Aesthetic Preference in the Transverse Orientation of the Occlusal Plane in Rehabilitation: Perspective of Laypeople and Dentists

International Journal of Environmental Research and Public Health Article Aesthetic Preference in the Transverse Orientation of the Occlusal Plane in Rehabilitation: Perspective of Laypeople and Dentists Ana Lidia Carvalho 1, Liliana Gavinha Costa 1, Joana Meneses Martins 1, Maria Conceição Manso 1,2 , Sandra Gavinha 1, Mariano Herrero-Climent 3, Blanca Ríos-Carrasco 4,* , Carlos Falcão1,3 and Paulo Ribeiro 1,3   Citation: Carvalho, A.L.; Costa, L.G.; Martins, J.M.; Manso, M.C.; Gavinha, S.; Herrero-Climent, M.; Ríos-Carrasco, B.; Falcão, C.; Ribeiro, P. Aesthetic Preference in the Transverse Orientation of the Occlusal Plane in Rehabilitation: Perspective of Laypeople and Dentists. Int. J. Environ. Res. Public Health 2021,18, 12258. https:// doi.org/10.3390/ijerph182212258 Academic Editor: Paul B. Tchounwou Received: 11 October 2021 Accepted: 20 November 2021 Published: 22 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1FP-I3ID, FCS, Universidade Fernando Pessoa, 4249-004 Porto, Portugal; [email protected] (A.L.C.); [email protected] (L.G.C.); [email protected] (J.M.M.); [email protected] (M.C.M.); [email protected] (S.G.); [email protected] (C.F.); [email protected] (P.R.) 2FP-I3ID, Instituto de Investigação, Inovação e Desenvolvimento, Universidade Fernando Pessoa, 4249-004 Porto, Portugal 3Porto Dental Institute, 4150-518 Porto, Portugal; [email protected] 4Department of Periodontology, Universidad de Sevilla, 41009 Seville, Spain *Correspondence: [email protected]; Tel.: +34-667760101 Abstract: The present study had a convenience sample with 236 laypeople and 242 dentists who completed an online questionnaire to choose the most attractive image among six pairs for comparison. Control image: symmetric (parallelism between occlusal plane (OP), commissural line (CL), and interpupillary line (IL)). Change of Control, obtaining three images with a 3-degree inclination of the labial commissures. Image A: OP parallel to IL; Image B: OP parallel to CL; Image C: OP at 1.5 degree mean angulation between IL and CL. Non-parametric comparison (IBM © SPSS Statistics vs. 27.0, p< 0.05 ). The “Dentists” group’s decreasing order of preference (attractiveness) of the images is: Control > A > C > B (p< 0.05). In the “Lay” group, it is: Control > A > (C not 6= B). Dentists significantly prefer more the Control and Image A than laypeople (p< 0.001). Sex (single exception in laypeople), age, and dentist’s area of activity did not interfere in the perception of attractiveness. Dentists and laypeople preferred the Control when compared to images with CL canted. In the existence of CL inclination, the preference of the groups was the IL as a reference for OP orientation, with the mean angulation or coincident with the CL being considered less aesthetic. Keywords: prosthodontics; dental esthetics; occlusal plane canting; commissural lines; interpupillary line; facial asymmetry; laypeople; dentists 1. Introduction Whenever the rehabilitation of a patient is at stake, an effective integration of the dental prosthesis into the face is a determining factor for achieving a successful final result [ 1 ]. To achieve this goal, one of the key factors in determining the prognosis of totally edentulous patients is the establishment of an occlusion compatible with the movements produced by the stomatognathic system [ 2 ]. There is also the need to respect the patient’s craniofacial structures and neuromuscular mechanism [3]. One of the three definitions of the occlusal plane (OP) presented in the 9th Edition of the Glossary of Prosthodontic Terms, states that this is “the average plane established by the incisal and occlusal surfaces of the teeth; generally, it is not a plane but represents the planar mean of the curvature of these surfaces” [ 4 ]. The correct orientation of the OP results in better stability of the total denture, which allows avoiding the transfer of undue stresses to the underlying residual bone crests, delaying its resorption. Furthermore, it improves the aesthetic performance of the smile and function of the final prosthesis [5,6]. Int. J. Environ. Res. Public Health 2021,18, 12258. https://doi.org/10.3390/ijerph182212258 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021,18, 12258 2 of 13 According to Ahmad, facial analysis methods that are based on mathematical principles of beauty assessment, such as anatomical landmarks and imaginary facial