E08. Compare the reactions speeds of a light-cured composite with a chemically cured composite, and discuss how curing methods affects the ability of stress compensation and shrinkage direction.

A light-cured material is activated when it is exposed to light of a particular wavelength. Thus, as long as the material has not been exposed to this wavelength, the material will not cure. However, as soon as photons of the right wavelength interact with the champheroquinone, free radicals will form. Since modern light sources have rather high intensities (often more than 500 mW/cm²), a very large number of free radicals form, particularly closest towards the light tip. This result in a very fast cure closest to the light tip, and material located further away that have not yet set will shrink toward the light tip if the material is bonded closer toward the light tip. The entire curing time takes only 20-40 s after the material has been exposed to light.

WARNING! The operatory light contains different wavelengths, including the 468 nm wavelength. Fortunately, since this light source is rather far away, the light intensity is not very high at the surface of the composite. However, as time goes on, the light intensity is high enough to start a slow curing process.

Cold-cured composites start curing as soon as the two pastes are mixed. The set is slowed down initially by the presence of inhibitors (often different quinones). As soon as a free radical is formed, it reacts with a quinone, and the free radical is "killed." It is first after all quinone molecules have been consumed as the chain growth can occur. Thus, by changing the quinone concentration, the manufacturer can control the working time. When all quinone molecules have been consumed, the material sets. In other words, the setting time (from the formation of the first free radical until termination) of a cold-cured material is longer than that of a light-cured material. During the setting reaction of any methacrylate, heat is released. The highest temperature rise of a chemically cured material will occur in the center of the material because of its poor heat conductivity. This means that the setting is accelerated and occurs first in the center of the material, which causes the material to shrink toward the center when it cures.

From the above we can conclude that any bonded composite will try to pull away from the margins when it sets and shrinks. This pull induces tensile stresses at the tooth-resin interface, stresses that can cause debonding and/or crack formation. Such cracks often occur in the enamel adjacent to the composite and show up as whitish looking margins. By using a technique that causes the material to shrink towards the margins rather than away from the margins one can reduce the stress level that develops around the margins.