Chapter 3

Esthetics in Tooth Display and Smile Design

Bjørn U. Zachrisson

The orthodontic profession has always been in pursuit of the ideal dentition. At present, we are on the threshold of a paradigm shift that apparently will change the fundamental conceptual basis of orthodontics and the traditional emphasis in diagnosis and treatment planning.1,2 The former emphasis on dental and skeletal components is still valid but greater attention than before is now required to the soft tissue aspects of orthodontics. In an effort to create natural esthetics, the orthodontist must give careful consideration to the patient in his or her entirety. Individual attributes of a tooth or segment of teeth may represent only part of the story, because teeth do not exist individually and separate from the patient to whom they belong. Combinations of tooth positions can create an effect that is greater than, equal to, or less than the sum of the parts.3 The discipline of esthetics in orthodontics can be broken down into at least four parts: microesthetics (the elements that make teeth look like teeth), gingival esthetics, macroesthetics (the principles that apply when groupings of individual teeth are considered), and facial esthetics.3 This chapter will focus on the principles of macroesthetics and facial esthetics in orthodontics and how to apply them clinically. The dynamic relationship between the teeth and the surrounding soft tissues during and after orthodontic corrections and the patient's facial characteristics will be emphasized. The display (amount and shape of crown structure that shows in various views and lip positions) will be related to age, sex, and facial characteristics. The purpose is to provide guidelines for orthodontists on how to analyze esthetic factors by viewing the patient from the front and to discuss some new concepts on how to achieve the desirable characteristics of tooth display in the vertical and transverse dimensions during normal social interaction.

Evaluating Esthetics in the Dentist's Chair

Esthetics, which is derived from the Greek word for “perception,” deals with beauty and the beautiful. It may be divided into two dimensions: objective (admirable) and subjective (enjoyable) beauty.4 Objective beauty implies that the object possesses properties that make it unmistakably praiseworthy. Subjective beauty is value-laden and is related to the tastes of the person contemplating it. Contemporary techniques in orthodontics should lend objective esthetics to the entire orofacial complex, involving unity, form, structure, balance, color, function, and display of the dentition. In addition, the creation of subjective beauty according to an individual orthodontist's preferences may enhance the cosmetic value of the treatment rendered to each patient.

In discussing the principles of visual perception and its clinical application to dentofacial esthetics, Lombardi5 remarked that detailed esthetic judgments can be made only by viewing patients from the front, in conversation, and observing facial expressions and smiling. The traditional dentist's view from above and behind the patient is skewed, differing markedly from the “true” perception of the patient in a mirror or by other persons during normal social interaction. For example, it is not possible to gain adequate information on such details as midline alignment (maxillary and mandibular relative to facial) and right-left symmetry of canine and premolar crown torque (Fig. 3-1, A) unless the patient is observed directly from the front (Fig. 3-1, B). A direct “eye-to-eye” view of the dentition can, in fact, be obtained when the patient is sitting in the dental chair6; the trick is to move the patient's head to the side of the headrest (see Fig. 3-1, B). With this method it is possible to analyze important esthetic factors during the treatment (Fig. 3-1, C and D), such as:

It is important for a successful outcome that the clinician has learned to look at all of these aspects and has the ability to make the necessary corrections while the fixed appliances are still on, because “you can only change what you see.”7 After carefully studying the patient's dentition, the orthodontist can make the required finishing archwire bends and perform any other esthetic procedures needed. For analysis of tooth display when speaking and smiling and viewing the buccal corridors on smiling, a better impression is obtained when the patient is sitting up or standing in front of the dentist.5,6

Standards of Normality

It is useful for esthetic oral rehabilitations to describe some average desirable characteristic features of smiles. These normative standards may serve as a guideline for enhancement of esthetics for the anterior component of the dentition.

