Access Cavity guide for the INBDE

Over time several access cavity designs have been promoted for root canal treatment, balancing the evolving understanding of variations in root canal morphology with the concept of  tooth preservation. With the increased popularity of minimally invasive access cavity preparations, here is a review of the fundamentals of access cavities. 

 

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.

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