Introduction
Working length (WL) is measured after access and orifice enlargement and sets how deep every file, irrigation needle and the root filling may go. This topic covers the apical end point, the reference point and the measuring methods. An error here is carried through the whole treatment.
Definition and apical end point
Working length is the distance from a coronal reference point to the point at which canal preparation and obturation should end.
Three landmarks lie at the root end (anatomy in topic 34):
- Radiographic apex – the root tip as seen on the radiograph.
- Apical foramen (major diameter) – where the canal leaves the root, often beside the apex rather than at it, so a file that reaches the radiographic apex may already be through the foramen.
- Apical constriction (minor diameter) – the narrowest part of the canal, about 0.5–1 mm coronal to the foramen, near the cementodentinal junction.
Preparation and filling should end at the apical constriction, usually taken as 0.5–1 mm short of the radiographic apex. The shaped canal ends here in an apical stop that holds the root filling. As commonly taught (values differ between textbooks):
- Vital pulp – the apical tissue is not infected: about 1–2 mm short, leaving a healthy pulp stump.
- Necrotic, infected pulp – bacteria reach the apical canal: about 0.5–1 mm short, to the constriction.
Why the exact length matters
- Over-instrumentation destroys the constriction (no apical stop) and pushes debris, sodium hypochlorite and filling material into the periapical tissues: pain, flare-up, foreign-body reaction, delayed healing.
- Under-instrumentation leaves infected tissue and debris apically; dentin chips pack into a plug, ledges form and the filling ends short: persistent or new periapical lesion.