WORKING LENGTH DETERMINATION

Outline

Introduction

Definition & significance

Historical background

Anatomic considerations & terminologies

Stop attachments

Methods for calculation of WL

- Radiographic methods

- Digital tactile method

- Apical periodontal sensitivity

- Paper point evaluation

- Electronic apex locators

- Future trends: Xeroradiography

direct digital radiography

Surgical operating microscope

Laser optical disk storage

- Causes of loss of WL

- Prevention of loss of WL

- References

- Conclusion

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Introduction-

The removal of all pulp tissue , necrotic material & microorganisms is essential for

endodontic success. This can only be achieved if the length of the tooth & the root

canals is determined with accuracy. Traditionally, the point of termination for

endodontic instrumentation & obturation has been determined by taking radiographs.

The development of the electronic apex locators has helped make the assessment of

WL more accurate.

Definition -

Working length is defined in endodontic glossary as “The distance from a

coronal reference point to point at which canal preparation and obturation

should terminate”.

Significance

Determines how far into canal the instruments are placed and worked.

Limits depth to which canal filling may be placed.

Will affect degree of pain and discomfort.

It calculated with in correct limits, plays an important role in determining success

of treatment.

Historical background-

At the end of the 19 th century:

WL was calculated to the site where pt experienced feeling of an instrument.

In 1899 :

Kell introduced application of X-ray to dentistry.

In early 1900’s :

In early 1900s, the popular opinion was that pulp extended through the tooth, past

the apical foramen into the periapical tissue & the narrowest diameter of the apical

portion of the root canal was precisely at the site where the canal exits the tooth at the

extreme apex.

In 1920’s : Grove, et al., contradicted filling till the radiographic apex

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Grove concluded that pulp tissue could not extend beyond the CDJ because the

cells unique to the dental pulp, the odontoblast was not found beyond the CDJ.

Hatton & Grove advised that preparation beyond the CDJ would result in injury to the

periapical tissues.

In 1955 – Kuttler, reported the most comprehensive anatomic microscopic study of the

root tip. he stated filling to the radiographic apex was unwise, contributing to post

operative pain & lowering the production of successful cases. He undertook research on

the anatomy of the apex, studying several thousand teeth under a light microscope &

reported a myriad of measurements.

In 1957- Ingle used pretreatment radiographs in a mathematical procedure for

determining WL.

Anatomical considerations & terminologies-

The apical foramen is the main apical opening of the root canal. It is frequently

eccentrically located away from the anatomic or radiographic apex.

An accessory foramen is an orifice on the surface of the root communicating

with a lateral or accessory canal

Anatomic apex and Radiographic apex

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

Note : Root morphology and radiographic distortion cause the location to vary

Apical foramen vs. Radiographic apex

Kuttler’s investigation showed that this deviation occurred in 68 to 80% of teeth

in his study.

Cementodentinal Junction

0.5-3 mm short of the anatomic apex

1. Is junction of dentin and cementum , it is a histologic landmark .

2. Langeland reported that the Cementodentinal junction does not always coincide

with the apical constriction.

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3. In 6% of cases, the constriction may be blocked by cementum.

4. Ranges from 0.5 to 3.0 mm short of the anatomic apex

Apical constriction( minor apical diameter)

The apical constriction is the apical portion of the root canal having the narrowest

diameter.

This position varies usually 0.5 to 1.0 mm short of the center of the apical foramen.

Less in Young and anterior teeth (0.5)(0.8).

Reference Points – Coronal & Apical

• Measurement is the difference between two points

• In ant teeth incisal edges , post teeth cusp tips.

• In Severely broken down teeth

• A coronal reference point should be identified and monitored accurately.

Stop Attachments

Silicone rubber stops

Metal

Plastic

Directional stops are also available & are useful in indicating the direction of curvature,

etc.

