A surgery-first approach using single-jaw rotational mandibular setback in low-angle mandibular prognathism

For the treatment of low-angle mandibular prognathism, rotational mandibular setback surgery is usually performed with Le Fort I maxillary osteotomy to rotate the maxillomandibular complex simultaneously. However, this maxillary surgery can be replaced with the orthodontic intrusion of maxillary posterior teeth. Single-jaw rotational mandibular setback surgery can be done with a surgery-first approach by planning orthodontic rotation of the maxillary occlusal plane with the simulation of the postsurgical forward mandibular rotation. This case report describes this approach applied to a 19-year-old female patient with low-angle mandibular prognathism but without maxillary deficiency. A Class II open bite was formed by the rotational setback surgery. During postsurgical orthodontic treatment, the maxillary total arch was distalized with maxillary molar intrusion using palatal mini-implants and lever. This case report demonstrates that orthodontic rotation of the maxillary occlusal plane and simulation of mandibular rotation can replace maxillary surgery and enable single-jaw rotational mandibular setback surgery with a surgery-first approach.

Highlights

  • Single-jaw rotational mandibular setback surgery can be done with a surgery-first approach.

  • Orthodontic rotation of the maxillary occlusal plane can replace maxillary posterior impaction surgery.

  • Maxillary occlusal plane can be rotated orthodontically after the surgery is done first.

  • The forward rotation of the mandible in harmony with the postsurgical intrusion of maxillary posterior teeth should be expected and Class II open bite should be made right after surgery.

  • Double-jaw-surgery-like effect can be achieved by single-jaw rotational mandibular setback surgery, even with surgery-first approach.

Mandibular setback surgery is used to correct mandibular prognathism. This mandibular setback is usually performed along the occlusal plane to prevent bite opening at the anterior or posterior teeth. During this setback along the occlusal plane, the vertical bony step (VBS) develops inevitably at the mandibular border between the proximal and distal bony segments because of the difference between the occlusal plane and the mandibular plane. The VBS stretches the pterygomasseteric sling, causing postsurgical forward rotation of the mandible. In patients with a low mandibular plane angle, simple mandibular setback along the occlusal plane cannot improve the prominence of the chin and mandibular border relative to the basal bone. In addition, the smile arc, which is deficient in most low-angle mandibular prognathism patients, may not be improved. , Instead, mandibular setback with a backward rotation is required in these patients to improve a square-looking mandible. To rotate the mandible during mandibular setback, maxillary Le Fort I osteotomy is generally performed to rotate the occlusal plane clockwise. , However, maxillary molar intrusion during presurgical orthodontic treatment also can change the maxillary occlusal plane, allowing rotational mandibular setback along the changed maxillary occlusal plane. ,

The demand for the surgery-first approach is increasing in favor of early surgical correction. Because presurgical intrusion of the maxillary molars is not available with the surgery-first approach, double-jaw surgery, including posterior impaction of the maxilla and rotational mandibular setback, is usually performed in patients with low-angle mandibular prognathism. However, intruding maxillary molars is possible during postsurgical orthodontic treatment after single-jaw mandibular setback surgery with a backward rotation of the distal segment of the mandible. This case report demonstrates that the surgery-first approach can be applied successfully for single-jaw rotational mandibular setback surgery.

Diagnosis and etiology

A 19-year-old female presented to our department with the chief complaint of a protruded chin ( Fig 1 ). Her profile was concave with a protruded chin. Her maxillary incisor exposure was deficient in both resting posture and smiling. In addition, there were Class III end-on molar relationships on both sides.

Cone-beam computed tomography showed that the patient’s maxillary dental midline coincided with the midsagittal reference plane, and her mandibular dental midline shifted 1.5 mm to the left, and the pogonion also shifted 2.5 mm to the left ( Fig 2 ). Her left gonion was positioned 3.3 mm buccally relative to her right gonion. As a transverse dentoalveolar compensation to the mandibular shift to the left, her maxillary left first molar was positioned 2.5 mm more buccally than the maxillary right first molar. Her airway was quite broad ( Fig 2 , B ), and there was no snoring or sleep apnea.

Fig 1
Pretreatment facial and intraoral photographs.

Fig 2
Pretreatment records: A, lateral cephalogram; B, lateral and frontal cone-beam computed tomography (CBCT) views; C, panoramic view constructed from CBCT; D, maxillary digital model superimposed on the CBCT model shows transverse dentoalveolar compensation to the mandibular shift to the left; E, CBCT model shows mandibular translation to the left.

Lateral cephalometric analysis showed low-angle skeletal Class III with a normal maxilla and prognathic mandible ( Table ). She had a flat occlusal plane angle, proclined maxillary incisors, and retroclined mandibular incisors as an anteroposterior dentoalveolar compensation to the mandibular prognathism. Because of these excellent dentoalveolar compensations, there was no crossbite.

Table
Cephalometric measurements
Parameter Norm Pretreatment Postsurgery Posttreatment 2-year retention
SNA (°) 81.1 80.6 80.7 80.8 80.5
SNB (°) 79.2 87.3 ∗∗ 77.5 80.4 80.2
ANB (°) 2.5 −6.7 ∗∗∗∗ 3.2 0.3 0.3
Wits (mm) 0.0 −14.5 ∗∗∗∗ 0.5 ∗∗∗∗ −6.3 ∗∗∗∗ −8.3 ∗∗∗∗
FMA (°) 29.6 14.6 ∗∗ 30.4 26.5 26.7
Occlusal plane (Tweed) to FH (°) 10.0 6.5 8.9 12.3 11.1
Maxillary occlusal plane to FH (°) 14.0 8.4 7.5 13.9 13.1
Occlusomandibular plane angle (°) 19.6 8.2 21.5 13.2 15.6
U1 to SN (°) 105.3 118.2 117.1 100.5 102.8
U1 to FH (°) 113.8 125.8 124.3 108.1 110.6
IMPA (°) 91.6 62.9 ∗∗∗∗ 69.4 ∗∗∗∗ 80.2 ∗∗ 80.8 ∗∗
Interincisal angle (°) 125.4 156.7 ∗∗∗ 135.8 146.2 ∗∗ 141.9
Upper lip to E-plane (mm) −0.8 −4.9 −0.2 −2.4 −3.5
Lower lip to E-plane (mm) 0.1 −3.7 −2.9 −5.0 ∗∗ −5.7 ∗∗
A′B′ to FH (°) 81.0 101.7 ∗∗∗∗ 76.7 81.9 82.1

FMA , Frankfort mandibular plane angle; IMPA , incisor mandibular plane angle.

>1 standard deviation from the norm; ∗∗>2 standard deviations from the norm; ∗∗∗>3 standard deviations from the norm; ∗∗∗∗>4 standard deviations from the norm.

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Oct 30, 2021 | Posted by in Orthodontics | Comments Off on A surgery-first approach using single-jaw rotational mandibular setback in low-angle mandibular prognathism
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