The two most popular materials used for dental fillings are amalgam or silver alloy and composite resin. Gold or porcelain can be used as a filling too. Called inlays or onlays, made in a dental laboratory, cement in place, making them a permanent restoration. There are many reasons for failure of these restorations. Most failures are a result of mishandling or contamination of the restoration area by saliva or other contaminants.
Amalgam Filling Failure
Amalgam is silver alloy containing other materials for strength. Mercury is the liquid ingredient that binds everything together. Amalgam gets mixed in small pre-filled capsules that get attached to a machine called an amalgamator. The capsule is vigorously shaken by the amalgamator, making the amalgam the right consistency to be placed in the tooth and compacted with other instruments into a filling. There are many reasons for amalgam to fail. The most common failure is the amalgam is too wet or too dry. This happens when the amalgamator is not calibrated properly, mixing the material for the wrong amount of time. Another failure comes from amalgam setting too quickly during use. The most common reason for failure is checking the bite. If the bite is too high the amalgam can fracture; necessitating removal and the process repeated.
Composite Filling Failure
Composite resin fillings are more complicated than amalgam fillings. They require the cavity prep to be etched with acid. After the acid comes an application of bonding agent to the surfaces receiving the composite resin. The bonding agent gets cured or hardened with a handheld ultraviolet light. Composite is then placed in the prep with a composite gun and again light cured. The filling is then smoothed with a diamond burr on a high-speed hand piece. The most common causes of failure in composite are contamination of the area with saliva. Once the etching is complete, even the most minimal amount of saliva will spoil it, causing failure of the filling, nothing will bond to contaminated etch. The same is true of the bonding agent, if it becomes contaminated with saliva, the composite will not bond. Another reason for failure is if composite is old. It loses its flexible texture and starts to thicken.
Failure of Onlay and Inlays
Onlays or inlays are essentially cast gold or porcelain fillings. An onlay is a cast filling that only involves surfaces of the tooth, not between teeth. Cast inlays are fillings that include proximal -- between the teeth, surfaces. These fillings require impressions and a dental lab to make them. They get cemented into the mouth either with cement or composite bonding. The most common failure for these fillings is a distortion in the impression that causes the inlay or onlay not to fit properly. New impressions must be taken and the filling remade. Other failures are contamination of the area with saliva, especially when using the composite bonding technique. Contamination of the acid etch or bond will cause failure to maintain the inlay or onlay in place. Failure also comes from cement. If the cement is not measured in the proper dimensions, or not given appropriate time to set before attempting to clean the excess, the cement can fail to hold the filling.
Crown or Bridge Failure
Cast crown restorations made of gold or porcelain on metal cover the entire tooth they are restoring. Cast bridges behave the same as crowns, with the added benefit of replacing a missing tooth between two crowns. Both types of restoration require impressions and a dental laboratory to make them. The most common failures are a distorted impression, causing the crown or bridge not to fit, causing the need for a remake. Other failures come from saliva contaminating the tooth right before cementation of the restoration. Another cause for failure is improper dimensions measured of cement. The measurement must be correct for the ultimate adhesive strength when used. Outdated or expired cement is also a cause for failure.