Esthetic Factors Involved in Anterior Tooth Display and the Smile: Vertical Dimension

In an extensive discussion of the principles of visual perception and their clinical applications to denture esthetics, Lombardi pointed out that detailed esthetic judgments can only be made by viewing patients from the front, in conversation, facial expressions, and smiling³ (Fig. 1A). The dentist's view from above and behind the patient is skewed, differing substantially from the "true" perception of the patient in a mirror or by other people during normal social activities (Fig. 1B,C).

For example, it is impossible to gain adequate information on such details as dentofacial midline alignment and right-left symmetry of canine and premolar torque unless the patient is observed directly from the front. Therefore, it is fundamentally important that esthetic factors such as the following be analyzed by sitting or standing in front of the patient (Fig. 1D):

• Crown lengths of maxillary and mandibular incisors.
• Incisal edge contours (before and after recontouring by grinding).
• Axial inclinations of all maxillary and mandibular incisors.
• Midlines (upper, lower, labial, and facial).
• Crown torque (canines, premolars, and molars on both sides).
• Smile line (rest position and full smile).
• Right-left symmetry of crown shapes and sizes and gingival margin levels.

After carefully studying these features, the orthodontist can make the required finishing archwire bends and perform any other esthetic procedures needed.

Smile Type

In visual evaluation of the full smiles of 454 dental and dental hygiene students, age 20 to 30, from the Los Angeles area, Tjan and colleagues found that 11% had a high smile, revealing the complete cervicoincisal length of the maxillary incisors and a contiguous band of gingiva.⁴ An average smile, revealing 75-100% of the maxillary incisors, was found in 69% of this population, and a low smile, displaying less than 75% of the maxillary incisors, in 20%.

Lip coverage of the maxillary incisors tends to increase with age, and therefore the percentage of high smiles may be greater among younger age groups^5,6 and smaller among older adults.⁷ There is also a sex difference in smile type: low smile lines are a predominantly male characteristic, and high smile lines predominantly female.⁴

Maxillary Incisal Curve and Lower Lip

The Tjan survey also revealed that 85% of the students had a maxillary incisal curve parallel to the inner contour of the lower lip, 14% showed a straight rather than a curved line, and 1% had a reverse smile line⁴ (Fig. 2).

Since parallelism is the "normal" finding in untreated persons, it would seem to be an optimal goal for objective beauty in all kinds of esthetic oral rehabilitations,^6,7 including orthodontic (Fig. 3A,B) and orthodontic-prosthetic treatment⁸ (Fig. 3C,D).

A straight or reverse smile line may contribute to a less attractive facial appearance.^3,8 In addition, the reverse curve is often associated with marked abrasive wear of the maxillary incisors (Figs. 2B and 4A).

Vertical Position of the Incisors

Normal Age Changes

Peck and colleagues recently measured maxillary incisor exposure, at rest position and full smile, in a sample of 15-year-old males and females⁵ (Table 1). Vig and Brundo reported a gradual decrease in maxillary incisor exposure for each increase in age group from under 30 to over 60⁷ (Table 2).

This progressive change is caused by the effects of gravity on upper and lower lip positions. The sagging of the perioral soft tissue is partly due to the natural flattening, stretching, and decreasing elasticity of the skin.¹⁰ Display of the maxillary incisors during normal conversation thus indicates youth, while display of only the mandibular incisors indicates age. The importance of the vertical dimension in tooth display has been demonstrated in prosthetic dentistry^8,9 and in orthognathic surgery involving inferior maxillary repositioning (Fig. 4).

Sexual Dimorphism

Several studies have found significant sex differences in anterior tooth display.^5-7 Generally, females have significantly more maxillary and less mandibular tooth exposure than males at all ages (Table 1). In an adult sample, Vig and Brundo found almost twice as much maxillary anterior tooth display with the lips at rest in women (3.4mm) as in men (1.9mm).⁷The men displayed much more of the mandibular incisors (1.2mm to .5mm). As noted above, a high smile type is twice as prevalent in women.^5,6 The reason for this dimorphism remains largely unexplained.

Standardized Extraoral Photography

A standardized procedure for recording tooth display in rest position and full smile, in conjunction with the standard full-face, three-quarter, and profile views, will help the clinician avoid undesirable treatment effects on the smiles of older patients. Estimates of method errors using double determinations show mean differences and standard deviations consistent with expectations.^5,6 Potential errors are immense, as dramatized by Burstone's data on the interlabial gap in adolescents: in centric occlusion the lip gap was 1.8mm (S.D. 1.2mm), but in rest position the gap was 3.7mm (S.D. 1.6mm).¹⁵

Each patient should be coached and asked to achieve the same lip position at least twice in succession before a photograph is taken (Table 3).^5,6 A short video sequence will be helpful in demonstrating rest position, normal conversation, and smiling. Most people are able to attain reasonably reproducible lip positions.

In rest position the teeth should be slightly apart, and the perioral soft tissue and mandibular posture must both be unstrained. At full smile the teeth should be lightly closed (Fig. 5).

