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Corresponding author at: Department of Orthodontics, The Institute of Craniofacial Deformity, Yonsei University College of Dentistry, 50-1 Yonsei-ro, Seodaemun-Gu, Seoul 03722, South Korea.
Excessive gingival display during smile, also referred to as gummy smile, is often perceived as esthetically unpleasant. While different etiological factors are known to be associated with gummy smile, it eventually results from an imbalance between the dentoalveolar hard tissue and the covering soft tissue. There are a number of different approaches to gummy smile correction. They range from a relatively simple and quick injection of Botulinum-toxin in the hyperactive lip elevator muscles to more invasive procedures such as maxillary impaction by orthognathic surgery. In the meanwhile, the surgical repositioning of the maxilla can be replaced by the intrusion of the whole maxillary arch with the help of orthodontic miniscrews. This article conveys the rationale, biomechanical evidence and clinical effect of total arch intrusion applied to the correction of gummy smile accompanied by lip incompetence.
Introduction
Gummy smile in terms of soft tissue paradigm
The latest paradigm shift in clinical orthodontics includes the shift from Angle to soft tissue paradigm, which emphasizes obtaining soft tissue morphology in balance with the underlying hard tissue morphology.
The core concept may be the coordination of the hard tissue structures according to the soft tissue contours. In this context, an excessive gingival display upon smiling, commonly known as gummy smile, can be recognized as insufficient soft tissue dimension in relation to the underlying dentoalveolar structures. Considering that the arbitrary enlargement of the soft tissue dimension is largely impossible, the treatment modality would have to focus on how to reduce the hard tissue dimension or volume according to the available soft tissue dimension. The majority of conventional orthodontic appliances and technics are intended for selective control and relative movement of a tooth or tooth segment within the arch. In case of a major discrepancy, displacement of the whole maxillary and/or mandibular basal bone may be necessary to improve the soft tissue problems, for which surgical intervention has often been addressed.
On the other hand, the arbitrary displacement of the whole dental arch with the application of orthodontic miniscrews has been shown to be successful in overcoming the limitations of conventional treatment methods.
Gummy smile can be caused by different etiological factors such as hyperactive muscles of the upper lip, altered passive eruption, and vertical excess of the maxilla. Although the origin of problem may vary, the amount of gingival display is eventually determined by the vertical relationship between the soft and the underlying hard tissue rather than the absolute vertical length of a specific structure. In other words, a gummy smile may still be present, if vertically short dentoalveolar area coexists with even shorter upper lip length.
The etiological factor of a gummy smile can be inferred by further diagnostic parameters such as the amount of incisor exposure at rest and the presence of lip incompetence. An excessive incisor display at rest signifies that gummy smile is likely caused by vertical excess of the maxilla rather than hyperactivity of the upper lip elevator muscles. The more serious clinical phenotype may be gummy smile accompanied by lip incompetence, which is often associated with the soft-hard tissue imbalance in both the maxilla and mandible. In patients with this phenotype, mentalis muscle hyperactivity at lip closure can be observed, as the mentalis muscles attempt to compensate for the lips, which are too short in relation to the underlying maxilla and mandible.
The treatment strategy must be tailored according to the etiology of the gummy smile. In case of hypermobile upper lip, the amount of gingival display can be reduced by the injection of Botulinum-toxin in the upper lip elevator muscles. Hwang et al. investigated the topographic relation and morphological variations of the three lip elevator muscles using adult cadavers and identified a reliable and easily reproducible injection point for gummy smile correction, which is referred to as the ‘Yonsei point’.
It is located lateral to the ala, where all the three muscle vectors converge, and can be easily localized on the facial soft tissue. A single dose injection of Botulinum-toxin at the Yonsei point was shown to be safe and effective in clinical application.
Those modalities may be applied only when the incisor exposure at rest is within normal range.
If the gummy smile is associated with the vertical excess of the maxillary hard tissue, gingival display can be reduced by selective orthodontic intrusion. Orthodontic intrusion of the maxillary anterior segment using intrusion arch or orthodontic miniscrews may lead to clinically successful outcome in terms of gummy smile correction.
