Abstract
Background
Thermoforming of clear aligners causes non-uniform material thinning that may influence the structural and mechanical characteristics of the aligners. While greater model base height (MBH) has been shown to increase thinning in single-layer aligners, its effect on dual-layer materials remains unclear. This study aimed to evaluate the effect of four MBH levels (2, 4, 6, and 8 mm) on the thickness distribution of thermoformed Zendura FLX dual-layer clear aligners.
Methods
Forty Zendura FLX aligners were thermoformed over standardized 3D-printed maxillary models with MBH of 2, 4, 6, and 8 mm (
n
= 10 per group). Thickness was measured at multiple tooth-specific sites across four representative teeth (central incisor, canine, first premolar, and first molar) using a digital caliper. Linear mixed-effects models and generalized linear mixed-effects models with a gamma distribution were used for statistical analysis. Estimated marginal means (EMMs) and 95% confidence intervals (CIs) were derived from the fitted models to summarize group-level thickness estimates.
Post-hoc
pairwise comparisons were performed using Tukey's test, with significance set at
p
Results
EMM thickness decreased progressively with increasing MBH: 446 µm (95% CI 432–460) at MBH 2 mm, 386 µm (95% CI 374–399) at MBH 4 mm, 329 µm (95% CI 319–340) at MBH 6 mm, and 268 µm (95% CI 260–277) at MBH 8 mm (all pairwise comparisons
p
Conclusions
Increasing MBH resulted in a progressive reduction in the thickness of thermoformed Zendura FLX dual-layer clear aligners across all tooth types and measurement sites. Among the four levels tested, an MBH of approximately 4 mm appeared to provide a more balanced thickness distribution. These findings suggest that MBH should be considered as an important fabrication parameter in clear aligner production.