Numerous studies have proven divergence in the perception of aesthetically appealing smiles, with laypeople accepting a wider range of deviations from common aesthetic norms than dental professionals [11, 24,25,26,27,28,29,30].

The objective of this study was to study whether there is a difference in the perception of tooth shade change and anatomical variations of anterior maxillary teeth among dental students depending on the level of education, gender, experience in assisting in a dental office and the presence of a dentist in the immediate family. The research was conducted among preclinical and clinical students evaluating the aesthetic acceptability of a smile on the unaltered, reference photograph (RP) and on the other 28 modified photographs (MP).

The results of the present study proved that preclinical students did not notice small modifications or accepted a wider range of tooth shade, length, and position modifications in comparison to clinical students. The knowledge of preclinical students is scarce, they have not had the opportunity to practice their perception in preclinical education. However, the “ideal” (RP) was best rated in both groups without significant difference. Omar and Tai [27] also showed that dental students rated the ideal photograph with the highest score, and significantly lower photographs with esthetic deviations. Obviously, targeted clinical education influenced clinical students in the present study to notice changes earlier and/or adopt more critical aesthetic norms. Similar conclusion was made by Mannaa [28], stating that dental students’ exposure and awareness of esthetic dentistry topics increased with academic progression.

This study also showed that MPs with the highest deviations towards lighter spectrum in the category of tooth shade were rated with the lowest scores, especially in clinical students. Relatively high scores were assigned to photographs showing small deviations in the category of tooth shade change towards darker spectrum (WB 5900 K), without significant difference between clinical and preclinical students, which can be attributed to the natural, yellowish shade of the teeth, especially canines, which are slightly darker in natural dentition. However, larger modifications towards darker spectrum were scored worse in both groups. Some studies proved that color-matching skills can be improved by education, knowledge [31,32,33,34] and targeted color matching training [35], which is in line with our results, especially for the perception of lighter shades of clinical students. Practice and education sharpen students’ clinical ability to recognize different tooth shades and match shades with restorative and prosthetic materials. Although clinical students gave lower scores to all MPs than preclinical students, the lowest scores in both groups were given to the lightest shades and the midline diastema. Noureddine et al. [36] examined multiple diastemas of anterior teeth and midline diastema and found out that the midline diastema was rated worst, suggesting that negative impact was related to the width of a midline diastema. In similar study clinical students ranked a midline diastema to be significantly lower than preclinical students who perceived the smile with retroclined incisors as significantly less pleasing than the clinical students. Together with midline diastema, open bite was perceived as an unfavorable feature by both year-groups [37].

A significant difference in the evaluation of tooth rotations between clinical and preclinical students could not be compared with other studies as no documented studies were found in the available dental literature. Alhammadi et al. [38] examined the perception of different aesthetic parameters of the orofacial region in students of the preclinical and clinical groups and found that clinical students were more successful in recognizing minimal deviations as opposed to preclinical students, which is in line with our results. Khalaf et al. [39] also showed that the higher the level of dental education (dental specialists, dentists, dental students, and laypersons), the lower the aesthetic perception and that dental educational level influences the aesthetic perception of anterior crowding and spacing, which may have an impact on treatment planning and need for orthodontic intervention. Although they have proven that education (students vs. dentists) has an impact on aesthetic perception, they did not make a detailed analysis between students of different years, and only clinical (fourth, fifth and sixth year) were included.

Professional knowledge can, besides targeted academic education, also be obtained from other sources such as assisting in a dental office or from a presence of a dentist in the immediate family [40, 41]. Therefore, this study also aimed to examine students’ knowledge acquired by assisting or from a dentist – a close family member. No significant effect of assisting or having a dentist in the immediate family on perception of color, shape and tooth position was observed (p > .05). Furthermore, as clinical students who reported having experience in assisting acquired knowledge also in academic education, only preclinical students were selected and there was no significant difference between those who assisted or did not (t-test, p > .05). Obviously, targeted academic education was more efficient in acquiring knowledge and skills required to notice deviations and modifications related to teeth shade, dimensions and positions, which was confirmed by significantly lower scores of clinical students in this study.

The only significant effect of gender was that females gave significantly lower scores for MPs towards lighter spectrum than male students, while the perception of dimension or position modifications showed no significant effect. In similar studies that dealt with the difference in the perception of aesthetic deviations, a greater criticism of female participants was also proven [42]. Color recognition difficulties are more common in men than in women [43, 44]. In investigating the tolerance thresholds of aesthetic deviations, the results of previous studies are contradictory. While some emphasize the important role of practice and experience in accurately determining and thus perceiving color [45,46,47,48], others argue that experience has minimal impact on the ability to accurately determine tooth shade [49,50,51].

It is important to discuss certain limitations of this study. To ensure good color vision the exclusion criteria was set at the error score > 20 (FM hue test). However, it allowed some small differences in color vision between gender. Some other factors like influence from the social media, fashion changes, etc. have not been included in this research. Studies on the aesthetics of smiles differ greatly depending on how data are collected (online surveys, self-smile evaluation, photographs, software photo manipulation), choice of parameters for evaluation, and evaluators’ subjective esthetics perception, making it difficult to compare results [42]. This study arbitrarily selected the characteristics of a smile that the authors considered important in the perception of a smile as aesthetically appealing. Other limitations of the present study are mood swings, tiredness, lack of motivation, and different type of PC screens during assessments.

The strength of the research is including the tooth rotations in smile aesthetic evaluation (which has not been included in previous studies), a sufficient number of participants in the preclinical and clinical groups, an equal ratio of female and male students similar to the proportion of gender distribution in the respective dental school, and evaluation of tooth shade, length, and tooth position modifications, analyzing mutual effects of belonging to the clinical or preclinical group, gender and possible knowledge acquired from other sources. One of the biggest strengths of the study is the use of the intraoral scanner and its software to simulate tooth rotations on a 2D photograph, which is novelty, and it has not been reported yet in the dental literature.

It is important to monitor students’ professional development and compare the impact of education on the criticism towards deviations from aesthetic norms. Understanding students’ perception and level of criticism depending on their level of education are of great importance because educators can get insight into the progression of their education. Senior students should be able to make critical clinical decisions considering the aesthetic components of a smile.