Smile Type: Incisor and Gingival Display

Lip coverage of the maxillary incisors in full smiles is generally distinguished into three types: low, average, and high smiles.8 The most frequent type8,9 (found in about 70% of the young adult population) is the average smile that reveals 75% to 100% of the upper incisors. The low smile displays less than 75% of the maxillary incisors in a full smile and is found in about 20% of the population, whereas the high smile reveals the complete cervicoincisal length of the upper incisors and a contiguous band of gingiva and occurs in about 10% of the U.S. population.8 A fourth type of lip line height may be defined as a “gummy” smile, which occurs when patients show more than 4-mm of gingiva on smiling (discussed later in the chapter).10,11

Upper lip coverage will always increase with age and therefore the percentage of high smiles may be greater among younger age groups12,13 and smaller among older adults.9,14 There is also a sexual dimorphism in that low smile lines are predominantly a male characteristic and high smiles are predominantly a female characteristic.8

It is noteworthy that some marginal gingival display in smiling is not as objectionable to laypersons as orthodontists may imagine.10,13 We should therefore look on a moderately high smile type as an acceptable anatomic variation well within the usual range of lip-tooth-jaw relationships, especially among women.12,13

Smile Arc15,16 (Smile Curve)

The relationship of the maxillary incisal curve to the inner contour of the lower lip can be divided into three types: parallel, straight, and reverse. In a survey of young adults in the Los Angeles area, Tjan et al.8 found that a great majority (85%) had a maxillary incisal smile curve parallel to the inner contour of the lower lip, 14% showed a straight rather than a curved line, and only 1% had a reverse smile curve. Since parallelism is the normal finding in untreated persons, it is an optimal goal for objective beauty in esthetic oral rehabilitations,17,18 including orthodontic (Figs. 3-2 and 3-3) and orthodontic-prosthetic treatments (Fig. 3-4). A straight or reverse smile curve may contribute to a less attractive facial appearance.5,17 The reverse curve is frequently associated with marked abrasive wear of the upper incisors.

Vertical Position of the Incisors

Normal Age Changes in Lip-Incisor Relationship

Progressive changes with age on upper and lower lip positions are caused by the effects of gravity. Some normative studies on the optimal vertical incisor positions in the faces of persons in different age groups are available. Peck et al.13 showed that the normal display of maxillary incisors with relaxed lips at 15 years of age was 4.7-mm (standard deviation [SD] 2.0-mm) for boys and 5.3-mm (SD 1.8-mm) for girls. The sexual dimorphism is evident at all ages. For adults, Vig and Brundo14 have provided normative mean values for different age groups (Table 3-1). Dong et al.19 compared the age changes in maxillary and mandibular incisor display at rest and when smiling (Fig. 3-5) and confirmed the observations that the age changes with relaxed lips were dramatic (Fig. 3-6). Mandibular incisor display shows a corresponding increase with age. The amount of mandibular incisor display after age 60 is approximately equal to the amount of maxillary incisor display before age 30 (see Fig. 3-5 and Fig. 3-6, D).

There is a close correlation between the incisor exposure at rest and during normal speech.20,21 The tooth display in speaking may be every bit as important as the tooth display in smiling to express personality and age. The most important information related to esthetics in treatment planning is obtained when the patient is observed during normal conversation. The tooth display on smiling will not provide the same information since when a person is smiling, the upper lip is raised actively by three different muscle groups.22 For this reason, nearly everyone, irrespective of age, will display the maxillary incisors nicely in the full smile, even if only the mandibular incisors are visible during their speech. In other words, the age changes in incisor display are much more pronounced and apparent with the lips relaxed and when the patient is speaking than when he or she is smiling (Fig. 3-7).15

The sagging of the perioral soft tissue is partly due to the natural flattening, stretching, and decreasing elasticity of the skin.23 The upper lip becomes longer and hides more and more of the maxillary incisors, while the drooping of the lower lip gradually exposes more of the mandibular incisors. As a consequence, show of maxillary incisors with relaxed lips signifies youth and beauty, whereas display of mandibular incisors is a characteristic of the elderly (see Fig. 3-6). The importance of the vertical dimension in tooth display has been demonstrated in prosthetic dentistry13,14 and in orthognathic surgery involving maxillary repositioning.24,25

The guide for tooth position planning in orthodontic treatment should start with an appraisal of the position of the maxillary central incisal edge relative to the upper lip.26,27

