The four premolars compared
| Feature | Max 1st PM (#5, #12) | Max 2nd PM (#4, #13) | Mand 1st PM (#21, #28) | Mand 2nd PM (#20, #29) |
|---|---|---|---|---|
| Cusps | 2; buccal clearly taller | 2; nearly equal heights | 2; large buccal, small nonfunctional lingual | 3 (most common) or 2 |
| Roots | Usually 2 (buccal and lingual) | Usually 1 | 1 | 1 |
| Occlusal outline | Hexagonal; long central groove | Oval; short central groove, many supplemental grooves | Diamond; transverse ridge splits mesial and distal fossae | 3-cusp: square, Y groove; 2-cusp: round, H or U groove |
| Signature feature | Mesial crown concavity and mesial marginal groove | Most 'symmetrical' of the maxillary premolars | Mesiolingual groove; crown tilted lingually, canine-like | Mesiolingual cusp larger than distolingual |
Shared features
- Premolars develop from four lobes (three buccal, one lingual) except the 3-cusp mandibular second premolar (five lobes: two lingual).
- Buccal cusps of mandibular premolars are supporting (centric holding) cusps; on maxillary premolars the lingual cusps are supporting.
- Maxillary premolars are wider buccolingually than mesiodistally; the maxillary first premolar's lingual cusp tip is offset to the mesial.
Clinical connections
- The mesial concavity of the maxillary first premolar crown and root makes matrix adaptation and root planing harder there.
- Bifurcated maxillary first premolar roots make the tooth a common site for root fractures and two-canal endodontic access.
- Mandibular second premolars are among the most commonly congenitally missing teeth (after third molars; order with maxillary laterals varies by source).
- The mental foramen usually lies near or between the apices of the mandibular premolars and can mimic a periapical lesion on a radiograph.
⚠️ Common traps
- Giving the maxillary SECOND premolar two roots: the first premolar is the two-rooted one.
- Calling the lingual cusp of the mandibular first premolar functional: it is small and usually nonfunctional.
- Calling the 2-cusp mandibular second premolar the most common type: the 3-cusp (Y groove) type is more common.
- Placing the mesial marginal groove on the mandibular first premolar: the mesial marginal developmental groove belongs to the maxillary first premolar (the mandibular first has a mesiolingual groove).
Mini-quiz
- A premolar has two roots and a concavity on the mesial surface of its crown. It is most likely a:
- Which premolar most resembles a canine?
- A mandibular second premolar with a Y-shaped groove pattern has how many cusps?
- Which statement about maxillary second premolars is correct?
- Which premolar cusps are supporting (centric holding) cusps?
Answer key: 1 A · 2 A · 3 B · 4 C · 5 C