الفهرس 
Materials and methodsOnly 14 pages are availabe for public view
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Abstract
The aim of the present study was to investigate the effect of the degree of polymerization on intrinsic discoloration of resin based composites.
A total of 90 composite resin disc specimens were prepared for this study divided into 3 main groups, 30 specimens each, according to the type of resin composite material used hybrid resin composite(Te-Econome), microfilled resin composite (Heliomolar) and nanofilled resin composite (Filtek Supreme). Each group was further subdivided into three sub groups, 10 specimens each, according to time of curing (C); where the first sub group (C1) was cured for 20 seconds, the second sub group (C2) for 40 seconds, and the third sub group (C3) for 60 seconds. Half of the specimens in each sub group, 5 specimens, where stored in dry condition (baseline specimens) while the other half was stored in coffee (test specimens).
The baseline specimens were stored in dry tightly sealed dark bottles for 24 hours at room temperature in dark environment. The test group specimens were stored in tightly sealed dark bottles for 7 days at room temperature. Baseline specimens were removed from the dry containers after 24 hours and color was assessed spectrophotometerically. After one week storage of the specimens in the assigned media, the color of the specimens was assessed spectrophotometerically and the tristimulus value of L a b color parameters was calculated. Moreover, the change in color (?E) was calculated according to the equation:
?E = {(?L) ² + (?a) ² + (?b) ²} ½
The results of the current study revealed that higher degree of discoloration occurs with the nanofilled resin composite (Filtek Supreme) compared to the microfilled resin composite (Heliomolar) which was higher by its turn than the hybrid resin composite (Te-Econome) at different curing times (T1, T2, and T3), higher degree of polymerization occurred with the Te-Econome compared to the Heliomolar which was higher by its turn than Filtek Supreme at different curing times. Concerning the relation between the intrinsic discoloration and the degree of polymerization reverse relationship was found between them.
Conclusions:
Under the conditions of the current study, the following conclusions could be derived:
1- Degree of polymerization of the resin composite is material and time of curing dependant.
2- Hybrid resin composite showed the most degree of polymerization.
3- Discoloration properties of the resin composite are material dependant.
4- Hybrid resin composite showed the least discoloration.
5- Reverse relationship was found between the intrinsic discoloration and the degree of polymerization of light activated resin composite.
Clinical recommendation:
It is recommended that on using the quartz tungsten halogen light curing unit the curing time should not be less than 40 seconds to ensure better color stability of different light activated resin composites