Indirect fillings

What Are Indirect Fillings? Indirect fillings are similar to composite or tooth-colored fillings except they are made in a dental laboratory and require two visits before being placed. Indirect fillings are considered when not enough tooth structure remains to support a filling but the tooth is not so severely damaged that it needs a crown.

 During the first visit, decay or an old filling is removed. An impression is taken to record the shape of the tooth being repaired and the teeth around it. The impression is sent to a dental lab that will make the indirect filling. A temporary filling (described below) is placed to protect the tooth while the restoration is being made. During the second visit, the temporary filling is removed, and the dentist will check the fit of the indirect restoration. Provided the fit is acceptable, it will be permanently cemented into place.
There are two types of indirect fillings -- inlays and onlays.

• Inlays are similar to fillings but the entire work lies within the cusps (bumps) on the chewing surface of the tooth.
• Onlays are more extensive than inlays, covering one or more cusps. Onlays are sometimes called partial crowns.

Inlays and onlays are more durable and last much longer than traditional fillings -- up to 30 years. They can be made of tooth-colored composite resin, porcelain, or gold. Inlays and onlays weaken the tooth structure, but do so to a much lower extent than traditional fillings.
Another type of inlay and onlay -- direct inlays and onlays -- follow similar processes and procedures as the indirect, but the difference is that direct inlays and onlays are made in the dental office and can be placed in one visit. The type of inlay or onlay used depends on how much sound tooth structure remains and consideration of any cosmetic concerns.

http://www.webmd.com/oral-health/guide/dental-health-fillings