What is a crown?
A crown is an artificial ‘cap’ or ‘helmet’ that covers the entire tooth after removal of existing decay.
What is an onlay?
Onlays are often referred to as partial crowns. When a tooth is too damaged to support a normal filling but not damaged enough for a crown, you end up with something in between the two, that is a ‘partial crown’, also known as an onlay.
What is the difference?
Making a full crown may sacrifice more healthy tooth structure than required. Having said that, the retention of a crown is superior to that of an onlay.
Modern dentistry places a lot of emphasis on being as minimally invasive as possible. Preservation of tooth structure is now seen as more important than the durability of the restoration. Tooth structure that is removed with the drill is permanently lost whereas typically, dental restorations can be replaced easily. This is the reason onlays are popular nowadays.
When is it needed?
Both crowns and onlays serve to protect the remaining tooth structure from deeper cracks. For patients with severe attrition or with a history of cracked teeth, onlays/crowns are often advocated even before cracklines appear, as they can reduce the amount of future damage done to the tooth and the pulp (nerve) of the tooth.
All back (posterior) teeth that have undergone a root canal treatment will require an onlay or a crown. This is because root canal-treated teeth are more fragile and have a higher risk of fracture.
What materials are used?
The main categories of of materials for crowns and onlays available are:
- all-ceramic materials (which are highly aesthetic)
- porcelain-fused-to-metal (PFM)
- base metal alloys
The material of choice is selected on a case-by-case basis.