How deep a chamfer or shoulder is needed for full thickness zirconium or lithium disilicate crown?


 Dentists, like most people, are creatures of habit. For many years we have been told by laboratories to provide 1mm shoulder or chamfers for our ceramic crowns. While this may make it easier for our labs to fabricate restorations, but now that they are predominantly fabricating milled zirconium and lithium disilicate restorations this amount of chamfer depth is more than the minimum needed. For those of my readers who are wondering what a chamfer or shoulder refers to, it is a shelf that provides the finishing line for this type of restoration.  Since this area is not the part of the restoration that has to deal with occlusal forces, probably all that is needed is about  a .5mm deep chamfer (or shoulder).  Since conserving tooth structure can ultimately influence the success or failure of a crown and the tooth supporting, preserving tooth structure is very desirable. The deeper our chamfers are, the closer we get to a tooth pulp when we are preparing a vital tooth (one without a root canal). Also, the teeth supporting teeth are subject to flexing forces and are more likely to sustain a fractue when deeper chamfers are used. This is especially true when we are dealing with a tooth that has a root canal, since it already has been hollowed out by the required hole created to provide endodontic access. 

An argument could be made to make a slightly deeper chamfer to allow for additional digital relief of undercuts caused by preparations with small undercuts so possibly .75 mm could be an advisable depth to compensate for small undercuts at the base of a preparation.  That being said, a far better time to correct undercuts is before taking a a final impression. This can be accomplished with small additions of bonded composite to eliminate undercuts in the preparation.

Clearly it is time for dental schools to change their core curriculums to encourage shallower more conservative crown preparations especially for all ceramic crowns. Yes, most crowns still benefit with more generous occlusal clearance (1.25-1.5 mm) and most lab technicians still prefer that 1 mm of clearance should be ideally provided for the area of the incisal 1/2 of the preparations used for all ceramic restorations. They feel this amount is desirable because it helps them create more esthetic restorations as well as making them less prone to fracture.




from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2022/02/how-deep-chamfer-or-shoulder-is-needed.html - http://lspindelnycdds.blogspot.com/

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