METHODS FOR CALCULATING THE WORKING LENGTH

Ideal Requirements

Rapid and accurate location of apical constriction,

Easy measurement ,

Rapid and periodic monitoring ,

Patient and clinician comfort ,

Minimal radiation to patient,

Ease of use in special patients ,

Cost effective.

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Prerequisite

Good, undistorted, preoperative radiographs showing the total length and all roots

of the involved tooth.

Straight-line access.

Small stainless steel K-file facilitates the process and the exploration of the canal.

A definite, repeatable plane of reference, it should be noted on the patient’s

record.

Knowledge of the average length of all of the teeth.

Which file to use?

Size: must be small enough to negotiate the total length of the canal but large enough not

to be loose in the canal

Material: ss or NiTi

Different techniques

Radiographic methods

Digital tactile method

By apical periodontal sensitivity

Paper point evaluation

Electronic apex locators

Future trends- Xeroradiography

Direct digital radiography

Surgical operating microscope

Laser optical disc storage

1. RADIOGRAPHIC METHODS-

Use of r.g. apex as termination point

A specific distance From the r.g. apex-INGLE’S METHOD

Acc to studies of Kuttler

Others

Differences Between Parallel & Bisecting Angle Techniques

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Parallel Bisecting

1. Greater geometric

accuracy

2. Reproducibility

3. Fewer retake’s

4. Lower radiation dose

5. Superior images of upper

molar roots

6. Superior image of bone

margins

1. Quick and easy

2. Comfortable for all patients

Use of radiographic apex as the termination point

Although used many years ago it is still used by a no. of clinicians. Many teeth so

treated either accidentally or purposely have showed perfect healing. Those who endorse

this concept state that It is impossible to locate CDJ & radiographic apex is the only

reproducible site available.

They stated that by calculating the length of the tooth to the radiographic apex,

keeping the distance patent & using larger files a bit shorter, the most ideal preparation is

developed.

Is radiographic apex reproducible?

Its position depends on many factors-

Angulation of the tooth

Position of the film

Holding agent for the film ( finger, x ray holder, hemostat, cotton roll)

Length of the x-ray cone

positioning of the cone

Anatomic str adjacent to the tooth.

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Advantages :

Many dentists are compulsive & actively seek to eliminate unwanted, possibly

diseased materials. So they are not satisfied unless some excess material is pushed

out through the apical foramen, to indicate that the apical portion of the canal has

been filled & thus sufficiently cleaned.

Less chances of under obturation due to small error in length calculation.

Disadvantages :

• ¯ in success

• Post operative pain.

• Tear drop prep

• ¯ in healing

Specific distance from the radiographic apex-

INGLE’S METHOD

1. Measure the tooth on the preoperative radiograph

2. Subtract at least 1.0 mm “safety allowance” for possible

image distortion or magnification.

3. Set the endodontic ruler at this tentative working length and adjust the stop on the

instrument at that Level.

4. Place the instrument in the canal until the stop is at

the plane of reference unless pain is felt.

5. Expose, develop, and clear the radiograph.

6. measure the difference and correct the working length by adding or subtracting the

difference.

7. From this adjusted length of tooth, subtract a 1.0 mm “safety factor”

8. Place the instrument with adjusting the stopper

9.Take a confirmatory radiograph

10.Record this final working length and the coronal point of reference on the patient’s

record.

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11. final working length may shorten by as much as 1 mm as a curved canal is

straightened out by instrumentation. --à reconfirmed after instrumentation is

completed.

In Resorptive Cases(As modified by Franklin S Weine)

If radiographically, there is no resorption of root end or bone, shorten the length by

the standard 1mm. If periradicular bone resorption is apparent, shorten by 1.5mm, & if

both bone & root resorption are apparent, shorten by 2mm.

The reasoning behind this suggestion is thoughtful. If there is root resorption, the

apical constriction is probably destroyed. Also, when bone resorption is apparent, there is

probably root resorption also, even though it maynot be apparent radiographically.

Variations : SLOB technique

Accuracy ?

