The analysis of shrinkage stresses of selected orthodontic adhesive systems

Introduction.
The phenomenon of polymerization shrinkage, characteristic for the cross-linking process of composite materials, causes stresses that appears at the interface between tooth tissues and the composite. In the case of reconstruction of enamel and dentin lost due to traumas or caries, this process is responsible for detachment of the filling from cavity walls immediately after its placement. Orthodontic adhesives, similar in their chemical structure to restorative materials, also undergo this phenomenon, which results in the formation of cracks and leaks between tooth enamel and bracket bases.

Aim.
The aim of the study was to assess the shrinkage stresses arising under laboratory conditions during polymerization of selected orthodontic adhesive systems with visible light.

Material and methods.
4 orthodontic adhesives were used in the study: LC Orthodontic Adhesive (Prime-Dent, USA), Enlight (Ormco, Netherlands), Light Bond (Reliance, USA) and Alpha-Dent (Dental Technologies, USA). The shrinkage stresses were assessed using the elasto-optic method. The stresses of composite materials, visualized in the form of bands on photosensitive plates, were visualized using the FL200/Gunt circular polariscope (Germany).

Results.
The average values of reduced shrinkage stresses recorded for each of the tested orthodontic adhesives ranged from 7.64 to 11.54 MPa.

Conclusions.
1.) Under the conditions of this study, during polymerization with visible light the study orthodontic adhesive systems generated stresses, the values of which could cause detachment of the composite material from the hard tissues of the tooth under clinical conditions. 2.) The value of internal forces generated in the material mass during its cross-linking may depend on the composite chemical composition.