Occlusal height difference between maxillary central and lateral incisors: should aesthetic perception influence bracket placement?

Features that contribute to an aesthetic smile have been well documented before [1‐4]. The height difference between maxillary central and lateral incisors is one of them and most contemporary protocols for bracket placement prescribe a difference of 0.5 mm [5, 6].

However a number of authors [7‐10], suggested that a maxillary central-to-lateral incisor occlusal height difference (OHD) of greater than 0.5 mm might improve aesthetics and anecdotal evidence from assessing smile lines displayed in advertisements appears to confirm this.

The aim of this study was to verify the maxillary central-to-lateral incisor OHD in females as a single feature in the aforementioned media and to discuss possible implications for individualized orthodontic treatment planning.

Maxillary central-to-lateral incisor differences (Δ) calculated using an average of 10.5 mm, 10 mm and 11 mm as base values for crown height of the maxillary central incisors are presented in Table 1. These deviations increased for larger OHDs.

The results for the different groups are in Table 2 and Fig. 2. The average maxillary central-to-lateral OHD was more than 1 mm for all groups: 1 (dental), 2 (fashion) and 3 (orthodontics). The difference between these groups were not statistically significant (P >  0.05).

The results of this study revealed that maxillary central-to-lateral OHDs are usually greater than 1 mm for images in dental, fashion and orthodontic advertisements (1.39 mm, 1.34 mm and 1.23 mm). In contrast, the maxillary central-to-lateral incisor OHD was very close to 0.5 mm for orthodontic textbooks.

A number of investigators [7‐10, 19, 20] found that the maxillary central-to-lateral OHD has an impact on perception of smile - aesthetics and this feature has been evaluated before. Ker et al. [7], using data from the Ohio state study, suggested a maxillary central-to-lateral incisal height difference of more than 0.5 mm was most aesthetic. In their computer-based investigation laypersons were asked to change the maxillary central-to-lateral incisal height difference in 0.1825 mm increments, until optimal aesthetics were achieved. Results exhibited values from 0 to 2.9 mm; negative values were disallowed.

Chan [21] found an ideal value of 1.4 mm OHD when changing the maxillary central-to-lateral incisal OHD in 0.18 mm increments. In an investigation by Machado et al. [9] images were digitally altered in order to create six different central incisor vertical positions in 0.5-mm increments. As a result, a maxillary central-to-lateral OHD of 1.5 mm was found most aesthetic. Bukhary et al. [10], also digitally manipulated dental configurations that were assessed. The length of the lateral incisor was altered in 0.5 mm increments to produce a total of five images with the lateral incisor 0.5 mm, 1 mm, 1.5 mm, 2 mm and 2.5 mm shorter than the adjacent central incisor. The authors found that an anterior dental arrangement with the maxillary lateral incisors 1.5 mm shorter than the adjacent central incisor was preferred by most assessors. In an investigation by King et al. [20], the vertical position of the maxillary lateral incisors was digitally morphed from a position of approximately 1.4 mm past the level of the central incisors to approximately 2.2 mm above the level of the central incisors. An animation of 43 frames in length were produced and assessed. A maxillary central-to-lateral incisor height difference of 0.5 mm was found to give best aesthetics. Brisman [22] used different non-digital dental setups and concluded that patients preferred an arrangement whereby the anterior teeth that are almost at the same horizontal plane; i.e. a maxillary central-to-lateral OHD of 0 mm. Interestingly, for studies where jurors preferred a maxillary central-to-lateral OHD of 1.5 mm, it made no difference whether aesthetics were changed by small increments or in 0.5 mm intervals [7, 10].

A number of factors need to be reviewed critically when assessing our investigation: In contrast to standardized clinical photography [23], acquisition of suitable photographic material from promotional photography, i.e. ‘genuine’ frontal views, was challenging. Variation of head position, can lead to parallax distortion and this can have an effect on linear measurements [24]. It is not questionable that advertisement photography is not standardized. Still, for best results professional photographers prefer focal lengths 85 mm in full-frame photography that show hardly any parallax distortion. This setting is preferred by most professionals and more important than a specific camera brand. Even an oblique shot with a portrait focal length with a full-frame camera would not have much influence on crown height perception. Wide angle lenses would create distortion if used close-up and are thus unsuitable for facial fashion photography. They would produce facial – and dental – features that could not possibly be regarded as attractive. It would appear nonsensical to use those pictures for measurement purposes. Therefore, it was the appearance of what advertising photography was willing to consider publishable was what we wanted to investigate; in other words, the looks rather than the exact measurement.

By calculating the differences (Δ) we found only minimal deviations from the values calculated using the 10.5 mm standard value. Although these deviations increased for larger OHDs, differences remained very small (please see Table 1) and hence have not compromised our results.

