Electronic apex locators

Question 1

Determination of working length

Answer 1

 To effectively undertake a root canal treatment, the
clinician must accurately determine the ap`ical limit of the root canal system as well as the position of the
canal terminus
 Studies have shown that root fillings which extend
beyond the apex or are more then 2mm short of the apex are associated with a higher chance of failure

Question 2

Anatomical apex definition

Answer 2

The end of the root as determined from a macro perspective

 Not particularly useful as it doesn’t take into account the micro anatomy

Question 3

Radiographic apex definition

Answer 3

Similar to the anatomical apex, but as determined by radiography
 Inaccurate, as the foramen is often located to one side of the radiographic apex
 This often varies from 0.3-0.6mm but can be up to 3mm
 Deposition of cementum at the apex can cause further discrepancies
 Pathology, such as external resorption can also change the position of the foramen

Question 4

Major apical constriction

Answer 4

The widest point of the
foramen where it exists the
root
 Considered to be entirely
within cementum
 Not an ideal point to finish a root filling as it does not produce a natural stop for the filling material

Question 5

Minor apical constriction

Answer 5

The narrowest point of the
foramen, also known as the
apical constriction
 The cementum narrows up from the major apical foramen to the minor apical foramen
 The location is variable (in some canals it may not even be present)
 Usually 0.5-1mm short of the radiographic apex
 At (or close to) the point where the cementum fuses with the dentine (CDJ)

Question 6

CDJ definition

Answer 6

The point where the cementum fuses with the radicular dentine
 Considered to be the point where the root canal system finishes and periodontium begins
 Considered to be the ideal limit of an RCT
However:
 Can only be detected histologically
 Can be irregular – one side may be up to 3mm higher than the other
 Therefore not considered a practical landmark to use in endo

Question 7

Which landmark should we use

Answer 7

 The minor apical constriction is the narrowest point of the root canal system and forms a good
natural barrier
 Consistent anatomical reference point
 Most practical landmark

Question 8

Different minor apical constrictions

Answer 8

Can vary in anatomy

• traditional single constriction
• tapering constriction
• multiconstricted
• parallel constriction

Question 9

Methods for determining working length

Answer 9

 Tactile feedback from instruments
 The paper point technique
 Radiographic determination of working length
 Electronic apex locators

Question 10

Tactile feedback

Answer 10

 Tactile sensation is an essential aspect of endodontic treatment
 However, even experienced endodontists could only detect the apical constriction in 60% of cases by tactile sensation alone
 Made even more complicated by sclerosis, resorption and anatomical differences
 Not a recommended technique for determining WL

Question 11

The paper point technique

Answer 11

 Based on premise that the root canal system of an uninfected tooth is dry whilst the periodontium is wet (hydrated tissue)
 The wet/dry interface is therefore used as the
reference point
 Paper points are placed at incrementally shorter lengths until no moisture is detected
 Can be useful for teeth with open apices or
abnormal apical anatomy
 However, complicated by apical exudate in infected cases and when patency is not achieved
 No good evidence to support the use of this
technique

Question 12

Radiographic determination of WL

Answer 12

 Probably most commonly used technique for determining working length
 Must be undertaken with paralleling technique
 Uses the premise that the apical constriction is on average 0.5-1mm short of
the radiographic apex
 This is inherently flawed however as apical constriction can be significantly further away resulting in over-instrumentation of apex
 Can be difficult to determine due to
overlying structures or superimposition of
canals
 Exposes pt to ionising radiation

Question 13

Electronic apex locators

Answer 13

 Have become a commonly used method for WL determination
 Modern apex locators are very reliable
 Can be used in conjunction with other methods of WL determination, or if the
reading is reliable, can be used as the sole method of WL determination
 Uses resistance/impedance of the root canal and PDL
red line –> deduct 0.5mm

Question 14

How do electronic apex locators work?

Answer 14

 Electrical circuit is
formed, starting at the AL,
running through a clip on the file, through RC, through apical constriction, out the PDL and finally through mucosa and to clip on the pt’s lip and back to the AL, completing circuit
 The tooth can be thought of as a capacitor
 Dentine and cementum are insulators of current
 The PDL, apical constriction and file in
the canal are all conductors of electricity
 The advancing file in the canal, and the PDL will act as conductors in the capacitor
 The dentine, cementum (and any associated fluid) will act as the insulator

Question 15

First generation apex locators

Answer 15

 Resistance-based apex locators
 Resistance between an endodontic instrument at the apical foramen and an electrode on the oral mucosa is approximately 6.5 kW
 The apex locators were set-up to detect this value
 Accurate when dry, but significantly affected by
exudate, pulp tissue, haemorrage or elctrolytes
 Sometimes resulted in a small electric shock to the
pt!

