The Truth About Depth of Cure

Let me ask a question: Shouldn’t a measurement for depth of cure (DOC) measure how much resin has actually polymerized (cured) at a certain depth? Thinking logically, the answer is obviously yes. So one would logically expect the so-called International Standards Organization (ISO) Standard No. 4049 test for DOC does just that, right? Incredibly, it does not. In fact, it doesn’t measure the amount of polymerization (carbon conversion) at all. That’s why I called it a “so-called” DOC. All one has to do is look at how the test is done to understand what it does and does not do. The ISO Standard No. 4049 test cures a column of composite resin, scrapes away the uncured composite, and measures the distance from the top surface to this point. This distance is then divided by 2 to determine the DOC (Figures 1a to 1g). So why isn’t this measurement the true DOC for a given composite resin product? One reason is that dividing the length of the hard composite column by 2 is arbitrary, but the real reason is that there is no determination of the amount of polymerization at this level. Thus, it could easily underestimate the true DOC, and we now know that it usually does. I am not condemning the ISO Standard No. 4049 as a totally useless test. Whereas it doesn’t give a true DOC measurement, it does allow the relative comparison of curing potential (or “curability”) among different composites. An example of this appears in a recent ADA Professional Product Review publication.¹ In this report, a study by Tiba, et al, entitled “A Laboratory Evaluation of Bulk-Fill versus Traditional Multi-Increment-Fill Resin-Based Composites,” tested DOCs for various products according to the ISO Standard No. 4049 and according to the Knoop Hardness Ratio test. An abstract of this report also appeared in the Journal of the American Dental Association [JADA]).² In the more complete report,1 the authors tested Filtek Supreme Ultra (3M ESPE), Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) and SonicFill (Kerr). The DOCs determined using the ISO Standard No. 4049 method were 2.63 mm, 3.32 mm and 3.67 mm respectively. What this means is that SonicFill cures to a relatively deeper level than Tetric Evo­Ceram Bulk Fill, which cures to a relatively deeper level than Filtek Supreme Ultra. It does not mean that these materials have a specific level of polymerization or carbon conversion at those depths. In my opinion, this is a serious flaw in using this test method when determining actual DOC. It simply does not tell the clinician the thickness that a given layer of composite can be placed, light-cured, and expect the bottom of the layer to exhibit adequate cure (polymerization). There is a DOC test that will do that and is, therefore, clinically relevant. It’s the Knoop Hardness Ratio test. The results using this test were also determined in the same report.¹ To review, this test cures a column of composite and measures the hardness at different depths. The depth at which the measured hardness value is 80% of the top hardness value is considered the DOC (Figures 2a to 2e). The validity of this test rests on the paper by Bouschlicher et al.³ These investigators found that there was 90% carbon conversion, a true measurement of polymerization, at the point where the hardness value was 80% of the top hardness value. The bottom line is that this test does tell clinicians the actual thickness of a layer of composite that can be placed which results in the bottom of the layer being adequately cured. Dentists accept the fact that it is clinically acceptable if the bottom of the composite layer placed cures to 90% when compared to the top of the layer. According to the investigators of the above report, the hardness value at the bottom of a 5-mm column of SonicFill is 98 +/-1% the hardness value at the top, and for a 4-mm column of x-tra fil (VOCO), it was 98 +/-1%. These values are remarkable when compared to the limited DOCs for previous generations of composites. Also, composite resin products like these with values significantly higher than the minimum 80% (90% carbon conversion) give a large margin of safety to practitioners. This is especially important if compromised curing lights or curing techniques are used. Of course, other factors such as managing shrinkage stress, adaptation, and overall physical properties are equally important when selecting a composite resin for posterior direct restorations.

Finally, although the ADA Professional Product Review1 evaluated DOCs for various composite resin products using both testing methods, there was no mention of the relative merits of the 2 testing methods. It is important to note that the same authors of the report1 presented a paper⁴ at the 2013 International Association of Dental Re­search meeting earlier this year in Seattle. The final statement in this abstract says, “This study shows some limitations of ISO 4049 for testing the DOC in relation to the more important hardness ratio for bulk fill composite materials.”
I have written this editorial because this is an important distinction and one which, in my opinion, readers of ADA Professional Product Review¹ and the shorter JADA² abstract would benefit by being made aware of.
Bulk fill products have added a new dimension (literally) to the placement of posterior composite restorations, and manufacturers of these products are to be commended for investing in the necessary breakthrough research to develop them. Whereas the ISO standard No. 4049 seemed to serve adequately in the age of limited DOC, it no longer does so with these new composite resins. It is hoped that organized dentistry and especially the ISO will recognize this and make the change to a single and more accurate method for measuring DOC that is necessary to keep up with evolving technology. Dentists need a clinically relevant method to rely on when selecting and using these new products.

References

1. Tiba A, Zeller GG, Estrich CG, Hong A. A laboratory evaluation of bulk-fill versus traditional multi-increment-fill resin-based composites. ADA Professional Product Review. 2013;8(3):13-17.
2. Tiba A, Zeller GG, Estrich CG, Hong A. A laboratory evaluation of bulk-fill versus traditional multi-increment-fill resin-based composites [Abstract taken from ADA Professional Product Review. 2013;8(3):13-17]. J Am Dent Assoc. 2013;144(10):1182-1183.
3. Bouschlicher MR, Rueggeberg FA, Wilson BM. Correlation of bottom-to-top surface microhardness and conversion ratios for a variety of resin composite compositions. Oper Dent. 2004;29(6):698-704.
4. Tiba A, Zeller GG, Estrich CG, Hong A. Examining the Depth of Cure for Bulk Fill Composite Materials, Abstract No. 2435. Paper presented at: 91st General Session and Exhibition of the International Association of Dental Research; March 22, 2013; Seattle, Wash.

Dr. Jackson is a 1972 graduate of West Virginia University School of Dentistry. He has published many articles on aesthetic adhesive dentistry and lectures across the United States and abroad. He is a Fellow in the AGD, a Fellow in the American Academy of Cosmetic Dentistry, a Diplomate in the American Board of Aesthetic Dentistry, and is director of the Mastering Dynamic Adhesion Program at the Las Vegas Institute for Advanced Dental Studies. He practices in Middleburg, Va, emphasizing comprehensive restorative and cosmetic dentistry. He can be reached at (540) 687-8075 or ron@ronjacksondds.com.

Disclosure: Dr. Jackson was a consultant in the development of SonicFill (Kerr) and has a financial interest in the product.