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Orthodontists should be familiar in examining all the components of a smile. A parallel position of the facial aspect of the incisor crown to the face is considered to be the most aesthetic position of the maxillary incisor. A more anterior position of the maxillary central incisors has been shown to be more acceptable than a more posterior position. Exposition of the incisor should not be larger than 3 mm in relationship to the upper lip. The lack of objective criteria to position incisors may be a first step to make the patient become dissatisfied with the treatment result. This paper discusses the ideal buco-ligual inclination for maxillary central incisor, and their horizontal and vertical position. A simple way to determine a virtual goal for maxillary incisor position, using digital tools, will be explained as well as how to transfer the occlusal plane to 3D planning softwares. A clinical case is presented using the Glabellar vertical line, the Mid-forehead vertical line and the Functional Esthetical Oclusal Plane as tools to determine where the maxillary central incisors should be positioned for optimal esthetic results.
Introduction
The potential beneficial effect of orthodontic treatment on smile and facial attractiveness is clear to layman, dental practitioners, and orthodontists. However, differences on treatment goals challenges the communication among dental specialties. In general, prosthodontist and maxillofacial surgeons plan their treatments around the maxillary central incisor, while orthodontists focus majorly on the position of the mandibular incisors. While this paper does not intend to define who holds the truth, let us assume that further communication among specialties would largely benefit patients.
The objective for good aesthetic results makes it mandatory that orthodontists clearly understand societal preferences for smile attractiveness. When observing a picture of a fellow human being, we tend to focus our vision on the eyes.
This is the main reason why orthodontists should understand the importance of placing the maxillary incisor in the most aesthetic position as possible while achieving its objectives to mandibular incisor position and to the occlusion function.
In traditional orthodontics, maxillary incisors goals are generally based in cephalometrics, where the maxillary central incisor long axis position is guided in relation to the cranial base or the maxilla. For restorative dentists and prosthodontist, however, the maxillary central incisor crowns are supposed to be parallel to the face, where light will reflect in the middle of them. Many orthodontists have brought these concepts from restorative dentistry, knowing that a crown parallel to the face of the patient will be more appreciated in a smile and studies have been shown that bucco-lingual inclination and antero-posterior position of the maxillary incisors has a major effect on smile attractiveness.
However, there is no objective widespread standard for the maxillary incisor position in orthodontics.
The lack of objective criteria to position incisors may be a first step to make the patient become dissatisfied with the treatment result. A good example is the need of overjet to treat a Class II malloclusion. To get the clearance for correction, orthodontists may procline maxillary incisors, retrocline mandibular incisors or have a mixture of both. The incorrect decision may remove the maxillary incisor from an esthetic position or keep it from being placed at it.
The literature can give us information on where in the next to ideal spatial position of the upper incisors in regards to its inclination, anteroposterior and vertical position. First of all, proclined or retroclined incisors were found by laypeople to be one factor leading to an unpleasant smile. It is known that smiles are graded as most attractive when the crowns of the incisors are more or less parallel to the face of the patient.
In this concept, the center of the facial aspect of the crown of the maxillary incisor, or de facial axis point (FA) should be touching a virtual line, called goal anterior-limit line (GALL), which is parallel to the facial plane and passes through a line between the forehead's anterior-limit line (FALL) and the glabellar vertical line (GVL). The incisor should be ideally touching the GALL or at least located between FALL and GVL (Fig. 1).
Figure 1Points and linear measurements: Trichion (T), Superion (S), Glabella (G), the Forehead's Facial-Axis (FFA) point, the Facial Axis (FA) point of the upper central incisor, the Goal Anterior-Limit Line (GALL), and the Glabellar vertical Line (GVL). Observe how close are the lines from each other. Repriented with permission from Currieteiro et al.
The forehead landmarks are not easy to be defined, specifically the forehead's facial-axis point (FAA), the landmark of the FALL, because its position depends on the patient's anatomy. It is located between Trichion and Glabella, in flat foreheads, or between Superion and Glabella, in round foreheads.