In this case the vertical position of the posterior segment as well as that of the anterior segment have to be evaluated properly. An effective intrusive displacement of both the anterior and posterior segments of the maxilla may be shown after superior repositioning of the maxilla by LeFort I osteotomy. However, due to the risk and morbidity of surgical intervention, surgery under general anesthesia for the sake of gummy smile may not be readily chosen as an option. Alternatively, simultaneous intrusive displacement of the of the whole upper dental arch with the use of miniscrews, which is referred to as total arch intrusion, can be considered. If the gummy smile is accompanied by lip incompetence, a treatment modality, which reduces the vertical dimension of both arches would be useful for the simultaneous correction of gummy smile and lip incompetence. The counterclockwise rotation of the mandible following the total arch intrusion results in the reduction of anterior facial height and increased chin prominence, which contribute to the improvement of facial and lip profile, especially in patients with lip incompetence. Above all, it is conceivable that majority of gummy smile cases may necessitate the intrusion of the whole arch rather than a segment of teeth.
Biomechanical considerations for total arch intrusion
In order to achieve total arch intrusion with the help of orthodontic miniscrews, an understanding of the biomechanical principles is fundamental. Previous studies have investigated the location of the center of resistance of the entire maxillary and mandibular dentitions using the finite element method.
Jo A.-R., Mo S.-S., Lee K.-J., Sung S.-J., Chun Y.-S. Finite-element analysis of the center of resistance of the mandibular dentition. Korean J Orthodont 2017;47: 21-30.
According to these studies, the center of resistance in the maxilla is estimated to be 11 mm apical and 26.5 mm posterior from the incisal edge of the central incisors. This is located around the middle area of the second premolar.
In the mandible it is located 13.5 mm apically and 25.0 mm posteriorly from the incisal edge, which is approximately between the second premolar and the first molar.
Jo A.-R., Mo S.-S., Lee K.-J., Sung S.-J., Chun Y.-S. Finite-element analysis of the center of resistance of the mandibular dentition. Korean J Orthodont 2017;47: 21-30.
Theoretically, vertical forces that pass through these points would induce intrusion of the entire dentition. The reported locations are, however, only estimations based on the finite element analyses. In clinical situations, it is not possible to exactly localize the center of resistance, as it varies depending on various factors, such as the root length, amount of periodontal support and length of dental arch. For example, in premolar extraction cases, the center of rotation is located more posteriorly. If the distance between the line of force and the center of resistance is large, an undesired rotation of the entire arch can occur.
After all, it is important to continuously evaluate the displacement pattern of the arch during total arch intrusion, and to adjust the force vector accordingly.
Case review
Case 1: Gummy smile accompanied by hyperdivergent face and deep bite
A 25-year-old male patient presented with facial asymmetry, excessive gingival display and lip protrusion. His chief complaints were gummy smile and upper and lower lip protrusion. While the patient was not very concerned about the retrognathic mandible, he wished to improve his smile esthetics and lip profile. An excessive incisor display was observed at rest, and some hyperactivity of the mentalis muscles was noted at lip closure as a result of lip incompetence (Fig. 1A). The cephalometric analysis revealed a hyperdivergent skeletal Class II relationship with a deep curve of Spee (Fig. 1C). Despite the hyperdivergent facial pattern, he had a severe deep bite with barely visible lower incisors. In the lower jaw, crowding of approximately 2 mm and congenital missing of the left second premolar were observed (Fig. 2). According to these findings, it was concluded that he required total arch intrusion and anterior retraction in both arches for improvement of the gummy smile and facial and lip contours. Since the facial asymmetry was not very pronounced at the apical base level, orthodontic camouflage of the asymmetry was planned.
Fig. 1Facial photographs of case 1 at A, pretreatment; B, posttreatment. Lateral cephalograms at C, pretreatment; D, posttreatment; and E, superimposition.
The patient wished a treatment with lingual brackets. Self-ligating lingual brackets with 0.018-inch slot were bonded in the mandible first due to deep bite. For improvement of the lip profile and orthodontic space closure for the congenitally missing tooth, the first premolars in the maxilla, and the second premolar and the persisting second deciduous molar in the mandible were extracted. In order to decrease the overbite without further worsening of the already hyperdivergent facial pattern, intrusion of the lower anterior teeth was planned using miniscrews located distal to the lower canines (Fig. 3A). After flattening of the curve of Spee, lingual brackets were also bonded in the upper arch. Following leveling and alignment a miniscrew was inserted between the first premolar and the first molar on the lingual side in each quadrant for total arch intrusion and anterior retraction. The total arch displacement was performed on a 0.016×0.022-inch rectangular stainless steel archwire with 25 degrees of extra torque for the incisors to prevent lingual tipping during retraction (Fig. 3B). The force vector was directed obliquely towards the estimated center of resistance in both arches for simultaneous total arch intrusion and anterior retraction (Fig. 3C). The amount of anterior retraction and anterior torque were controlled throughout the treatment, and the extraction spaces were closed reciprocally. After 33 months of treatment all spaces were closed and adequate occlusal relationship was attained (Fig. 4). Furthermore, the dental midlines coincided and the amount of gingival display was reduced to an optimal level, while the smile arc was maintained consonant (Fig. 1B). Overall, the smile esthetics and the facial profile improved dramatically, and the patient was satisfied with the treatment results. Superimposition of the pre- and posttreatment lateral cephalograms showed intrusive retraction of the upper and lower incisors with a stable interincisal angle. The total arch intrusion in both arches led to a counterclockwise rotation of the mandible and reduction of the anterior facial height, resulting in a significant improvement of the facial profile (Fig. 1E).