lines, are currently used in oral rehabilitation to reestablish OP [7]. The present literature largely explores the principles to be followed by dentists when it comes to reestablishing the OP in a sagittal perspective, valuing the use of different parameters such as the Camper Plan, the Frankfurt Plan, and the Natural Head Position [8–11]. In order to maintain facial harmony in the transverse analysis of the OP, from a frontal perspective, it is known that it must be perpendicular to the facial midline and parallel to the horizontal facial reference lines, such as the interpupillary line (IL) and the line of the labial commissures (CL) [12,13]. Perfect facial symmetry is a theoretical concept, as all human faces including those considered “most attractive” are asymmetrical [ 14 , 15 ]. Thus, it is difficult for the clinician to establish a threshold that separates pleasant asymmetrical faces from those that need intervention [16–18]. Facial asymmetries existing in the CL and the inclination of the OP were described as common conditions that affect the aesthetics of the smile. When faced with these asymmetries, the clinician must choose the reference line with which to guide the transversal OP in the planned oral rehabilitation [ 19 ]. However, the relationship between the position of the labial commissures and the OP has not been sufficiently investigated and clarified [1,20]. Currently, the scientific literature disagrees as to which horizontal reference to use in framing the OP in the facial context, and there is also a need to determine within what limits are visually accepted discrepancies in the transverse plane. Thus, the need for further studies to help clarify the dentists’ decision-making process during the rehabilitative diagnosis process is urged [1,12,13,20]. The present study examined what is considered more aesthetic in the opinion of dentists and laypeople when comparing asymmetric smiles to potentially serve as guidance regarding the rehabilitation of patients with CL inclinations and also to investigate the differences in the perception of attractiveness between groups. Finally, it was analyzed if the sex, age, and dentist’s area of activity influenced this perception. 2. Materials and Methods Data Collection Tool In this cross-sectional study, data were collected using an online questionnaire consisting of two parts. Initially, socio-demographic issues were addressed (age group, sex, whether layperson or dentist, and in the latter case, the area of activity). Then, the second part included the identification of the preferred image in a total of 6 comparisons of 2 images. The images used in the questionnaire were obtained from a facial photograph acquired in a photographic studio (Figure 1a), with a Nikon D750 © camera (Nikon, Tokyo, Japan) and AF-S VR Micro-Nikkor 105 mm f/2.8 G IF-ED lens (Nikon, Tokyo, Japan). Next, a symmetrical facial model was created (digitally) in which the image designated as Control has parallelism between the OP, the IL, and the CL (Figure 1b). Subsequently, the Control image was altered in order to obtain three new images with a 3-degree CL tilt, counterclockwise (Figure 1c). The 3-degree angle was chosen based on a previous study that found this angle noticeable by most laypersons [ 19 ]. These three images differ from each other by the orientation of the OP. Image A has the OP parallel to the IL ( Figure 1d ), Image B has the OP parallel to the CL (Figure 1e), and Image C has the OP in a mean angulation formed between the angle of the IL and the CL, which is equivalent to 1.5 degrees ( Figure 1f ). The 4 images (Control, Image A, Image B, and Image C) were arranged in 6 pairs (Control and Image A; Control and Image B; Control and Image C; Image A and Image B; Image A and Image C; Image B and Image C). All manipulations made to the images were carried out in Adobe Photoshop © (Adobe Inc, San Jose, CA, USA) by the main author. Int. J. Environ. Res. Public Health 2021,18, 12258 3 of 13 Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 3 of 13 images were carried out in Adobe Photoshop© (Adobe Inc, San Jose, CA, USA) by the main author. (a) (b) (c) (d) (e) (f) Figure 1. (a) Original asymmetric image; (b) Symmetrical image—Control; (c) 3-degree CL tilt; (d) OP parallel to the IL; (e) OP parallel to the CL; (f) OP mean between the IL and the CL equivalent to 1.5 degrees. Figure 1. ( a ) Original asymmetric image; ( b ) Symmetrical image—Control; ( c ) 3-degree CL tilt; ( d ) OP parallel to the IL; ( e ) OP parallel to the CL; ( f ) OP mean between the IL and the CL equivalent to 1.5 degrees. 