CHAPTER 7: TOOTH MORPHOLOGY AND ACCESS CAVITY PREPARATION (part I)

from book

main objectives of root canal therapy

thorough shaping and cleaning of all pulp spaces

complete obtura- tion of these spaces with an inert filling material

Diagnostic measures in locating root canal orifices

multiple pretreatment radiographs or cone beam CT

examining the pulp chamber floor with a sharp explorer

troughing grooves with ultrasonic tips

staining the chamber floor with 1% methylene blue dye

sodium hypochlorite “champagne bubble” test

visualizing pulp chamber anatomy & root canal bleeding points

recommended for effective cleaning and drying of the pulp chamber floor before visual inspection of the canal system

17% aqueous ethylenediami-netetraacetic acid (EDTA)

95% ethanol (using the Stropko irrigator fitted with a 27-gauge notched irrigating needle)

dental operating microscope (DOM)

an important aid for locating root canals

introduced into end- odontics to provide enhanced lighting and visibility

enhances the clinician’s ability to remove dentin with great precision, thereby minimizing procedural errors

improves the clinician’s ability to locate and negotiate canals

by magnifying and illuminating the grooves in the pulpal floor and by distinguishing the color differences of the dentin of the floor and walls

sodium hypochlorite (NaOCl)

allowing this to remain in the pulp chamber may help locate a calcified root canal orifice

tiny bubbles may appear in the solution, indicating the position of the orifice

root canal system

The entire space in the dentin where the pulp is housed

The outline of this system corresponds to the external contour of the tooth

factors potentially modify its dimensions such as:

• physiologic aging

• pathosis

• occlusion

• production of secondary/tertiary dentin & cementum

two portions of root canal system

pulp chamber — in the anatomic crown of the tooth

pulp or root canals — found in the anatomic root

Other features of root canal system

pulp horns

accessory canals

lateral canals

furcation canals

canal orifices

apical deltas

apical foramina

root canal

begins as a funnel-shaped canal orifice

generally at or just apical to the cervical line and ends at the apical foramen, which opens onto the root surface at or within 3 mm from the center of the root apex

Nearly all root canals are curved, particularly in a faciolingual direction

These curva- tures may pose problems during shaping and cleaning proce- dures because they are not evident on a standard facial radiograph.

Angled views are necessary to determine their presence, direction, and severity

A curvature may be a gradual curve of the entire canal or a sharp curvature near the apex

Double S-shaped canal curvatures also can occur.

In most cases the number of root canals corresponds to the number of roots; however, an oval root may have more than one canal

Accessory canals

are minute canals that extend in a horizontal, vertical, or lateral direction from the pulp to the periodontium

In 74% of cases they are found in the apical third of the root, in 11% in the middle third, and in 15% in the center of the root apex.

contain connective tissue and vessels but do not supply the pulp with sufficient circulation to form a collateral source of blood flow

formed by the entrapment of periodontal vessels in Hertwig’s epithelial root sheath during calcification

they are sig- nificant because they serve as avenues for the passage of irri- tants, primarily from the pulp to the periodontium

furcation canals

Accessory canals may also occur in the bifurcation or trifurcation of multi-rooted teeth

form as a result of the entrapment of periodontal vessels during the fusion of the diaphragm, which becomes the pulp chamber floor

occur in three distinct pattern for mandibular molars

4 to 720 μm

the diameter of furcation openings in mandibular molars according to scanning electron microscopy (SEM)

none to more than 20 per specimen

range of number of furcation canals

Mandibular teeth

have a higher incidence of foramina involving both the pulp chamber floor and the furcation surface

3 distinct patterns accessory canals occur in MN first molars

single furcation canal extends from the pulp chamber to the intraradicular region

lateral canal extends from the coronal third of a major root canal to the furca- tion region (80% extend from the distal root canal)

have both lateral and furcation canals.

basic requirement for endodontic success.