• Depends on radiographic technique used

• Forsberg et al., demonstrated that paralleling technique was significantly more

reliable than bisecting angle technique. they found it was 82-89% accurate.

According to studies of KUTTLER- Minor diameter: Acc to studies of Kuttler, the

narrowest diameter of the canal is definitely not at the site of exiting of the canal from the

tooth but usually occurs within the dentin prior to the initial layers of cementum. This

was referred to as CDJ or minor diameter.

Major diameter twice the minor diameter.

• Average distance b/w major & minor diameter:

0.524mm(18-25 yrs)

0.659mm(>55 yrs)

This means that the longitudinal view of the canal as tapering funnel to the root tip is

incorrect. The funnel tapers to the distance short of the site of exiting & then widens

again. Because the adjacent walls of cementum are slightly convex or hyperbolic or

funnel shaped when viewed in longitudinal section, the configuration of the area between

major & minor diameter resembles that of a MORNING GLORY FLOWER.

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Advantages:

Most scientific method for the calculation of WL.

If slight errors are made, less than 1mm in either direction the variance rarely

causes a serious problem & usually will give a calculation similar to other

methods of WL.

Major advantage of this method is it allows rapid development of a solid

apical dentinal matrix.

It improves the opportunity for demonstrated lateral canals.the dense matrix

doesn’t allow excess sealer to exit through the tip during condensation.

Disadvantages:

Most complicated.

Lot of time

Requires radiographs of excellent quality with magnification.

Step by step technique:

1. Prepare proper access cavity, remove pulp tissue & debris.

2. Locate the major or minor diameter on the pre operative radiographs.

3. Estimate the length of the root either by measuring with a mm ruler on the pre

operative radiograph. If the tooth is longer than average- 95th percentile

If average- use average length

If shorter than average- use 5th percentile.

4. Estimate the width of the canals on the radiograph.

If narrow- use 10 to 15 file

Average- 20 or 25 file

If wide – 30 or 35

If very wide – 50 or larger

5. Using the file selected by step 4 , set the stop for the WL as estimated in step 3.

place the file into the access cavity & take an initial radiograph. If the file seems

to stop at a length that could be accurate, stop & take a radiograph.

6. If the file appears too long or too short by more than 1mm from the minor

diameter, adjust the file accordingly & retake the film to verify the accuracy.

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7. If the file appears too long or too short from the minor diameter by less than 1

mm, make the changes & use that as the calculated WL.

8. If file reaches the major diameter exactly subtract 0.5mm if patient is 35 years old

& 0.67 mm if older.

9. If the file is exactly at the minor diameter use that as the WL.

Effect of periapical radiolucency with resorption-

If a definite periapical radiolucency is present with radiographic indication of apical

resorption, apical preparation must end an additional 0.5mm from the calculated WL.

If radiographically visible apical root resorption is sxtensive, shorten by 2 mm or more.

Other methods of determining the tooth length

GROSSMAN’S METHOD

An instrument extending to the apical constriction is placed in the root canal, determined

by digital tactile sense & a radiograph is taken. A mark or stopper is placed at the

occlusal or incisal reference point which will also be detectable on the radiograph. By

measuring the length of the radiographic images of both the tooth & measuring

instrument as well as actual length of the instrument, the clinician can determine the

actual length of the tooth by a mathematical formula.

BREGMEN’S METHOD

Bregmen (1950) has advocated another method in which a 25mm length flat probes are

prepared & each has a steel blade fixed with acrylic resin as a stop, leaving a free end of

10mm for placement into the root canal. This probe is placed in the tooth until the

metallic end touches the incisal edge or cusp tip of the tooth. Then a radiograph is taken.

In the radiographic image the following are measured.

CRD – Real tooth length

CRI – Real instrument length

CAD – Apparent tooth length

CAI – Apparent instrument length

CRD = CRI x CAD

CAI

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Other methods-

Best (1960): used SS pin measuring 10mm fixed to the labial surface of the tooth

with utility wax keeping the pin parallel to the long axis of the tooth & a

radiograph was obtained. From the radiograph the length is determined with the

aid of a special ruler, the BW gauge.

