An onlay is a type of cusp-covering dental restoration that is made in a laboratory or by a computer controlled milling machine from either ceramic, composite resin, gold, titanium or other metals.
Onlay is usually needed in those cases where there is an extensive damage to the tooth involving 2 or more cusps.
Inlays and Onlays are applied in two dental visits. At the first visit, the old filling, or decay, is removed, and the tooth is prepared for the inlay / onlay. The dentist will then make an impression of the tooth, and send this impression to a dental laboratory. This impression will be used by the laboratory to construct a custom-made porcelain, or gold inlay / onlay. At this time the dentist will place a temporary sealant on your tooth and schedule a second appointment.
Onlays differ from inlays in that onlays cover one or more of the tooth’s cusps, and achieve their retention to the tooth primarily from features cut in the exterior walls of the tooth.
Onlays differ from crowns mainly in the amount of the tooth covered by the restoration.With an onlay the restoration overlaps the cusp tips minimally, extending only about a third of the way down the side of the tooth.
Crown fabrication usually needs a more extensive tooth preparation.
Steps in preparation
The steps in the fabrication of an onlay is more or less similar to that of an inlay with some slight variations.
Before starting of the procedure a general history of the patients health is taken.
1. Local Anaesthesia
Tooth to be restored is usually anaesthetized by using injectable LA around the nerves that supply sensation to the tooth.
2. Impression before the treatment
A preliminary impression (or “mold”) is made of the teeth before they are altered. The material used most for onlay/partial crown impressions is polyvinyl siloxane, a dimensionally stable and extremely accurate elastomer (meaning it’s “stretchy”, but returns to the shape it takes when it cures after a minute or two).
3. Matching of shade
Before starting any treatment for the restoration a matching of the shade has to be done. This will reduce the chances of the restoration colour being different from the natural teeth. All the shade selection procedure is to be carried out in natural light.
4. Isolation
Isolation of the operative area should be carried out by the help of rubber dam.
5. Core Preparation
The tooth is prepared by removing old restorative materials (if necessary), removing any decay, and (if necessary) filling in any deep holes or missing corners of the tooth.
It may be necessary to place small metal (normally titanium) pins in the tooth to rebuild the portion of the tooth that will be covered with the onlay/partial crown. The dentist may use any of a variety of filling materials to rebuild the tooth, including composite resin, glass ionomer, and silver amalgam.
6. Onlay Tooth Preparation
Outer surface of the tooth is reduced in all dimensions (biting surface and sides) by 0.75mm to 3mm to make room for the material that will be placed on the tooth.
The walls of the preparation are tapered to allow the onlay/partial crown to be slipped down over the tooth. Sometimes internal walls will be prepared in the tooth to provide “inlay retention”
7. Post Preparation Impression
An impression of the prepared tooth and the teeth that bite against it is made, taking care to gently reflect the gum tissue away from the prepared tooth.
Dentist sometimes will place “retraction cord” in the trough between the gums and prepared tooth prior to making the impression. The retraction cord coated with special types of chemicals reduces the bleeding or the flow of the salive and there by help in recording the sub gingival margins of the preparation accurately.
The cord will be removed after the impression is made.
8. Placement of temporary crown
9. Lab Work and the final Delivery of the crown
Lab work involves many procedures like cast fabrication , articulation, die fabrication and preparation of wax pattern , production of the actual crown and finally the finishing.
10. Finally the onlay is delivered to the patient and cemented on to the tooth . Before cementation if needed then local anaesthesia is administered to reduce the irritation due to soft tissue manipulation. For cementation usually Luting GIC is used andd the excess cement is removed.
11. Test for High Points and proper Occlusion.
Before sending the patient home a through check up for any high points should be done and it must be made sure that the patient is biting in his previous occlusion.
