Many times there is too much missing tooth structure for a filling but enough remaining to contraindicate a crown. Fillings should occupy less than 30% of the top of the tooth.  You can see in the pictures at the right a molar and a premolar tooth with approximately 30% of the tooth covered by a filling.

The advantage of a filling is that they are quick and easy to place in addition they are relatively inexpensive.  In order to have the ability to place the filling material as a putty and "cure" it with an ultraviolet light certain chemical properties are necessary.   In order to have the desirable properties we have to accept certain physical properties.

Stregnth
Fillings lack the strength to properly replace more than 30% of a tooths structure.

Durability
Fillings wear away faster than natural tooth structure.

Stability Over Time
Fillings shrink or expand with time, in addition they are subject to shape changes in response to temperature changes in the mouth, called thermo cycling.  While natural teeth also thermo cycle, the rates of shape change for fillings and teeth are different.

Size of the Filling
The size of the filling determines how much these properties will effect the lifespan of the filling.  The larger the filling, the shorter it's lifespan.  As stated before if a filling occupies less than 30% of a tooth the undesirable physical properties are mitigated.

Inlays & Onlays
Between 30% to 60% of the tooth is ideal for an INLAY or ONLAY.  Inlays and onlays are made from gold, ceramic or composite. They are much stronger and more stable than filling material.

Because they are made in a lab they do not shrink or expand with time, in addition the physical properties are much like natural tooth structure.  With proper care an inlay or onlay will last as long as a crown.  In fact, I have seen gold inlays that were placed in the 1950's that are in perfect condition today!

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Lower molar 30% of tooth with most structure intact, perfect for a small filling.	Lower molar 40% of tooth with most structure intact, perfect for a small filling or a conservative inlay.
Lower molar 50% of tooth missing with all cusps intact, gold or ceramic inlay preferred. The major load bearing part of the tooth is being replaced.	Lower molar 50% of tooth with one cusp missing. Gold or ceramic onlay preferred to replace missing cusp and load bearing center of tooth.
Bicuspid with 30% of tooth structure.	Bicuspid with 50% of tooth structure missing, Conservative inlay can be used but is best served with a full coverage crown due to small size of tooth.