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Evaluation of Dental Midline Position Jeffrey W. Beyer and StevenJ. Lindauer

Maxillary midline position relative to the facial midline is stressed as an important diagnostic feature in orthodontic treatment planning. Depending on the patient, however, movement of the dental midline to be coincident with the facial midline may be difficult to achieve. In addition, evaluation of dental midline position may be complicated if other midline facial structures are not well aligned. The two objectives of the current study were to determine how far the maxillary dental midline could deviate from the facial midline and still be considered aesthetically acceptable, and to determine how the position of various midline facial landmarks affect overall facial aesthetics. One hundred twenty individuals, including orthodontists, gen- eral dentists, orthodontic patients, and parents of patients, evaluated digitally altered images of two patient-subjects to rate the acceptability of dental midline deviations and to prioritize the importance of location of various midline facial structures. The mean threshold for acceptable dental midline deviation was 2.2 -+ 1.5 mm. There was a significant difference in deviation thresholds between the two patient-subjects (P < .05). Orthodon- tists and dentists were significantly less tolerant of midline deviations than were patients (P < .001), with the tolerance of parents in between. When deviations of various midline facial structures were evaluated, photographs with maxillary midline and/or nose deviations were considered less aes- thetic (P < .001). There were no apparent differences noted among orthodon- tists, dentists, patients, and parents in this part of the study. (Semin Orthod 1998;4:146-152.) Copyright© 1998 by W.B. Saunders Company

F acial aesthetic evaluation is an impor tan t part of the or thodont ic t rea tment-planning process. O n e

of the pr imary goals of o r thodont ic t rea tment is the a t ta inment of the best facial aesthetic appearance for a given patient. Arguably, the evaluation of a pat ient 's frontal symmetry is the most critical aspect of diagno- sis because this is the perspective that the pat ient will be seeing most often. Some degree of facial asymme- try commonly occurs in virtually all individuals. TM

Deviation of midl ine structures to the r ight or left, such as the nose, chin, or dental midline, however, is no t considered to be normal. ~ In patients present ing for o r thodont ic t reatment , the maxillary a n d / o r man- dibular dental midlines often are not co inc ident with

From the Department of' Orthodontics, School of Dentistry, Virginia Commonwealth University, Richmond, VA.

Address co~wespondence to Steven J. Lindauer, DMD, MDSc, Associate Pro[esso~; Department of Orthodontics, School of Dentistry, Virginia Commonwealth University, 520 N 12th St, Rm 118, Richmond, VA 23298-0566.

Copy~gght © 1998 by W.B. Saunders Company 1073-8746/98/0403-000458.00/0

each o ther or with the facial soft-tissue midline. This may be because of skeletal asymmetries in which the maxilla a n d / o r mandib le are malposi t ioned relative to the facial skeleton, or because of dental asymme- tries resulting f rom displacement or distort ion of the upper or lower dental arches, or asymmetric crowd- ing, spacing, tooth rotations, or tooth size discrepan- cies.2,6, 7

The location of the maxillary midl ine relative to the facial soft-tissue midl ine is often stressed as an impor tan t factor in or thodont ic diagnosis and treat- ment -p lanning procedures , e~v Depend ing on the pa- t ient and their presentat ion, however, substantially moving the maxillary midl ine dur ing t rea tment may entail complex procedures out of p ropor t ion with the potent ial benefi t to the patient. A.symmetric t rea tment mechanics are often chal lenging and may increase the total t rea tment time, whereas unilateral extract ion o r surgical in tervent ion might be considered in more ex t reme cases. 2,~

Much has been written regarding the diagnosis of dentofacial asymmetries, but the aesthetic significance of dental symmetry and midl ine location has no t been

146 Seminars in Orthodontics, Vol 4, No 3 (September), 1998: pp 146-152

Dental Midline Position 147

determined conclusively. ~,l°,lt In a study reporting on dental aesthetic self-evaluation by 10- to 13-year-olds, Graber and Lucker 12 found that overjet and dental

crowding or spacing were considered to be more significant factors than midline deviations in determin- ing self-satisfaction with dental appearance. In con- trast, Hulsey 13 found that symmetry was one of the most important factors in defining an attractive smile. The degree of asymmetry that falls outside of the

limits of aesthetic acceptability is presently based only on subjective opinion because no absolute or even accepted standards exist by which a judgment of abnormality can be made. Articles in the literature discuss methods for correcting dental asymmetries without defining a threshold of deviation at which treatment should be initiated, l°,H The importance of the dental midline location, specifically for evaluating overall facial aesthetics, has never been studied.