However, it cannot be denied that the inclination of the optical axis towards the motive (teeth in that case) leads to a certain amount of distortion on form of object lengthening or shortening, resulting in different perception of any given object. However, when it comes to reproduction of “beauty” as desired in advertisement and fashion photography, the shooter will instinctively cut out individual creativity through distortion but rather do everything for the outcome that suits the taste of the target group. Simply because the image has to be sold. Naturally, if the investigator is not the same person as the photographer, differences will remain, and an error must result. If related to OHD, this means that the measurement outcome will include certain errors. However, even with a slight amount of distortion, the difference between an OHD of 0.5 mm and 1.5 mm will still be detectable.

Our study assumed an average maxillary central incisor crown height of 10.5 mm [2, 18] and this was used as the standard value to calculate the maxillary central-to-lateral incisal height difference. This can lead to over - or underestimation of the calculated incisal height. However, the results for group 4 (orthodontic textbooks; mean 0.62 mm) suggest that the height difference could have been only (0.12 mm) overestimated; we also calculated results using different incisor heights and as per our calculation (Table 1), over - or underestimation of the maxillary central-to-lateral incisor OHD was very small and this could not have affected our results significantly. Image distortion due to non - standardized photographic technique is unlikely to have caused significant bias in our study; it is rather more likely that distortions cancelled each other out on average.

The results of our three advertisement groups are in agreement with those studies preferring a maxillary central-to-lateral incisal OHD of more than 1 mm [7, 9, 10, 21]. Not completely unexpected, the results for group 4 (orthodontic textbooks) showed a mean value of 0.62 mm for maxillary central-to-lateral OHD, which is close to the 0.5 mm used for standard protocols for bracket placement [5, 6].

Physical attractiveness is commonly used as an advertising tool [25]: “what is beautiful is good” [26]. A smiling face is known for its advertising appeal and the effect on customers has been evaluated before: the industry recommends use of smiling images over to non-smiling faces [27]. One would assume that for purposes of marketing, the more attractive the smile, the greater the effect on the customer. Several studies found maxillary central-to-lateral incisal height difference greater than 1 mm to be the most attractive [7, 9, 10, 21] and we assume that the advertisers’ selections of smiles were, consciously or sub - consciously influenced by this factor.

In order to avoid potential biased selection of the photographs, both lay persons as well as dental professionals were requested to contribute images. Consecutively, two investigators selected the photographs for the different study groups. Hence, a selection could be assumed due to personal preferences [28]. However, decision for inclusion or exclusion was made according to pre-set criteria, unlikely influencing our results.

Treatment planning in aesthetic dentistry usually begins at the maxillary central incisor area [29], and this applies to orthodontics [4]. To create a particular central-to-lateral incisor OHD, the orthodontist must either define the bracket position on the incisors as required or use artistic bends of the archwire. However, ‘white aesthetics’ are one of the considerations for achieving an aesthetic appearance: gingival tissues are generally known to follow individual tooth movements within reason [30‐33] and that, in turn, will change the contour of the anterior gingiva, potentially necessitating minor gingivoplasty to achieve optimal aesthetics. Machado et al. [9], found that when the gingival margin of the central incisor matched the laterals it was rated most aesthetic. However laypersons are not able to differentiate gingival asymmetry of 0.5 to 1.5 mm between maxillary incisors [7, 29, 34, 35], suggesting that the vertical position of the incisors can be varied to some extent without causing dissatisfaction with orthodontic treatment.

The need for customized orthodontic (and potentially restorative) treatment applies to patients with missing maxillary lateral incisors, whose treatment plan may include space closure [36‐38]. Re-arrangement of the maxillary incisor display may be required and to optimize aesthetics and deviation from commonly used bracket placement protocols should be considered, modifying the extent of central-to-lateral OHD. Bukhary et al. [10] found that hypodontia patients seem to prefer a maxillary central-to-lateral OHD of 1 mm. This value lies between the 0.5 mm, commonly used in standard protocols for bracket placement [5, 6] and values of greater than 1 mm, suggested by other investigators [9, 19, 21].

Although “ideal” values for OHD of about 1.5 mm were found in the literature, there seems to be a considerable diversity depending on individual preference. Ker et al. [7] showed that values ranging from 0 to 2.9 mm were acceptable suggesting: “Beauty is in the eye of the beholder” [28]. Patients’ opinions should be taken into account to avoid dissatisfaction with orthodontic treatment and OHD between central and lateral incisors should be considered at the treatment planning stage [39, 40].

We recommend assessing patient opinion regarding maxillary central-to-lateral OHD in females at the treatment planning stage or at least during the final treatment stages including finishing and final adjustments. Modification of commonly used bracket placement protocols may be helpful achieving the desired aesthetic outcome.