Bramante employed SS probes of various calibers & lengths.

Bramante and Berbert reported great variability in formula determined working

lengths , with only a small percentage of successful measurements

Everett & Fixott(1963) designed a diagnostic x ray grid system. It consists of lines

1mm apart running lengthwise & crosswise. Every 5th mm is accentuated by a

heavier line to make reading easier on radiograph.

LIMITATIONS OF RADIOGRAPHY

Apical foramen may exit buccally or lingually .

Dense bone and Anatomic structures superimpose R-C files obscuring the apex.

Super imposition of zygomatic buttress has been shown to interfere radiograph in

maxillary molar apices.

Provides 2 dimensional image of 3 dimensional structure.

Technique sensitive in both its exposure and interpretation.

Digital tactile method

Accuracy ?

Seidberg – 64% accuracy

In vivo – 25% accuracy

Preflared canals – 75% accuracy

Clinicians should be aware of this and this should be in conjunction with other methods.

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Disadvantages :

Ineffective in root canal with immature apex.

Inaccurate if canal is constricted throughout its length or it curvature is present.

By apical periodontal sensitivity

Always working length determinants should be painless.

In inflamed tissue, hydrostatic pressure developed may cause moderate to severe

instantaneous pain.

When pain is afflicted in this manner, little useful information is gained by

clinician, and considerable damage is done to patients trust.

Canal contents totally necrotic – mild awareness or no reaction

Paper point evaluation

Indications :

• Root canal with immature apex

• Cases in which apical constriction has been lost.

Paper point evaluation, by addition of mm markings 18, 19, 20, 22, 24mm

from tip and can be used to estimate working length.

ELECTRONIC APEX LOCATORS

History –

In 1918, Cluster was first to report the use of electric current to determine WL.

Suzuki in 1942 stated that E. resistance between pdl and oral mucosa was a constant

value of 6.5 kW.

Sunada adopted the Suzuki principle & was the first to describe the detail of a simple

clinical device to measure WL in patients. He used a simple direct current ohmmeter to

measure a constent resistance of 6.5 kW. between oral mucosa & PDL regardless of the

age of the patient or the shape & type of the teeth. The device used by Sunada in his

research became the basis for most apex locators.

In 1987, Huang reported that this principle is not a biologic characteristic but rather a

physical principle.

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Inoue incorporated audiometric component that permitted the device to relate the canal

depth to the operator via low frequency audible sounds. In the 1970s & 1980s , sono-

explorer was developed using this modification.

In 1975,Neosono resistance type apex locator became available. They have improved

circuitry, are more compact & are easier to operate.

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 to patient’s body. The circuit is completed when the endodontic instrument is

advanced apically inside the root canal until it touches the periodontal tissues.

Classification of EAL-

Based on the type of current flow , the opposition to the current flow,& no. of

frequencies involved

FIRST GENERATION EAL-

Resistance type

Measure opposition to the flow of DC.

Locates apical foramen

When the tip of the reamer reaches the apex in the canal, the resistance value is

6.5 kW. Eg.Root canal meter (1969) – 150 Hz

Endodontic meter

Dento meter

Disadvantages : yield inaccurate results in presence of

Electrolytes

Excessive moisture

Vital pulp tissue

Exudates or hemorrhage are present in the canals.

SECOND GENERATION EAL-

Impedance type

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Measure opposition to the flow of AC. The electronic mechanism is that the

highest impedance is at the apical constricture, which is the narrowest portion of

the canal where the impedance changes drastically, when a canal is thought of

being a long hollow tube.