This assessment is made with the patient's upper lip at rest, using a millimeter ruler or periodontal probe. This incisor position can be either acceptable or unacceptable, depending on the patient's age. The clinical guideline should be that the maxillary incisors should be moved in the vertical direction that improves their relationship to the resting lip position relative to the patient's age (Figs. 3-8 and 3-9; see Fig. 3-7). The relaxed lip position is the most reproducible functional position. It can and should be used as the guide,26,27 whereas forced or posed smiles may vary considerably in the same patient and cannot be considered an accurate guide for incisor positioning.27

Spontaneous smiling records may also be recommended for diagnostic purposes. Because of the dynamic nature of spontaneous smiling, dynamic video recording of the spontaneous smile should be preferred.9,20,21 Lip-tooth relationships follow a consistent pattern in natural rest position and during speech and spontaneous smiling.

Sex Differences

The sexual dimorphism in anterior tooth display implies that females have significantly more maxillary and less mandibular tooth exposure than males at all ages. In a group of adults, Vig and Brundo14 found almost twice as much maxillary anterior tooth display with the lips at rest in women (3.4-mm) as in men (1.9-mm). Men displayed much more of the mandibular incisors (1.2-mm compared to 0.5-mm).

Standardized Extraoral Records

A standardized procedure for recording the incisor display in (1) rest position (Fig. 3-10; see Figs. 3-4 to 3-9) and (2) a posed smile (see Figs. 3-2 to 3-4) both before and after orthodontic treatment is recommended and will help the clinician to avoid undesirable treatment effects on the maxillary incisor reveal. Each patient should be coached and asked to achieve the same lip position at least twice in succession before a photograph is taken. In rest position (instruct the patient to say “Emma” or “Mississippi”),6,28 the teeth should be slightly apart and both the perioral soft tissue and the mandibular posture must be unstrained. In the posed smile (instruct the patient to bite together, smile, and say “Cheee …”),6 the teeth should be lightly closed. As mentioned, the analysis of incisor display during spontaneous smiling should be made with video records.

Clinical Implications for Deep Overbite Correction

Average and Low Smile Types

The correction of deep anterior overbite can be made by various combinations of incisor intrusion and molar extrusion.29,30 The treatment concepts for deep overbite cases have changed significantly during the past 10 to 15 years. This is due to the increasing attention now being paid to the esthetic importance of the vertical display of the maxillary incisors during normal speech and with relaxed lips.6,7,26 While active intrusion of maxillary incisors with intrusion arches à la Burstone, utility arches à la Ricketts, overlay base-arches, and similar approaches has previously been considered a cornerstone of deep bite correction, the risk of too much intrusion (so-called “overintrusion”) with such approaches is apparent.28,30 Overintrusion may tend to hide the maxillary incisors behind the upper lip when the patient is speaking. Lindauer et al.30 compared the outcomes from two common procedures used to reduce deep overbite: maxillary incisor intrusion using an intrusion arch and posterior eruption using an anterior biteplate. Both the intrusion arch and the biteplate procedures effectively reduced overbite significantly over a relatively short period of treatment. The intrusion arch patients displayed significant reductions in maxillary incisor display (mean change from 5.4- to 3.0-mm) accompanying documented incisor intrusion. This means that a maxillary intrusion arch will lead to a premature aged oral appearance. Such a mistake can go undetected by the orthodontist unless the upper central incisors show on speaking and smiling is recorded and analyzed properly. With increasing age of the patient and concomitant drooping of the upper lip, an unesthetic incisor display at adolescence will predictably worsen with time.9,21

In a child or adolescent patient, maxillary incisor intrusion beyond 4- to 5-mm below the upper lip at rest represents overintrusion of these teeth and unwanted aging of the patient. In a young adult between 20 and 30 years of age, there should be at least 3-mm of maxillary incisors showing. For an adult 30 to 40 years of age, approximately 1.5-mm of the maxillary incisors should show at rest position of the lips, and at age 40 to 50 years, about 1-mm should show. In patients more than 50 to 60 years of age, the maxillary incisors normally should not show at all when the lips are relaxed (see Fig. 3-6). According to Frush and Fisher,31 an optimal incisor position for adults occurs when the maxillary lateral incisors show “when the patient is speaking seriously.” The tips of the lateral incisors should show in varying degrees according to the sex and age requirements.

