Endodontic Access Cavity Preparation

INTRODUCTION :

A well-designed access preparation is essential for a quality endodotic result.

The objective of an access preparation is to create a smooth, straight-line path to the canal system and ultimately, the apex.

Careful cavity preparation and cavity obturation are the keystones of ideal root canal therapy success.

Access preparation may be divided into visual and the assumed. The coronal anatomy is the first indication of the assumed and is the first key to the root position and root canal system. Before entry, the clinician must visualize the expected location of the coronal pulp chamber and canal orifice position to avoid unnecessary tooth removal.

DIVISIONS OF CAVITY PREPARATION :

For sake of convenience, two anatomic divisions of cavity preparations are :

1.       Coronal Preparation.

2.      Radicular preparation.

Simply put, the principles of cavity preparation by Black can be applied as :

   outline form – entire length of preparation.

   convenience form – modification of outline form to accommodate canal anatomy/instruments.

   retention form – for gutta-percha point.

   resistance form – developing an “apical stop”.

CORONAL CAVITY PREPARATION :

Instruments :

Initial entry is made by round-end carbide bur to cut through dentin till a ‘drop’ into the pulp chamber is felt.

#2 round bur :  for mandibular anteriors and maxillary premolars.

#4 round bur : for maxillary anteriors and mandibular pre-molars. Occasionally for ‘young’ maxillary premolars and ‘adult’ molars.

#6 round bur :  mandibular molars with large pulp chambers.

#1 round bur :  to seek out additional canals.

Fissure bur   :  to finish the slope sidewalls in visible position of preparation.

The alliance between endodontic cavity preparation and pulp endodontic cavity preparation and pulp anatomy is inspirable. Modifications in cavity preparation are dictated by the number or anatomy of the canals.

Principles :

G.V. Black’s principles are applied as :

I. OUTLINE FORM :

The objective is to establish complete access for instrumentation from cavity margin to apical foramen.

The internal anatomy dictates the external outline. Hence, preparation is done from inside to outside with the bur (in contrast – the preparation in operative dentistry – the outline is related to the external anatomy).

FACTORS DETERMINING OUTLINE :

1. size of pulp chamber :

Extensive preparation done for young patients compared to elderly.

2. Shape of pulp chamber :

outline should reflect shape of pulp chamber. Eg: in maxillary premolar, the coronal pulp is flat mesiodistally but wide bucco-lingually making the outline an elongated oval bucco-lingually (in contrast operative outline extends mesiodistally).

3. Number, position and curvature of root canal :

outline should allow an-unstrained instrument approach to the apical foramen/ ‘Shamrock preparation’ done in severally curved canal.

II. CONVINIENCE FORM :

The objectives behind this are :

a.       to gain unobstructed access to the canal orifice – enough tooth structure should be removed so as to visualize and instrument all the canals certain modifications have to be done to search out extra canals such as the ‘Shamrock preparation’ where in only a part of the wall is extended to search out a canal giving a clover-leaf appearance to the outline.

b.      To get direct access to apical foramen – for case of instrumentation.

c.       Expansion to accommodate filling techniques – such as warm vertical method in which a wider preparation is necessary.

d.      To attain complete authority over enlarging instrument – or else errors may result such as perforation, ledges, zipping etc.

III. REMOVAL OF REMAINING CARIOUS DENTIN/DEFECTIVE RESTORATION :

The objective behind this is to :

        i.            Eliminate mechanically as many bacteria as possible from the interior of the tooth.

      ii.            Eliminate the discolored tooth structure that may cause staining of the crown.

    iii.            Eliminate the possibility of any bacteria laden saliva leaking into the prepared cavity.

If any carious perforation occurs during removal, then it should be restored with cement from the inside.

IV. TOILET OF THE CAVITY :

All of the caries, debris and necrotic material must be removed from the chamber before the radicular portion is begun or else it will cause obstruction during canal enlargement/increase in bacterial population/stain the crown. NaOCl irrigation is done.

ACCESS PREPARATION GUIDELINES (Cohen) :

  as internal anatomy dictates the access whape, visualization of the location of pulp space is important by direct vision/tactile/radiographs. A decision about bur penetration is thus made.

  restorations impinging a straight-line axis; and caries is removed.

  roof of pulp chamber is perforated at its largest part using a round bur. In calcified/multirooted teeth, direction of access is towards largest canal.

  roof of pulp chamber is removed using the belly of the bur on the out stroke with frequent NaOCl irrigation.

  Endodontic explorer/pathfinder is used to locate canal orifices.

  canals once located are instrumented with #10/15 K-file to determine potency. Tooth length may be determined at this point.

  radicular access is initiated next.

  final outline is established with a round tip, tapered, diamond bur after the canals have been located and the initial opening have been completed.

According to R.E. Walton, the 3 main objectives of access preparation are :

1.    Straight line access : Helps in

a.       Improved instrument control.

b.      Improved obturation.

c.       Decreased procedural errors.

2.   conservation of tooth structure : Helps  in

a.       minimal weakening of tooth.

b.      Prevention of perforation.

3.    unroofing of chamber and exposure of pulp horns : Help in

a.       maximum visibility.

b.      Location of canals.

c.       Aesthetic considerations.

MAXILLARY ANTERIOR TEETH PREPARATION :

1.       entrance is always gained thorough lingual surface just above the cingulum at an angle perpendicular to the surface using a #4 round bur (Grossman) or a round-point tapering fissure bur (Ingle). Only enamel is cut at this stage at the high-speed.

