Korean J Orthod.  2022 Jul;52(4):249-257. 10.4041/kjod21.269.

Comparison of dimensional accuracy between direct-printed and thermoformed aligners

  • 1Department of Orthodontics, Saint Louis University, Saint Louis, MO, USA
  • 2Department of Orthodontics, Graduate School of Dentistry, Kyung Hee University, Seoul, Korea


The purpose of this study was to evaluate and compare the dimensional accuracy between thermoformed and direct-printed aligners.
Three types of aligners were manufactured from the same reference standard tessellation language (STL) file: thermoformed aligners were manufactured using Zendura FLX TM (n = 12) and Essix ACETM (n = 12), and direct-printed aligners were printed using Tera HarzTM TC-85DAP 3D Printer UV Resin (n = 12). The teeth were not manipulated with any tooth-moving software in this study. The samples were sprayed with an opaque scanning spray, scanned, imported to Geomagic® Control XTM metrology software, and superimposed on the reference STL file by using the best-fit alignment algorithm. Distances between the aligner meshes and the reference STL file were measured at nine anatomical landmarks.
Mean absolute discrepancies in the Zendura FLXTM aligners ranged from 0.076 ± 0.057 mm to 0.260 ± 0.089 mm and those in the Essix ACETM aligners ranged from 0.188 ± 0.271 mm to 0.457 ± 0.350 mm, while in the direct-printed aligners, they ranged from 0.079 ± 0.054 mm to 0.224 ± 0.041 mm. Root mean square values, representing the overall trueness, ranged from 0.209 ± 0.094 mm for Essix ACETM , 0.188 ± 0.074 mm for Zendura FLXTM , and 0.140 ± 0.020 mm for the direct-printed aligners.
This study showed greater trueness and precision of direct-printed aligners than thermoformed aligners.


Aligner; Physical property; Resin; Three-dimensional scanner


  • Figure 1 Anatomical landmarks on the reference model. Incisal/occlusal: mid-incisal edge point of the lateral incisors (MI), midpoint on the central groove of the second premolars (PG), mesio-lingual cusp tips of the first molars (ML). Mid-crown: functional axis of clinical crown points of central incisors (FACC), midpoint on the palatal surfaces of the first premolars (MP), the buccal pit of the second molars (BP). Gingival: the gingival zenith of the central incisors (GZ), the highest point on the palato-gingival margin of the first premolars (HP), the central point on the gingival margin of the first molars (MC).

  • Figure 2 Aligner mesh (purple) superimposed over the master stereolithography (STL) mesh (blue) by using the best-fit algorithm from Geomagic® Control XTM (3D Systems, Morrisville, NC, USA). Some portions of the aligner mesh are behind the master STL (– values), while some are in front (+ values).

  • Figure 3 Representative heatmaps of an Essix ACETM (Dentsply Sirona, Sarasota, FL, USA) aligner, a Zendura FLXTM (Zendura Dental, Fremont, CA, USA) aligner, and a direct-printed aligner (from left to right). Green areas represent areas where the aligner did not deviate from the model by more than 0.25 mm while bluer areas represent deviations in the negative direction and redder areas represent deviations in the positive direction.

  • Figure 4 Modified Bland–Altman plots of landmark measurements for all samples. Different colors denote different samples. A, Zendura FLXTM (Zendura Dental, Fremont, CA, USA) aligners. B, Essix ACETM (Dentsply Sirona, Sarasota, FL, USA) aligners. C, Direct-printed aligners.


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