Overlays – Biomimetic Crown
- The Overlay, also known as the “crown-lay”, “occlusal veneer”, “partial coverage crown”, and sometimes “Onlay” – can be considered the modern adhesive alternative to the full crown preparation. At our practice, the only time we place full coverage crowns is to replace existing ones. An overlay is a minimally invasive restoration used in place of a full coverage crown for most of the same indications.
- What is an Overlay? First, let’s explain what a crown is…… Historically, whenever all of the cusps (the pointy parts of teeth) needed to be covered due to extensive internal damage or fracture, a crown was placed. The reason for this is that when covering the entire top of the tooth is necessary prior to the invention of dental adhesives, the only way to keep the cover in place was through mechanical retention. The best way to keep the cover in place was to “cap” the tooth which is retained much like the cap on a person’s head. Resistance from the side walls much like the side of one’s head keeps the cap in place. The dental cement simply increased the friction to hold the “cap” in place. This proved to be a very durable restoration, and when done right could stay in place for decades. Unfortunately, in order to prepare a tooth for a crown, healthy tooth structure on the sides of of the tooth needed to be removed down to the gum line to make room for the metal crown. Like most traditional that used metals and required mechanical retention to keep them in place, excessive healthy tooth structure needed to be removed to ensure the longevity of the restoration.
- In the 21st century, we can use modern dental adhesives to bond ceramic and composite restorations too the tooth, no longer requiring removal of excessive healthy tooth structure for mechanical retention of our restorations. Therefore, from a minimally invasive perspective, when the entire occlusal surface needs to be covered an overlay (occlusal veneer) can be used in place of a full coverage crown. The overlay design simply covers the top of the tooth and some of the sides leaving 1/3-2/3 of the front an back of the tooth untouched. This has several advantages:
A full ceramic crown preparation removes 70-75% of the weight of the coronal tooth structure
Full crown preparations generally result in the removal of the hard outer enamel of the tooth
An Overlay preparation removes in 32-47% of weight of the coronal tooth structure and results in greater distance between the restoration & the nerve – both factors are correlated with a decrease in risk of future root canal treatment
Cusp sparing Onlay preparations result in even less tooth structure removal
Here is what a tooth looked like underneath an old relatively well “sealed” metal crown
Here is what a tooth looked like underneath a 15 year old fractured onlay – notice the tooth is still clean and sealed from the adhesive resin used for bonding
Here is a metal crown that had a lot of decay underneath, unfortunately even though this had been leaking for years, the mechanical retention of the crown keptunderneath it in place masking the extent of decay growth
Overlays & non-retentive ceramic Onlays will debond (pop off) or break if a significant gap is formed & leakage occurs – additionally X-rays can see through ceramic restorations unlike metal, so decay processes can be spotted more early
Which of these two preparation designs look better able to handle a load? Which looks stronger and more resilient to fracturing?