Orthodontic Terminology, Anatomy, and Tooth Movement Principles
What this domain tests
Chairside language only works when the vocabulary, the anatomy behind it, and the biology of force are joined together. Expect items on directional terms, Angle classification, cephalometric values, and the pressure–tension theory.
Vocabulary and classification
- Mesial/distal, buccal/labial, lingual/palatal, occlusal/incisal, apical/cervical. "Facial" covers labial (anterior) and buccal (posterior).
- Angle class is set at the first molars: Class I - mesiobuccal cusp of the upper first molar sits in the buccal groove of the lower first molar; Class II - lower molar distal to that; Class III - mesial.
- Class II division 1 has proclined upper incisors and large overjet; division 2 has retroclined centrals and a deep bite.
- Overjet is horizontal (mm); overbite is vertical (mm or percent). They are never interchangeable.
- Leeway space (about 1.5 mm per side in the mandible, less in the maxilla) plus mixed-dentition prediction (Tanaka–Johnston, Moyer's) explains late crowding.
- Bolton analysis compares summed mesiodistal widths; an overall ratio near 91% and anterior near 77% indicate proportional tooth size.
Cephalometrics and landmarks
Common norms: SNA about 82°, SNB about 80°, ANB about 2°. An ANB above roughly 4° suggests a skeletal Class II pattern; below 0° a Class III pattern. Chairside landmarks to know: mucogingival junction, attached gingiva, frenula, incisive papilla, palatal rugae, pterygomandibular raphe, retromolar pad, hamular notch, external oblique ridge, coronoid process.
How teeth actually move
Light continuous force compresses the periodontal ligament (about 0.25 mm wide) on the pressure side and stretches it on the tension side, producing frontal resorption by osteoclasts and direct apposition. Heavy force occludes the PDL, causes hyalinization, and forces undermining resorption from adjacent marrow - the lag phase. Movement averages about 1 mm per month in three phases: initial, lag, progressive.
The center of resistance lies near the middle third of the root. The moment-to-force ratio decides tipping, controlled tipping, bodily translation (roughly 10:1 for a single-rooted tooth), or torque. Anchorage is graded maximum, moderate, or minimum by how much reactive tooth movement is acceptable.
Worked example
Records show 8 mm overjet, Class II molars bilaterally, and ANB 6°. The molar relationship sets the Angle class, so this is Class II. The 6° ANB says the discrepancy is at least partly skeletal, and 8 mm of overjet says the incisors are also protrusive. Round-wire alignment alone would not address that pattern; growth modification or extraction decisions belong to the orthodontist's plan, not the assistant's.
Traps that cost marks
Calling maxillary palatal surfaces "lingual"; swapping overjet and overbite; assuming more force moves teeth faster (it causes hyalinization and root resorption); and forgetting that bone resorbs on the pressure side while it is deposited on the tension side.