In orthodontic diagnosis, the extent to which the maxillary midline deviates from the facial soft-tissue midline is commonly recorded, presumably because an objective will be for the two midlines and the mandibular midline to be coincident after treatment. In addition, facial landmarks, such as the nose, phil- trum, and chin, often used as references for maxillary midline positioning, may not themselves be centered on the face or with each other. Because the locations of these midline landmarks are not generally altered as a result of ordmdontic treatment, it would be useful to know their relative importance for determining optimum aesthetic goals for positioning of the dental midline. Arnett and Bergman s noted that the phil- trum is usually a reliable midline structure and can, in most instances, be used as the basis for midline assessment. 8

The aims of the present study, therefore, were to determine the extent to which the dental midline can deviate from the facial midline and still be considered aesthetically acceptable, and to evaluate the relative importance of various facial midline landmarks in influencing dental midline aesthetics. A large group of individuals, including general dentists, orthodon- tists, adolescent patients, and the parents of patients, participated in this study to evaluate the influence of midline structure positioning, including the dental midline, on overall facial aesthetics.

Methods

The study was conducted in two parts. The first part was designed to quantify the extent to which devia- tions of the maxillary dental midline from the facial soft-tissue midline would be tolerated by different groups of evaluators. The goal of the second part was

to determine the influence of positioning of various facial soft-tissue midline landmarks on overall facial

aesthetics.

Dental Midline Deviations

Frontal face video images of one male and one female volunteer were captured and stored electronically by using the Dolphin Digigraph System (Dolphin Imag- ing, Valencia, CA). These images were digitally altered to create seven additional images by moving the maxillary dental midline incrementally 0.7 mm fur- ther off center in each one. Increments of 0.7 mm were chosen because of the limitations of the system with regard to the fixed focal distance of the video camera. Thus, there were a total of eight photographs of both the male and female patient-subjects, with

midline deviations ranging between 0 and 4.9 ram, as shown in Figure 1.

A panel of 120 individuals, including 30 general dentists, 30 orthodontists, 30 adolescent padents, and 30 parents, (15 men and 15 women in each group) served as evaluators allowing for comparisons to be made among groups, between male and female evalu- ators, and between male and female patient-subjects. Each panelist determined a threshold for acceptable dental midline position by viewing each image individu- ally in a predetermined order without the assistance of

calibrated measuring instruments. A response of "ac- ceptable" or "not acceptable" was required before proceeding to the next image. Evaluators were not permitted to revisit a photograph once they had proceeded. In addition, five of the male patient- subject photographs with midline deviations between 0.7 mm and 3.5 mm were duplicated and interspersed to test for reliability of the method. Results from a pilot study using 30 dental students as evaluators determined that there were no significant differences in perception of midline acceptability dependent on

the order in which the facial photographs were viewed. 1-~ The pilot study also showed that repeatabil- ity of determining the acceptability, of midline devia- tions of 0, 4.2, and 4.9 mm was near 100%.

"Acceptable" or "not acceptable" ratings by each evaluator were recorded for each of the 21 photo- graphs viewed: 13 for the male photographs and 8 for the female photographs. Each evaluator's threshold for midline deviation was recorded as the largest "acceptable" value, in millimeters, for both the male and female images. Mean acceptable midline devia- tion thresholds were compared among groups, be- tween male and female evaluators, and between the male and female images by using a two-way multivari- ate ANOVA followed by the Tukey-Kramer HSD mul- tiple comparisons test, when indicated, lntraexaminer

148 Beyer and Lindauer

0:0

2:8

010

3:5

0:7

4.2

I A 2,1

4i9

0:7 1.4 2!!

Figure 1. Video images of the (A) male and (B) female patient-subjects. Una l te red images are labeled 0.0. Altered images are labeled with the amoun t of dental midl ine deviation in millimeters, ranging f rom 0.7 m m to 4.9 mm.

reliability was evaluated using the Kappa statistic, based on the ag reemen t of acceptability o f the five repea ted male photographs.

M i d l i n e L a n d m a r k D e v i a t i o n s

The same 120 panelists part icipated in the second part of the study. From the original male pat ient image, seven addit ional photographs were created in which the location of one or more facial midl ine features were altered. The result ing eight photographs are shown in Figure 2. The nose, maxillary denti t ion, phi l t rum, and chin locations were moved a lone or in combina t ion with one ano the r 2.8 m m to the r ight of

the facial midline, as described in Table 1. Results f rom the pilot study showed that 2.8-mm deviations would be detectable by most evaluators. 15,16

Evaluators were shown the entire series of eight individual photographs and asked to place them in order f rom most to least acceptable, based on overall facial appearance. Once complete , each pho tograph was given a numerical rank f rom 1 (best appearance) to 8 (worst appearance) . Average ranks for each pho tograph were compared using the Kruskal-Wallis rank sum test to de t e rmine statistically significant differences between photographs. The data were also examined to detect any differences a m o n g evaluator groups or between male and female examiners.

