Mandibular second premolar: FDI 35 and 45
The lower second bicuspid: a wider table than the first premolar, with either two or three cusps.
Quick answer
- FDI 35 and 45, Universal 20 and 29, Palmer LL5 and LR5.
- Mandibular second premolar erupts at 11 to 12 years.
The numbers on both sides
| Tooth | FDI | Universal | Palmer |
|---|---|---|---|
| Lower left second premolar | 35 | 20 | LL5 |
| Lower right second premolar | 45 | 29 | LR5 |
In FDI the first digit is the quadrant and the second is the position counted from the midline. 35 is quadrant 3, position 5: the lower left second premolar. 45 is the same position in quadrant 4, on the lower right. Universal numbers adult teeth from 1 at the upper right third molar around to 32 at the lower right third molar, which makes this tooth 20 on the lower left and 29 on the lower right. Palmer writes the position inside a bracket that shows the quadrant. In plain text that gives LL5 and LR5.
Crown surfaces: mesial, distal, buccal, lingual and occlusal.
The ADA chart also uses the older name second bicuspid.
Development and eruption
| Stage | Age |
|---|---|
| Calcification begins | 2.25 to 2.5 years |
| Crown completed | 6 to 7 years |
| Erupts | 11 to 12 years |
| Root completed | 13 to 14 years |
Calcification starts at 2.25 to 2.5 years and the crown is complete at 6 to 7 years. The tooth erupts at 11 to 12 years, and the root closes at 13 to 14 years, at the later end of the premolars.
Read on 24 October 2026. Sources: Human tooth development, tables citing Ash and Nelson, Wheeler's Dental Anatomy, Physiology and Occlusion (Wikipedia); American Dental Association, Tooth development charts (primary and permanent). Figures are usual values, and teeth vary from person to person.
Crown
The crown is larger than the first premolar's, and the broader table makes the tooth work more like a molar. The common three-cusp form has a buccal cusp, a larger mesiolingual cusp and a distolingual cusp, separated by a Y-shaped groove with a central pit.
A two-cusp form with a U-shaped or H-shaped groove also occurs, less often. A crown with one cusp is not typical, and four cusps is a molar pattern. The distal contact with the first molar is broader cheek to tongue than the mesial contact.
Read on 24 October 2026. Sources: Nagas and others, The Permanent Maxillary and Mandibular Premolar Teeth, in Dental Anatomy (IntechOpen, 2018); MolarPath note Permanent tooth morphology (built on Wheeler's Dental Anatomy, 11th ed., and Sturdevant's Operative Dentistry, 8th ed.). Figures are usual values, and teeth vary from person to person.
Roots and canals
The root is single, tapered and wider and longer than the first premolar's, with a distal curve and longitudinal grooves on both proximal faces. Ingle's Endodontics tabulates one root and one canal.
In the pooled analysis of 15,981 teeth from 30 studies, 78.63 percent were single rooted and 91.82 percent were Vertucci type I. Single canals ranged from 92 to 98.8 percent across studies. C-shaped canals were 1.4 percent in the C-shaped canal meta-analysis, well below the lower second molar.
Read on 24 October 2026. Sources: Nagas and others, The Permanent Maxillary and Mandibular Premolar Teeth, in Dental Anatomy (IntechOpen, 2018); Number of roots and canals, from Ingle's Endodontics (animated-teeth.com); Karobari and others, Roots and Root Canals Characterization of Permanent Mandibular Premolars, J Clin Med 2023; Yousefi and others, Prevalence of c-shaped canal morphology in premolar and molar teeth assessed by CBCT, BMC Oral Health 2025. Figures are usual values, and teeth vary from person to person.
Neighbors and the baby tooth before it
The first premolar has a mesiolingual groove and a single lingual tubercle; the second premolar has two lingual cusps in its common form. The mandibular tooth is larger than the first premolar, the reverse of the upper arch where the first premolar is larger.
It replaces the primary mandibular second molar, which resembles a small permanent first molar with five cusps. The primary molar is wider than the premolar. After the second primary molar is lost, the first permanent molar drifts forward into the leeway space.
Read on 24 October 2026. Sources: Nagas and others, The Permanent Maxillary and Mandibular Premolar Teeth, in Dental Anatomy (IntechOpen, 2018); MolarPath note Primary tooth morphology (built on McDonald and Avery's Dentistry for the Child and Adolescent, 11th ed.); MolarPath note Eruption sequence and chronology (built on McDonald and Avery, 11th ed., and Proffit's Contemporary Orthodontics, 7th ed.). Figures are usual values, and teeth vary from person to person.
Local anesthesia
The mental foramen most often lies below this tooth or between the apices of the two premolars, and its position shifts with age and tooth loss.
The mental nerve block is therefore given in the fold at the second premolar. It numbs the lip and chin. The inferior alveolar block is the usual method for pulpal anesthesia of the tooth in an adult, because in adults lower infiltration works mainly for teeth in front of the molars.
Read on 24 October 2026. Sources: MolarPath note Mandibular techniques, local anesthesia (built on Malamed's Handbook of Local Anesthesia, 7th ed.). Figures are usual values, and teeth vary from person to person.
Clinical and exam points
The AAPD names the mandibular second premolar as the most frequently missing permanent tooth when third molars are left out. Hypodontia has a prevalence of 4.4 to 13.4 percent in the permanent dentition depending on the continent.
When the premolar is absent, the primary second molar can stay for years. Ankylosis, most common in primary molars, is reported in 7 to 14 percent of the primary dentition. An ankylosed primary molar stays at the same level while the neighbors erupt, and it appears to sink. Management is to follow the tooth and extract it when it interferes with eruption or when neighboring teeth tip.
Read on 24 October 2026. Sources: American Academy of Pediatric Dentistry, Management of the Developing Dentition and Occlusion in Pediatric Dentistry (2024 revision); MolarPath note Eruption sequence and chronology (built on McDonald and Avery, 11th ed., and Proffit's Contemporary Orthodontics, 7th ed.). Figures are usual values, and teeth vary from person to person.
Questions and answers
What are the numbers for the mandibular second premolar?
FDI 35 and 45, Universal 20 and 29, Palmer LL5 and LR5. The ADA also uses the name second bicuspid.
How many cusps does it have?
Three is the most common form, with one buccal and two lingual cusps. Two-cusp forms with a U or H groove occur less often.
Which tooth is most often congenitally missing?
Leaving out the third molars, the mandibular second premolar, according to the AAPD.
Where is the mental foramen?
Most often below the second premolar.
Read on 24 October 2026. Sources: American Dental Association, Tooth development charts (primary and permanent); American Academy of Pediatric Dentistry, Management of the Developing Dentition and Occlusion in Pediatric Dentistry (2024 revision); MolarPath note Mandibular techniques, local anesthesia (built on Malamed's Handbook of Local Anesthesia, 7th ed.); Nagas and others, The Permanent Maxillary and Mandibular Premolar Teeth, in Dental Anatomy (IntechOpen, 2018). Figures are usual values, and teeth vary from person to person.
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