The role of liquid resin in orthodontic bonding : toxicological, bond strength and clinical aspects

Dental acrylic composites are rarely fully polymerized in situ. Their unpolymerized components are related to undesirable occupational exposure to dental, particularly orthodontic, personnel. The aim of this study was to design a simple, compatible and reproducible cytotoxicity test which allows the evaluation of the cytotoxicity of orthodontic bonding composites. Based on these data, an alternative bracket bonding technique, which possibly reduces the dose of the irritants in these composites, was studied.

Four experimental designs of cell/material exposure patterns were evaluated. Standardized sterile glass slices were placed directly above, directly below and in tissue culture inserts above cell cultures of human oral fibroblasts (HOFs). The vitalities and viabilities of these designs were compared to that of the control HOFs by tetrazolium bromide reduction (MTT) and 3H-thymidine uptake (TdR) assays for up to six days. The reproducibilities of all the designs were compared to that of HOFs. The effects of six commercial acrylic resins on the vitalities and viabilities of HOFs were assessed by MTT and TdR assays over a 6-day period. The use of tissue culture inserts in our experimental design, assessed by MTT assay, was shown to be a simple, compatible and reproducible test for cytotoxicity screening of dental acrylic resins.

To compare the cytotoxic properties of the oxygen inhibited layer (OIL) and other leachable components of orthodontic bonding composites, seven commercial products were tested. In the absence of OIL, orthodontic bonding composites were not significantly cytotoxic. The presence of OIL caused an average of 33% reduction in the vitalities of HOFs. In the absence of OIL, light cured and chemically cured 2-paste materials were not cytotoxic in vitro. However, chemically cured liquid-paste materials significantly reduced the average vitalities of HOFs.

An alternative bonding technique, which possibly reduces the occupational exposure of orthodontic personnel to the cytotoxic liquid resin, was studied. Two orthodontic bonding composites were used in an in vitro mechanical test. In the control group, metal brackets were bonded to extracted healthy human premolars with these materials according to the manufacturers' instructions. In the test group, the liquid resins supplied with these products were not used to bond orthodontic brackets. The specimens were tested for shear bond strength after a standardized storage procedure. Enamel adhesions produced by these two commercial materials with and without the use of liquid resins were similar in their early in vitro shear bond strength.

The relevance of the in vitro mechanical data was substantiated by a retrospective clinical study. In the control group, bonding composite and its liquid resin were used to bond fixed appliances onto patients' upper teeth. In the test group, bonding composite without liquid resin was used. The duration of appliance survival was statistically analyzed. Supra-gingival orthodontic metal brackets seemed to bond to and work on healthy enamel surfaces equally well in both test and control groups.

List of scientific papers

I. Tang AT, Li J, Ekstrand J, Liu Y (1999). Cytotoxicity tests of in situ polymerized resins: Methodological comparisons and introduction of a tissue culture insert as a testing device. J Biomed Mater Res. 45(3): 214-222.
https://doi.org/10.1002/(SICI)1097-4636(19990605)45:3<214::AID-JBM9>3.0.CO;2-Y

II. Tang AT, Liu Y, Björkman L, Ekstrand J (1999). In vitro cytotoxicity of orthodontic bonding resins on human oral fibroblasts. Am J Orthod Dentofacial Orthop. 116(2): 132-138.
https://pubmed.ncbi.nlm.nih.gov/99364904

III. Tang AT, Björkman L, Adamczak E, Andlin-Sobocki A, Ekstrand J (1999). In vitro shear bond strength of orthodontic bondings without liquid resin. Acta Odontol Scand. [Accepted]

IV. Tang AT, Björkman L, Isaksson L, Lindbäck KF, Andlin-Sobocki A, Ekstrand J. Retrospective study of orthodontic bonding without liquid resin. Am J Orthod Dentofacial Orthop. [Accepted]