Root Canal Treatment
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The mains goals of conventional root canal treatment include:
- Alleviating current symptoms and preventing future symptoms.
- Debriding and shaping the root canals, which involves:
- Removing compromised pulpal wall dentine.
- Widening the canals enough to allow for adequate irrigation, drying and obturation.
- Minimizing tooth removal to maintain maximal structural integrity.
- Creating the radiographic appearance of a well-obturated root canal system.
- Maintain health and promote healing of periradicular tissues.
ENDODONTIC INSTRUMENTS
Broach – a tapered metal instrument with sharp projections/barbs along its length, used to remove pulp tissue, cotton pellets, or other soft materials. A broach is not used for canal enlargement.
Reamer – a tapered metal instrument with sharp cutting edges, used with a reaming motion (half-turn, twist, pull) to enlarge root canals. A reamer differs from a K-type file in having a lower number of flutes.
NiTi file – a tapered metal instrument composed of a nickel-titanium alloy. These can be used as hand files and are commonly used as rotary files. NiTi files are less likely to fracture and are better able to remain centered in the canal, resulting in a lower chance of canal transportation. NiTi files are more flexible than their stainless steel counterparts. Differing characterics of files include
- Size (e.g size 10, 15, 20).
- Taper (e.g. 0.02, 0.04, 0.06).
- Length (measured in mm).
Larger files are less flexible than smaller files. More tapered files are stronger and more aggressively remove tooth structure in the coronal area. This will allow easier cleaning, irrigation and obturation, but will increase structural compromise.
K-file – a tightly spiraled tapered metal cutting instrument with edges that cut in a reaming, push-pull filing, or watch-winding motion. There are many different cross-sectional designs that are machined or twisted.
Hedstrom file – a tapered metal instrument with elevated cutting edges that appear to form a series of intersecting cones used with a pulling motion. A Hedstrom file cuts more aggressively than a K-file.
Gates Glidden drills – a slender stainless steel shank with a flame-shaped cutting head and a non-cutting pilot-tip. These drills are mainly used to widen and taper the root canal orifice. They are designed sto fracture near the hub rather than between the shank and the head. The diameter of the cutting head comes in sizes 1-6, ranging from 0.5 to 1.5 mm, corresponding to ISO sizes 50–150. Many NiTi rotary file systems include a file specifically designed to open the canal orifice like Gates Glidden drills, but the latter is much cheaper.
ACCESS CAVITIES
An access cavity preparation aims to obtain the following:
- Facilitate complete removal of the entire pulp chamber content.
- Provide complete visualization pulp chamber floor.
- Provide straight line access for the introduction of instruments.
There is great individual variation when it comes to tooth and root morphology. However, the following are the most likely pulpal canal configurations and corresponding access cavities appropriate to perform orthograde endodontic procedures. Statistics are approximate, rounded to be easier to memorize. Third molar anatomy is too varied and not included, but tends to be similar to their immediate neighbor.
Single canal – central and lateral incisors, canines, and maxillary second premolars, and mandibular first and second premolars. However:
- 25% of maxillary second premolars have more than one canal.
- 10-20% of mandibular premolars have a second canal.
- 5% of mandibular central incisors have a second canal.
- 30% of mandibular lateral incisors have a second canal.
- 5% of mandibular canines have a second canal.
Two canals – maxillary first premolar, though 20% have a single canal, and around 5% have 3.
Three canals – maxillary second molar and lower molars. However:
- 30% of maxillary second molars have 4 canals.
- 30% of mandibular first molars have 4 canals.
- 5% of mandibular second molars have 4 canals, and 5% have C-shaped canals.
Four canals – maxillary first molar, though 40% only have 3 canals (singe mesio-buccal canal).
MAXILLARY TEETH
Central incisors
- Triangular access cavity (to include mesial and distal pulp horns).
- Single orifice on the pulp chamber floor.
- Single canal.
- Single canal at the apex.
Lateral incisors
- Ovoid access cavity.
- Single orifice on the pulp chamber floor.
- Single canal.
- Single canal at the apex.
- There is usually a distal curvature in the apical third of the root.
Canines
- Ovoid access cavity.
- Single orifice on the pulp chamber floor.
- Single canal.
- Single canal at the apex.
First premolar
- Ovoid access cavity.
- 80% present with two orifices on the pulp chamber floor, 15% with a single, 5% with three.
- In almost 20% of teeth the two pulpal canals coalesce to a single apical opening (Type II).
Second premolar
- Ovoid access cavity
- 75% present with a single orifice on the pulp chamber floor, 24% with two, 1% with three.
- In almost 20% of cases the two pulpal canals coalesce to a single apical opening (Type II).
First Molar
- Triangular access cavity.
- 60% (maybe greater) present with a second mesio-buccal canal (MB2), which combined with the palatal and disto-buccal canals makes 4 orifices on the pulp chamber floor. 40% lack the MB and only present with three.
- Two-thirds of MB2 coalesce with MB1 to form one canal with a single apical opening.
- Less than 20% of mesio-buccal roots present with two apical openings.
