UNSUPPORTED ENAMEL! We could have all left Op1 lecture much sooner last Tuesday morning if any of the students had figured this out. We assistant professors in the back of the auditorium had our fingers crossed! But just repeating a phrase without understanding what it means won't help anyone succeed as an operative dentist. Perhaps it's hard to understand enamel - and its need for support - when the teeth you treat are plastic. (I must add that after yesterday's lab assignment, the concept of unsupported enamel is already making more sense!) So let's dive into the textbook definition of unsupported enamel and see how it relates to the all important concept of margin strength in tooth preparation. This next edit is a rather extreme example, but if you create too much of an undercut in your prep, and the wall is too convergent, you'll have unsupported enamel. See the cut rods highlighted in orange? They are not supported by dentin and can easily fracture away. I really love Dr. Mark Winkler's analogy using carpet to explain this whole concept. I'll see if I can illustrate it here. Think of the carpet backing as the DEJ - the dentinoenamel junction - and think of the carpet fibers as the enamel rods. Where the cavosurface angle is greater than 90 degrees - in green - there is no unsupported enamel, and all of those fibers, even the cut fibers, can remain firmly attached to the backing. In the purple highlighted area, those fibers have been cut away from the backing and can easily be vacuumed right up. *I'll go into more detail on proper resistance form in future posts.
Carpet graphic borrowed from this site.
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AuthorA dentist and educator, sharing resources and experiences with her colleagues and students. Archives
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Side note - whenever I refer to Sturdevant or Summitt, I'm citing one of these two electronic textbooks:
- Heymann, Harold, et al. Sturdevant's Art and Science of Operative Dentistry. 6th ed., Mosby, 2012. - Hilton, Thomas J. Summitt's Fundamentals of Operative Dentistry: A Contemporary Approach. 4th ed., Quintessence, 2013. |