Direct bonded composite restorations are a healthy, beautiful way of restoring a tooth with a cavity or when a silver/mercury filling has been removed. It has many advantages over a silver/mercury filling besides being healthier. It bonds to the tooth, strengthening it internally. Silver/mercury does not bond to the tooth so bacteria can creep in an attack the tooth internally. It expands and contracts at a rate similar to the tooth so there is no potential of it cracking the tooth like silver/mercury, which expands and contracts much differently than the tooth.
Direct bonded composite restorations are very technique sensitive, and require a protocol much different than silver/mercury. The tooth must be completely isolated from saliva and bacteria contamination during the cleaning, disinfecting and bonding process. The composite is built up in small increments, restoring the tooth in small sections. This technique puts much less stress on the surrounding tooth structure. And last, the bite must be meticulously refined so the tooth/composite bond stays intact for the long term.
There are limitations in terms of the size that the restoration can be for maximum longevity. If it is a situation that exceeds what bonded composite can predictably do, a porcelain onlay may be recommended.