WORKING LENGTH DETERMINATION
CONTENTS
INTRODUCTION AND HISTORY DEFINITION CLINICAL CONSIDERATIONS
CONTENTS
INTRODUCTION AND HISTORY DEFINITION CLINICAL CONSIDERATIONS
RA RADI RADIOGRAPHI DIOG OGRA RAPH PHI I C METHOD Gros Gr Grossman ossm sman an Me Meth Method thod od Ingl In Ingle·s gle· e·ss Me Meth Method thod od Wein We Weine·s ine· e·ss Modif Mod Modification ific icati ation on Direct digital radiography or xeroradiography
Radiovisiography (R.V.G.)
NON
RADIOGRAPHIC MET HOD
Digital tactile sense Apical periodontal sensitivity Paper point measurement
DETERMINATION OF WORKING LENGT H BY ELECT RONICS CONCLUSION REFERENCES
INTRODUCTION AND HISTORY Determination of an accurate working length is one of the most critical steps of endodontic therapy. Cleaning
shaping and obturation of the root canal system cannot be accomplished accurately unless working length is determined precisely.
Thus we can say that predictable endodontic success demands accurate determination of working length of root canal and strict adherence to it, in order to create a small wound site and good healing conditions.
HISTORICAL PERSPECTIVES At the end of 19th century
Working length is usually calculated when file is placed in the canal and patient experiences pain .
1899--Kells 1899
Introduced in XX- rays in dentistry
1918-- Halton 1918
Microscopically studied the diseased periodontal tissues.
1929 - Collidge
Studied the anatomy of root apex in relation to treatment problem
1955 - Kuttler
Microscopically investigated the root apices
1962 - Sunada
Found electrical resistance between periodontium and oral mucous memberane
1969 - Inove
Significant contribution in evolution of Electronic Apex Locator
DEFINITION According to Endodontic glossary working length is defined as ´ the distance from a coronal reference point to a point at which canal preparation and obturation should terminate.
Diagram showing working length (1mm from the radiographic apex)
Reference Point Reference point is that site on occlusal or the incisal surface from which measurement are made. A reference point is chosen which is stable and easily visualized during preparation Usually this the highest point on incisal edge of anterior teeth and buccal cusp of posterior teeth. Reference point should not be changed between appointments.
Anatomic Apex is the tip or the end
of the root determined morphologically Radiographic Apex is the tip or end
of the root determined radiographically Root morphology and radiographic distortion may cause the location of the radiographic apex to vary from the anatomic apex.
Apical Foramen is main opening of the root canal which may be located away from anatomic or radiographic apex. The apical constriction (minor apical diameter) is the apical portion of the root canal having the narrowest diameter. This position may vary but is usually 0.5 to 1mm short of the center of the apical foramen. The minor diameter widens apically to the foramen i.e. major diameter and assume a funnel shape.
The average distances between major and minor diameters are .524mm in teeth examined in an 1818-25 year old group and 0.659mm in a 55 year old and older group. This means that longitudinal view of the canal as a tapering funnel to tip of the root is incorrect but funnel tapers to a distance short of the site of exiting and then widens again.
The location of the cementodentinal junction also ranges from 0.5 to 3 mm short of the anatomical apex. Therefore, it is generally accepted that the apical constriction is most frequently located 0.5 to 1 mm short of the radiographic apex, but with variations. Ingle
Variation in location of CDJ
Guldener suggested choosing a working length which corresponds to the tooth length less 0.5 mm for cases with a necrotic pulp. In cases of vital pulp extirpation, he recommended an additional reduction of 0.5-1 mm short of the tooth length. Taylor pointed out a narrower spot at the apical level called minor diameter which he believed to correspond histologically to the CDJ. IEJ, 1998, 31, 384-93
Clinical
considerations
Working length determines how far into canal, instruments can be placed and worked. It affects pain and discomfort which patient will experience following appointment by virtue of over and under instrumentation. If placed in correct limits ,it plays an important role in determining the success of treatment.
Before determining a definitive working length, the coronal access to the pulp chamber must provide a straight line pathway into the canal orifice.
Once the apical restriction is established, it is extremely important to monitor the working length periodically since the working length may change as a curved canal is straightened (´a straight line is the shortest distance between two pointsµ).
The loss may also be related to the accumulation of dentinal and pulpal debris in the apical 2-3 mm of the canal or other factors such as Failing to maintain foramen patency Skipping instrument sizes or Failing to irrigate the apical one third adequately.
