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?
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.
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.
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”
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.
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.