Mandibular second molar: FDI 37 and 47
The lower twelve-year molar: four cusps in a cross pattern, and the molar most prone to C-shaped canals and enamel projections.
Quick answer
- FDI 37 and 47, Universal 18 and 31, Palmer LL7 and LR7.
- Mandibular second molar erupts at 11 to 13 years.
The numbers on both sides
| Tooth | FDI | Universal | Palmer |
|---|---|---|---|
| Lower left second molar | 37 | 18 | LL7 |
| Lower right second molar | 47 | 31 | LR7 |
In FDI the first digit is the quadrant and the second is the position counted from the midline. 37 is quadrant 3, position 7: the lower left second molar. 47 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 18 on the lower left and 31 on the lower right. Palmer writes the position inside a bracket that shows the quadrant. In plain text that gives LL7 and LR7.
Crown surfaces: mesial, distal, buccal, lingual and occlusal.
Development and eruption
| Stage | Age |
|---|---|
| Calcification begins | 2.5 to 3 years |
| Crown completed | 7 to 8 years |
| Erupts | 11 to 13 years |
| Root completed | 14 to 15 years |
Calcification starts at 2.5 to 3 years and the crown is complete at 7 to 8 years. The tooth erupts at 11 to 13 years, a little earlier than its upper counterpart, and the root closes at 14 to 15 years.
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 has four cusps, two buccal and two lingual, and no distal cusp. It is smaller than the first molar by a fraction of a millimeter in every dimension, shorter from biting surface to neck and narrower front to back. The outline is often rectangular, and the distobuccal lobe is sometimes more pronounced than the mesiobuccal.
A single buccal developmental groove separates the two buccal cusps, and the groove pattern is cross shaped.
Read on 24 October 2026. Sources: Craciunescu and others, Dental Anatomy and Morphology of Permanent Teeth, in Human Teeth, From Function to Esthetics (IntechOpen, 2023); 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
Two roots and three canals, two mesial and one distal, are usual. The roots lie close together and are straighter and sometimes shorter than the first molar's. They may fuse into a C shape.
The C-shaped canal is most frequent on this tooth. A systematic review of 101 studies pooled a prevalence of 17.3 percent for the mandibular second molar, from 75 studies, against 0.8 percent for the maxillary first molar. A C-shaped orifice leads to a ribbon-shaped canal system, and the cross-section may change from a full C to separate canals along the root.
Read on 24 October 2026. Sources: Craciunescu and others, Dental Anatomy and Morphology of Permanent Teeth, in Human Teeth, From Function to Esthetics (IntechOpen, 2023); MolarPath note Permanent tooth morphology (built on Wheeler's Dental Anatomy, 11th ed., and Sturdevant's Operative Dentistry, 8th ed.); Yousefi and others, Prevalence of c-shaped canal morphology in premolar and molar teeth assessed by CBCT, BMC Oral Health 2025; MolarPath note Pulp chambers and root canal morphology (built on Cohen's Pathways of the Pulp, 12th ed.). Figures are usual values, and teeth vary from person to person.
Neighbors and why no baby tooth comes first
The first molar has five cusps; this tooth has four. The third molar behind it is more variable and often has more cusps. The mandibular second molar is the tooth where cervical enamel projections are most frequent.
No baby tooth precedes it. The primary mandibular second molar, which looks like a small first permanent molar, is replaced by the second premolar, and this molar comes in behind the first molar.
Read on 24 October 2026. Sources: Craciunescu and others, Dental Anatomy and Morphology of Permanent Teeth, in Human Teeth, From Function to Esthetics (IntechOpen, 2023); Chussid, Anatomy of Primary Teeth, Columbia University College of Dental Medicine lecture; Bhusari and others, Prevalence of enamel projections and its co-relation with furcation involvement in maxillary and mandibular molars, J Indian Soc Periodontol 2013. Figures are usual values, and teeth vary from person to person.
Local anesthesia
An inferior alveolar block gives pulpal anesthesia. The long buccal nerve needs its own injection when the procedure touches the buccal gingiva, as it does for a rubber dam clamp, an extraction or a crown margin.
Failed blocks have recognizable causes: anatomic variation, accessory innervation through the mylohyoid nerve, inflamed tissue and the wrong depth or direction. A bifid mandibular canal can defeat a conventional deposit, and a higher technique such as the Gow-Gates block may be chosen.
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
In a dry-skull study of 944 molars, cervical enamel projections were present on 14.7 percent of mandibular second molars, the highest rate, and 87.5 percent of molars with a projection had furcation involvement. The enamel replaces cementum there, so no connective tissue fibers attach and the epithelial attachment is the only seal.
Because a C-shaped system changes along the root, treatment has to allow for isthmus areas between the canals and for the shape that varies from the orifice to the apex.
The buccal and lingual furcations of lower molars are probed from the buccal and lingual sides. A small angle between the roots combined with a long root trunk makes the entrance narrow and places it more apically, and in a biometric study a curette blade about 1 mm wide could not enter a furcation whose entrance was under 1 mm or whose roots diverged by under 30 degrees.
Read on 24 October 2026. Sources: Bhusari and others, Prevalence of enamel projections and its co-relation with furcation involvement in maxillary and mandibular molars, J Indian Soc Periodontol 2013; MolarPath note Pulp chambers and root canal morphology (built on Cohen's Pathways of the Pulp, 12th ed.). Figures are usual values, and teeth vary from person to person.
Questions and answers
What are the numbers for the mandibular second molar?
FDI 37 and 47, Universal 18 and 31, Palmer LL7 and LR7.
How is it different from the first molar?
It has four cusps and one buccal groove, where the first molar has five cusps and two buccal grooves. Its two roots usually sit closer together.
How common are C-shaped canals?
A pooled analysis gives 17.3 percent, the highest of any tooth in that review.
When does it erupt?
The ADA chart gives 11 to 13 years.
Read on 24 October 2026. Sources: American Dental Association, Tooth development charts (primary and permanent); Craciunescu and others, Dental Anatomy and Morphology of Permanent Teeth, in Human Teeth, From Function to Esthetics (IntechOpen, 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.
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