TL;DR

Key Facts

What Is It?

The maxillary first molar is the sixth tooth from the midline in the upper arch and the largest tooth in the maxillary dentition. Together with its mandibular counterpart, it erupts around age 6–7 years as the first permanent molar in the mouth, earning the shared nickname “six-year molar.” Its three roots — mesiobuccal, distobuccal, and palatal — span a wide footprint in the posterior maxillary alveolus, often in close proximity to the maxillary sinus, a relationship that has significant clinical implications for endodontic, periodontic, and surgical procedures.

The crown of the maxillary first molar is distinguished by the oblique ridge, a diagonal eminence that crosses the occlusal surface from the mesiopalatal (MP) cusp to the distobuccal (DB) cusp. This ridge is unique to maxillary molars and is one of the most reliable features for identifying maxillary molar teeth. Additionally, an accessory fifth cusp — the cusp of Carabelli — is found on the lingual surface of the mesiopalatal cusp in a substantial percentage of individuals (reported in 10–60 % depending on definition and population). While typically non-functional, the cusp of Carabelli is a classic dental anatomy landmark.

Of all the teeth in the mouth, the maxillary first molar generates the most endodontic research publications, and for good reason: its mesiobuccal root contains a second canal (MB2) with reported frequency as high as 96 % in CBCT studies, yet this canal is identified and treated in only a fraction of those cases by practitioners who do not actively search for it. The consequences of a missed MB2 — persistent infection, failed root canal treatment, and eventual tooth loss — make it one of the most clinically important anatomical facts in all of dentistry.

Why It Matters (Clinical + Exam Context)

The maxillary first molar is clinically significant across endodontics, periodontics, operative dentistry, prosthodontics, and oral surgery. Its early eruption, complex anatomy, and proximity to critical structures mean it demands mastery across virtually every dental discipline.

Clinical Relevance

• MB2 canal — the most missed canal in dentistry: The second canal in the mesiobuccal root of the maxillary first molar is frequently missed because its orifice is positioned palatally and slightly mesially to the MB1 orifice — often hidden beneath a dentinal shelf. Studies consistently show that when clinicians actively look for MB2 (using magnification and illumination), they find it far more often than when they do not. A four-canal treatment must be the goal for every maxillary first molar endodontic procedure unless CBCT definitively confirms MB1 alone.
• Maxillary sinus proximity: The roots of the maxillary first molar — particularly the palatal root — frequently project into the maxillary sinus or are separated from it by only a thin membrane. Periapical pathology can push into the sinus, and root canal irrigants or fractured instruments can accidentally enter the sinus. Oroantral fistulae can form after extraction if the sinus membrane is perforated.
• Periodontal furcation — three-way complexity: With three roots, the maxillary first molar has a trifurcation rather than a bifurcation. Class II or Class III furcation involvement in any of the three furcal areas (buccal, mesial, and distal-palatal) dramatically worsens periodontal prognosis and complicates hygiene maintenance and surgical management.

Crown Morphology

The crown of the maxillary first molar is the widest mesiodistally and the largest volumetrically in the maxillary arch. Its occlusal surface features four main cusps, a defining oblique ridge, central and secondary fossae, and the potentially present cusp of Carabelli on the mesiopalatal cusp.

The Four Main Cusps

The four cusps are arranged in two buccopalatal rows. The buccal row contains the mesiobuccal (MB) and distobuccal (DB) cusps; the palatal row contains the mesiopalatal (MP) and distopalatal (DP) cusps. Of the four, the mesiopalatal cusp is the largest — it is the primary functional cusp occluding in the central fossa of the mandibular first molar. The distobuccal cusp is the smallest of the four main cusps.

The Oblique Ridge

The oblique ridge is the hallmark of maxillary molar occlusal anatomy. It runs diagonally from the distal cusp ridge of the MP cusp to the mesial cusp ridge of the DB cusp, crossing the central groove at an oblique angle. This ridge divides the occlusal surface into a mesial and a distal portion, creating separate mesial and distal fossae connected by the central groove that passes under the ridge. The oblique ridge is exclusive to maxillary molars and is absent in mandibular molars.

The Cusp of Carabelli

On the mesiopalatal cusp, on the fifth lingual surface of the crown, a range of accessory cusp morphologies may be present — from a simple groove or pit (the most common expression) to a fully developed fifth cusp. This is the cusp of Carabelli. When fully expressed, it creates a distinctive “fifth cusp” appearance on the palatal surface of the MP cusp and may need to be accounted for when designing crown preparations or indirect restorations.

Root Morphology

The maxillary first molar has three roots: the mesiobuccal (MB), the distobuccal (DB), and the palatal. The three roots diverge widely from the root trunk, which is approximately 3–4 mm in height. The wide divergence of the roots provides excellent anchorage but also creates complex furcation anatomy.

The Mesiobuccal Root and MB2 Canal

The mesiobuccal root is the most complex of the three roots. It is broad buccolingually, flattened mesiodistally, and frequently contains two canals — the MB1 (main, buccal canal) and the MB2 (second, palatal canal). The MB2 orifice is located approximately 1–3 mm palatal to the MB1 orifice, often covered by a dentinal shelf that must be removed to expose it. The two canals may join at the apex (Vertucci Type II or IV) or remain separate (Vertucci Type IV — two separate foramina).

