Optical Changes of Dental Clear Thermoplastics Related to Multiscale Roughness

Objectives: The aim of this in vitro study was to assess the optical changes and topographical development of thermoplastic materials for clear appliances, associated with low temperature degradation (LTD).
Methods: Three PET-G polyester thermoplastic materials were taken into consideration: Leone (L), Erkodur (E), and Duran (D). Samples (n=60) were divided into a control group (immersed in artificial saliva) and the LTD group, further subject to thermal cycling in two water baths (5-55°C) with 30 s of dwell time (+10 s of transfer time), in order to simulate 14 days of intraoral use (400 cycles). Related to thermal cycling, optical CIE L*a*b* coordinates were recorded by a spectrophotometer, translucency parameters (TP) and levels of color changes (NBS ΔE) were achieved. Three-dimensional AFM profiles were generated, surface nanoroughness and microroughness was recorded. Statistical analyses were performed (One-Way ANOVA and Post Hoc Tests, paired t-Tests, p<0.05, Pearson).
Results: D registered significant higher TP values than L, respectively E, after LTD. LTD increases TP significant for D and decreases for E. The TP variations are strongly correlated to L* and moderate to a* coordinates. Color changes varied from slight (E), perceivable (D) to marked (L). A significant increase in microroughness was observed for L, reaching significant differences between L and D. On nanoscale significant differences are reached between all materials. A strong negative correlation between surface roughness parameters measured over length scales of nanometer and micrometer was observed.
Conclusions: LTD of clear thermoplastic materials may be associated with up to marked color changes after a period of 14 days in a simulated intraoral environment. Color changes are very strong positive correlated to microroughness.