Residual thickness of root in first maxillary premolars with post space preparation

It has been suggested that, to ensure tooth strength, a minimum of 1 mm of root wall thickness should be left after post preparations. The purpose of this study was to determine the instrument diameter that will not affect this measurement in maxillary first premolars. Post preparations were made in 106 teeth with one and two root canals at a working depth equal to the anatomical crown length, with 0.70, 0.90, 1.10, 1.30, 1.50, and 1.70 mm diameter instruments. Sections were cut perpendicular to the long axis of the tooth at the cervical and apical ends of each preparation, and the minimum width of residual root was measured on each wall, at both sites. A binocular microscope with a micrometer eyepiece was used. At the cervical level of the preparation, no group showed a wall thickness < 1 mm. Data for the apical sections was statistically analyzed, and the corresponding confidence limits were calculated with 95% confidence on the mean. The results show that the minimum residual thickness was only preserved when 0.70 mm instruments were used in single-canal roots and when 1.10 mm or smaller instruments were used for two-canal roots. This seemingly anomalous result occurs because fluting on both the mesial and distal sides of the root impinge on single canals, whereas dual canals are buccally or lingually displaced to an area of thicker root diameter.