CLEANING AND SHAPING THE ROOT CANAL SYSTEM l Preclinical Endodontics MCQs

CLEANING AND SHAPING THE ROOT CANAL SYSTEM l Preclinical Endodontics MCQs (multiple choice question) for dental students

Working length estimation

Q: why do you need to find the working length ?

• because it is the length at which canal preparation and obturation will be done.
• Working length [WL] = the distance from the incisal edge or the cusp tip to 0.5 – 1mm short of the radiographical apex
• The apical end of the root canal [ cemento dentinal junction ] is 0.5 -1 mm SHORT from the radiographical apex but sometimes the foramen is located laterally → more than 1 mm away from radiographical apex.
• Reference point : site on the tooth from which measurements are made [ usually the tip of the cusp or the highest point of the incisal edge] – must be easily visualized during prep and stable [ does not change between appointments]

Q: How can you determine the working length?

1.                         A> From pre op radiograph
2.                         B> Using electronic devices [ apex locator ]
3.                         C> Tactile sensation
4.                         D> Bleeding on paper point [ in case of open apex]

• WL should be measured after gaining straight line access to the canals – pre op WL estimation and actual WL might be different because much of the coronal deflection will be eliminated [ mostly length will change in the mesial canals of molars ]

Procedure:

1.     Estimate the WL from the pre op radiograph – you should also know the avg length for each tooth
2.     Get a straight line access and place a file to the estimated length with the rubber stopper on the ref point
3.     Take radio graph to verify if the length is correct or needed adjustment

 

N.B

• if file is more than 2 mm away from desired position → adjust and take another radiograph
• Initial size / initial file : the largest file that can go to the full WL [ should have slight resistance at the apical 3rd ]
• Apexlocator: electronic device that has a probe placed on the pt’s lip and a clip that touches the shaft of a file inserted in the canal. Once the file is closer to the apex the resistance changes and this is displayed on the monitor. – to confirm if your reading is accurate you can check the reading with different file sizes – confirm the length by taking a radiograph.
• BEST WL ESTIMATION PROTOCL = APEXLOCATOR CONFIRMED BY RADIOGRPAH

 

Indications of using an Apexlocator:

1.     Pregnant patients to reduce radiation
2.     Children who can’t tolerate taking radiographs
3.     Disabled or heavily sedated pts
4.     Pt’s who can’t tolerate radiograph because of gag reflex
5.     Apex is obstructed by [ tori, impacted tooth , shallow palatal vault , zygomatic arch , overlapping roots, excessive bone density]

Biomechanical preparation

Objectives of biomechanical preparation:

1.     Obtain a continuous tapered conical form that mimics that natural shape of the canal
2.     Remove all necrotic tissue, pulp, bacterial toxins from the root canal space
3.     Provide enough space inside the canals for irrigation solutions and intracanal medications

 

 Outline form :

• the RC prep should be wider coronally than the middle and apical parts

 Retention form :

• provided by the master cone tug back apically

 Resistance form :

• provided by keeping the apical constriction as narrow as possible – to prevent overfilling

 Extension for prevention :

• to locate any additional canals and remove all pulp debris

 

Instrumentation Motions:

1.  Filing = push and pull
2.  Reaming = push rotate pull
3.  Watch winding = file rotated 2- 3 quarter turns clockwise then anticlockwise then retracted [ most useful for initial canal negotiation]
4.  Balanced force = with flex O and Flex R files – insert with quarter turn clockwise + apical pressure and cutting with counterclockwise rotation with apical pressure

Instrumentation techniques

Step back technique [ telescopic preparation] :

 

Phase I [ apical preparation]:

• Start with the initial file [ the largest file that goes to the full WL] then enlarge 3 sizes to reach the master apical file [ MAF] – in between each file recapitulate by inserting a small size file and removing the debris.
• All the 3 files reach the full working length

Phase II [ preparation of the remaining of the canal ]

• After reaching the MAF enlarge the file size 3 times while reducing 1mm with each size to get the continuous taper of the canal.
• Ex:

1.                    WL = 20 mm initial file is 15
2.                     you enlarge with 20 then 25 then 30 – 30 will be the MAF [ all of those files will reach the full WL ]
3.                   Enlarge with file 35 [ WL = 19 ] file 40 [ WL = 18 ] file 45 [ WL = 17 ]

Advantages

1. Good apical stop
2. Good coronal flare

Disadvantages

1. File tends to straighten in the canal
2. Loss of WL
3. debris can block the canal

 

Crown down technique [ pressure less technique] :

• Coronal flaring with gates glidden then incrementally remove dentine from coronal to apical area
• Start with large k – file [ ex: size 60 ] with reaming action and no apical pressure , then use sequentially smaller files as you proceed apically
• Take radiograph when you reach estimated WL

Balanced force technique : [ Roane technique]

• Coronal and middle 3rd are shaped with Gates Glidden , apical part Is shaped by hand files
• Position and pre load the instrument [ engage the dentine] with clockwise rotation then cut and shape with counterclockwise rotation while maintaining apical pressure [otherwise the file will come out]
• The apex is prepared much larger than all the other techniques

 

Q: what are the problems that can occur during instrumentation?

1. Loss of working length → due to canal blockage with debris if you don’t recapitulate in between files
2. Ledge formation → not following thr canal curvature or precurving the files
3. Zipping [ widening the apex ]
4. Stripping [ lateral perforation]
5. Over instrumentation [ instrumenting beyond the apex and injury to the PA region]
6. Over preparation [ widening the canal prep too much ]
7. File breakage

N.B:

• Niti files cause less canal transportation and ledge formation.

 

 

Irrigation

Importance of irrigation :

1. Flush out bacteria and debris that form from bio mechanical preparation out of the canals
2. Lubricates the canals during instrumentation and increase efficiency of files / instruments
3. Remove debris from lateral / accessory canals/ fins / deltas and other areas files can’t reach and clean
4. Dissolves necrotic tissue / pulp remnants and remove smear layer
5. Use solutions that have antibacterial properties to improve sterilization of canals and overall success of the RCT

 

Ideal irrigant solution:

1. Should have broad spectrum antimicrobial properties / inactivate bacterial endotoxins
2. Ability to dissolve necrotic tissue or debris
3. Good lubricant
4. Low surface tension to flow into inaccessible areas
5. Dissolves the smear layer

•  5.25% NaOCl has better tissue dissolving capacity
•  warming NaOCl syringes in a water bath at 60-70°C → increases it’s effectiveness

 

Q: how can you check if the canals are clean or not yet?

• Place a gauze near the access cavity and irrigate then check the gauze to see how clean the solution is and if there are any debris.