The tooth-to-lip position at rest should be monitored constantly throughout the orthodontic treatment. Since no orthodontist would wish to make his or her patients look older than they really are, it is important to carefully analyze each patient's tooth display on speaking before deciding whether maxillary intrusion mechanics should be used.28 In some deep overbite cases, extrusion rather than intrusion of the maxillary incisors may be indicated (see Fig. 3-7) or a combination of orthodontics and prosthetic crown lengthening with porcelain laminate veneers will be the method of choice (see Fig. 3-4).

From an esthetic point of view, the best treatment strategy in the majority of deep overbite cases is to actively intrude the mandibular rather than the maxillary incisors. This is particularly true when the curve of Spee is marked and when the six mandibular anterior teeth are above the functional occlusal plane at the start of treatment. Mandibular incisor intrusion can be achieved with segmented intrusion arches, utility arches, overlay base-arches, etc. The rate of intrusion can be controlled by recording the position of the maxillary central incisal edges relative to fixed points on the mandibular appliances. Using overlay base-arches (see Figs. 3-7 to 3-9), the mandibular incisor intrusion usually occurs at a rate of 0.5-mm per month. It should be emphasized that it is not possible to effectively intrude mandibular incisors with one continuous archwire. Compared to a conventional continuous archwire, segmented mechanics (Burstone) will produce overbite correction by (1) more incisor intrusion and (2) less molar extrusion and subsequent posterior rotation of the mandible.32,33

Another situation that calls for the use of segmented archwires happens in children with reduced anterior overbite and maxillary canines erupting in a high position.28 If a continuous leveling archwire is used, the intrusive counterforce on the incisors may overintrude them into functionally and esthetically unacceptable positions. In such instances, the first molars should be connected with a solid transpalatal bar to yield a reliable posterior anchorage unit and a cantilever wire from the extra molar tube used to bring down the canines and secure an optimal vertical incisor display after treatment.7

An alternative to incisor intrusion for deep overbite correction may be active molar extrusion. Such effects can be obtained with functional appliances, biteplates, headgears, etc.34 Molar extrusion may be of merit in a growing child with normal or low-angle face type and vertical growth pattern28 but it would be calamitous in a high-angle case and cannot be recommended in adults because of stability concerns.34

The second most common mistake in orthodontic treatment and finishing in the vertical plane is to create a straight smile line rather than an incisal smile curve.15,28,35,36 Undesirable arc flattening is probably underestimated in orthodontics. Ackerman et al.36,37 reported that as many as 32% of their patients got a flattening of the smile arc during orthodontic treatment. One reason why such changes may go unnoticed by orthodontists is that they are observed only when the patients are examined from the front.

It may seem difficult to achieve the desired parallelism between the maxillary incisors and the lower lip in smiling. In clinical practice, however, this appearance can readily be achieved if the maxillary central incisors are symmetrically positioned 0.5- to 1.5-mm longer than the lateral incisors38 (see Fig. 3-10). If the lower lip shows a marked curvature in smiling, the distoincisal edges of the maxillary central incisors can be ground slightly with a diamond instrument, as this procedure will not affect the functional occlusion.33

It is particularly undesirable to combine maxillary incisor overintrusion with a straight rather than a curved arrangement of these teeth. This display gives the impression of a typical static denture that results in the so-called “denture mouth.”28,31

High Smile Types: “Gummy” Smiles

As mentioned, a fourth category of smile line types is the “gummy” smile, defined as show of more than 4-mm of maxillary gingiva in full smiling.11 This smile type has provoked considerable interest and concern among orthodontists. Its biological mechanism appears to involve the combined effects of anterior vertical excess, an increased muscular capacity to raise the upper lip in smiling, short upper lip, and associated factors such as excessive interlabial gap at rest and excessive overjet and overbite.18,37