Dental Midline Position 149

N 0 P O

R S T U

Figure 2. Altered images of the male patient-subject in which various facial midl ine structures were moved, alone or in combinat ion, 2.8 m m right of the facial midline. For a descript ion of the labeled photographs , see Table 1.

Results

Dental Midline Deviations

In t raexaminer reliability, based on repeatability of assessments made by evaluators on the five male subject photographs with midl ine deviations f rom 0.7 m m to 3.5 ram, as assessed by the Kappa statistic, was j udged to be "m ode ra t e " to "substantial" (K = 0.60). 17 Overall ag reemen t between duplicate photographs was 80%. Percent ag reemen t was greatest for the 0.7-mm and 3.5-ram deviation photographs (93% and 85%, respectively) and least for the 2. l - ram deviation photographs (63%). Of the 120 evaluators, seven rated all of the photographs as " n o t acceptable" in e i ther the male or female pho tograph series or both. Four teen evaluators found at least one pho tog raph with no midl ine deviation to be " n o t acceptable ,"

Table 1. Facial Landmarks Altered to Create Eight Images of the Male Patient-Subject for Use in Evaluating the Significance of Midline Landmark Locat ion in Inf luencing Facial Aesthetics

On With the Photo Facial Midline 2.8 mm Right

N Nose, philtrum, max dent Chin O Nose, philtrurn, chin Max dent P Philtrum, chin Nose, max dent Q All features None R Nose, max dent, chin Philtrum S Nose, philtrum Max dent, chin T Philtrum, max dent, chin Nose U Nose, chin Philtrum, max dent

Abbreviation: Max dent, maxillary dentition.

while rating at least one pho tograph with a deviation as "acceptable ."

The mean threshold for "acceptab le" maxillary midl ine deviat ion was 2.2 -+ 1.5 ram. A graphic descript ion of the examiners ' ratings is shown in Figure 3. Evaluators were somewhat less tolerant of deviations in the female patient-subject photographs (2.0 --- 0.9 mm) than in the male patient-subject pho- tographs (2.4 -+ 1.0 mm; P < .005). The re were no significant differences between male and female evalu- ators ( P > .05), Statistically significant differences, however, were found between dentists, orthodontists, adolescent patients, and parents ( P < .001). Mean values for acceptable midl ine deviations for the male and female photographs for these groups of evalua- tors are compared in Table 2. Statistically significant differences were found between orthodontis ts versus patients for the male pho tograph series and for or thodontis ts and dentists versus patients for the female series. In general , orthodontists and dentists were less tolerant o f dental midl ine deviations than were adolescent patients, with parents ' ratings in between.

Midline Landmark Deviations

Results for the second part of the study, which evalu- ated the effects of varying the posit ion of various facial midl ine landmarks on overall facial aesthetics, are shown in Figure 4. The Kruskal-Wallis rank sum test showed that there were statistically significant differ- ences in tile rankings of tile various photographs by the evaluators (P < .001). In order, photographs N, Q,

150 Beyer and Lindauer

Figure 3. Acceptable mid- line deviation thresholds as de te rmined by each of the 120 evaluators for both the male and female patient- subjects (total = 240). Six evaluators found none of the photographs in e i ther the male or female series to be acceptable. O n e evalua- tor found none of the pho- tographs to be acceptable in both the male and fe- male series.

and R (deviation of the chin only, no deviations, and deviation of the ph i l t rum only, respectively) were rated bet ter than photographs T, U, P, and O (devia- tion of the nose only, deviation of the ph i l t rum and dental midl ine together, deviation of the nose and dental midl ine together, and deviation of the maxil- lary dent i t ion alone, respectively), which were rated better than pho tograph S (deviation of the maxillary dent i t ion and chin together) . All groups ranked S as the worst photograph. Generally, photographs with deviated maxillary denti t ions a n d / o r nose deviations were considered less aesthetic. There were no appar- ent differences in rankings based on evaluator sex or among general dentists, orthodontists , patients, and parents, a l though orthodontis ts were the only group to rate the pho tograph with no deviations as the best overall.