Trichion, which is the hairline point, and Superion, which is the most superior-anterior point of the forehead are not that easy to be defined as well because of anatomical variations. Therefore it seems easier to simply use GVL, a true vertical line passing through the Glabella, for the goal for upper incisor crown position, especially because there is no detrimental effect to the smile if incisors are moved anteriorly without proclination.
besides the fact that Glabella is easily and reliably determined. Moreover, the distance between GVT and GALL appear to be clinically insignificant (Fig. 1).
With those two parameters in hand, regarding the buco-lingual inclination and AP position of the maxillary incisor, the vertical position goal remains to be defined. The incisal edge of maxillary incisors below the relaxed upper lip is said to be between 2.0 and 3.4 mm.
It is very important consider that these values should comprehend the final planed incisal edge of the incisors. Therefore, if restauration is deemed necessary, due to the length of the crown, compensation is needed.
We conclude that a desired position for maxillary central incisors at the end of treatment is:
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With their facial aspect of the crown parallel, or close to parallel to the frontal plane or GVL;
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With their crowns positioned at GVL;
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With their incisal edge below the relaxed lip by 2-3 mm.
A simple way to determine a virtual goal for maxillary incisor position using digital tools
A simple way of defining how distant the pretreatment upper central incisor is from the incisor goal is to superimpose the cephalometric radiography of the patient over the profile picture. The radiography can be modified using the transparency tool of any imaging software, such as Office Powerpoint (Microsoft Co,. Redmonton, Wa) to be placed over the profile picture. It is very important that the profile picture has been taken with the patient at natural head posture (NHP), or digitally oriented as such. (Fig. 2A)
Figure 2A. Superposition of the transparent lateral cephalometric radiography on the patient's profile picture. It can be observed that the radiography was rotated to fit over the picture, which is oriented in the NHP. B. The GVL (thick green line) is a vertical line that passes though Glabella (green point), while the mid-forehead vertical line (MFVL) (thin green line) passes through a point in the middle of the forehead (MF) (yellow). The facial aspect of the maxillary incisor's crown is touching GVT and is parallel to it. It can be concluded that the maxillary incisor are in a good position and that MFVL limits the retraction amount. C. The FAOP (red line) is drawn from the average contact point of the first molars and the upper lip stomion. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Two true vertical lines to the plane of the face should be traced for reference (hence the importance of NHP): GVL, which passes through the Glabella point and a mid-forehead vertical line (MFVL), which passes through a point in the middle of the forehead(MF) (Fig. 2B). Ideally, the incisor's crown should be parallel and touching GVT, and MFVL limits how far back the crown can be brought posteriorly without affecting the smile. Orthodontists should keep in mind two important information. First, the maxillary incisors can be protracted in relation to GVT, but that will only go unnoticed if the crown is maintained parallel to GVT.
Apparently, orthodontists and non-dentists are more sensitive to changes in buco-lingual inclination than AP movement of incisors. Second, retraction of maxillary incisors appear to be more visible and detrimental to the smile than protraction.
The vertical relationship of the incisal edge can be evaluated by observing and measuring how distant the incisal edge (or the “future” incisal edge, if it is to be restored) is from the Functional Aesthetic Occlusal Plane (FAOP).
FAOP is line traced form the average point of contact between maxillary and mandibular first molars and the upper lip stomion. The incisal edge should be positioned 2-4 mm below this line (Fig. 2C).
How can you take these objective aspects and incorporate them into your orthodontic treatment?
The current technique of trimming models, with the occlusal plane parallel to the ground overestimates maxillary and underestimate mandibular incisor proclination. However, the whole dentition of a patient is three-dimensionally positioned at an angle to the face, and not normally positioned parallel to the ground. Therefore, it is very hard to look at models, digital or cast, and have the same perspective as when we are seating straight to a patient smiling. This means that we, as orthodontists, may decide upon treatment options differently, if we look to the patients smile straight on then when we look into trimmed models. Smile arc and buco-lingual inclinations of incisors, all of which directly affect aesthetics, will not be evaluated correctly, and may let to incorrect decisions.
