Do Dental Composites Always Shrink Toward the Light?

David,

Watched your video of restoring a biscuspid, very interesting system. Did you bulk cure the restoration, and if so did you use the curing light on each side in an attempt to pull the material outward towards the light to compensate for shrinkage? I believe studies have shown that composite does not shrink towards the light, it shrinks towards the strongest bond in the prep boundaries also influenced by the C factor.

Do Dental Composites Always Shrink Toward the Light?: J Dent Res 77(66) June 1998

David

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DR. CLARK'S RESPONSE:

Dear David

You are correct, we do not have any evidence that the composite shrinks toward the light. There is, however, some consensus that the composite that cures first has the first foothold and that it may have an advantage. Because the gingival margin is always the weak link, It doesn't hurt to do all that we can to help it. C factor, the amount of enamel rods and the angle of engagement (saucer versus GV Black walls) are probably more important to determine which margin experiences the most "suck back".

The C factor of the Clark Class II is so low that we can fill in one increment. I am currently doing an article on these topics at the request of 3M which I will send the final draft of if you promise NOT TO DISSEMINATE until it publishes. My 2 hour lecture on the topic will be available shortly on dentaledu.tv (Just taped it this weekend). It gives a very thorough discussion of C-factor and other topics that you have interest in. I am so happy that you have such a good feel for these issues, not too many dentists take the time. Good for you.

Warm regards,
David

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David,

Thanks for the response. I do like playing devil's advocate on stuff like this. I didn't watch all of the videos so I don't know what you do with say an MOD. The case shown is saucer but in restorations that extend well into the central fossa areas it might be a higher C factor. Also when you create the flare for the saucer the ends of polymerization are farther apart so potentially more shrinkage between ends. I remember someone years ago was putting composite over flow and curing together. I cure the flow first so it gets a maximum foothold before the filled resin.

I promise not to share any of the article with anyone until published and would be glad to read it and give you my feedback. There is a huge need for the average general dentist to improve direct composites and I'm glad dentists like you are thinking about it and making it better.

David

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DR. CLARK'S RESPONSE:

For all medium, large, and amalgam retreatment cases I suggest rebuilding the occlusal separately, then prepping the interproximal and restoring each saucer independently so the C factor remains favorable. As far as a larger size impacting shrinkage, everything that we have seen shows that a feather edge on a flattened restoration (assuming enamel margins) will not have any problems. My theory is that a feather edge over a flat surface area encourages the composite to dissipate the stress and shrinkage by simply shrinking toward the tooth instead of toward the center of the composite mass and since there are no opposing walls in the prep, the tooth is too strong to accept any stress,

So in the end, no white lines no micro fracturing,
Think about placing a large thin composite on the facial of an upper incisor to cover decalcification or stain. The margins look perfect year after year.

Dentin is another story.

Cheers