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"Soft-start Polymerization of Resin-Composite Restorations
(Updated Dec 2005) 2003)

Question: What can you tell me about "soft-start" polymerization?

Answer: "Soft-start" polymerization is a method recently advocated to reduce polymerization contraction stresses in resin-composite restorations. During early polymerization, the resin composite cross-linking network is relatively weak - allowing "flow" to fairly easily accommodate for stresses and prevent damage to adhesive bonds. With further polymerization, contraction and flow decrease, while stiffness and stress increase. This may cause adhesive failure. The bond strength must exceed the contraction stress to provide a stable marginal adaptation.1 "Soft-start" polymerization proposes that a slower rate of conversion will allow better flow of resin with a decrease in contraction stress. "Soft-start" polymerization may be divided into three separate techniques: stepped, ramped, or pulse-delay. A stepped program emits a low irradiance for 10 seconds and then increases immediately to a maximum value for the duration of the exposure. In a ramped program, the irradiance gradually increases from a low value to maximum intensity over a 10-second period, after which it remains constant for the duration of the exposure. Pulse delay uses a short low-level burst, a delay for polishing, and finally a long exposure at full intensity. The majority of laboratory research suggests that "soft-start" polymerization may be beneficial,2-17 but several studies have found no difference.18-23 Also, the limited clinical trials available have shown no significant difference between the "soft-start" technique and conventional cure.24,25 More in vivo research is desperately needed to substantiate the potential benefits of this concept.

References
1. Davidson CL, de Gee AJ. Relaxation of polymerization contraction stress by flow in dental composites. J Dent Res 1984;63:146.
2. Bouschlicher MR, Rueggeberg FA. Effect of ramped light intensity on polymerization force and conversion in a photoactivated composite. J Esthet Dent 2000;12:328-339.
3. Bouschlicher MR, Vargas MA, Boyer DB. Effect of composite type, light intensity, configuration factor and laser polymerization on polymerization contraction forces. Amer J Dent 1997;10:88-96.
4. Bouschlicher MR, Rueggeberg FA, Boyer DB. Effect of stepped light intensity on polymerization force and conversion in a photoactivated composite. J Esthet Dent 2000;12:23-32.
5. Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without "softstart-polymerization". J Dent 1997;25:321-330.
6. Unterbrink GL, Muessner R. Influence of light intensity on two restorative systems. J Dent 1995;23:183-189.
7. Yoshikawa T, Burrow MF, Tagami J. A light curing method for improving marginal sealing and cavity wall adaptation of resin composite restorations. Dent Mater 2001;17:359-366.
8. Feilzer AJ, Dooren LH, de Gee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995;103:322-326.
9. Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440-444.
10. Sakaguchi RL, Berge HX. Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dent 1998;26:695-700.
11. Kanca J, Suh BI. Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. Am J Dent 1999;12:107-112.
12. Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mat 2000;16:292-296.
13. Koran P, Kurschner R. Effect of sequential versus continuous irradiation of a light-cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization. Am J Dent 1998;10:17-22.
14. Goracci G, Mori G, Casa de'Martinis L. Curing light intensity and marginal leakage of resin composite restorations. Quint Int 1996;27:355-362.
15. Suh BI, Feng L, Wang Y, Cripe C, Cincione F, de Rjik W. The effect of the pulse-delay cure technique on residual strain in composites. Compend Contin Educ Dent 1999;20:4-12.
16. Lim BS, Ferracane JL, Sakaguchi RL, Condon JR. Reduction of polymerization contraction stress for dental composites by two-step light-activation. Dent Mater 2002;18:436-444.
17. Sahafi A, Peutzfeldt A, Asmussen E. Effect of pulse-delay curing on in vitro wall-to-wall contraction of composite in dentin cavity preparations. Am J Dent 2001;14:295-296.
18. Friedl KH, Schmalz G, Hiller KA, Markl A. Marginal adaptation of Class V restorations with and without "softstart-polymerization". Oper Dent 2000;25:26-32.
19. Hasegawa T, Itoh K, Yukitani W, Wakumoto S, Hisamitsu H. Effects of soft-start irradiation on the depth of cure and marginal adaptation of dentin. Oper Dent 2001;26:389-395.
20. Sahafi A, Peutzfeldt A, Asmussen E. Soft-start polymerization and marginal gap formation in vitro. Amer J Dent 2001;14:145-147.
21. Yap AU, Soh MS, Siow KS. Post-gel shrinkage with pulse activation and soft-start polymerization. Oper Dent 2002;81-87.
22. Muangmingsuk A, Senawongse P, Yudhasaraprasithi S. Influence of different softstart polymerization techniques on marginal adaptation of Class V restorations. Am J Dent 2003;16:117-119.
23. Hofmann N, Siebrecht C, Hugo B, Klaiber B. Influence of curing methods and materials on the marginal seal of Class V composite restorations in vitro. Oper Dent 2003;28:160-167.
24. Oberlander H, Friedl KH, Schmalz G, Hiller KA, Kopp A. Clinical performance of polyacid-modified resin restorations using "softstart-polymerization". Clin Oral Investig 1999;3:55-66.
25. Brackett WW, Covey DA, St. Germain HA, Jr. One-year clinical performance of a self-etching adhesive in class V resin composites cured by two methods. Oper Dent 2002;27:218-222.

(Col Vandewalle)