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Getting the Most Out of Your Apex Locator
Electronic Apex Location (EAL) has revolutionized endodontic treatment. While it has not completely replaced imaging techniques for determination of working length, it provides a very accurate adjunctive method of determining where the canal ends. From the earliest resistance based models to today's most modern multi-frequency and impedance-based units, many clinicians have been frustrated with having perfect readings at one time and poor results the next. Regardless of what the manufacturers may indicate, here are some tips as to how to get the best, most efficient use of your EAL.

1. Drier is Better
The EAL works best in a relatively dry environment. Although many units claim to work "in any fluid", the reality is that they work best when the canals are dry. Since we do NOT want to routinely insert instruments in dry canals (even if just for the lubricating effect of the irrigant) a compromise is made. The access should be irrigated and the canal broached first (see The Broach for tips on how to use a Broach effectively) The access is lightly suctioned to remove the majority of the NaOCl. The electrolytic effect of the NaOCl touching alloys and restorations is the cause of most problems with initial readings. (Buccal and palatal/lingual amalgams are an especially difficult area in molar teeth.) It is important to provide the file with a clear path to dentin with no alloy contact.

2. Select the right file
The second most common error involves file selection. It is important to remember that EALs work on the basis of contact with the canal walls and periapex. The better the adaptation of the file to the canal walls the better the reading. Using a #10 file in a teenage Maxillary central incisor will give inaccurate readings. Once the file size approaches that of the diameter of the foramen, the accuracy improves. Therefore it is important to try to get a reasonably good file fit in larger canals. The file size should be fairly close to that of the foramen. Having said that, the EAL can work accurately and predictably in calcified canals when used with a file as small as a size #06. Proper file selection comes with experience.

3.Using the EAL with Multiple Files
The EAL can be used with as many as 4 files at a time in a tooth. This is how it is done:

  1. Remove the Clip attachment from the EAL. Always use the "Fork". Ground the patient with the lip clip.
  2. Insert the first instrument with your right hand; hold the "Fork" of the EAL in your left. Watch for the typical transition. EAL readings should show a smooth transition as you approach the apex. IGNORE THE NUMERICAL VALUES, THEY ARE NOT ACCURATE. I prefer to use an audio signal because I do not need to see the instrument to know where I am in the canal. As the "beeps" get more rapid, I wait for the silence that indicates the apex. Pushing the file in deeper than that results in a solid tone that indicates I am "long". If so, I rotate the file slightly counter clockwise, thereby "unscrewing" it away from the apex. (Never rotate a file that is located near the apical foramen more than a few degrees! You risk breakage.) The beeping returns and then I again slightly rotate clockwise/insert and wait for the silence. By using the file with this motion it is possible to reproduce the tones and know exactly where the apex is. I then use the Fork to slip the rubber stop down to the point or cusp of reference.
  3. Insert the second instrument and proceed in the same manner. Look carefully at the instrument shafts and make sure that they do not contact each other or any alloy restorations. If necessary, as you insert the file use the Fork to (a) apply very slight pressure to the shaft or handle of the instrument you are measuring OR (b) use the insulated portion of the fork to push away the shaft of an adjacent file. (Just enough to clear contact!) This prevents "shorting out" of the file you are measuring with adjacent files or an alloy. Continue as above. You occasionally may hear the EAL "squawk" as the "shorting" occurs intermittently. Ignore this noise and listen for the transition beeps and the silence.

  4. (The key with this technique is the ability to distinguish "real" readings from the "interference". A good analogy is the ability to concentrate on one voice when 3 or four people are talking. If you can do this, then the EAL "squawk" is not something to be worried about.)

  5. Once all instruments have been placed, they will invariably be "crossed over" each other and the shafts will be touching in most cases. It is a simple matter to "isolate" each file MOMENTARILY, take a reading with the Fork, listen for the silence that indicates you are at the apex (or look for the "AT APEX" indication) and then slide the stop to the reference point if it is not already there.
With this method, virtually all 3 (and some 4) canal molar working lengths can be initially be obtained in as little as 30 seconds (if the canal angles are "cooperative"). Once they are at length they can literally be checked in seconds. Since curved canals shorten as they are straightened, EALs are an excellent way of confirming final canal length after cleaning and shaping. EALS can also help determine the approximate size of the apical foramen (MAF).

By knowing the working lengths AND the apical foramen size, fitting a Gutta percha cone becomes much easier. If you find that the apical cone diameter you select is drastically smaller that the apical foramen size, it is likely that you do not have enough taper (the cone is binding somewhere higher up in the body of the canal). Similarly, selecting too large an apical diameter for your cone will also "hang it up" or buckle short of the working length because it is too wide.

Always perform cleaning and shaping procedures in a thoroughly irrigated environment. If necessary, suction the area and take the EAL reading in a relatively dry field. Remember to confirm all lengths radiographically. Teeth with accessory canals, bifidities and multiple canal systems can have unusual ramifications, not apparent to the locator.