Limitations of traditional working length assessment
Tactile sensation, although useful in experienced hands, has many limitations.
The anatomical variations in apical constriction location, size, tooth type and age make working length assessment unreliable.
In some cases the canal is sclerosed or the constriction has been destroyed by inflammatory resorption (Stock 1994).
Seidberg et al. (1975) found that, even among experienced clinicians, only 60% could locate the apical constriction by using tactile sense.
In a simulation study, Chandler & Bloxham (1990) found substantial variation between subjects asked to detect a resistance in a model root canal. Marked intra-subject differences were also a feature of their experiment.
In a clinical study in adult patients it was found that preflaring of the canal significantly increased the ability to determine the apical constriction by tactile sensation, and helped to detect the apical constriction 75% of the time (Stabholz et al. 1995).
Radiographic determination of working length has been used for many years. The radiographic apex is defined as the anatomical end of the root as seen on the radiograph, while the apical foramen is the region where the canal leaves the root surface next to the periodontal ligament (American Association of Endodontists 1984).
When the apical foramen exits to the side of the root or in a buccal or lingual direction it
becomes difficult to view on the radiograph. Olson et al. (1991) found that when placing files to the foramen in extracted teeth, only 82% appeared to be at the apical foramen.
Dense bone and anatomical structures can make the visualization of root canal files impossible by obscuring the apex. The superimposition of the zygomatic arch has been shown to interfere radiographically with 20% of maxillary first molar apices and 42% of second molar apices (Tamse et al. 1980).
The deposition of secondary dentine and cementum can move the apical constriction further from accepted limits causing preparation errors (Stein & Corcoran 1990, Chong & Pitt Ford 1994).
A radiograph provides a two-dimensional image of a three-dimensional structure and is technique sensitive in both its exposure and interpretation.
Cox et al. (1991) found that when adjustments were required to working length 68% of examiners agreed when adjustments up to 0.5 mm were needed, but there was only 14% agreement when adjustments greater than 1.0 mm were required.
With concerns over radiation exposure and the increased use of electronically stored patient records, several types of digital radiography machines have been introduced.
These use sensors instead of film, and have several advantages over conventional radiographs such as reduced radiation exposure, speed of image acquisition and the possibility of enhancing or editing the image (Shearer et al. 1991). Cederberg et al.
(1998) found that storage-phosphor digital imaging performed equally as well as Ektaspeed Plus film for working length assessment and had better results with its enhancement features.
Friedlander et al. (2002), however, found that the clarity of fine endodontic files and periapical lesions was less with phosphor-plate digital images than with conventional film, due in part to the pixel size of 64 lm being larger than the tip of
the 06 K-file. Currently, direct digital radiography has not been shown to exceed conventional radiography in quality, even with enhancement and measuring features, but is useful for its speed and lower doses of radiation (Martinez-Lozano et al. 2001).
The preoperative radiograph is essential in endodontics to determine the anatomy of the root canal system, the number and curvature of roots, the presence or absence of disease, and to act as an initial guide for working length.
The electronic apex locator is an instrument, which used with appropriate radiographs, allows for much greater accuracy of working length control (McDonald 1992, Pratten & McDonald 1996, Segura-Egea et al. 2002).