BSSO - Tricky Viva Questions for Your Seminar

1. HISTORY & EVOLUTION (Often asked to trip up candidates)

The earliest intraoral description of what became the modern BSSO was by Schuchardt in 1942 (described in German literature). The landmark modern description was by Trauner and Obwegeser in 1957. Dal Pont (1961) modified it by extending the vertical cut anteriorly on the buccal cortex to the first molar region, increasing the bone contact area.

Hunsuck shortened the medial horizontal cut so it did not extend to the posterior border of the mandible - this reduced medial stripping and neurovascular bundle manipulation. Epker further limited periosteal stripping of the masseteric and medial pterygoid muscles, preserving blood supply to the proximal segment and reducing postoperative swelling and bone resorption.

Spiessel introduced it in 1976. Before this, patients required 5-6 weeks of intermaxillary fixation (IMF) with wires, and sagittal relapse rates were 23-45%. Rigid internal fixation dramatically reduced relapse (down to 10-18%) and improved patient comfort and early function.

2. ANATOMY (This is where HODs attack)

The IAN enters the mandibular foramen on the medial surface of the ramus, approximately 1 cm above the occlusal plane. It travels through the mandibular canal in the body of the mandible. During BSSO, the critical step is the horizontal medial cut - the nerve must be visualized or at least its position respected when placing the medial horizontal osteotomy superior to the lingula. The nerve can end up in the proximal fragment after splitting (occurs in ~39% of cases) - this is a key anatomical hazard.

• Proximal segment: Masseter (lateral), Medial pterygoid (medial/inferior), Temporal muscle (via coronoid process). These provide blood supply and pull the proximal segment.
• Distal segment: Carries the dentition, geniohyoid, mylohyoid, digastric (anterior belly), and the suprahyoid group.

The lingula is a bony projection on the medial ramus that acts as a landmark for the mandibular foramen. The medial horizontal cut should be placed superior and posterior to the lingula to avoid entering the inferior alveolar canal and damaging the IAN.

In a bad split, the cortical fracture propagates unpredictably - often through the inferior border, the condylar neck, or the coronoid. If the split goes inferior to the mandibular canal, the IAN stays in the distal (tooth-bearing) fragment instead of the proximal fragment. This is dangerous and may require conversion to a different procedure.

3. INDICATIONS & CONTRAINDICATIONS

• Mandibular prognathism (Class III, most common setback use)
• Mandibular retrognathia (Class II, mandibular advancement)
• Mandibular asymmetry
• Open bite correction (combined with other procedures)
• Hemimandibular hyperplasia / elongation
• Obstructive sleep apnea (advancement >10 mm, often combined with Le Fort I)

IVRO (intraoral vertical ramus osteotomy) may be preferred when:

• Setback required is >7-8 mm (large setbacks are difficult with BSSO due to unfavorable condylar torque)
• Patient has pre-existing TMJ disease (IVRO allows condyle to seat passively)
• Risk of bad split is high (e.g., thin mandible, impacted third molars)
• BSSO has the advantage of rigid fixation and no IMF; IVRO requires IMF but has less IAN risk.

Yes. Third molars should be removed at least 6-9 months before BSSO to allow adequate cortical bone healing in the region of the osteotomy. Retained third molars, especially impacted ones, weaken the osteotomy site and increase the risk of a bad split.

4. STEPS OF THE PROCEDURE (Must know in sequence)

1. Mucosal incision - external oblique ridge incision with flap elevation
2. Expose the lateral ramus, external oblique, and anterior border of ramus
3. Medial dissection - subperiosteal stripping above the lingula to identify the medial ramus (Hunsuck modification: limited dissection)
4. Three osteotomy cuts:
   • Medial horizontal cut - above the lingula, just through the medial cortex
   • Vertical/anterior cut - on the lateral surface of the mandibular body, distal to the second molar (Dal Pont position, at level of first molar)
   • Connecting sagittal cut - along the external oblique ridge connecting the above two cuts
5. Splitting - using osteotomes/spreaders to achieve the sagittal split between cortices
6. IAN position identified; nerve repositioned if trapped in proximal segment
7. Distal segment placed in pre-planned occlusion with surgical splint + IMF
8. Condyles seated in fossa, rigid internal fixation applied
9. IMF released, occlusion verified, wound closure

The condyle must be in its centric relation position - fully seated, most superiorly and anteriorly in the glenoid fossa. This is achieved by:

• Manual pressure on the gonial angle directing the condyle superiorly
• Using a condylar positioning device (e.g., Epker's condylar positioning jig)
• Failure to seat the condyle properly leads to condylar torque, TMJ symptoms, and postoperative relapse.

5. FIXATION - TRICKY AREA

1. Bicortical position screws (lag screws) - 2.0 mm screws placed transorally at the superior border; 2-3 screws per side. Gold standard for mandibular advancement. Risk of lateral condylar displacement if gaps exist.
2. Miniplates with monocortical screws - used when gap exists at osteotomy site, safer for condyle position, preferred in setback.
3. Bioresorbable plates and screws - eliminate need for hardware removal but have lower strength.
4. Wire fixation - historical only; requires 6 weeks IMF, high relapse.

Relapse rates are comparable (10-19% for both). However, bicortical screws carry a higher risk of condylar displacement in advancements due to segment compression, while miniplates offer better three-dimensional control. Most centers use 3 bicortical position screws per side for advancements as per Van Sickels' technique.

