Primary mandibular second molar: FDI 75 and 85

The numbers on both sides

In FDI the first digit is the quadrant and the second is the position counted from the midline. 75 is quadrant 7, position 5: the lower left primary second molar. 85 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 K on the lower left and T on the lower right. Palmer writes the letter for the position inside a bracket that shows the quadrant. In plain text that gives LLE and LRE.

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

Development and eruption

Calcification starts at about 18 weeks in utero and the crown is complete at 10 months. The tooth erupts at 23 to 31 months, and the root is complete at 3 years. It is shed at 10 to 12 years, among the last of the baby teeth to go.

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 resembles a small permanent mandibular first molar, with five cusps. Unlike the upper second molar, it carries no cusp of Carabelli. It is larger than the first primary molar and wider than the premolar that replaces it.

The table is five-cusped, the contacts are broad and flat, and the crown bulges above a pinched neck, most on the cheek side.

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, flare apart to leave room for the premolar crown. In the Salama study of 10 second molars, canal lengths were 15.8 mm for the mesiobuccal canal, 14.4 for the mesiolingual and 14.9 for both the distobuccal and distolingual canals. The number of canals varied from three to four across the mandibular molars studied.

A micro-CT study of 38 extracted primary molars found accessory canals in 6 of them (15.79 percent), and physiologic resorption affected less than two thirds of the root length.

Read on 24 October 2026. Sources: Salama and others, Anatomy of primary incisor and molar root canals, Pediatric Dentistry 14(2), 1992; 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.

Neighbors and the tooth that replaces it

The first molar in front of it is smaller and atypical. The permanent first molar comes in behind this tooth at about 6 years, and the second premolar takes its place at 11 to 12 years.

The lower first permanent molar pushes the primary molars forward into the primate space as it erupts, an early mesial shift. When the primary second molars are lost, the first permanent molars drift forward into the leeway space, which is about 3.4 mm in the lower arch for both sides combined.

Read on 24 October 2026. Sources: MolarPath note Eruption sequence and chronology (built on McDonald and Avery, 11th ed., and Proffit's Contemporary Orthodontics, 7th ed.); MolarPath note Primary tooth morphology (built on McDonald and Avery's Dentistry for the Child and Adolescent, 11th ed.); American Dental Association, Tooth development charts (primary and permanent). Figures are usual values, and teeth vary from person to person.

Local anesthesia

Buccal infiltration with 4% articaine works for pulp therapy on primary mandibular molars. A block is harder to place in a small child, and it numbs the lip and tongue for longer, with a risk of biting.

Intraligamentary injection reaches the roots of primary molars, but the follicle of the second premolar sits beneath this tooth, and buccal infiltration avoids the pressure injury that an intraligamentary injection risks.

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

Ankylosis most often involves primary molars, with an incidence of 7 to 14 percent in the primary dentition. The tooth stays at the same vertical level and appears to sink as its neighbors erupt. The AAPD advises keeping an ankylosed molar that has a successor until eruption is blocked or adjacent teeth tip, then extracting it.

Infraocclusion has a worldwide prevalence of 1.3 to 8.9 percent. Where the second premolar is missing, which is the most common absent permanent tooth after third molars, a sound primary second molar can serve for many years, and the AAPD lists the factors for deciding between keeping it, reducing it and extracting it.

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 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); 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.