Electronic Apex Locators

Question 1

What is required to effectively undertake root canal treatment?

Answer 1

The clinician must accurately determine the apical limit of the root canal system as well as the position of the canal terminus
Root canal fillings which extend beyond the apex or are more than 2mm short of the apex = higher chance of failure

Question 2

What is the anatomical apex?

Answer 2

The end of the root as determined from a macro perspective

- Doesn’t take into account the microanatomy

Question 3

What is the radiographic apex?

Answer 3

Determined by radiography
Inaccurate as the foramen is often located to one side of the radiographic apex
Often varies from 0.3-0.6mm but can be up to 3mm
Cementum deposition at the apex can cause further discrepancies
Pathology e.g. external resorption can change the position of the foramen

Question 4

What is the major apical constriction?

Answer 4

Widest point of the foramen where it exists in the root
Entirely in cementum
Not ideal point to finish root filling as does not produce a natural stop for the filling material

Question 5

What is the minor apical constriction/apical constriction?

Answer 5

Narrowest point of the foramen
Cementum narrows from the major apical foramen to the minor apical foramen
Location is variable
Usually 0.5-1mm short of the radiographic apex
At the point where the cementum fuses with the dentine (CDJ)

Question 6

What is the cemento-dentinal junction (CJD)?

Answer 6

The point where the cementum fuses with the radicular dentine
Considered to be where the root canal system finishes and periodontium begins
Ideal limit of RCT

Question 7

Why is the CJD not a practical landmark to use in endodontics?

Answer 7

It can only be detected histologically

Can be irregular - one side may be up to 3mm higher than the other

Question 8

What landmark should be used to end the RCT?

Answer 8

Minor apical constriction
= Narrowest part of the RCS and forms a good barrier
- Consistent anatomical reference point and most practical landmark
- Anatomy can vary

Question 9

Methods for determining working length?

Answer 9

Tactile feedback from instruments
Paper point technique
Radiographic determination
Electronic apex locators

Question 10

What complicates apical constriction detection via tactile feedback?

Answer 10

Sclerosis, resorption and anatomical differences

Question 11

What is the paper point technique?

Answer 11

Uninfected tooth is dry and periodontium is wet
Wet/dry interface is used as the reference
Can be useful for teeth with open apices or abnormal apical anatomy
Complicated by apical exudate in infected cases and when patency is not achieved

Question 12

Radiographic WL?

Answer 12

Most common WL determination
Assumes that the apical constriction is 0.5-1mm short of the radiographic apex
BUT apical constriction can be further away = overinstrumentation at the apex
Can be difficult due to overlying structures or canals
Exposes the pt to ionising radiation

Question 13

Apex locators?

Answer 13

Modern ones are very reliable
Can be used with or without other WL determination methods
Uses resistance/impendence of the root canal and PDL

Question 14

How do apex locators work?

Answer 14

Electrical circuit is formed starting at the AL, through a clip on the file, through the RC, the apical constriction, out the PDL and through the mucosa and to the clip on the pt’s lip and back to the AL

Question 15

First generation apex locators?

Answer 15

Resistance based
Resistance between an endo instrument at the apical foramen and an electrode on the oral mucosa is approx 6.5kW
Accurate when dry but affected by exudate, pulp tissue, haemorrhage or electrolytes
Electric shock to pt

Question 16

2nd generation apex locators?

Answer 16

Impedance based apex locators
More accurate than resistive devices
Affected by presence of electrolytes
Not accurate enough

Question 17

3rd generation apex locators?

Answer 17

Use multiple frequencies to determine distance between an endo instrument and the end of a canal
Calibrated to detect the apical constriction
Improved in accuracy by presence of electrolytes
Accuracy decreased if canal dry or attached to rotary instruments
Accuracy of 83-100%

Question 18

4th generation apex locators?

Answer 18

Use 2 frequencies to detect the apical constriction

Studies fail to show improved accuracy - 3rd and 4th generation locators have the same accuracy

Question 19

Problems with apex locators?

Answer 19

Metallic restorations will short circuit the apex locator = mis-reading

• Ideally replace metallic restorations prior to tx
• If not possible - ensure the file does not touch the restoration and the pulp chamber is not flooded with irrigating solution

Perforations will trigger an apex response when the file touches the perforation
= Prevents accurate apex locator reading from a perforated canal until the perforation has been repaired
BUT can be a positive as can help confirm a perforation

Large canals may cause misreading
- But should be easy to detect as the apex locator will give the reading too soon

Question 20

Tips for apex locator success?

Answer 20

Radiographs 
Access cavity
Irrigating media
Endodontic file
Apex reading
Re-checking the WL
The battery
Unstable readings

Question 21

Why is a pre-op radiograph essential?

Answer 21

Obtain info about the root canal’s shape and anatomy

Question 22

How can the access cavity aid apex locator success?

Answer 22

Remove any metallic restorations prior to tx
No fluid in the pulp chamber - gently drying with a 3 in 1
Modern apex locators work best in a moist canal - use paper points but not until canal is completely dry

Question 23

How can irrigation aid apex success?

Answer 23

Irrigant not flooded into the chamber

Moist canal

Question 24

How can the endo file aid apex locator success?

Answer 24

Choose one which will contact the walls of the RC
Narrow file will be less accurate
Metal of the file does not affect the accuracy

Question 25

How to know when the apex locator has located the apex?

Answer 25

Advance the file until the visual display reads apex/0, record the length of the file, then manually subtract 0.5mm from the length = ensures file is within the RC but still close to the PDL
Most accurate when it’s giving an apex or o reading and is contacting the PDL

Question 26

How to recheck the WL?

Answer 26

With the apex locator after the coronal 2/3 of the canal has been shaped
WL will decrease from the initial reading due to a decrease in the curvature of the canal
Must be checked prior to shaping the apical terminus

Question 27

How can the battery impact the apex locator?

Answer 27

Low voltages can cause electronic errors

Ensure well charged before use

Question 28

When to accept the apex locator reading?

Answer 28

Only accept the reading when the scale bar is stable, moves in sympathy with the movements of the file

Question 29

When to not accept the apex locator reading?

Answer 29

When the scale bar:

• Flashes intermittently
• Moves erratically
• Displays no bars

Question 30

What can cause faulty readings?

Answer 30

Faulty machine
Obliterated anatomy
Excessive exudate

Question 31

What to do when the apex locator displays an unstable reading?

Answer 31

Use other WL determination methods
OR
Dress the canal and re-check on a subsequent visit

Question 32

Summary of how to locate the apex?

Answer 32

Study high qual radiographs
Magnification of radiographs and blocking out light
Keep apical anatomy in mind
Use tactile sense to locate apical constriction
Observe blood/fluids on instrument tip or anywhere on a paper point
Use and understand apex locator