Problem solving endodontics – looking at working length estimation

endodonticsJohn Rhodes presents an interactive, practical and problem-solving solution in endodontics. This month, he looks at working length estimation.

The question of how to get reliable results from an apex locator is often posed to me. Whether it is necessary to take a diagnostic radiograph when using one. In this problem-solving, interactive article I will be looking at how to estimate working length. Also, how to use an apex locator predictably and provide some troubleshooting tips.

There are several ways of establishing the working length. Using various approaches helps to ensure the canals are instrumented, disinfected and obturated to the correct length.

Generally, preparation takes place as near to the apical constriction as possible. However, this landmark only exists in about 50% of roots and is not visible on a radiograph. Therefore the apex locator is an invaluable tool for locating the cemento-dentine junction (CDJ) and periodontal ligament. Modern apex locators can achieve this with more than 95% accuracy.

Root filling teeth to the radiographic apex will result in over-extended filling material the majority of the time. You can complete preparation to the zero reading of the apex locator in infected cases. Patency is very important to ensure the entire canal is disinfected. When the pulp is vital, and won’t get contaminated, preparation is up to 0.5mm-1mm shorter (Figure 1).

Figure 1: Basic apical anatomy. The root filling is normally finished at the level of the apical constriction. The distance between the radiographic terminus and apical foramen varies between 0-3mm and the distance between apical constriction and apical foramen 0.5mm-1mm

First estimation of length

The preoperative CBCT or digital radiograph can provide the first estimate of length. Most digital software comes with a calibration and measuring tool. The digital ruler is calibrated with a known length like crown height or width. This will make future measurements more accurate.

The operator can compare the length with anatomical knowledge and the general appearance of the tooth; how do the root lengths compare to the crown height? Are they likely to be longer or shorter than expected?

Case example

Figure 2: The maxillary right first and second molars. It is difficult to identify the individual roots on the radiograph as they are superimposed and the buccal bone is thick and obscures the fine detail. For this reason a preoperative small volume CBCT was exposed

In this case, the maxillary right first and second molar teeth required root canal treatment. They appear necrotic and associated with chronic abscesses (Figure 2).

Preliminary measurements were taken from the preoperative CBCT (Figure 3).

Figure 3: Preliminary measurements were taken from the preoperative CBCT

Access was made through the bonded crown on UR7 and after locating the canal orifices and completing coronal flaring, the electronic apex locator was used to measure the working lengths to a reproducible reference point. Not flaring the coronal aspect, not achieving patency and obstructions in the root canal can all affect apex locator readings. The zero reading is the only point of interest and indicates when the file tip is at the CDJ or periodontal ligament.

After preliminary preparation with Waveone Gold instruments (Dentsply Sirona), a diagnostic radiograph was exposed. In maxillary molar teeth where the buccal bone can be thick and in mandibular molars with a dense cortical plate the files will need to be sufficiently large to be able to visualise the tip on radiograph. In this case I used a size 20 Flexofile (Dentsply Sirona).

The diagnostic working length radiograph shows consistency with the apex locator readings (Figure 4). The UR6 had been root treated at a previous appointment.

To see how to apply these steps, visit the Youtube link at the end of the article.

  • Figure 4: The diagnostic working length radiograph shows consistency with the apex locator readings


After completing preparation with Waveone Gold instruments and disinfecting with 3% sodium hypochlorite the canals  were obturated with gutta percha and AH Plus sealer (Dentsply Sirona).

The postoperative radiograph shows a good coronal – apical seal and length control (Figure 5).

A distal angled view shows the completed root fillings in UR6 and UR7 (Figure 6). There are three individual canals in the mesio-buccal root of the UR6.

As always, most techniques in endodontics require patience, practice and perseverance. Although, once mastered, the apex locator is an invaluable tool. It can be used predictably to estimate the working length of the majority of root canal.

This article first appeared in Endondontic Practice magazine. You can read the latest issue here.

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