A different treatment strategy may be needed for patients with high lip lines than for those with average or low smile types. Treatment alternatives include various combinations of orthodontic, periodontal, and surgical therapy. The differential diagnosis should take into consideration both the amount of maxillary incisor display at rest position of the lips and the amount of gingiva shown on smiling. If the maxillary incisor show at rest is optimal, active upper incisor intrusion should not be initiated. Instead, local gingivectomies or surgical crown lengthening with removal of crestal alveolar bone39–41 should be done. Such procedures are particularly indicated in cases with altered passive eruption, excessive marginal gingivae, and short clinical crowns, since they will expose more of the anatomic crowns. When crestal alveolar bone is removed during surgical crown lengthening, the gingival margin will stabilize within 6 months at about 3-mm from the new bone level.41 Attempts to inject botulinum toxin (Botox) for neuromuscular correction of excessively gummy smiles caused by hyperfunctional upper lip elevator muscles have been reported to be effective for about 6 months but the effect is transitory.42

Treatment of the most severe gummy smiles may require maxillary superior repositioning surgery (Le Fort I osteotomy), along with reduction of the associated vertical maxillary excess.13

Midlines

Dental to Facial Midline Positions

As mentioned, it is virtually impossible to check the relationship of the upper and lower midlines to the facial midline in the normal dentist-patient positions, since these allow only a view from the side (see Fig. 3-1). However, moving the patient's head to the side of the headrest will allow direct frontal observation.6

Midline Guides

The most practical guide to locate the midline of the face is to draw an imaginary vertical line that extends through soft tissue nasion and the midpoint of the Cupid's bow in the upper lip.3,43

A line between these landmarks not only locates the position of the facial midline but also determines the direction of the midline. Whenever possible, the maxillary dental midline should be coincident with the facial midline (Figs. 3-10 to 3-13). If this is not possible, the midline between the maxillary central incisors should be strictly vertical and parallel to the facial midline.3,10,44,45

Esthetically Acceptable Midline Deviations

Kokich Jr. et al.10 reported an interesting interaction between maxillary central incisor midline deviation and crown angulation. As long as the dental midline was parallel to the facial midline, even a 4-mm maxillary midline deviation was not detected on photographs by dentists and laypersons. Similar findings from laypersons' perspectives were made by Springer et al.46 However, all evaluators rated a 2-mm deviation in incisor angulation (canted midline) as noticeably unattractive. A significantly slanted dental midline is displeasing and will be noticed easily. These data demonstrate that a precise dental midline is not necessary for optimal esthetics as long as the incisal crown angulation is not canted (see Figs. 3-8 to 3-13). On the other hand, if the midaxis between the right and left maxillary central incisors is markedly canted, it will look unacceptably skewed even if the incisor contact point is located in the middle of the face.

While alignment of the maxillary and mandibular dental midlines is desirable in orthodontic treatment for occlusion reasons, the mandibular midline becomes a lesser issue in esthetics. The narrowness and uniform sizes of the mandibular incisors make visualization of their midpoint more difficult.

Connector Area Versus Contact Point

Morley and Eubank3 introduced the term connector areas as a useful tool and a visual goal to optimize smile esthetics in dental patients. The connector areas are larger, broader areas than the contact points between teeth and can be defined as the zone in which two adjacent teeth appear to touch. The most esthetic relationship between the maxillary anterior teeth is referred to as the 50-40-30 rule. This rule defines the ideal connector area between the two maxillary central incisors as 50% of their clinical crown length (Fig. 3-14; see Figs. 3-2, 3-12, and 3-13). The ideal connector area between a maxillary lateral incisor and a central incisor is 40% of the central incisor's clinical crown length, and between a lateral incisor and a canine is 30% of the clinical crown length of the central incisors3 (see Fig. 3-13).

The most important connector area is the one between the two maxillary central incisors. Since it should be quite long in orthodontically well-treated cases, it is clinically important to carefully check the clinical midaxis between the mesial surfaces of the central incisors before appliance removal. In cases of pre-treatment crowding it is almost always necessary to recontour these mesial surfaces by slight grinding (see Figs. 3-1, 3-3, 3-12, and 3-13). This will relocate the contact point in an apical direction to reduce or avoid interdental gingival recession47 (dark triangles between the teeth due to loss of the gingival papillae) and result in an optimally long and vertical connector area (see Figs. 3-2 and 3-12 to 3-14). If the connector area is canted undesirably, it can be improved in the finishing stages of treatment using small artistic archwire bends. A prerequisite for detecting the need for such corrections is that the patient's dentition is studied from the front.