Condyle sag is inferior displacement of the condyle from the glenoid fossa during rigid fixation. If the condyle is not actively seated when screws are placed, the condyle sags inferiorly. Postoperatively, when the condyle "springs back" up, it causes an anterior open bite and relapse of the mandible.

6. COMPLICATIONS (The HOD's favorite)

Intraoperative:

• Bad split / unfavorable fracture (most common serious intraoperative complication)
• IAN injury / transection (2-3.5% transection; 10-30% some form of long-term neurosensory deficit)
• Lingual nerve injury
• Bleeding - inferior alveolar artery or masseteric artery
• Condylar fracture

Postoperative Early:

• Swelling, trismus
• Infection
• Hardware failure
• Malocclusion

Postoperative Late:

• Neurosensory disturbance / paresthesia (most common late complication; up to 30%)
• Relapse (10-19% with rigid fixation)
• Condylar resorption (especially in females with pre-existing TMJ disease, Class II deep bite)
• TMJ dysfunction / worsening
• Avascular necrosis of proximal segment
• Lingual nerve paresthesia (3% at 5 years)

A bad split is an unfavorable, uncontrolled fracture through bone during the splitting step. Types include:

• Fracture through the condylar neck
• Fracture through the inferior border
• Fracture through the coronoid
• Buccal plate fracture

Management depends on type and location: small buccal plate fractures can be stabilized with a plate; condylar neck fractures may require conversion to IVRO or ORIF; inferior border fractures need plating. The case may need to be abandoned and rescheduled.

• Young females (especially 15-25 years, hormonal influence)
• Pre-existing TMJ disease / condylar osteoarthritis
• Class II deep bite with retrognathia
• Large mandibular advancements (>8-10 mm)
• Counter-clockwise rotation of the occlusal plane
• High mandibular plane angle

• Subjective: Two-point discrimination, brush stroke direction test, light touch
• Objective: Nerve conduction studies, current perception threshold (CPT) testing, Semmes-Weinstein monofilaments
• Recovery is graded using the MRC sensory grading scale or Zuniga and Trapp's classification.

7. RELAPSE

• Inadequate rigid fixation
• Unsupported condyle / condyle sag at time of fixation
• Muscle pull (masseters, pterygoids) - especially in large advancements
• Condylar resorption
• High mandibular plane angle (greatest predictor of relapse in advancements)
• Large magnitude of movement (>8 mm advancement has higher relapse)
• Counter-clockwise rotation increases relapse risk

Setbacks are more stable than advancements. Muscle pull from the masticatory muscles (masseter, medial pterygoid) acts in a direction that tends to push the mandible anteriorly, opposing a setback and aiding retention. In advancements, the same muscles pull posteriorly and contribute to relapse.

8. COMBINED PROCEDURES (Bimaxillary surgery)

When there is a significant sagittal and/or vertical discrepancy in both jaws (Class III with maxillary hypoplasia, or Class II with maxillary excess), or when the mandibular movement alone would compromise the airway (BSSO setback causing posterior airway space narrowing). Bimaxillary surgery allows more precise correction, shorter individual jaw movements, and better facial aesthetics and airway outcomes.

The standard sequence is maxilla first (Le Fort I fixated), then mandible (BSSO) with the final surgical splint. This is because the maxilla provides the stable reference for the mandible. The counter-argument for "mandible first" exists but is less common.

9. PRE-SURGICAL WORKUP

• Orthopantomogram (OPG) - nerve canal position, third molars, bone quality
• Lateral cephalogram - cephalometric analysis (ANB angle, SNA, SNB, Wits appraisal, IMPA)
• PA cephalogram - for asymmetry cases
• CBCT / 3D CT - for severe asymmetry, TMJ assessment, virtual surgical planning (VSP)
• Dental models with face-bow transfer for articulator mounting and model surgery

ANB = SNA - SNB. Normal is 2-4 degrees. A negative ANB indicates Class III skeletal relationship; a markedly positive ANB (>5) indicates Class II. The magnitude of BSSO movement is planned to normalize the ANB and achieve a Class I occlusion. The surgeon must determine whether the discrepancy is primarily mandibular, maxillary, or combined.

It screens for obstructive sleep apnea (OSA). Some BSSO setback patients may have borderline airway compromise pre-op, and setback will worsen the posterior airway space. Conversely, BSSO advancement (MMA - maxillomandibular advancement) is a definitive surgical treatment for OSA with a success rate of ~80-90%.

10. WILDCARD / INTEGRATIVE QUESTIONS

BSSO is intraoral (no external scar), allows rigid internal fixation, preserves the condyle-ramus relationship in the proximal segment, provides a large bone contact area for healing, and can correct horizontal, vertical, and rotational deformities simultaneously.

• Condyle sag (most likely if early onset)
• Condylar resorption (progressive, more common in young females)
• Relapse due to inadequate fixation
• Muscle pull/stretch on distal segment
• Missed pre-existing TMJ disease Management: evaluate with MRI of TMJ; if condylar resorption suspected, wait until stable before any revision; CBCT to compare condylar positions; may need total joint replacement in severe resorption.

• Position screws: Threads engage both cortices equally; the screw maintains the gap between segments and fixes position. Used to prevent movement.
• Lag screws: Near cortex is over-drilled (glide hole), far cortex is tapped; tightening the screw compresses the two segments together. Used when passive bone contact exists. In BSSO advancements, gaps often exist anteriorly - using lag screws here causes lateral condylar displacement (a common exam trap).