That means, at the apical constriction the ratio tends towards a small value (Kobayashi & Suda 1994), however, this phenomenon is related to the morphology of the constriction. Lack of a constriction because of open apices (Hu¨lsmann & Pieper 1989, Goldberg et al. 2002) or an impenetrable canal (Rivera & Seraji 1993, Ibarrola et al. 1999) have been reported as an impediment to determine the position of the canal terminus (Oishi et al. 2002).