Occasionally, working length is lost owing to ledge formation or to instrument separation and blockage of the canal.
Failure to accurately determine and maintain working length may result in the length being too long and may lead to perforation through the apical constriction. Destruction of the constriction may lead to overfilling or overextension and an increased incidence of postoperative pain.
Failure to determine and maintain working length accurately may also lead to shaping and cleaning short of the apical constriction. Incomplete cleaning and underfilling may cause persistent discomfort, often associated with an incomplete apical seal. Apical leakage may occur into the uncleaned and unfilled space short of the apical constriction.
During this procedure inaccurate measurements may occur because of the following: Inaccurate adjustment of the stopper to the reference point Movement of the stopper during measuring procedure Lack of parallelism between the long axis of the measuring file Inaccurate identification of the file length IEJ 2006,39,108-112
Stop attachments Devices have been developed that assist in adjusting rubber stops on instruments. It is critical that the stop attachment be perpendicular and not oblique to the shaft of the instrument.
There are several disadvantages to using rubber stops. Not only is it time consuming, but rubber stops may move up or down the shaft, which may lead to preparations short or past the apical constriction.
To achieve the highest degree of accuracy in working length determination, a combination of several methods should be used. The most common methods are Radiographic Method Non Radiographic Methods Electronic Methods
Radiographic method The following materials are essential to perform this procedure: Good, undistorted, preoperative radiographs showing the total length and all roots of the involved tooth. Adequate coronal access to all canals.
An endodontic millimeter ruler. Working knowledge of the average length of all of the teeth. A definite, repeatable plane of reference to an anatomic landmark on the tooth, a fact that should be noted on the patient·s record.
Do not use weakened enamel walls or diagonal lines of fracture as a reference site for length of tooth measurement Weakened cusps or incisal edges are reduced to a well supported tooth structure. Diagonal surfaces should be flattened to give an accurate site of reference.
Grossman·s method An instrument extending to the apical constriction is placed in the root canal which can be determined by digital tactile sense and a radiograph is taken.
Stopper is placed at the level of incisal or occlusal reference point Measure the length of the x-ray images of both the tooth and measuring instrument as well as actual length of the instrument in the canal.
Ingle·s method
VARIATION ²When two canals of a maxillary first premolar appear to be superimposed, much confusion and lost time may be saved .Occasionally it is advantageous to take individual radiographs of each canal with its length of tooth instrument in place. A preferable method is to expose the radiograph from mesial horizontal angle. This causes the lingual canal to always be more one in the image. ACCURACY ² Accuracy depends on the radiographic technique used. Paralleling Technique was significantly more reliable than bisecting angle technique.
Weine·s modifications
If radiographically there is no resorption of root end or bone ,shorten the length by 1mm,If periapical bone resorption is apparent ,shorten by 1.5mm and if both root and bone resorption is apparent, shorten by 2mm.
Direct digital radiography or xeroradiography Not widely used in endodontics They record images produced by an X-ray but differ from conventional radiography is that it does not require a wet chemical processing or dark rooms
Radiovisiography (R.V.G.) It has three components Radio Visio
: :
Graphy :
has sensitive intraoral sensor video monitor display processing unit high resolution printers
Advantages It produces useful images at low radiation It provides an instantaneous image on a monitor Elimination of X-ray film Ability to enlarge special areas Potential for computer storage
Non
Radiographic method
Digital tactile sense Apical periodontal sensitivity Paper point measurement
Digital tactile sense method
If the coronal portion of the canal is not constricted, clinician may detect an increase in resistance as the file approaches the apical 2-3 mm. This detection is by tactile sense. In this region the canal frequently constricts before exiting the roots. There is also tendency for canal to deviate from radiographic apex.
If the canals were preflared, it was possible for an expert to detect the apical constriction in about 75% of cases.
Disadvantage This method is ineffective in root canals with an immature apex and is highly inaccurate if the canal is constricted throughout its entire length or if the canal has excessive curvature. This method should be considered as supplementary to high quality, carefully aligned, parallel, working length radiographs and an apex locator.
Determination of working length by apical periodontal sensitivity If an instrument is advanced in the canal toward inflamed tissue, the hydrostatic pressure developed inside the canal may cause moderate to severe, instantaneous pain. At the onset of the pain, the instrument tip may still be several millimeters short of the apical constriction.
When the canal contents are totally necrotic the passage of instruments into canal and past the apical constriction may evoke only mild pain or no reaction at all, bcoz when periradicular lesion is present tissue is not richly innervated.
