July 1, 2004
by Oral Health
By Daniel Fortin, DMD, MS; Genevive Guertin, DMD, MS; Athena Papadakis BDS (Hons), FDS, RCS (Eng), M.Sc., FRCDC
Clinical management of patients with spacing of the anterior teeth following orthodontic treatment is often necessary. When creating space orthodontically, a number of important questions arise as restorative treatment will often require orthodontic space redistribution prior to the placement of the final restorations. Successful treatment of these cases is therefore dependant not only on the correct treatment plan, but also on the correct sequencing of treatment between the different members of the dental team. This article will discuss multiple factors such as tooth proportion, tooth position and gingival architecture that must be considered in order to obtain the best esthetic and functional result following orthodontic treatment in space management cases.
In the past century, dentistry and patients’ understanding of the dental profession have evolved tremendously. In fact, patients seek orthodontic treatment not only to improve their function, but also to improve their smile esthetics. In essence, the primary goal of modern orthodontics is to establish the best possible occlusal relationship between the maxillary and mandibular arches while maintaining or enhancing facial esthetics.1 In order to do this, the orthodontist will often have to rely on restorative procedures to obtain the optimal esthetic result.2
The management of a patient with a tooth size discrepancy in the so-called “smile zone” is a challenge for both the orthodontist and the restorative dentist. As early as the initial consultation, the orthodontist should be able to determine what the outcome of his treatment will be and should be in a position to discuss the treatment options available to his patient for the ultimate management of the tooth size discrepancy. If an excess of space is expected at the end of treatment, then two treatment options need to be presented to the patient before the treatment is initiated. The orthodontist needs to decide to either aim for total orthodontic space closure or to maintain the spaces open for conventional prosthetic restorations to be placed at the end of treatment. The decision needs to be based on several major factors such as: the type of occlusion, the amount of space available and the width and shape of the existing incisors and canines.3
In many cases, patients show a tooth size discrepancy within the arch with a lack of harmony between tooth sizes in upper and lower arches. The Bolton analysis4,5 may be used to measured tooth size discrepancies from cuspid to cuspid or from first molar to first molar6 (to follow).
The exact amount of the discrepancy can be calculated in millimeters. This analysis also discerns whether the excess will be in the mandibular or maxillary arch. It is widely accepted that the presence of anterior dental spacing in the maxilla is considered unattractive.
In order to thoroughly evaluate, diagnose, and resolve the esthetic problems caused by a tooth size discrepancy, an interdisciplinary approach is needed. Communication between the orthodontic and restorative disciplines throughout the orthodontic treatment is imperative in order to achieve the best esthetic and functional outcome for the patient.
WHAT MAKES A SMILE ATTRACTIVE?
In the past century, the complete denture literature has identified some of the major factors that play a role in the achievement of an attractive and natural smile such as tooth form and proportion, occlusal plane, overall symmetry and gingival architecture. With dentate patients on the other hand, the dental team has to work with an existing smile and optimize its esthetic appearance by creating a balance between the lips, the teeth and the gingiva. When setting goals for treatment and its sequence for this group of patients, the dental team will have to “begin with the end in mind” and identify the objectives they are aiming for before the treatment is initiated.7
TOOTH SHAPE, PROPORTION AND POSITION
When looking at a smile from a facial aspect, central incisors are usually the first teeth noticed and therefore dominate the smile. As a general rule, in order to be esthetically pleasing, the central incisors should be a mirror image of one another with symmetry in shape and position of their long axis.8 There are as many tooth shapes as there are individuals but generally speaking, the unworn incisors have a trapezoidal form with an average 78 percent width to length ratio. With tooth wear and degree of eruption, this ratio may increase to up to 87 percent9 and will vary within individuals, their race and gender.
Lateral incisors are usually smaller in size than the central incisors. The long axis of the lateral incisors can be slightly different from one another but should show a slight mesial tip.8 Many suggest the use of the divine or golden proportion as a guideline to determine the width of the lateral incisor.10 According to a study by Magne9 on anatomic crowns, this 61.2 percent ratio would be incorrect as they found that the mean width of lateral incisor is approximately 78 percent the mean width of the central incisor (Fig. 1).
Canines play an important role in occlusion and esthetics as they control the width of the arch and buccal corridor. From a facial perspective, the long axis of the canine tooth has a greater mesial tip than the lateral incisor.
