Impact of Gingival Margin Asymmetries on the Smile Esthetic Perception of Dental Specialists, Doctors of Dental Medicine, Students, and Laypeople: a Comparative Pilot Study

Abstract

Objective

To evaluate the perceptions of altered incisor gingival position among dental specialists, dentists, dental students, and laypeople.

Materials and methods

Four digital smile photographs with altered gingival margin position of the right maxillary incisor (0, 1, 2, and 3 mm) were presented to a sample of 232 respondents (71.1% female; 28.9% male): 42 dental specialists, 63 dentists, 33 dental students (1^st to 3^rd year), 38 dental students (4^th to 6^th year) and 56 laypeople. The questionnaire consisted of four randomly displayed photographs, administered via Google Form, and respondents were asked to rate the images on a scale from 1 to 5, from the least attractive to the most attractive. A statistical analysis was performed using the TIBCO Statistica program (v. 13.3. 0, TIBCO Software Inc., Palo Alto, CA, USA; 2017). According to the Shapiro- Wilk's test, the data were not distributed normally. The Kruskal-Wallis test with post hoc multiple comparisons with the Bonferroni adjustment were used to compare group esthetic scores and to determine the threshold levels of deviation at which each group was discriminated between esthetic and non-esthetic situations.

Results

Median values of esthetic scores decreased in all groups as the gingival asymmetries increased. Dental professionals were significantly more critical of esthetics than laypeople in all levels of asymmetry. The greatest difference was found for 2 mm of gingival asymmetry (p=0.002).

Conclusion

From the results of our study, we can conclude that the perception of gingival asymmetries in the esthetic zone of smile differs among dental specialists, doctors, students, and laypeople. Dental specialists, doctors, and clinical students were more critical of these asymmetries, while preclinical students and laypeople noticed only 2 mm or more of gingival asymmetry of central incisors.

Introduction

It is said that beauty is in the eyes of the beholder, it is not unified, but it depends on the person who is observing it. The criteria for a beautiful smile vary from person to person even when they are of the same profession. An attractive smile should have symmetric and minimal gingival display, teeth in correct proportion, form, and position, proper shade and color, no interproximal spaces, and harmony between the lower lip and incisal edges of maxillary anterior teeth (1). The ideal shape of the gingival margin is characterized by several factors, one of which is that the gingival margin of the upper central incisors should be at the same level (2). Although we learn to apply these principles in our daily work during our education, it is difficult to apply them if they are not quantified and supported by scientific research. Numerous studies have been conducted to examine the perception of smile among specialists, dentists, and laypeople (1, 3-5). Kokich reported that orthodontists notice specific dental discrepancies easier than laypeople, while dentists and laypeople are similarly critical to maxillary arch deviation, marginal gingival distance, and the gingiva-lips distance (4).

Gingival asymmetry can lead to an unbalanced appearance of a smile (6). The objectives of many studies are to determine the acceptable level of asymmetry and when to perform either periodontal or orthodontic treatment. It has been reported that laypeople do not notice the 1.5 mm of gingival asymmetry, therefore the procedures such as periodontal surgery or orthodontic tooth movement accompanied by composite restoration are often indicated by excessive specialist care, rather than a real esthetic problem. For asymmetries exceeding 2 mm, these procedures are recommended, with an orthodontic approach being preferred because the bone and periodontal tissue move together with the intrusion or extrusion of the tooth (3). In a similar study conducted by Kokich and Kiyak laypeople failed to detect gingival asymmetry of maxillary central incisors from 1.5 mm to 2 mm, while an asymmetry of 0.5 mm was noticed by orthodontists (7).

The aim of this study was to examine how dental education in Croatia affects smile perception. The hypothesis was that laypeople would be less concerned about deviations from ideal gingival symmetries than clinical students, dentists, and dental specialists, while the perception of preclinical students would be similar to that of laypeople.

Materials and methods

The research was conducted after the approval of the Ethics Committee of the School of Dental Medicine, University of Zagreb. The image of an attractive female smile, according to objective criteria (1) was taken with a Nikon D750 camera with a macro 105 mm lens, and digitally processed in the computer program Adobe Photoshop 6 (version 6.0, San Jose, California, USA) wherein three modifications were made. The levels of maxillary central incisors gingiva are symmetrical on the original image. In the first modified image, the gingival level of the right central incisor was lowered by 1 mm, 2 mm in the second image, and 3 mm in the third image, all relative to the left incisor (Figure 1). In order to transfer the values ​​to the image in the correct ratio, a millimeter-scale was transferred to the image, also using a Photoshop computer program (Figure 2). A total of 232 respondents: 42 dental specialists, 63 dentists, 33 dental students (1^st to 3^rd year), 38 dental students (4^th to 6^th year), and 56 laypeople participated in the study. Voluntary participants were recruited on social media during August and September of 2021. Exclusion criteria for laypeople were professional dentistry knowledge, such as dental technicians and dental hygienists. The questionnaire consisted of 7 questions about demographic and socio-economic characteristics of respondents and 4 photographs (Figure 2), displayed in random order.

