# A Comparison of Dental Arch Width and Length on 3D Digital and Plaster Models
## 2 Materials and Methods  
The study was approved by the Ethics Committee of the School of Dental Medicine, University of Zagreb, Croatia (05-PA-15-12/2017 on 14 December 2017), and it was conducted in accordance with the Declaration of Helsinki. All subjects provided written informed consent. The study involved 30 randomly selected subjects, each with no missing permanent teeth from the first molar to the first molar in both jaws and without abnormal morphology or defects that could affect the crown diameter. A minimum sample size of 24 study models per group was estimated using a power of more than 80% to detect a difference of 0.05 mm, with a standard deviation of 0.06 mm and a significance level of 0.05. An online calculator was used ([[https://statulator.com/SampleSize/ss2M.html](https://statulator.com/SampleSize/ss2M.html)]([https://statulator.com/SampleSize/ss2M.html](https://statulator.com/SampleSize/ss2M.html)); accessed on 2 February 2019). Full-arch impressions of the maxillary and mandibular were taken with alginate (Orthotrace alginate, Cavex, Haarlem, Holland) and silicone (3M ESPE, Seefeld, Germany) material. The impressions were poured after half an hour with a type IV dental stone (Sherakfo-gips, Shera, Lemfoerde, Germany) according to the manufacturer's guidelines. After 40 min, the impressions were removed from the plaster models. The plaster models were stored at room temperature for 72 h before being scanned using a laboratory Neway scanner (Open Tech 3D, Brescia, Italy) to create digital models according to the manufacturer's instructions. The same plaster model obtained from the alginate impression was used for manual and digital measurements. Direct intraoral digital impressions of both the arches and occlusion were taken using the Trios 3 intraoral scan device (3 Shape, Copenhagen, Denmark). The scanning process began with the occlusal surface of the left first molar, followed by slow progression towards the incisors and continuing until the last molar on the right side. Then, the scan probe was rotated to the lingual side, scanning along the lingual surface until reaching the last molar on the opposite side. Subsequently, the scan probe was smoothly rolled to the buccal side, ensuring comprehensive coverage of the entire arch to the opposite side. When scanning the maxillary arch, a similar procedure was followed, starting from the occlusal side, progressing to the buccal, and concluding with the palatal side. Four sets of study models of each subject were taken by the same person using conventional and digital impression techniques. The workflow of the procedures is shown in Figure 1.  
The measurements were simultaneously and independently performed by two examiners. After one month, both examiners independently repeated measurements of 5 randomly selected models to evaluate measurement error (both random and systematic error). On the plaster model, measurements were performed manually using the digital caliper Mitutoyo (Ontario, Canada). Measurements of digital models were made using Ortho Analyzer software (version 1.5.1.7, 3 Shape, Copenhagen, Denmark). The measurements made on both arches (Mx-maxillary, Md-mandibular) included the following:  
* ICW--transversal width between the tips of left and right canines (Figure 2);
* IMW--transversal width between the mesiobuccal cusp tips of the left and right first permanent molars (Figure 2);
* AP--curve line connecting the mesial contact point of the permanent first molars along the buccal cusps of the premolars and incisal edges of the anterior teeth. The arch perimeter on the plaster models was determined as the length of a soft wire (e.g., brass wire) which was modeled to the individual shape of the maxillary and mandibular arch (Figure 3). Statistical analysis was performed using the IBM SPSS Statistics software (Version 22.0, IBM SPSS Inc., Chicago, IL, USA). After verifying the normality of the distribution using the Shapiro-Wilk test, parametric and non-parametric statistical tests were performed.