2.      rotate the hand piece, so that bur parallels the long axis of the tooth. Slow-speed is utilized as better tactile sense is achieved with it to gain entry into pulp chamber a ‘drop’ is feet.

3.      preliminary cavity outline is funded and fanned incisally with fissure bur. Enamel has short bevel towards incisal.

4.      the overhanging enamel and dentin of the lingual roof of  pulp chamber are removed including the pulp horns with #4 round bur at slow speed to attain a straight line access.

5.      a Gates-Glidden drill of size #4 is used remove the lingual shoulder by working on the outstroke.

6.      occasionally #1 / #2 round bur is used laterally and incisally to eliminate pulpal horn debris.

7.      in calcified teeth, the access cavity takes on an avoid shape with the greatest diameter incisogingivally  convenience extension must be advanced further incisally to allow bur shaft and instruments to operate in central exist.

 

RADICULAR CAVITY PREPARATION :

Objectives :

        i.            thorough debridement of root canal system  is achieved by skillful instrumentation coupled with liberal irrigation to eliminate bacterial contaminants, necrotic debris and dentin.

      ii.            The specific shaping of the root canal preparation to receive a specific type to filling. This is done by either the ‘step back’ or the ‘step-down’ approach.

Principles :

G.V. Black’s principles are applied as :

              I.      Outline form : the entire length of the cavity falls under the rubric outline form. The canal preparation shape is governed by the anatomy of the root canal.

           II.      Convenience form : the canal is optimally enlarged to receive instruments as well as irrigants and filling materials.

         III.      Toilet of the cavity : Meticulous cleaning of the walls until they feel glassy smooth, accompanied by continuous irrigation will ensure thorough debridement certain nooks and crannies of the root canal system cannot by instrumented and thus the importance of thorough irrigation.

        IV.      Retention form : the nearly parallel walls ensure firm seating of the master gutta-percha cone Meticulous care during preparation of the apical 2-3 mm of the canal is done to ensure sealing against future leakage. Incidence of accessory canals is also more here. Flaring starts from here is the coronal direction.

           V.      Resistance form : ensures resistance to overfilling or over-insrumentation. It helps to maintain the integrity of the natural constriction of the apical foramen against which gutta-percha cones can be compacted.

        VI.      Extension for prevention : the extension of the cavity preparation throughout its entire length and breadth is necessary to prevent future problems. Peripheral enlargement of the canals to remove all the debris, followed by total obturation is the primary preventive method.

Instruments :

A variety of instruments, both hand and rotary, are available in the market ranging from files, reamers, burs to ultrasonic equipment. Depending on their efficiency and convenience, dentists utilize a variety of techniques to attain as perfect a canal preparation as possible to obtain a successful obturation later.

ACCESS IN SPECIAL SITUATIONS (Walton & Torabinejan/Cohen) :

Usually, the access preparation is performed under less than ideal conditions as teeth may be affected by caries, fractures, restoration etc. It is critical that operator recognizes these variations and plans the access accordingly :-

1. Caries :

ideally, caries removed before entering pulp chamber to avoid contaminating canals with bacteria and to give a proper temporary seal.

One exception : it is permissible to temporarily leave a small remnant of caries if its removal may lacerate the gingiva and/ or compromise isolation at the gingival margin.

When removing caries in proximal region, a small portion of enamel can be left undermined to retain filling.

2. Existing Permanent Restoration :

its presence provides more flexibility in designing preparation as removing only restorative material allows for wider opening of the access.

a.       Ideal situation :

-          Preferably entire restoration (Small occlusal amalgam/full crown) is removed to improve visibility.

-          Retention interproimal portion of Class-II restoration that extends     sub-gingivally to aid in isolation.

-          A crown that is still serviceable is not removed.

b.      Extent of opening :

If restoration is to be retained and repaired, wide access can be made at the expense of the restoration rather than dentin a full gold crown may have very wide access without compromising retention or strength.

c.       Defective Restoration :

Defective part/entire restoration should be removed if open margin recurrent caries may lead to communication between pulp space and oral cavity (especially in sub-gingival area). Determine the defects prior to access preparation.

3. Existing temporary restoration :

a.       crowns : better to remove temporary crowns as they get frequently loosen and compromise seal.

b.      Bands : Safer to remove unless it is retaining temporary or not hindering clamp placement. Copper bands can be retained if well adapted and cemented.

4. Previous partial Endodontic treatment :

mandatory to take a periapical radiograph before treatment. Modifications can be made accordingly after judging the prognosis.

5. Altered internal anatomy :

external factors and age change the morphologic features of pulp space. Such alterations include :

-                irregular secondary dentin.

-                Calcified canals.

-                Pulp stones.

-                Dens in dente.

-                Dens evaginatus.

-                Restorations.

ERRORS IN CAVITY PREPARATION :

1. perforations : caused to failure to recognize inclinations; depth of pulp chamber; assuming canal is straight.
2. gouging overextension : caused due to failure to recognize inclinations; - unsuccessful search for canals or receded pulp.
3. under extension : Entire roof of pulp chamber not removed lingual shoulder not removed leading to curved access.
4. ledge : caused due to loss of instrument control.
5. discoloration : incomplete removal of pulp debris.
6. missed canals : small access can cause this or eisarientation of access outline.
7. Broken Instruments : occurs in cross-over canals due to failure in extending outline/internal prep.