I read the article “Oral epithelial cell reaction after exposure to Invisalign plastic material” in the January issue (Premaraj T, Simet S, Beatty M, Premaraj S. Am J Orthod Dentofacial Orthop 2014;145:64-71). The authors declared that Invisalign plastic powder eluate with saline solution can cause cytotoxic effects on keratinocytes in vitro. First, I want to express my appreciation to the authors for their great effort in the experiment. However, I have a few concerns, mainly about the method that had been used in the study.

In Invisalign eluate, the particle distribution varies from 1.0 to 4.5 mm. How can this be divided equally among the needed groups? If they weren’t equally divided, would the amount of dissimilarity affect the final result? In the article, it was mentioned that 2-week and 4-week eluates did not demonstrate consistent results for MTT and flow cytometric assays. Is it possible that in the 8-week eluate, the particles of Invisalign plastic had degraded or precipitated?

The measured resistance and impedance of keratinocytes in the different groups were shown in normalized form, whereas the absolute value may be critical to evaluate the changes in the saline-eluate group. The mixture of saline solution and Invisalign might change in an electrochemical way, which could create confounding factors. I think it’s necessary to show the initial data and the relevant analysis of the experiment in the article.