Residency · Residency · Oral Maxillofacial Surgery

Bilateral Sagittal Split Osteotomy

Overview

The bilateral sagittal split osteotomy (BSSO) is the most commonly performed mandibular osteotomy in orthognathic surgery. It allows advancement, setback, rotation, and asymmetry correction of the mandible. Originally described by Trauner and Obwegeser in 1957, it was subsequently modified by Dal Pont (1961), Hunsuck (1968), and Epker (1977). The osteotomy splits the mandible sagittally between the lateral cortex (the proximal or condyle-bearing segment) and the medial cortex with the teeth (the distal or tooth-bearing segment).

Indications

The BSSO is indicated for mandibular prognathism (Class III, requiring setback), mandibular retrognathism (Class II, requiring advancement), mandibular asymmetry, open bite correction (clockwise rotation), obstructive sleep apnea (mandibular advancement as part of maxillomandibular advancement), and in combination with Le Fort I osteotomy for bimaxillary surgery.

Relevant Anatomy

The inferior alveolar nerve (IAN) enters the mandibular foramen on the medial ramus and courses through the mandibular canal within the body; the osteotomy splits around this nerve, carrying a 1-5% risk of injury. The lingula is a bony projection on the medial ramus marking the entrance of the IAN into the mandibular foramen, and the medial horizontal cut is positioned above it. The mandibular foramen is located approximately at the center of the ramus in the anteroposterior direction, at the level of the occlusal plane. The antilingula is a palpable bony prominence on the lateral ramus opposite the lingula, used as an external landmark for the medial cut level. The retromolar fossa lies posterior to the last molar and marks where the sagittal cut begins. The external oblique ridge is a lateral cortical ridge extending from the ramus to the body that the sagittal cut follows. The masseteric sling, formed by the masseter and medial pterygoid attachments to the angle, can influence proximal segment positioning.

Surgical Technique (Obwegeser-Dal Pont Modification)

Exposure

The incision is placed along the external oblique ridge in the retromolar region and extends anteriorly into the buccal vestibule to approximately the first molar region. A full-thickness mucoperiosteal flap is elevated on the lateral ramus and body. Medial dissection involves subperiosteal stripping of the medial ramus above the lingula while protecting the IAN at the mandibular foramen. A channel retractor is placed on the medial ramus for protection during the medial cut. Lateral dissection exposes the external oblique ridge and the buccal cortex of the body.

Osteotomy Cuts

Three cuts are made. The medial horizontal cut passes through the medial cortex of the ramus above the lingula, using a Lindemann side-cutting bur or reciprocating saw. This cut penetrates only the medial cortex and extends from the anterior border of the ramus to approximately the posterior border. The sagittal cut (body cut) follows the external oblique ridge from the medial cut anteriorly to the region of the second or first molar, passing through only the lateral cortex down to the level of the IAN canal. The Dal Pont modification extends this cut more anteriorly and inferiorly along the buccal cortex, creating a larger bone contact area. The vertical or inferior cut (Dal Pont modification) is made at the anterior extent of the sagittal cut through the inferior border of the mandible, connecting the sagittal cut to the inferior border and creating a defined anterior limit to the split.

Splitting

After completing the three cuts, a sagittal splitting osteotome is inserted into the sagittal cut. Gentle mallet taps advance the osteotome, propagating the split along the cancellous bone between the two cortices. The split should follow the path of least resistance through the cancellous bone around the IAN canal. A Smith spreader or Obwegeser spreader may be used for controlled separation. The IAN should remain with the distal (tooth-bearing) segment in its bony canal. The proximal segment (condyle, ramus, and angle) is separated from the distal segment.

Nerve Identification

After splitting, the IAN is visualized in the distal segment within its bony canal. If the nerve is adherent to the proximal segment, it is carefully dissected free. Stretching or compressing the nerve during segment manipulation must be avoided.

Positioning and Fixation

The distal segment is positioned into the planned occlusion using the splint and intermaxillary fixation. The condyles must be seated in the fossae before fixation, which is critical to prevent condylar sag. Rigid fixation most commonly employs three bicortical positional screws placed in an inverted-L or triangular pattern, using a trocar or transbuccal approach for screw insertion in the angle/body region. Screws are typically 2.0 mm in diameter and 10-16 mm in length depending on anatomy. Semi-rigid fixation uses a miniplate across the osteotomy with monocortical screws on each segment. Wire fixation with prolonged MMF is historical and rarely used alone today but may supplement rigid fixation.

<image>Step-by-step surgical illustration of the bilateral sagittal split osteotomy (Obwegeser-Dal Pont technique) showing: (A) the three osteotomy cuts (medial horizontal, sagittal along the external oblique ridge, and vertical through the inferior border), (B) the sagittal split with osteotome, (C) separation of proximal and distal segments with the IAN visible, and (D) fixation with three bicortical positional screws</image>

Modifications

Obwegeser (Original)

The original Obwegeser technique placed the sagittal cut along the full length of the lateral ramus, producing a large overlap area but less bone contact in the body.

Dal Pont Modification

Dal Pont extended the sagittal cut anteriorly and added the vertical inferior border cut. This created a greater bone-to-bone contact area and improved stability. It is the most commonly used technique today.

Hunsuck Modification

Hunsuck shortened the medial horizontal cut so it did not extend to the posterior border of the ramus, reducing medial dissection and potentially lowering nerve injury risk.

Epker Modification

Epker emphasized minimal periosteal stripping on the lateral surface, preserving the periosteal blood supply to the proximal segment and reducing the risk of condylar resorption.