Figure 1.

Original image and 3 modifications of right central incisor gingiva

Figure 2.

Original image with a millimeter scale

Before selecting the appropriate testing, the equivalence of survey responses on different media was assessed. Twenty-four test respondents needed to rate the same image on a computer, mobile phone, and paper with an interval of 7 days between tests. Since there were no statistically significant differences between the answers on different media (p=0.63, repeated-measures ANOVA) and a good level of agreement with average measure Infraclass Correlation Coefficient (ICC) of 0.864 with a 95% confidence interval from 0.709 to 0.944 was reached, we decided to use either computer or mobile devices for testing.

The questionnaire was formed using Google Forms and distributed via social media applications. Respondents were asked to rate the images from 1 to 5, or from least attractive to most attractive. They were instructed to rate the photos based on their first impressions and not to return to previous questions.

Statistical analysis

A statistical analysis of the data was performed using the TIBCO Statistica program (v. 13.3. 0, TIBCO Software Inc., Palo Alto, CA, USA; 2017). According to the Shapiro- Wilk's test the data were not distributed normally. Descriptive statistical data were reported as medians and interquartile ranges. The Kruskal-Wallis test including post hoc multiple comparisons with Bonferroni adjustment was used to compare group esthetic scores and to determine the threshold levels of deviation at which each group discriminated between esthetic and un-esthetic situations. The tested hypothesis was that there was no difference between the esthetic scores for digitally altered smiles for each group of raters (p =0, 05).

Results

According to the results of this study, 71.1% of the participants were female, and 28.9% of them were male. The median age of the participants was 33 years (IQR 25-36).

There was no difference in esthetic scores between groups for photographs with symmetrical gingiva of central maxillary incisors, but esthetic scores for all levels of gingival asymmetries differed significantly between participant groups. Descriptive statistics and median values of esthetic scores of images with 0, 1, 2 and 3 mm of gingival asymmetries are presented in Table 1.

Table 1. Descriptive statistics.

Asymmetry	Group	N	Median*	IQR	Mean	Std. Dev.	95% CI	Min.	Max.	P
(0 mm)	Specialists	41	5.0	4.0-5.0	4.6	0.5	0.4-0.6	4.0	5.0
	Dentists	66	5.0	4.0-5.0	4.5	0.7	0.6-0.9	2.0	5.0
	Students (1-3y)	32	5.0	4.0-5.0	4.5	0.7	0.5-0.9	3.0	5.0
	Students (4-6y)	37	5.0	4.0-5.0	4.7	0.5	0.4-0.7	3.0	5.0
	Laypeople	56	5.0	4.0-5.0	4.5	0.7	0.6-0.9	2.0	5.0	0.063
1 mm	Specialists	41	4.0ab	3.0-4.0	3.5	0.9	0.7-1.1	1.0	5.0
	Dentists	66	3.0b	3.0-4.0	3.5	0.7	0.6-0.9	2.0	5.0
	Students (1-3y)	32	4.0ac	3.0-5.0	4.0	0.8	0.6-1.0	3.0	5.0
	Students (4-6y)	37	4.0ab	3.0-4.0	3.8	0.9	0.7-1.1	2.0	5.0
	Laypeople	56	4.0ac	3.5-4.0	3.9	0.8	0.7-1.0	1.0	5.0	0.001
2 mm	Specialists	41	2.0a	2.0-3.0	2.2	0.9	0.7-1.1	1.0	5.0
	Dentists	66	2.0a	2.0-3.0	2.4	0.7	0.6-0.9	1.0	4.0
	Students (1-3y)	32	3.0ab	2.0-3.0	2.8	0.9	0.7-1.2	1.0	5.0
	Students (4-6y)	37	3.0ab	2.0-3.0	2.4	0.8	0.6-1.0	1.0	4.0
	Laypeople	56	3.0b	2.0-4.0	2.9	1.0	0.8-1.2	1.0	5.0	<0.001
3 mm	Specialists	41	1.0a	1.0-2.0	1.6	0.8	0.7-1.0	1.0	5.0
	Dentists	66	2.0ab	1.0-2.0	1.9	0.8	0.7-0.9	1.0	4.0
	Students (1-3y)	32	2.0ab	1.0-3.0	2.1	0.8	0.7-1.1	1.0	3.0
	Students (4-6y)	37	2.0ab	1.0-2.0	1.8	0.8	0.7-1.0	1.0	4.0
	Laypeople	56	2.0b	1.5-3.0	2.2	0.9	0.7-1.1	1.0	4.0	0.003

* Kruskal-Wallis test with post hoc multiple comparisons with Bonferroni adjustment were used to compare group esthetic scores. Shared superscripts (a, b, c) indicate p>0.05 with no significant differences between variables, whereas values that do not share letters in superscript within the same asymmetry amount (1mm/2mm/ 3 mm) differ significantly (p<0.05).

