MAXILLARY FIRST MOLAR

1. GENERAL FEATURES / INTRODUCTION

• Largest tooth in the maxillary arch and the largest of all permanent teeth.
• It is not a succedaneous tooth - it has no deciduous predecessor; it erupts posterior to the second deciduous molar.
• Eruption age: 6 years (hence also called the "6-year molar" or "cornerstone of the dental arch").
• Crown is wider buccolingually than mesiodistally (extra BL dimension ~1 mm).
• It is considered the cornerstone of the dental arches due to its position, function, and strategic role in occlusion.

2. CROWN - GENERAL FEATURES

• Crown is relatively short but broad both mesiodistally and buccolingually, giving the occlusal surface generous dimensions.
• Has four well-developed functional cusps + one supplemental cusp (cusp of Carabelli) of little practical use.
• Four cusps: Mesiobuccal (MB), Distobuccal (DB), Mesiolingual (ML), Distolingual (DL).
• The mesiolingual cusp is the largest; followed by mesiobuccal, distolingual, and distobuccal.
• Crown is wider mesially than distally and wider lingually than buccally.

3. CROWN - BUCCAL ASPECT

• Two buccal cusps: mesiobuccal and distobuccal, separated by a buccal developmental groove.
• Mesiobuccal cusp is broader and more prominent than distobuccal cusp.
• Buccal surface shows a buccal developmental groove running from the occlusal surface toward the cervix.
• Cervical line is fairly regular, curving slightly toward the root.
• Crown is wider at the cervix than at the occlusal.

4. CROWN - LINGUAL ASPECT

• Two lingual cusps: mesiolingual (largest cusp of the tooth) and distolingual.
• Cusp of Carabelli may be present on the mesiolingual cusp - it is a supplemental cusp on the lingual surface of the mesiolingual cusp; considered a trait of the tooth (5th cusp).
• Lingual developmental groove separates the two lingual cusps.
• The mesiolingual cusp tip extends lingually beyond the lingual groove of the crown.
• The distolingual cusp is smooth and rounded when viewed from the lingual aspect and describes the arc of an ellipse.

5. CROWN - MESIAL ASPECT

• The outline from the mesial is roughly trapezoidal.
• The mesiobuccal and mesiolingual cusps are visible; mesiolingual cusp is clearly taller.
• The cervical line curves slightly occlusally on the mesial surface (more than on the distal).
• Mesial contact area is at the junction of occlusal and middle thirds.
• Mesial marginal ridge is well defined and connects the mesial slopes of the MB and ML cusps.

6. CROWN - DISTAL ASPECT

• The distal surface is smoother and more convex than the mesial.
• The distobuccal cusp is visible from the distal.
• Distal marginal ridge is less prominent than the mesial marginal ridge.
• Distal contact area is at the middle third.
• The distolingual cusp outline shows a slight concavity when viewed from the distal.

7. CROWN - OCCLUSAL ASPECT (Most Important for Exam)

• Occlusal outline is roughly rhomboidal (4-sided).
• Four major fossae:
  • Central fossa (largest, roughly triangular, mesial to oblique ridge)
  • Distal fossa (triangular, distal to oblique ridge)
  • Mesial triangular fossa (near mesial marginal ridge)
  • Distal triangular fossa (near distal marginal ridge)
• Oblique ridge - characteristic feature of maxillary molars: runs from the mesiolingual cusp to the distobuccal cusp obliquely across the occlusal surface; divides the central and distal fossae.
• Central developmental groove runs from the central pit mesially toward the mesial triangular fossa.
• Distal oblique groove - runs from the distal fossa toward the lingual, connecting with the lingual developmental groove at the cusp ridges; it is called distal oblique groove and is the deepest groove on the occlusal surface.
• Buccal developmental groove radiates buccally at the bottom of the buccal sulcus from the central fossa.
• Central fossa has a sulcate developmental pit (central pit) at its approximate center.
• Mesiobuccal primary cusp triangle - the characteristic triangular figure made by tracing the cusp outlines of the three mesial cusps, the mesial marginal ridge, and the oblique ridge - representative of all maxillary molars.

8. ROOTS

• Three roots: Mesiobuccal (MB), Distobuccal (DB), and Lingual (Palatal).
• Lingual root is the longest and largest of the three.
• Mesiobuccal root is the broadest buccolingually; often curved.
• Distobuccal root is the smallest and most round in cross section.
• Roots trifurcate from a common broad base above the crown.
• The trifurcation/furcation is located well up on the root trunk - well separated from the CEJ.
• From the buccal view, the lingual root tip can be seen between the two buccal roots as it extends lingually.
• Root trunk is broader at the base in all directions.

9. PULP CAVITY

• The pulp chamber is broad and low, conforming to the shape of the crown.
• Four pulp horns corresponding to the four main cusps:
  • Mesiobuccal pulp horn (largest)
  • Mesiolingual pulp horn
  • Distobuccal pulp horn
  • Distolingual pulp horn
• Three root canals (corresponding to 3 roots):
  1. Mesiobuccal canal - may have 2 canals (MB1 and MB2); MB2 is a common anatomical variation, clinically significant in endodontics.
  2. Distobuccal canal - single, relatively straight.
  3. Lingual (Palatal) canal - widest and most accessible; usually single, relatively straight and round.
• The pulp chamber floor is roughly flat with canal orifices at the corners.
• Canal orifices form a rhomboidal/trapezoidal pattern on the chamber floor.
• Mesiobuccal root has the most complex canal anatomy of any root in the maxillary arch.

10. BLOOD SUPPLY & NERVE SUPPLY

• Arterial supply: Branches of the posterior superior alveolar artery (from maxillary artery) supply the molar region; the infraorbital artery (via anterior/middle superior alveolar branches) may also contribute.
• Venous drainage: Via the posterior superior alveolar veins draining into the pterygoid venous plexus.
• Nerve supply:
  • Posterior superior alveolar nerve (PSA) - supplies the mesiobuccal and distobuccal roots and the distobuccal portion of the crown.
  • Middle superior alveolar nerve (MSA) - when present, contributes to the mesiobuccal root supply.
  • Anterior superior alveolar nerve - may contribute a minor portion.
  • The palatal root and lingual aspect of the crown are supplied by the greater palatine nerve (branch of V2).
• Lymphatic drainage: Submandibular and deep cervical lymph nodes.
• Clinical note: The PSA nerve block is the routine injection for anesthetizing the maxillary first molar, but the mesiobuccal root often requires additional infiltration or MSA block due to variable innervation.

QUICK REFERENCE - KEY DISTINGUISHING FEATURES