small cavities.

Types of Inlays and Onlays

Dental inlays and onlays can be fabricated from porcelain, metal, or composite material. Porcelain is the leading choice in terms of durability and aesthetics. However, it does have some drawbacks. It is more expensive because the fabrication process must be outsourced to a lab and the porcelain can aggravate tooth wear on your opposing teeth. The life longevity expected from these restorations can easily exceed ten years, so they are a very worthy consideration.

Metal inlays and onlays are made from a metal alloy, typically one comprised of high noble metal content like gold. These can be extremely durable but many not appeal to everyone on an aesthetic level. Cheaper varieties of these restorations may induce unwanted allergic reactions and will generally have a shorter lifespan.

Composite resin inlays and onlays are fabricated of a material similar to that of white fillings, although the composition may be different to account for higher occlusal (biting) stress. Unlike conventional white fillings, composite inlays and onlays are cured outside the mouth. This presents a major advantage in terms of minimizing leakage of the restoration. Composite resin undergoes a process known as polymerization when cured, and as a result composite restorations shrink. This can compromise the bonding strength of direct composite restorations and leave the margins vulnerable. Otherwise, inlays and onlays of this variety present the same issues as conventional white fillings – reduced durability in comparison to porcelain and potential for stain retention and absorption. However, they are also much more easily repaired than porcelain restorations and are offered at a lower pricing point. You can expect to get approximately five years out of these restorations.

The Office Visit

You shouldn’t expect any surprises in getting a tooth outfitted with an inlay or onlay. The process is much the same as a regular filling, although you may have to return for a second visit.

The dentist will first numb your tooth and remove the existing restoration. If the dentist is drilling out an amalgam filling, extra precautions will be taken to isolate the rest of your mouth from mercury vapor and particles. Once the old restoration has been removed, underlying decay will be treated. The tooth is then properly shaped to accept the inlay or onlay.

At this point, the procedure can go one of two ways. If an indirect fabrication technique is being used, the dentist will take an impression of the tooth. This impression will then be transferred to a cast or “mockup” of your teeth, where a wax pattern or replica of the final restoration will then be created. This wax pattern will be transferred to the final restorative material by means of “investment”. In the case of porcelain restorations, this process is handled by the dental lab. Composite restorations can be outsourced as well or done by the dentist, in which case a single-appointment treatment is possible.

If the dentist’s office is equipped with a CAD/CAM system like the CEREC system, indirect porcelain restorations can be completed the same day. The CEREC system utilizes a three-dimensional camera to accurately capture the prepared tooth structure and surrounding teeth. Impressions of the surrounding teeth are used as a basis for the new restoration. The software analyzes the occlusal (biting) surface of these existing teeth and renders the restoration based on a special algorithm. The dentist in return can modify the design, by reducing the intensity of biting stress on certain contact points, for instance.

Once the design is completed, the system then mills the restoration out of a block of porcelain. This fabrication method yields a stronger final restoration, a tighter fit and less tooth preparation (both attributable to the high accuracy of this system).

The indirect technique requires the use of provisionals to reduce tooth sensitivity and restore functionality to the teeth in the interim. The provisional can be made directly on the tooth or based on an impression (which yields a more realistic representation of the final result).

Direct fabrication is only possible for composite restorations. In this method, the composite material is directly applied to the tooth’s surface (with a decoupling agent in-between) and then removed, fully cured, and internally etched. It is then fitted on the tooth, adjusted, and bonded into place. The tooth is then finished by means of removing excess bonding agent and polishing.

The Delivery Appointment (Indirect Inlays/Onlays)

The second or delivery appointment involves the removal of the temporary filling material and cementation of the final restoration. The dentist will first clear out the provisional filling and clean the tooth. The restoration will be tried-in with careful attention to fit. The restoration and tooth are then etched, and bonding and luting agents are applied. The bonding agent creates a chemical bond between the restoration and tooth, while the luting agent creates mechanical retention by means of “stuffing” open voids. The restoration is pushed into place, the bonding and luting agents slightly cured and the excess cleaned. The tooth is then polished.

Final Thoughts

Although inlays and onlays can pose a substantially higher investment than direct restorations like composite and amalgam fillings, they offer a longer life span, improved aesthetic results, better protection against decay and reduced likelihood of future tooth trauma. If you have aging fillings that are due for replacement, ask your doctor about this option.


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