Radiographic determination of working length has been used for many years. This method of determining the working length is based on the concept that it is impossible to locate the CDJ clinically and that the radiographic apex is the only reproducible site available in this area. The radiographic apex is defined as the anatomical end of the root as seen on the radiograph. However, although it is generally accepted that the minor apical foramen and apical constriction is on average located 0.5-1.0 mm short of the radiographic apex, there are wide variations in the relationship of these landmark. Also when the apical foramen exits to the side of the root or in a buccal or a lingual direction, it becomes difficult to locate its position using radiographs.
You must also keep in mind that the position of the radiographic apex depends on many factors: the angulation of the tooth, the position of the film, the holding agent for the film (finger, X-ray holder, hemostat, cotton roll), the length of the X-ray cone, the vertical and horizontal positioning of the cone, the anatomic structure adjacent to the tooth, and many other factors. A radiograph is a two-dimensional image of a three-dimensional structure and is technique sensitive in both its exposure
Procedure of Calculating the Working Length
In order to establish the length of the tooth, you would normally require a reamer or a file with a rubber instrument ‘stop’ on the shaft, an endodontic millimeter ruler, and a good, undistorted, pre-operative radiograph showing the total length and all the roots of the involved tooth. The exploring instrument size must be small enough to negotiate the total length of the canal, but not so small as to be loose in the canal.
1) Measure the tooth on a pre-operative radiograph.
2) Subtract atleast 1mm “safety allowance” for possible image distortion or magnification.
3) Set the endodontic ruler at this tentative working length and adjust the stop on the instrument at that level.
4) Place the instrument in the canal until the stop is at the plane of reference, unless pain is felt. In case pain is felt the instrument is left at that level and the rubber stop readjusted to this new point of reference.
5) Expose, develop and clear the radiograph.
6) On the radiograph, measure the difference between the end of the instrument and the end of the root. Add this amount to the original measured length of the instrument extended into the tooth. If through some oversight the exploring instrument has gone beyond the apex, subtract this difference.
7) From this adjusted length of the tooth subtract a 1.0 mm “safety factor” to conform to the apical termination of the root canal at the cementodentinal junction. According to Weine F if radiographically there is no resorption of root end or bone, shorten the length by the standard 1.0 mm. If peri-radicular bone resorption is apparent, shorten by 1.5mm, and if both root and bone resorptions are apparent, shorten by 2.0mm.
|Measurement of apical foramen for taking working length|
8) Set the endodontic ruler at this new corrected length and readjust the stop on the exploring instrument.
9) A confirmatory radiograph of the adjusted length is desirable because of the possibility of radiographic distortion, sharply curved roots, and sometimes the measuring error.
The working length can be determined by a number of methods. Let us read more about each in the following subsections.
|Working length radiograph|