diagnosis and treatment planning

knowledge of common root canal morphology & its frequent variations

the exception rather than the rule

a root with a tapering canal & a single foramen

Vertucci’s classification type I

A single canal extends from the pulp chamber to the apex (1)

Vertucci’s classification type II

Two separate canals leave the pulp chamber and join short of the apex to form one canal (2-1)

Vertucci’s classification type III

One canal leaves the pulp chamber and divides into two in the root; the two then merge to exit as one canal (1-2-1)

Vertucci’s classification type IV

Two separate, distinct canals extend from the pulp chamber to the apex (2)

Vertucci’s classification type V

One canal leaves the pulp chamber and divides short of the apex into two separate, distinct canals with separate apical foramina (1-2)

Vertucci’s classification type VI

Two separate canals leave the pulp chamber, merge in the body of the root, and redivide short of the apex to exit as two distinct canals (2-1-2)

Vertucci’s classification type VII

One canal leaves the pulp chamber, divides and then rejoins in the body of the root, and finally redivides into two distinct canals short of the apex (1-2-1-2).

Vertucci’s classification type VIII

Three separate, distinct canals extend from the pulp chamber to the apex (3).

Weine’s Classification type I

(Single canal, single exit) 1

Weine’s Classification type II

(Two separate canals, merging to one exit) 2-1

Weine’s Classification type III

(Two canals throughout, twO separate exits) 2-2

Weine’s Classification type IV

(One single canal, splitting into two exits) 1-2

orifice

If only one of this is found and it is not in the center of the root, another one probably exists

The closer they are, the greater the chance the two canals join at some point in the body of the root

as the distance between of this in a root increases, the greater the chance is that the canals will remain separate.

The more separation between orifices the less the degree of canal curvature

3 anatomic & histologic landmarks where classic concept of apical root anatomy is based

minor foramen / apical constriction (AC)

cementodentinal junction (CDJ)

major foramen / apical foramen (AF)

apical constriction (AC)

narrowest part of the canal

tapering 0.5-1.5mm coronal to the apical foramen

generally is considered the part of the root canal with the smallest diam- eter

also is the reference point clinicians use most often as the apical termination for shaping, cleaning, and obturation

Pulp blood vessels are narrow at the AC, which makes success- ful treatment of inflammation in the canal difficult

Posttreat- ment discomfort generally is greater when this area is violated by instruments or filling materials, and the healing process may be compromised.

cementodentinal junction (CDJ)

approximately 1 mm from the apical foramen

the point in the canal where cementum meets dentin

it is the point where pulp tissue ends and periodontal tissues begin

The location in the root canal varies considerably

It generally is not in the same area as the apical constriction

considered just a variable junction at which two histologic tissues meet in the root canal

apical foramen (AF)

circumference or rounded edge like a funnel or crater, that differentiates the termination of the cemental canal from the exterior surface of the root

From apical constriction, the canal widens as it exits the root at this point

does not normally exit at the anatomic apex but rather is offset 0.5 to 3 mm

funnel shaped / hyperbolic / shape of a morning glory

The space between the major and minor diameters has been described as ___

mean distance between the major and minor apical foramen

0.5 mm in a young person

0.67 mm in an older individual

—distance is greater in older individuals because of the buildup of cementum

diameter of the foramen

502 μm in individuals 18-25 years of age

681 μm in those over age 55

—which demonstrates the growth of apical foramen with age

diameter of the canal at the CDJ

353 μm for the central incisors

292 μm for the lateral incisors

298 μm for the canines

maxillary premolars

had the most and the largest accessory foramina (mean value, 53 μm) and the most complicated apical morphologic makeup.