Sono explorer : Sono explorer MK III

Endocater : contains sheath over probe

Apex finder : visual digital LED indicator

Endo analyzer : A. locator + pulp tester

Digiapex I, II, III : A. locator + pulp tester

Exact-A - pex : LED bar graphs

Foramatron IV

Impedance is defined as effective opposition offered by inductance, capacitance and

resistance when they are connected in series to flow of A.C

Note : Root canal are to be reasonably free of electro conductive materials to obtain

accurate readings. Presence of pus & electroconductive irrigants leads to inaccurate

usually shorter measurements. This created a “catch 22” situation. Should canals be

cleaned & dried to measure WL, or should WL be measured to clean & dry the

canals.

III GENERATION APEX LOCATORS

Introduced in early 1990s. In biologic settings, the reactive component facilitates

the flow of AC, more for higher than for lower frequencies. Thus, a tissue through

which 2 AC of different frequencies are flowing will impede the lower frequency

current more than higher frequency current. The reactive component of the circuit

may change, for eg. As the position of the file changes in the canal.

It measures the impedence difference between the two frequencies or the ratio

of two electrical impedances.

They use multiple frequencies to determine the distance from the end of the

canal.

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These units have more powerful microprocessors & are able to process the

mathematical quotient & algorithm calculations required to give accurate

reading.

In 1990, Yamashita reported a device that calculates the difference between 2

impedances from 2 different frequencies which were generated with

composite sine wave current source & marketed as Endex.

Endex :

• Uses very low current and two frequencies of 5 & 1 kHz

• Accurate when fill with electrolyte

Neosono ultima Ez & copilot

Justy II : 500 & 2000 Hz

Apex finder AFA : Wet / dry

Uses 5 frequencies & comparative impedance principles in its electronic circuitry.

It is accurate regardless of irrigants or fluids.

Root ZX : 8 & 0.4 kHz Microprocessor

The shortcomings of early apex locators was overcome by the introduction of ratio

method & development of self calibrating Root ZX.

Capacitance increase at apical constriction. Quotient of impedance decreases rapidly as

the apical constriction is reached. The change in the electrical capacitance at the apical

constriction is the basis for Root ZX.

Combination Apex Locator and Endodontic Handpiece.

Tri Auto ZX

Has 3 automatic safety mechanisms

1. Auto start mechanism

2. Auto torque reverse mechanism

3. Auto apical reverse mechanism.

IV GENERATION APEX LOCATORS

Bingo 1020 / Raypex 4

• Frequencies of 400 Hz and 8 kHz

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• Easier for beginner to use in Preflared canals.

Elements diagnostic unit & apex locator-

Measures capacitance & resistance

Uses composite waveform of 2 signals, 0.5 & 4 KHz

Less sampling error

More consistent

V GENERATION APEX LOCATORS

Raypex 5

Has a unique feature of apex zoom

4 blue – Beginning of apical region.

3 green – Apical constriction region

4 yellow – Adjacent to apical foramen

1 red – Reached apical foramen

&

Red dot – Apex has been passed.

Erroneously positive-

Short circuit

Metal fillings

Parapulpal post

Gingival proliferations

Liquid

Accessory canals

Root perforations/ fracture

Erroneously negative-

Circuit cannot be closed

Obliterated RC

Retreatment

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OTHER USES OF APICAL LOCATORS

• To detect and locate root perforations

• Diagnosis of external and internal resorption.

• Prepared pin holes can be checked.

• Horizontal or vertical root fracture

• Can be used even in deciduous teeth

• Number of radiographic can be reduced

• Vitality testing

COMMON PROBLEMS ASSOCIATED WITH APEX LOCATORS

Inflammation can still have effect on accuracy?

Intact vital tissue, inflammatory exudates and blood can conduct electric current

and cause inaccurate readings.

Other conductors that cause short circuiting are metallic restorations, caries, saliva

and instruments in 2nd canal

Lack of patency, calcifications can also effect.

Contraindicated in patients with some pacemakers

Size of the apical foramen influences WL.

Huang (1987)found that when the size of the major foramen<0.2mm were not

affected, even in the presence of conductive irrigants, but as it>0.2mm measured

distances from the foramen increased.