- The MB2 canal is located just lingual to the MB1.
- These teeth have the highest endodontic failure rate.
Second molar
- Triangular access cavity.
- 70% present with a single mesio-buccal canal, which combined with the palatal and disto-buccal canals makes 3 orifices on the pulp chamber floor. 30% have an MB2 like the first molar and present with four canals.
- Half of MB2 coalesce with MB1 to form one canal with a single apical opening.
- 10% of mesio-buccal roots present with two apical openings.
MANDIBULAR TEETH
Central incisor
- Ovoid access cavity.
- 95% present with a single canal orifice on the pulp chamber floor, 5% with two.
- 20% of teeth have a single orifice on the pulp chamber floor, but the canals splits into 2 canals, before joining back together as one (Type III).
Lateral incisor
- Ovoid access cavity.
- 95% present with a single canal orifice on the pulp chamber floor, 5% with two.
- 20% of teeth have a single orifice on the pulp chamber floor, but the canals splits into 2 canals, before joining back together as one (Type III).
Canine
- Ovoid access cavity.
- 95% present with a single canal orifice on the pulp chamber floor, 5% with two.
- 15% of teeth have two orifices on the pulp chamber floor, but the canals join into a single canal before joining back together as one (Type III).
First premolar
- Ovoid access cavity.
- Care must be taken since the crown of a mandibular premolar is tilted lingually relative to the root.
- 75% present with a single canal, 25% with two canals.
- In the majority of teeth with two canals, there is only a single canal visible on the pulp chamber floor, and the canal bifurcates further down the root (Type V). Only about 2% may present with more than one orifice.
Second premolar
- Ovoid access cavity.
- Care must be taken since the crown of a mandibular premolar is tilted lingually relative to the root.
- 98% single canal orifice on the pulp chamber floor, 2% with two.
First molar
- Trapezoid access cavity.
- Orifices are usually distributed symmetrically either side of an imaginary mesio-distal line through the middle of the crown.
- 75% of the mesial roots will have two orifices on the pulp chamber floor. 25% will have a single canal. In about half the teeth with two mesial canals they will merge to a single apex.
- 80% of the distal roots will have one orifices on the pulp chamber floor. 20% will have two canals. A distal canal further from the midline indicates a second canal.
- These teeth are statistically the most likely to require root canal treatment.
Second molar
- Trapezoid access cavity.
- Orifices are usually distributed symmetrically either side of an imaginary mesio-distal line through the middle of the crown.
- 65% of the mesial roots will have two orifices on the pulp chamber floor. 35% will have a single canal. In about half the teeth with two mesial canals they will merge to a single apex.
- 95% of the distal roots will have one orifices on the pulp chamber floor and one canal. 5% will have two canals.
- 5% of teeth will not have the traditional canal configuration and might instead only have a C-shaped canal.
WORKING LENGTH
Working length determination requires the selection of a reference point and the determination of the end/stop that the canal is to be prepared and obturated to. Working length determination often follows access cavity preparation and is accomplished using small (10 or 15) hand files, apex locators, and radiographs. With the improvement of the accuracy and reliability of apex locators, many operators are comfortable going off apex locator measurements only, but radiographs are recommended to confirm the working length. An apex locator is an electronic instrument that operates on the principles of resistance, frequency, or impedance. The tactile feel for a file at the apical constriction is sometimes used to verify the correct position.
When referring to the apex there are multiple landmarks that could be referenced:
- Anatomic apex (radiographic apex) – the most apical point of the root.
- Apical foramen – the opening at the apex of the root of a tooth through which the nerve fibers and blood vessels pass. The apical foramen rarely coincides with the anatomic apex. The mean distance between anatomic apex and apical foramen is about 0.5 mm.
- Apical constriction – the narrowest area at the apex, usually located about 0.5 mm from the apical foramen. This narrowing provides a natural stop in root canal treatment and can be reliably detected by an apex locator.
The root canal instrumentation and obturation is commonly stopped about 1.0 mm short of the radiographic apex (at the apical constriction) since the apical constriction sits 0.5 mm from the apical foramen, which sits 0.5 mm from the radiographic apex.
Working films are radiographs used to establish or confirm the working length, that is, the distance from a reference point to the apical stop. A master cone (master point) refers to the largest gutta-percha point that can be placed to full working length.
CLEANING AND SHAPING
The goal of this stage is to remove all necrotic pulpal tissue, the compromised dentine wall, and open up access to any lateral canals or apical delta. Progressively larger files are used to clean and shape each individual canal. Rotary instruments are most commonly used during this stage, and variations in techniques are dictated by specific manufacturer’s instructions. Coronal preparation usually precedes apical preparation to avoid excessive friction, file binding, unnecessary fatigue or potential failure. The sequence is determined by the manufacturer and the clinician.
The crown-down technique of canal preparation involves initial shaping and flaring of the coronal third of the root canal, followed by incremental shaping and cleaning further down. The step-back technique follows the reverse order. Smaller, more flexible files are used to shape and clean the apical third, followed by mid and coronal root preparation. The hybrid technique is a combination of the two.
