Permanent molars compared
| Feature | Max 1st molar (#3, #14) | Max 2nd molar (#2, #15) | Mand 1st molar (#19, #30) | Mand 2nd molar (#18, #31) |
|---|---|---|---|---|
| Cusps | 4 + cusp of Carabelli (on the mesiolingual cusp) | 4 (distolingual smaller) or 3 | 5: MB, DB, D, ML, DL | 4 |
| Cusp size order | ML > MB > DB > DL > Carabelli | DL much reduced | MB widest buccal cusp; D smallest | Nearly equal |
| Roots | 3: MB, DB, palatal (palatal longest) | 3, closer together, may fuse | 2: mesial (usually 2 canals) and distal | 2, closer together; C-shaped canals possible |
| Groove pattern | Oblique ridge (ML to DB) crosses the occlusal table | Oblique ridge smaller | Y-5 pattern; two buccal grooves | Cross (+) pattern; one buccal groove |
| Crown shape | Rhomboid; wider buccolingually than mesiodistally | More rhomboid, heart-shaped if DL is absent | Pentagonal; wider mesiodistally than buccolingually | Rectangular |
Patterns that apply to all molars
- Maxillary molar crowns are wider buccolingually; mandibular molar crowns are wider mesiodistally.
- Mandibular molar crowns tilt lingually on their roots; lingual cusps of mandibular molars are taller and sharper than the buccal (supporting) cusps.
- Cusp size, root spread and crown size decrease from first to third molars; third molars are the most variable teeth.
- Root trunks get longer from first to third molars, so furcations are harder to reach but less easily exposed by bone loss.
Clinical connections
- The MB root of the maxillary first molar often has a second canal (MB2), commonly missed at access.
- Maxillary first molar furcations: buccal from the buccal; mesial best from the palatal; distal from the buccal or palatal. Mandibular molars: buccal and lingual furcations.
- Enamel projections into the buccal furcation of mandibular molars favor plaque and furcation involvement.
- C-shaped canal systems are most common in mandibular second molars.
⚠️ Common traps
- Ranking the cusp of Carabelli as large: it is the smallest cusp and may be absent.
- Placing the oblique ridge from MB to DL: it runs from the mesiolingual to the distobuccal cusp.
- Calling the mandibular first molar's largest cusp the distal: the distal cusp is the smallest.
- Giving mandibular molars a palatal root: they have mesial and distal roots.
Memory aids
- Maxillary first molar cusp size: 'My Mom Doesn't Drive Cars' (ML, MB, DB, DL, Carabelli).
Mini-quiz
- Which is the largest cusp of the maxillary first molar?
- The oblique ridge of a maxillary first molar connects which cusps?
- A mandibular molar has four nearly equal cusps and a cross-shaped groove pattern. It is most likely the:
- From which side is the MESIAL furcation of a maxillary first molar best probed?
- Which root of the maxillary first molar most often has two canals?
Answer key: 1 A · 2 B · 3 B · 4 B · 5 B