Introduction
Bimaxillary orthognathic surgery represents the most comprehensive approach to correcting complex dentofacial deformities involving both the maxilla and mandible. Combined maxillary (Le Fort I osteotomy) and mandibular (bilateral sagittal split osteotomy/BSSO) surgery enables correction of anterior-posterior, vertical, and transverse skeletal discrepancies that cannot be adequately treated with single-jaw procedures. Success requires sophisticated surgical planning incorporating cephalometric analysis, three-dimensional (3D) imaging, model surgery, virtual surgical planning, precise intraoperative technique, and methodical postoperative orthodontics to stabilize the surgical correction. This review addresses the surgical planning process, anatomical considerations specific to bimaxillary procedures, fixation methodology, and factors affecting long-term stability.
Preoperative Assessment and Planning
Cephalometric Analysis
Cephalometric radiography (lateral skull radiograph standardized for magnification and patient positioning) remains fundamental for orthognathic treatment planning, though increasingly supplemented by 3D imaging. Key measurements for bimaxillary surgery planning include:
Maxillary position: Anterior-posterior position (distance from point A to nasion perpendicular or vertical reference line), assessed relative to cranial base (SNA angle: normal 82±4 degrees). Vertical position evaluated via anterior nasal spine to palatal plane relationship to maxillary molars. Mandibular position: Anterior-posterior position (point B distance), assessed via SNB angle (normal 80±4 degrees). Vertical dimensions: gonial angle (normal 120±7 degrees, <120 indicates hypodivergence, >130 indicates hyperdivergence), posterior facial height (inferior anterior facial height/total anterior facial height ratio), and facial axis of growth. Occlusal plane angulation: Normal occlusal plane intersects nasion-sella-point A line at approximately 7 degrees. Steep occlusal planes (>15 degrees) indicate vertical maxillary excess; flat planes (<5 degrees) indicate vertical maxillary deficiency. Chin position: Pogonion (most anterior point of mandibular symphysis) position relative to cranial base and maxilla. Pogonion-to-nasion perpendicular distance determines chin projection. Menton (most inferior point of mandible) vertical position relative to maxilla determines lower posterior facial height.Three-Dimensional Imaging and Virtual Planning
CBCT imaging (cone beam computed tomography) provides superior bony anatomical detail compared to 2D cephalometry, enabling visualization of: (1) transverse asymmetries not apparent on sagittal cephalometry, (2) vertical discrepancies and asymmetries, (3) posterior anatomical structures (ramus, condyle, posterior body), and (4) impaction patterns or supernumerary dentition. CBCT protocols for orthognathic planning utilize 0.4-0.5 mm slice thickness for maximal skeletal detail. 3D virtual planning software (Dolphin 3D, OrthoAnalyzer, Proplan, others) enables: (1) 3D virtual models of maxilla and mandible from CBCT data, (2) simulation of planned surgical movements, (3) prediction of dental occlusion post-surgery, (4) assessment of airway dimension changes (critical in vertical maxillary excess cases with potential for sleep apnea), and (5) generation of surgical guides and wafers.Virtual planning process: (1) CBCT is segmented (computer identifies maxillary and mandibular bone from soft tissue), (2) dental models are registered/aligned to CBCT data, (3) planned surgical movements are simulated in 3D (e.g., maxilla advanced 8 mm, rotated counterclockwise 2 degrees; mandible advanced 5 mm, rotated 1 degree), (4) resultant occlusion is assessed for anterior-posterior and vertical dimension changes, and (5) surgical wafers/guides are generated based on planned movements.
Model Surgery
Model surgery (surgical rehearsal on dental casts) enables: (1) verification of planned movements' feasibility, (2) assessment of molar and incisor relationship changes, (3) identification of interferences or undercuts, and (4) prediction of surgical changes relative to the planned vector. Process: (1) maxillary and mandibular casts are mounted on an articulator in pre-treatment occlusion, (2) maxillary cast is separated from the articulator at the planned osteotomy level (typically 5 mm above apical margin of teeth), (3) maxillary cast is moved to the planned position and blocked in place with wax, (4) mandibular cast is then repositioned into the new maxillary position, and (5) surgical wafer is fabricated from the post-treatment casts to guide intraoperative positioning.
