Orthognathic virtual treatment planning for functional esthetic results

Abstract

Virtual three dimensional (3D) facial diagnosis and treatment planning has the potential to improve outcomes for orthognathic surgery patients. It is necessary to appreciate the inadequacies of traditional two dimensional treatment planning and how that may adversely affect the virtual 3D treatment planning process. Virtual 3D planning greatly improves the clinician’s ability to diagnose and treat facial form and position problems that prevent the patient from accomplishing the three Functional Facial Keys (eating, breathing and communicating). It also enhances facial-skeletal orientation, maxillary and mandibular positioning, correction of asymmetries and occlusal plane management. This article presents a systematic approach to treat functional facial problems using virtual 3D treatment planning for consistent esthetic facial results.

Introduction

We have been treatment planning facial esthetics three dimensionally (3D) for as long as we have been performing orthodontics and orthognathic surgery. When we discuss 3D treatment planning as a new method of approaching our orthognathic surgery patients we are referring to the capture and manipulation of 3D volumes and surfaces in a computer environment. The irony is that at any given moment while using most 3D software and computer imaging, we are only seeing in two dimensions (2D) through standard monitors. Therefore, an important question is “how has a 3D image capture and viewing software changed the practice of dentistry and-in particular-orthognathic surgery?”

Traditional

To answer the question, clinicians will use their education (academia, books, publications) and clinical experience (observing and treating patients). The problem with using traditional treatment approaches when planning in the virtual 3D environment is traditional diagnostic and treatment planning methods are frequently not applicable. Two dimensional measurements such as Frankfort Horizontal and Rickett’s E-plane are absent realistic facial functionality and esthetics. Many of the studies evaluating skeletal, dental, and facial positions do so in a normative manner. To analyze 100–200 people who are considered “normal” and then apply those findings to an actual clinical patient is full of complicating factors. The patient that is being treated was not part of the cohort that was analyzed and the average findings of the studies may not represent the optimal result for any single individual. If we apply averages to our patients, we will, at best, have average results.

Functional

A better approach to 3D diagnosis and treatment planning and our patients is to ask better questions. What is the purpose of the face? If I change the position or shape of a particular structure of the face, how will it affect its purpose and the other structures that are adjacent. Esthetics is really just a measure of function. We are neurologically wired to be attracted to other humans who function appropriately. This is a protective mechanism for the species as we will not be inclined to share our genetic material with someone who is not functioning correctly. Stated simply, if the face works right, it looks right. The three Functional Facial Keys of the face define the purpose of the face which is to: (1) breathe, (2) eat and (3) communicate. When growth and development prevents the proper function of these three keys, the system will compensate in adaptive (mild form and position changes) or maladaptive ways (pain and tissue breakdown). So our goal in 3D facial planning is to provide an esthetic functional result by restoring natural forms and establishing proper positions for each part of the individual’s face. This will then allow the facial system to accomplish all three Facial Functional Keys and thereby be esthetic.

Orientation

Orientation of the head is the most important step in facial treatment planning. If this step does not receive adequate attention, the orthodontic and surgical results will be compromised from the outset. It is surprising that surgeons spend very little time reviewing the orientation of the skull when using 3D virtual treatment planning services. Moorees quoting Schmidt from 1876 insisted “the horizontal positioning of the head is a physiologic concept, which we must find by observation of the living.”

The clinician needs to realize that head posture is dictated by the muscles of the head and neck. These muscles can respond dysfunctionally to growth disturbances or the compensations that arise from such growth problems. Interferences in the occlusion, a constricted airway and muscular pain can cause inappropriate posture and altered head position. For example, a patient who has a small airway will unconsciously open their airway by rolling their head forward over their shoulders to position their jaw down and forward, thereby opening the airway ( Fig. 1 ). Since the occlusion, face and airway are being treated to correct these kinds of compensations, we expect posture to restore naturally after surgery. For this reason, we should treat to a corrected head position as opposed to a strict natural head position.

Figure 1
The patient’s head posture in the clinical photo represents her natural head position. She extends her neck and elevates her head to breathe more easily. The digital image shows the corrected sagittal plane orientation.

3D viewing allows for head positioning (orientation) without muscular influence. Straight posture can be assigned as the patient is digitally made to look to the horizon. The corrected head position should not be determined by viewing each two dimensional plane of orientation statically. Using 3D software, the head can and should be moved through ranges of positions, along each planar axis, to reveal deviations from the desired corrected head posture.

Sagittal plane (profile view)

Traditional

Orientation of the face in the sagittal plane (profile) is traditionally performed by leveling Frankfort horizontal (Porion to Orbitale) to the horizon. There are no studies that articulate Frankfort horizontal parallel to the horizon as the natural sagittal plane posture for every individual. The literature reports deviations between plus or minus eight degrees relative to natural head position. In our experience, leveling the head to Frankfort horizontal commonly causes an upward gaze as opposed to the natural and functional eyes forward head position ( Fig. 2 ).

Figure 2
The patient’s digital image has been leveled to Frankfort Horizontal (blue line) which results in the unnatural elevation of his gaze (black arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The technique for setting the sagittal orientation using a plumb line to the face was described by Lundstrom and Lundstrom in 1992. This approach is far superior to Frankfort horizontal for facial treatment planning. It requires the clinician to find the most naturally appearing sagittal head position without using any internal landmarks.

Functional

This technique can be misleading. There are enumerable points of view available to the clinician with the patient in the correct sagittal head position. The forward gaze “natural head position” may appear inappropriate in a 3/4 position compared to the profile ( Fig. 3 ). Any disturbance in the frontal orientation will make the right and left profile/sagittal orientations different. The best technique is to set the eyes to the horizon in profile and then rotate the 3D object around the x -axis and observe if the forward gaze changes. If it lifts or falls, the frontal plane should be adjusted until the eyes maintain a correct forward gaze in all points of axial rotation.

