Inlays and onlays

In dentistry, inlays and onlays are used to fill cavities, and then cemented in place in the tooth. This is an alternative to a direct restoration, made out of composite, amalgam or glass ionomer, that is built up within the mouth.Inlays and onlays are used in molars or premolars, when the tooth has experienced too much damage to support a basic filling, but not so much damage that a crown is necessary. The key comparison between them is the amount and part of the tooth that they cover. An inlay will incorporate the pits and fissures of a tooth, mainly encompassing the chewing surface between the cusps. An onlay will involve one or more cusps being covered. If all cusps and the entire surface of the tooth is covered this is then known as a crown.
Historically inlays and onlays will have been made from gold and this material is still commonly used today. Alternative materials such as porcelain were first described being used for inlays back in 1857. Due to its tooth like colour, porcelain provides better aesthetic value for the patient. In more recent years, inlays and onlays have increasingly been made out of ceramic materials. In 1985, the first ceramic inlay created by a chair-side CAD-CAM device was used for a patient. More recently, in 2000, the CEREC 3 was introduced. This allows for inlays and onlays to be created and fitted all within one appointment. Furthermore, no impression taking is needed due to the 3D scanning capabilities of the machine.

Inlays

Sometimes, a tooth is planned to be restored with an intracoronal restoration, but the decay or fracture is so extensive that a direct restoration, such as amalgam or composite, would compromise the structural integrity of the restored tooth or provide substandard opposition to occlusal forces. In such situations, an indirect gold or porcelain inlay restoration may be indicated.

Comparison of inlays and direct fillings

When an inlay is used, the tooth-to-restoration margin may be finished and polished to a very fine line of contact to minimize recurrent decay. Opposed to this, direct composite filling pastes shrink a few percent in volume during hardening. This can lead to shrinkage stress and rarely to marginal gaps and failure. Although improvements of the composite resins could be achieved in the last years, solid inlays do exclude this problem. Another advantage of inlays over direct fillings is that there is almost no limitations in the choice of material.
While inlays might be ten times the price of direct restorations, it is often expected that inlays are superior in terms of resistance to occlusal forces, protection against recurrent decay, precision of fabrication, marginal integrity, proper contouring for gingival health, and ease of cleansing offers. However, this might be only the case for gold. While short-term studies come to inconsistent conclusions, a respectable number of long-term studies detect no significantly lower failure rates of ceramic
or composite
inlays compared to composite direct fillings. Another study detected an increased survival time of composite resin inlays but it was rated to not necessarily justify their bigger effort and price.

Inlay types, main uses of inlays

Inlays and Onlays are similar as they are a type of indirect restoration. However, the difference is that Inlays are indirect restorations which do not have cuspal coverage and are within the body of the tooth. Onlays are indirect restorations that cover both body and cusps of teeth.
Inlay Indications:

Extensively restored or weakened teeth
Repeated fracture or failure of previous direct restoration
Restorations within body of tooth which do not require cuspal coverage
Difficulty achieving good contour, contact point or occlusion using direct restorations
Advantages of Inlays

Inlays are a type of indirect restoration that is used to restore extensively damaged or decayed teeth. When compared to conventional fillings, inlays have several advantages:

Inlays are extremely strong and durable: well-made gold inlays, in particular, have exceptional longevity with proper care
Inlays can give the restored tooth a natural, aesthetic appearance: ceramic inlays allow an excellent shade match that makes the restoration almost indistinguishable from the surrounding natural tooth
Ceramic inlays have better physical properties than traditional resin composite fillings for posterior teeth
Inlays may allow the dentist to achieve better contours, contact points, and occlusion than direct fillings because they are custom-made for the patient in a laboratory
Resin inlays have less microleakage and less post-operative sensitivity than direct resin composite fillings
Disadvantages of Inlays

Inlays are a hugely popular restorative technique, used in cases to preserve tooth substance and provide a greater strength than restorations. Inlays are most commonly constructed in composite resin and ceramic materials. Both types have varying properties, however the general downsides to inlays include:

