Access Cavity Preparation an Anatomical and Clinical Perspective June 2011

1FAMDENT PRACTICAL DENTISTRY HANDBOOK Vol. 10 Issue 3 Jan. - Mar. 2010

ACCESS CAVITY PREPARATION – AN ANATOMICAL AND CLINICAL PERSPECTIVEDr. V. Gopi Krishna, MDS, FISDR is a clinician, researcher and academician of national acclaim. He is the Co-Editor of Grossmans Endodontic

Practice - 12th Edition (Wolters Kluwer – Lipincott) and Editor of Preclinical Manual of Conservative Dentistry (Elsevier). He is also the Editor-

in-Chief of the Journal of Conservative Dentistry (www.jcd.org.in) and is working as Professor, Dept. of Conservative Dentistry & Endodontics

at Thai Moogambigai Dental College. He runs the Root Canal Centre - an exclusive endodontic training and treatment centre at Chennai,

which mentors more than 100 dentists every year in improving their endodontic skills. For more information on microscope aided clinical

endodontic training modules with live patient demonstrations you can contact Dr. Gopi Krishna at [email protected] (Ph: 91-

9840218818) and on Facebook Root Canal Centre.

“You don’t know how much you know until you know

how much you don’t know”

“Access is success”. These three words in a nutshell convey the

significance of a proper access opening on the overall quality and

success of endodontic treatment. In this era of instrument driven

endodontics it is important that one understands that the efficacy

of cleaning and shaping is effective only if employed after a good

access opening.

Anatomy of Pulp Cavity

The pulp cavity is the central cavity within a tooth and is entirely

enclosed by dentin except at the apical foramen (Fig. 1). The pulp

cavity may be divided into a coronal portion, the pulp chamber,

and a radicular portion, the root canal. In anterior teeth, the pulp

chamber gradually merges into the root canal, and this division

becomes indistinct. In multi-rooted teeth, the pulp cavity consists

of a single pulp chamber and usually three root canals, although

the number of canals can vary from one to four or more. The roof

of the pulp chamber consists of dentin covering the pulp chamber

occlusally or incisally (Fig. 1).

A pulp horn is an accentuation of the roof of the pulp chamber

directly under a cusp or developmental lobe. The term refers more

commonly to the prolongation of the pulp itself directly under a

cusp. The floor of the pulp chamber runs parallel to the roof and

consists of dentin bounding the pulp chamber near the cervix of

the tooth, particularly dentin forming the furcation area. The canal

orifices are openings in the floor of the pulp chamber leading into

the root canals. The canal orifices are not separate structures, but

are continuous with both pulp chamber and root canals. The walls

of a pulp chamber derive their names from the corresponding walls

of the tooth surface, such as the buccal wall of a pulp chamber. The

angles of a pulp chamber derive their names from the walls forming

the angle, such as the mesiobuccal angle of a pulp chamber.

(Fig. 1)

Anatomy of Root Canals

The root canal is that portion of the pulp cavity from the canal

orifices to the apical foramen. It may be divided for convenience

into three sections, namely: coronal, middle, and apical thirds.

Accessory canals, or lateral canals, are lateral branching of the

main root canal generally occurring in the apical third or furcation

area of a root (Fig. 1). A distinction sometimes made between an

accessory canal and a lateral canal is that a lateral canal is an

accessory canal that branches to the lateral surface of the root and

may be visible on a radiograph. Lateral canals occur 73.5% of the

time in the apical third, 11.4% of the time in the middle third and

6.3% of the time in the cervical third of the root. The apical foramen

is an aperture at or near the apex of a root through which the

blood vessels and nerves of the pulp enter or leave the pulp cavity.

Accessory foramina are the openings of the accessory and lateral

canals in the root surface (Fig. 1).

Although variations are the norm in root canal configurations;

various researchers have classified them according to the number

of canals, intracanal branching & fusion and exit from the canal.