Fixation Controversy

Rigid Fixation (Bicortical Screws)

Three positional screws provide absolute stability. Advantages include no need for postoperative MMF, faster return to function, and more stable positioning. Disadvantages include potential nerve injury during screw placement, condylar displacement if condyles are not properly seated, and the need for a transbuccal approach. Condylar seating must be ensured before tightening screws, as improper seating leads to early relapse.

Semi-Rigid Fixation (Miniplates)

A 4-hole or longer miniplate with monocortical screws offers no risk of screw injury to the IAN, allows some functional movement (physiologic settling), and facilitates easier hardware removal if needed. Disadvantages include the potential need for a short period of MMF with guiding elastics and theoretically less rigidity.

Wire Fixation

Wire fixation is a historical technique rarely used as sole fixation. It requires prolonged MMF of 4-6 weeks, causing nutritional compromise, hygiene difficulty, and patient dissatisfaction. It may be used adjunctively in resource-limited settings.

Complications

Bad Split (Unfavorable Fracture)

Bad splits occur in 2-5% of cases. Types include buccal plate fracture extending below the IAN canal, lingual plate fracture extending to the inferior border, condylar neck fracture, and fracture through a tooth socket. Risk factors include third molars in the osteotomy line (which should be removed 6 or more months prior or at the time of surgery), unerupted teeth, a thin mandible, excessive force during splitting, and unfavorable anatomy. Management depends on the fracture pattern and may require additional fixation, lag screws, or a reconstruction plate.

Inferior Alveolar Nerve Injury

Temporary neurosensory disturbance occurs in 30-80% of cases (the wide range reflecting variable assessment methods), while permanent disturbance occurs in 1-5%. Causes include nerve stretching during the split, compression from fixation screws, and entrapment between segments. Risk factors include large advancement (greater than 7 mm), older patient age, and unfavorable (laterally positioned) nerve. Most patients recover within 6-12 months; those with persistent deficits at 12 months are unlikely to recover fully.

Condylar Resorption (Idiopathic Condylar Resorption)

Progressive condylar resorption leads to relapse with open bite and retrognathia. Risk factors include young female patients, high mandibular plane angle, large advancement, condylar torque during surgery, pre-existing condylar disease, and use of rigid fixation with improper condylar positioning. Incidence is 1-7% depending on the patient population and surgical technique. Prevention includes short-period postoperative MMF or guiding elastics, careful proximal segment positioning, and counterclockwise rotation of the distal segment to reduce condylar loading. Severe cases may require secondary surgery or alloplastic TMJ replacement.

Relapse

Advancement is relatively stable with rigid fixation, with less than 10% significant relapse for movements under 7 mm. Setback is more prone to relapse due to suprahyoid muscle stretching. Large advancements exceeding 10 mm have higher relapse rates, and distraction osteogenesis may be considered as an alternative. Clockwise rotation has a higher relapse rate than counterclockwise movements.

Other Complications

Additional complications include hemorrhage (from the inferior alveolar artery or retromandibular vein), infection (1-3%), hardware failure or irritation requiring removal, tooth devitalization adjacent to the osteotomy line, non-union or delayed union (rare), and TMJ dysfunction (click, pain, limited opening).

<image>Diagram showing the types of unfavorable (bad) splits in BSSO, including buccal plate fracture, lingual plate fracture extending to the inferior border, and condylar neck fracture, with annotations of risk factors and management strategies for each pattern</image>

Third Molars and BSSO

Third molars in the osteotomy line increase bad split risk. Most surgeons prefer to remove third molars 6-12 months before BSSO to allow bone healing at the extraction site. Removal at the time of BSSO is an option (single anesthetic) but complicates the split and increases infection risk. Leaving third molars in situ is only appropriate when they are not in the osteotomy path, which is rare.

Clinical Pearls

Condylar positioning is the single most critical step -- improper seating leads to condylar sag, early relapse, and possible idiopathic condylar resorption. A condylar positioning device or manual verification (tactile seating of the condyle into the fossa) should be used before fixation. The split should propagate smoothly with minimal force; if resistance is encountered, the osteotomy cuts should be rechecked rather than applying excessive force. Periosteal attachment to the proximal segment should be maintained as intact as possible (Epker modification) to preserve blood supply and reduce condylar resorption risk. In large advancements (greater than 7 mm), neurosensory disturbance is almost certain in the short term, and patients should be counseled accordingly. The IAN position should be evaluated on CBCT before surgery, as a laterally positioned canal is at higher risk during the sagittal split. Postoperative guiding elastics should be used for 4-6 weeks even with rigid fixation to guide occlusal settling.

References

• Trauner R, Obwegeser H. The surgical correction of mandibular prognathism and retrognathia. Oral Surg Oral Med Oral Pathol. 1957.
• Dal Pont G. Retromolar osteotomy for correction of prognathism. J Oral Surg. 1961.
• Epker BN. Modifications in the sagittal osteotomy of the mandible. J Oral Surg. 1977.
• Joss CU, Vasalli U. Stability after bilateral sagittal split osteotomy advancement surgery with rigid internal fixation. J Oral Maxillofac Surg. 2009.
• Al-Moraissi EA, Ellis E. Is there a difference in stability or neurosensory function between rigid and semi-rigid fixation for BSSO? J Oral Maxillofac Surg. 2016.
• Wolford LM, et al. Condylar resorption. Oral Maxillofac Surg Clin North Am. 2014.

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