Amalgam fillings do not stick to the tooth. To retain the filling in the tooth, the cavity must be prepared with 'undercuts'. These undercuts not only lock in the amalgam filling but also cut off the nutrient supply to the dentine above the cut.

Therefore the tooth structure above and to the side of the filling becomes brittle.
All metals in the mouth will undergo some corrosion. Amalgam also corrodes at a reasonably fast rate. When amalgam corrodes it also expands and it does so in all directions.

The force created by this expansion will often create minute fractures in the tooth that is already more brittle due to the shape of the cavity preparation. At this stage the patient returns to the dentist to report that all they were eating was some soft bread and the tooth broke!

To repair such a problem, dentist will usually drill a small hole into the dentine and insert a self-tapping screw - called a pin. The pin is reinforcement for the amalgam filling which will go back in. Even if this pin is made of titanium it will undergo corrosion when in contact with amalgam.

Again the corrosion will cause an enlargement of the pin (sometimes up to five times its diameter) which will then crack the tooth further - but this time lower down the root surface.

This tooth is now a candidate for a crown because the filling, which has to go back into the tooth, is now so large that it cannot sustain the forces of chewing for very long.

Composite fillings do stick to the tooth. They are bonded chemically and mechanically to the tooth. They do not require a cavity, which is undercut and therefore do not require such a large or damaging cavity. In fact a composite filling can be used to rebuild a broken cusp without the use of pins or other mechanical support. I personally have not used a pin for years and have had great success with such restorations.

Studies comparing the fracture resistance of the tooth when filled with amalgam or composite indicate that amalgam will weaken the tooth structure whereas bonded composite fillings will strengthen the tooth. There is absolutely NO reason to continue the use of mercury amalgam!

Secondary Decay Under Fillings
Another bit of misinformation, which is often touted about by the dental associations, is that secondary decay is much greater with composite fillings than amalgam. This is completely false. When amalgam corrodes it not only does so on the chewing or exposed surfaces, but also corrodes on the side, which is in contact with the tooth- the deep part of the cavity.

The corrosion products react with the calcium and phosphorous in the tooth, with the formation of hydrochloric acid. This acid then dissolves the tooth structure which is called secondary decay. The newer term for this is Crevice Corrosion. This does not happen with composites.

Toxicity
Mercury is one of the most toxic substances known to man. Amalgam is made of 50% mercury which leaches from the set amalgam all of the time. Recent research is indicating that the breakdown products of composites and glass ionomer cements are between 300 times and 1.6 million times below the Tolerable Daily Intake levels. By comparison the mercury from amalgam is about 4 times greater than the Tolerable Daily Intake levels.
Although different people may show sensitivity to different composites, they are not subjected to the high level of poisoning as with dental mercury amalgam.