The first step in cleaning and shaping is working length determination. The word “length” is defined as the distance between two fixed points.
There are two apices recognized for any root: 1) Radiographic apex: which is the external border of the root tip & is seen radiographically. 2) Anatomical apex: which is the natural apical constriction formed by the cemento-dentinal junction (narrowest part in the canal).
The tooth length is the distance between a reference point coronally (such as the incisal edge for anterior teeth and the cusp tip for posterior teeth) and the radiographic apex. The working length is the distance between the reference point coronally and the anatomical apex.
Significance of working length determination: 1) Determine the instrument length in the canal. 2) Limits the depth to which the canal filling maybe placed. 3) Limits the postoperative pain & discomfort as instrumentation shorter than the apical constriction leaves uncleaned space, while beyond the apical constriction irritate the periapical tissues, violate the apical zone and affect the compaction of the filling material against the apex. 4) Determination of the success of treatment.
Variation in the position of the apical constriction in relation to the radiographic apex: The distance between the anatomic and radiographic apex ranges from 0.5-2mm, depending on; a) Apical foramen is usually found 0.5-1mm short from the radiographic apex ( no bone or root resorption). b) Bone but no root resorption 1.5mm from the apex. c) Bone and root resorption: 2mm from the apex.
Methods of working length determination: 1) Radiographic method : - Proper preoperative radiograph is taken. - Proper sized file is inserted into the root canal from the reference point to the: * Estimated average length. * Apical constriction as felt by the dentist. * Patient sensation if healthy apical tissues are present.
- Provisional working length x ray is taken while the file is inserted in the canal. - Adjustment for the working length is made by subtraction or addition to the used file length according to the proximity of the file to the anatomical apex. - Adjusted working length should be checked radiographically.
This method is used for the working length determination in multi-rooted teeth.
If the radiograph is taken with zero horizontal angulation this will lead to superimposition of the buccal and palatal canals on the film.
This is why the buccal object rule is used through directing the x ray cone mesially or distally.
- It states that “the most distant object from the cone (always the lingual) moves towards the direction of the cone in the film. - Meaning that when the x ray cone is directed from the mesial side, the lingual canal comes mesially on the film while if the x-ray is directed from the distal, the lingual canal becomes distally on the film. N.B: same lingual opposite buccal.
3) Electronic apex locators: - They are electronic devices which are designed to determine the canal length by giving a reading when the file tip is at the apical foramen as it reaches vital tissues. - The principle is based on the electrical resistance of different tissues. When the circuit is complete, resistance decreases and current begins to flow.
Old types were affected by the presence of saliva, blood inside the canal while recent types are not affected by them and work efficiently in their presence. As recent typesdepend on the electrical impedance which is the different electric resistance between the cervical and apical dentin, Apex locators are accurate but not superior to radiograph.