Gingival asymmetry of 2 mm was necessary before laypeople rated it non-esthetic. The threshold for pre-clinical students was also 2 mm of asymmetry. Specialists, dentists, and students from 4^th to 6th year were able to detect gingival asymmetry of 1 mm and rated it significantly less esthetic (p < 0,003).

Discussion

Batra et al. reported: “The untrained eye seemed to be more sensitive to changes in the central incisors than in the lateral incisors or canines and when the changes were unilateral rather than bilateral.” (8). The goal of modern dental professionals is to satisfy the esthetic requirements of patients in addition to treatment (9). Accepting these requirements can be a guide to improve dental esthetics (10). Gingival asymmetry can be treated by a multidisciplinary approach, but it is necessary to know how much asymmetry is acceptable to our patients. Previous studies showed that dental specialists were more critical of gingival asymmetries than laypersons (3, 7, 11). The results of our study are in agreement with these findings as we reported that specialists could detect 1 mm of gingival asymmetry, unlike laypeople who were unable to see gingival asymmetry less than 2 mm. Dental students present a higher level of awareness towards dental esthetics (12), and their perception is different depending on their stage of education. Preclinical students rated the asymmetry of 2 mm non-esthetic, similarly to laypeople. This may be due to a lack of dental courses in their first years of studies because students are then mostly focused on general medicine courses. From 4th to 6th year, students attend more dental courses and clinical practicals; hence they are more critical to small deviations from an ideal smile. Many studies were conducted to investigate the students’ perceptions of altered smile esthetics (13-15) but these studies comprised only symmetric situations. The results obtained in our study show that we should have a very good communication with patients to avoid “overtreating” them if they have gingival asymmetry of 2 mm or less. If gingival asymmetry of central maxillary incisors exceeds 2 mm, a treatment is needed- either orthodontic or periodontal. The orthodontic intrusion of the teeth has been considered problematic for many years, but today it is a safe and commonly used procedure during fixed orthodontic treatment. Intrusion of a single tooth requires light continuous forces in young patients and light interrupted forces in adult patients, as the apical bone region of some adult patients tend to be more compact (16). Intrusion of a tooth should be followed by a composite restoration or veneer. From the periodontal point of view, the esthetics of soft tissues around the teeth is greatly impacted by undisturbed supracrestal tissue attachment, previously known as biologic width, which consists of junctional epithelium and supracrestal connective tissue attachment (17). The clinical dimension can vary among patients for same group types of teeth, and can show higher clinical value than histologically accepted mean dimension of 3 mm, as demonstrated by Perez in 2008 (18). This should be taken into consideration when planning clinical crown lengthening procedures, which is mostly performed through periodontal surgery and can demonstrate significant soft tissue rebound through a healing period of 6 to 12 months (19, 20) or gingival recession (21). These procedures include apically repositioned flap that usually includes bone resection, gingivectomy, and gingivoplasty which can be performed with conventional scalpel, electrosurgery, or dental laser (22).

The tested hypothesis has been supported by findings in this research. The threshold level for gingival asymmetry of central incisors was 2mm for laypeople and preclinical students, while the threshold level for clinical students, dentists, and dental specialists was 1mm. Our results indicate that dental education in Croatia affects students’ perception of smile. On a higher level of education, dental students notice smaller deviations from an ideal smile.

The first limitation of this study is a small number of participants in groups with students and specialists, which is not consistent with similar studies (23). The second limitation is the unfavorable distribution of participants’ age and gender. It was not possible to test whether there are differences between genders, or different ages of participants within groups (specialists, dentists, students, and laypeople).

Conclusions

From the results of our study, we can conclude that perception of gingival asymmetries in the esthetic zone of smile differs among dental specialists, doctors, students, and laypeople. Dental specialists, doctors, and clinical students are more critical of these asymmetries, while preclinical students and laypeople notice only 2 mm or more of gingival asymmetry of central incisors. We should, therefore, communicate with our patients in order to find the best way to finish our dental treatment, without “overtreating” them but trying to please both their demands and ours.