1. Field of the Invention.
The present invention relates generally to endodontic procedures and methods for enlarging and shaping the root canals of teeth in preparation for filling and sealing.
2. Description of the Related Art.
A relatively common but often difficult dental procedure is the cleaning, shaping and filling of the root canal of a patient's tooth. In the performance of a root canal procedure, a hole is first cut in the crown or exposed portion of the tooth, typically either in the biting surface of the tooth, for posterior teeth, or in the side of the tooth on the interior of the jaw for incisor teeth. Small endodontic instruments known generally as root canal files are then used to clean out the material present in the root canal, and to impart a tapered shape to the root canal so that filling material may be fully inserted into the root canal to thoroughly seal it.
For a great many years, the preparation of root canals has been achieved with the use of files of standard taper. Conventionally when a root canal is being cleaned and shaped, a series of files having increasing diameters is used to gradually enlarge the root canal. The files are held between the thumb and forefinger of one hand by the dentist. Each file in a set of the known prior art has an identical taper from one end to the other. For example, in a typical K-type file set the taper is 0.32 millimeters on every file over the standard 16 mm length of cutting flutes, or 0.02 mm of taper/mm of flute length. This taper is sometimes referred to as a standard ISO (International Standards Organization) taper. Although these file sets have identical tapers, they come in a number of sizes. The size number characterizing the file is the diameter of the file at the tip in hundredths of a millimeter, and the diameter of the file at the large end is thus 0.32 millimeters greater than this tip diameter. A complete set will include sizes 06, 08, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, 120, 130, and 140, while sizes 08-60 will typically be used. Some manufacturers make certain half-sizes, or off-standard sizes.
Hedstrom-type instruments similarly come in sets of increasing size, typically from 0.10 to 1.40 millimeter tip size, with 0.15-0.60 millimeter tip sizes being most commonly used. Both the K-type and Hedstrom files manufactured to ISO standards, whether twisted or lathe-cut, have flute pitches and frequencies which vary little or none in some sizes (large), but quite a bit in other sizes (small).
One specific procedure for preparing a root canal by using a series of root canal files having decreasing size to clean out the material present in the root canal is known as the "Crown-Down" technique. This technique was developed at the Oregon Health Sciences University Graduate Endodontic Program. It involves proceeding from the top of the tooth, the crown, down toward the root canal terminus employing a series of successively smaller burs and files.
The crown-down technique has been found to be prone to the formation of undesirable apical ledging. Apical ledging occurs when the tip of a file does not follow the curvature of the root canal and instead bores a passage branching out from the root canal. An example is shown in the accompanying FIG. 2A at 24. The occurrence of apical ledging often requires surgical correction.
Schilder in U.S. Pat. No. 5,017,138 describes step-back shaping with multiple recapitulations through a series of instruments, each time from small to large. This technique is referred to as serial step-back shaping. The serial step-back technique is, at best, a difficult and time-consuming method, as the dentist must indirectly gauge the rate of taper in the root canal preparation by the distance interval of step-back of the progressively larger instruments as they fit further back from the canal terminus. Accordingly, a steep learning curve is associated with this technique.
Both of these prior art techniques, crown-down and serial step-back, have attendant disadvantages of requiring numerous repetitive steps (as many as 50 steps) and requiring the use of a large number of instruments (approximately 18) in order to achieve a satisfactory final result. In addition, the skills required to create conservative but adequately tapered shapes in root canals are usually developed only after treating hundreds of clinical cases. Finally, these prior art methods are extremely time-consuming and therefore expensive for the practitioner.
In order to reduce the risk of apical ledging, the initial crown-down technique was modified by the inventor by adding subsequent steps employing a series of successively larger files to step-back from the canal terminus. This modified technique is referred to as the "Crown-Down/Step-Back" technique.
As practiced, both the crown-down and serial step-back procedures generally employ root canal files of fixed taper. In my prior patent application Ser. No. 08/234,290, the disclosure of which is incorporated by reference herein, I disclosed an endodontic treatment system involving variably tapered files and matching variably tapered auxiliary implements and preparation materials which present substantial advantages over the standard fixed taper files.