Fig. 3Treatment progress of case 1. A, force vector used for the correction of deep bite by intrusion of the lower incisors at 5 months of treatment; B, total arch intrusion in progress with the help of palatal and lingual miniscrews at 17 months of treatment; C, schematic diagram showing the oblique force vectors used in the present case for total arch intrusion. The force vectors are directed towards the estimated center of resistance of the upper and lower dentitions, which are presumably located further posterior due to premolar extraction.
Case 2: Gummy smile following Phase I treatment in a growing patient
An 8-year-old female patient presented with the chief complaint of mandibular prognathism (Fig. 5A). She had a maxillary transverse deficiency with bilateral crossbite and congenitally missing upper first molars (Fig. 6A). The cephalometric analysis indicated a hyperdivergent skeletal Class III relationship (Fig. 7A). In the facial photographs, it was apparent that she also had severe lip incompetence, which was expressed as hyperactivity of the mentalis muscles at lip closure (Fig. 5A).
Fig. 5Facial photographs of case 2 at A, pretreatment; B, end of Phase I treatment; and C, posttreatment.
For Phase I treatment growth modification with the help of bonded RPE (rapid palatal expander) and facemask was planned. Although she also had a severe vertical problem, the purpose of Phase I treatment was to mainly correct the transverse and anteroposterior problems. Orthopedic appliances used for the correction of Class III malocclusion in growing patients usually work by extrusive mechanism. Therefore, it is difficult to correct the anteroposterior and vertical problems simultaneously during growth modification. Following the treatment by RPE and facemask the transverse and anteroposterior relationship was improved, but the vertical problem was exacerbated, as anticipated. At the end of Phase I treatment, she showed severe gummy smile (Fig. 5B), and the superimposition of the lateral cephalograms indicated unfavorable growth with clockwise rotation of the mandible (Fig. 7B).
After the eruption of all permanent teeth, Phase II treatment was initiated. The aim of Phase II treatment was to compensate for the unfavorable growth and vertical changes that occurred during and after Phase I treatment. For orthodontic camouflage of the Class III malocclusion, the lower first premolars were extracted. Following leveling and alignment, multiple interradicular miniscrews were placed for total arch intrusion in both arches and reciprocal space closure with translatory retraction of the incisors in the mandible. After 37 months of Phase II treatment, the gummy smile was corrected, and the facial profile was improved significantly (Fig. 5C). Superimposition of the lateral cephalograms obtained before and after Phase II treatment showed that intrusion of the maxillary and mandibular dentitions and counterclockwise rotation of the mandible occurred as intended (Fig. 7D).
Conclusions
An imbalance between the dentoalveolar hard tissue and the covering soft tissue can manifest not only as gummy smile, but also as further unesthetic facial features such as lip protrusion and incompetence. Therefore, the target segments of intrusion for gummy smile correction should be determined in consideration of the overall facial and lip contours. The simultaneous displacement of the whole dental arch with the help of orthodontic miniscrews has been shown to be effective in reducing the amount of gingival display, while achieving an ideal smile arc. In the presence of lip incompetence, the facial profile can be improved by total arch intrusion of both the maxillary and mandibular arches.
Patient consent
Consent to publish the case report was obtained.
Funding
No funding or grant support.
Author contributions
All authors attest that they meet the current ICMJE criteria for Authorship.
Declaration of competing interest
The authors reported no competing financial interests or personal relationships that could appear to influence the work reported in this paper.
References
Ackerman J.L.
Proffit W.R.
Sarver D.M.
The emerging soft tissue paradigm in orthodontic diagnosis and treatment planning.
Jo A.-R., Mo S.-S., Lee K.-J., Sung S.-J., Chun Y.-S. Finite-element analysis of the center of resistance of the mandibular dentition. Korean J Orthodont 2017;47: 21-30.
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