Discussion

O n e of the goals of this study was to establish general guidelines for evaluating maxil lary dental midl ine to facial soft-tissue midl ine discrepancies. Based on the data collected, a deviation o f greater than approxi- mately 2 m m would be considered unaesthet ic by most people. More ambiguity in decision making would be expected, and was found, close to the threshold of

Table 2. Average Midline Deviation Thresholds of Tolerance by the Four Groups of Examiners

Examiner Group Male Photograph Fema~ Photograph

Orthodontists 1.88 _+ 0.80* 1.68 -+ 0.77* Dentists 2.33 -4- 0.98 1.82 + 0.72* Patients 2.80 -+ 1.23 t 2.43 +- 0.99-~ Parents 2.48 +_ 0.86 2.02 -+ 0.88

NOTE. Values are in millimeters. *Significantly lower threshold (P < .05). tSignificantly higher threshold (P < .05).

acceptability. The 2.1-mm deviation photograph, very close to the acceptability threshold, showed the lowest reliability of j u d g m e n t among the photographs tested; repeatability was jus t 63%. Judgmen t s of acceptability in general were otherwise surprisingly reproducible. Photographs fur ther away f rom the average threshold were progressively more reliably judged .

There was a statistically significant difference be- tween the average acceptability thresholds for midl ine deviations between the male and female subject photo- graph series, with somewhat less tolerance of midl ine deviation in the female subject. Because there were only one male subject and one female subject whose photographs were al tered for use in this study, it would be difficult to general ize that there is more tolerance of imperfec t ion for males versus females. It is possible, however, to conclude that the difference found in this study shows that varying thresholds of acceptability of midl ine deviation exist among individuals; the same 2-ram or 3-mm deviation may be considered accept- able in one person and unacceptable in another, depend ing on o ther facial characteristics.

Significant differences in midl ine tolerances were found among general dentists, orthodontists, adoles- cent patients, and parents. The patients were gener- ally more tolerant of midl ine deviations than were the orthodontis ts and dentists. This is in ag reemen t with previous studies that have found that orthodontists were more critical of facial appearance than patients or their parents. 18-2° Orthodont is ts as a group consis- tendy had the lowest threshold for midl ine acceptabil- ity, followed, in order, by general dentists, parents, and patients.

The second part of the study was designed to address the way in which deviation of various facial midl ine landmarks affect overall facial aesthetics. None of the photographs that were most highly ranked by participants in the study included those with the nose

Dental Midline Position 15 1

Figure 4. Mean overal l rankings of the images in which various facial midl ine landmarks were moved 2.8 m m to the right. *Rated significantly be t t e r (P < .05). **Rated significantly worse ( P < .05).

5

2

N O P Q R S T U Image

or maxillary midl ine deviated f rom the facial midline. It is interest ing to note that, of the eight photographs evaluated, four conta ined deviations of the maxillary midline, and these were the four most unfavorably' ranked photographs. This might be because maxillary midl ine deviations are considered especially unaes- thetic, but this ou tcome may also have been influ- enced by the study be ing conduc ted within a dental school setting, with the participants especially aware of the appearance of the denti t ion.

In an ideal situation, facial midl ine landmarks such as the nose, phi l t rum, and chin would be al igned with the facial soft-tissue midline, and the goal after orth- odont ic t rea tment would be for the dental midl ine to be coincident. In patients in whom the locations of the nose, phi l t rnm, and chin are not aligned, it may be more difficult to establish goals for m o v e m e n t of the maxillary midline. Results f rom this study suggest that the ideal location of the maxillary dental midl ine should be de te rmined independent ly of the locat ion of o ther specific facial landmarks, because all photo- graphs in which the maxillary midl ine was deviated, alone or in combina t ion with ano ther facial landmark, were rated poorly. Consistently rated worst of all was the pho tograph in which the dental midl ine was al igned with the chin point, which was 2.8 m m right of the facial soft-tissue midline. A similar clinical out- come might result if a maxillary midl ine is moved dur ing or thodont ic t rea tment to coincide with a mandibular dental midl ine that deviates f rom the facial midl ine because of a mandibular skeletal asym- metry.

Conclusion

Midline asymmetries warrant special considerat ion in the or thodont ic diagnosis and t rea tment-planning process. Acceptability of dental midl ine deviations

depends on individual factors, including asymmetries associated with o ther facial midl ine structures and on the person evaluating the asymmetry. Generally, a 2-ram or greater deviation of the dental midl ine appears to be easily detectable by most individuals and, therefore, should be considered when formulat- ing an or thodont ic t r ea tment plan. The extent to which t rea tment should be directed at addressing the asymmetry depends on tim magni tude of the devia- tion, o ther characteristics of the individual, and the risks and costs associated with the various t rea tment options p roposed to correct the detected deviation.

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