Location of contact areas on rotary Profile instruments in relationship to the forces developed during mechanical preparation on extracted teeth

The aim of this study was to locate the areas of direct instrument contact with dentine in the root canal system during rotary preparation and to analyse the relationship between these areas and the vertical forces and torque developed during the preparation. Canal preparations were performed by endodontists either with the step-back (SB) or the crown-down (CD) technique. In order to locate the areas of contact, the instruments were coated with two layers of gold by electro-deposition. They were photographed before and after use, and a coding system, based on mm from the instrument tip, was devised to designate areas of gold removal or instrument wear due to friction. To standarise the conditions of instrument manipulation, the teeth were fixed in the Endographe holder, and this device was used to measure vertical forces and torque. The results showed that the first series of instruments used for the CD technique (taper 0.06) left 2 +/- 1 mm of the tip with the gold intact, indicating that these instruments and this step of the CD technique are the safest part of the preparation. For all other instruments (taper 0.04 series), the areas of gold removal involved the 3 mm around the tip and this finding was independent of the order of instrument use and preparation technique (SB or CD). The differences between the two techniques were significant in terms of the mean area of decolouration and the mean force and torque values. For the SB and CD techniques, the contact areas were, respectively, 10 +/- 3 and 7 +/- 2 mm. The forces and torque values were correspondingly higher for SB vs. CD; the mean values were, respectively, 19 and 21 N for vertical forces and 16 10(5) and 13 10(5) Nm for torque. The recorded torque values and the location on the instruments of the areas of contact with dentine during this development of torque i.e. at or near the tip, indicate that great caution should be used with the rotary technique, particularly with the taper 0.04 instruments, regardless of preparation technique.