Chris J. Lampert, DMD Printer Friendly Version

Determining Endo Working Length

An accurate working length is one of the most important criteria for achieving successful endodontic results and minimizing post-operative discomfort.  An erroneous working length, either long or short, can compromise the outcome of the case from the beginning.  An erroneously short working length leaves un-cleaned and unfilled canal space in the apical region.  An erroneously long working length will lead to over instrumentation and overextended obturation, causing significant post-operative discomfort.  That being said, in my opinion, it is better to be slightly long than slightly short.

There are different opinions throughout endodontics on where the canal terminus is.  Furthermore, there is also debate on how close to the canal terminus you should clean and shape.  Do you work short of the canal terminus at the expense of leaving pulp tissue and debris in the canal?  Do you achieve patency and clean the entire canal?  I am definitely a proponent of achieving patency and cleaning the entire canal. Leaving tissue in the apical region is setting the case up for failure at a later date when the remaining tissue becomes necrotic.

The three currently accepted methods for determining root canal length are:

1. Radiographic Working Length
Radiographs are probably the most common method used for determining working length.  I have learned that you get three pieces of information regarding working length from a radiograph; lies, lies, and damn lies.  Although radiographs are important in endodontic treatment, primarily for assessing canal curvature, they have limitations regarding length determination. This is because the radiographic apex (the end of the root on the radiograph) and canal terminus (where the canal exits the root) often do not coincide. The one absolute that can be proven by a radiograph is when a file is out the apex.  At that point there is no doubt you are patent. If the file is right at the radiographic apex you are usually patent.  If the file is short of the apex on a radiograph you still might be patent. This is why radiographs can be used to confirm patency, but are not the most accurate method for determining working length.

2. Electronic Apex Locators
Currently, apex locators are the most accurate length determining method we have.  All electronic apex locators work by the same method of measuring the multi-frequency impedance changes that occur when the file reaches the periodontal ligament.  Electronic apex locators are really “patency detectors” and they are accurate at determining the point of patency.  Apex locators are most accurate when the canal is dry and the file fits tightly in the canal. Even though the manufacturers state their apex locators can be used in wet canals, they work best in dry canals. Good electrical contact between the file and the canal wall produces a stable reading and is achieved when the file fits tightly in the canal terminus.  The remedy for a jumpy reading is to use a larger file size.

3. Paper Point Method
The late Dr. David Rosenberg taught me and many other clinicians the paper point method.  It is very accurate for determining the final canal length prior to obturation.  This technique is used following cleaning and shaping and involves placing a feather tipped paper point through the terminus of a patent canal. The tip of the paper point becomes moist where it exits the canal and this moisture spot is the true working length for obturation. This method is perhaps the most accurate for post-shaping measurement, but it is also technique sensitive and requires a lot of practice to master. The paper point method requires a patent canal terminus and great care must be taken during instrumentation to prevent apical bleeding.

I often use this method to compare pre-shaping working length from an electronic apex locator to the post-shaping working length. In curved canals, the degree of curvature is usually less following instrumentation.  This reduction in curvature means there is a straighter and shorter distance to the canal terminus resulting in a shortening of the true working length. I rely on and trust the electronic apex locator over all other length determining methods.  Once you develop confidence in your electronic apex locator, it will shorten your treatment time by requiring fewer working films. Ultimately, it will produce better end results with less post-operative discomfort for your patients. 

Chris J. Lampert, DMD maintains a fulltime Endodontic practice in Portland, Oregon, and is involved with research, testing, and development of new endodontic products. Dr. Lampert received his DMD from Oregon Health and Sciences University and his Endodontic post-graduate degree from Boston University. 

Dr. Lampert can be reached at drlampert@thedentistsnetwork.net

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