A Modification of a Direct Technique for the Fabrication of Posts and Cores Using a Wax Pattern

Many direct and indirect techniques exist for fabrication of posts and cores which emphasize accurate fitting, chairside time reduction, retention, and resistance to rotation.¹ The most remarkable advantage of indirect methods is that they can promise optimal design and parallelism.² Mekayarajjananonth, et al³ believe that the dowel patterns fabricated in the prepared canals can directly lead to their accurate and passive fit.
In 1975, Shadman, et al4 presented a direct technique for fabrication of posts and cores. They completed the dowel part by pouring the molten blue inlay wax over a barbed broach wrapped in cotton fibers, and then inserting it into the canal. After forming the core, the handle of the broach was cut with a wire cutter followed by spruing, investing, and casting. This technique requires no special armamentarium and is appropriate for various clinical situations. However, the residual barbed broach has to be pulled out during the burnout stage. In addition, like other direct techniques, the direction of the core is limited by the placement of the broach. Therefore, under some conditions the cores fabricated with this technique cannot meet the requirements of restoration, especially when they act as abutments in complicated fixed partial denture restoration.
The purpose of this article is to present a method that is a modification of the technique introduced by Shadman, et al.⁴ The new method retains its advantages but avoids its disadvantages. The cores fabricated in this direct technique can meet the requirements of the restoration.

Figure 1. File is rotated from left to right, clockwise, wrapping cotton fiber sheet around it.	Figure 2. Cotton fibers are wrapped around file and the file supports all the cotton. Length of cotton wrapping the file is longer than the sum of the expected length of core and post. The extension of the cotton covers the whole surface of the wrapped file with a thin layer.
Figure 3. Cotton-wrapped file is soaked into molten blue inlay wax, picked out, and cooled to form prefabricated dowel.	Figure 4. Cotton-wrapped files are covered with wax pattern of finished post and primary core.
Figure 5. Reseat patterns with primarily amended core contour.	Figure 6. Schematic diagram shows the incisal-anticlockwise direction rotating the file out of the wax pattern. When one hand is pulling the file, the other hand should fix the wax core pattern firmly within the canal.
Figure 7. Files have been pulled out from wax patterns. Surplus cotton fibers of one pattern have been cut off.	Figure 8. Core patterns are carefully amended according to requirements of restoration, including common path of insertion.

Figure 9. Finished metal posts and cores.

TECHNIQUE

Step 1. Prepare the space of the dowels within the root canals in the usual manner.⁴

Step 2. Spread cotton fibers in the shape of a square, with the sides about 1/2-inch in length.

Step 3. Place a screwed file (K-Files, DENTSPLY Asia) at the left edge of the cotton piece and rotate the file from left to right to wrap the cotton around the file (Figure 1). Cut off the surplus cotton that is not supported by the file. The length of the cotton wrapping the file should be longer than the sum of the expected length of core and post. The extension of the cotton should cover the whole surface of the wrapped file with a thin layer (Figure 2).

Step 4. Soak the cotton-wrapped file into molten blue inlay wax (Kewei Dental China). Then cool it to form a prefabricated dowel (Figure 3).

Step 5. Carry the mobile wax with the prefabricated dowel into the canal. Insert a hot explorer beside the file into the canal to melt the wax again. Remove the explorer before the wax condenses. Build up the core pattern on the post to reinforce the strength of the whole wax pattern to make the removal of the post pattern from the canal easier.

Step 6. Draw out the file as well as the strengthened solidified wax pattern (Figure 4). Check for defects and correct them in the usual way.4

Step 7. Seat the pattern again and amend the core pattern briefly according to the requirements of the restoration (Figure 5).

Step 8. Fix the pattern firmly within the canal with one hand by its labial and lingual surfaces and rotate the handle of the file anticlockwise with the other hand to draw the file out of the wax pattern (Figure 6). Cut off the abundant cotton outside the core (Figure 7).

Step 9. Amend the core pattern carefully according to the requirements of the restoration, including the common path of insertion (Figure 8). Connect the wax rod as a sprue onto the core and send the whole pattern to the laboratory. At the next appointment, insert the metal posts and modify the cores in routine fashion (Figure 9).

DISCUSSION

The advantage of this method is that it can form the dowel in the actual canal, and guarantees that the fabricated cores are parallel to the other abutments. Because the file is separated from the wax post pattern by a thin layer of cotton and no metal portion contacts the wax, it is possible to remove the file from the finished post pattern without damaging the integrity of the post wax pattern. The incisal-anticlockwise rotation of the file handle during the removal stage is along the direction of the file screw threads and is contrary to the rotation direction during the cotton-wrapping stage. This is the key point that guarantees the file’s harmless removal. With-out the disturbance of the prefabricated dowels, dentists can form the contour of the cores in a rational direction. This is beneficial especially when the direction of the core needs to be changed greatly from the long axis of the root according to the requirement of the restoration, or when multiple roots need to be restored together. In addition, it is believed that the dowel pattern fabricated with this direct technique can attain accurate and passive fit.3 Using this kind of direct technique, technicians need not remove a broach during the burnout stage. This will protect the inner surface of the investment and preserve it intact.

SUMMARY

This new direct technique for the fabrication of posts and cores using a wax pattern is easy to manipulate. It requires no special armamentarium and is appropriate for various situations. It provides the advantage of a direct technique and ensures the post pattern’s precise fit. It also avoids the disadvantage of other direct techniques, the limitation of the direction of the cores by the placement of the broaches, the pins, or the prefabricated dowels.

References

Dr. Sun received his DMD degree from the West China College of Stomatology, Sichuan University, and his Bachelor of Stomatology degree from the College of Stomatology, West China University of Medical Science. He is a lecturer at the College of Stomatology, Shandong University. He can be reached at whitedove69@163.com.

Professor Ji received her Bachelor of Stomatology degree at Shandong University of China. She is a professor of Prosthodontics at Shandong University. She can be reached at jiping@sdu.edu.cn.

Professor Zhao (corresponding author) received his Bachelor of Stomatology from Shandong University of China, and is an associate professor of Prosthodontics
at the University. He can be reached at zhaohq@sdu.edu.cn.