Assist prof. Dr.Emad Farhan Alkhalidi B.D.S., M.Sc., PhD. Conservative Department
Working lengthRoentgenography in Endodontics: There are many applications of radiographs in endodontics which are as follows: 1) Aid in the diagnosis and localization of hard tissue alteration of the tooth (sclerosis and resorption) and periradicular structures.
2) Determine the number, location, size, shape and direction of roots and root canals. 3) Estimate and confirm the length of root canals prior to instrumentation. 5) Determine the relative position of structures in facial or lingual dimensions. 6) Confirm the position and adaptation of the filling points. 7) Evaluate in recall films the success or failure of endodontic treatment.
Determination of an accurate working length is one of the most critical steps of endodontic therapy. The cleaning, shaping and obturation of root canal system cannot be accomplished accurately unless working length is determined precisely.
Determination of the working lengthThe working length is the distance from the incisal edge of the tooth or tip of the cusp to 0.5-1 mm short of the radiograph apex.According to endodontic glossary working length is defined as:“the distance from a coronal reference point to a point at which canal preparation and obturation should terminate”.
Objective of the working length To establish the length of the tooth at which the canal preparation and subsequent obturation are to be completed. The apical end of the root canal is the CDJ, which is usually 0.5-1mm short of the radiographic apex. Sometimes the apical foramen is laterally positioned so it would be more than 1 mm from the radiographic apex.
Reference point: Reference point is that site on occlusal or the incisal surface from which measurements are made. A reference point is chosen which is stable and easily visualized during preparation. Usually this is the highest point on incisal edge of anterior teeth and buccal cusp of posterior teeth. Reference point should not change between the appointments. Therefore in case of teeth with undermined cusps and fillings, they should be reduced considerably before access preparation.
Anatomic apex & radiographic apex:Anatomic apex : is“tip or end of root determined morphologically”.Radiographic apex: is “tip or end of root determined radiographically”.Apical foramen is main apical opening of the root canal which may be located away from anatomic or radiographic apex.
Apical constriction (minor apical diameter) is apical portion of root canal having narrowest diameter. It is usually 0.5 -1 mm short of apical foramen. The minor diameter widens apically to foramen, i.e. major diameter
Cementodentinal junction is the region where cementum and dentin are united, the point at which cemental surface terminates at or near the apex of tooth. It is not always necessary that CDJ always coincide with apical constriction. Location of CDJ ranges from 0.5 - 3 mm short of anatomic apex.
Significance of working length determination: cleaning and shaping procedure should be kept within the root canal space to avoid over or Under instrumentation. What if the length was longer ? Perforation through the apical constriction. Enlargement of the apical narrowing may lead to overfilling or over extension and subsequently to increased incidence of postoperative pain. In addition, one might expect a prolonged healing period and-lower success rate.
What if it was short ? Failure to determine and maintain WL accurately may also lead to shaping and cleaning short of the apical constriction. Incomplete cleaning and underfilling. May cause persistent discomfort. May support the continued existence of viable bacteria, and thus contribute to a continued periapical lesion and ultimately a lowered rate of success.
Techniques of WL determination: I. Radiogrpahic method (conventional or digital) of length determination: 1. Measure the estimated working length from preoperative periapical radiograph. 2. Adjust stopper of instrument to this estimated working length and place it in the canal up to the adjusted stopper. 3. Take the radiograph. 4. On the radiograph measure the difference between the tip of the instrument and root apex. Add or subtract this length to the estimated working length to get the new working length. 5. Correct working length is finally calculated by subtracting 1 mm from this new length.
II. Paper point: When canal preparation is complete, a paper point should be dry when removed from a canal. A blood-soaked or moist tip suggests an overextended preparation or seepage of fluids into the canal due to apical zip or perforation. Not always reliable (bleeding from the pulp?).
III. Electronic method of WL determination apex locators (EAL): Electronic apex locators (EAL) are used for determining working length as an adjunct to radiography. They are basically used to locate the apical constriction or cementodentinal junction or the apical foramen, and not the radiographic apex. Measures the electrical impedance, resistance and frequency differences between the file and the mucosa. Uses the human body to complete electrical circuit. One side of apex locator’s circuit is connected to endodontic instrument by file clip. The other side is connected the patient’s body by contact to the patient’s lip (lip clip).