Higher cost compared to a restoration, due to the need for a dental laboratory
Lengthier process as two appointment required i.e. increased chair time for patient
Over time, they can present complications such as marginal leakage and staining by foods
Tricky to ensure the absence of undercuts in the tooth preparation stage & highly technique sensitive
Composite or ceramic inlays have a lower strength than materials such as a gold
Risk of loss of vitality with tooth removal
Last longer than restorations but may need replacement
Ceramics can be brittle
Difficult repairs
Onlays

When decay or fracture incorporate areas of a tooth that make amalgam or composite restorations inadequate, such as cuspal fracture or remaining tooth structure that undermines perimeter walls of a tooth, an onlay might be indicated. Similar to an inlay, an onlay is an indirect restoration which incorporates a cusp or cusps by covering or onlaying the missing cusps. All of the benefits of an inlay are present in the onlay restoration. The onlay allows for conservation of tooth structure when the only alternative is to totally eliminate cusps and perimeter walls for restoration with a crown. Just as inlays, onlays are fabricated outside of the mouth and are typically made out of gold or porcelain. Gold restorations have been around for many years and have an excellent track record. In recent years, newer types of porcelains have been developed that seem to rival the longevity of gold. If the onlay or inlay is made in a dental laboratory, a temporary is fabricated while the restoration is custom-made for the patient. A return visit is then required to fit the final prosthesis. Inlays and onlays may also be fabricated out of porcelain and delivered the same day utilizing techniques and technologies relating to CAD/CAM dentistry.
A systemic review found that the most common cause of onlay failure is ceramic fracture, followed by ceramic de-bonding from the tooth structure, and the occurrence of secondary caries which is seen as a discolouration at the margins of the restoration. High failure rates were associated with teeth that had previous root canal treatment, and with patients who exhibit para-functional habits such as bruxism, or teeth clenching.

Indications

Inlays/onlays are indicated when teeth are weakened and extensively restored. There are no obvious contrast between the two.
Inlays are usually indicated when there has been repeated breach in the integrity of a direct filling as metal inlays are more superior in strength. It is also indicated when placement of direct restoration may be challenging to achieve satisfactory parameters. They are usually reserved for larger cavities as tooth conservation is paramount in current practice and small cavities can be restored with direct composites instead.
Onlays are indicated when there is a need to protect weakened tooth structure without additional removal of tooth tissue unlike a crown, e.g. restoring teeth after root canal treatment to give cuspal coverage. It can also be used if there is minimal contour of remaining coronal tooth tissue with little retention.

Contraindications

Poor oral hygiene

Contraindications to providing Onlays and Inlays include plaque and active caries. It is important to ensure adequate oral hygiene before providing any indirect restoration as failure to manage the caries risk of an individual may result in recurrent caries. Caries risk is defined as "a prediction as to whether a patient is likely to develop new caries in the future". The restoration itself does not alter the risk, which allows subsequent caries to develop around the indirect restoration placed. This may be caused by plaque retentive features of the restoration, or if the restoration is poorly bonded to the tooth. However, in the main the subsequent caries around a restoration is because the caries risk has not been reduced. Reducing the patients risk of further disease prior to treatment provides predictable results and helps prevent further restorative procedures.¹

Parafunctional habits and heavy occlusal forces

Inlays and Onlays are contraindicated in patients with parafunctional habits and heavy occlusal forces. A Parafunctional habit refers to abnormal functioning of oral structures and associated muscles, for example patients who clench or grind their teeth. Occlusal forces are greater on molars when compared to premolars. Evidence has shown greater failure of Onlays and Inlays in molars than premolars over an 11.5 year period. The most common cause for failure was porcelain fracture. To ensure longevity it is beneficial to avoid heavy occlusal forces. If a cuspal coverage onlay is required porcelain should be used as cuspal coverage with composite is contraindicated.