Maxillary third molar: FDI 18 and 28

The numbers on both sides

In FDI the first digit is the quadrant and the second is the position counted from the midline. 18 is quadrant 1, position 8: the upper right third molar. 28 is the same position in quadrant 2, on the upper left. 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 1 on the upper right and 16 on the upper left. Palmer writes the position inside a bracket that shows the quadrant. In plain text that gives UR8 and UL8.

A plain number can name two different teeth, so the system matters. Universal 18 is the mandibular left second molar, which is FDI 37. Universal 28 is the mandibular right first premolar, which is FDI 44.

Crown surfaces: mesial, distal, buccal, palatal (lingual) and occlusal.

The ADA chart also uses the older name wisdom tooth.

Development and eruption

Calcification starts at 7 to 9 years and the crown is complete at 12 to 16 years. The tooth erupts at 17 to 21 years, if it erupts, and the root is not complete until 18 to 25 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 shorter from biting surface to neck and narrower front to back than the second molar's. The occlusal outline is heart shaped with three functional cusps, two buccal and one lingual. Usually one large lingual cusp stands in place of two, with no lingual groove, though a poorly developed distolingual cusp and groove can appear.

Many accidental grooves mark an unworn surface. The tooth is often a developmental anomaly, varying in size, contour and position.

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). Figures are usual values, and teeth vary from person to person.

Roots and canals

Ingle's Endodontics tabulates three roots and three or more canals for upper molars, but the third molar departs from the rule. A study of 592 extracted maxillary third molars from northern Jordan found one root in 10.8 percent, two in 9.8 percent, three in 69.9 percent, four in 9.1 percent and five in 0.34 percent.

The canal counts in the same sample were one in 5.91 percent, two in 11.48 percent, three in 52.36 percent, four in 28.2 percent and five in 2.03 percent. The mesial root is often fused and divides in the apical third, and the roots are short for the size of the crown.

Read on 24 October 2026. Sources: Number of roots and canals, from Ingle's Endodontics (animated-teeth.com); Al-Qudah and others, Root and canal morphology of third molar teeth, Scientific Reports 2023; Craciunescu and others, Dental Anatomy and Morphology of Permanent Teeth, in Human Teeth, From Function to Esthetics (IntechOpen, 2023). Figures are usual values, and teeth vary from person to person.

Neighbors and why no baby tooth comes first

Compared with the second molar, the third molar is smaller, with fewer cusps and often fused roots. It has no predecessor and no replacement.

It comes in last, at 17 to 21 years, with a root that finishes at 18 to 25 years. Space is often inadequate, which is why impaction is common.

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 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

The posterior superior alveolar nerve supplies the pulp and the buccal tissue, and a posterior superior alveolar block is the standard approach. The greater palatine block covers the palate for extraction.

The buccal tissues and the palatal tissues of upper molars have separate innervation, so surgery needs anesthesia on both sides of the alveolar process.

Read on 24 October 2026. Sources: MolarPath note Maxillary 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 2024 meta-analysis of 98 studies and 183,828 subjects found third molar impaction in 36.9 percent of people and 46.4 percent of teeth, with impaction more frequent in the mandible than in the maxilla. Only 4 of 53 studies that reported by arch showed a slight lead in the maxilla.

The AAPD advises that third molars be monitored for position and space, and that the early adult dentition stage includes evaluating them. Pericoronitis is inflammation of the soft tissue around the crown of an erupting or incompletely erupted tooth.

Read on 24 October 2026. Sources: Pinto and others, Worldwide Prevalence and Demographic Predictors of Impacted Third Molars, J Clin Med 2024; American Academy of Pediatric Dentistry, Management of the Developing Dentition and Occlusion in Pediatric Dentistry (2024 revision); MolarPath note Eruption sequence and chronology (built on McDonald and Avery, 11th ed., and Proffit's Contemporary Orthodontics, 7th 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); Al-Qudah and others, Root and canal morphology of third molar teeth, Scientific Reports 2023; Pinto and others, Worldwide Prevalence and Demographic Predictors of Impacted Third Molars, J Clin Med 2024. Figures are usual values, and teeth vary from person to person.