They are the most conservative indirect restorations designed to replace missing tooth structure where placing direct restorations (such as composite or amalgam fillings) is not a good option.
When the defect in the tooth is so big or located on the tooth unfavorably and presents a high risk of the tooth cracking in the future, the tooth may require a type of restoration, that grasps and reinforce the remaining tooth structure from outside of the tooth. Unlike the crowns, onlays allow to preserve more of your tooth enamel, making the remaining tooth structure stronger, while still performing as a support for the weaker part of the tooth.
Why would the tooth need an onlay? In the 9 out of 10 cases, the tooth may need an onlay due to the tooth decay. Because of that, the process starts with the decay excavation and leveling all the hole walls with the restorative material, so the decay is contained and the tooth is not sensitive later. Then the tooth structure is reduced in a most conservative way allowing the remaining healthy enamel to stay on the tooth. Then the impression is made and the onlay is ordered in the lab or cut with the CAD/CAM technology. After the onlay has been made, the fit is checked and the onlay is cemented with the permanent cement. After cementation, the bite is checked by your dentist and the instructions are given.
A very similar construction as onlay, that is designed to serve a different purpose. Why the tooth need to be inlaid? The dental decay very often starts in between the teeth and ruins the contacts of one tooth to another one, allowing the food impaction. So, the inlay is used, when it is not possible to create the good tooth-to-tooth contact with the regular filling. Because regular filling is made in the actual tooth, it is difficult to simulate the convexity of the natural tooth outline. It is even more difficult to make that restoration slightly wider than the available space, which is important, so the tooth remains in a tight contact with it’s neighbor after being restored. Insufficient contacts will lead to food impaction, food retention, pain while chewing foods, gum inflammation and possibly new caries. Because the inlay is made outside of the mouth (in the lab or by CAD/CAM technology), it is possible to build any thickness or contact pressure in it.
The process of fabricating the inlay is identical to making an onlay, except for minor details of cavity geometry. After it has been made, it is tried in in the tooth and cemented with the permanent cement. The bite is checked and the instructions are given.
How long do the inlays and onlays last? Statistically, teeth-born indirect restorations last for 7-9 years, however it greatly depends on the tooth surrounding factors. The porcelain or metal do not catch the cavity, but the tooth may. This is why it is important to maintain the good oral hygiene levels. If the caries risk is low and oral hygiene is great, a properly made inlay or onlay may last you your lifetime.
Common problems with inlays and onlays.
Increased tooth sensitivity to cold. This may be completely normal for several days, up to two-three weeks. If the sensitivity stays longer, you need to give your dentist a call. If the tooth feel a heavy throbbing pain, or the pain that stays longer than 2-5 seconds, it may need a root canal treatment. Contact your dentist and discuss the symptoms.
Food impaction. This should not be happening. Contact the dentist and let him/her know about it. One of the main purposes of the inlay or other indirect restorations is the contact restoration. This is a much more complicated issue to fix than it may look like. So contact your dentist and follow the plan. The inlay or onlay may need to be replaced.
Bite change. Just like in case with the teeth sensitivity, the bite may feel funny for a day or two. If you feel the newly made inlay or onlay does not allow your other teeth meet when biting, please inform your dentist right away.
Transition line. This is totally expected. The line of transition between the inlay/onlay and the tooth enamel is planned to be above the gum level, many times being on the most visible tooth area. Depends on the type of the cement used, it could be more or less noticeable.