Access Cavity Preparation and Root Canal Morphology
This domain tests whether you can find every canal without destroying tooth structure.
Laws of canal location
- Law of symmetry: the second canal is a mirror image of the first.
- Law of colour change: the floor is darker than the walls at the level of the cementoenamel junction.
- Law of concentricity: chamber walls and canal walls are concentric at the CEJ.
- Law of orifice location: orifices lie at the junction of walls and floor; follow the dentin map with a sharp explorer or ultrasonic tip rather than cutting blindly.
Anatomy that is examined repeatedly
- Maxillary first molar: MB2 in roughly 60-70% of teeth, located 1-2 mm palatal to MB1, usually joining it.
- Mandibular incisors: two canals in about 30-40%.
- Maxillary first premolar: two roots in about half of teeth, with a mesiobuccal and palatal canal.
- Mandibular second molar: C-shaped canals in a few percent, with thin danger zones on the distal wall of the mesial root and the mesial wall of the distal root.
- Vertucci types I to VIII describe canal configurations; type IV (two separate canals to the apex) and type II (two canals joining) are the most commonly tested.
Access principles
Complete roof removal, straight-line access to the apical third, no unsupported cusps left, no uncontrolled troughing into the furcation. Conservative access preserves pericervical dentin but must not compromise canal negotiation.
Worked example
Maxillary first molar with a buccal and palatal canal located. Trough with an ultrasonic tip from MB1 towards the mesial, 1-2 mm palatal, following the groove until the MB2 orifice is entered. Verify with a file and radiograph; if the orifice is not found, do not excavate deeper into the floor.
Traps
Confusing a developmental groove with a canal; over-flaring and perforating the furcation; failing to check chamber depth and perforating the floor.