The effect of instrument type and preflaring on apical file size determination

The aim of this study was to investigate the effect of instrument type (K-files and Lightspeed (LS) instruments) and the impact of preflaring on the first file size that binds at working length (WL) in a range of canal types of varying sizes and curvatures. One hundred and twenty-one canals from 60 extracted intact human maxillary and mandibular premolars and molars were utilized. After standard access cavities, a size 6 K-file was inserted into each canal until the tip of the file was visible outside the apical foramen. WL were set 0.5 mm short of these measurements. Each canal was sized using consecutively larger K-files and hand-held LS instruments until one bound at WL. The file sizes were recorded. The same procedures were repeated after coronal and middle third flaring using Profile rotary instruments. Statistical analysis (univariate analysis of variance: ANOVA) showed that overall, the estimate of mean apical diameter with the LS instrument was larger than with the K-file by 9.4 x 10(-2) mm (P < 0.001; 95% confidence interval: 8.7 x 10(-2), 10.2 x 10(-2)), i.e. almost 2 ISO file sizes larger. Flaring of the cervical and middle thirds of the canal had an impact on apical sizing by both types of instruments, with an average increase of 5.3 x 10(-2) mm (P < 0.001; 95% confidence interval: 4.5 x 10(-2), 6.0 x 10(-2)), i.e. approximately 1 ISO file size larger. Preflaring and use of hand-held LS instruments resulted in an increase in the instrument size that bound at WL. If an operator wishes to determine an accurate master apical file size, canal orifice enlargement should be performed first before the placement of the assessment file.