The most widely accepted clinical classification was proposed by

Vertucci and his classification is as follows: (Fig. 2)

GROSSMAN’S Corner

Dr. V. Gopi Krishna

Fig. 1 Various views of the root canal system: (a) Labial view of a central incisor. (b) Apical third of a root. (c) Buccal view of a maxillary first molar. (d) Buccal view of a mandibular first molar.


3. To de-roof the pulp chamber

4. To remove the coronal pulp tissue including vital pulp, necrotic

pulp and pulp stones.

5. To locate all the root canal orifices

6. To achieve straight line access into the root canal

7. To remove the dentinal shelves between the canal orifices

Armamentarium

I would like to classify the instruments and equipments required for

access cavity preparation into two separate lists:

I. Must Have Instruments & Equipments

a. Burs

i. Round burs ( #2, #4 and #6) (Fig. 3)

ii. Tapering fissure bur with a round head (Fig. 4)

iii. Safe end burs e.g.: Endo Z bur (Fig. 5)

iv. Transmetal bur e.g.: Trihawk burs (Fig. 6)

• TypeI(Fig.2a) Single canal extends from the pulp chamber

to the apex

• Type II (Fig. 2a) Two separate canals leaving the pulp

chamber and join short of the apex to form one canal (2-1)

• Type III (Fig. 2a) 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)

• Type IV (Fig. 2b) Two separate, distinct canals extend from

the pulp chamber to the apex

• Type V (Fig. 2b) One canal leaves the pulp camber and

divides short of the apex into two separate, distinct canals with

separate apical foramina (1-2)

• Type VI (Fig. 2b) Two separate canals leave the pulp

chamber, merge in the body of the root, and re-divide short of

the apex to exit as two distinct canals (2-1-2)

• Type VII (Fig. 2b) One canal leaves the pulp chamber,

divides and then rejoins in the body of the root and finally re-

divides into two distinct canals short of the apex (1-2-1-2)

• Type VIII (Fig. 2c) Three separate, distinct canals extend

from the pulp chamber to the apex (3)

Thus, a clinician must be familiar with the various pathways the root

canals take to the apex.

Fundamental Objectives of Access Cavity Preparation

The following are the fundamental objectives which one has to keep

in mind while preparing the pulpal access cavity after achieving

profound local anesthesia:

1. To remove all decay, leaking restorations and undermined tooth

structure

2. To conserve healthy tooth structure

INDUSTRY WATCHACCESS CAVITY PREPARATION – AN ANATOMICAL

AND CLINICAL PERSPECTIVE

Fig. 2 Vertucci’s classification.

Fig. 3 Round bur

Fig. 5 Endo Z – Safe end cutting bur

Fig. 6 Trihawk bur

Fig. 4 Tapering fissure bur with a round head


b. DG – 16 Endodontic Explorer (Fig. 7)

c. Endodontic Spoon excavator (Fig. 8)

d. Orifice Enlargers

i. Gates Glidden Drills (Fig. 9)

e. Patency files

i. K Files ISO sizes #06, #08 and #10

II. Nice To Have Instruments & Equipments

a. Burs

i. LN Bur (Dentsply Maillefer) (Fig. 10)

ii. Munce Discovery Burs (CJM Engineering) (Fig. 11)

b. Ultrasonic Tips

i. Start X tips (Dentsply Maillefer) (Fig. 12)

c. Orifice Enlargers

i. X Gates (Fig. 13)

ii. Rotary orifice shapers

iii. Endodontic Micro Openers (Dentsply Maillefer)

(Fig. 14)

d. Dental Operating Microscope for Magnification and Illumi-

nation (Fig. 15)

The use of surgical telescopes of greater than 2.5X magnification

(ideally 4X or greater) and accompanying coaxial high-intensity

illumination or an operating microscope is highly recommended.

