Lateral Canals

Accessory (lateral) canals branch from the main root canal, with diameters ranging from over 100 μm to a common minimum of 10 μm [107]. Such narrow oriÞces create a surface tension barrier that does not allow adequate mixing between the irrigant and the liquid within the canal. The narrowing of the root canal apically (toward the root) poses a similar barrier. Any ßuid ßowing down the accessory canals from the root canal will be laminar ßow; turbulent ßow will be not be achievable due to the very low Reynolds numbers inherent at such small ÒpipeÓ diameters, where edge effects and viscosity become the major factors affecting ßuid dynamics [76, 108]. The lateral canals may contain bacteria/bacterial bioÞlm which may cause lateral, periradicular bone lesions. Histological sections of extracted teeth have indicated that the lateral canals are not completely cleaned and, after root Þlling, they often still contained vital or necrotic pulp tissue and bacteria [109]. As long as there is no method to completely and predictably clean and disinfect lateral canals, microbes in the lateral canals remain one possible reason for posttreatment endodontic disease.

The small number of studies on irrigant action in lateral or accessory canals is probably due to the difÞculty of such studies, as the accessory canal position and status before treatment are difÞcult to determine. Consequently, there is a need for standardized models that simulate accessory

canals. Models of artiÞcially created lateral canals in cleared teeth or an epoxy resin have recently been developed to evaluate efÞcacy of irrigant penetration [88, 110].

Smear Layer

Use of any kind of metallic instrument in the root canal results in a smear layer wherever the instrument comes into contact with the root canal wall [111, 112] (Fig. 4.5). Smear layer is a 1Ð2-μm-thick, amorphous, irregular, and granular layer with a deeper part that can penetrate up to 40 μm into the dentinal tubules. The penetration is hypothesized to be the result of capillary action and adhesive forces between the dentinal tubules and the smear layer [113, 114]. Others have estimated the layer to be up to 5 μm thick with inorganic particles of 0.05Ð0.15 μm diameter [115Ð117]. Essentially, the smear layer is a complex mixture of inorganic and organic particles, proteins, pulp tissue, blood cells, and, in infected canals, bacteria and fungi [118, 119]. As the irrigation needle is likely to follow the path created by the endodontic instruments, delivery of irrigants to areas covered by the smear layer is usually unproblematic except perhaps in the most apical canal. Irrigation with the needle introduced only to the coronal or middle parts of the root canal (needle too big in size or apical canal not sufÞciently enlarged) will result in incomplete removal of the smear layer in the apical root canal.