Inlays and onlays are known as indirect fillings because unlike a standard filling that is done in a dentist's office, both are made in a laboratory and cemented or bonded to the surface of the tooth during a second visit to the dentist.
And unlike standard fillings, inlays and onlays do not weaken the tooth structure, but actually strengthens it. After the procedure, the tooth can bear up to 50 - 75% more chewing force.
An inlay is done when the tooth structure replaced is within the cusp tips of the tooth. If the damage is more extensive and the new structure covers the entire chewing surface including one or more tooth cusps, the procedure is called an onlay.