MandibularTrapezoidal

Mandibular First Molar

Mandibular First Molar - Axial View showing trapezoidal access with MB, ML, and distal canal orifices

Average Length21.0 mm

Root Development9-10 years

Universal #19, 30

FDI #36, 46

Access Cavity Design

ShapeTrapezoidal/Rectangular

OrientationBase faces mesial, shorter side faces distal

Entry PointMesial two-thirds of occlusal surface

Do Not Invade

Mesial marginal ridge

Canal Anatomy

Configurations

2 roots, 3 canals (MB, ML, D)~67%

2 roots, 4 canals (MB, ML, DB, DL)~28-30%

3 roots (Radix Entomolaris)3-6% globally; 5-30% in East Asian populations

Canal Positions

MBMesiobuccal

Slightly buccal to mesiobuccal cusp tip

MLMesiolingual

Slightly lingual, connected to MB by isthmus

DDistal

Central when single (oval), or DB + DL when two

MMMiddle MesialUp to 46.2% with magnification

Between MB and ML, often below pulp floor level

RERadix Entomolaris3-6% globally; 5-30% in East Asian populations

Extra distolingual root with sharp buccal curvature

Danger Zones

⚠Mesial furcation perforation
⚠Stay within mesial two-thirds of occlusal surface

Clinical Tips

⚠️

Warning

Access shape is TRAPEZOIDAL/RECTANGULAR, NOT triangular

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Technique

Middle mesial canal: Use microscope + trough isthmus between MB and ML

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Tip

Microscope increases MMC detection from 6.6% to 46.2%

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Technique

For Radix Entomolaris: Extend access distolingually, use flexible NiTi files

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Technique

SLOB technique + 20-30° mesial or distal angulation reveals superimposed RE

Anatomical Variations

Middle Mesial Canal (MMC)

Up to 46.2% with magnification

Between MB and ML canals, often only accessible below pulp floor

Access Modification: Trough along developmental groove connecting mesial canals

Radix Entomolaris

3-6% globally; 5-30% in East Asian

Extra distolingual root with sharp buccal curvature and apical 'kick'

Access Modification: Extend distolingually, trapezoidal shape with distal base

Ethnic Variation

East Asian (Chinese, Korean, Eskimo)5-30%

Caucasian, African, Indian<5%