Mandibular central incisor: FDI 31 and 41

The numbers on both sides

In FDI the first digit is the quadrant and the second is the position counted from the midline. 31 is quadrant 3, position 1: the lower left central incisor. 41 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 24 on the lower left and 25 on the lower right. Palmer writes the position inside a bracket that shows the quadrant. In plain text that gives LL1 and LR1.

A plain number can name two different teeth, so the system matters. Universal 31 is the mandibular right second molar, which is FDI 47.

Crown surfaces: mesial, distal, labial (facial), lingual and incisal.

Development and eruption

Calcification starts at 3 to 4 months. The tooth erupts at 6 to 7 years, alongside the first molar, and the root is complete by about 9 years, earlier than any other incisor.

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 the smallest of any tooth: in the tabulated means it is 9.0 mm long and 5.0 mm wide at contact, narrowing to 3.5 mm at the cervical line. It is bilaterally symmetrical, with the sharpest line angles of any tooth and the shallowest grooves on the front.

The lingual surface is smooth, with a slight fossa and a faint cingulum. After the mamelons wear off, the incisal edge is straight and sits at right angles to the long axis. Both contacts lie at the same height in the incisal third.

Read on 24 October 2026. Sources: Grawish and others, Permanent Maxillary and Mandibular Incisors, in Dental Anatomy (IntechOpen, 2017). Figures are usual values, and teeth vary from person to person.

Roots and canals

The single root is straight and flattened from side to side, with shallow developmental depressions on the mesial and distal faces. These depressions are easy to miss during debridement. Ingle's Endodontics tabulates one canal, but the flat root can hold two, one labial and one lingual.

A Chilean CBCT sample of 336 teeth found every central incisor single rooted and 30.7 percent with two canals. A pooled analysis of three Chinese CBCT studies gave 5.6 percent, which shows how much the figure depends on the population.

Read on 24 October 2026. Sources: Grawish and others, Permanent Maxillary and Mandibular Incisors, in Dental Anatomy (IntechOpen, 2017); Number of roots and canals, from Ingle's Endodontics (animated-teeth.com); Betancourt and others, CBCT assessment of root and canal morphology in mandibular anterior teeth in a Chilean subpopulation, BMC Oral Health 2026; Aung and Myint, Evidence of Second Canal between Permanent Mandibular Central and Lateral Incisors in China, Int J Dent 2020; 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.

Neighbors and the baby tooth before it

The mandibular lateral incisor is slightly wider, the reverse of the upper arch, where the central incisor is the wider tooth. Only the central incisor has a single opposing tooth, the maxillary central incisor.

It takes the place of the primary mandibular central incisor, which looks symmetrically flat from the front. The primary tooth is the first to erupt in the whole dentition, at 6 to 10 months. The permanent incisor develops lingual to it.

Read on 24 October 2026. Sources: Grawish and others, Permanent Maxillary and Mandibular Incisors, in Dental Anatomy (IntechOpen, 2017); 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; American Dental Association, Tooth development charts (primary and permanent). Figures are usual values, and teeth vary from person to person.

Local anesthesia

In adults, infiltration works mainly for teeth in front of the molars. A standard inferior alveolar block reaches the mandibular teeth to the midline and numbs the lip and chin.

An incisive nerve block gives pulpal anesthesia in front of the mental foramen without numbing the tongue. For all the lower front teeth, one incisive block on each side avoids two inferior alveolar blocks and the numbness on both sides of the tongue.

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

A lower incisor that was treated years ago and still has an apical radiolucency, despite a good-looking filling on a straight-on film, may have a missed lingual canal. An angled film or CBCT shows the canal, and the access should reach toward the incisal edge to remove the lingual shoulder.

The AAPD notes that mandibular incisors have a high relapse rate for rotations and crowding. The permanent lower incisors are about 6 mm wider in total than the primary ones they replace, in one set of figures, which is why a spaced primary arch is a good sign.

Read on 24 October 2026. Sources: MolarPath note Pulp chambers and root canal morphology (built on Cohen's Pathways of the Pulp, 12th ed.); American Academy of Pediatric Dentistry, Management of the Developing Dentition and Occlusion in Pediatric Dentistry (2024 revision); MolarPath note Primary tooth morphology (built on McDonald and Avery's Dentistry for the Child and Adolescent, 11th ed.). Figures are usual values, and teeth vary from person to person.

Questions and answers

Read on 24 October 2026. Sources: American Dental Association, Tooth development charts (primary and permanent); Betancourt and others, CBCT assessment of root and canal morphology in mandibular anterior teeth in a Chilean subpopulation, BMC Oral Health 2026; Aung and Myint, Evidence of Second Canal between Permanent Mandibular Central and Lateral Incisors in China, Int J Dent 2020; 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.