Effect of 3D-printed model base height on thickness distribution of thermoformed zendura FLX dual-layer clear aligners: an in vitro study

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 < 0.05. 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 < 0.001). This decreasing trend was consistent across all tooth types and measurement sites, though the magnitude of reduction varied by site. Palatal sites consistently exhibited greater thickness than facial sites. For most tooth type–site combinations, all MBH contrasts were statistically significant, with limited exceptions observed at specific sites for anterior teeth, molars, and premolars. 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.

The authors' abstract, as published at the source. Frontiers in Dental Medicine, 2026 · DOI ↗