Determining of canine position by multiple facial landmarks to achieve natural esthetics in complete denture treatment

Abstract

Statement of problem.

Anterior tooth selection is an important step in complete denture treatment as it plays a pivotal role not only in esthetics but also in mastication and pronunciation. However, conventional methods for tooth selection are not well established and rely on facial measurements and proportions, which vary among different ethnicities.

Purpose.

The purpose of this clinical study was to investigate the relationship between interalar width and intercanine distance and to compare different clinical methods for determining the position of the canine tooth.

Material and methods.

Two hundred Thai participants (100 men and 100 women) aged 18 to 25 years old with 6 full maxillary anterior teeth were enrolled in this study. The interalar width and intercanine distance were measured with digital vernier calipers and compared by using the paired-samples t test. To determine the canine position, 2 reference lines—the alar line (A line) and the inner canthus of the eye to alar line (IA line)—were drawn through the canine on both sides. The horizontal distances from each reference line to the canine cusp tip and distal contact point were evaluated and then analyzed using the 1-sample t test.

Results.

All measurements were significantly different between men and women (P<.01). Interalar width was greater than intercanine distance in both sexes. In men, the A line coincided with the canine distal contact point (P>.05). In contrast, the IA line was distal to the canine distal contact point by 3.5 ±3.6 mm on the left side and by 3.9 ±3.4 mm on the right side. In women, the A line was situated between the canine cusp tip and distal contact point. It was mesial to the distal contact point by 2.0 ±2.0 mm on the left side and by 1.8 ±2.0 mm on the right side. The IA line was distal to the canine distal contact point by 1.2 ±2.6 mm on the left side and by 1.6 ±2.7 mm on the right side.

Conclusions.

The interalar width is greater than the intercanine distance in both sexes. The A line is more clinically relevant than the IA line for predicting canine position. The A line can directly determine the distal contact point of the canine in edentulous male patients. However, in women, a distance of approximately 2 mm should be added distal to the A line to locate the distal contact point of the canine on both sides.

INTRODUCTION

Patients who have lost their teeth experience not only compromised mastication and pronunciation but also the emotional damage of losing their natural physical appearance.¹ Thus, the goal of a complete denture includes helping the patient to regain confidence in their appearance. This requires knowledge and understanding of both the physical and biological factors.² The best way to select artificial teeth is from pre-extraction records, including diagnostic casts, radiographs, and recent images showing a natural smile. Patients without pre-extraction records will present a more challenging tooth selection process.^2–4 Error in the selection of the artificial teeth will affect correct tooth alignment.^3,5 Consequently, this will potentially compromise the patient’s appearance and ability to form natural facial expressions, damage the remaining gingival line, and impair their ability to pronounce words correctly.^5,6

In patients without pre-extraction records, clinicians may measure the nose to estimate the distance between the maxillary canines. This is based on the observation that during embryogenesis of the face, the premaxilla that supports the anterior maxillary teeth and the alar of the nose develops from the same region.^7,8 Therefore, theoretically, the interalar width and intercanine distance should be related.

Hoffman et al⁹ reported that the intercanine distance is approximately 3% greater than the interalar width. While the intercanine distance and interalar width were highly correlated in an Indian population,^10,11 studies conducted in Brazil, England, and the USA reported that these 2 measurements were not significantly related and that using this relationship may result in misaligned teeth.^12–14 This lack of correlation can be observed in different ethnicities, sexes, ages, and geographical regions.¹⁵ In a Thai cohort, the interalar width was reported to be greater than the intercanine distance by 3.14 ±3.00 mm in men and by 2.61 ±2.54 mm in women.¹⁶ In another study of a Thai cohort, a low but statistically significant level of correlation was found between the distance of the canine distal contact points and the interalar width (r=.271, P<.001).¹⁷

Two traditional methods have been used clinically to determine the position of the maxillary canine cusp tips. The first method uses an imaginary vertical line perpendicular to the floor starting from the widest part of the ala of the nose and then straight down to the maxillary occlusal rim^3,18—called the alar line (A line). The second method uses an imaginary diagonal line that begins at the inner canthus of the eye and passes through the widest part of the ala of the nose to the upper occlusal rim¹⁹—called the inner canthus to alar line (IA line). Although these 2 methods draw different lines on the face, both estimate the same location as the canine cusp tips or midline vertical axis of the canine. However, the 2 approaches may lead to confusion among dentists in the selection of the appropriate treatment method. Nevertheless, because the distance between the distal contact points of the left and right canines is used for artificial tooth size selection from a tooth mold chart,⁵ the clinical relevance point in the tooth selection step should ideally not be the canine cusp tip but the canine distal contact point.

