Polymerization shrinkage: effects of constraint and filling technique in composite restorations.
OBJECTIVE
To evaluate the linear polymerization shrinkage (LPS) and its effect upon mean gap width, bond strength and cohesive strength of a composite placed under different constraints (C-factors--CF) and filling techniques.


METHODS
Composite was placed in cavities sized 4 x 4 x 2 mm3 (CF = 3) or on flat dentin surfaces (CF = 0.3) of bovine incisors, after adhesive application. They were inserted in one or three increments, and light cured (600 mW/cm2) for 80 s. The LPS was measured by placing a probe on the top surface of the composite in order to measure its dislodgment in the top-bottom direction. Half of the sample was sectioned to obtain composite resin sticks subjecting them to tensile forces at 0.5 mm/min. The other half of the sample was sectioned and the mean gap width was measured in both sides of the sections. Then the sections were sliced again to obtain composite/dentin sticks. The mean gap width in the sticks was performed before subjecting them to tensile forces at 0.5 mm/min. Data was analyzed by a two-way ANOVA and the correlation between the bond strength and gap width was analyzed by simple linear regression.


RESULTS
(1) Linear polymerization shrinkage: significant differences were observed for the interaction (p < 0.05). Under the low constraint, the LPS were similar for both filling techniques. Under higher constraint, polymerization shrinkage was lower for the incremental technique. (2) Gap width and bond strength: no difference was detected either for interaction, or for technique (p > 0.05). Under higher constraint, the gap width was higher and the bond strength lower. (3) The cohesive strength of composite resin was similar for all groups (p > 0.05). No correlation between bond strength and gap width was found (p = 0.17).


CONCLUSIONS
The effects of polymerization shrinkage were not reduced by the filling technique under the different cavity constraints tested.
Loguercio
Alessandro D
AD
School of Dentistry, University of Oeste de Santa Catarina, Campus Joaçaba, R. Getúlio Vargas, 2125, Bairro Flor da Serra, CEP Joaçaba, SC 89600-000, Brazil. aloguercio@hotmail.com
Reis
Alessandra
A
Ballester
Rafael Y
RY
D000704:Analysis of Variance; D000818:Animals; D017438:Bisphenol A-Glycidyl Methacrylate; D002417:Cattle; D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D001840:Dental Bonding; D018772:Dental Marginal Adaptation; D019232:Dental Restoration Failure; D003793:Dental Restoration, Permanent / Q000379:methods; D003799:Dental Stress Analysis; D003804:Dentin; D017220:Dentin-Bonding Agents; D008027:Light; D016014:Linear Models; D008422:Materials Testing; D015394:Molecular Structure; D044367:Phase Transition; D011108:Polymers / Q000737:chemistry; D018709:Statistics, Nonparametric; D013718:Tensile Strength
D003188:Composite Resins; D017220:Dentin-Bonding Agents; C444701:Filtek Z250; D011108:Polymers; C115612:single bond; D017438:Bisphenol A-Glycidyl Methacrylate