Purpose: to locate the areas of direct instrument contact with dentine in the root canal system during rotary preparation & to analyze the relationship between these areas & the vertical forces & torque developed during the preparation.
N= 40 freshly extracted mandibular incisors with straight canals were used in this study.
•Canal preparations were performed by 5 endodontists either with the step-back (SB) or the crown-down (CD) technique. ( 8 teeth per operator. 4 with SB & 4 with CD)
•In order to locate the areas of contact, the instruments were coated with two layers of gold by electro-deposition.
•They were photographed before & after use, & 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 standardize the conditions of instrument manipulation, the teeth were fixed in the Endographe holder, & this device was used to measure vertical forces and torque.
Most highlighted Results:
- The first series of instruments used for the CD technique (taper 0.06) left 21 mm of the tip with the gold intact – these instruments & 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 – 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, the mean force & torque values.
- The size of the contact areas constituted a significant difference between SB and CD, with the SB technique involving a greater contact area.
- The forces and torque values were correspondingly higher for SB vs. CD.
The recorded torque values and the location on the instruments of the areas of contact with dentine during this development of torque ie 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