What is the difference between an onlay and a filling?

An onlay is an restoration that is made in a dental laboratory that is designed to protect a tooth that whoose tooth structure has been compromised by decay or by the preparation made for prior fillings. Usually onlays cover the chewing surface of the tooth including the cusps . The design of an onlay can protect a tooth and make it less likely to suffer a future fracture. When designing an onlay, a dentist must create enough clearance between the tooth being restored and the opposing arch. The ideal amount of reduction of the occlusal reduction is approximately 1.5 -2mm,; enough thickness to allow for a durable restoration. When designing the onlay preparation a dentist should remove any prior tooth structure as well and often onlays can become partial coverage crowns. They differ from crowns in that more tooth structure is left intact on a tooths buccal and lingual surfaces.

It used to be that onlays were a two sitting procedure, but now with the advent of cad cam technology, it is possible for a 'high tech' dentist to prepare and place an onlay in one sitting. This technology is expensive and most dentists have not yet invested in purchasing an on site cad cam milling machine.

Onlays can be cemented in with a number of different types of dental cements. At present most ceramic or composite onlays are 'bonded in' with a composite resin cement. When cementing in gold onlays dentist have a wide variety of possible cements to choose from. I personally use old fashioned, but tried and true dental cements that are resin free.

Fillings, on the other hand, are usually inserted directly in the tooth during one sitting. They are not made on a bench or in a cad cam milling machine, but placed in the mouth by the dentist. They can be made from a variety of materials, including amalgam, composite, glass ionomer, and even gold. With care, they can also be used to onlay and protect cusps, but because of the difficulty in placing well made large multi surface fillings, usually laboratory onlays or crowns are a better choice for a dentist to use when restoring a tooth with compromised tooth structure.