As early as in the 1980s it was reported that oxygen inhibits the radical polymerization of dental composites. During the polymerization process, the radicals react not only with the double bonds of the monomers but also with oxygen from the air. This leads to the formation of what is known as an oxygen inhibited layer on the surface of the mixed luting composite. This layer is sticky and gives the user the impression that the material has not polymerized adequately.
In practical use, the luting material placed between the tooth structure and the restoration polymerizes without being hampered by atmospheric oxygen because there is no ingression of oxygen into these areas. In this case, the inhibited layer is mostly noticeable on the excess luting material around the cement joint.
The self-cured polymerization reaction is relatively slow compared with light-cured polymerization. For this reason, self-cured polymerization results in a thicker inhibition layer than light-cured polymerization. In the literature¹, an inhibition layer of approx. 30 μm is reported for light-cured polymerization and up to 100 μm for self-cured polymerization.
Recent investigations carried out by Ivoclar Vivadent² have shown that the inhibition layer of current composite-based luting materials can be as high as around 200 μm in purely self-cured specimens.
Composite-based luting materials that are not fully cured are prone to absorbing water. Water absorption may lead to discolourations and changes in translucency. Furthermore, material that is incompletely cured may be washed out more easily, which has an adverse effect on the quality of the restoration margins.
How can oxygen inhibition be avoided?
As early as in 1991, Roulet et al.³ demonstrated that the application of glycerine gel to the surface of the cement joint prior to the final curing process can prevent oxygen inhibition on the surface area and thus promotes the optimum quality of the restoration margins. For this reason, Ivoclar Vivadent generally recommends applying Liquid Strip glycerine gel to the cement joint before the final curing process. The glycerine gel is water soluble and can be easily rinsed off with water spray once the polymerization is complete. The use of Vaseline for covering the cement joint is not recommended because Vaseline is hydrophobic and can be removed only with difficulty.
¹Ruyter IE; Unpolymerized surface layers on sealants. Acta Odontol Scand 1981;39 (1):27-32
²Gianasmidis A, Brot A, Grabenbauer B, Burtscher P; Inhibition layers of radical polymerized luting materials; Oral Presentation No. 0190, IADR General Session 2017, San Francisco
³Bergmann P, Noack MJ, Roulet JF; Marginal adaptation with glass-ceramic inlays adhesively luted with glycerine gel. Quintessence Int. 1991 Sep;22(9):739-44