Mandibular Setback
Haithem M. Elhadi Babiker
Deepak G. Krishnan
DEFINITION
Surgical correction of maxillofacial skeletal deformities dates back to the late 19th century.
Numerous techniques have been described for repositioning of the maxilla and mandible through various osteotomies.
The bilateral sagittal split osteotomy (BSSO) is the most common surgical procedure for correction of mandibular deformities.
It was first described by Obwegeser in 1957.1
The BSSO setback is a procedure used for correction of mandibular prognathism and asymmetries through a series of connected osteotomies that posteriorly reposition the mandible.4
ANATOMY
The mandible is a U-shaped bone forming the lower jaw. It is the only movable bone of the skull, articulating with the temporal bones at the temporomandibular joints.
It consists of a horizontal horseshoe-shaped body and two broad and oblong rami projecting upward. The alveolar processes of the mandible house and support the mandibular teeth.
The inferior alveolar nerve enters the mandibular foramen on the medial surface of the ramus near its center. It then extends downward and forward within the body of the mandible; it exits at the mental foramen, located on the external surface midway between the alveolar process and the lower border, in the interval between the premolar teeth.
The anteromedial margin of the mandibular foramen is guarded by an upward tonguelike projection, the lingula. Along its course, the inferior alveolar nerve innervates the mandibular teeth and terminates as the mental nerve, which supplies sensation to the skin and mucous membrane of the lower lip and chin.
The muscles of mastication directly attach to the surface of the mandible. The masseter and medial pterygoid muscles attach to the lateral and medial surfaces of the ramus.
These two muscles form the pterygomassetric sling, which straddles the mandibular ramus.
The lingual nerve, before reaching the tongue, comes in close contact with the inner surface of the alveolar border of the mandible in close proximity to the mandibular third molar.
PATHOGENESIS AND NATURAL HISTORY
Mandibular prognathism is a condition in which the mandible is positioned anterior to the maxilla, referred to as class III malocclusion.
In the NHANES study (1989-1994), 0.3% of the US population was reported to have mandibular prognathism severe enough to require orthodontic-surgical treatment.5
About 14% of youth (less than 18 years of age) in the United States have a class III malocclusion. Roughly a quarter of these patients have isolated mandibular prognathism, whereas the remaining three-quarters have a combination of maxillary deficiency, in addition to mandibular protrusion.
Mandibular prognathism causes aesthetic concerns, as well as chewing and speaking difficulties, prompting patients to seek surgical treatment at a younger age.
In cases of severe mandibular prognathism, bimaxillary surgery is the preferred method of treatment (BSSO setback combined with maxillary advancement). This is because excessive mandibular setback may result in obstructive sleep apnea and is also subject to higher rates of relapse.
PATIENT HISTORY AND PHYSICAL FINDINGS
Patients usually present during the teenage years.
The main complaints are difficulty with chewing and speech, and the aesthetic concerns of a large mandible. Often, the malocclusion also presents as an open bite.
Patients are usually first seen by an orthodontist and eventually referred to a maxillofacial surgeon when ready for surgery.
Orthodontic treatment is usually undertaken for approximately a year and serves to align the teeth within the dental arches and corrects any dental compensation.
Surgery is undertaken when the patient reaches skeletal maturity and there is proof of growth cessation (girls 16-18 years, boys 18-20 years).
Preoperative workup includes obtaining
Dental models
Cephalometric analysis
Face bow transfer to record the relationship of the jaws to the skull base
Model surgery to fabricate the surgical splints
In the case of single-jaw surgery, a single splint is fabricated to help position the mandible.
In the case of double-jaw surgery (LeFort I advancement combined with BSSO setback) for severe class III malocclusion, two splints are made: an intermediate splint used intraoperatively to position the maxilla in its final position and a final splint to position the mandible.
If third molar teeth are present, they may be extracted 6 months prior to surgery.
IMAGING
Lateral cephalometric radiographs are obtained for cephalometric analysis.
Panoramic radiographs are obtained to determine the presence and location of mandibular third molar teeth and also level of the inferior alveolar canal.
A CT scan with 3D images is obtained and can be used for virtual surgical planning (VSP) in lieu of traditional model surgery.
DIFFERENTIAL DIAGNOSIS
Class III skeletal relationship can be secondary to mandibular prognathism, maxillary hypoplasia, or a combination of both.
Accurate diagnosis is important to determine the appropriate surgery.
The diagnosis is confirmed by detailed clinical evaluation combined with cephalometric analysis. Often, a midface deficiency is wrongly diagnosed as mandibular prognathism, whereas in reality upon cephalometric investigation, the mandibular position may be normal.