Mounting casts on an articulator or even trimming models parallel to Frankfort horizontal (which would be a representation of a “true horizontal line” to the NHP) can give the orthodontist a real representation on how the incisors of the patient are positioned to the face. However, this will bring much work and have proven to be impractical over the years. An excellent idea, suggested over a decade ago,
would be for the different 3D planning softwares to allow for occlusal plane orientation.
Today, most of the orthodontic softwares available (for treatment planning, aligner or indirect bonding design) allow 3D scans to be oriented following the oclusal plane characteristics of the patient as if it were mounted on a virtual articulator. This can be done upon insertion and orientation of the 3D scans or later, according to the preference of the doctor. The whole procedure is very easy and quick to accomplish.
The first step for this procedure requires definition of the NHP of the patient. The doctor needs two good extra-oral pictures of the patient, lateral and smiling, both need to be taken at the NHP or at the position of choice which will guide treatment. The orientation of these pictures is very important, because a wrong orientation would compromise the transfer of the 3D occlusal plane from the physical world to the digital one.
The profile picture will be used to orient the lateral cephalometric radiography, which should be modified using the transparency tool of any imaging software. The transparency will allow the radiography to be correctly superimposed to the profile picture, transferring the NHP from one to the other. The functional occlusal plane is then drawn with a line tool and its angulation measured to any horizontal line. A virtual protractor can be easily used for that purpose (Fig. 3).
Figure 3Superposition of the transparent lateral cephalometric radiography to the patient's profile picture. It can be observed that the radiography was rotated to fit over the picture, which is oriented in the NHP. A. The functional occlusal plane is drawn and its angulation can be measured relative to any true horizontal line (yellow color) B. The occlusal plane angle can be measured using the Iconico Screen Protractor Software (Iconico, Inc. Code & Design). This software can be downloaded at www.iconico.com. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Once acquired, the angulation can be transferred to any 3D software. In some softwares this is done during the loading of the intraoral scans, such as in Archform (Archform, San Jose, CA, USA) or using an occlusal software function, such as in the Clincheck Software (Align Tecnology, San Jose, CA, USA) (Figure 4, Figure 5). This procedure is used to orient digitally the “Pitch” of the model.
Figure 4Occlusal plane angle transfer technique using the Archform Software. A. Oclusal plane parallel to the ground, as the 3D scans are inserted to the software. B. Colored gizmo are used to orient models according to the occlusal plane angle found in the patient's oriented cephalometric radiography.
Figure 5Occlusal plane angle transfer using the Invisalign Clincheck Software. A) The option occlusal plane inclination should be chosen for virtual model orientation. B. Virtual model oriented parallel to the ground, as it is originally in the software; C) Virtual model oriented occlusal plane already oriented using the occlusal plane angle found in the patient's oriented cephalometric radiography.
The next step in the orientation process is the “Roll” and “Yaw” model orientation. An appropriate overlay software called WindowTop (BiGilSoft.com) can make any of the orthodontic planning softwares transparent, and the software's window can be fitted over the smile picture of the patient, which was taken at NHP. The picture can be resized, as can the 3D models, to match both in size and the rotational controls of the softwares can adjust the row and yaw of the models (Figure 6, Figure 7). Once models are saved or the position is applied, the 3D model orientation will be saved and the models mounted to the NHP can be used for diagnostics and treatment planning. This digitally mounted position allows the buco-lingual inclination of incisors to be seen and taken into consideration for treatment planning, as well as the smile arc. (Figure 8, Figure 9) The smile arc is the relationship of the curvature of the maxillary anterior tooth edges to the curvature of the lower lip during the social smile, which is influenced by the occlusal plane (OP) angle. It is also imperative that orthodontist knows the occlusal plane changes can affect the smile arc. Flattening a patient's smile arc results in a less esthetic smile.
Conventional orthodontic mechanics can induce small changes in the occlusal plane angle, and a flat occlusal plane or too steep occlusal plane can be considered less attractive.
