Tooth Morphology and Access Cavity Preparation

Access is the crucial first step in root canal treatment. A well-designed access cavity is essential for successful endodontic results.
Access cavity is the endodontic coronal preparation enabling unobstructed access to canal orifices, a straight-line path to the apical foramen, and precise control over instrumentation and obturation.
Successful endodontic therapy hinges on: cleaning and shaping, disinfection, and precise three-dimensional obturation of the root canal system.
The main goal of access cavity preparation is smooth, straight-line access to the canal system and the apex.
An ideal access preparation ensures: clear vision into the canal, files can pass through the canal without obstruction, and no remaining caries within the cavity.
Obturating instruments should also pass through the canal without touching the access cavity.
A pyramid diagram shows the hierarchical order of procedures, starting with the access cavity preparation followed by cleaning and shaping, tooth length, and finally obturation.

A diagram illustrates the pulp horn, pulp chamber, orifice, lateral canal, root canal, and apical foramen within a tooth section.

Outline form: Establishing the cavity shape by projecting the internal anatomy onto the external surface. Factors include pulp chamber size (young patients - extensive, old patients - limited), and pulp chamber shape (Anterior - triangular, Premolars - oval or ovoid, Molars - triangular).
Convenience form: Designing the cavity for optimal visibility, instrumentation, and obturation by creating a straight-line path from occlusal to apical foramen. Benefits include unobstructed access to the orifices, direct access to apical foramina, full control over instruments, accommodation of fillings.
Removal of remaining caries and defective restorations: Removing caries and restorations eliminates bacteria, discoloration, and risk of coronal leakage. If the pulp chamber is not completely deroofed, contamination can occur and discoloration of the treated tooth may result.
Toilet of the cavity: Careful removal of all caries, calcified debris, and necrotic material from the pulp chamber before radicular preparation, using irrigation, avoids canal obstruction.

Low-speed contra-angled handpieces and high-speed contra-angled handpieces are used for various procedures.
Different bur sizes are used, including round burs.
Other instruments include specific exploring tools, tapered stones, endo Z burs, pulpout burs, and endo access burs.
Surgical operating microscopes, and loupes are used for magnified visual assistance.

Central and lateral incisor access cavity outline forms are triangular, with the base of the triangle towards the incisal edge and the apex toward the cingulum.

Entrance is gained through the middle of the middle third of the palatal surface.
Initial entrance is prepared with a round bur at a high-speed (90 degrees) until enamel is penetrated.
The bur is angled 45 degrees to the long axis of the tooth and is advanced to enter the pulp chamber.
Removal of the pulp chamber (deroofing).
Removal of the lingual shoulder.

Gouging of labial wall (failure to recognize 29-degree lingual-axial angulation).
Gouging of distal wall (failure to recognize 16-degree mesial-axial inclination).
Perforation at the labiocervical (failure to complete convenience extension).
Missed canal (insufficient convenience extension).
Crown discoloration (failure to remove pulp debris and insufficient incisal extension)
Ledge formation (using an uncurved, oversized instrument).

Illustration showing upper molars with trapezoid shapes and lower molars with trapezoid shapes.

Buccal canal is under the buccal cusp tip.
Palatal canal is at the base of the palatal cusp.
Tips on extension of orifices to the axial walls preventing "mouse hole effect" by ensuring appropriate access cavity extensions and a straight-line path for instrument operation.

Initial penetration should be parallel to the long axis of the tooth, precisely along the center of the central groove.

A round bur is used to open the pulp chamber; the bur will "drop" into the chamber when the chamber is reached.

A round bur is used to deroof the pulp chamber; improper deroofing can contaminate the space and discolor the tooth.

Perforations in the furcation area are a concern. Also crown/root perforation can occur.

Description of the orifices location in maxillary first premolars, mesio-buccal canals in molars, palatal canals, disto buccal canals of maxillary molars and the orifice of the distal and mesio lingual canals of mandible molars, and their related relationships.

Law of centrality: pulp chamber floor located at center.
Law of cementoenamel junction: distance consistent.
Law of concentricity: consistent with external tooth surface.
Law of color change: dark floor, light walls.
Law of symmetry: canal orifices are equidistant from a line.
Law of orifice location: canals are at junction of walls and floor, and at the terminus of developmental fusion lines.

Extension of orifices to the axial walls causes a "mouse hole effect." Under-extension can lead to procedural errors. Straight-line access is confirmed by passively inserting a file to the apex or first curvature without deflection.

Additional information on specific instruments, locations of canals for upper and lower teeth (with diagrams), and specific types of errors frequently made