Primary mandibular first molar: FDI 74 and 84

The numbers on both sides

In FDI the first digit is the quadrant and the second is the position counted from the midline. 74 is quadrant 7, position 4: the lower left primary first molar. 84 is the same position in quadrant 8, on the lower right. Universal gives baby teeth the letters A to T, from the upper right second molar around to the lower right second molar, which makes this tooth L on the lower left and S on the lower right. Palmer writes the letter for the position inside a bracket that shows the quadrant. In plain text that gives LLD and LRD.

Crown surfaces: mesial, distal, buccal, lingual and occlusal.

Development and eruption

Calcification starts at about 15.5 weeks in utero and the crown is complete at 5.5 months. The tooth erupts at 14 to 18 months, and the root is complete at 2.5 years. It is shed at 9 to 11 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 is atypical. It has a prominent mesial marginal ridge and a transverse ridge that divides the table, and it resembles no permanent tooth. A primary anatomy lecture gives it four cusps.

The mesiobuccal cervical bulge is large.

Read on 24 October 2026. Sources: MolarPath note Primary tooth morphology (built on McDonald and Avery's Dentistry for the Child and Adolescent, 11th ed.); Chussid, Anatomy of Primary Teeth, Columbia University College of Dental Medicine lecture. Figures are usual values, and teeth vary from person to person.

Roots and canals

Two roots, mesial and distal, carry the tooth. In the Salama study, 8 of 10 first molars had four canals and 2 had three. The mean canal lengths were 16.4 mm for the mesiobuccal canal, 14.2 for the mesiolingual, 13.1 for the distobuccal and 12.7 for the distolingual. The mesiolingual canal was the most variable, with a standard deviation of 1.96.

Cross-sections showed thin, ovoid canals, with the mesial canals wider cheek to tongue than the distal canals. Most variation lay in the buccolingual dimension, which a radiograph does not show. The distance from the occlusal surface to the pulp floor averaged 5.79 mm in first and second molars together.

Read on 24 October 2026. Sources: Salama and others, Anatomy of primary incisor and molar root canals, Pediatric Dentistry 14(2), 1992. Figures are usual values, and teeth vary from person to person.

Neighbors and the tooth that replaces it

The lower second molar behind it is larger and looks like a small permanent first molar with five cusps. The first molar has no such counterpart.

The first premolar replaces it at 10 to 12 years. When one primary molar is removed, a premolar speeds up once its root is half to two thirds formed, but earlier removal lets bone grow over the crypt and delays it.

Read on 24 October 2026. Sources: 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.); American Dental Association, Tooth development charts (primary and permanent). Figures are usual values, and teeth vary from person to person.

Local anesthesia

In children, buccal infiltration with 4% articaine matches an inferior alveolar block for pulp therapy on primary mandibular molars. Infiltration is preferred in young children because the effect is shorter, the lip usually stays less numb than after a block and the risk of lip biting falls.

Canadian labeling does not recommend articaine for children under 4 years. The mandibular foramen of a young child sits at or below the occlusal plane, so a block has to go lower than in an adult.

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

An abscessed primary molar shows radiolucency in the furcation, often with a buccal swelling, and no periapical lesion. The pulpal floor is thin and porous, and accessory canals connect the pulp and periodontium in some teeth (6 of 38 extracted primary molars in one micro-CT study). Follow-up of a pulpotomy therefore looks at the furcation first.

The pulp horns lie close to the surface, and the mesial horns are higher than the distal ones. Mesial and distal chamber walls are thinner than the buccal walls, so proximal preparations need careful depth control.

Read on 24 October 2026. Sources: MolarPath note Primary tooth morphology (built on McDonald and Avery's Dentistry for the Child and Adolescent, 11th ed.); MolarPath note Pulp chambers and root canal morphology (built on Cohen's Pathways of the Pulp, 12th ed.); Kirilova and others, Micro-CT Assessment of Dentin Mineral Density and Furcation Morphology in Primary First and Second Molars, Dentistry Journal 2026. 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); Salama and others, Anatomy of primary incisor and molar root canals, Pediatric Dentistry 14(2), 1992; 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.