The pulp chamber in the coronal part of a tooth consists of a single cavity with projections (pulp horns) into the cusps of the tooth (Fig. 1). With age, there is a reduction in the size of the chamber due to the formation of secondary dentine, which can be either physiological or pathological in origin. Reparative or tertiary dentine may be formed as a response to pulpal irritation and is irregular and less uniform in structure.
The entrances (orifices) to the root canals are to be found on the floor of the pulp chamber, usually below the centre of the cusp tips. In cross-section, the canals are ovoid, having their greatest diameter at the orifice or just below it. In longitudinal section, the canals are broader bucco-lingually than in the mesiodistal plane. The canals taper towards the apex, following the external outline of the root. The narrowest part of the canal is to be found at the apical constriction , which then opens out as the apical foramen and exits to one side between 0.5 and 1.0 mm from the anatomical apex. Deposition of secondary cementum may place the apical foramen as much as 2.0 mm from the anatomical apex. It must be realized, however, that the concept of a single root canal with a single apical foramen is mistaken. The root canal may end in a delta of small canals, and during root canal treatment cleaning techniques should be employed to address this clinical situation.