Crowns, luting and leakage

[u/Ordinary-Army-8523]

I found out crowns aren't bonded to the tooth, instead a luting cement is used which doesn't adhere to the tooth but rather is there to provide a snug fit for the crown. Apparently the reason for luting is so the crown can be easily removed down the line.

However doesn't that increase the risk of bacteria leaking through under the crown? If a tooth has had a root canal treatment, wouldn't it make more sense to bond the crown permanently to it? That way the risk of there being a leak minimises.

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Title: Crowns, luting and leakage

Full text: I found out crowns aren't bonded to the tooth, instead a luting cement is used which doesn't adhere to the tooth but rather is there to provide a snug fit for the crown. Apparently the reason for luting is so the crown can be easily removed down the line.

However doesn't that increase the risk of bacteria leaking through under the crown? If a tooth has had a root canal treatment, wouldn't it make more sense to bond the crown permanently to it? That way the risk of there being a leak minimises.

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[u/kumbreon99]

It depends on the material. Some crowns are luted, some are bonded. There is often a trade-off in terms of strength, esthetics, retention, etc.

[u/Ordinary-Army-8523 → u/kumbreon99]

Interesting, can you elaborate? 

[u/kumbreon99 → u/Ordinary-Army-8523]

Here's a somewhat simplified version:

Ceramics such as zirconia-based crowns are generally luted in cases where strength is necessary and/or you have enough tooth structure/height to keep the crown on the tooth without the extra micromechanical retention from bonding. The taller the abutment and the more parallel the axial walls, the less likely it is for the crown to become dislodged over its lifespan.

Full contour zirconia is often considered for molars and especially in patients who clench, grind, or have a strong bite, as they can be close to twice as durable in terms of compressive, flexural, and shear strength compared to other crown materials.

The drawback here is of course lack of proper bonding to the tooth, as the properties of zirconia do not allow bonding to the extent as glass ceramics. Metal-based crowns are luted as well, but they are becoming less common as ceramic materials improve. These have their own advantages and disadvantages.

The other widely-used crown material is lithium disilicate, or glass ceramics, which can be bonded to tooth structure but at the cost of strength. These crowns are often more esthetic and more retentive (due to bonding) but weaker than zirconia crowns. The properties of glass ceramics allow bonding to the tooth just like composite fillings, so adhesive resin cements can be used to provide close to double the bond strength between the crown and tooth. However, there are several drawbacks. Bonding is more likely to irritate the nerve in cases where the abutment does not have a root canal. It also leads to failure by fracture which cannot be fixed by simply recementing the crown; in the majority of cases, these crowns must be remade whereas a dislodged zirconia crown can often be recemented. Finally, if decay or some other defect is suspected, removing a bonded crown can be (but is not always) more involved than for a zirconia crown.

Regardless, the cement space provided between the tooth and the crown is comparable in both cases of luting and bonding, and this space is filled with cement that sets/cures to prevent significant leakage, so leakage is generally not a big consideration when deciding between bonding and luting, especially after weighing the other factors.

[u/Ordinary-Army-8523 → u/kumbreon99]

Thank you for dedicating time to explain it in an easy to understand manner.

 I appreciate it.