Therefore, in view of the ambiguity of these concepts and the difference in results among different ethnicities, the present study aimed to analyze the difference between interalar width and intercanine distance quantitatively and to determine whether the A line or IA line can be used to estimate the position of the canine cusp tip and canine distal contact point. The research hypothesis was that the A line or the IA line can be used to estimate the position of the canines. A better understanding of the most appropriate methods of selecting and positioning canine teeth will aid in fabricating dentures for edentulous patients.

MATERIAL AND METHODS

This study was approved by the Institutional Review Board of Mahidol University (certificate No. MU-IRB 2009/134.2506). A total of 200 adult Thai participants (100 men and 100 women; age 18–25 years) were enrolled in this study. Participants from this age group were enrolled because no significant developmental facial changes have been reported to occur after approximately 17 years old.²⁰ All participants had 6 completely erupted natural maxillary anterior teeth with a vertical overlap of 1 to 3 mm, a horizontal overlap of approximately 20% to 30% of the height of the mandibular incisors, and an occlusion classified as Angle class I. Their teeth had no cracks, artificial crowns, proximal restorations, crowding, rotation, or malalignment. Participants who had previously undergone orthodontic treatment or who had a facial abnormality (congenital, traumatic, or surgical), were excluded from the study.

Each participant sat face-forward in a relaxed position, and 2 horizontal facial lines were measured by using digital vernier calipers with a resolution of 0.01 mm (Mitutoyo Corp) without accounting for the curvature of the face. The first line—the interalar width—was the widest part of the nose (Fig. 1A). The second line—the intercanine distance—was the distance between the maxillary canine cusp tips (Fig. 1B).

Figure 1.

Measurements of 2 distances. A, Widest part of nose (interalar width). B, Distance between maxillary canine cusp tips (intercanine distance), without accounting for curvature of face.

To determine the relative position of the A lines with regard to the position of the maxillary canine, a stainless-steel rod was set perpendicular to the floor and passively placed against the edge of the widest part of the ala of the nose to represent the A line (Fig. 2A). The distances between the rod and the 2 landmarks, the canine cusp tip (CT) and the canine distal contact point (CD), were measured on the left and the right sides separately with digital vernier calipers. When the rod was in line with the target landmark, the measurement was considered as 0. When the rod was aligned distal to the landmark, the measurement was considered a positive value and negative value when aligned mesial to the landmark (Fig. 3).

Figure 2.

Demonstration of 2 reference lines. A, Alar line (A line). B, Inner canthus to alar line (IA line).

Figure 3.

Measurement of distance from reference line to 2 positions of canine. A, To cusp tip (CT). B, To canine distal contact point (CD).

To evaluate the IA line and the maxillary canine, the position of the stainless-steel rod was changed such that its slope provided a reference IA line which connected the inner canthus of the eye to the widest part of the ala of the nose (Fig. 2B). The horizontal distances between the rod and the 2 landmarks—the CT and CD—were examined using the same criteria as those used to measure the A line. All measurements were made by 2 investigators (N.S., T.A.), and the mean values were calculated.

The paired-samples t test was used to compare the interalar width and the intercanine distance, while the independent-samples t test was used to analyze the differences between men and women for each measurement (α=.05). For the measurements of the A and IA lines-to-CT or to-CD positions, the 1-sample t test, adjusted with the Bonferroni correction, was analyzed with the 0 value to evaluate the coincidence of facial landmarks with the canine position, and a 2-way repeated measures ANOVA was applied to compare the data between men and women. Moreover, the interrater reliability was tested using an intraclass correlation coefficient (α=.05).