Maxillary Surgery: Le Fort I Osteotomy
The Le Fort I osteotomy (transpalatal maxillary osteotomy) separates the maxilla above the apical margins of teeth and above the roots of the zygomatic process, enabling movement in all three dimensions. Anatomical landmarks for incision placement and osteotomy design include:
Incision placement: Intraoral vestibular incision (buccal mucosa) extending from the distal aspect of one first molar to the contralateral first molar, approximately 4-5 mm apical to the mucogingival junction. Placement 4-5 mm apical to the junction preserves attached gingiva and reduces recession risk. Osteotomy design: Horizontal cut through the maxillary facial process above the apices of the maxillary teeth (typically 5-7 mm apical to the cementoenamel junction, ensuring clearance from apical foramina of anterior teeth). Vertical cuts through the lateral maxillary wall between first and second molars. Palatal cut completes the Le Fort I; three approaches exist: (1) complete palatal osteotomy (cutting through entire palate horizontally 5 mm apical to palatine sutures), (2) partial palatal osteotomy (cutting anterior hard palate but preserving posterior hard palate connective tissue), and (3) palatal split (preserving palatal mucosa and splitting posterior palate between teeth). Partial/split approaches reduce postoperative palatal sensation loss (occurring in 20-30% of patients with complete osteotomy) and reduce palatal scarring/contraction risk. Anatomical hazards: Infraorbital neurovascular bundle (identified at the infraorbital foramen, located ~12 mm inferior to the orbital rim; careful elevation of periosteum and gentle soft tissue handling preserve sensation). Pterygoid plates (posterior attachment; controlled fracture of pterygoid plates occurs as maxilla is mobilized, not requiring surgical fracture). Nasal septum (preserved intact to prevent airway compromise). Maxillary mobilization and positioning: Gentle elevation using Le Fort osteotome or broad periosteal elevator mobilizes the maxilla after osteotomy completion; excessive force causes pterygoid plate fracture or unnecessary vascular trauma. The maxilla is repositioned using surgical guides (based on model surgery or virtual planning), ensuring 3-dimensional accuracy. Positioning is verified by examining molar relationships and anterior incisor overlap.Mandibular Surgery: Bilateral Sagittal Split Osteotomy (BSSO)
The BSSO separates the mandible along the sagittal plane within the body/angle, enabling anterior-posterior advancement or setback. This remains the most common mandibular procedure for orthognathic surgery.
Anatomical landmarks: Inferior alveolar nerve location (within the mandibular canal, running from the lingula medially toward the inferior alveolar foramen anteriorly). The canal is identified on CBCT; average depth below the exterior cortical surface varies by anatomical location (deeper in posterior body, shallower in anterior symphysis). Preoperative CBCT enables precise identification of canal location and enables surgery planning to preserve neurovascular structures. Incision and access: Intraoral approach via mucosa over the external oblique line (buccal cortex), with incision placed 5-10 mm buccal to the external oblique ridge. Incision extends from the distal aspect of first molar to approximately the canine region, with careful mucoperiosteal elevation to expose the external cortex while preserving the attached gingiva. Sagittal split osteotomy: (1) sagittal (vertical) cut is made through the buccal cortex at the anterior aspect of the mandible, approximately 5-8 mm anterior to the first molar, extending from the apical margin of teeth to the inferior border of the mandible (depth ~1-1.5 cm below the external surface, positioning the cut within the cancellous bone between cortical plates); (2) horizontal cut connects the sagittal cut to the internal oblique ridge; (3) the mandible is gently split along the sagittal plane using controlled fracture (osteotome and mallet) or motorized oscillating saw, creating a split that extends posteriorly along the mandibular ramus (avoiding the inferior alveolar nerve and vascular bundle, which remain in the proximal segment). Proximal segment preservation: The medial aspect of the ramus and the inferior alveolar neurovascular bundle remain attached to the proximal segment (posterior mandible), ensuring blood supply preservation and minimizing neurosensory morbidity. Anterior segment mobilization: The anterior mandible (symphysis, body to first molar, with attached teeth and gingiva) is mobilized anteriorly or posteriorly into the planned position, typically 5-10 mm advancement being the maximum comfortable advancement without significant neurovascular compromise.Fixation Methods
Intermaxillary Fixation (IMF)
IMF maintains established occlusion between maxilla and mandible during bone healing. Methods include: (1) arch bars (stainless steel bars cemented to teeth with composite or bonded with composite resin), connected with elastic bands or wire, applied preoperatively before jaw surgery; (2) composite bonded splints (tooth-colored material bonded to lingual surfaces of maxillary and mandibular anterior teeth, faster and simpler than arch bars, reversible); (3) screw-supported occlusal plane guides (titanium screws placed in anterior maxilla/mandible with maxilla-mandible connectors, enabling precise occlusal control).
