Dental Anatomy: Numbering, Morphology and Clinical Correlates
Dental Anatomy
Dental Anatomy in INI-CET tests precise morphology, tooth identification across numbering systems, eruption chronology and the clinical consequences of that anatomy - access cavity shape, canal numbers, contact and embrasure design.
Core rules
- FDI system: two digits, the first is the quadrant (1–4 permanent, 5–8 primary), the second the tooth position 1–8 counting from the midline. Universal system runs 1–32, maxillary left to right then mandibular left to right.
- Primary teeth begin calcification around 14 weeks in utero; permanent first molars begin at birth; the mandibular central incisor erupts near 6 months and the primary dentition is complete by about 2.5–3 years.
- Maxillary first permanent molar: three roots, five cusps, oblique ridge, cusp of Carabelli; the MB2 canal is found in roughly 60–70% of teeth.
- Mandibular first permanent molar: two roots, with the mesial root carrying two canals in about two-thirds of cases (Vertucci type IV).
- Primary molars show bulbous crowns, marked cervical constriction, thin enamel and flared roots that cradle the permanent successor.
Worked example
Identify tooth 36 and predict its canal anatomy. Quadrant 3 is the patient's lower left, and position 6 from the midline is the first molar - so FDI 36 is the lower left first permanent molar, which is universal number 19. Expect two roots and three canals: mesiobuccal, mesiolingual and distal. This is why the access outline is trapezoidal and the two mesial orifices are located just beneath the mesial marginal ridge, well mesial to the centre of the chamber floor. Missing the mesiolingual orifice is the classic cause of a 'missed canal' failure here.
Traps that cost marks
- Reversing FDI quadrants on the left side: 2 is upper left, 3 is lower left.
- Transferring permanent molar morphology onto the primary second molar, which closely resembles a permanent first molar.
- Answering 'widest contact' or 'embrasure' questions from memory rather than from the functional principle that contact form protects the interdental papilla and shunts food.
- Forgetting that primary incisors are narrower mesiodistally than their permanent successors, which is the basis of primate space.