When your decayed tooth needs more than a filling but less than a crown to repair it, your dentist will use either an inlay or an onlay to bring your tooth back to a healthy state. Inlays and onlays are often referred to as partial crowns, and use the existing tooth as a base and fit the inlay or onlay onto the tooth. This is done to strengthen the tooth, restore its shape, and prevent further damage.

An onlay extends beyond the rim of the cusp, going over the tooth cusp or involving more than one cusp. Onlays are used when the cavity or damage reaches one or more cusps of the tooth or teeth. This often involves reconstruction of the cusp, which has been worn or broken away.
If the cavity or tooth damage is more severe, effecting more than the area within the cusps or just beyond, it may be necessary to have a crown placed. However if it is confined, a less invasive onlay or inlay may be used.

Unlike a traditional filling, inlays and onlays are created outside the mouth. Their shape is formed and hardened. Then it is adhered to the tooth, allowing precise shaping and design. Traditional fillings are placed in the tooth and then allowed to harden there. The other benefit of working outside the mouth first is that dentists can ensure that the composite used to create the inlay or onlay matches the color of your natural teeth. This gives a seamless transition between your teeth and the restoration, giving you a more comfortable, confident smile.