RESULTS

The intraclass correlation coefficient was .92, indicating a high interrater reliability. A significant difference was found between the sexes in all measurements (Tables 1–3), which indicated that men and women needed to be analyzed separately. The results indicated that the interalar width was significantly larger than the intercanine distance in both sexes (P<.001).

Table 1.

Relationship between interalar width and intercanine distance (mm)

Sex	Interalar width	Intercanine distance	Difference	95% CI of Difference		P *
	Mean ±SD	Mean ±SD	Mean ±SD	LB	UB
Men	40.2 ±2.4	36.4 ±2.0	3.8 ±3.1	3.2	4.4	<.001
Women	36.6 ±2.2	35.5 ±2.0	1.1 ±2.6	0.6	1.6	<.001
P #	<.001	.002

Difference, Mean difference between interalar width and intercanine distance

^P*, P value of paired-sample t test for comparison of interalar width and intercanine distance

^P#, P value of independent-samples t test for comparison between men and women

CI, confidence interval; LB, lower bound; SD, standard deviation; UB, upper bound

Table 3.

Analysis of coincidence between IA line and canine position (mm)

	A line-to-CT				A line-to-CD
Sex	Left		Right		Left		Right
	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *
Men	7.5 ±3.9 (6.7, 8.2)	<.001	8.0 ±3.7 (7.3, 8.8)	<.001	3.5 ±3.6 (2.8, 4.2)	<.001	3.9 ±3.4 (3.3, 4.6)	<.001
Women	4.7 ±2.7 (4.2, 5.3)	<.001	5.2 ±2.9 (4.7, −5.8)	<.001	1.2 ±2.6 (0.7, 1.7)	<.001	1.6 ±2.7 (1.1, 2.2)	<.001
P #		<.001				<.001

^P*, P value of 1-sample t test for comparison between distance and 0 value

^P#, P value of 2-way repeated measures ANOVA for comparison between men and women

CD, canine distal contact point; CI, confidence interval; CT, cusp tip; IA line, inner canthus to alar line; SD, standard deviation

For the A line-to-canine position, a statistically significant difference was found in the A line-to-CT measurements on both the left and right sides in both sexes (P<.001). All A line-to-CT measurements were significant positive values, which indicated that the A line was positioned distally to the canine cusp tip in both sexes (Table 2). However, the A line-to-CD measurements differed between the sexes. In women, the A line-to-CD measurements on both the left and right sides were significantly negative values, which indicated that the A line passed mesial to the CD on both the left and right sides (P<.001). However, in men, the A line was coincident with the CD on both the left (P=.287) and right sides (P=.352).

Table 2.

Analysis of coincidence between A line and canine position (mm)

	A line-to-CT				A line-to-CD
Sex	Left		Right		Left		Right
	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *	Mean±SD (95% CI)	P *
Men	3.8 ±2.8 (3.2, 4.3)	<.001	4.1 ±2.6 (3.6, 4.6)	<.001	−0.3 ±2.4 (−0.7, 0.2)	.287	−0.2 ±2.6 (−0.7, 0.3)	.352
Women	1.8 ±2.1 (1.4, 2.2)	<.001	2.1 ±2.2 (1.6, 2.5)	<.001	−2.0 ±2.0 (−2.4, −1.6)	<.001	−1.8 ±2.0 (−2.2, −1.4)	<.001
P #		<.001					<.001

^P*, P value of 1-sample t test for comparison between distance and 0 value

^P#, P value of 2-way repeated measures ANOVA for comparison between men and women

A line, alar line; CD, canine distal contact point; CI, confidence interval; CT, cusp tip; SD, standard deviation

With regard to the relation of the IA line-to-canine position, the results of statistical analysis revealed that the measurements of the IA line-to-CT or to-CD yielded significant positive values in both sexes on both sides, which indicated that the IA line was positioned distally to the canine cusp tip and the canine distal contact point (Table 3).

