[Determination of the apical limits of instrumentation by subtraction using the paralleling radiographic technic. (Study contribution)].

The objective of the present work was verifying the apical position of the file when a technique based on subtraction and parallelism principle is used for determining the actual length of instrumentation. For this purpose, 24 natural teeth, in a total of 51 root canals, were used from patients under treatment in Endodontics Course of the Fundação para o Desenvolvimento Científico e Tecnológico da Odontologia. The teeth were previously radiographed with the parallelism technique. For each canal it was subtracted 3 mm in cases of vital pulp and 1.5 mm in necrotic form the length seen in X-ray. This measure was transferred to the endodontic instrument and with the file in the interior of the canal another X-ray was taken. The distance between the tip of the instrument and the root radiographic apex was measured. The results showed the presence of distortion in the original value with the greatest index to those teeth with necrotic pulp.