Access Cavity Guidelines For Permanent Teeth

I. Maxillary Incisors

Steps of access cavity preparation:

• Divide the tooth into nine boxes. The box in the centre is the

region for penetration of the bur. This is usually just incisal to the

cingulum,

• The lingual surface is entered perpendicular to the lingual

surface,

• The bur head is then oriented parallel to the long axis to the

tooth,

Fig. 7 DG 16 Fig. 13 X gates

Fig. 14 Micro opener

Fig. 15 Dental operating microscope greatly enhances both the illumination and magnification during access cavity preparation

Fig. 8 Endodontic excavator

Fig. 10 LN bur

Fig. 12 Start X ultrasonic tips

Fig. 9 Gates Glidden drills

Fig. 11 Munce discovery burs




• A round bur on slow speed contra-angled handpiece is used to

de-roof the pulp chamber,

• Completed access preparation must have a triangular outline

with the base towards the incisal edge and apex towards the

cingulum.

Key Rules for Maxillary Anteriors:

1. Entry slightly incisal to the cingulum

2. “Go palatal”- Always ensure a Palatal orientation of the bur

after crossing the DEJ

3. No variations – Single orifice with a single canal

II. Mandibular Anteriors

Fig. 16 a – f: Steps of access cavity preparation for mandibular anteriors

Fig. 18 Maxillary canine. (a) Ovoid funnel-shaped preparation. (b) Clinical image

Fig. 19 Mandibular central and lateral incisor. The preparation is ovoid in shape, which is more lingual in order to ensure the tracing of the second lingual canal

Fig. 20 Mandibular canine. The preparation should be ovoid, funnel-shaped. The cavity should be extended inciso-gingivally for room to find the orifice and enlarge the apical third without interference.

Fig. 17 Maxillary lateral incisor. It is similar to maxillary central incisor, but the size is smaller in dimensions.

(F) Clinical Image

(a) (b)

(a) (b)

(a) (b)




Key rules for mandibular anteriors

1. Change the bur Use the smaller round diamond No. 2

Preserves the tooth structure

2. Change the operator seating position 12 o’clock position

Better orientation

3. Change the radiographic position (exaggerated mesial shift)

Helps in identification of extra lingual canal

III. Maxillary Premolars

Key Rules:

1. Bucco - lingual orientation of cavity preparation

2. Oval shaped access

3. In case of bulkier roots suspect extra canals

IV. Mandibular Premolars

Key rules:

1. Ovoid / Round shaped access’

2. In case of bulbous roots suspect bifurcation of canals

V. Maxillary Molars

Maxillary molars generally have three roots and can have as many

as three mesial canals, two distal canals, and two palatal canals.

The mesiobuccal root of the maxillary first molar has generated

more research and clinical investigation than any root in the mouth.

It generally has two canals and they are called mesiobuccal (MB-1)

and second mesiobuccal (MB-2)

Fig. 21 Maxillary premolars. The ovoid coronal preparation need not be as long bucco-lingually as the pulp chamber. Final preparation should provide unobstructed access to canal orifices. Cavity walls should not impede complete authority over enlarging instruments.

Fig. 23 Mandibular premolars. The preparation is ovoid in shape which is less extensive bucco-lingually than that of the maxillary premolar

Fig. 24 Maxillary first molar. The outline is trapezoidal in shape with the broader base towards the buccal surface. The cavity is entirely within the mesial half of the tooth and need not invade the transverse ridge but is extensive enough, buccal to lingual, to allow positioning of instruments

Fig. 22 (a) Incomplete de-roofing of a maxillary premolar access cavity preparation(b) De-roofing allows access to the isthmus area which contains pulp tissue(c) Completed access opening allowing straight line access to both the buccal and palatal canals

(a) (b)

(c)




Key Rules for tracing the canals:

1. All canals are present in mesial 60% of tooth

2. Trace the palatal canal under the palatal cusp

3. Straight line up from palatal canal orifice towards buccal

wall Disto buccal canal

4. Mesial line from Disto buccal canal Mesio buccal canal

Most Common Variations:

i. If Mesio-buccal canal not present at the expected area then

trough for the canal more buccally towards buccal wall

ii. If Mesio-buccal canal is present near the buccal wall, trough for

the second canal MB2 below it. MB2 is consistently located

mesial to or directly on a line between the MB-1 and the palatal

orifices, within 3.5mm palatally and 2 mm mesially from the

MB-1 orifice.

iii. If Disto-buccal canal is not present at the expected area, always

trace it towards the mesial direction, towards a line joining

Mesio-buccal canal and palatal canal.