DISCUSSION

From the present results, the research hypothesis was rejected. The interalar width was significantly larger than the intercanine distance. Neither the A nor the IA line can accurately estimate the position of the CT in either sex. However, the A line could determine the CD in men but was located on average 2 mm mesial to the CD in women. These findings will be beneficial for tooth selection when complete denture treatment is undertaken for individuals without pre-extraction records.

Artificial teeth should replicate the size and positional arrangement of the natural teeth. Methods for selecting the artificial teeth have been investigated,^9–14,16,17 including the estimation of tooth size from the interalar width. Most of these studies focused on the association between the interalar width and intercanine width. However, the authors are unaware of a previous study that scrutinized the direct relationship between the position of the ala of the nose and the canine tooth, as was done in the present study, rather than the proportions of the facial measurements.

This present study revealed that both the intercanine distance and the interalar width was significantly greater in men than in women, which was consistent with previous studies.^9,10 The differences between male and female participants were because of the differences in facial proportions and therefore the data for men and women were treated separately. The interalar width was found to be larger than the intercanine distance in the present study, which is consistent with the findings of Miranda and D’Souza.¹¹ However, Hoffman et al⁹ reported that the interalar width was shorter than the intercanine distance. These conflicting results might be attributed to ethnic differences—an Asian cohort was used in the present study, as well as in that of Miranda and D’Souza, whereas Hoffman et al used a white cohort. Therefore, the data from one ethnicity should not be generalized to other ethnicities. Rather, studies for specific ethnicities should be encouraged to improve satisfaction in both the esthetic and functionality of complete denture therapy in each population.

The combined width of the anterior maxillary teeth from the distal contact point of the canine to that of the contralateral canine is required for selecting artificial teeth from a denture tooth mold chart.¹⁹ However, many studies have only made evaluation between the interalar width and the intercanine distance.^9–13,16,17 Additionally, Mavroskoufis and Ritchie¹⁴ recommended that the addition of 7 mm to the interalar width is necessary to estimate the combined 6 anterior maxillary teeth, whereas Mccord and Grant¹⁹ recommended the addition of 8 to 10 mm to the nasal width to obtain the distance between the distal surfaces of the canines. Consequently, the present study aimed to examine the direct association between the ala of the nose not only with the canine cusp tip but also with the canine distal contact point.

The results of the present study showed that both the A and IA lines failed to estimate the position of the canine cusp tip, which was contrary to conventional complete denture teaching.^3,18,19 The IA line was located distally to the distal contact point of the canine in both sexes, which indicated that this line was not suitable for estimating the position of the canine tooth. In contrast, the results of statistical analysis supported the observation that the A line coincided with the distal contact point of the canine in men; however, it was located approximately 2 mm mesial to the canine distal contact point in women. Therefore, it is suggested that the A line was a more appropriated guideline to determine the combined width of the 6 anterior maxillary teeth for Asian people despite the fact that other factors such as a jaw ridge or the esthetic need of patients must be considered.

Limitations of this study included that it was limited to a Thai population that might have shown different craniofacial dimension from other population groups. Therefore, further studies with diverse populations and also comparison among groups of different ethnicities are recommended to improve complete denture prosthodontics.

CONCLUSIONS

Based on the findings of this clinical study, the following conclusions were drawn:

1. Among Thai people, the interalar width was greater than the intercanine distance in both sexes.

2. However, both the A and IA lines failed to determine the canine cusp tip location, which was contrary to conventional complete denture teaching.

3. The A line is more clinically relevant than the IA line to predict canine position.

4. The perpendicular line from the ala of the nose can directly determine the distal contact point of the canine in male edentulous patients.

5. In women, approximately 2 mm should be added distally to the A line to locate the distal contact point of the canine on both the left and right sides.

CLINICAL RESEARCH.

Determining canine position by using multiple facial landmarks to achieve natural esthetics in complete denture treatment

CLINICAL IMPLICATIONS.

The clinical canine cusp line methods, which rely on facial landmarks to determine the position of the canine cusp tips, are inaccurate for use in Asian patients. When using the A line method, the line should match with the canine distal contact point in men; in women, it should be moved 2 mm distally to reach the canine distal contact point.