Question 16

Second generation apex locators

Answer 16

 Impedance-based apex locators
 Theoretically more accurate than the solely resistive devices
 However, still affected by the presence of electrolytes
and tended to be highly variable between teeth
 Not accurate enough for reliable clinical use

Question 17

Third generation apex locators

Answer 17

 Use multiple frequencies to determine distance
between an endodontic instrument and the end of a canal
 Usually calibrated to detect the apical constriction (but if apical constriction is abnormal this can result in misreadings)
 Are often IMPROVED in accuracy by the presence of
electrolytes (eg NaOCl) and accuracy is decreased if the
canal is dry
 Have an accuracy of 83-100%
 Can be attached to rotary instuments to give instant
readings, but this may decrease accuracy

Question 18

Fourth generation apex locators

Answer 18

 Uses two frequencies to detect the apical constriction, but only uses one at a time
 This apparently reduces ‘noise’ from using multiple
frequencies and therefore negates the need for a filter
 The manufacturers claim fourth generation apex
locators are therefore more accurate
 However, studies have failed to show improved
accuracy, with third and fourth generation apex
locators both proving to be equally as reliable

Question 19

Problems with apex locators

Answer 19

 Metallic restorations will short-circuit the apex locator resulting in a mis-reading
 Perforations will trigger an apex response when the file touches the perforation
 Large lateral canals may cause a mis-reading

Question 20

Problems with apex locators: metallic restorations

Answer 20

 Ideally replace metallic
restorations prior to treatment
 If this is not possible, ensure the file does not touch the restoration and that the pulp chamber is not flooded with irrigating solution
 Large lateral canals MAY cause a mis-reading

Question 21

Problems with apex locators: perforations

Answer 21

 This prevents an accurate apex locator reading from a perforated canal until the perforation has been repaired
 This effect can be advantageous – if you are unsure if a perforation has occurred an apex locator will help confirm this

Question 22

Problems with apex locators: lateral canals

Answer 22

 Large lateral canals MAY cause a mis-reading

 This should be relatively easy to detect as the apex locator will give an apex reading too soon

Question 23

Tips for clinical success

Answer 23

1. Radiographs
2. The access cavity
3. The irrigating media
4. The endodontic file
5. The apex (or 0) reading
6. Re-checking the WL
7. The battery
8. Unstable readings

Question 24

Radiographs

Answer 24

 A pre-operative radiograph is essential to obtain information about the root canal’s shape and anatomy
 Using an apex locator does not replace the need for a pre-operative radiograph

Question 25

The access cavity

Answer 25

 Ideally remove any metallic restorations prior to
treatment
 No fluid in the pulp chamber – gentle drying with 3 in 1 syringe
 Modern apex locators work best with a ‘moist’ canal – use paper points but not until the canal is completely dry

Question 26

Irrigating media

Answer 26

 The different irrigating solutions do not impact
significantly on the performance of modern apex locators
 Just make sure the previous rules are followed, ie not flooded into the chamber and moist canal

Question 27

The apex (or 0) reading

Answer 27

 Advance the file until the visual display reads ‘apex’ or ‘0’
 The apex locator is most accurate when it is giving an
‘apex’ or ‘0’ reading and contacting the PDL
 Earlier readings (such as ‘0.5’ or ‘1’) do not indicate the distance in mm from the apex
 Therefore, the best approach is to advance the file until it gives an ‘apex’ or ‘0’ reading, then manually
subtract 0.5mm from the length
 This will ensure the file is within the root canal, but
still close to the PDL

Question 28

Re-checking the WL

Answer 28

 The WL should be re-checked with the apex locator after the coronal 2/3 of the canal have been shaped
 The working length will decrease from the initial
reading due to a decrease in the curvature of the canal
 This must be checked prior to shaping the apical
terminus

Question 29

The battery

Answer 29

 Low voltages cause electronic errors

 Therefore, ensure the apex locator’s batteries are well charged before use

Question 30

Unstable readings

Answer 30

Apex readings should only be accepted when the scale bar is:
 Stable
 Moves in sympathy with the movements of the file
If the visual scale bar does the following things, the apex reading
should not be accepted:
 Flashes intermittently
 Moves erratically
 Displays no bars
Unstable readings can be due to a number of reasons, including a faulty
machine, obliterated anatomy, excessive exudate etc
If this occurs either use other methods of determining WL or dress the
canal and re-check on a subsequent visit

Question 31

Locating the apex

Answer 31

 Careful study of high quality radiographs.
 Magnification of radiographs and blocking out of extraneous light.
 Keeping apical anatomy foremost in your mind.
 Use tactile sense to locate apical constriction.
 Observe blood/fluids on instrument tip or anywhere on a paper point.
 Use and understand your apex locator