Acromegaly and pituitary hyperactivity should be suspected in mandibular prognathism that presents later in life, and appropriate workup should be undertaken.
Unilateral hyperplasia of the mandible can cause an asymmetric prognathism and malocclusion. A neoplasm of the mandibular condyle should be ruled out in these cases. Unilateral condylar hyperplasia is an idiopathic rare condition that can also cause an asymmetric mandibular prognathism.
NONOPERATIVE MANAGEMENT
Mild class III malocclusion can sometimes be treated with orthodontic management alone.
True skeletal class III relationship can only be treated by surgical intervention.
SURGICAL MANAGEMENT
The BSSO setback procedure is usually performed during the teenage years after skeletal maturity.
The main objective of the operation is to correct mandibular prognathism by cutting the mandible and repositioning it posteriorly to meet the maxilla in a normal class I relationship.
The main steps of the procedure are as follows:
Make bilateral intraoral incisions to expose the posterior aspect of the mandible.
Perform bilateral sagittal split osteotomies of the mandible (FIG 1).
Position the mandible into a class I relationship using an occlusal splint.
Use wires to perform maxillomandibular fixation.
Remove any bony interference between the two cut segments of the mandible.
Establish a proper position of the condyles in the fossae bilaterally.
Rigidly fixate the mandible in its new position.
 FIG 1 • The BSSO procedure includes a medial ramus osteotomy, an anterior ramus horizontal osteotomy, and a vertical osteotomy. |
Preoperative Planning
The risks, benefits, and complications are discussed with the patient prior to surgery. Discussing the risk of damage to the inferior alveolar nerve is mandatory.
The mandibular third molar teeth are preferably extracted 6 months prior to the surgery to allow adequate time for healing.
If they have not been extracted previously, they may be removed during the procedure.6
The occlusal splints are tested on the patient preoperatively to ensure adequate fit.
Preoperative photos are obtained for documentation.
Positioning
The patient is positioned supine with the arms tucked in.
They are intubated nasally, and the endotracheal tube is sutured to the nasal septum and secured appropriately to the forehead (FIG 2).
Approach
Surgery is performed through an intraoral approach.
 FIG 2 • The patient is positioned supine and the endotracheal tube is sutured to the nasal septum and then secured to the forehead. |
TECHNIQUES
▪ Exposure
Clean the oral cavity with chlorhexidine or an equivalent antiseptic mouth rinse, and place a throat pack.
Inject local anesthetic containing a vasoconstrictor to infiltrate the area of dissection.
Insert a bite block on the opposite side to adequately open the mouth.
Place two Langenbeck retractors to pull away the cheek.
Place a tongue retractor to protect the tongue (TECH FIG 1A).
Make a 3-cm incision along the external oblique ridge of the mandible from midway on the ramus to the level of the second molar tooth (TECH FIG 1B).
If the mandibular third molar is present and indicated for removal, the incision is modified to incorporate the tooth.
A sufficient cuff of gingiva (5 mm) is maintained adjacent to the molar teeth for a relaxed wound closure.
Sharp dissection is extended through mucosa, muscle, and periosteum to raise a full-thickness mucoperiosteal flap, exposing the entire ascending ramus and posterior body of the mandible (TECH FIG 1C).
Care is taken not to dissect the lateral muscular attachments along the anterior ramus ensuring adequate blood supply to the proximal segment of the mandible.
TECH FIG 1 • A. Exposure is obtained by Langenbeck retractors to pull away the cheek and a tongue retractor to protect the tongue. B. An incision is made along the external oblique ridge halfway up the ramus to the level of the second molar tooth. Leave enough gingival cuff to facilitate easy closure. C. Sharp dissection is undertaken to raise a full-thickness mucoperiosteal flap to expose the ramus and posterior body. D. Using a sharp elevator, the masseter muscle fibers are stripped to expose the planned vertical osteotomy site.
Expose the inferior border of the mandible in the area of the planned vertical osteotomy.
Keep the periosteum intact to prevent excessive bleeding and herniation of the buccal fat.
Strip enough of the masseter muscle to adequately visualize the planned vertical osteotomy site (TECH FIG 1D). Keep as much of the masseter as possible attached to the proximal segment of bone to prevent bony devascularization. At the anterior extent of the incision, the depressor anguli oris attaches tenuously to the inferior border of the mandible. This area must be stripped off sternly to adequately mobilize the mandible after the osteotomy.Related posts:
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