There are a couple reasons that old filling will fall out. Your dentist will run several tests to help know if this is going to happen. This can be accomplished through x-rays, air tests, and other tests that are done to check for weaknesses in the fillings.

Decay is a major reason that old fillings will fall out. This decay can be caused from several different reasons.

When chewing there is a large amount of pressure that will also over time create a weakened state of filling and will cause it to become loose and possibly fall out.

An average healthy filling will last about seven years. If however there are problems like a leak, crack or if it just falls out. Fillings just do not last forever.

There are also other reasons that fillings fall out. If a tooth was not repaired correctly, or if the filling was not put in correctly, it will fall out. An amalgam filling will fall out if they are not placed in correctly as it is the tooth structure that holds the filling in.

Component fillings are bonded to the tooth, so they are less likely to fall out from improper placement, however they can still where or break.

Saliva is another cause that a filling will fall out. This is because the saliva has bacteria, and therefore if the saliva gets in the way of the filling before it is completed, it will disrupt the bonding process of the filling.

Cosmetics are another reason that a person would have an old filling removed. The discoloration and look of an old filling may cause self-conscious problems and would then be beneficial for removal and replacement.

If a filling is not shaped right, whether it is an amalgam filling or a component filling, it can crack and then fall out. If the filling is higher than the rest of the tooth, this can happen. The regular force of chewing can cause this to happen.

There are also problems where a large amalgam filling is in need for a crown to protect them. With out the crown, the filling is likely to fall out over time.

Fillings tend to pull away from the enamel of the tooth. This is called leaking. Leaking is another problem that can cause a filling to fall out. Leaking is a problem that occurs because the side of the tooth does not meet up with the filling correctly. This can allow bacteria, food, saliva and other contaminants to get into the filling, and then will cause decay that will result in the possibility of the filling falling out.

In order to help fillings last, it is important to follow good oral hygiene steps. This would include going to the dentist regularly, brushing and flossing three times daily, and if you see a crack, feel sensitivity, or become aware of any missing parts to your filling make an appointment with your dentist right away.

White fillings tend to need to be replaced more often than the standard silver.
Whether your filling last for five years or fifteen years, it is important to keep your visits with your dentist so that when the time comes, they will know when the filling needs to be repaired or replaced. The tooth will be assessed at the time of replacement and you may find that the dentist may use the same or maybe a new better filling option.