Lecture five------د.احمد غانم
Working Length Determination
Every part of endodontic treatment is controlled by a measurement of instruments penetration depth into the canal. This length is typically determined in millimeters. It is measured from a reference point on the tooth’s cavosurface that is within the clinician’s field of view. It varies from the complete canal length to some arbitrary determined point near the termination of the canal space.
Biologic Rationale for Working Length
WL determines the extent of canal cleaning and shaping that will be accomplished. The most clinically relevant working length landmark is the apical constricture, regardless of whether it is in dentin or Cementum. The constricture is the narrowest point within the canal and therefore the narrowest diameter of the blood supply. Beyond the constricture the canal widens and develops a broad vasculature supply. Therefore from a biologic perspective the constricture is the most rational point to end the canal preparation. Intraradicular termination of the cleaning process leaves a canal content interface equal in area to the total inflammatory capable vital tissue surface (1:1). Termination beyond the constricture provides a greater area of vital tissue than irritant interface. Extraradicular termination can theoretically provide a hemisphere of vascular support to the inflammatory process. Such a condition alone gives the inflammatory process a volume, area, and numerically superior relationship. The surrounding vital tissues should have more capacity to destroy irritant and return the area to a biologically functional and repaired state. Thus cleaning and shaping through the apical constricture completely eliminate pathogenic canal content and allows the inflammatory healing mechanism to progress.
Procedurally, it is advantageous to treat to the constricture because it is a morphologic landmark that can be identified and some times felt by experienced clinician. As the canal is shaped, coronal to the apical constricture, it becomes progressively easier to locate the constricture with a small potency file and tactile sense. The experienced hand can detect an abrupt increase in resistance followed by a rapid decline as the instrument tip passes beyond the constricture. Clinician are urged to treat root canals to this apical end point because lateral and accessory canals are more common near the apex. Considering the possibility of an accessory canal, treating just 1-2mm short of the apical constricture can leave 2-4mm of untreated canal system. Such a length could significantly increase the chances for persistent periapical infection and inflammation.
The significances of WLD are:
1. The calculation determines how far into the canal the instruments are placed and worked and thus how deeply into the tooth the tissues, debris, metabolisms, end products, and other unwanted items are removed from the canal.
2. It limits the depth to which the canal filling may be placed.
3. It will affect the degree of pain and discomfort that the patient will feel following the appointment.
4. It will play an important role in determine the success of the treatment and conversely, if calculated incorrectly may doom the treatment to failure.
Method of WLD
ONLY by correlating many confirming piece of evidence can clinicians visualize the true terminus of root canals.
Radiographic
The most commonly used method determining the length of a canal is radiograph. The clinician starts by placing a file to an estimated length and then exposed the film. From the radiographic tooth measurement, 3mm is subtracted fro the estimated working length. This takes into account the following: the relation of radiographic apex to the actual apical foramen or constricture (approximately 1mm), the magnification effect of the radiograph. Magnification of 2mm (due to divergence of central beam) is allowed for all teeth. This 3mm decrease should generally leave the initial instrument placement slightly short of working length. For example:
26mm=length measured on radiograph.
-1mm=difference between radiographic apex and actual foramen.
-2mm=magnification factor.
23mm=estimated WL.
Selection of File for Working Length Measurement
A file must now be selected for determining the actual working length radiographically.
In general, single-rooted teeth have large canals. Therefore, it will often be possible to use a fairly large file for working length determination, especially in the maxillary anterior teeth. The instrument of choice here would be the file that would fit to the desired length (estimated working length) snugly enough to prevent it from dropping out. If you use small instruments that are loose in the canal for working length determination in maxillary anterior teeth, the following might result:
In the clinic the file might fall completely out of the canal while the patient is walking to the X-ray machine.
The file might slip partially out of the canal and thus give an inaccurate and misleading radiograph. An instrument dropping out of the tooth is not a problem in working length determination for mandibular teeth, but the instrument could still be moved or disturbed by pressure from the rubber dam or from files in other canals in the case of a multirooted tooth.
Generally in maxillary anterior teeth a size #25 or #30 K-file can be used for your initial working length radiograph.
Electronic
Apex locators may be used to determine the canal length. The unit leads are connected to a file that is inserted into the canal and to a clip that contacts the oral mucosa. The pulp is extirpated, the canal is irrigated and dried, and the attached file inserted to the terminus. A dry canal and chamber eliminates ionic conduction, which can cause a premature indication that the apex has been reached. This always necessary when the apex locators works on a resistance principle. Impedance and frequency type models are not sensitive to ionic solutions like the resistance-based units, however, only the canal should contain fluid and the chamber should dry to prevent conduction through metal restorations to gingival tissues.
Tactile
The experienced clinician develops keen sense and gains considerable information from the passage of an instrument through a canal. Once radicular access has removed dentinal interface from the coronal third of a canal, the observant clinician can detect a sudden rise of resistance as a file approaches the apex. Careful study of the apical anatomy discloses 2 facts to make tactile identification possible: (1) the unresorbed canal commonly constricts just before exiting the root, and (2) it frequently changes course in the last 2-3mm. both situations apply pressure to the file. After preparation develops space in the coronal 2\3 (radicular access) the quality of tactile information improves as the file will bind only in the apical area.
Paper point evaluation
Ones the preparation is complete, a paper point may yield more than a dry canal. After the canal is rendered dry, an additional paper point may be used to seek out apical moisture or bleeding. A bloody or moist tip suggests an overextended preparation or seepage of fluid into the canal. The point of wetness gives an approximate location to the actual canal end point.
It is important to remember that all methods are used in harmony to ensure that final shaping and Obturation will extend the full length of a canal.
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