Transverse Dimension

Most orthodontists are familiar with the fact that too little lingual root torque of the maxillary central incisors during treatment will have a negative esthetic effect in most patients. Partly due to a different reflection of the incoming light, patients with optimal incisor crown inclination are considered to look more attractive than patients who finish orthodontic treatment with large interincisal angles. The current evidence regarding the most desirable esthetic labiolingual crown inclination of the maxillary canines, premolars, and molars is limited. Thus any discussion relative to what constitutes the most esthetic positions of the upper canines and posterior teeth in different patients will be subjective.

As discussed and illustrated elsewhere,7,48,49 the fullness of the smile should be sought through adjustment of the clinical crown torque of the maxillary canines and premolars to their most esthetic appearance in different face types, rather than through nonextraction heroics or unnecessary lateral expansion and labial tipping of the maxillary dentition. Some important elements for the transverse dimension in orthodontics are:

Terminal Tooth on Smiling

Generally speaking, about 90% of people show either the first or the second premolar as the last tooth when they are smiling.8,16 To create the illusion of smile fullness, the last premolar displayed should be positioned relatively upright42,43 (see Figs. 3-2, 3-10, and 3-11). It is particularly important to avoid a lingual inclination of the maxillary premolars in patients with a relatively small maxillary apical base (see Fig. 3-14) and in premolar extraction cases (Fig. 3-15; see Figs. 3-2, 3-12, 3-13).48,49 When there is crown inclination asymmetry between the right and left last premolar on smiling, the smile invariably appears narrower on the side where the premolar has more tilt.47

The torque prescriptions for most preadjusted appliance systems tend to create too much lingual crown inclination of maxillary and mandibular canines and posterior teeth.50 The view that the canines and premolars should have considerable lingual crown inclination (Fig. 3-16) in an optimally treated orthodontic case51 irrespective of tooth size, jaw size, face type, and facial expressivity is disputed from an esthetic perspective.

image

Figure 3-16 The optimal lingual crown inclination in normally occluded (A) upper and (B) lower posterior crowns according to Andrews.51 C, Progressive medial tipping of the axial inclinations of the teeth according to Morley and Eubank.3

The normative crown inclination values proposed by Andrews51 (see Fig. 3-16) that have influenced available preadjusted appliance systems were based on careful study of 120 nonorthodontic patients with normal occlusion and teeth that were “straight and pleasing in appearance” and 1150 successfully treated orthodontic cases. Although this information has been of great value to our profession, study of already treated orthodontic cases may not be the optimal basis for optimal crown torque estimations from an esthetic perspective. Indeed, laypersons prefer straight upper canines and premolars according to an extensive computer-based survey based on slider technology on a large number of U.S. patients (n = 243).52

Crown Inclination Symmetry on Contralateral Teeth

Crown inclination symmetry of the contralateral teeth on the right and left sides of the maxilla and mandible will contribute to an optimally esthetic appearance (see Figs. 3-11 to 3-15). It is generally easier said than done to obtain bilateral crown inclination symmetry on canines and premolars (see Fig. 3-15). A prerequisite to detect asymmetries is to view the patient directly from the front at the start of orthodontic therapy so that the necessary correction bends can be made in the archwires.

Front-to-Back Progression

The front-to-back progression of the maxillary canine, premolars, and molars is a critical factor for the display of the dentition when the patient is speaking and smiling. The principles of gradation and smoothness must be observed, so the decrease in size and detail occurs gradually.5 Lombardi5 noted that the teeth should have a harmonious perspective from the dominant central incisors transitioning posteriorly, with each tooth in harmonious proportion to those adjacent. The apparent widths may or may not be in “golden proportion”53,54 to one another. Analyzing students in California, Preston54 found the golden proportion between the perceived width of the maxillary central and lateral incisors in only 17% of the cases and between any perceived lateral incisor and canine widths in none of the cases. He claimed that there is nothing mystical or exclusively correct about the use of the golden proportion. Such use may well provide a pleasing outcome, as might many other approaches (see Figs. 3-10 to 3-15 and 3-18). From a clinical point of view, it may be more important to avoid any interruption of the harmony and gradual smoothness. This implies that canines and premolars with excessive lingual tipping (Figs. 3-17 and 3-18) or a premolar placed too far buccally5 will lessen the harmony of the tooth display curve laterally and reduce the esthetic impression.