Figure 3
The center image shows the head canted down on the right side. In the right profile image, the sagittal plane orientation looks appropriate whereas the left profile orientation appears head down.

Frontal plane (frontal view)

Traditional

Many techniques have been described for leveling the head in the frontal view, such as leveling the orbits, leveling the ears, leveling the globes, leveling the zygomatic arches, etc. All of these techniques are fraught with complicating factors as few humans have eyes, ears or jaws which are parallel to the floor ( Fig. 4 ).

Figure 4
Frontal plane orientation is influenced by multiple measurements. In these images the eyes, ears and canines are observed to be on three different planes, possibly complicating orientation.

Functional

The clinician needs to decide the facial level for each patient. This can be difficult as our eyes are influenced by the symmetry and/or the asymmetry of adjacent structures. Uneven eyes, for example, might cause us to view canted maxillary canines as level if they match the pupillary cant. We are capable, however, of interpreting all sensory input from a patient’s face and determining the correct level for the entire complex shape that is the face. The clinician should use all available structures of the face to decide the proper frontal plane orientation. Here are important observations. Are the eyes level? If not, how far off? Is there a cant to the maxillary canines? If so how much? Are the ears level? If not, how far off are they? Do the pupils look level? If not, how far off are they? Once all possible measurements are noted, the face is then positioned to fulfill as many, if not all of the individual measurements.

Axial plane

Traditional

This orientation has often been taken for granted when looking at a patient’s two dimensional radiograph. We assume we are viewing perpendicularly to their facial frontal plane. Radiographically, the frontal plane is defined by the position of the film plate relative to x -ray beams as they pass through the face. The assumption is that the patient is positioned looking forward, but it is not necessarily true in a cephalostat or other head positioning device. Any axial deviation of a two dimensional image creates errors in a frontal and a profile measurement and the positioning error is not very evident when examining the radiograph.

Functional

Axial orientation is complicated by the fact that a face does not grow in a straight forward direction. Because the cranial sutures can fuse at different times, the face may deviate in many directions as develops. The best technique is to measure and observe multiple locations to best project the face in a most forward position. Relying solely on bisecting the foramen magnum is rarely correct by itself. Combining the observations of the foramen magnum, the position of the mid palatal suture, the outline of the frontal bones and the forward projection of the lateral orbital rims will improve the accuracy of the axial orientation ( Fig. 5 ).

Figure 5
During axial plane orientation, the midsagittal plane can be observed to bisect the foramen magnum and the midpalatal suture, though this is not always the case. This patient also shows even lateral orbital rim projection but has an uneven frontal bone with the right brow projecting forward more than the left.

The midline

Traditional

Traditional methods for defining the facial midline include: the philtrum, the tip of the nose and the midline of the distance between the inner canthi, the zygomas or the pupils. All of these methods can be influenced by scar tissue, septal deviation, nasal bone asymmetry (from growth or rapid palatal expansion), orbital dystopia and other issues. A negative influence to defining the facial midline is a mandibular deviation. A significant mandibular deviation will distort the tissues of the mid face in the direction of the deviation. If the mandibular asymmetry is corrected, the upper lip and face will move in the direction of the correction and change the origin of the incorrectly diagnosed midline.

Functional

Grybauskus et al. described a technique referred to as “forced symmetry” where CBCT’s are taken in the natural head position and then with the mandible “forced” into the facial midline position. A simple approach during the clinical exam is to ask the patient to hold their mandible close to the facial midline and observe the upper lip and nose for changes ( Fig. 6 ).

Figure 6
The patient places her mandible to right, more coincident with the facial midline which reveals a significant correction of her upper philtrum. Relying on the philtrum in this patient, without centering the mandible, would have resulted in a significant positioning error.

The clinician must establish a hierarchy of trust in regarding those references he/she will use to choose the facial midline. The mid sagittal plane is then chosen and checked against the other planes of orientation. It is important to realize that each change in an orientation plane affects the other two planes. When one plane is adjusted the other two are changed and might require re-orientation. 3D facial orientation is a complex process that cannot be relegated to a single-measurement-fits-all approach. The more data considered during the orientation process, the more likely the final head orientation will be correct.

Final occlusion

After orientation, the next step in virtual orthognathic treatment planning is placing the patient’s dentition together into a finished occlusion. There are numerous issues to address during this step such as establishing esthetic anterior tooth inclinations and angulations, decompensating posterior molars and establishing esthetic and functional tooth forms. Optimized tooth interdigitation promotes a stable surgical result with functional uncompressed temporomandibular joints. Arch form symmetry and a level occlusal plane (OP) help to insure correct positioning of the underlying bones. Orthodontically uncorrected dental cants should be noted but not corrected in the virtual environment. Such a correction would lead to an asymmetric skeletal-facial result. Overbite and overjet should be appropriate anteriorly and posterior with the occlusal contacts verticalized. This approach prevents “flowering” of the maxillary segments during multi-segment surgery which can cause asymmetric mid-face projections ( Fig. 7 ). The final occlusal is either accomplished on stone dental casts, scanned and merged into the virtual plan or it is set in the digital environment. Once the final occlusal position is established, the maxillomandibular complex (MMC) can be addressed. We leave the maxillary central incisors in the zero/neutral position vertically and anterioposteriorly with the patient’s dentition in final occlusion at the outset of treatment planning.

Jan 9, 2020 | Posted by in Orthodontics | Comments Off on Orthognathic virtual treatment planning for functional esthetic results
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