VI. Mandibular Molars

Key Rules:

1. Quadrilateral / Trapezoidal shape access

2. Suspect for the fourth canal

Common variations:

1. Possibility of a 2nd distal canal which can be in the following

configuration

• ‘C’shapedcanals

• 2canalswithorificesveryclosetogether

• 2distinctorifices

Fig. 25 The two most common locations of the MB2 canal

Fig. 27 Quadrilateral outline form reflects the anatomy of the pulp chamber. Both mesial and distal walls slope mesially. The cavity is primarily within the mesial half of the tooth but is extensive enough to allow positioning of instruments. Further exploration should determine whether a fourth canal can be found in the distal. In that case, an orifice will be positioned at each angle of the rhomboid.

Fig. 28 Single distal canal – oval shaped

Fig. 29 Single Distal canal - C shaped

Fig. 26 Clinical image of a maxillary first molar with four canals




2. Possibility of 2nd mesial canal (Between the 2 mesial canals)

3. Possibility of an extra root having a canal which is referred to as

Radix entomolaris / Radix paramolaris

4. Teeth having two canals in a straight line do not have any third

or extra canals and this configuration is seen in the second and

the third molars.

Steps of Access Opening

We can divide the complete process of access cavity preparation

into seven distinct steps :

STEP # 1 Relieve the tooth out of occlusion “Reduce

Post – operative Discomfort”

STEP # 2 Use caries as a guide “Chase the caries”

STEP # 3 Laws of Access Opening “Know your

Geography”

STEP # 4 De-roofing of pulp chamber “Most important

rule of access opening”

STEP # 5 Observe the color change “Color is the

language of endodontics”

STEP # 6 Remove dentinal shelves “Establish straight

line access”

STEP # 7 Locating the Canal Orifices “Eureka

Moment!!!”

STEP # 1 Relieve the tooth out of occlusion “Reduce

Post – operative Discomfort”

This is the first step of access cavity preparation in relation to

premolars and molars. This crucial step is recommended for the

following reasons:

i. Establishes stable occlusal reference points for working length

determination

ii. Reduces post operative discomfort by minimizing trauma to the

apical periodontium from occlusal loads

iii. Improves convenience form for the operator

STEP # 2 Use caries as a guide “Chase the caries”

The thumb-rule to follow while doing access opening in a cariously

involved tooth is to start removing the dental decay immaterial of

the location of the decay. Invariably the dental decay would lead

into the pulp chamber. Hence in cases of a tooth with distal decay

the access opening commences from the distal side towards the

mesial pulp chamber.

STEP # 3 Laws of Access Opening “Know your

Geography”

The biggest fear an operator has while preparing an access cavity

is the “Fear of Perforation”. This fear stems from the fact that many

operators are not clear of the internal map / geography of the pulp

chamber. One of the key landmarks, which help the operator in

avoiding procedural errors and helps in determining the location of

Fig. 30 Two distal canals – Orifices next to each other

Fig. 32 Single mesial canal which is in line with the distal canal in a mandibular second molar.

Fig. 33 Relieving the occlusion with a Tapering Fissure diamond

Fig. 34 Occlusal view after completion of relieving the occlusion

Fig. 31 Two distal canals – Distinctly separate orifices




the pulp chamber and root canal orifices, is the Cemento-enamel

Junction. (Fig. 35)

Krasner and Rankow proposed guidelines or laws that are of

immense help to a clinician during access cavity preparation. The

laws are:

i. Law of Centrality: The floor of the pulp chamber is always

located in the center of the tooth at the level of the CEJ

ii. Law of Concentricity: The walls of the pulp chamber are always

concentric to the external surface of the tooth at the level of the

CEJ, that is, the external root surface anatomy reflects the

internal root canal anatomy.

iii. Law of the CEJ: The distance from the external surface of the

clinical crown to the wall of the pulp chamber is the same

throughout the circumference of the tooth at the level of the

CEJ, making the CEJ the most consistent repeatable landmark

for locating the position of the pulp chamber.