Buccal Corridors

Several studies have reported laypersons' perceptions of and reactions to the buccal corridors.31,35,46,52,55 None found a specific relationship between this trait and smile esthetics. However, excessive buccal corridors are unesthetic to both orthodontists and laypersons.46,55,56 Frush and Fischer31 considered the buccal corridor to be a normal feature of a dentition that prevents the “sixty tooth smile” that is often characteristic of a denture. Apparently, the labiolingual inclination of the maxillary canine and premolars is as important as or more important than the presence or absence of dark buccal corridors for reflection of dental arch fullness (Figs. 3-19 and 3-20; see Fig. 3-18). It should be emphasized that dark shadows between the buccal surfaces of the dentition and the corners of the mouth are more apparent on frontal photographs than in real life, as they are generally due to inadequate flash lighting of the posterior areas of the mouth in routine photography.36,37 If the pre-treatment maxillary dental arch is acceptable with regard to shape and width, it may seem preferable in terms of long-term stability to obtain smile fullness by intentionally adding buccal crown torque to lingually inclined canines and premolars during treatment (see Figs. 3-10 to 3-13 and 3-18). Maxillary lateral expansion is indicated whenever the maxillary and mandibular dental arches are notably constricted in the beginning, whether or not posterior crossbite is present. Excessive intentional lateral expansion of the dental arches is likely to introduce a disequilibrium and long-term relapse.48,49

The principles and considerations discussed above may be supplemented by a discussion of the optimal crown torque from an esthetic perspective for individual teeth, as follows.

Maxillary Canine

There is a very large individual variation in labiolingual inclination of the maxillary canines and premolars between patients. The patient in Figure 3-19, A and C, has a broad maxilla and marked lingual tipping of all teeth. In marked contrast, the patient in Figure 3-19, B and D, has a narrow maxillary apical base and a pronounced labial tilt of all clinical crowns. The crown torque discrepancies between these two patients demonstrate why it is unrealistic to try to produce the same orthodontic treatment result for both of them; it is also obvious that they cannot be treated to optimum esthetics using the same preadjusted bracket prescription system without marked third-order archwire bends. An individualized esthetic goal should be set for each patient before the orthodontic treatment is started. Excessive lingual tipping of the maxillary canine is undesirable for esthetic reasons, whether it occurs unilaterally or bilaterally (see Figs. 3-17 and 3-18).52

It is surprisingly common that at the end of fixed appliance treatment, maxillary canine and premolar crowns are tipped lingually because the prescription in many modern brackets provides negative torque (lingual crown torque) for these teeth.57,58 This affects smile esthetics negatively, especially in patients with narrow and tapered arch forms, by making the canines less prominent (see Fig. 3-17) and causing the first premolars to almost disappear on smile (see Fig. 3-20). To obtain a broader arch and more pleasing smile, the solution is not further lateral expansion across the premolars, but to use lingual root torque so that the crowns are uprighted. This gives the appearance of a broader smile without the risk of relapse that accompanies such expansion. Existing data confirm that the inclinations of these teeth remain as they were at the end of treatment.58,59

Uprighting the canines and premolars in this way does elongate their lingual cusps and potentially could lead to occlusal interferences that would be difficult for patients to tolerate. If this occurs (which is unlikely), reduction of the height of the lingual cusps is indicated.

To achieve symmetry of labiolingual crown inclination between the teeth on the right and left sides of the mouth, intentional individualization is needed (see Fig. 3-15). The easiest and most practical way to achieve symmetry is to carefully study each case from the front and make the necessary archwire bends early in treatment (see Fig. 3-15).

