Endodontics: Working Length Determination

Working Length Determination

• Working length is the length to which the root canal preparation and obturation will terminate.
• It's measured from an external reference point on the crown to the cemento-dentinal junction (CDJ) of the root.
• The CDJ, or apical foramen, is typically 0.5 to 1 mm shorter than the radiographic apex.

Biological Rationale

• Establishing a good working length is crucial for cleaning and shaping the root canal.
• An accurate working length ensures proper instrumentation and filling of the root canal.
• The optimum length is at the apical constriction.

Over Instrumentation

• Over instrumentation can result in perforation of the apical foramen.
• This can cause periapical tissue trauma, inflammation, pain, and swelling.
• Necrotic material can get forced periapically with acute inflammation.
• Irrigant solutions/intercanal medicaments can leak through the foramen, leading to irritation.
• Filling materials can extrude from the foramen, causing periapical tissue irritation.

Under Instrumentation

• Under instrumentation occurs when the instrument length is shorter than the cemento-dentinal junction.
• This can lead to ledge formation in the canal.
• Bacteria or necrotic pulp can be left beyond this point and lead to failure.
• Short root canal filling can result in microleakage.

Radiographic Method

• Radiographs are used to determine the number, size, shape, curvature, and angulation of roots.
• Radiographs check for periapical pathosis (and degree of resorption if present).
• They identify root obstructions (e.g., pulp stones).
• The presence of vertical or horizontal roots/fractures, or root resorption are also noted.
• Radiographs can estimate the tooth's working length.
• An estimated length should be determined by an initial radiograph.
• If the estimated length is 21 mm, the stopper must be adjusted on the initial file shaft to 21 mm from the file tip then the file inserted in the canal and a confirmatory radiograph is taken.

Considerations for Radiographic Estimation

• Curved canals require instrumentation with bends.
• If the canal is elongated or shortened, re-radograph is required.
• Examine the canal size/shape to ensure instrument access.
• The radiographic apex should be 1 mm shorter than the working length.
• Account for bone and root resorption.

Selection of Initial Files

• Multi-rooted teeth use smaller files (e.g., #10 or #15) to prevent ledge formation.
• Single-rooted teeth use larger files.
• Instrumental selection should allow snug fit to the desired length to prevent dropping.
• Improper length determination can cause file displacement.

Selection of External Reference Point

• Anterior teeth use incisal edge.
• Posterior teeth use cusp tips.
• Reference point should be easily checked and positioned on sound tooth structure, avoiding inclined planes.
• The file stopper should rest straight on the reference point.
• Bent files are used if canals are curved.

Initial Radiograph with File in Canal

• Three possibilities can be observed:
• The file tip is just on the cemento-dentinal junction (1 mm shorter than radiographic apex).
• The file tip is at the radiographic apex.
• The file tip is shorter than the radiographic apex by more than 1 mm..

Confirming Working Length

• Reset the stop to the estimated length.
• Reinsert the file.
• Take a new radiograph to confirm the length.
• Verify the working length is correct(just to the cemento-dentinal junction).
• Additional adjustments to working length may be needed if not achieved.

Disadvantages of Radiographic Method

• Radiographs are 2-D representations of 3-D objects causing possible superimpositions, leading to inaccurate working length measurements.

Tube-Shift Localization (Clark's SLOB Rule)

• Used to determine buccal or lingual location of objects (like impacted teeth or root canals) in relation to a reference object.
• Same direction of movement: Lingual location
• Opposite direction of movement: Buccal location

Electronic Apex Locators

• Determine working length.
• Reduce x-ray exposure.
• One side of the apex locator's circuitry is connected to the patient's body.
• The circuit is complete when the endodontic instrument touches the periodontal tissues.

First/Second Generation Apex Locators

• First generation: measure direct current resistance.
• Problems with inaccuracies.
• Second generation: measures alternating current impedance.
• More accurate than first gen, more reliable..

Third Generation Apex Locators

• Based on reactive biologic components facilitating alternating current flow at higher frequencies.
• Reactive component changes in relation to file position.
• Canal must be flooded with electrolyte.
• Maximum impedance difference at the apex is indicated by an audio alarm.

Other Uses of Apex Locators

• Detect root/root perforations.
• Diagnose incomplete root formation.
• Detect external/internal resorption.

Contraindications to Apex Locators

• Cardiac pacemakers.
• Electrosurgical instruments.

Limitations of Apex Locators

• Effective but not the sole method for working length determination.
• Early devices inaccurate if periapical pathosis/dry canals.
• More advanced devices remain helpful in these circumstances but are not always the sole method of measurement.

3-Digital Tactile Sensation Method

• Ineffective for immature apices, excessive root curvature, or constricted canals.

4-Paper Point Method

• Used for immature apices and apical root resorption.
• Paper point passed into the canal after profound anesthesia.
• Moisture/blood on point beyond apex estimation of working length/junction.

5-Apical Periodontal Sensitivity Method

• Patient response to pain is a factor in working length determination.
• Hydrostatic pressure may cause pain when instrument reaches inflamed tissues.
• Passage past apical constriction (if necrotic) may cause minimal or no reaction.