Surgical-Orthodontic Treatment of a Patient with Severely Asymmetrical Skeletal Class III Dentofacial Deformity

Compared with a typical skeletal Class III mandibular protrusion, an asymmetrical skeletal Class III is a more complex three-dimensional craniomaxillary deformity, involving unilateral excessive mandibular growth or deficient maxillary growth in the sagittal dimension, bilateral inconsistency of maxillary and mandibular height in the vertical dimension, and inconsistency of the maxillary and mandibular widths in the transverse dimension.^1,2 Comprehensive treatment including presurgical orthodontics, orthognathic surgery, and postsurgical orthodontics is required for these defects to be completely resolved.³

The long-term stability of conventional surgical-orthodontic treatment for a severely asymmetrical skeletal Class III dentofacial deformity has been considered problematic because of the possible inadequacy of dental decompensation before surgery, along with complications due to condylar changes, airway conditions, and muscle pull and function.⁴ Considering these factors, it is important to carefully evaluate such a skeletal deformity and the dental compensation mechanisms to plan presurgical orthodontics that can achieve the desired goals and ensure stability. The magnitude and amount of surgical correction will largely depend on the efficacy of the presurgical orthodontic decompensation, which should be planned in all three planes of space.^5,6 In the sagittal dimension, the proclined upper incisors and retroclined lower incisors must be decompensated to place them in ideal positions with respect to the jaw; in the transverse dimension, the arches must be coordinated while controlling the torque of the posterior segments; in the vertical dimension, the overerupted teeth must be intruded to establish an appropriate vertical anterior tooth display after surgery.^3,7

This case report demonstrates comprehensive surgical-orthodontic treatment of a patient with severe skeletal discrepancies in all three planes: sagittal (Class III malocclusion), transverse (facial asymmetry), and vertical (long face and open bite).

Cone-beam computed tomography (CBCT) showed inconsistency in the morphology of the TMJ and widening of the anterior space on the left (Fig. 2). Functional examination found no signs of TMD, although the mandible deflected to the right when opening.

The diagnosis was a skeletal Class III dentofacial deformity with facial asymmetry, attributable to abnormal development of the maxilla and mandible in three dimensions; a congenitally missing upper right first premolar; an abnormality in the upper left second molar; and chronic generalized periodontitis.

Treatment objectives were to correct the asymmetrical skeletal Class III deformity, thus achieving a harmonious facial appearance; align the dental midlines with each other and with the facial midline; establish ideal overjet and overbite with proper interdigitation of the posterior teeth; improve the periodontal condition; and facilitate good oral hygiene.

Combined surgical-orthodontic treatment was the only viable approach to achieve these objectives. Presurgical orthodontics would decompensate the dentition to achieve a maximum skeletal correction and stable postsurgical occlusion. A Le Fort I osteotomy was planned to advance, slightly rotate, and center the maxilla, with the advancement more pronounced on the left and the impaction on the right and in the posterior regions. In addition, a bilateral sagittal split ramus osteotomy would set back and rotate the entire mandible counterclockwise, with more movement on the right to level the mandible with the maxilla. Postsurgical orthodontics would finish and detail the occlusion.

The presurgical orthodontic decompensation was carefully planned to coordinate with the planned surgical movements (Table 4). Maxillary premolars are commonly extracted to help decompensate proclined upper incisors and coordinate the midline between the upper dentition and the maxilla. In this patient, because of the missing maxillary right first premolar, the upper dental midline was deviated to the right compared with the maxillary midline. This was a challenging situation, since the upper dental midline and facial midline typically deviate to the same side. We decided to extract the upper left first premolar and to retract the upper left incisors with strong anchorage from a miniscrew in the left maxilla. The patient agreed to this plan and to extraction of the lower left and right third molars, followed by two-jaw surgery to correct his skeletal deformity.

The patient was closely monitored for two months after surgery. Postsurgical orthodontic treatment was then initiated, using .019" × .025" stainless steel archwires to finish and detail the occlusion.

After a total 31 months of treatment, all appliances were removed. Considering the typical bilateral discrepancy in masticatory muscle tension in a patient with asymmetrical skeletal Class III dentofacial deformity, we designed special removable acrylic retainers for nighttime wear to help prevent relapse (Fig. 5).

Treatment Results

Post-treatment records confirmed good esthetic and occlusal results (Fig. 6A and 6B).

The CBCT image of the TMJ showed slight growth of the right condyle during treatment (Fig. 10).