The optimal vertical reference position for the maxillary incisal edge in orthodontic treatment planning is with relaxed lips (Fig. 6). Useful information can be obtained when the patient is observed in normal conversation. The full smile will not provide the same information, partly because of high individual variability in upper lip movement from rest position to full smile. This movement ranges from 2mm to 12mm, with an average of 7-8mm, and is not correlated with age.¹⁶

The smile is apparently formed in two stages, the first raising the lip to the nasolabial fold, and the second involving further superior raising of the lip and the fold by three muscle groups.⁵ Nearly everyone, irrespective of age, will display the maxillary incisors nicely on maximum smiling, even if only the mandibular incisors are visible during conversation (Fig. 6).

Clinical Implications for Low and Average Smile Types

The correction of deep overbite can be accomplished by various combinations of intrusion of the anterior teeth and extrusion of the posterior teeth.¹⁷ From the esthetic perspective, a serious mistake commonly made in orthodontic practice is "overintrusion" of the maxillary incisors (Figs. 7 and 8). In most deep overbite cases, this will tend to hide the maxillary anterior teeth behind the upper lip in normal conversation. Such a mistake can go undetected by the orthodontist unless the patient's tooth display and smile are analyzed from the front. With increasing age and concomitant drooping of the upper lip,⁷ an unesthetic anterior tooth display may worsen.

The maxillary incisors should be moved in the vertical direction that improves their relationship to the resting lip position, and the tooth-to-lip position should be monitored constantly throughout treatment. In some deep overbite cases, this may actually mean extrusion rather than intrusion of the maxillary incisors.^18,19 In most orthodontic patients, except those with marked "gummy" smiles, active intrusion of the maxillary incisors is undesirable.

The best treatment strategy in the majority of deep overbite cases is to actively intrude the mandibular incisors, using double tubes on the mandibular first molars and continuous or segmented base arches¹⁷ or utility arches²⁰ (Fig. 9). In a young patient with a short lower face, extrusion of the posterior teeth might correct a deep overbite, but the stability of such correction is uncertain, especially with less-than-adequate growth during and after treatment.¹⁷

Another common mistake in orthodontic finishing is to create a straight (or even reverse) maxillary incisal curve relative to the smile line (Fig. 7B,D). Parallelism of the incisal curve and the inner contour of the lower lip in smiling may seem difficult to produce. In practice, however, this appearance can readily be achieved if the maxillary central incisors are symmetrically positioned .5-1mm longer than the lateral incisors²¹ (Figs. 3B,C and 4B,D). If the lower lip shows a marked curvature in smiling, the distoincisal edges of the maxillary central incisors can be ground slightly without affecting functional occlusion¹² (Fig. 3B).

It is particularly undesirable to combine maxillary incisor overintrusion with a straight arrangement of these teeth. Figure 7 shows a young patient who received what Lombardi calls a "denture mouth".³

Clinical Implications for High Smile Types

The "gummy" smile, which can be defined as 2mm or more of maxillary gingival exposure in full smiling (Figs. 10 and 11), has provoked considerable interest and concern among clinicians.^{5,6,18,19,23,26} Its biological mechanism appears to involve the combined effects of anterior vertical excess, an increased muscular capacity to raise the upper lip in smiling, and associated factors such as excessive interlabial gap at rest and excessive overjet and overbite.⁹ Surprisingly, upper lip length, clinical crown height of the incisors, and mandibular and palatal plane angles do not appear to relate to the gingival smile line.⁵ The sexual dimorphism in smile types indicates that females are twice as likely as males to have gummy smiles.^4-6,27

A different treatment philosophy is needed for patients with high lip lines than for those with average or low smile types. Active maxillary incisor intrusion should be the goal in this category of patients. Treatment alternatives include various combinations of orthodontic, periodontal, and surgical therapy.^5,22,26

Intrusion base arches or utility arches may succeed in reducing a gummy smile orthodontically in some cases (Fig. 10). Such treatment can produce a remarkable change in facial appearance.¹⁹ Selective intrusive and restorative techniques can also be used to improve the final esthetic result in patients with fractured or overerupted and abraded incisors.^22,28

In other instances, gingival display can be eliminated by a simple gingivectomy (Fig. 11) or surgical crown lengthening with removal of crestal alveolar bone.^22,26,29 Such procedures are particularly indicated in cases with altered passive eruption, excessive gingival margins, and short clinical crowns, because they will expose more of the anatomical crowns. When crestal alveolar bone is removed during surgical crown lengthening, the gingival margin will stabilize within six months at about 3mm from the new bone level.²⁹

The type of gingival surgery depends on the relationship between the alveolar bone crest and the cementoenamel junction. At the end of orthodontic treatment, a labial gingivectomy to the bottom of the clinical pocket may produce a net gain of 1mm or more in crown length, or about half the probing length^30,31 (Fig. 11). The gingivectomy is especially useful in eliminating the accumulation of hyperplastic gingiva often associated with fixed appliance therapy. Wennström has demonstrated that even if the gingivectomy is extended into the alveolar mucosa, the regenerated tissue will still be normal gingiva with keratinized epithelium.³² When a local labial gingivectomy is performed on an adult, the cut should be reduced mesiodistally to eliminate the risk of interdental recession³¹ (Fig. 11C).