It is apparent from these and other cases that (1) the preferred labiolingual crown inclination of the maxillary canine from an esthetic viewpoint in a majority of cases is relatively upright7,52 and (2) that pre-treatment asymmetries in crown inclination between right and left canines will remain after orthodontic therapy if no intentional measures are taken to correct them. Such adjustments may involve the use of individual archwire bends at different periods during treatment or possibly the use of custom brackets specifically engineered for the individual needs of each patient.

Maxillary First and Second Premolars

Premolars with an upright position will produce a broader smile than premolars that are tipped lingually. Particularly when orthodontic treatment has resulted in lingual tipping of the maxillary premolars behind upright canines, the smile becomes undesirably narrow in the posterior segments and does not reach an optimal fullness (see Fig. 3-20).

For this reason, the esthetically preferable first and second premolar crown torque for most patients is around 0 degrees.7,52 For patients with a wide maxillary apical base, a few degrees of lingual crown torque may be desirable. For patients with a small maxillary apical base, upright premolar crowns or even some labial crown inclination may produce a nice display of their dentitions (see Fig. 3-14).

Maxillary First Molar

Only a small percentage of a population show the maxillary first molars when smiling.8 For these patients, the molars should be relatively straight to contribute to a full smile (Fig. 3-21).

Mandibular Canine

The optimal outcome of orthodontic treatment regarding the mandibular canines is to achieve (1) relatively upright positions when viewed from the front (see Figs. 3-11 and 3-21) and (2) bilateral crown inclination symmetry (see Figs. 3-9, 3-11, and 3-13). Upright rather than lingually tipped mandibular canines allow more labial crown torque of the maxillary canines, producing a broader smile. Many preadjusted prescription appliances place lingual crown torque in the mandibular canines, which appears undesirable both esthetically and functionally. Particularly when the mandibular canines are tipped lingually before orthodontic treatment is started, appliances with built-in lingual crown torque tend to produce an excessive lingual tipping of the canines (Fig. 3-22). The correction of such tipping toward the lingual is time-consuming and difficult and may remain unnoticed at the end of treatment (see Figs. 3-14 and 3-22). A mandibular canine torque prescription that induces some labial crown torque (lingual root torque) will counteract such side effects.

Mandibular Premolars and Molars

A common side effect of routine orthodontic treatment with both preadjusted and standard edgewise appliances is that the crowns of the mandibular posterior teeth tend to tip lingually.50 This is undesirable not only from an esthetic point of view, but also for functional reasons. If the mandibular premolars and molars receive marked lingual crown torque, the lingual cusps of the maxillary posterior teeth may erupt into occlusion with their lingual cusps hanging down. This arrangement may cause balancing side interferences in side movements of the mandible. This side effect can be avoided by careful clinical observation and archwire bending and/or by using nontorqued attachments for the mandibular posterior teeth, including the second molars.

How to Achieve Both an Optimal Smile and Dental Stability

The available research evidence regarding long-term stability of orthodontic treatment results implies that the patient's pre-treatment mandibular intercanine width and mandibular arch form may be the optimal guide to future dental and arch form stability.60–63

If these measures are within a normal range, which implies that the intercanine width is around 25- to 26-mm, the mandibular incisors are in front of the A-pogonion line, and the arch form is symmetrical, with no need for transverse uprighting (buccal crown torque) of premolars and molars, then the mandible should be treated orthodontically without incisor proclination and lateral expansion (see Figs. 3-9, 3-12, 3-13, and 3-18). Mild to moderate dental crowding is managed by mesiodistal enamel reduction (stripping).

The maxillary dental arch is placed on top of and coordinated with the mandible. The original maxillary arch form is respected but frequently has to be rounded and slightly expanded posteriorly (see Fig. 3-13) to occlude properly with the mandibular teeth.48,49 With this approach, smile fullness is sought not through lateral expansion or tipping of the maxillary dentition but rather through intentional adjustment of the labiolingual crown inclination of the maxillary canines and premolars to their most esthetic appearance (see Figs. 3-11 to 3-13).48 Maxillary lateral expansion is indicated, however, whenever the maxillary and mandibular arches are notably constricted in the beginning, whether or not a posterior crossbite is present.

Clinical Guidelines

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