Apical fit of initial K-files in maxillary molars assessed by micro-computed tomography

To two- and three-dimensionally assess the fit of the first K-file binding at working length after a crown-down canal procedure.

Twelve maxillary molars with fully developed roots and four separate root canals were selected. Canals were pre-flared using ProFile 0.04 instruments to three quarters of estimated working length (WL). WL was electronically determined using an ISO 06 K-file. Progressively larger K-files were inserted passively to WL. The first binding file was termed initial apical file (IAF). On micro-computed tomography (microCT) scans, cross-sectional areas of IAFs and canals and the largest and smallest root canal diameters were measured 1 mm from instrument tips. Volumes of the apical 2 mm of the instruments and corresponding root canal sections were calculated, and file binding was assessed.

IAF sizes ranged from ISO-size 0.08-0.30 and were lowest in second mesiobuccal and highest in palatal canals. Files bound in the apical 2 mm in 96% of the canals. The mean canal area filled by the instruments 1 mm from the tip was below 40% in all canal types, the filled volume below 50% (anova, P > 0.05). The mean ratio (+/-SD) between largest and smallest canal diameter was 3.0 +/- 2.6 for first mesiobuccal, 3.3 +/- 2.6 for second mesiobuccal, 4.1 +/- 3.5 for distobuccal and 1.6 +/- 0.4 for palatal canals, indicating oval to flattened cross-sections.

Two- and three-dimensional analysis of microCT scans revealed that whilst IAFs bound in the apical area, their fit was poor, because their shape does not correspond to maxillary molar canal anatomy.