On other hand, Langeland and associates reported vital pulp tissue with nerves and vessels may remain in the most apical part of the main canal even in the presence of a large periapical lesion. This suggests a painful response may be obtained inside the canal even though the canal contents are ´necroticµ and there is a periapical lesion.
Determination of working length by paper point measurement In a root canal with an immature (wide open) apex, the most reliable mea ns of determining working length is to gently pass the blunt end of a paper point in to the canal after profound anesthesia has been achieved.
The moisture or blood on the portion of the paper point that passes beyond the apex may be an estimation of working length or the junction between the root apex and the bone.
In cases in which the apical constriction has been lost owing to resorption or perforation, and in which there is no free bleeding or suppuration into the canal, the moisture or blood on the paper point is an estimate of the amount the preparation is overextended
A new dimension has recently been added to millimeter markings. Markings are at 18,19,20,22,24mm from the tip. These paper points were designed to ensure that they be inserted fully to the apical constriction.
Determination of working length by electronics Apex locators
In many of these instruments, the event is signalled by A beep Flashing lights and Digital reading.
All apex locators function by using the human body to complete an electrical circuit. One side of the apex locator·s circuitry is connected to an endodontic instrument. The other side is connected to the patient·s body either by a contact to the patient·s lip or by an electrode held in the patient·s hand.
The electrical circuit is complete when the endodontic instrument is advanced apically inside the root canal until it touches periodontal tissue. The display on the apex locator indicates that the apical area has been reached.
Classification and accuracy of apex locators The classification is based on the type of current flow and the opposition to the current flow, as well as the number of frequencies involved.
First
generation Apex locators
First generation apex location devices, also known as resistance apex locators, measure opposition to the flow of direct current or resistance. When the tip of the reamer reaches the apex in the canal, the resistance value is 6.5 kilo-ohms (current 40 mA)
Trade names Sonoexplorer
Advantage Used where radiographs can·t be used accurately.
e.g. in palatal canals of maxillary molars and premolars Used in Patient with gag reflex
Disadvantage Many factors can lead to wrong readings like low battery, tissue present in canal, wet canal, too narrow canal with blockage and problems of lip clip.
Second generation apex locators It is also known as impedance apex locators, measure opposition to the flow of alternating current or impedance.
Various Second Generation Apex Locators Sono Explorer The Apex Finder Endo Analyzer Digipex Digipex II Formation IV
Disadvantage The root canal has to be reasonably free of electroconductive materials to obtain accurate readings. The presence of tissue and electroconductive irrigants in the canal changes the electrical characteristics and leads to inaccurate, usually shorter measurements.
Endo analyzer It is combined apex locator and pulp tester
Digipex It is visual LED digital indicator and an audible indicator. It requires calibration
Digipex II is a combination apex locator and pulp vitality tester. Exact-A-Pex has an LED bar graph display and does not require calibration.
The Apex Finder - It has visual digital LED indicator and is self caliber. Pio apex locator has an analog meter display and audio indicator. It has an adjusting knob for calibration.
Formatron IV has a flashing LED light and a digital LED display and does not require calibration.
Third generation apex locators The principles are in biologic settings, the reactive component facilitates the flow of alternating current, more for higher than for lower frequencies Thus, a tissue through which two alternating currents of differing frequencies are flowing will impede the lower frequency current more than the higher frequency current.
The reactive component of circuit may change, for example, as the position of a file changes in a canal. When this occurs, the impedances offered by the circuit to currents of differing frequencies will change relative to each other. This is the principle on which the operation of the third generation apex locators.
Endex The original third generation apex locator was described by yamaoka et al. In Europe and asia this device is available as the APIT. It uses a very low alternating current.
The signals of two frequencies (5 and 1 kHz) are applied as a composite waveform of both frequencies. As the attached to endodontic reamer enters coronal part of canal , the difference in impedences at two frequencies is small. As instrument is advanced apically ,difference in impedence values begins to change.
When endodontic reamer is reached apical constriction, the impedance values are at their maximum difference, and these differences are indicated on the analog meter and audio alarm.
This impedance difference is the basis of the difference method.
The unit must be reset (calibrated) for each canal. The device operates most accurately when the canal is filled with electrolyte ( normal saline or sodium hypochlorite). Guttapercha must be removed from the canals in re- treatment cases before electronic working length determination is made.
ultima ez apex locator is a third generation device that supersedes the second generation sono explorer line. Neosono
It works best in the presence of sodium hypochlorite.