IMF duration varies by fixation technique: (1) rigid fixation (plates securing both jaws) enables IMF removal immediately postoperatively or after 2-4 weeks; (2) non-rigid fixation (elastic bands) typically requires 4-6 weeks IMF. Modern technique prefers early IMF release (2-3 weeks) to enable function and begin postoperative orthodontics while maintaining some soft tissue healing.
Rigid Fixation (Internal Fixation)
Titanium plates (2.0 mm dynamic compression plates, 1.8-2.0 mm spacing between screw holes) secure the osteotomy segments in three dimensions, enabling immediate function and eliminating need for prolonged IMF.
Maxillary fixation: Typically 2-4 plates (one-plate approach places the plate on the facial surface of the maxilla at the Le Fort osteotomy level; two-plate approach places a plate on the facial surface and a second plate on the nasal aperture margin, providing superior rotational control). Plate placement across the planned osteotomy ensures segment stability. Mandibular fixation: Typically 2 plates (one plate on each side of the sagittal split, securing the proximal segment to the anterior segment). Plate placement medial to the inferior border of the mandible preserves the bony contour of the inferior border.Bimaxillary Surgery Sequencing
Controversy exists regarding maxillary-first versus mandibular-first sequencing. Maxillary-first approach: (1) maxilla is mobilized and fixated first, establishing maxillary position relative to the cranial base; (2) mandible is then mobilized into relationship with the new maxillary position, determining final mandibular position. This approach is anatomically logical (maxilla is relatively fixed structures, mandible adjusts to maxilla). Mandibular-first approach: (1) mandible is mobilized and fixated first; (2) maxilla is positioned relative to the new mandibular position. This approach is mechanically advantageous in some cases (reducing relapse risk) but less commonly employed.
Postoperative Orthodontics
Postoperative orthodontics (typically 3-8 months duration) refines the occlusion achieved by surgery. Preoperative orthodontics (typically 12-18 months) aligns teeth within their respective dental arches, eliminating dental compensations and enabling the surgeon to focus on skeletal correction. Without preoperative orthodontics, the surgeon must correct both dental compensation and skeletal deformity.
Post-op orthodontic goals: (1) establish optimal interdigitation of maxillary and mandibular teeth, (2) achieve optimal anterior-posterior and vertical relationships, (3) establish proper overjet (4-6 mm) and overbite (2-3 mm), and (4) achieve coincident midlines and minimal rotations. Braces remain affixed for 3-6 months postoperatively, using light forces (25-50g for incisors, 50-100g for molars) to enable active movement while accommodating healing tissues.Stability and Relapse
Long-term skeletal stability (5-10 year follow-up) demonstrates that surgical corrections are largely stable. Relapse rates (percentage of surgical movement that is lost postoperatively) typically range from 10-20% for maxillary movements and 5-15% for mandibular movements. Factors affecting stability include: (1) magnitude of movement (larger movements show slightly higher relapse), (2) direction of movement (vertical movements show higher relapse than anterior-posterior), and (3) surgeon technique and rigidity of fixation.
Condylar seatedness (position of the mandibular condyle in the temporal fossa) is critical for stability. Surgery performed with the condyle seated in centric relation, followed by proper IMF and postoperative guidance, results in lower relapse rates (5-10%) compared to surgery with mandible in habitual closure (relapse 15-25%).Complications
Neurosensory morbidity: Inferior alveolar nerve injury results in temporary hypoesthesia/paresthesia in 30-50% of BSSO patients, with permanent sensory loss in 2-5%. Greater palatine nerve injury (from Le Fort I) causes palatal sensation loss in 20-50% of patients, usually resolving within 6-12 months. Condylar sag: Improper surgical technique or IMF loss can result in condylar descent (inferior positioning), causing open bite relapse or TMJ dysfunction. Airway changes: Maxillary advancement can reduce nasopharyngeal airway dimension by 10-30% in some cases; conversely, mandibular advancement may improve airway. Preoperative sleep apnea assessment (sleep study) is indicated in obstructive sleep apnea patients before maxillary advancement.Conclusion
Bimaxillary orthognathic surgery represents a sophisticated surgical discipline requiring comprehensive preoperative planning (cephalometric analysis, 3D imaging, model surgery, virtual planning), meticulous intraoperative technique, appropriate fixation methodology, and systematic postoperative orthodontics. Understanding the anatomical constraints, neurovascular relationships, and healing biology enables surgeons to achieve predictable skeletal corrections with minimal morbidity. Long-term stability of surgical corrections (relapse rates 10-20%) with proper technique and postoperative management enables achievement of esthetic and functional outcomes that significantly improve patients' quality of life and psychological well-being.