Influence of light intensity on composite resin restoration Done by : Rofaidh Wali Teaching assistant, Umm Al-Qura University, Faculty of Dentistry Makkah, Saudi Arabia

Light curing units * Systems designed to control the curing time of material that hardens over a period of time * They emit a beam of light with a wavelength range between 400 and 700 nm that activates polymerization initiators in a materials, the most important of which are composite resins.

The amount of radiation reaching a given point in the material will depend on several factors * lamp output intensity. * Exposure time. * distance from light source to material. * curing depth. * presence of elements between light source and target material.

Types of polymerization lamp Halogen lamps: * use an incandescent light-bulb consisting of a tungsten filament surrounded by the halogen gases. * emit white light. * use of the filter wastes a great deal of radiation. * low energy performance. * generate high temperature. * some loss of power. * required considerable maintenance.

Plasma arc lamps: * use a fluorescent bulb containing plasma * not emit white light. * filter are used * the disadvantages of these lamps are similar to those of halogen lamps.

Laser lamps: * Provide narrow beam of coherent blue light. * High intensity lamps * Work over a very limited range of wavelengths * Do not require filters.

LED lamps: * Medium intensity lamps. * Based on light emitting diodes for generation of blue light. * Provide high energy performance. * Ensure constant effectiveness. * Not generate high temperature. * No filters are required.

Mechanisms of photopolymerization The light emitted by these lamps activates polymerization initiators in composite resins. The radiation absorption spectrum of the initiators must therefore match the lamp emission spectrum. Three major initiators are available : camphorquinones, PPD(fenilpropanodione) and lucerin.

Effectiveness of different types of polymerization lamp * Generally agreed that plasma arc units provide good results and significant time saving when used for cementing brackets and orthodontic bands. * Laser based curing units provide :- - higher conversion factor. - greater curing depth. - required less exposure time. - developed enhanced mechanical properties.

However, - they associated with material wear. - greater polymerization shrinkage. - greater marginal leakage. but no loss of power over distance. Therefore, it is technique of choice when the light source cannot be brought close to resin surface.

Different polymerization technique Different polymerization technique We have 2 main categories:- 1- Stepped or soft start technique curing start at low intensity then intensity is increased. Properties: - satisfactory - not reduce polymerization shrinkage. - generate less stress - less marginal leakage.

2- Pulse-delay technique - Incorporates a waiting period between exposures. - Curing start at low intensity and then stopped for a given period then applied at high intensity in 1 or more pulses - Greatest reduction of shrinkage stress is achieved with a waiting period.

Conclusion * The most reliable curing unit for any type of composite resin is the high density halogen lamp, fitted with a programming device to enable both pulse- delay and stepped techniques. * With any other type of curing unit is used, it must be compatible with the composite material to be used. * Further researches is required to identify the most reliable light curing techniques.

Silikas.N, Eliades.G et.al(2000) ˈ ² ˈ studied The light intensity effects on resin- composite degree of conversion and shrinkage strain. Aim of study :- investigate relationship between DC and shrinkage strain of two composites when exposed by two light intensity levels for different time period

They used: * Z100 and Tetric Ceram Composite material. * The material was placed between two translucent polyethylene strips and plastic blocks to produce a very thin film. * The material then polymerized by using the Elipar Highlight Unit under four different curing modes: - standard ( full intensity for 40s) - two step ( soft start) - low intensity for 40s - low intensity for 10s

* FTIR spectra measure the degree of conversion * The bonded-disk technique was used to measure polymerization shrinkage strain. * The measurements were made continuously for 30 minutes from photoinitiation and three repetitions were made per material. * All the data were analyzed by ANOVA.

Degree of conversion

Shrinkage strain

They found:- * Reduction of light intensity would not significantly reduce the degree of conversion values but reduce the shrinkage strain. * This decrease in shrinkage, if sustained, could lead to better marginal adaptation. * However, the full-intensity phase of the ‘soft-start’ mode II, led to final shrinkage strains that not statistically significantly lower than standard one.

Bouschlicher. M.R, Rueggeberg.F.A et.al(2000) ˈ ³ ˈ studied The effect of stepped light intensity on Polymerization Force and conversion in a photoactivated composite. Bouschlicher. M.R, Rueggeberg.F.A et.al(2000) ˈ ³ ˈ studied The effect of stepped light intensity on Polymerization Force and conversion in a photoactivated composite. Aim of study: Evaluate effect of stepped light intenisity on polymerization shrinkage forces and DC of a hybride composite

They used:- - Hybrid composite samples with two threaded steel rods for each. - The flat bonding surfaces of rods were microetched with 30 μ silanized silica and air dried for 60s. - Then the rods were attached to the load cell and crosshead of a universal testing machine. -An unfilled bonding resin was applied to the silanated rod ends and light cured for 20 seconds.

Following surface treatment, - loading hybrid composite on rod tips. - samples were polymerized using two exposure durations (40 s and60 s) in conjunction with the two curing modes (STD and 2-STEP).

* The mean maximum shrinkage force and the maximum rate of force development were compared and the degree of conversion was measured. * The power density was measured with radiometer.

They found: - Maximum shrinkage force and maximum rate of force development during the first 300 seconds of polymerization were not influenced by a standard or 2-STEP cure mode of similar exposure duration.

- The 2-step cure mode result in longer delays before polymerization shrinkage force was recorded. - Both duration of exposure resulted in statistically significant differences in the overall degree of conversion but the mode of cure had no effect on degree of conversion.

References: 1- Amparo Jimenez-Planas,juan Martin, camilo Abalos and Rafael liamas: developments in polymerization lamps. quintessence international. Volume (38). No.(10). Page(e74-e84), N. Silikas, G.Eliades, D.C. Watts: light intensity effects on resin- composite degree of conversion and shrinkage strain. Dental materials. Volume(16). No.(4). Page( ), Murray R. Bouschlicher, Frederick A. Rueggeberg, Daniel B. Boyer: effect of stepped light intensity on Polymerization Force and conversion in a photoactivated composite. J Esthet Dent. Volume(12). No.(1). Page(23-32),2000