As previously discussed, lateral canals form channels of communication between the main body of the root canal and the periodontal ligament space. They arise anywhere along its length, at right angles to the main canal. The term accessory is usually reserved for the small canals found in the apical few millimetres and forming the apical delta (Fig. 2).
Both lateral and accessory canals develop due to a break in Hertwig s epithelial root sheath or, during development, the sheath grows around existing blood vessels. Their significance lies in their relatively high prevalence, Kasahara et al.finding 60% of central incisors with accessory canals, and 45% with apical foramina distant from the actual tooth apex. Kramer found that the diameter of some lateral canals was often wider than the apical constriction.
Lateral canals are impossible to instrument and can only be cleaned by effective irrigation with a suitable antimicrobial solution. Consequently, sealing such canals is only moderately successful.
The following descriptions of normal canal morphology and access cavities are illustrated in Figure 3 (maxilliary) and Figure 4 (mandibular).