The aim of this study was to compare the effects of five commonly used cleaning protocols on the optical properties of thermoformed and direct 3D-printed clear aligners (CAs).

A total of 100 unused aligners (25 per brand: Invisalign®, ClearCorrect™, Smartee® [thermoformed], and 4D Aligner™ [direct 3D-printed]) were randomly assigned to five cleaning groups: (1) water rinse, (2) brushing, (3) brushing with toothpaste, (4) cleaning tablet solution, and (5) mild colourless soapy water. Each protocol was applied once daily for 7 days. Colour and translucency parameters were measured using a spectrophotometer at baseline (T1), after 48 hours of cleaning (T2), and after 7 days of repeated daily cleaning (T3). Colour difference (ΔE) and translucency difference (ΔTP) were then calculated for the T1–T2 and T1–T3 intervals. Data were analysed using two-way ANOVA to assess the effects of aligner brand, cleaning protocol, and their interaction on ΔE and ΔTP.

Invisalign® and ClearCorrect™ aligners showed significantly lower colour change than Smartee® and 4D Aligner™ at T2 and T3 ( P < 0.001), with Smartee® exhibiting the highest discoloration over time. Water rinsing and brushing alone resulted in significantly smaller ΔE and ΔTP compared with cleaning tablet solutions and mild liquid soap ( P < 0.01–0.001). Notably, the magnitude of these differences depended on the aligner brand, as certain cleaning protocols caused pronounced ΔE and ΔTP in Smartee® and 4D Aligner™ but had more limited effects on Invisalign® and ClearCorrect™ ( P < 0.001).

Colour stability of CAs appears to depend not only on the cleaning method but also on the interaction between aligner brand and cleaning protocol, highlighting the importance of considering material-specific factors when providing cleaning advice.