Overlays (Biomimetic Crown Alternative)

Overlays – Biomimetic Crown
Alternative

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?

After removal of this failing filling notice the large crack underneath the lingual cusps.

Dr. Schiffenhaus removed the large cracks in dentin as well as decay. He then prepared the tooth for an Overlay – sparing a large amount of healthy tooth structure and enamel. The tooth was bonded with the Immediate Dentin Sealing technique & a ceramic Overlay was bonded over the top. Note the amount of remaining tooth vs a traditional crown preparation.

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The more natural tooth left behind the stronger the remain tooth is and less likely to need a root canal. In fact, for onlays and overlays the weight and volume of tooth removed is 25-40% compared to 65% for an All Ceramic Crown.

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Overlay Preparations spare tooth structure at the base of the tooth. Most engineers will tell you the strength in an load bearing structure is at the base and in the outer walls. The bottom 2/3’s of the tooth are the widest and help the tooth resist damage to the forces of chewing. The outer part of the tooth, enamel, is the hardest substance in the human body and provides the tooth much needed strength. The dentin-enamel complex is an important stress shielding and crack stopping apparatus located at the border of enamel and the inner dentin. Both of these important structures at the base of the tooth are removed during a crown preparation but are conserved for an overlay preparation.

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The Overlay design is a much more fracture resistant design for posterior ceramic restorations. The leading cause of ceramic restoration failure in the posterior is fracture. The crown preparation was originally designed for metals. Metals are not brittle and have some give. Glass ceramics are very strong in compression but are very brittle. Complex geometries, edges, corners, steep walls, and sudden transitions all exacerbate stress and decrease the fracture resistance of glass ceramics. The non-retentive overlay design is an open geometry with a rounded and flowing design minimizing stresses and maximizing ceramic fracture resistance. The upper part of any dome structure is mostly in compression which ceramics handle well. The lower base of the dome experiences much more tension and flexing which is damaging to glass ceramics. The microstructure of enamel and the dentin-enamel complex are designed to mitigate these stresses. Placing bonded ceramic in this lower region can lead to high flexing-tensile stresses on the ceramic and bonding resin that can lead to fractures, debonding, and/or leakage. This is avoided in overlays that stay above this area unlike ceramic crowns. These concepts are expertly covered by Graeme Milicich here for more reading”

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The Ceramic Overlay Preparation stays further away from the nerve by not removing tooth at the gum line. All Ceramic Crown preparations require removal of 1-1.2mm of tooth at the gum line and reduce the remaining dentin thickness to the nerve more than anywhere else in the preparation. The closer to the nerve a preparation gets the higher the risk of pulpal injury and/or future microleakage impacting the pulp health.

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Protection from hidden failures – one of the greatest weaknesses of the overlay preparation is one of its greatest strength. Because the overlay relies mostly on adhesion, if a dentist does not execute the technique sensitive nature of bonding the overlay properly it can pop off over the years. However, a crown has mechanical retention, so if the bond fails it will stay in place. This is not always a good thing…… A restoration that fails internally but stays in place can trick a dentist and patient into thinking everything is ok, and it may not be until the disease process becomes large enough or the tooth fails in a catastrophic way the intervention is performed. It is a philosophical difference but a Biomimetic Dentist prefers the restoration to fail not the tooth. We prefer the tire to go flat ASAP vs blow out doing 70 MPH on the highway. An advantage to the overlay is that if it is leaking or has a poor bond it will come off before much damage is done to the tooth underneath. Even if it doesn’t, it is easier to spot decay with margins high above the gum line and materials that can be easily seen through on X-rays. A crown that stays in place despite the bond failing is no longer stabilizing the tooth and could be significantly leaking. Generally, ceramic crowns with failed bonds typically will break, but metal crowns are cemented and will stay generally stay in place.

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Overlays (Biomimetic Crown Alternative)