3. Process of Editing Images in Adobe Photoshop© 3.1. Creation of Symmetrical Facial Model: Control The facial midline was correctly identified using the program’s ruler. The half of the image that was chosen to be duplicated was selected (rectangular marquee tool). After the selected half was duplicated in a second layer, the images were mirrored to make a Int. J. Environ. Res. Public Health 2021,18, 12258 4 of 13 single fully symmetrical image. Finally, the two layers were joined to make a single final image. No changes were made to the color and shape of the teeth. The original dental characteristics of the voluntary model were preserved, since maintaining a realistic image was a priority, so as not to cause strangeness or a sensation of artificiality in front of the study participants. 3.2. Creation of the Asymmetric Facial Model The selection of the entire contour of the lip vermilion, inner and outer line was made with the polygonal lasso tool, so that a 3-degree rotation to the top and to the left could be performed (free transformation and rotation). The teeth were not selected and therefore were kept in the same position. Color, texture, fill, and luminosity adjustments were made to correctly adapt the OP, gums, and lips to their new positions (polygonal lasso tool, content aware, mixer brush tool, eyedropper tool, history brush tool). From the asymmetrical facial model, the following 3 images were made with OP deviation. 3.3. Creation of Image A: Occlusal Plane Parallel to the Interpupillary Line Two parallel lines were drawn to be used as a reference in the OP adjustment. One of the lines crossed the two pupils, and the other was rested under the OP. The OP did not need adjustment, as the image was already parallel to the LI. 3.4. Creation of Image B: Occlusal Plane Parallel to the Labial Commissures A line that passed through the two labial commissures was drawn to be used as a reference in the OP adjustment. All teeth were selected at the same time (polygonal lasso tool) from the inner line of the lip vermilion. The selected fragment of the image was rotated 3 degrees to coincide with the left labial commissure (free transformation and rotation). Thus, when tracing the second line under the OP, it was found that both lines were parallel to each other. Color, texture, fill, and luminosity adjustments were made to correctly adapt the OP, gums, and lips to their new positions (polygonal lasso tool, content aware, mixer brush tool, eyedropper tool, history brush tool). 3.5. Creation of Image C: Occlusal Plane Mean between the Interpupillary Line and the Commissure Line Equivalent to 1.5 Degrees All teeth were selected at the same time with the polygonal lasso tool starting from the inner line of the lip vermilion. This selected image fragment was rotated 1.5 degrees toward the left labial commissure (free transformation and rotation). Color, texture, fill, and luminosity adjustments were made to correctly adapt the OP, gums, and lips to their new positions (polygonal lasso tool, content aware, mixer brush tool, eyedropper tool, history brush tool). 4. Sample The participants included in this study were laypeople, as follows: patients from the Pedagogical Dental Clinics of the Universidade Fernando Pessoa and other participants who voluntarily, through a link, agreed to answer the questionnaire shared through social networks. Dentist professors related to the Fernando Pessoa Foundation were also invited to answer the questionnaire as were other dentists who agreed to answer the shared questionnaire through the link in private groups of dentists. All participants had to master the Portuguese language, as the questionnaire was written in Portuguese. The sizing of the sample (of the two groups) was calculated based on the method of estimation by power analysis for the difference of proportions in two groups, with the following assumptions: the proportion of respondents in a group (Dentist or Lay) that prefers an image was 0.5 (50%); 2% difference in the preference of the two groups was considered as a significant difference between the evaluated groups; a probability of a type I error of 0.05 and a type II error of 20% was considered. In this case, using the expression: n= [Zα×√2×p×(1 −p) + Zβ×√p1×(1 −p1) + p2×(1 −p2)](p1−p2)2. (1) Int. J. Environ. Res. Public Health 2021,18, 12258 5 of 13 It was calculated that each of these groups should have a size greater than 64 participants (n> 64). Data collection took place from February to May 2021, obtaining 478 participants: 236 laypeople (49.4%) and 242 MD (50.6%). 