Methods for Calculation of Working Length

There are a number of methods that can be employed for calculating the working length. These are :

1) Radiography

2) Digital Tactile Sense

3) Apical Periodontal Sensitivity

4) Paper Point Measurement, and

5) Apex Locators.

Anatomy of the Apical Foramen

To appreciate fully the concept of working length, it is important to understand the apical anatomy. The root canal has two main sections: a longer conical section in the coronal region consisting of dentin and a shorter funnel-shaped section consisting of cementum located in the apical portion. The shape of this apical portion is considered to be an inverted cone: its base being located at the major apical foramen. The apex of the inverted cone is the minor foramen that is often thought to coincide with the apical constriction regarded as being at or near the cementodentinal junction (CDJ). This means that the most apical portion of the root canal system narrows from the opening of the major foramen, which is within cementum, to a constriction (minor foramen) before widening out in the main canal. The major apical foramen does not have a uniform shape but can be asymmetrical and its position on the root tip varies. The deviation of the major foramen from the root tip can occur as a result of increasing age or due to pathological changes, the most common being external root resorption. It also depends on the type of tooth. The root canal terminus is often considered to be CDJ. In many instances, the CDJ coincides with the pulp and periodontal tissue junction, where the pulp tissue changes into apical periodontal tissue.

Theoretically, the CDJ is the appropriate apical limit for root canal treatment as preparation to this point results in a small wound site and optimal healing conditions. The term ‘theoretically’ is applied here because the CDJ is a histological site and it can only be detected in extracted teeth following sectioning; in clinical situation it is impossible to identify its position. The minor apical foramen is a more consistent anatomical feature that can be regarded as being the narrowest portion of the canal system and thus the preferred landmark for the apical end-point for root canal treatment.

Anatomy of apical foramen