Residency · Residency · Plastic Surgery
Orthognathic Surgery Principles
Introduction
Orthognathic surgery corrects dentofacial deformities that cannot be addressed by orthodontics alone. Requires coordinated planning between orthodontist and surgeon. Goals: functional occlusion, balanced facial proportions, stable airway, improved aesthetics. Performed after skeletal maturity (females 16-17, males 17-19 years).
Most common indications: skeletal Class II and III malocclusion, facial asymmetry, obstructive sleep apnea, cleft-related maxillary hypoplasia.
Cephalometric Analysis
Key Landmarks
Sella (S): center of sella turcica. Nasion (N): most anterior point of frontonasal suture. Point A (Subspinale): deepest concavity of anterior maxilla. Point B (Supramentale): deepest concavity of anterior mandible.
Pogonion (Pog): most anterior point of chin. Menton (Me): most inferior point of mandibular symphysis. Gonion (Go): most posteroinferior point of mandibular angle. ANS/PNS: anterior and posterior nasal spine.
Key Measurements
| Measurement | Normal Value | Significance |
|---|---|---|
| SNA angle | 82 ± 2° | Maxillary AP position relative to cranial base |
| SNB angle | 80 ± 2° | Mandibular AP position relative to cranial base |
| ANB angle | 2 ± 2° | Maxillomandibular relationship |
| FMA (Frankfort-mandibular plane angle) | 25° | Vertical facial pattern |
| Upper incisor to SN | 104° | Upper incisor inclination |
| IMPA (lower incisor to mandibular plane) | 90° | Lower incisor inclination |
SNA angle: maxillary anteroposterior position relative to cranial base (normal: 82 +/- 2 degrees). SNB angle: mandibular anteroposterior position (normal: 80 +/- 2 degrees). ANB angle: maxillomandibular relationship (normal: 2 +/- 2 degrees). Positive ANB >4: skeletal Class II.
Negative ANB: skeletal Class III. Wits appraisal: perpendicular projections of A and B points onto the occlusal plane. FMA (Frankfort-mandibular plane angle): vertical facial pattern (normal: 25 degrees). High angle (>30): long face, open bite tendency.
Low angle (<20): short face, deep bite tendency. Upper incisor to SN: upper incisor inclination (normal: 104 degrees). Lower incisor to mandibular plane (IMPA): lower incisor inclination (normal: 90 degrees).
Facial Analysis
Vertical thirds: trichion to glabella, glabella to subnasale, subnasale to menton (ideally equal). Horizontal fifths: five equal segments of facial width. Profile: convex (Class II), straight (Class I), concave (Class III). Lip position: relative to Ricketts E-line (tip of nose to soft tissue pogonion). Nasolabial angle: 90-110 degrees (increases with maxillary advancement).
Virtual Surgical Planning (VSP)
Process
CT scan with dental models (or cone beam CT with integrated dental scan). 3D virtual skull model created. Osteotomies planned digitally with precise movements in all three planes. Occlusal splints (intermediate and final) fabricated by 3D printing or CAD/CAM. Cutting guides can be printed for intraoperative use.
Advantages
Improved accuracy and predictability. 3D visualization of complex movements (especially asymmetry cases). Precise splint fabrication. Reduced operative time. Enhanced communication between surgeon and orthodontist.
Presurgical Orthodontics
Duration: 12-18 months typically. Goals: dental decompensation (reverse tooth movements that compensated for skeletal discrepancy), level and align arches, coordinate arch widths. Dental decompensation may temporarily worsen the patient's appearance and occlusion (important to counsel patients). Surgical hooks placed on archwires for intraoperative fixation.
Surgery-first approach: emerging trend where surgery is performed before decompensation; orthodontics then faster due to regional acceleratory phenomenon (RAP).
Le Fort I Osteotomy
Anatomy
Osteotomy at the Le Fort I level: across the piriform aperture, lateral maxillary wall, pterygomaxillary junction, and nasal septum. Blood supply: descending palatine artery (preserved), ascending pharyngeal artery contributions. Key structures at risk: infraorbital nerve, descending palatine artery, nasal mucosa.
Technique
Circumvestibular incision in the maxillary buccal sulcus. Subperiosteal exposure of the anterior and lateral maxilla. Horizontal osteotomy from piriform aperture to pterygomaxillary junction bilaterally. Pterygomaxillary disjunction with curved osteotome.
Nasal septum separated from the maxillary crest. Maxilla downfractured and mobilized. Repositioned into planned position using intermediate splint. Fixed with four L-shaped or straight miniplates (two at piriform and two at buttress).
Movements
Advancement: most common in Class III; increases SNA; widens alar base. Impaction: for vertical maxillary excess (gummy smile); bone removed superiorly. Inferior repositioning (downgraft): for vertical maxillary deficiency; bone grafts may be needed. Segmental osteotomy: multi-piece Le Fort I for transverse discrepancies (widening) or anterior open bite closure. Rotation: differential impaction/downgraft for cant correction in asymmetry.