Discussion

Asymmetrical skeletal Class III dentofacial conditions with complex craniomaxillary deformities and dental compensation mechanisms in three dimensions are inevitably complex, usually requiring combined surgical-orthodontic treatment to achieve the best results in terms of occlusal function, facial esthetics, and long-term stability. An accurate evaluation of the facial soft tissues and skeleton, dental compensation mechanisms, TMJ function, and airway conditions is important in determining the appropriate treatment plan and achieving stable long-term results.^8,9

Presurgical orthodontic mechanics should intentionally avoid any movements that might contribute to relapse. Kim and Baek maintained that adequate decompensation of the incisors is mandatory, so that mandibular setback surgery can retrude the mandible into a more ideal position with a more esthetic profile.¹⁰ In the case shown here, we used U1-PP rather than U1-SN to determine the inclination of the maxillary incisors for presurgical decompensation, since a proper U1-PP angle means the incisors are in normal positions with respect to the maxilla. Therefore, because the dentition and the maxilla are moved as a whole during surgery, the incisors will be in the right place when the maxilla is moved into the desired position, and a proper upper incisor display will be obtained. Presurgical objectives should also include decompensation of the extrusive teeth in an open bite; this promotes mandibular rotation in surgery while avoiding an increase in posterior facial height, thus contributing to stability.³ Without proper vertical control, especially in cases with intermaxillary occlusal interferences, the bite closure during postsurgical orthodontics would create a more forward chin projection.

Because patients with high-angle Class III malocclusions tend to have thin alveolar bone labial and lingual to the lower incisor apices,¹¹ excessive labial proclination of the lower incisors can increase the risk of gingival recession. Troy and colleagues advised caution when decompensating the incisors in patients with severe skeletodental dysplasias.⁵ In our patient, considering the severe periodontal condition of his lower anterior teeth and his thin gingival biotype, it was crucial to protect the anterior teeth from tongue pressure during labial proclination of the lower incisors. In addition, thin alveolar bone brings the roots closer to cortical bone, with a consequent risk of orthodontically induced inflammatory root resorption (OIIRR) of the anterior teeth. Other potential contributors to OIIRR include treatment duration, magnitude of applied force, amount of tooth movement, and severity and type of malocclusion.¹² Despite these risk factors, the post-treatment root resorption in our patient was minor to moderate except for the upper left second premolar. One study showed that a hypofunctionally nonoccluding tooth can accelerate the root destruction resulting from the mechanical stress of orthodontic force.¹³ Our patient’s pretreatment occlusal trauma and poor periodontal condition could also have contributed to the root resorption of the upper left second premolar.¹⁴

In recent years, several authors have expressed concern about the negative effects of a significant mandibular setback on the posterior airway space.¹⁵ This may result from the hyoid bone following the direction of the surgical movement as the mandible is displaced downward and backward.¹⁶ Studies have also demonstrated the constrictive effect of an insufficient maxilla on the upper airway in patients with skeletal Class III deformities.^9,17 Hyperdivergent patients have a narrower anteroposterior pharyngeal dimension, and patients with long faces tend to have extremely narrow airways, both anteroposteriorly and coronally.¹⁸ Considering the risk of upper airway obstruction, it is essential to evaluate the upper airway morphology when formulating a treatment plan for an asymmetrical skeletal Class III dentofacial deformity. In our patient, to avoid deleterious positioning of the hyoid bone after the mandibular setback, we performed a genioplasty to advance pogonion by 4mm—not only for esthetic reasons, but to bring the genioglossal muscle forward, pulling the tongue away to avoid upper airway obstruction in the hypopharynx.¹⁹ Our 3D airway volume analysis demonstrated enlargement of the palatopharynx. Advancement of the maxilla and counterclockwise rotation of the maxillomandibular complex may also have helped reduce airway obstruction.²⁰ Total airway volume did not change after surgery, and the patient showed no symptoms of obstructive sleep apnea. Considering his hypertrophy and low tongue position, however, we advised the patient to perform tongue exercises throughout treatment to promote adaptation to a reduced capacity of the oral cavity after surgery.

Asymmetrical growth of the condyle may continue in some cases after stabilization of a developmental mandibular deformity, leading to postsurgical relapse of mandibular asymmetry.²¹ In our patient, post-treatment CBCT imaging of the TMJ showed slight growth of the right condyle, which could have resulted either from asymmetrical growth of the condyle or from hyperplasia caused by the mild mandibular trauma before treatment. Technetium ⁹⁹m-methyl diphosphonate (⁹⁹mTc-MDP) is generally used to evaluate developmental characteristics of an asymmetrical mandibular condyle and to provide diagnostic guidance for treatment of these patients.²² Our patient declined a ⁹⁹mTc-MDP examination because of the expense and inconvenience. To prevent a relapse of the mandibular asymmetry, we built in some overcorrection and designed special retainers that would adapt to the postsurgical jaw position.

Conclusion

In comprehensive surgical-orthodontic treatment of a patient with severely asymmetrical skeletal Class III discrepancies, accurate presurgical decompensation in all three dimensions is the key to success. Careful analysis and planning will determine the magnitude and type of surgical correction. U1-PP should be used rather than U1-SN to define the inclination of the maxillary incisors and ensure ideal positions after surgery. Periodontal and airway conditions and TMJ function are other important factors to consider in establishing an overall treatment plan.

ACKNOWLEDGMENTS: This work was supported by the National Natural Science Foundation of China (NSFC 81801005 and NSFC81870795) and the Jilin Provincial Development and Reform Commission (20181793146).