Vertical fracture resistance of roots after ultrasonic removal of fractured instruments

Abstract

To investigate ex vivo root resistance to vertical fracture after fractured instruments were ultrasonically removed from different locations in the root canal. Fifty-three canine roots were weighed and divided into four groups. Eight roots served as a control group in which canals were instrumented to a size F5-ProTaper instrument. In the experimental groups, F5-ProTaper fragments were fractured in the coronal, middle and apical one-thirds, and then removed ultrasonically. The time required for removal was recorded. Roots were reweighed, and canals were shaped to a size F5-ProTaper and filled with GuttaFlow. After incubation, roots underwent a vertical fracture test in which the force at fracture was recorded. The difference in root mass before and after treatment (fractured file removal or canal preparation) was calculated. Data were analysed using the Kruskal-Wallis, Mann-Whitney post-hoc and regression tests at P < 0.05. The highest root-mass loss was recorded when fragments were removed from the apical one-third (46.04 mg) followed by the middle and coronal (27.7 and 13.5 mg, respectively); these differences were significant (P < 0.05). There were significant differences in the force required for vertical fracture amongst the experimental groups (P < 0.05) with the lowest mean force recorded in the apical-third group (107.1 N) followed by the middle and coronal (152.6 and 283.3 N, respectively). The highest mean force was recorded in the control group (301.5 N) which was not significantly different from that in the coronal group (P = 1.00). A negative exponential correlation (r = 0.669) existed between the root-mass loss and the force required to fracture the roots. Whilst removal of fractured instruments from the coronal one-third of the root canal can be considered as a safe procedure, removal from deeper locations renders the root less resistant to vertical fracture.