The actual mesio-distal measurement of the canine is on average 7 to 8mm.11,9 However, when visualized from the front, only the mesial aspect of the tooth is visible, making it appear slightly narrower than the adjacent lateral incisor.
Consequently, the dental team should aim for:
Trapezoidal shaped incisors of greater length than width;
Gradual increase in the mesial tip from the central incisors to the canines;
Dominant central incisors, smaller lateral incisors and canines with an approx 78 percent decrease width from a facial perspective.9
NB: When performing diastema closure, the clinician will want to avoid creating a cuboidal tooth form, which is often esthetically unpleasant.
When the spacing is too large to be closed with one tooth in the quadrant, then multiple teeth will need to be altered.
A diagnostic wax-up is very useful in determining the likely final esthetic outcome and can be performed before any treatment is initiated.
In health and proper tooth positioning, the gingiva has a pink stippled appearance with relatively symmetrical bilateral sinuous architecture.
The gingival height of contour of the central incisors and the canines stands at the distofacial line angle, and the gingival height of contour of the lateral incisors is generally centered mesio-distally.13
When planning to optimize the esthetic outcome, the orthodontist should not only reposition the teeth in the ideal position, but must also reposition the bone and soft tissues optimally. Intrusive or extrusive movements should be used primarily to develop symmetry of the gingival margins12,7 rather than to harmonize the incisal edges that can later be modified with restorative dentistry.
What to aim for:
Bilateral symmetry in the position of the gingival height of contour;
Gingival plane parallel to the interpupillary line;
Lateral incisor height of contour in the range of 0.5mm apical and 2mm coronal to the height of contour of the adjacent teeth12 (Fig. 2).
THE SMILE LINE
The smile line is the curvature obtained by joining the incisal edges of the central incisors and canines. As a general rule, the curvature of the smile line should parallel the inner curvature of the lower lip14 also called the lip line. This harmony between the lip line and the smile line (positive smile line) is considered to be a sign of youth and attractiveness.8 As patients get older, the general curvature of the smile line usually becomes flatter as a result of wear. The smile is considered unattractive when the curvatur
e is reversed when compared to the lip line.15
Note: The incisal edge of the lateral incisor does not usually fall within the smile line. It usually will be approximately 1mm apical to the curvature obtained by the incisal edges of the central incisors and canines forming a gull-wing pattern (Fig. 3).
What to aim for?
A positive smile line curvature adapted to the lip line and to the age of the patient. Avoid a reversed smile line.
It is generally accepted that the maxillary dental midline should coincide with the facial midline14 (to follow).
But in certain orthodontic cases, correction of a dental to facial midline discrepancy is not straightforward, and may increase both the complexity and duration of the orthodontic treatment.16
When is a discrepancy in midline position acceptable?
A study17 investigated the perception of discrepancies between the dental and facial midlines by orthodontists and lay people. The author had shown that as the size of the dental to facial midline discrepancy increased, both the orthodontists and the lay people found the subjects in the photos to be less attractive. The attractiveness score decreased as the midline discrepancies got larger. The scores were not affected by the direction of the midline discrepancy (left or right) or by the gender of the judge. It was estimated that the probability of a lay person recording a less favorable attractiveness score when there was a 2mm discrepancy between the dental and facial midlines was 56 percent.16
It was also observed18 that the vertical relationship of the dental midline appears to be much more critical than the mesio-lateral position of the incisors. In fact, a maxillary midline deviation of 4mm was necessary before orthodontists rated it significantly less esthetic. In comparison, the general dentists and lay people were unable to detect even a 4mm midline deviation. Yet, all three groups were able to distinguish a 2mm discrepancy in incisor crown angulation. It is therefore important to note that although even a large midline deviation is relatively undetectable, a minor incisor crown angulation discrepancy is easily noticeable and found to be unattractive and less tolerable.
It can therefore be concluded that we should aim for a straight vertical midline that keeps the central incisor symmetry without compromising the normal proportions of length and width.