—mandibular premolars had strik- ingly similar characteristics, a possible reason why root canal therapy may fail in premolar teeth

termination points

1 mm from the apex — no bone or root resorption has occurred

1.5 mm from the apex — only bone resorption has occurred

2 mm from the apex — both bone and root resorption have occurred

may cause a severe inflammatory reaction

Sealer or gutta-percha (or both) in the periradicular tissues, lateral canals, and

apical ramifications

Successful treatment depends on:

the anatomy of the root canal system, the dimensions of the canal walls, and the final size of enlarging instruments.

maxillary first molar

has a very complicated canal shape at the apical limit and that this anatomy makes shaping, cleaning, and obturation difficult, particularly in the MB-1 and distobuc- cal canals.

isthmus

a narrow, ribbon-shaped communication between two root canals that contains pulp or pulpally derived tissue

must be found, prepared, and filled during surgery, because they can function as bacterial reservoirs

Any root with two or more canals may have this so this should be suspected whenever multiple canals are seen on a resected root surface

methylene blue dye

used to aid visualization of the outline of the resected root surface and thus detection of an isthmus

objectives of access cavity preparation

(1) to remove all caries,

(2) to conserve sound tooth structure,

(3) to completely unroof the pulp chamber,

(4) to remove all coronal pulp tissue (vital or necrotic),

(5) to locate all root canal orifices,

(6) to achieve straight- or direct-line access to the apical foramen or to the initial curvature of the canal

(7) to establish restorative margins to minimize marginal leakage of the restored tooth

properly prepared access cavity creates:

a smooth, straight-line path to the canal system and ultimately to the apex, or position of the first curvature

allows complete irrigation, shaping and cleaning, and quality obturation

Ideal access results in straight entry into the canal orifice, with the line angles forming a funnel that drops smoothly into the canal(s)

Straight line access

provides the best chance of debridement of the entire canal space and reduces the risk of file breakage

isthmus classifications type I

an incomplete isthmus

it is a faint communication between two canals.

isthmus classifications type II

characterized by two canals with a definite connection between them (complete isthmus).

isthmus classifications type III

a very short, complete isthmus between two canals.

isthmus classifications type IV

complete or incomplete isthmus between three or more canals

isthmus classifications type V

marked by two or three canal openings without visible connections.

indicates the location of the line angles

Projection of the canal center line to the occlusal surface of the tooth

step in preparing an access cavity

visualization of the position of the pulp space in the tooth

Evaluation of the Cementoenamel Junction and Occlusal Anatomies

Preparation of the Access Cavity Through the Lingual and Occlusal Surfaces

Removal of All Defective Restorations and Caries Before Entry Into the Pulp Chamber

Removal of Unsupported Tooth Structure to prevent tooth fracture.

Delay of Dental Dam Placement Until Difficult Canals Have Been Located and Confirmed

Location, Flaring, and Exploration of All Root Canal Orifices

Inspection of the Pulp Chamber, Using Magnification and Adequate Illumination

Tapering of Cavity Walls and Evaluation of Space Adequacy for a Coronal Seal

cementoenamel junc- tion (CEJ)

the most important anatomic landmark for determining the location of pulp chambers and root canal orifices.

importance of Diag- nostic radiographs

help the clinician estimate the position of the pulp chamber,

the degree of chamber calcification,

the number of roots and canals

the approximate canal length

—Palpation along the attached gingiva aids the determination of root location and direction.

results when passage of instru- ments down the canal is not guided by the walls of the root canal

treatment errors

root perforation

misdirection of an instrument from the main canal (ledge formation)

instru- ment separation

creation of an incorrect canal shape (apical transportation)

Micro-Openers

excellent instruments for locating canal orifices when a dental dam has not been placed.

flexible, stainless steel hand instru- ments have #.04 and #.06 tapered tips

have offset handles that provide enhanced visualization of the pulp chamber.

—dental dam must be placed once the roof of the pulp chamber has been penetrated and the canals identified.

endodontic explorer

a sharp instrument used to locate canal orifices and to determine their angle of departure from the pulp chamber

Congealed pulp tissue

may form a collagen plug that blocks the apex, preventing complete shaping and cleaning.

reducing the risk of bacterial contamination of the canal system

composite, glass ionomer, and mineral trioxide aggregate

Round carbide burs #2, #4, and #6

used extensively in the preparation of access cavities.

