Assessing colour stability of direct printed and thermoformed aligners: An in vitro study on aesthetics longevity

To evaluate and compare the colour stability of direct-printed and thermoformed clear aligners after exposure to common staining agents.

Clear aligners fabricated using two direct-printed resins, TC85 and TA28 (Graphy Inc., Seoul, Republic of Korea), and one thermoformed aligner, Erkodur (Erkodent, Germany), were tested for colour stability in coffee, curcumin, and 0.2% chlorhexidine mouthwash, with distilled water as control. A total of 132 specimens (n=11 per group) were immersed in the respective solutions at 37°C for 7 days. Colour variation (ΔE*) was measured using a spectrophotometer, while molecular and surface changes were analysed using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM). Data were analysed using two-way ANOVA with Tukey's post hoc test (P<0.001).

Thermoformed aligners demonstrated significantly greater colour stability (ΔE*=2.56; 95% CI: 2.27-2.85; P<0.001), smoother surface morphology, and superior chemical integrity compared to direct-printed aligners - TC85 (ΔE*=9.82; 95% CI: 9.53-10.11) and TA28 (ΔE*=3.59; 95% CI: 3.30-3.88).

Direct-printed aligners exhibited inferior optical and surface properties, influenced by material composition and printing technology. PETG-based thermoformed aligners showed enhanced colour and structural stability, suggesting superior long-term aesthetics performance.

Discolouration and increased surface roughness in clear aligners can compromise aesthetics, patient compliance, and biomechanical efficiency, underscoring the importance of material selection in aesthetics orthodontic therapy.