In vitro enamel caries formation and orthodontic bonding agents

To examine, in vitro, the caries-like lesion formation in enamel adjacent to fluoride-releasing orthodontic bonding agents using polarized light microscopic techniques.

40 human extracted permanent third molars with sound enamel smooth surfaces were divided into two treatment groups: Light Bond group, a fluoride-releasing filled resin orthodontic bonding agent (n=20); and Pro Seal group, an orthodontic bonding agent with glass-ionomer (n=20). Prior to bonding agent placement on the buccal surfaces, acid-resistant varnish was applied to the molar teeth leaving a 2 mm (occlusal-cervical direction) by 5 mm (mesial-distal direction) exposed sound enamel window on the buccal surfaces of each molar tooth. The exposed window had the assigned orthodontic bonding agent applied, according to the manufacturer's instructions. With each specimen, a 1 mm (occlusal-cervical direction) by 5 mm (mesiodistal direction) sound enamel window was exposed by selectively removing the acid-resistant varnish on the opposing lingual or palatal surface with each molar tooth, and serving as a matched internal control with each molar. The molar teeth were then sectioned into buccal and lingual/palatal tooth halves. Acid-resistant varnish was applied to the cut surfaces. Each group underwent synthetic saliva rinsing for 2 weeks prior to in vitro caries formation using a modified ten Cate solution over a 2-week lesion initiation period. Longitudinal sections (three per treatment and control groups) were taken for polarized light study. The remaining tooth portions were exposed to synthetic saliva rinsing for 1 week and then exposed to the in vitro caries solution for an additional 1-week period to allow for lesion progression within the exposed enamel windows (progression 1 period). Longitudinal sections (three per treatment and control groups) were taken at the end of lesion progression 1 for polarized light study. After lesion progression 1, the remaining tooth portions were exposed to synthetic saliva rinsing for 1 week and then exposed to the in vitro caries solution for 1 week to allow for additional lesion progression within the exposed enamel windows (progression 2 period). Longitudinal sections (three per treatment and control groups) were taken at the end of lesion progression 2 for polarized light study. All longitudinal sections from the treatment and control groups at all three time periods (lesion initiation, progression 1 and progression 2) were imbibed with water and examined with polarized light microscopy to determine lesion depths and evaluate the enamel-resin interface. Mean (SD) lesion depths were determined and compared (ANOVA, t-test).

Following lesion initiation, lesion progression 1 and lesion progression 2 periods, both treatment groups exhibited significant reductions in mean lesion depth when compared with the matched no treatment control group (P< 0.05). The Pro Seal group exhibited statistically significant reductions in mean lesion depths when compared with those for the Light Bond group at lesion initiation, lesion progression 1 and lesion progression 2 periods (P< 0.05, ANOVA, t-test). Both orthodontic bonding agents showed intact and intimate enamel-bonding agent interfaces with no lesion formation within the underlying bonded enamel. Caries-like lesions were only present in the exposed enamel windows adjacent to the orthodontic bonding agents.