however, the capacitive characteristic of the impedance starts to appear. The influence of the capacitance on the overall impedance is proportional to the frequency of the measurement as shown in equation 4. At high frequencies (fH) the overall impedance value will be much lower than at low frequency (fL). 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).