Bilateral Sagittal Split Osteotomy (BSSO)
Anatomy
Osteotomy through the mandibular ramus splitting it into proximal (condyle-bearing) and distal (tooth-bearing) segments. Inferior alveolar nerve runs in the distal segment canal. Key structure at risk: inferior alveolar nerve (IAN).
Technique (Obwegeser-Dal Pont modification)
Intraoral sagittal ramus incision. Medial osteotomy above the lingula on the medial ramus. Lateral osteotomy on the buccal cortex from the second molar region to the ramus. Connecting osteotomy through the inferior border.
Split completed with osteotomes; IAN identified and protected. Distal segment advanced or set back into planned position. Proximal segment (condyle) passively seated. Fixed with bicortical screws (3 screws per side) or miniplates.
Movements
Advancement: for mandibular deficiency (Class II). Setback: for mandibular prognathism (Class III); less common in modern practice. Rotation: for asymmetry correction.
IAN Injury
Neurosensory disturbance in 30-70% immediately postoperative. Most resolve within 6-12 months. Permanent deficit in 3-5%. Risk increases with large movements and patient age.
Genioplasty
Indications
Chin deficiency (microgenia) or excess. Facial asymmetry at the chin level. Adjunct to orthognathic surgery to refine lower facial proportions.
Technique
Intraoral degloving incision at the labial sulcus. Horizontal osteotomy below the mental foramina. Segment mobilized: advancement, setback, vertical reduction, or lengthening. Fixed with a single plate or step plate. Can be combined with any orthognathic procedure.
Movements
Advancement: most common; up to 10-12 mm. Vertical reduction: for long chin. Lengthening: interpositional bone graft. Lateral shift: for chin-point asymmetry.
Bimaxillary Surgery
Combined Le Fort I and BSSO in the same operation. Indicated for severe Class II or III, bimaxillary protrusion, significant asymmetry, open/deep bite deformities. Sequence: maxilla first (positioned with intermediate splint against mandible), then mandible positioned with final splint against repositioned maxilla. Allows larger corrections with better stability than single-jaw surgery. Commonly combined with genioplasty.
Stability and Relapse
| Movement | Stability | Relapse Rate |
|---|---|---|
| Maxillary impaction | Most stable | <5% |
| Mandibular advancement | Stable | 5-10% |
| Maxillary advancement | Moderate | 10-15% |
| Mandibular setback | Less stable | 10-20% |
| Maxillary downgraft | Least stable | 15-25% |
Most stable movements: maxillary impaction, mandibular advancement. Least stable movements: maxillary downgraft, mandibular widening. Rigid fixation (plates and screws) has significantly improved stability. Skeletal relapse rates: 10-20% for mandibular setback, 5-10% for mandibular advancement.
Cleft patients have higher relapse after Le Fort I advancement. Distraction osteogenesis reduces relapse for large movements.
Complications
Hemorrhage (internal maxillary artery, descending palatine artery). Inferior alveolar nerve injury (BSSO). Infraorbital nerve injury (Le Fort I). Bad split (unfavorable fracture pattern during BSSO). Condylar sag/resorption (idiopathic condylar resorption). TMJ dysfunction. Relapse. Nonunion (rare).
Tooth root damage. Alar base widening after Le Fort I (managed with alar cinch suture). Velopharyngeal insufficiency after maxillary advancement (rare in non-cleft). DVT/PE (low risk; ambulatory same-day in most cases).
Clinical Pearls
Virtual surgical planning has become the standard of care for complex cases; it improves accuracy particularly for asymmetry correction where 3D movements are difficult to plan on 2D cephalometry. Always perform an alar cinch suture after Le Fort I osteotomy to prevent postoperative alar base widening; a V-Y closure of the vestibular incision prevents upper lip thinning. When performing BSSO, the key to avoiding bad splits is adequate exposure, correct osteotomy placement (medial cut above lingula), and controlled splitting with osteotomes — never force the split.
The surgery-first approach can reduce total treatment time by 6-12 months but requires close surgeon-orthodontist coordination and careful case selection. In Class III cleft patients, consider distraction osteogenesis over conventional Le Fort I for advancements >10 mm to reduce the significant relapse risk. Condylar position must be verified intraoperatively (passive condylar seating); failure to seat the condyle properly leads to early relapse and TMJ symptoms.
References
- Proffit WR, White RP Jr, Sarver DM. Contemporary Treatment of Dentofacial Deformity. Mosby; 2003.
- Posnick JC. Orthognathic Surgery: Principles, Planning and Practice. Elsevier; 2014.
- Epker BN, Stella JP, Fish LC. Dentofacial Deformities: Integrated Orthodontic and Surgical Correction. Mosby; 1998.
- Swennen GR, Mollemans W, Schutyser F. Three-dimensional treatment planning of orthognathic surgery in the era of virtual imaging. J Oral Maxillofac Surg. 2009;67(10):2080-2092.
- Pannaci M, et al. Surgery-first approach in orthognathic surgery: a systematic review. Int J Oral Maxillofac Surg. 2017;46(6):752-757.