While inlays are placed in the middle of the tooth, and onlays fills in the damaged tooth in place where the cusps of the tooth have to be augmented,dental overlays cover both inner and outer sides of the cusps of the tooth.
These three restorations are considered indirect prosthetic fillings, which are made in a dental laboratory on the basis of the teeth impression. They are only differentiated by the size and the amount of the tooth covered.
Although a dental overlays covers the whole cusp, it is more conservative than a conventional dental crown, and has better properties of biocompatibility because it is a supragingival restoration.
Like inlays and onlays, the placement of overlays can be performed in two dentist visits. A temporary prosthetic is placed during the preparation of the final overlay, which is permanently cemented during the second appointment, after being checked whether color and shape are suitable.
Dental overlays allow maximum preservation of healthy tooth tissue because this technique requires less tooth grinding. They are stronger and last longer than conventional fillings.
It is recommended to use a dental prosthetic that suits to the amount of tooth that needs to be restored, because using a dental filling of a size bigger than required can weaken the tooth structure, causing the tooth to break. This is why the dentist has to be very accurate when choosing whether to place an inlay, onlay, overlay or a crown.