Basic Procedure for Access Cavity Preparation

Guidelines for cutting the access cavity are:

1) Start Radiograph

Begin with an X-ray of the unprepared tooth. This start with X-ray is important in making a diagnosis, as well as estimating the initial measurement of the canals. You must study the radiograph as a blueprint to establish the size, shape and location of the pulp canal(s) and their relative positions.

2) Place Rubber Dam

Rubber dam application is an essential prerequisite for providing non-surgical endodontic treatment. The benefits of using a rubber dam for an endodontic treatment include: It provides a dry, clean, and disinfected field; it protects the patient from the possible aspiration or swallowing of tooth and filling debris, bacteria, necrotic pulp remnants, and instruments or operating materials; it protects the patient from rotary and hand instruments, drugs, irrigating solutions, and the trauma of repeated manual manipulation of the oral soft tissues; and it is faster and more convenient than the repeated changing of cotton rolls and/or saliva ejectors.

3) Cut through the Centre

First, remove all caries and fillings that obstruct the view or that can cause leakage. Undermined enamel should also be removed together with any parts of the crown that may cause accessibility to the canal(s) difficult. Using a high-speed fissure bur or round bur held perpendicular to the lingual surface in case of anterior teeth and occlusal surface for posterior teeth, cut just through the enamel in the centre of the pencil-marked area. A common error is to begin cavity too far gingivally. Do not force the bur.

4) Extend Access

Extend the opening laterally to the designated outline by maintaining the point of the bur in the central cavity and rotating the handpiece toward the incisal so that the bur continues to parallel the long axis of the tooth. If there is pulp exposure, it should be widened with the handpiece in order to properly determine the extension of the pulp chamber.

5) Cut through Dentin

With a low-speed long-shank #4 or #6 bur (depending on the size of the pulp chamber), make a cut with the long axis of the tooth and cut directly through the dentin into the large pulp horn, or the largest area of the pulp chamber. The bur should be used with a pull stroke from the chamber and out.

6) Explore Access

Use the endodontic explorer (DG 16) to check for the canal. If the explorer meets constant resistance, the pulp chamber has not yet been reached.

7) Cut into the Pulp Chamber

Continue drilling apically through the dentin. You will feel a slight drop as the bur breaks through the roof and drops into the pulp chamber.

8) Explore Access

When the pulp chamber has been penetrated, probing with the explorer will often produce a “catch” along the ledges, or overhangs, created by the lingual walls or roof of the pulp chamber. The location of the orifice can often be determined by the colour of the dentin, sometimes referred to by the term dentinal mapping. The orifice may appear as a whitish spot in a general field of yellowish dentin.

9) Remove Undercuts

Expand the coronal cavity access slightly. Avoid perforating the floor of the pulp chamber. Penetrate the pulp chamber using a slow-speed long-shank round bur (No. 4 or No. 6, depending on size of chamber).Working from inside the chamber to outside in a sweeping motion, remove undercuts, or lingual and labial walls of pulp chamber.

Additional bevelling of the incisal wall is also completed by working from inside to outside to remove the lingual “shoulder” of the canal, thus allowing for the continuous access from the coronal cavity into the canal. The ideal access consists of smooth walls without ledges. The use of fissure burs very often creates ledges in the floor and walls of the cavity access preparation, which can make canal instrumentation more difficult. Moreover, ledges in the dentin can diminish the tensile strength of the tooth. In general, the No. 2 is used for working within the canals, while Nos. 4 and 6 are for working within the chamber, using a sweeping motion to avoid gouging the floor of the pulp chamber and creating the illusion of a canal which may lead to perforation. Remove debris from the chamber as you proceed, using a No. 2, No. 4 or No. 6 bur to eliminate pulpal horn debris and bacteria. If the canal or chamber is calcified, remove dentin with the slow speed hand piece and appropriate bur.

10) Irrigate

Irrigate periodically to flush out debris. Fill an irrigating syringe with “sodium hypochlorite” and attach an irrigating needle, the tip of which should be bent at approximately a 45 degree angle to the long axis of the needle. The distance from the bend to the tip of the needle should equal 20 mm. Use this needle to gently flush fillings and debris from the chamber. The needle should fit in the canal very loosely, and the solution should be introduced very slowly, so that it can run back out of the access opening and is not forced through the apex.

11) Straight-line Access

The resulting cavity should be smooth and continuous, flowing from cavity margin to canal orifice; this is referred to as straight-line access. Verify that you have achieved straight-line access by rotating a file within the canal. The file should have direct and unimpeded access to the canal, achieving 360 degrees of unrestrained motion; you should be able to rotate the file 360 degrees about the cavity outline without encountering resistance within the pulp chamber due to ledges or ridges.