STEP # 4 De-roofing of pulp chamber “Most important

rule of access opening”

Any permanent tooth not worn down occlusally/incisally has a pulp

chamber that is situated approximately 7 mm from a cusp tip or an

incisal edge. To slowly gain depth by small degrees leading up to

7 mm is needlessly inefficient, but to go beyond 7 mm in one fell

swoop is needlessly dangerous. You have a landmark: 7 mm. By

sticking to it, you will gain access in a predictable way without the

concern of perforating the floor of the chamber.

Once we enter into the roof of the pulp chamber then the operator

has to change to a lateral cutting motion instead of proceeding

in an apical direction. Care must be taken to slowly completely

remove the roof over the pulp chamber.

Fig. 35 Cemento-enamel junction is the key anatomical landmark during access cavity preparation

Fig. 36 Starting the access cavity preparation with a tapering fissure bur with a round head

Fig. 37 Initial access cavity outline. Note the color change as we enter into the dentin

Fig. 39 A, B, C & D: Sequential de-roofing the pulp chamber using a lateral cutting motion

Fig. 38 Note the grayish color change as we gradually near the roof of the pulp chamber.

39 (a) 39 (b)

39 (c) 39 (d)




STEP # 5 Observe the color change “Color is the

language of endodontics”

Table 1: Color chart of endodontic access cavity preparation

Law of color change: The color of the pulp chamber floor is always

darker than the walls.

Pulp stones take on a yellow / pearly white color and while in

intimate mechanical relationship with the floor of the pulp chamber,

they do not fuse with the floor like secondary dentin does. For this

reason, they can often be picked away from the floor with a sharp

explorer. Once through the secondary dentin and pulp stones, the

canals of a calcified canal may still not be readily apparent.

STEP # 6 Remove dentinal shelves “Establish straight

line access”

STEP # 7 Locating the Canal Orifices “Eureka

Moment!!!”

Enamel White

Dentin Yellow

Floor of the pulp chamber Gray

Root Canal Orifice Dark gray or black

Pulp stone Pearly white / Dark Yellow

Fig. 40 Pearly white color of the pulp stone

Fig. 43 The probe pointing into the distal canal while straight line access is not present for the mesial canals

Fig. 44 The DG 16 endodontic explorer is pointing towards the Dentinal Shelves protruding from the respective walls

Fig. 45 Refining the access preparation with the help of Start X ultrasonic tips to remove the dentinal shelves and in planing of the walls. These tips can also be used to trace extra canals and to dislodge calcified pulp stones.

Fig. 42 Note the dome shaped floor having a distinct gray color after the partial removal of pulp stones

Fig. 41 Pulp stone, which is more yellowish than the surrounding dentinal walls.




The following laws will help the clinician in locating the canal

orifices:

Law of symmetry 1: Except for maxillary molars, the orifices of the

canals are equidistant from a line drawn in a mesio-distal direction

through the pulp chamber floor.

Law of symmetry 2: Except for maxillary molars, the orifices of the

canals lie on a line perpendicular to a line drawn in a mesio-distal

direction across the center of the floor of the pulp chamber.

Law of orifices location 1: The orifices of the root canals are

always located at the junction of the walls and the floor.


located at the angles in the floor-wall junction.


located at the terminus of the root developmental fusion lines.

The above laws were found to occur in 95% of the teeth examined.

Five percent of mandibular second and third molars did not

conform to these laws because of the presence of C-shaped canal

anatomy.

Conclusion:

Cleaning and shaping constitutes the most important phase of

endodontics. However, it is the access cavity preparation that lays

the foundation for successful cleaning and shaping. In my opinion,

mastering the art of access cavity preparation is the single most

important operator variable that ultimately determines both the

prognosis and quality of the endodontic therapy

Fig. 46 Access cavity preparation completed

Fig. 47 Under higher magnification

Fig. 49 Representation of first and second laws of symmetry and first, second and third laws of orifice location

Fig. 48 Mesial canal orifices



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Access Cavity Preparation an Anatomical and Clinical Perspective June 2011