They are used to remove caries and to create the initial external outline shape.

They also are useful for penetrating through the roof of the pulp chamber and for removing the roof

fissure carbide and diamond round-end burs

can be used for some of the axial wall extensions of the access cavity preparation.

can be used to level off cusp tips and incisal edges, which are used as reference points for the working length determination.

However, when these burs are used for this purpose by inexperienced clinicians, their cutting ends can gouge the pulp floor and axial walls

—Fissure carbide and diamond burs with safety tips (i.e., noncutting ends) safer choices for axial wall extensions. They can be used to extend and favorably orient the axial walls of the pulp chamber. Because they have no cutting end, the burs can be allowed to extend to the pulp floor

Round diamond burs (#2 and #4)

needed when endodontic access must be made through porce- lain or ceramometal restorations

—Diamond burs are less trau- matic to porcelain than carbide burs and are more likely to penetrate the porcelain without cracking or fracturing it

Gates-Glidden burs

used during shaping and cleaning to enter the canal(s) easily

used to flare or enlarged and blend the orifices into the axial walls of the access cavity

This process permits the intracanal instruments

starting with smaller sizes and progressing to the larger sizes

DG-16 endodontic explorer

used to identify canal orifices and to determine canal angulation.

JW-17 endodontic explorer

serves the same purpose as DG-16 endodontic explorer

its thinner, stiffer tip can be useful for identifying calcified canals.

useful for detecting any remaining pulp chamber roof, particularly in the area of a pulp horn

endodon- tic spoon

used to remove coronal pulp and carious dentin

fine Ultrasonic tips

used to trough and deepen developmental grooves to remove tissue and explore for canals

are smaller than conventional round burs

their abrasive coatings allow clinicians to sand away dentin and calcifications conservatively when exploring for canal orifices.

Evaluation of straight-line access

• Insert the largest file that fits passively to the apical foramen or first canal curvature.

• Measure internal canal length from a pretreatment periapical radiograph.

• Gently insert and withdraw the file to feel for binding or deflection.

If deflection occurs:

• Reassess lingual shoulder removal.

• Avoid prematurely extending the incisal edge.

• If lingual shoulder is adequate but the file still binds, extend the incisal wall until straight-line access is achieved.

Final incisal wall position is determined by:

1. Complete removal of pulp horns

2. Achievement of straight-line access

Access preparation

• #2 round bur for premolars

• #4 round bur for molars

—Penetrate 1 mm into dentin

Shape considerations

• Premolars: oval, widest buccolingually

• Molars: initially oval, widest:

  • Maxillary molars: buccolingual

  • Mandibular molars: mesiodistal

Final molar outline

triangular (three canals) or rhomboid (four canals), with corners determined by canal orifice positions.

Penetration angles of premolars

parallel to the long axis of the root in both mesiodistal and buccolingual directions to avoid gouging or perforation, as roots are often tilted relative to the occlusal plane.

Penetration angles of molars

toward the largest canal, since the pulp chamber is usually widest just above that orifice:

• Maxillary molars → palatal canal

• Mandibular molars → distal canal

“mouse hole” effect

an orifice extends onto an axial wall indicating internal underextension and impeding straight-line access.

internal impediments to straight-line access in posterior teeth

cervical dentin bulges and the natural coronal canal constriction.

Cervical dentin bulges

are dentin shelves that may overhang canal orifices, restricting access and accentuating canal curvatures.

Removal methods for Cervical dentin bulges

Safety-tip diamond or carbide burs

Gates-Glidden burs

Instruments are placed at the orifice level and leaned toward the bulge to remove overhanging dentin.

Removal methods for Coronal flaring:

• Use Gates-Glidden burs in a sweeping upward motion with lateral pressure away from the furcation.

• Alternatively, #10 or #12 tapered NiTi rotary files can shape the upper canal.