The ultima ez is mounted with a root canal graphic showing file position as well as an audible signal. The ultima-ez also comes with an attached pulp tester called the co-pilot.
or justy II is another third generation apex locator. Justwo
The device uses frequencies of 500 and 2000 Hz in a relative value method. Two electric potentials are obtained that correspond to two impedences of root canal.
These two potentials are converted to logarithmic values, and one is substracted from other. The result drives the meter. The unit determines working length in the presence of electrolytes. Although no calibration is required, a calibration is recommended.
Apex finder A.F.A. (´All fluids allowedµ)
uses multiple frequencies and comparative impedance principles in its electronic circuitry.
It has a liquid crystal display (L CD) panel that indicates the distance of the instrument tip from the apical foramen 0.1mm increments. It also has an audio chime indicator.
The display has a bargraph ´canal condition indicatorµ that reflects canal wetness and allows the user to improve canal conditions for electronic working length determination.
Endo analyzer 8005 combines electronic apex location and pulp testing in one unit.
Root ZX, a third generation apex locator
that uses dual frequency and comparative impedance principles.
The root ZX is mainly based on detecting the change in electrical capacitance that occurs near the apical constriction. Advantages It requires no adjustment or calibration and can be used when the canal is filled with strong electrolyte or when the canal is empty and moist.
The meter is an easy to read L CD. The position of instrument tip inside the canal is indicated on the LCD meter and by the monitor·s audible signals. The root ZX allows shaping and cleaning of the root canal with simultaneous continuous monitoring of the working length.
The position of the tip of the rotary instrument is continuously monitored on the LED control panel of the handpiece during the shaping and cleaning of the canal.
Combination apex locator and endodontic handpiece Tri auto ZX is a cordless electric endodontic handpiece with a built in root ZX apex locator.
The tri auto ZX has three automatic safety mechanisms. Auto start stop mechanism The handpiece automatically starts rotation when the instruments enter the canal and stop when the instrument is removed.
Auto torque reverse mecha nism
The handpiece automatically stops and reverses the rotation of the instrument when the torque threshold is exceeded. This mechanism developed to prevent instrument breakage
Auto apical reverse mechanism
A mechanism controlled by the built in root ZX apex locator and developed to prevent instrumentation beyond the apical constriction.
The Tri Auto ZX has four modes The Electronic Measurement of Root (EMR) mode , a lip clip , hand file , and file holder are used with the apex locator in the handpiece to determine working length. The handpiece doesn·t operate in this mode. In LOW mode the torque threshold is lower than in HIGH mode. The LOW mode is used with small to midmid-sized instruments for shaping and cleaning the apical and mid--third sections of root. mid
All three mechanisms are functional in this mode
In HIGH mode torque threshold is higher than LOW mode but lower than MANUAL mode The HIGH mode is used with midmid-size to large instruments for shaping and cleaning in the mid--third and coronal third sections of root mid canal. All three mechanisms are functional in this mode.
MANUAL mode offers highest threshold of torque. In MANUAL mode the autoauto-start start--stop and auto--torque auto torque--reverse mechanism do not function. The autoauto-apical apical--reverse mechanism does not function. MANUAL mode is generally used with large instruments for coronal flaring.
Other
apex locating handpiece
Sofy ZX uses the root ZX to electronically monitor the location of the file tip during all instrumentation procedure.
The devices minimizes the danger of over-instrumentation.
Endy 7000
It is available in Europe It is an endodontic handpiece connected to an Endy apex locator that reverses the rotation of the endodontic instrument when it reaches a point in the apical region preset by the clinician.
Contraindications The use of apex locators and other electrical devices such as pulp testers, electrosurgical instrument is contraindicated for patients who have cardiac pacemakers. Electrical stimulation to the pacemaker patient can interfere with pacemaker function.
Studies have concluded that when apex locators are used in conjunction with radiographs , there is a reduction in number of radiographs required, and that some of the th e problems associated with radiographic working length estimation can be eliminated.
Conclusion
The correct working length is a crucial factor for the successful preparation of the root canal and thus the long term success of the root canal therapy. The traditional method for the determination of root canal length is the radiograph, but accuracy is difficult to achieve because of the apical constriction can not be identified, and variables in technique, angulation and exposure distort this image and lead to error.
The electronic method eliminates many of the problems associated with radiographic measurement. Its most most impor importan tantt advan advantag tage e over over radiography is that it can measure the length of the root canal to the end of the apical foramen, not to the radiographic apex IEJ 1999,32,103 1999,32,103-107 -107
