# Posterior Tooth Morphology > Posterior tooth morphology in BDS Oral Anatomy: premolar and molar cusps, roots, canals, oblique ridge, cusp of Carabelli and eruption chronology. - Canonical URL: https://prepelephant.com/topics/bds/oral-anatomy/tooth-morphology-posterior - Exam / course: BDS · Subject: Oral Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Posterior Tooth Morphology", PrepElephant, https://prepelephant.com/topics/bds/oral-anatomy/tooth-morphology-posterior ## Direct answer From the first premolar backwards, teeth are built for grinding: broad occlusal tables carrying cusps, ridges, fossae and grooves, with multiple roots that anchor them against heavy masticatory loads. The maxillary first premolar is the only premolar that is usually bifurcated; the mandibular first molar is the largest tooth of the mandibular arch with five cusps; and the maxillary first molar is the largest maxillary tooth, its four cusps plus a cusp of Carabelli seated on three diverging roots. Canal counts follow root anatomy loosely and then betray it — the maxillary first molar hides a second mesiobuccal canal in about 70% of teeth. Chronology anchors everything: the first permanent molar erupts at six years, second molars near 12, third molars anywhere from 17 to 21. ## What you must remember - **Maxillary first premolar:** two roots (buccal and palatal) in most teeth, two canals in the great majority; carries a mesial developmental depression on the crown root continuation — unique among teeth and a favourite identification point. - **Maxillary second premolar:** single root, one canal in about 90% of teeth; the mesial surface lacks the first premolar's concavity. - **Mandibular premolars:** the first has a dominant buccal cusp with a tiny lingual cusp; the second commonly shows three cusps (one buccal, two lingual) with a Y-groove pattern. - **Maxillary first molar:** four cusps plus cusp of Carabelli (on the mesiolingual cusp, in about half to two-thirds of teeth); three roots — two buccal, one palatal; usually four canals because the second mesiobuccal (MB2) canal is present in about 70% of pooled anatomical studies. - **Mandibular first molar:** largest mandibular tooth, five cusps (three buccal, two lingual), two roots, typically three canals — two mesial, one distal; the first permanent tooth to erupt at six years ("six-year molar"). - **Maxillary molars:** oblique ridge running from the mesiolingual to the distobuccal cusp is a maxillary-molar signature; second molar crowns are more symmetric, roots more fused and distally curved. - **Chronology (permanent):** first premolars 10-11 years, second premolars 11-12, first molars 6, second molars 11-13, third molars 17-21 — the sequence, not the exact month, is what vivas test. - **Primary molars:** the primary mandibular second molar mimics a five-cusp permanent first molar; the primary maxillary first molar is the most oddly shaped tooth in the primary dentition. ## Reading an unfamiliar posterior tooth Start with cusp count on the occlusal table, because that single observation splits the possibilities fast. Two cusps means a premolar; three cusps means either a mandibular second premolar (two cusps lingually) or, with all cusps large and the table square, a maxillary second molar reduced to four cusps. Five cusps with a Y groove pattern is almost always a mandibular first molar. Next feel the roots: three roots, with one clearly palatal, is a maxillary molar; two robust mesial-distal roots under a five-cusp crown is the mandibular first molar; two slim roots pressed buccopalatally under a two-cusped crown is the maxillary first premolar. Then check side using the relative cusp heights — in mandibular molars the buccal cusps are pointed and the lingual blunt — and the groove pattern, because the buccal groove of a mandibular molar notches the buccal surface at the mesial two-thirds. Finally age the tooth by attrition and root closure, since a first molar extracted from a teenager still shows flared, incomplete apices. ## How the exam frames it BDS practicals love the "identify and justify" format, and the justifications that score are the mesial depression of the maxillary first premolar, the oblique ridge of maxillary molars, the cusp of Carabelli and the five-cusp arrangement of the mandibular first molar. Theory papers recycle three short-note classics: the oblique ridge, the cusp of Carabelli (state its position on the mesiolingual cusp and its frequency), and the root canal configuration of the maxillary first molar, where quoting the roughly 70% MB2 prevalence — higher on CBCT than on plain films — separates a textbook answer from an updated one. ## Frequently asked questions ### Which tooth is the largest in the maxillary arch? The maxillary first permanent molar, with four functional cusps plus the cusp of Carabelli and three diverging roots. ### Why does the maxillary first premolar often need two access canals? It has two roots or a two-canal single root in the great majority of teeth, so a single round access misses the palatal canal. ### What is the oblique ridge and where is it found? A raised enamel ridge running from the mesiolingual cusp to the distobuccal cusp, characteristic of maxillary molars and absent in mandibular molars. ### Which is the first permanent tooth to erupt and which is the largest mandibular tooth? Both answers are the same tooth: the mandibular first molar, erupting at about six years, which is why it is called the six-year molar. ### How commonly is an MB2 canal present in the maxillary first molar? Pooled anatomical studies report it in roughly 70% of teeth, with CBCT-based series reporting even higher detection than plain radiographs.