Mandibular Permanent First Molar

Mandibular Permanent First Molar

• It is the first permanent molar to calcify and erupt.
• Also known as the "six molar" tooth.
• Designated as number "6" in the lower arch.
• Eruption usually occurs around six years of age.
• Functionally, it is the key of occlusion.

Differences in Molars

• Mandibular molars are mesiodistally wider than buccolingually, while maxillary molars are the opposite.
• Mandibular molar crowns tilt lingually over the root, while maxillary molar crowns are centered.
• Mandibular molars have 4 or 5 cusps, while maxillary molars have 3 or 4 cusps.
• Lingual cusps of mandibular molars are mesiolingual and distolingual .
• Mandibular molars have a rhomboidal proximal form, differing from the trapezoid form of maxillary molars.
• The occlusal shape of mandibular molars are pentagonal or rectangular.
• Mandibular molars have 2 roots (1 mesial, 1 distal), unlike maxillary molars which typically have 3 (2 buccal, 1 lingual).

Number of Surfaces and Roots

• Mandibular permanent first molars have five surfaces: buccal, lingual, mesial, distal, and occlusal.
• These molars have two roots.

Buccal Aspect

• The geometric outline is trapezoidal, tilted distally.
• The mesial outline is concave at the cervical ⅓ and convex at the middle and occlusal ⅓.
• The distal outline is straight or convex until the contact area beneath the distal cusp ridge.

Contact Areas

• The mesial contact area is located at the junction of the occlusal and middle thirds.
• This area is centered buccolingually and below the marginal ridge.
• The distal contact area is located in the middle third and is buccal to the center.

Anatomical Landmarks on Buccal Surface

• The buccal surface of the tooth is smooth and convex, with maximum convexity at the cervical third.
• There are two developmental grooves: mesiobuccal (MB) and distobuccal (DB).
• There may be a deep pit at the cervical end of the mesiobuccal groove.
• Cervical ridge is at the cervical 1/3
• A developmental depression runs mesiodistally in the middle third.

Occlusal Outline (Buccal)

• Formed by three cusps: MB, DB, and D.
• The mesiobuccal cusp is the largest.
• The distal cusp is smaller.
• The cervical outline is straight with dipping into the root bifurcation.

Lingual Aspect

• Exhibits lingual convergence.
• Mesial outline is straight or convex until the contact area at the junction of the occlusal and middle 1/3
• Distal outline is convex or straight until the contact area beneath the distal cusp ridge.

Occlusal Outline (Lingual)

• Formed by two lingual cusps: ML and DL of nearly equal width.
• The mesiolingual cusp is usually the widest and highest.
• The cervical outline is irregular with dipping into the root bifurcation.

Anatomical Landmarks (Lingual)

• Lingual Developmental groove

Mesial Outline

• Geometric outline is rhomboid and inclined lingually.
• The mesiolingual cusp is more pointed than the mesiobuccal cusp because it is out of occlusion.
• The cervical line slants occlusally from the buccal to the lingual side.
• Mesial marginal ridge is concave
• The occlusal profile is marked by the mesiobuccal and mesiolingual cusps.
• Cusp height is from longest to shortest: mesiolingual, distolingual, mesiobuccal, distobuccal, and distal cusp.

Buccal and Lingual Outlines

• The buccal outline is convex in the cervical third (cervical ridge), then concave or straight in the middle and occlusal thirds.
• The lingual outline is convex, with its crest of curvature in the middle third.
• The mesial surface is concave in its cervical third, then convex in its middle and occlusal thirds.

Mesial Aspect Landmarks

• The contact area is centered bucco-lingually below the crest of the marginal ridge.
• A deep developmental depression is on the mesial surface of the root.

Distal and Mesial Aspects

• The crown is shorter distally.
• There is a distal tilt.
• The distal marginal ridge (DMR) is U-shaped, shorter, and more cervical than the mesial marginal ridge (MMR).
• The distal contact area is located below the distal cusp.
• The cervical line is usually straight.

Occlusal Aspect

• The geometric outline is hexagonal or pentagonal or rectangular.
• Crown measurements are larger mesiodistally than buccolingually.
• The crown converges lingually and distally.

Elevations and Landmarks

• The occlusal table has 5 cusps with 5 triangular ridges.
• Mesial marginal ridge (MMR) and distal marginal ridge (DMR).
• The MMR may be crossed by an insignificant marginal ridge groove.
• Two less prominent transverse ridges are formed by triangular ridges of mesial and distal cusps.

Depressions on Occlusal Surface

• There is one major central fossa and pit.
• Two minor fossae (mesial and distal triangular fossae + pits).
• Presence of developmental grooves: Y5 pattern.
• The central groove is zigzag in its mesiodistal course.
• Mesiobuccal (MB) groove, distobuccal (DB) groove, and lingual groove are present.

Roots

• The roots are approximately twice the length of the crown.
• The axis of the roots are inclined distally.
• The root trunk measures 3 mm.
• A deep developmental groove extends on the root trunk.
• The root trunk appears longer on the lingual surface due to the occlusal position of cervical line

Root Characteristics

• Roots are widely separated.
• The mesial root is larger than the distal root.
• The mesial root is curved and in line with the mesiobuccal (MB) cusp.
• The distal root is straight.
• The tooth has a deep developmental depression on the mesial surface of the root.

Variations

• The mandibular first molar can exhibit five cusps.
• In 19% of cases, it presents with 4 cusps due to the distal cusp being missing.
• In large molars, the distal cusp may be joined by a sixth cusp, referred to as the 'cusp six' or tuberculum sextum.
• The grooves may be type Y, type + and type X

Pulp Cavity

• The pulp chamber is broad mesiodistally and almost rectangular in shape.
• There is a pulp horn beneath each cusp, accounting for 5 horns.
• There are 3 main root canals.
• The mesial root has 2 root canals, while the distal root has 1 root canal.

Root Canal Cross-Sections

• In cross-section, the mesial root canals are round.
• The distal canal is oval.
• In X-rays, the two mesial root canals may appear overlapping.

Pulp Cavity and Root Canals

• Buccolingually, the pulp chamber is square.
• Mesiodistally, the pulp chamber is rectangular but shorter.
• There is a pulp horn beneath each cusp, resulting in 4 pulp horns.
• The mesial root usually has 2 root canals, while the distal root has 1 root canal.
• At the mid-root section, the mesial root is kidney-shaped with two small canals, and the distal root is oval with one large, rounded canal.

General Variation

• The third molar shows the most variation in size and shape.
• The pulp cavity varies greatly in size and shape.
• The pulp chamber of an 8-year-old is larger than other molars because it is younger by 9-11 years.