5. Ethical Considerations The study was approved on 10 February 2021 by the Ethics Committee of the Universidade Fernando Pessoa (FCS-MED—127/21) and by the Clinical Direction of the Pedagogical Dental Clinics. Due to the use of a voluntary model for the study, a written authorization was also requested before the images were obtained by camera. The informed assent filled in by the participant at the beginning of the questionnaire did not imply the collection of his/her name, so it is considered that the questionnaires were filled out anonymously. The complete questionnaire can be read in Supplementary File S1. All the information collected was treated as a group and, in this way, the anonymity of the participants was guaranteed, even in the face of an eventual crossing of sociodemographic data. 6. Data Analysis The data collected from the questionnaires were organized and stored in Excel automatically and the statistical analysis was performed using the IBM © SPSS Statistics vs. 27.0 software (IBM Corp. released 2017, Armonk, NY, USA). For the purposes of data processing and with regard to the areas of activity of the Dentists, the responses obtained were categorized into two groups: “areas related to aesthetics” and “areas not related to aesthetics”. The first group mentioned above includes Dentistry, Prosthodontics, and Orthodontics, while all other areas were included in the second group (Oral Surgery, Implantology, Oral Medicine, Pediatric Dentistry, Periodontology, Endodontics, Hospital Dentistry, Health Oral Public, and Generalists). The description of the results obtained was performed using absolute (count of answers) and relative (percentage) frequencies. Chi-square tests were performed to detect significant differences in the choice of the most attractive image by the two groups as well as to identify whether the sex and age of the participants and the area of activity of the dentists were relevant factors in the perception of attractiveness. The comparison of image choice/preference (in each group) was performed using the binomial test. The analysis was performed considering a significance level of 5%. 7. Results It is observed (Table 1) that the decreasing order of preference of the images in the Dentists group is: Control > A > C > B. In the Laypeople group, there is a similar situation; however, no significant difference was detected in the preference (attractiveness) between the B and the C image. Thus, in the Laypeople group, the decreasing order of preference (attractiveness) in the images is Control > A > (C not 6= B). There were also significant differences in the perception of the attractiveness of the images between the groups. In short, Dentists significantly prefer the Control image more than Laypeople did (90.0% vs. 80.9% and 89.3% vs. 73.3%) and the A image more than Laypeople did (86.4% vs. 70.8%). In Table 2, no significant differences were detected in the preference according to sex (t. Chi-square ,p> 0.05), except for the comparison between Image A and C for the Lay group (p= 0.009), in which women significantly prefer Image A more than men (69.2% vs. 50%). In the Dentist’s group (Table 2), in both sexes, it is observed that the Control image is significantly more preferred than the A, B, and C images (binomial t, p< 0.001 for the three comparisons). In addition, they significantly prefer Image A over Images B and C ( p< 0.001 ), but there is no significant difference between Images B and C (p= 0.077 and 0.133). Thus, in this group and for each sex, it can be said that the decreasing order of preference (attractiveness) in the images is: Control > A > (C not 6= B). In the lay group (Table 2), in each sex, the preference is not homogeneous. Thus, for women, the descending order of preference (attractiveness) in the images is: Control > A > (C not 6= B). For lay men, Int. J. Environ. Res. Public Health 2021,18, 12258 6 of 13 the descending order of preference (attractiveness) in the images is: Control not 6= A but >C > B, A not 6=C but both >B. Table 1. Comparison of the perception of attractiveness in each group and between groups. Image Dentists n = 242 Laypeople n = 236 p** Control A p* 176 (72.70%) 66 (27.30%) <0.001 169 (71.60%) 67 (28.40%) <0.001 0.785 Control B p* 220 (90.90%) 22 (9.10%) <0.001 191 (80.90%) 45 (19.10%) <0.001 0.002 Control C p* 216 (89.30%) 26 (10.70%) <0.001 173 (73.30%) 63 (26.70%) <0.001 <0.001 A B p* 209 (86.40%) 33 (13.60%) <0.001 