6.2 Eruption Sequences & FDI Numbering

Quick Answer: Use FDI only: permanent quadrants 1–4, primary 5–8, tooth 1–8 (or 1–5 primary) from the midline. Permanent first molars erupt ~6 years; primary dentition completes ~2.5–3 years; mixed dentition ~6–12 years.

Eruption timing and tooth ID are pure recall items on the AFK when written well—and applied items when a radiograph description or pediatric scenario is attached. NDEB’s official system is FDI two-digit notation (ISO 3950).

FDI Numbering System (Official for NDEB)

Each tooth has two digits:

First digit = quadrant (patient’s right/left as viewed from the front, clinician’s opposite):

Second digit = tooth position from the midline:

Examples: 16 = permanent maxillary right first molar; 36 = permanent mandibular left first molar; 51 = primary maxillary right central; 84 = primary mandibular right first molar; 23 = permanent maxillary left canine.

Exam traps: Do not use Universal (1–32) or Palmer notation on NDEB exams. Quadrants run clockwise from the patient’s upper right for permanent (1→2→3→4), not the clinician’s left-to-right reading order without thinking. Primary second digit 4 and 5 are molars, not premolars—there are no primary premolars.

Permanent Dentition — Eruption Chronology

Ages are means; girls often slightly earlier than boys; individual variation of ±6–12 months is common. AFK expects relative sequence more than exact months.

Classic sequence logic (often taught):

1. Permanent first molars erupt distal to primary second molars (~6-year molars)—no predecessors.
2. Incisors replace primary incisors (centrals before laterals; mandibular before maxillary for centrals).
3. Mandibular canines often erupt before premolars in the lower arch.
4. In the maxilla, first premolars often erupt before the canine (canine frequently last of the anteriors/premolars—impaction risk if space is lost).
5. Second molars (~12-year molars) after premolars/canines.
6. Third molars much later if at all.

Rule of thumb many candidates use: 6-year molars → lower 1s → upper 1s / lower 2s → upper 2s → lower 3s → premolars → upper 3s → 12-year molars. Memorize that maxillary canines are late relative to premolars.

Root completion typically trails clinical eruption by about 2–3 years. Radiographic open apex after eruption is therefore normal for a period.

Primary Dentition — Eruption and Exfoliation

Primary sequence (typical): mandibular centrals → other incisors → first molars → canines → second molars. Primary canines erupt after first primary molars—unlike the permanent maxillary pattern where canines are late among permanent successors but permanent first molars still lead the permanent wave.

Complete primary dentition: usually by 30 months (range ~24–36 months)—20 teeth.

Natal/neonatal teeth (present at birth or first month) are usually mandibular incisors and may be supernumerary or early primary; management is clinical, but recognition is high-yield.

Delayed eruption beyond expected windows raises concern for systemic disease, obstruction (supernumerary, cysts), ankylosis, or trauma history—link to pediatric/oral pathology thinking without deep diving here.

Mixed Dentition

Mixed dentition begins with eruption of the first permanent molars and/or mandibular permanent centrals (~6 years) and ends when the last primary tooth exfoliates and permanent successors (through second molars) are in—roughly 12–13 years.

Key mixed-dentition concepts for AFK:

• Ugly duckling stage: temporary midline diastema and flared maxillary laterals while permanent canines erupt—often self-corrects; do not overtreat as pathology solely on that basis.
• Leeway space: difference between combined mesiodistal width of primary canine + molars vs permanent canine + premolars; greater in mandible; helps late mesial shift of permanent molars into Class I.
• Early loss of primary second molars risks mesial drift of permanent first molars and space loss for premolars—space maintenance logic.
• Ectopic eruption of permanent first molars can resorb primary second molar distal roots.

Putting FDI and Eruption Together (Worked Examples)

• A 7-year-old with 16, 26, 36, 46 erupted and 31/41 erupted, still has primary maxillary centrals: normal—maxillary permanent centrals often follow mandibular.
• FDI 63 is primary maxillary left canine—expect eruption around 16–22 months and exfoliation ~10–12 years as 23 erupts late.
• FDI 75 is primary mandibular left second molar—exfoliates ~10–12 years for 35.
• Question stem “tooth 14” means permanent maxillary right first premolar (~10–11 years), not a primary tooth.

AFK Exam Traps

1. Primary quadrants 5–8, not “A–T” or Universal primary letters on NDEB.
2. Second digit 4/5 in primary = molars, not premolars.
3. Permanent maxillary canines erupt after first premolars in the usual sequence—space loss after early primary canine/molar loss endangers 13/23.
4. First permanent molars have no primary predecessors—they erupt distal to primary second molars.
5. Ages are approximate; order beats memorizing a single month number when two answers differ only by sequence logic.

Drill FDI conversion until automatic, then layer chronology. Most scoring mistakes are quadrant errors (confusing left/right or primary/permanent) rather than deep morphology.