COMMON CLINICAL SITUATIONS
Orthodontic treatments are often used to close median diastema or to redistribute interdental spaces for subsequent restorative treatment. The etiology of such spacing can be developmental, pathological or iatrogenic.19
As discussed earlier, incisor tooth size discrepancies need to be identified before the onset of treatment and the treatment options explained fully to the patient and the parents. Initial treatment planning and tooth size analysis by the orthodontist thus can predict if there will be any spacing after treatment. If this is overlooked at the onset, orthodontic treatment may be finalized functionally but esthetic deficits may still remain.
Management of the median diastema
The Bolton analysis can also be used to evaluate the impact of a build-up on smaller than average teeth. If the central incisors are small, it is possible to input larger values for these teeth. Subsequently, a Bolton analysis is performed making it possible to evaluate whether there will be adequate coordination of the upper and lower tooth sizes after the build-ups.6
Care must be taken to close the diastema while maintaining adequate tooth proportions and a vertical midline (Figs. 5 & 6).
Management of spacing in the lateral incisor region
Missing or peg-lateral incisors
It is estimated that two percent of the population is missing one or both maxillary lateral incisors and that there exists a familial tendency for both peg shaped and missing laterals.20,21
A peg-shaped maxillary lateral incisor is a developmental anomaly characterized by an alteration in coronal morphology. Typically, these teeth have a reduced mesio-distal diameter with the proximal surfaces converging markedly towards the incisal region.22
In the past, peg-lateral incisors were the teeth of choice in extraction as part of the treatment plan for orthodontically correcting crowded dentitions. Nowadays, modern restorative materials offer a numbers of more conservative options such as resin build-ups, porcelain veneers or crowns.
In the case of missing laterals, maintaining space for restorative procedures such as a Maryland bridge, a conventional bridge or an implant are all suitable alternatives.
If the decision is taken to close the spaces with the existing canines by lateralizing them, then alternative esthetic restorative procedures may likely be required in order to improve the anterior esthetics. The restorative dentist will have to find ways to create the optical illusion of balance and harmony in the smile zone. This can be achieved through various methods such as: bleaching, reshaping and/or veneering the transformed canine. It is important to note that the buccal surface of the canine has a curvature that does not reflect light in the same manner as a lateral incisor would. In fact, canine labial surfaces should be reduced with care, as the procedure can result in a yellower or grayer tooth23 (Fig. 7). Furthermore, there often exists a color contrast between the cuspids and the central incisors. Usually modern bleaching techniques can attenuate the darker-yellow appearance of canines.
Optimal positioning of the peg-lateral incisor to optimize the esthetics of the final restorative procedure
In the presence of one or more narrow lateral incisors, the clinician needs to evaluate the remaining dentition in order to arrive to the most appropriate treatment option. If the size of the lateral incisor is closer to the optimal dimension of the space, the orthodontist should consider interproximal reduction of the mandibular incisors to balance the anterior tooth size discrepancy. If the tooth size discrepancy is very large, resin bonded build-ups should be considered.
If only a small amount of space needs to be built-up, the peg-lateral incisor should be positioned nearer the central incisor than the canine since the contour of the mesial surfaces of lateral incisors are relatively flat and the distal surfaces are more convex.7
This position should create the most optimal situation for resin build-up reconstruction limiting the restorative procedure to one surface only. It should also allow the preservation of optimal soft tissue papilla contour (Figs. 8 & 9).
If the peg-lateral is very narrow, the tooth should be positioned closer to the center of the space, mesio-distally, and maintained during the orthodontic treatment. When a very small lateral is build-up this way, soft tissue contours are not always predictable.
When patient treatment requires a comprehensive approach, communication among the disciplines is critical in achieving improved esthetic outcomes in the anterior maxilla. From the onset, it is important to establish realistic treatment objectives. A wide range of procedures are available. Advances in adhesive dentistry have enabled dentists to use free-hand bonding as an everyday part of their practice. Correct initial diagnosis and treatment planning involving both the orthodontist and the restorative clinician will improve the final result.
Minimally invasive procedures starting with bleaching and resin bonding to more involved and irreversible restorations like porcelain laminate veneers and crowns considered as part of the options available to the patient. Furthermore, soft tissue management considerations must play a major role in the final harmony of the smile.
Daniel Fortin DMD MS, Universit de Montral; Genevive Guertin DMD MS, Universit de Montral; Athena Papadakis BDS (Hons), FDS RCS (Eng), M.Sc., FRCDC, Universit de Montral.
Oral Health welcomes this original article.
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