Introduction
Molars have the most complex canal systems: several roots, easily missed extra canals, and fused or C-shaped variants. This note covers their root and canal anatomy and the rules for finding orifices on the pulp chamber floor. A missed canal is a main cause of failed molar treatment.
Pulp chamber floor and the laws of orifice location
In molars the roof of the pulp chamber lies at about the level of the cementoenamel junction (CEJ) and the floor below it, over the furcation. The darker floor carries developmental (root fusion) lines running between the orifices. Krasner and Rankow's laws (the access outline itself is topic 38):
- Centrality: the chamber floor is always in the centre of the tooth at the level of the CEJ.
- Concentricity: the chamber walls are concentric with the outer surface of the tooth at the CEJ.
- CEJ: the most consistent landmark for locating the chamber ; cusps may be worn or restored.
- Colour change: the floor is always darker than the walls; calcifications and reparative dentin are lighter.
- Symmetry (except maxillary molars): orifices are equidistant from a mesiodistal line through the centre of the floor and lie on a line perpendicular to it.
- Orifice location: at the junction of floor and walls, at its angles, and at the ends of the developmental lines.
Orifices lie where the dark floor meets the lighter walls – at its angles and at the ends of the dark lines. Follow the lines with an endodontic explorer (DG-16).