Cleaning and Shaping: Instrumentation and Irrigation
This domain tests the balance between adequate disinfection and preservation of canal anatomy.
Working length and glide path
Electronic apex locator plus a radiograph with the file in place. The apical constriction is the reference, generally 0.5-1 mm short of the radiographic apex. Establish a reproducible glide path with a size 10-15 K-file before introducing rotary instruments.
Instrumentation
Crown-down or apical-coronary sequences with NiTi rotary or reciprocating systems. Torque and speed matter: excessive torque on a curved canal causes torsional fracture; excessive rotation in a sharp curve causes cyclic fatigue fracture. Reuse, hypochlorite corrosion and canal curvature increase separation risk. Preparation should preserve curvature, avoid ledging, transportation, zipping and apical perforation, and enlarge the apical diameter beyond the first file that binds (often at least size 25-30 in narrow canals).
Irrigation
Hypochlorite is the main antimicrobial and tissue-dissolving agent; volume, concentration, temperature and refreshment matter more than the label. Activation with ultrasonic or sonic devices improves penetration into fins and isthmuses. EDTA 17% for 1 minute removes smear layer; a final hypochlorite rinse follows EDTA to limit dentin erosion. Do not mix hypochlorite and chlorhexidine.
Worked example
A curved mesiobuccal canal in a mandibular molar. Negotiate to the apex with a size 10 file, create a glide path, then use a small-taper rotary file with light pressure and frequent recapitulation. If resistance is felt, do not advance: irrigate, recapitulate, and move to a smaller or more flexible instrument.
Traps
Forcing a file past resistance; skipping patency and irrigation between files; relying on a single irrigant; forgetting that a smooth, over-flared canal may be a transported one.