Mandibular first molar: FDI 36 and 46
The lower six-year molar: five cusps, two roots and usually three canals, and the tooth that most often needs treatment.
Quick answer
- FDI 36 and 46, Universal 19 and 30, Palmer LL6 and LR6.
- Mandibular first molar erupts at 6 to 7 years.
The numbers on both sides
| Tooth | FDI | Universal | Palmer |
|---|---|---|---|
| Lower left first molar | 36 | 19 | LL6 |
| Lower right first molar | 46 | 30 | LR6 |
In FDI the first digit is the quadrant and the second is the position counted from the midline. 36 is quadrant 3, position 6: the lower left first molar. 46 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 19 on the lower left and 30 on the lower right. Palmer writes the position inside a bracket that shows the quadrant. In plain text that gives LL6 and LR6.
Crown surfaces: mesial, distal, buccal, lingual and occlusal.
Development and eruption
| Stage | Age |
|---|---|
| Calcification begins | At birth |
| Crown completed | 2.5 to 3 years |
| Erupts | 6 to 7 years |
| Root completed | 9 to 10 years |
Calcification begins at about birth and the crown is complete at 2.5 to 3 years. The tooth erupts at 6 to 7 years, as the first permanent tooth in the mouth, and replaces no baby tooth. The root closes at 9 to 10 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 five cusps: mesiobuccal, distobuccal, distal, mesiolingual and distolingual. Three buccal cusps show from the cheek side and two lingual cusps from the tongue side. It is the largest tooth of the lower arch.
The grooves are the central groove, two buccal grooves and a lingual groove. The central groove holds mesial, central and distal pits, and the part between the two central pits is the Lewis offset. The crown tilts toward the tongue over its roots. The buccal height of contour lies in the cervical third and the lingual height in the middle third.
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); Mandibular first molar (Wikipedia); 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, mesial and distal, support the tooth. The mesial root is broader, has a blunter apex and more prominent flutings, and usually has two canals. The distal root usually has one canal but often two, so three canals is the usual total.
A middle mesial canal can arise in the groove between the two mesial orifices. The distal wall of the mesial root has the thinnest dentin and forms the danger zone for strip perforation. A third, distolingual root called the radix entomolaris (Calberson and others, 2007) is under 5 percent in white, African and Indian populations, and 5 percent to over 30 percent in East Asian and Indigenous groups of the Arctic and the Americas. A second radiograph from a different horizontal angle helps reveal it. A pooled C-shaped canal prevalence is 2.5 percent in 19 studies.
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 Pulp chambers and root canal morphology (built on Cohen's Pathways of the Pulp, 12th ed.); Yousefi and others, Prevalence of c-shaped canal morphology in premolar and molar teeth assessed by CBCT, BMC Oral Health 2025; Calberson and others, The Radix Entomolaris and Paramolaris: Clinical Approach in Endodontics, Journal of Endodontics 33(1), 2007. Figures are usual values, and teeth vary from person to person.
Neighbors and why no baby tooth comes first
The mandibular second molar has four cusps in a cross pattern and a single buccal groove, against five cusps and two buccal grooves here. The upper first molar has three roots.
The primary mandibular second molar resembles a small version of this tooth with five cusps, and the permanent molar comes in behind it.
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.); Chussid, Anatomy of Primary Teeth, Columbia University College of Dental Medicine lecture. Figures are usual values, and teeth vary from person to person.
Local anesthesia
The inferior alveolar block misses the long buccal nerve, which supplies the cheek and the buccal gingiva of the molars. A separate injection behind and to the cheek side of the last molar is needed for an extraction, a rubber dam clamp or a crown margin.
In children, the mandibular foramen sits at or below the occlusal plane at first and rises with growth. The injection goes lower in a child than in an adult. In adults, lower infiltration works mainly 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
One encyclopedic summary calls the mandibular first molars the most common carious teeth and the most common to need endodontic treatment or extraction, with up to 45 percent of extracted teeth being lower first molars, citing a study of the extraction of endodontically treated teeth.
A summary of Ingle's Endodontics puts missed canals in about 42 percent of failed cases. In this tooth a middle mesial canal or the canal of a distolingual root can be missed.
Read on 24 October 2026. Sources: Mandibular first molar (Wikipedia); Number of roots and canals, from Ingle's Endodontics (animated-teeth.com). Figures are usual values, and teeth vary from person to person.
Questions and answers
What are the numbers for the mandibular first molar?
FDI 36 and 46, Universal 19 and 30, Palmer LL6 and LR6.
How many cusps does it have?
Five: mesiobuccal, distobuccal, distal, mesiolingual and distolingual.
How many canals does it have?
Usually three, two in the mesial root and one in the distal root. The distal root often has two.
What is a radix entomolaris?
An extra root on the distolingual side of a lower molar, most often the first molar.
When does it erupt?
The ADA chart gives 6 to 7 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); 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.
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