access preparation of premolar?

Answer 1

THIS PREPARATION INTO THE TOOTH PRIOR TO ROOT CANAL TREATMENT USUALLY TAKES THE SHAPE OF AN OVAL WITH THE NARROW PART OF THE SHAPE POINTING TOWARD THE BUCCAL AND TOWARDS THE LINGUAL.SEE THE LINK:
Access Preparation on Maxillary Premolars

The secret of gaining access to maxillary premolar pulp chambers is to first outline the cavosurface margin limits and then to proceed from there toward the pulp with burs whose long axes are directed toward the center of the root at the CEJ level.

The cavosurface outline of the access opening should reflect the cross-sectional shape of the underlying root and pulp chamber and is therefore oval in shape with the narrowest dimension of the oval being M-D. (Fig. 8).

This M-D dimension should be just large enough so that the head of a #6 round bur will just wedge into the center of the oval, but not rotate, when placed with the fingers (about 2 mm).
Caution: Use of a #6 bur to cut the preparation will result in too wide an access outline.

The B-L dimension of the access is determined by points one-half way up the buccal and lingual cusp ridges from the central groove as determined by viewing from directly over the occlusal surface. It may be helpful to mark the limits of your outline form on the tooth in pencil before beginning.

Using a #701 bur whose long axis is held parallel to the long axis of the tooth, penetrate the enamel into dentin in the central groove area and then extend your cavosurface outline out to your previously determined limits. Place the floor of this prep about 1 mm below DEJ in the central groove area (Fig. 9). Then change to a long shank #2 round bur and penetrate from your initial access preparation toward the pulp chamber in a conical fashion





The longer shank bur gets the handpiece head back away from the occlusal surface so that you get better visibility and it allows you to see more of the bur shank to help line it up with the center of the tooth. The angulation of the bur as you proceed toward the chamber is critical. As you can see in Fig. 10, maxillary premolars are considerably narrower M-D at the CEJ level than they are at the occlusal surface. The chamber lies in the center of the crown at CEJ level. Your objective is to make the center of the tooth at CEJ level your target as you penetrate with the bur. Use your perio probe to determine the mesial, distal, buccal and lingual aspects of the root at the CEJ level. It may be helpful until you get more experience to place pencil marks in the center of the root on the buccal and lingual at the CEJ level to use them as guidelines as you penetrate toward the chamber. You will be aiming for the center of an imaginary line connecting these two marks (Fig. 11). Remember, the angulation of your bur shank must be controlled in two dimensions, M-D and B-L (Fig. 10). It takes very little deviation from this path, particularly in the M-D plane to result in considerable gouging and even crown perforation (Fig. 12).



Figure 11. View of occlusal surface with outline of root structure at CEJ level superimposed. "X" is your target. Figure 12. Gouging and perforation of crown.


Due to the narrow M-D width of many maxillary premolar pulp chambers, you may not get the sensation of falling into the chamber with the bur. Therefore, as you approach the level at which you expect the chamber to be, you should frequently check the base of your prep with the endo explorer to see if it will drop into the chamber due to its fine point or will give you resistance to withdrawal (binds or a "tacky" feeling) because its tip is wedging into a small opening in some aspect of the chamber.


It is desirable to estimate the depth at which you expect to encounter the pulp. A good way to do this is to measure on the radiograph the distance from a cusp tip to the pulp chamber and then set this length on your endo explorer with a silicone stop. When placed to the depth of your preparation, the distance from the pulp equals the distance from the cusp tip to your rubber stop.


Penetrating too far into a tooth in search of the pulp is wrought with problems. In multirooted
premolars, you have a furcation which, depending on its position, could be perforated by excessive bur depth. (Fig. 13)


The deeper you get in a tooth in search of the pulp, the greater your chances of perforating the crown or root because it becomes increasingly difficult to control the direction of the bur.


Also, if you reach the level where you expect to find the pulp chamber and do not find it, this may be an indication that your bur path has deviated from the intended path and is actually at the level of the pulp,but off to the B, L, M, or D (i.e.,alongside the pulp).


If you find that you have lost your orientation in the crown or feel you are deviating from your intended path, a straight-on radiograph (angulation like a bite-wing) taken at that point may show the relationship of your access prep to the chamber -- at least in a M-D plane. You may benefit from removing the rubber dam and clamp before taking the radiograph so that the image of the clamp is not superimposed over the area you wish to see.

Once you have located the chamber, unroof it with the round bur and blend the chamber walls into the occlusal access margins (Fig. 14). Remember, unless obliterated by secondary dentin deposition, maxillary premolars have two pulp horns (B and L).

If the M-D width of your pulp chamber is so narrow that the use of even a #2 round bur would cause gouging of the chamber walls, you may want to unroof the chamber, once you have located it with burs, with small Hedstrom files. For this purpose, place a small Hedstrom file (#25 or 30) into the chamber opening, flood the chamber with irrigant, and file with a back and forth motion while flexing the file toward the B and L. Hedstrom files used in the coronal portion of the tooth are also quite helpful for blending or flaring the chamber smoothly into your cavosurface access outline. Remember to use copious amounts of irrigant throughout the access procedure.




Once you have opened up the pulp chamber, place a straight #10 K-file into the coronal 1/3 - 1/2 of each canal to determine if the shaft of the file binds significantly against the access wall. If it does, you will lose control of the apical end of the file during instrumentation, leading to "walking out" of the canal. If you have already extended the cavosurface margin of your access to 1/2 way up the B and L cusp ridges, you will need to extend the access further in the area where the file binds. Be sure to get your instructor's approval before extending the access further. This further extension of the access is usually only necessary in two-canaled maxillary premolars where the canals are relatively parallel to each other, rather than divergent. (Fig. 15).



Figure 15 a. File test indicates no further extentsion of access needed. Figure 15 b. Instance where buccal access margin had to be further extended (arrow) to prevent file buccal canal from binding. Note: Canal are more divergetn in (a) and more parallel in (b).

Answer 2

This would be done to get to the pulp chamber for a root canal or a filling.