INTRACANAL IRRIGANTS
Sodium hypochlorite (NaOCl, bleach) is a clear greenish-yellow solution that has a high pH and chlorine odor. The irrigant is a fantastic solvent for organic tissue and a potent antimicrobial agent, but does not remove the smear layer. NaOCl is considered the gold standard endodontic irrigant with common clinical solutions ranging in concentrations from 0.5 to 5.25%. It is not recommended to use off-the-shelf bleach for dental treatment. Sodium hypochlorite will inactivate EDTA.
Care needs to be taken using this irrigant. If spilled it will bleach clothing and it may cause severe adverse symptoms and reactions if extruded beyond the apex. Termed a sodium hypochlorite accident, this causes tissue damage (necrosis) in the area surrounding the apex of the tooth, usually leading to severe pain, rapid tissue swelling, and bleeding from the periapical area. Treatment following a hypochlorite accident may include:
- Administering long lasting local anesthetic like bupivacaine.
- Encouraging drainage through the canal.
- Oral steroids.
- Cold compresses.
- Post-operative antibiotics and analgesics.
Hydrogen peroxide (H2O2) is a colorless liquid biocide that has been extensively used for disinfection and sterilization, and can substitute as a canal irrigant. It is used in concentrations of 3-5% and is active against bacteria, viruses, and yeasts. H2O2 degrades to form water and oxygen, with hydroxyl free radicals (•OH) that destroys proteins and DNA. It is a much less solvent solution but can be used in combination with NaOCl. Hydrogen peroxide bubbles when it comes into contact with tissue and partially cleans with an effervescent effect.
Ethylene-diamine-tetra-acetate (EDTA) is an aminopolycarboxylic acid used as a chelating agent to bind iron and calcium ions. It binds these ions as a hexadentate aqueous solution and is used to demineralize and soften dentin, facilitating canal negotiation and preparation. EDTA is also used to remove the smear layer in root canals before obturation. EDTA is the active ingredient in RC-PREP, and can be used as an irrigant (in solution) or a lubricating gel. EDTA will inactivate sodium hypochlorite.
Chlorhexidine (CHX) is a bis-biguanide antiseptic antibacterial agent that is bacteriostatic at lower concentrations and bactericidal at higher concentrations. CHX is less toxic to patient tissues but does not have the solvent action of NaOCl.
OBTURATION
After the canal has been prepared, cleaned and sterilized, the canal(s) are filled. There are many materials and techniques used for this process. Almost universally, gutta-percha used in combination with a sealer is the gold standard for root canal obturation.
Gutta-percha (GP) is a tough rubbery material made from the latex of several Malaysian trees (especially Palaquium gutta) of the sapodilla family, and is used to obturate root canals in conjunction with sealers. Gutta-percha is pliable, radiopaque, and comes in various sizes. Chloroform is often used to remove existing gutta-percha during retreatment root canal therapy. Gutta-percha needs to be used with a sealer otherwise it will not attain a hermetic seal. The elasticity may cause rebound from dentine, and there is some shrinkage after cooling.
There are various methods of using gutta-percha to obturate a tooth. Cold lateral compaction was the gold standard for a long time. A master cone is placed, and after the length is confirmed radiographically, the walls are coated with sealer and the cone seated. The master cone is condensed against the lateral wall with a spreader instrument. When this instrument is removed, another (accessory) cone is placed and the spreader reinserted. This process is repeated until obturation is achieved.
Warm vertical compaction uses a heated instrument to sear gutta-percha to a point about halfway down the canal. The coronal section is removed, and the apical section heated and condensed. The coronal void is then backfilled using an injection system.
Carrier- based obturation uses plastic carriers coated with GP that are thermoplasticized in a special oven prior to insertion into the canal. A size verifier that will correspond to the correct size obturator is used prior to obturation. The plastic carrier is seated and subsequently severed at the canal orifice leaving the plastic carrier as part of the permanent filling.
The continuous wave of condensation technique is a hybrid of the cold lateral and warm vertical techniques. The technique uses a well-fit master apical cone and a heatsource at 200°C to a depth 3-mm short of working length. The canal is then backfilled with warm gutta-percha with an increment of up to 10 mm in depth.
Endodontic sealers are used to fill the discrepancies between the canal walls and gutta-percha. Sealers commonly used include:
- Zinc oxide-eugenol sealers.
- Calcium hydroxide sealers.
- Resin sealers.
- Glass ionomer sealers.
The optimum sealer should be:
- Non Irritating to periapical tissues (if extruded).
- Insoluble.
- Dimensionally stable.
- Able to create a hermetic seal.
- Radiopaque.
- Bacteriostatic.
- Easy to remove if necessary.
Chloroform is the reagent of choice used to dissolve gutta-percha during root canal retreatment. However, it needs to be used with caution since the vapor can be hazardous. Other chemicals used as an alternative to dissolve gutta-percha:
- Turpentine
- Methyl chloroform
- Carbon disulfide
- Benzene
- Halothane
