Orthodontics
Three numbers — SNA, SNB, ANB — and a small decision matrix for which space maintainer follows which extracted primary tooth. Master both and you cover most of the Ortho block.
6%
Decision Logic
01
Cephalometric Norms
Three angles you need cold. Memorize the normals; everything else is comparison.
SNA, SNB, ANB — measured from a lateral ceph
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Memory Hook · "82-80-2"
SNA 82°, SNB 80°, ANB 2°. ANB is the difference (SNA − SNB).
Full cephalometric reference
The OSCE orthodontic chart compares the patient's measurements to a North American mean. These four angles cover virtually every ceph question on the exam.
| Measurement | What it shows | Normal (mean) | If increased | If decreased |
|---|---|---|---|---|
| SNA | Maxilla A-P position relative to cranial base | 82° ± 2° | Maxillary protrusion (prognathism) | Maxillary retrusion |
| SNB | Mandible A-P position relative to cranial base | 80° ± 2° | Mandibular protrusion (prognathism) | Mandibular retrusion (retrognathism) |
| ANB | Inter-jaw relationship (SNA − SNB) | 2° ± 2° | Class II skeletal pattern | Class III skeletal pattern |
| SN‑MP Sella-Nasion to Mandibular Plane |
Vertical facial pattern / mandibular plane angle | 32° ± 5° | Hyperdivergent / long face / open bite tendency | Hypodivergent / short face / deep bite tendency |
| FH‑MP Frankfort Horizontal to Mandibular Plane |
Same vertical pattern, referenced to FH | 25° ± 5° | Steep mandibular plane (long face) | Flat mandibular plane (short face) |
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How OSCE charts read. The patient column is shown next to a "Mean Patient" column. If patient SNB is 5°+ below mean, suspect mandibular retrognathism. If SN-MP is 5°+ above mean, suspect a hyperdivergent (long-face) pattern — often paired with anterior open bite.
02
Skeletal Class via ANB
ANB tells you which jaw is "ahead." Then SNA and SNB tell you which jaw is the cause.
Class I
ANB = 0–4°
Normal skeletal relationship
Malocclusion is dental, not skeletal.
Class II
ANB > 4°
Maxilla forward (high SNA) or mandible back (low SNB)
"Overjet" appearance.
Class III
ANB < 0°
Maxilla back (low SNA) or mandible forward (high SNB)
"Underbite" appearance.
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OSCE pattern · Locating the cause
After ANB tells you the class, look at SNA and SNB individually. Class II with high SNA = max excess; with low SNB = mand deficient. Treatment differs (headgear vs. functional appliance).
03
Space Maintenance — Maxilla
When a primary tooth is lost prematurely, choose the maintainer by tooth, side, and abutment status.
| Lost tooth | Unilateral / Bilateral | Appliance |
|---|---|---|
| Primary 1st molar | Unilateral | Band-and-loop |
| Primary 1st molar | Bilateral | Nance appliance |
| Primary 2nd molar (1st perm molar erupted) | Unilateral | Band-and-loop on 1st perm molar |
| Primary 2nd molar (1st perm molar erupted) | Bilateral | Nance appliance |
| Primary 2nd molar (1st perm molar NOT erupted) | Unilateral | Distal shoe (guides eruption of 1st perm molar) |
| Primary 2nd molar (1st perm molar NOT erupted) | Bilateral | Distal shoe (one each side) |
| Primary canine | Either | Lingual arch / Nance |
| Primary anterior | Multiple | Hawley appliance with prosthetic teeth (esthetic) |
04
Space Maintenance — Mandible
Critical mandible difference: Lingual Holding Arch (LHA), not Nance, for bilateral cases.
| Lost tooth | Unilateral / Bilateral | Appliance |
|---|---|---|
| Primary 1st molar | Unilateral | Band-and-loop |
| Primary 1st molar | Bilateral | Lingual Holding Arch (LHA) |
| Primary 2nd molar (1st perm molar erupted) | Unilateral | Band-and-loop on 1st perm molar |
| Primary 2nd molar (1st perm molar erupted) | Bilateral | LHA |
| Primary 2nd molar (1st perm molar NOT erupted) | Unilateral | Distal shoe |
| Primary 2nd molar (1st perm molar NOT erupted) | Bilateral | Distal shoe (each side) |
| Primary canine | Either | LHA |
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Memory Hook · Maxilla = Nance · Mandible = LHA
For bilateral cases. The Nance has an acrylic palatal button (only fits maxilla); the LHA wraps lingually behind the mandibular incisors.
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Trap · Distal shoe contraindications
Distal shoe is contraindicated in patients at risk of bacterial endocarditis, immunocompromised, or with poor cooperation. Don't pick it for a "compromised heart" patient.
05
Malocclusion Patterns
Common Angle classifications and dentoalveolar findings.
| Pattern | Description | Treatment direction |
|---|---|---|
| Class I (Angle) | Mesiobuccal cusp of max 1st molar in mandibular 1st molar buccal groove | Address dental crowding/spacing |
| Class II div 1 | Distal molar relationship + protrusive maxillary incisors + ↑ overjet | Headgear (max excess) · functional appliance (mand deficiency) · Class II elastics |
| Class II div 2 | Distal molar + retroclined max central incisors + ↑ overbite | Open bite · upright incisors · address vertical overbite |
| Class III | Mesial molar relationship + edge-to-edge or anterior crossbite | Reverse pull headgear (face mask) · Class III elastics · surgery if severe |
| Anterior crossbite | Lower incisors anterior to upper | Tongue depressors · removable appliance with springs · fixed appliance |
| Posterior crossbite | Lower posteriors buccal to upper (reverse) | Maxillary expansion (RPE if pre-pubertal · SARPE if skeletal mature) |
| Open bite | Lack of vertical overlap of incisors | Habit cessation (thumb / tongue thrust) · myofunctional therapy · ortho |
| Deep bite | Excessive vertical overlap | Intrusion of incisors · extrusion of posteriors |
| Crossbite from sucking habit | Buccal crossbite + narrow max | Habit cessation · expansion if persists past age 3-4 |
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Key · Timing
Skeletal Class III + Class II from mandibular deficiency: best treated pre-pubertal growth spurt with functional appliances. Past growth, often surgical orthognathic.