Biocompatibility of light-cured orthodontic bonding adhesives: a preliminary study

Direct bonding of orthodontic brackets is an established procedure and has lead to increasing number of light-cured bonding materials produced with ever-increasing bond strength and ease of delivery. However, considerably less attention is given to biocompatibility of these materials. Both in vivo and in vitro studies indicate orthodontic bonding agents are cytotoxic.  Spectrophotometric analysis revealed that these resins contained a great amount of potentially toxic, leachable, un-polymerized material, even when fully cured.

Hypothesis: Different light-cured bonding agents may have different cytotoxic effects depending on their chemistry.

Objective: Compare and analyze the toxicity of three different light-cured bonding agents to a population of gingival cells as measured by cell growth, and viability in vitro.

Method: Light Bond®, MonoLok®, and Transbond XT® orthodontic adhesives were tested.  Human gingival cells were obtained from extracted third molars, pooled, and expanded until needed.  Cells were seeded in a 24-well plate at 5 x 10⁴ cells/well over plastic tissue culture treated coverslips containing bonded brackets and incubated for 48 hours.  For histological evaluation, cells were stained with 0.2% Toluidine Blue for 2 min.  Cells were photographed using light microscopy.  Cell viability/cytotoxicity assay was performed with 4µM EthD-1 and 2 µM calcein AM for 45 min. at 37°C and photographed using fluorescence microscopy.  2 x 10⁵ cells/well were seeded with brackets and counted for three days to establish cell growth curve compared to a non-bonded control.

Result: Toluidine blue stained cultures indicated sparse cell attachment close to MonoLok® and Light Bond® resins.  Whereas Transbond XT® showed increased biocompatibility.  These results were confirmed by cell viability/cytotoxicity assay as well as cell growth which showed a 1.5 fold increase between between 24 and 48 hours.

Conclusion: This data indicates the potential biocompatibility of Transbond XT® as compared to MonoLok® and Light Bond® and warrants further research.