# Reproducibility and Accuracy of Two Facial Scanners: A 3D In Vivo Study
## 4 Discussion
Additionally, it is important to consider how the scanning accuracy depended on the location of the scanned surface area, being more accurate on the middle of the face. In comparing surface areas, [PERSON] et al. confirmed that the forehead area had the lowest mean precision [34]. These findings are supported by the literature, which has demonstrated that the forehead showed an average overlap percentage of 54.46% within the highly reproducible band (0.5 mm to 0 mm and 0 mm to \(-\)0.5 mm), with previous studies confirming an even lower overlap percentage of 11.08% [9; 13]. The same difficulty in capturing images is also observed for the chin, as demonstrated by [PERSON] et al. The mandible, in fact, represents the only moving bone of the face, making the lower third less reproducible [35]. However, it is also important to emphasize that Vectra and Einscan are two different three-dimensional acquisition systems. The Canfield VECTRA M3 3D Imaging System is a static device that captures precise 3D models of the human face by combining synchronized 2D images with photogrammetric algorithms [36]. As demonstrated by a previous study, synchronized two-dimensional images of the subjects are captured within 0.75 milliseconds with Vectra 3D [37], making the system immune to subject movement and less susceptible to operator-dependent scanning errors. Moreover, it includes lighting specifically designed for clinical photography, which produces shadow-free images with colors that adjust to the patient's skin tones [38]. The EinScan H2 integrates structured light and infrared technology, offering versatility across various scanning applications and capturing the fine details of complex surfaces and dark materials with remarkable precision [39]. However, it has a scanning speed of 1,200,000 points/s and, as a portable device, it often requires an extensive learning curve to perform the scanning protocols optimally. These factors can lead to errors during the image acquisition process, resulting in lower accuracy and reproducibility in certain areas of the face [16; 39]. Despite the areas showing different percentages within the highly reproducible range (0.5 mm to 0 mm and 0 mm to \(-\)0.5 mm), it is very important to consider the 3D analysis of point measurements. Considering both the forehead and the chin, the trichion (Tr), nasion (Na\({}^{\prime}\)), and pogonion (Pog\({}^{\prime}\)) points did not show statistically significant differences in median values (\(p>0.05\)) between the two instruments, indicating a similarity between Vectra and Eiscan. These data allow us to highlight how, within the same area, some zones are more easily influenceable than others during the 3D scan. However, except for the forehead (82.52%), more than 90% of the averages of overlapping surfaces between the two scans fell within the tolerance band (1.5 mm to \(-\)1.5 mm). These data agree with the study by [PERSON] et al. [13], although in their study, the forehead also fell within this band. Notably, the mean percentages of areas within the highly reproducible band obtained in their study appear to be lower than those in ours. The limitations of this study include the high costs of the scanners, the need for a larger sample size to strengthen the obtained results, the challenge of obtaining a neutral facial expression while avoiding potential face movements to prevent image distortions, and the requirement for proper illumination during scanning. Moreover, it is necessary to consider that many factors influence the accuracy of the 3D scanner, such as the ability to record details, accuracy, scanning principles, span of scanning, size of the scanning area, arch length, surface irregularities, temperature, relative humidity, and illumination [19; 20; 21]. Additionally, it should be noted that only subjects without facial alterations or skin imperfections were selected. Furthermore, the sample lacks diversity in age and ethnicity, limiting the applicability of the results to a broader population.