167 (70.80%) 69 (29.20%) <0.001 <0.001 A C p* 174 (71.90%) 68 (28.10%) <0.001 153 (64.80%) 83 (35.20%) <0.001 0.096 B C p* 102 (42.10%) 140 (57.90%) 0.017 107 (45.30%) 129 (54.70%) 0.172 0.482 * Binomial test; ** Chi-square test. Bold pvalues stand for significative differences. Table 2. Comparison of the perception of attractiveness in each group and by sex. Image Dentists Laypeople Female n = 155 Male n = 87 p** Female n = 182 Male n = 54 p** Control A p* 115 (74.2%) 40 (25.8%) <0.001 61 (70.1%) 26 (29.9%) <0.001 0.494 135 (74.2%) 47 (25.8%) <0.001 34 (63.0%) 20 (37.0%) 0.077 0.109 Control B p* 143 (92.3%) 12 (7.7%) <0.001 77 (88.5%) 10 (11.5%) <0.001 0.330 152 (83.5%) 30 (16.5%) <0.001 39 (72.2%) 15 (27.8%) 0.002 0.064 Control C p* 142 (91.6%) 13 (8.4%) <0.001 74 (85.1%) 13 (14.9%) <0.001 0.114 138 (75.8%) 44 (24.2%) <0.001 35 (64.8%) 19 (35.2%) 0.041 0.108 A B p* 138 (89.0%) 17 (11.0%) <0.001 132 (72.5%) 50 (27.5%) <0.001 0.106 71 (81.6%) 16 (18.4%) <0.001 35 (64.8%) 19 (35.2%) 0.041 0.274 A C p* 112 (72.3%) 43 (27.7%) <0.001 62 (71.3%) 25 (28.7%) <0.001 0.869 126 (69.2%) 56 (30.8%) <0.001 27 (50.0%) 27 (50.0%) 1.000 0.009 B C p* 66 (42.6%) 89 (57.4%) 0.077 36 (41.4%) 51 (58.6%) 0.133 0.856 88 (48.4%) 94 (51.6%) 0.711 19 (35.2%) 35 (64.8%) 0.041 0.088 * Binomial test; ** Chi-square test. Bold pvalues stand for significative differences. In the results of Table 3, it is observed that in the Dentist’s group, the Control image is significantly more preferred than A, B, and C (p ≤ 0.003), and that they prefer A significantly more than B and C (p< 0.001), with the exception of the age group “>45 years old” in the comparison between A and C, in which there is no significant difference (p= 0.324) in the Int. J. Environ. Res. Public Health 2021,18, 12258 7 of 13 perception of the images. It is also noted that in the age group of “36 to 45 years”, Dentists significantly prefer C when compared to B (p= 0.028). Table 3. Comparison of the perception of attractiveness in each group by age group. Image Dentists Laypeople ≤35 Years Old n = 145 36 to 45 Years Old n = 60 >45 Years Old n = 37 p** ≤35 Years Old n = 172 36 to 45 Years Old n = 15 >45 Years Old n = 49 p** Control A p* 105 (72.4%) 40 (27.6%) <0.001 43 (71.7%) 17 (28.3%) 0.001 28 (75.7%) 9 (24.3%) 0.003 0.903 124 (72.1%) 48 (27.9%) <0.001 12 (80.0%) 3 (20.0%) n.a. 33 (67.3%) 16 (32.7%) 0.022 0.614 Control B p* 135 (93.1%) 10 (6.9%) <0.001 54 (90.0%) 6 (10.0%) <0.001 31 (83.8%) 6 (16.2%) <0.001 0.204 142 (82.6%) 30 (17.4%) <0.001 13 (86.7%) 2 (13.3%) n.a. 36 (73.5%) 13 (26.5%) 0.002 0.304 Control C p* 133 (91.7%) 12 (8.3%) <0.001 51 (85.0%) 9 (15.0%) <0.001 32 (86.5%) 5 (13.5%) <0.001 0.309 128 (74.4%) 44 (25.6%) <0.001 12 (80.0%) 3 (20.0%) n.a. 33 (67.30%) 16 (32.7%) 0.022 0.511 A B p* 126 (86.90%) 19 (13.1%) <0.001 52 (86.70%) 8 (13.3%) <0.001 31 (83.8%) 6 (16.2%) <0.001 0.883 117 (68.0%) 55 (32.0%) <0.001 12 (80.0%) 3 (20.0%) n.a. 38 (77.6%) 11 (22.4%) <0.001 0.311 A C p* 107 (73.8%) 38 (26.2%) <0.001 44 (73.3%) 16 (26.7%) <0.001 23 (62.2%) 14 (37.8%) 0.188 0.358 111 (64.5%) 61 (35.5%) <0.001 10 (66.7%) 5 (33.3%) n.a. 32 (65.3%) 17 (34.7%) 0.046 0.983 B C p* 66 (45.5%) 79 (54.5%) 0.319 21 (35.0%) 39 (65.0%) 0.028 15 (40.5%) 22 (59.5%) 0.324 0.373 74 (43.0%) 98 (57.0%) 0.079 8 (53.3%) 7 (46.7%) n.a. 25 (51.0%) 24 (49.0%) 1.000 0.497 * Binomial test; ** Chi-square test; n.a: not applicable. Bold pvalues stand for significative differences. Thus, in the Dentists group and in the age group “ ≤ 35 years old”, it can be said that the decreasing order of preference (attractiveness) in the images is: Control > A > (C not 6=B). In the age group from “36 to 45 years old”, the order of preference will be: Control > A > C > B. Finally, the age group “>45 years old” with Control > A > B, but A is not 6= C and B is also not 6=C. Regarding the Laypeople group, it is observed that only in the age group “ ≤ 35 years old” it is possible to establish a descending order of preference (attractiveness) in the images, as follows: Control > A > (C not 6= B). In the age group from “36 to 45 years old”, it was not possible to infer any statistical results due to the insufficient sizing of the sample. In the age group “>45 years old”, only the preference of A was detected when compared to B (p< 0.001). Finally, in both groups, no significant differences were detected in preference according to age group. It is possible to see in Table 4that the group of areas “not related to aesthetics” detects significant differences in attractiveness in all comparisons