Distobuccal and Palatal Roots

The distobuccal root is the shortest of the three, straight, and typically contains a single canal (Vertucci Type I). The palatal root is the longest (~14–16 mm), broadest, and most divergent — it angles palatally and slightly distally from the crown. The palatal root canal is large, round to ovoid, and usually single. Despite its large size, the palatal root may still curve in the apical third, which can complicate endodontic instrumentation.

Clinical Considerations

• Magnification for endodontics: Finding MB2 reliably requires dental loupes (minimum 2.5–3.5×) or, ideally, an operating microscope. The MB2 orifice is too easily obscured by the dentinal shelf to be found consistently with the naked eye or suboptimal lighting. The routine use of magnification and illumination for maxillary first molar root canal treatment is considered the standard of care in modern endodontology.
• Maxillary sinus assessment before extraction or surgery: Before extracting a maxillary first molar or performing periradicular surgery, the relationship of the roots to the maxillary sinus must be assessed with a periapical radiograph or CBCT. If roots protrude significantly into the sinus, extraction requires careful elevation and may result in oroantral communication — which must be diagnosed and managed immediately to prevent oroantral fistula formation.
• Oblique ridge in preparation design: When preparing a maxillary first molar for an inlay or onlay, the oblique ridge should be preserved wherever possible, as it contributes substantially to crown strength. When the preparation must cross the oblique ridge (e.g., Class II MO-DO preparations that involve both mesial and distal boxes), the remaining tooth structure is significantly weakened and full cuspal coverage should be considered.
• Implant placement after loss: When a maxillary first molar is replaced with an implant, sinus augmentation (lateral window or crestal approach) is frequently required due to sinus pneumatisation following tooth loss. The loss of the molar roots’ functional loading stimulus causes the sinus floor to expand inferiorly over time, reducing available bone height for implant placement.

Common Mistakes & Misconceptions

• Misconception: “The maxillary first molar has three canals — one per root.”
  Correction: The maxillary first molar most commonly has four canals: MB1, MB2 (both in the mesiobuccal root), DB1 (distobuccal root), and the palatal canal. The MB2 canal is present in the majority of teeth and must be actively sought.
• Misconception: “The cusp of Carabelli is always a fully formed fifth cusp.”
  Correction: The cusp of Carabelli represents a spectrum of morphological expression, ranging from a simple groove, pit, or tubercle to a full supplementary cusp. Most expressions are minor and may only be detected by careful examination; a fully formed fifth cusp is the least common form.
• Misconception: “The oblique ridge on the maxillary first molar is the same as the transverse ridge on the mandibular first premolar.”
  Correction: Both are ridges crossing the occlusal surface, but they differ in location, direction, and significance. The oblique ridge of the maxillary molar runs diagonally (mesiopalatal cusp to distobuccal cusp) and is a feature of maxillary molars. The transverse ridge of the mandibular first premolar runs buccolingually and is a distinguishing feature of that premolar specifically.
• Misconception: “If a maxillary first molar root canal treatment is asymptomatic and radiographically healed, all canals were treated.”
  Correction: Periapical healing can occur even with an untreated MB2 canal, if the bacterial load in the untreated canal is low. These cases may later reactivate. The absence of symptoms is not proof that all canals were identified and treated — only systematic examination during treatment can confirm this.

Related Topics

References & Sources

1. Woelfel, J.B. & Scheid, R.C. (2012). Dental Anatomy: Its Relevance to Dentistry, 8th ed. Lippincott Williams & Wilkins.
2. Nelson, S.J. (2020). Wheeler’s Dental Anatomy, Physiology, and Occlusion, 10th ed. Elsevier.
3. Vertucci, F.J. (1984). Root canal anatomy of the human permanent teeth. Oral Surgery, Oral Medicine, Oral Pathology, 58(5), 589–599.
4. Peiris, R., Takahashi, M., Sasaki, K., & Kanazawa, E. (2008). Root and canal morphology of permanent maxillary molars in a Japanese population. Odontology, 96(1), 36–43.
5. Weine, F.S., Healey, H.J., Gerstein, H., & Evanson, L. (1969). Canal configuration in the mesiobuccal root of the maxillary first molar. Oral Surgery, Oral Medicine, Oral Pathology, 28(3), 419–425.
6. Kim, S.G. (1998). Endodontic surgery using the operating microscope. Dental Clinics of North America, 41(3), 487–506.

Summary

The maxillary first molar is the largest and most anatomically complex tooth in the upper arch. Its three roots, four main cusps, oblique ridge, cusp of Carabelli, and — most critically — the MB2 canal in its mesiobuccal root make it a constant focus of dental education and research. The oblique ridge and cusp of Carabelli allow reliable identification of maxillary molar teeth. The proximity of the palatal root to the maxillary sinus demands careful pre-surgical assessment. Above all, the high incidence and clinical significance of the MB2 canal demands that every practitioner treating this tooth approach its endodontic anatomy with systematic vigilance, adequate magnification, and a commitment to finding and treating all canals before considering the procedure complete.

About the Author

Dr. Andries Smith

Founder, Dental Panda

Dr. Andries Smith founded Dental Panda in 2020. As an immigrant to the United States, he had to take the INBDE exam, even though he was practicing dentistry for over 10 years. This revealed an opportunity. Andries noticed that INBDE prep course companies were putting profit over students. With his expertise and experience in dentistry, he created free dental wiki resources for students and the general public to have access to.