Figure 6Snapshot of the Archform Software overlayed to the patient's smile picture (oriented according to NHP)and made transparent by Windowtop software (BiGilSoft.com). Original virtual model, with the occlusal plane oriented parallel to the ground; B) The model has been resized to fit the picture but hasn't yet had the pitch value inserted. C. Virtual model with the pitch tuned to the smile picture. A small roll and yaw of the model was also done to finely tune the model to the picture.
Figure 7Snapshot of the Clincheck Software. A) Original virtual model, with the occlusal plane oriented parallel to the ground; B) Image of the Clincheck software window overlayed to the patient's smile picture (oriented according to NHP)and made transparent by Windowtop software. The model hasn't yet had the pitch value inserted. C. Virtual model with the pitch finely tuned to the smile picture. A small roll of the model was also done. Unfortunately, the yaw of the model cannot be adjusted in the Clincheck model at the time of this publication. The grid option must be turned on to allow an orthographic view of the model with will match the smile picture.
Figure 8Virtual models seen laterally A. Without the occlusal plane orientation. B. Models oriented according to the patient's occlusal plane. Incisors buco-lingual inclination and posterios teeth's axial mesial distal inclination can be correctly evaluated.
Figure 9Virtual models seen from a frontal view A. Without the occlusal plane orientation. B. Oriented according to the patient's occlusal plane. At tis point, the smile arc can be evaluated correctly.
Figure 10Superposition of the transparent lateral cephalometric radiography on the patient's profile picture. It can be observed that the radiography was rotated to fit over the picture, which is oriented in the NHP. The GVL (green line) and MFVL (yellow line) show that the maxillary central incisors are excessively proclined and anteriorly positioned, while the FAOP (red line) shows that they are excessively extruded. To the left, smile photography show the clear detrimental impact that the characteristics described have on the smile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Figure 11A. Initial intraoral photography of the patient from Fig. 10. B. Straight wire mechanics supported by skeletal anchorage which aimed to retract and intrude the whole maxillary arch. C. Final introral photography.
Figure 12A Initial frontal intraoral photography of the patient from Fig. 10. B. Straight wire mechanics supported by skeletal anchorage which aimed to retract and intrude the whole maxillary arch. C. Final frontal introral photography.
Figure 13Superposition of the transparent lateral cephalometric radiography on the patient's profile picture. It can be observed that the radiography was rotated to fit over the picture, which is oriented in the NHP. The GVL (green line) and MFVL (yellow line) show that the maxillary central incisors where retracted. The facial aspect of the crown is parallel to GVT and the maxillary central incisors where moved to a place in between GVT and MFVL. The FAOP (red line) shows that they are positioned at a satisfactory position. To the left, the patient's photography shows the clear improvement of the smile caused by placing the maxillary central incisors in an optimal position. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Orthodontists and patients will benefit from this technique because it can be used for diagnostics and treatment planning. Additionally, this will not apply only to aligners but also when fixed appliances will be used because most softwares, will not charge the doctor (or will only charge a small fee) unless models are exported for printing.
Clinical case
A 20-year-old Class II patient with vertical maxillary excess sought orthodontics treatment. Using the aforementioned concepts, it was observed that the maxillary central incisors were ahead of the GVL and proclined.
The treatment objectives were to retract and intrude the whole maxillary arch and treat the Class II mallocusion. This treatment plan aimed to decrease the gummy smile and produce true forward mandibular rotation.
Treatment mechanics involved retraction and intrusion of the maxillary arch using skeletal anchorage. Four miniscrew implants were used, two extra-alveolar (1.8 mm x 13 mm) e two interradicular (1.5mm X 9 mm), in order to apply a resulting force to achieve the desired movements.
Retraction and intrusion of the maxillary arch was accomplished successfully and the case was treated in 36 months. Smile was largely improved and the maxillary central incisors were brought to a more esthetic position.
Conclusion
The use of information described in the literature with regards to the optimal position of central incisors can assist the orthodontist in producing aesthetic smiles while treating malocclusions. The GVT, MFVL and FAOP are excellent tools to be added to the orthodontists’ arsenal to treat malocclusions. The use of digital tools can give orthodontics objective guidelines on where the upper central incisors should be positioned if esthetic smiles are desired.
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.
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