Dentistry – Apparatus – Broach
Reexamination Certificate
1999-02-16
2001-09-25
Wilson, John J. (Department: 3732)
Dentistry
Apparatus
Broach
Reexamination Certificate
active
06293794
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of dental instruments. More particularly, the invention relates to those instruments used in the practice of endodontia to remove dead or damaged tissue from a tooth root canal preparatory to filling the root canal.
Extraction of a tooth may often be avoided by removing decayed, damaged, or dead tissue from the nerve canal within a tooth root. Typically, to remove decayed, injured or dead tissue from the root canal of a tooth, an endodontist will first drill into the tooth to locate the root canal and thereafter use instruments of small diameter, such as reams and files, to remove the decayed, injured or dead tissue from the nerve canal. The objective of tissue removal with the instrument is to produce a funnel-shaped canal, with the smaller end at the apical foramen, so as to provide the canal with a desirable form for filling. Such root canals are not necessarily straight. The instrument must be able to follow a curved path as it is moved through the canal for purposes of removing the tissue. Therefore, the instrument must be flexible yet possess sufficient strength so that it is not easily broken when stroked and rotated through a root canal.
Commonly, both files and reamers have flutes which spiral along a portion of the instrument length. Whether the instrument is considered to be a file or a reamer is determined by the pitch of the flutes. A greater pitch enables the instrument to cut better in a rotary mode. Hence, a large pitch instrument is characterized as a reamer. A lesser pitch to the endodontic instrument flutes enables the instrument to cut better in the reciprocating mode of motion. A low pitch instrument is therefore characterized as a file. During a cleaning and shaping operation performed with an endodontic file or reamer, the instrument is normally rotated and reciprocated into and out of the root canal along the longitudinal axis of the instrument. Therefore, even during rotational movement of a reamer through a canal the instrument is commonly reciprocated to a degree to effect the desired cut. Similarly, a file is commonly rotated slightly as it is reciprocated longitudinally.
In order for the instrument to extirpate root canal tissue as it is rotated and moved longitudinally, relatively high torsion forces must be typically applied to the instrument. These high torsional loads on the instrument may lead to torsional failure of the instrument. In addition, the curved path of common root canals requires that the instrument concurrently be subject to bending and torsional loads as it is moved along the canal. This combination of bending and torsional loads as the instrument is moved along the canal increase the likelihood of torsional failure.
Traditional ISO Standard files for endodontia have a taper angle of about two percent. This taper is usually continuous over the full working length of the file. Size of the file is based upon the file tip diameter. Using ISO Standard files, a “crown down” technique of radicular extirpation begins by first penetrating the root canal with a large diameter instrument followed sequentially by one or more files of diminishing diameter until the apical foramen is reached. As the root canal bore is extended toward the root apex, the active cutting proportion of the file working length increases while, at the same time, the file size and tip diameter is diminishing. This increase in the active cutting portion of the file necessarily results in an increase in the torque stress imposed on the file and on the probability of file failure.
Another approach for radicular extirpation is based upon use of several files having the same maximum diameter but with a graduated series of decreasing taper angles from the file tip that terminate at the maximum diameter. The canal is started with the file of greatest taper angle. The first file is advanced to a depth corresponding to the desired entrance diameter. The second file, having a smaller angle of tapered convergence from the tip to the maximum diameter, has a greater axial length from the maximum diameter base to the file tip. Hence, the second file will advance the canal depth from the same entrance diameter. This procedural sequence will continue down to the apical foramen. Similar to the “crown down” procedure, the decreasing taper angle method of extirpation imposes greater torque on the most fragile files.
It is therefore, an object of the present invention to provide an improved method of radicular extirpation that operatively limits the magnitude of torque imposed on a root canal file.
Another object of the invention is a set of root canal instruments for carrying out the improved method of the invention.
A further object of the invention is a root canal instrument set that operatively limits the magnitude of torque that is likely to be imposed on each instrument in the set.
SUMMARY OF THE INVENTION
These and other objects of the invention as will be apparent from the following description of the preferred embodiments are carried out by an endodontic procedure in which the magnitude of torque imposed on each instrument in a graduated set is inherently limited by the instrument design.
For reference, instruments according to the invention are rotary cutting devices having tissue cutting edges formed about the axis of substantially conical or cylindrical shaft to surfaces. That axial length portion of an instrument provided with cutting edges is normally characterized as the “working surface” of the instrument. A substantially smooth surface, axial extension of the instrument shaft from the working surface is often characterized as the “shank.” The functional purpose of a shank is for direct engagement with an instrument motor drive chuck. At the axial end of the instrument opposite from the shank is the leading tip of the instrument. The length of working surface behind the tip is characterized as “the cutting surface.” Although the instrument working surface includes cutting edges throughout, that portion of the working surface specifically intended for insitu tissue engagement represents the cutting surface. The cutting surface of an instrument may be all or less than the length of the working surface.
Each instrument in a set according to the invention is formed with a predetermined magnitude of cutting surface as distinguished from the working surface. With respect to each instrument in the set, the cutting surface is developed about the instrument axis at a predetermined taper angle. The axial length of the cutting surface is preferably determined between a minor (smaller) diameter and a major (greater) diameter respective to the instrument cross-section. The working surface of the instrument from the major diameter toward the instrument shank may continue with a second taper angle. Additionally, the minor diameter of each instrument in the sequential set substantially corresponds to the major diameter of the preceding instrument in the set.
The taper angle respective to the cutting surface of all instruments in the set may be substantially the same or graduated. Additionally, the length or area of a cutting surface for each instrument in a set is about the same as for other instruments in the same set.
In practice, the root canal may be initially opened by the first instrument with a pilot boring down to the apical foramen. This first instrument may have a shank and working surface diameter that is substantially the same as the cutting surface major diameter. The minor diameter is substantially the apex of the first taper angle. Accordingly, drive torque to the canal starting instrument is essentially limited to that exerted by the limited area of the first cutting surface. Moreover, this limited torque is transmitted along a substantially constant shaft diameter of relatively substantial size thereby minimizing the probability of shaft failure.
The second instrument in the set has a minor diameter at its tip that is substantially the same as the major diameter for the first instrument. T
Ormco Corporation
Wilson John J.
Wood Herron & Evans L.L.P.
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