made, except between Images B, and C. The Control image is significantly more preferred than Images A, B, and C ( p< 0.001 ). Furthermore, there is a significant preference for Image A when compared to Images B and C (p< 0.001). However, no significant difference is detected between Images B and C. Thus, for this group, it is possible to state that the decreasing order of preference (attractiveness) in the images is: Control > A > (C not 6=B). In the group of areas “related to aesthetics”, there are significant differences in attractiveness in all comparisons, with a significant preference for the Control image over Images A, B, and C (p= 0.003, p< 0.001, p< 0.001, respectively). Next, there is a preference for Image A over Images B and C (p< 0.001), and significantly more for Image C over B (p= 0.012). Thus, in this group, it is verified that the decreasing order of preference (attractiveness) in the images is: Control > A > C > B. Int. J. Environ. Res. Public Health 2021,18, 12258 8 of 13 Table 4. Comparison of the dentists’ perception of attractiveness by area of activity. Image Area of Activity Related to Aesthetics No n = 158 Yes n = 84 p** Control A p* 120 (75.9%) 38 (24.1%) <0.001 56 (66.7%) 28 (33.3%) 0.003 0.123 Control B p* 145 (91.8%) 13 (8.2%) <0.001 75 (89.3%) 9 (10.7%) <0.001 0.522 Control C p* 142 (89.9%) 16 (10.1%) <0.001 74 (88.1%) 10 (11.9%) <0.001 0.671 A B p* 132 (83.5%) 26 (16.5%) <0.001 77 (91.7%) 7 (8.3%) <0.001 0.080 A C p* 112 (70.9%) 46 (29.1%) <0.001 62 (73.8%) 22 (26.2%) <0.001 0.630 B C p* 72 (45.6%) 86 (54.4%) 0.301 30 (35.7%) 54 (64.3%) 0.012 0.139 * Binomial test; ** Chi-square test. Bold pvalues stand for significative differences. Finally, there are no significant differences between the dentist’s group (areas related to aesthetics and areas not related to aesthetics) in any of the six cases. 8. Discussion The results, when significant, showed a clear preference of dentists and laypeople to the completely symmetrical image when compared to the others, confirming once again the importance of facial symmetry in the perception of attractiveness reported in several studies over the years [ 21 – 23 ]. Still, it is important to note that while the brain may receive the image of facial symmetry as a very attractive visualization, all evidence points to varying degrees of asymmetry as the natural state of the human face, both anatomically and functionally [ 24 ]. It is up to the dentist to distinguish, from an aesthetic point of view, a pleasing asymmetric face from an asymmetric face that requires intervention [18]. In the present study, the preference found in both groups was for the image with the OP parallel to the IL, even in the presence of deviation in the CL, which goes along with what Ahmad says when he states that: “The interpupillary line is used as a reference for the occlusal and incisal plane orientations. The other horizontal lines can be eschewed and therefore do not act as definite references.” [ 7 ]. Even so, this result refutes Silva et al. in their online survey for lay participants when they report that in cases where there is a lack of parallelism between the IL and the CL, most laypeople prefer the transversal OP leaning in the same direction as the commissures [ 19 ]. However, the study warns that the degree of OP inclination must be determined for each patient individually, since approximately four out of 10 (40%) participants preferred the OP completely parallel to the IL. The dentists, on the other hand, possibly have their aesthetic perception molded to what is most commonly described in the available scientific literature, in which an example can be given by Fradeani, who defends the concept that the IL, if parallel to the horizontal plane, is the most adequate reference to perform facial analyses. In this context, he adds that ideally, the CL should also be parallel to contribute to general harmony [ 25 ]. Finally, a recent study by Farret points out that in cases of asymmetry in the lip architecture when smiling, this asymmetry should not be taken into account when defining the diagnosis and treatment plan; therefore, IL is a more reliable reference in these situations [26]. Int. J. Environ. Res. Public Health 2021,18, 12258 9 of 13 In cases where there was a difference between the groups in the perception of attractiveness, it was shown that dentists were more capable of opting for the symmetrical image than laypeople, which makes it possible to assume that dentists have more precision in the observation of facial planes