Is polymerization shrinkage stress a good thing?
Is polymerization shrinkage stress a good thing?
Apparently it depends on who you ask.
In 2004 Versluis et al published an 11 page paper titled Residual Shrinkage Stress Distributions in Molars After Composite Restoration. Within their carefully worded analysis, they emphasized “Calculation of shrinkage stresses in a tooth-restoration complex is not trivial.”
18 years later, Dr. David Clark teamed up with Dr. Richard Young at the Seattle Study Club Symposium and trivialized polymerization shrinkage stress to promote the so-called “Bioclear Method” using 3M Filtek One bulk fill composite and anatomic mylar matrices (Bioclear Matrix). Clark cited a rejected and unpublished study funded by 3M to try and convince the audience that “polymerization stress is a good thing.”
The study involved a series of finite element analysis (FEA) diagrams created by researcher Alex Fok, PhD, MSc and colleagues at the Minnesota Research Center for Biomaterials and Biomechanics. To captivate the audience, Clark made statements like “the bulk of the restoration is going into compression... that dark blue is compression.” He writes off the red “scatter” in areas of higher stress (pulpal floor) where you’re likely to see debonding of composite over dentin.
I sent Alex the YouTube video, which has since been removed, and asked him if he agreed with Clark’s analysis of his work and/or use of the diagrams to promote the Bioclear brand:
“I have no financial interest in Bioclear or their partnership with 3M, and I haven't been following David's presentations. But I've seen him misrepresent other people's work, notably the effect of crack length, rather than depth, on the propensity for crack propagation. Hopefully, with our paper published, people can make their own judgement… We only submitted the FEA portion for consideration. It was rejected for lack of experimental results to validate the FEA. That's what I'd need to add to the paper before resubmission.”
Regarding misrepresenting people’s work, Fok is referencing Clark’s popular reference to J.E. Gordon’s book The New Science of Strong Materials: Or Why You Don’t Fall Through the Floor when attempting to explain why teeth crack. Clark is notorious for misinterpreting the Inglis formula in which L represents the length of a crack proceeding inwards from a surface (crack depth), not the length of a crack across a surface, when attempting to corroborate his argument that antiquated tooth preparations cause cracked teeth.
It’s unlikely there’s a respected scientist or researcher in adhesive dentistry that agrees with Clark and his elementary idea that “...polymerization shrinkage stresses in these modern preps is a good thing,” even if you “change to a negative c-factor, less than one,” which is conceptually and mathematically impossible. And if that’s not confusing enough, there is no mention of “negative c-factor” in the draft of Fok’s study. Perhaps Clark misspoke, or maybe he simply doesn’t know what he’s talking about. Luckily, there are clinicians and researchers who do.
Academic clinician and researcher Alireza Sadr, DDS, PhD, who is Associate Director at Biomimetics Biomaterials Biophotonics & Technology (B4T) laboratory at University of Washington School of Dentistry, has published with colleagues a variety of interesting articles using state of the art Optical Coherence Tomography (OCT) to visualize polymerization shrinkage stress in real-time, and the conclusions are shocking.
In one sample shown below, a 2mm deep cavity is bulk-filled after application of Scotchbond Universal. Dentin delamination or gap formation (white line) occurs after just 12 seconds of curing and continues to increase even after curing ends. This sample closely resembles the bulk placement of composite in Clark’s “Bioclear Method,” though other samples can be viewed and appreciated on the B4T website. The ground-breaking article “Real-time in-depth imaging of gap formation in bulk-fill resin composites” was published in 2019.
It is clear that through a variety of well-documented and peer-reviewed research polymerization shrinkage stress in adhesive dentistry is and never will be a good thing. Uncontrolled polymerization shrinkage causes gap formation at the dentin interface which can lead to post-operative sensitivity, leakage, and recurrent decay over time. Dr. Sadr and colleagues have demonstrated this ad nauseam, as shown by over 60 years of research, and public records obtained from the University of Washington School of Dentistry reveal numerous dental professionals’ continued frustration with Clark’s claims, including fear of being attacked by him if they speak out.
As of this writing, no peer-reviewed studies supporting Clark’s claims can be found in any reputable journal or repository. Even Clark’s heavily cited FEA analysis, a study funded by 3M OCSD, was rejected after peer-review for “lack of experimental results to validate the FEA”, according to Alex Fok. “I have yet to revise and resubmit the manuscript after it was rejected the first time round. Other projects such as grant writing have taken priority”, said Fok in February 2023, stating he expected to due to so “in the next few months.”
While cavity preparations different from the traditional G.V. Black preparations are more favorable for composite, Sadr and many others have proven that changing the preparation alone does not mitigate the complexities of adhesive dentistry to the degree that Clark and his marketing team might prefer. To learn more, read “Modern Cavity Preparations.”
References
Hayashi J, Espigares J, Takagaki T, Shimada Y, Tagami J, Numata T, Chan D, Sadr A. Real-time in-depth imaging of gap formation in bulk-fill resin composites. Dent Mater. 2019 Apr;35(4):585-596. doi: 10.1016/j.dental.2019.01.020. Epub 2019 Feb 25. PMID: 30819550.
Versluis A, Tantbirojn D, Pintado MR, DeLong R, Douglas WH. Residual shrinkage stress distributions in molars after composite restoration. Dent Mater. 2004 Jul;20(6):554-64. doi: 10.1016/j.dental.2003.05.007. PMID: 15134943.
Do you have any comment on the use of BulkEZ for deep boxes, large restos, endo accesses, class 1 . I use either etch 20 sec + 2 cured layers of Photobond. Or etch + Prelude SE adhesive ( which is essentisally optibond Solo). I do not use Prelude SE as mcuh, and if needing BulkEZ I will spread uncured Link on the cured adhesive first. I tend to be confused whether Bulk EZ alone after disecting cracks can be just as good alone versus ribbond layered with LC resins and flows.