Steine et al (1990) as width of major foramen increases, distance between file tip

& foramen increased.

Blunderbuss apices tend to give short measurements electronically due to the

instruments not touching the apical dentine walls.(Berman & Fleischman

1984,Wu et al 1992)

Clinical acceptance-

Not still gained widespread acceptance worldwide. This may be due to early devices

which suffer from poor accuracy & didn’t function properly in the presence of common

irrigants. Cost is also a factor.

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NEW TECHNOLOGY

XERORADIOGRAPHY : It has been developed that appear to have a

potential use in endodontics. It differs from conventional radiography in that it

doesn’t require wet chemical processing.

It uses a uniformly charged selenium alloy photoreceptor plate held in a light proof

cassette. When exposed to x rays the charge on the photoreceptor is dissipated according

to tissue density & a latent electrostatic image is formed. This latent image is then

transferred into a visible image by deposition of specially pigmented particles attracted to

the photoreceptor plate. The visible image is then transferred to a base sheet to produce

film of appreciable quality which can be viewed with reflected or transilluminated light.

Advantages :

• Fine detail

• Pronounced edge enhancement

• High image contrast

• Low exposure

• Shows end of instruments and apex

Disadvantages :

Electrocurrent – discomfort

Process of development – 15 min

DIRECT DIGITAL RADIOGRAPHY

This system directly replaces conventional radiographic film with an electronic sensor

which is connected to the computer. The radiographic image is captured on an electric

clip called CCD ( charged coupled device ) & viewed on a high resolution monitor.

Advantages ¯ time between exposure and interpretation.

¯ radiation dose

Digital image enhancement

Patients can more easily, view and appreciate

Patients acceptance

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Disadvantages

Cost

Small sensor cant capture full image

Infection control is of concern as the sensor is not autoclavable. During use the

sensor must be protected with the latex sheath. After use atleast 6 inches of the

connecting cable must undergo disinfection.

Role of magnification

Increased visibility.

Good illumination.

Better depth of focus.

Reliability, reproducibility.

Saves time.

Greater comfort.

LASER OPTICAL DISK STORAGE

It is a useful medium to store radiographic images. An 8” optical disk can store

upto 10,000 images with a 0.5 sec interval & display time.

Image is recorded by a focused laser beam heating a thin film of tellurium

suboxide at specific points on the optimal disk. The heating process changes the

reflectivity of the tellurium oxide to a higher reflective value which institutes the

data storage. The recovery of the image from the optical disk is accomplished by

the detection of reflected light from the disk surface of the points with the higher

reflective value.

Advantage :

Images of superior diagnostic merit

Loss of working length can be due to:

Blockage

Ledges

Fractured instrument

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Accumulation of dentinal debris in the apical 1/3rd of the canal.

Prevention of loss of working length:

Reproducible reference point

Rubber stops at right angle to the shaft of the instrument

Precurve instrs

Continually observe instr stops

Directional instr stops should be used

Use constant radiographic angles

Maintain the original preoperative shape of the canal.

Use copious irrigation & recapitulation

Always use sequential file size

REFERENCES

Ingle’s endodontics, 5th ed

Cohen’s pathways of pulp, 9th ed

Grossman’s endodontic practice, 11th ed.

Weine’s endodontic therapy,5th ed

The electronic determination of working length,DCNA 1992 April;vol 36(2):293-307.

Endodontic working length determination-where does it end? General dentistry

1999;vol.47(3): 281-6.

Straight line access and coronal flaring : effect on canal length,JOE;2002:28:6:474-

476.

Electronic apex locators.IEJ2004;37:425-437,425-437.

The fundamental operating principles of electronic root canal length measurement

devices,IEJ 2006,39:595-609.

www.google.com

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CONCLUSION

The procedure for calculation of working length should be performed

with skill, using techniques that have been proven to give valuable and accurate

results and methods that are practical and efficacious. If performed in this manner,

dentists will produce many treatments of high quality and considerable longevity.

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