and assume them with a higher level of attractiveness, while laypeople tend to be more tolerant. The finding that professionals are able to perceive smaller differences in the OP is consistent with what Revilla-León et al. affirms, which further justifies this result by the fact that dentists are more used to seeing and treating small differences in facial appearance [ 1 ]. This finding is repeated in studies that analyzed the OP and other characteristics of the smile [27,28]. Due to the results of a research carried out by Ker et al., it was concluded that although dentists have greater knowledge about the aesthetics of the smile, they should not ignore the fact that laypeople are more tolerant than professionals to the different variations that may exist. The clinician must be careful when identifying the ideal for patients, when this knowledge could sensitize them to unrealistic or unattainable goals [29]. It is also verified in the results that the dentists significantly prefer more than laypeople the OP parallel to the IL when compared to the OP parallel to the CL (with 3-degree inclination), which can be explained by the possibility that the dentists were influenced in their choices in the academic training they had, as explained above, while the laypeople more unpretentiously voted for what seemed most aesthetic to them. Even so, the use of an OP parallel to the IL, as frequently advocated by prosthodontists, can result in suboptimal esthetics in the final restoration if the dentists do not take into account a series of facial asymmetries, in addition to the CL inclination, which can influence the choice of OP during prosthodontic treatment [ 30 ]. On the other hand, the use of the labial commissures as a guide for the orientation of the OP has the disadvantage of instability over time, since advancing age causes the corners of the mouth to fall or tilt, and in addition, the commissures are not observed in the same way in dentate and edentulous patients [ 2 ]. The dentists still reported more attractiveness in the OP mean between the IL and the CL equivalent to 1.5 degrees than when the OP parallel to the CL (3-degree inclination), while the laypeople were not able to significantly prefer between either of the two options. It is concluded that dentists, when faced with different OP inclinations, preferentially opt for the lowest inclination present in the comparison, not valuing the coincidence between the plane and the CL. As for the laypeople, a possible way of interpreting the observed insufficient preference can be based on the fact that in this group, the participants were possibly not able to distinguish between inclinations present in the two images, allowing the conclusion that inclinations of up to 3 degrees of the OP are not noticed by the laypeople. This contradicts the conclusions of the study by Geron and Atalia, in which it was found for the lay participants that the OP inclination from 2 degrees was perceived and considered unaesthetic [ 31 ]. Padwa et al., on the other hand, concluded that laypeople can identify OP angulations greater than 3 degrees 70% of the time [ 32 ]. Finally, still equally important to mention, there are two studies that state that laypeople were unable to detect this type of asymmetry up to 4 degrees (in the case of the first study, 3 mm inclinations equivalent to 4 degrees) [ 27 , 29 ]. Attention is also drawn to the fact that these studies did not take into account the relationship between the OP and the CL, which is why it is understood that in these cases, there were only inclinations of the OP, while the CL continued parallel to the IL. In general, sex was not a factor that affected the perception of attractiveness of each of the groups, which is similarly to the results obtained in the study by Jiménez-Castellanos et al. (for laypeople), in the study by Silva et al. in 2017 (for laypeople), Silva et al. in 2019 (for laypeople), and in disagreement with Revilla-León et al. (for laypeople, dentists, and dental students) [ 1 , 12 , 19 , 33 ]. The only exception regarding the participant’s sex occurred in the case of lay women, who significantly preferred the OP parallel to the IL than with a mean between the IL and the CL equivalent to 1.5 degrees, while lay men had no significant results that could declare the existence of some preference. These results can be interpreted in correlation with a study carried out by Koidou et al., in which they state, in agreement with previous findings, that dental appearance is more important for