Larger than a filling, smaller than a crown
Fillings are ideal when a tooth has a small cavity. But when tooth decay or fractures extend deeper into teeth, trying to patch it up with a filling only places the tooth at a higher risk of fracturing and cracking during everyday use. At this point, the majority of general and cosmetic dentists progress to a full coverage dental crown that protects the entire tooth, even if there is significant healthy structure remaining.
Sometimes fillings are not effective, but crowns are too aggressive than what’s truly necessary. It’s at this point where dental inlays and onlays come into play. An inlay or onlay offers greater protection and durability than a large filling but is significantly less invasive than a full coverage “cap” which requires reshaping of the tooth entirely. Yet, many dentists do not offer inlays and onlays in their practice.
Filling in the fissures/valleys and grooves
Inlays are sometimes called “lab made fillings” because of their purpose and design process. An inlay essentially fills in the valleys and hollows inside of a tooth, as long as the cusps around it are still intact. It’s almost like a large filling, except more durable than a standard composite or amalgam (metal) restoration.
Because inlays restore such a large portion of teeth, they’re made in a lab like a crown. First, the compromised tooth structure is removed, then an impression or digital scan is taken. The impression/scan is transferred to the lab, where a technician designs it by hand or has it milled electronically. Because it’s made from a solid piece of ceramic or zirconia, the inlay is extremely durable and acts to strengthen the tooth, rather than pose a risk to the enamel that’s left.
Covering the cusps and biting edges
Onlays cover the cusps and ridges (or biting edges) of teeth, rebuilding enamel that’s been chipped or broken off. When it comes to recreating the shape of compromised teeth, an onlay is more like a traditional crown, except it does not cover the full tooth surface.
Onlays are sometimes called “three-quarter crowns” because of their ability to cover such a large area, without encompassing the entire tooth. As such, they’re more conservative than traditional crowns, allowing you to preserve as much healthy, natural tooth structure as possible.