LIGHT POLYMERIZATION OF PHOTO-CURED COMPOSITE MATERIALS: MODERN APPROACHES AND APPLICATION PECULIARITIES

In present-day dentistry, the photo-cured composite materials are widely used for aesthetic dental restorations. There is a wide choice of photo-cured composite materials, which differ significantly in physical and working properties, as well as the technique of their application. Most of the modern photo-cured composite materials contain the light-sensitive catalyst camphorquinone, but lucerin and 1-phenyl-1,2-propanedione are also used as photoinitiators. The important condition for photocured composite materials polymerization is that it begins and is prolonged when the intensity of the light flux is acceptable to maintain the excited state of the photoinitiator. The polymerization process is accompanied by stress, which results in polymerization shrinkage. The various approaches are used to prevent the negative consequences, namely, directed polymerization, the effect of the light flux according to the soft start, pulsating and honeycomb polymerization. The lifetime of dental restorations is directly associated with the technical characteristics of photo-cured composite materials, among which there are halogen, LED, laser and plasma ones. The main features of halogen photopolymerizers include the wide range of rays, which creates the ability to polymerize composites with photoinitiator, different to camphorquinone, with significant amount of heat, and high power consumption. However, after the thermal exposure, the mechanical characteristics of photo-cured composites, in particular, plasticity and strength deteriorate significantly; the uneven polymerization of the material occurs, which leads to the local internal stresses and deformations. Halogen photopolymerizers are rarely used recently. In LED photopolymerizers, light is generated in a semiconductor crystal due to the energy of excited electrons. In the spectrum of the luminous flux of the LED photopolymerizer the thermal and ultraviolet components are absent, all the energy is in the range of blue light, which prevents the possibility of significant overheating of hard tissues and dental pulp. The disadvantages of LED photopolymerizers include the possibility of their application only for photo-cured composite materials polymerization, where only camphorquinone appears to be the photoinitiator as well as the high degree of emitted luminous flux dispersion. The important advantage is the stability of the light flux in time and the possibility of wireless structures application. LED photopolymerizers are used in clinical practice more often. In laser photopolymerization devices light is generated by transition of electrons in gas environment of argon from the unstable to stable state. The short exposure time of photo-cured composite material portion (2-3 sec) is the important property of laser photopolymerizers, but it is also the negative factor for the occurrence of polymerization stress in material, the high energy flux turns into a high level of stress in material without providing the corresponding mechanical characteristics and significant degree of conversion. Laser photopolymerizers also emit a significant amount of heat. The widespread clinical application of these devices is limited. Plasma photopolymerizers generate a rather bright light flux due to the high-voltage arc located between two electrodes in a medium of highly rarefied ionized gas. The source of bright light flux is xenon or argon lamp. Their advantages include the possibility of application with photo-cured composite materials containing various photoinitiators, but plasma photopolymerizers are bulky and uncomfortable in use, they have large thermal radiation, they are short-lived. Thus, there is a wide choice of restorative photo-cured composite materials and significant number of photopolymerization agents used for hardening of these materials during the dental restorations. It became possible due to significant advances in dental materials technology and engineering as well as restoration technologies. However, the application of certain materials and devices with definite characteristics in a particular clinical case requires numerous laboratory investigations and long-term clinical studies