Orthognathic Workup
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Direct answer
The osteotomy is the short act; the workup is the long one, and nearly every unhappy orthognathic outcome traces back to a shortcut taken during assessment. A complete workup runs: history and motivation, growth status and dental health; clinical facial analysis in frontal and profile views plus intraoral occlusal examination; radiographs headed by the lateral cephalogram, with the classic norms (SNA 82 plus or minus 2 degrees, SNB 80 plus or minus 2, ANB 2 plus or minus 2) quantifying jaw position; study models mounted in centric relation on an articulator with a facebow; standardised photography; model surgery generating intermediate and final splints; and assessment of airway and of psychological expectation. Elective surgery waits until facial growth is essentially complete.
What you must remember
- Cephalometric norms to quote: SNA about 82 degrees (maxillary position), SNB about 80 degrees (mandibular position), ANB about 2 degrees (the jaw relationship) — the skeleton is the map, but the soft tissue is the territory.
- Additional measurements examiners like: Wits appraisal as a supplement to ANB when cranial base rotation misleads, mandibular plane angle (SN-GoGn, about 32 degrees) for vertical pattern, and soft-tissue measurements such as lip position to the E-line.
- Why centric relation and not habitual posture: models must be recorded in the retruded, repeatable condylar position, since surgery repositions the jaws relative to a stable hinge axis — articulator mounting with a facebow transfers this relationship faithfully.
- Model surgery and splints: the planned movements are performed on mounted casts, generating an intermediate splint (positioning the first jaw moved against the unoperated jaw) and a final splint (the planned occlusion) — the operating theatre executes what the laboratory rehearsed.
- Growth timing: defer elective surgery until growth is essentially complete (serial cephalograms, hand-wrist radiograph or cervical vertebral maturation for skeletal age), because operating on a still-growing mandible invites relapse of the discrepancy.
- The orthodontic sandwich: presurgical orthodontics decompensates the dentition (undoing the camouflage) for 12-18 months, surgery corrects the skeleton, and postoperative orthodontics finishes the occlusion — patients consented for surgery must consent for full-period orthodontics too.
- Airway and systemic screen: sleep-disordered breathing and large planned mandibular setbacks raise airway concerns; smoking, vitamin D and bone health, and controlled diabetes affect healing; and photographic documentation is medico-legally mandatory.
- Psychological assessment: expectations and body dysmorphic concerns are screened before surgery — unrealistic expectations, not technical failure, drive most dissatisfaction with excellent surgical results.
A class III patient taken through the chain
A 19-year-old woman presents with a "protruding lower jaw" she has hated since school. The workup chain begins with her story — teasing, chewing difficulty — and motivation. Clinically: a concave profile, midface deficiency in the paranasal hollowing, class III molars and canines with reverse overjet, competent lips. The lateral cephalogram measures SNA 77 degrees (deficient maxilla), SNB 84 degrees (prominent mandible), ANB minus 7 — a bimaxillary discrepancy, not a mandibular problem alone; serial cephalograms confirm growth is complete. Models are recorded in centric relation — the wax record made with the mandible guided into retruded contact — and mounted with a facebow, alongside standardised photographs. Presurgical orthodontics decompensates her incisors for fourteen months, unmasking the full skeletal width of the discrepancy. Model surgery rehearses the plan — Le Fort I advancement of about 5 mm with mandibular setback, or maxillary advancement alone if the airway and profile favour it — and produces the intermediate and final splints. The psychological review confirms realistic expectations, and only now does the operation have a plan worth executing.
Where students slip
Examiners find the workup's weak points quickly. The cephalometric answer must be quantitative — SNA, SNB and ANB with the numbers attached — and interpreted ("ANB minus 7 means the mandible is relatively ahead of the maxilla"), since reciting landmarks without norms earns nothing. The centric relation question discriminates: candidates who mount models in habitual occlusion, or cannot say why the retruded contact position is used, reveal the articulator was never understood — surgery needs the repeatable condylar seat, and splints are only as valid as the record. The third trap is sequence: surgery before orthodontic decompensation, or splints without the intermediate one, shows a workflow memorised out of order. Indian exam convention also asks who decides — the joint orthodontic-surgical conference — and rewards the sentence that the soft-tissue profile and the patient's complaint lead the plan, with cephalometric numbers as servants, not masters.
Frequently asked questions
What are the normal values of SNA, SNB and ANB?
SNA about 82 plus or minus 2 degrees (maxilla), SNB about 80 plus or minus 2 degrees (mandible), and ANB about 2 plus or minus 2 degrees (the jaw relationship).
Why are records taken in centric relation rather than habitual occlusion?
Centric relation is the retruded, repeatable condylar position; jaw repositioning surgery must reference a stable hinge axis, so mounted models and splints are valid only from this record.
What is model surgery and what does it produce?
Rehearsal of the planned movements on articulated casts, producing the intermediate splint (positioning the first jaw moved) and the final splint (the target occlusion).
When is orthognathic surgery timed relative to growth?
Electively after facial growth is essentially complete, confirmed by serial cephalograms or skeletal maturity indicators, unless severe or syndromic deformity justifies earlier operation.
Why is psychological assessment part of the workup?
Dissatisfaction after technically successful surgery usually stems from unrealistic expectations or body dysmorphic disorder, which must be identified before, not after, the osteotomy.