Endodontic treatment system

Abstract: An endodontic file for use in preparing a root canal has a shank and a flexible working portion having cutting edges which may be dulled at along one side of the file. The working portion has a taper greater than the ISO Standard of 0.02mm per mm of axial length. The working portion is formed of nickel titanium alloy and may be pre-curved.

Description: This invention relates to endodontic instruments used in preparing the root canal of a human tooth for filling. A relatively common but 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 drilled in the tooth to access the root canal, after which root canal files are 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 inserted into the root canal to seal it. Two commonly used types of root canal files are the K-type file and the Hedstrom file. The K-type file is axially twisted and tapered and has a triangular or square cross-sectional shaft providing three or four spiral cutting edges along the tapered shaft with a conical tapered tip on the end thereof. K-type files have recently come to be manufactured with lathe-cut flutes as well. The Hedstrom-type instrument is a lathe-cut file having a round tapered shaft with one, two, or three spiral cutting flutes machined into the shaft all the way to the tip. The main difference resulting from the construction of the two types of files is that the K-type file will cut in either rotational direction, or when moved up and down, while the Hedstrom-type file will cut best when moved up and down in the root canal. When a root canal is being cleaned and shaped, a series of files having increasing diameters is used to gradually enlarge the root canal. Each file in a set has an identical taper from one end to the other, typically 0.32 millimeters on every file over the standard 16 mm length of cutting flutes for K-type files, 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). This variation in flute angle or pitch is most perpendicular to the long axis of the file near the shank end, changing to a lesser angle (more in line with the long axis) as the flutes approach the file tip. One particular Hedstrom-type file has been modified to provide a safe edge by removing or dulling the cutting edges of the flutes along one side of the file. There are also ground-flute instruments which have their flutes cut into tapered wire blanks which are round in cross-section. This results in the flutes being cut across the grain of the metal, thus achieving less strength than twisted-flute files and so being inadequate in the smaller sizes. Twisted-flute files are formed by first grinding a tapering cross-sectional shape that is triangular, square, or rhomboidal down the length of the wire, in line with the metal grain. Then this tapering wire blank is twisted, causing the corners of the tapered wire blank to become spiral cutting flutes. Because of the twisting process, the metal grain is thus-aligned with the helical cutting flutes. In the smaller sizes below #15 only the twisted-flute instruments are stiff enough, with a short #15 file being typically used to initially penetrate coronally occluded canals. Attempting to add strength to the tips of smaller ground flute instruments by increasing the flute pitch angle toward the instrument tip, thereby decreasing the depth of the flute spaces and increasing the mass of metal in the more fragile tip region, creates file tips which are too stiff and inefficient in the larger sizes. Unfortunately, even twisted-flute #15 files are often not strong enough to withstand the tremendous apical forces which dentists bring to bear when attempting to negotiate calcified canal orifices. These ISO tapered #15 files often buckle in use, requiring their disposal and the use of yet another #15 file. It is typical that dentists will destroy 10-20 #15 files before they can clear to the end of these difficult canals. Root canals are seldom straight and there is always the possibility of causing irreversible damage to roots during shaping procedures, depending on the thinness of the root and severity and location of the root curvatures. Root perforation, ledging and other mistakes in root shaping procedures may occur during the preparation of a root canal using currently available instruments. At worst, these mistakes may require tooth extraction or surgical correction; at least, they may result in root weakening with follow-on problems in the future. The art of root canal shaping is one which requires great skill to prevent damage to the tooth and to create a tapered canal preparation conducive to ideal filling of the canal. The technique used with a conventional set of files having identical tapers to clean and shape the root canal is referred to as the "step-back" technique. A series of file sizes from 08 to 60 (12 instruments) are introduced into the canal from smallest to largest with each successively larger file being used further back from the end of the canal. Additionally, between 4 and 6 sizes of Gates-Glidden or Peezo burs are similarly used in this step-back manner comprising a total of 16 to 18 instruments. This technique is, at best, a difficult and time-consuming method as the dentist must indirectly gauge the rate of taper in the preparation by the distance interval of step-back of the progressively larger instruments as they fit further back from the canal terminus. As only the ISO taper of 0.02 mm/mm is currently available in standard sizes to dentists in files, irrigation cannulas, condensation pluggers, heat carriers and injection needles, paper points, filling material, and restorative posts, the current shift to canal preparations of greater taper has created great difficulties for dentists who want to enhance their root canal shaping objectives. The skills required, with these relatively non-tapered instruments and materials, to create conservative but adequately tapered shapes in root canals and to easily and ideally fill them usually comes only after treating hundreds of clinical cases. US 5,213,499 discloses endodontic files in accordance with the pre-characterising portion of claim 1. The files have a taper of 0.0215 mm/mm. A file according to the present invention is characterised by the feature set out in the characterising portion of claim 1. The files of the present invention differ significantly and advantageously from the technology discussed above in a number of respects. 1. Rather than using a large series of files of differing sizes but the same taper, as is the case when using standard ISO-tapered files, use of the present system often allows full root canal shaping to be accomplished with as little as a single file. 2. Instead of indirectly creating a tapering canal preparation with the difficult and time-consuming serial step-back shaping technique necessary with ISO-tapered files, the present system requires only that the shaping file or files be worked to the terminus of the canal, thereby completing the preparation. 3. The present system provides variably-tapered negotiation files, allowing optimally varied stiffnesses as the tips of the instruments in the series increase in diameter. Furthermore, these files of increasing taper can have small, even increases in tip diameter allowing more passive negotiation to the terminus of constricted canals. 4. One version of these tapered files can be efficiently used with rotary cutting motions without increased breakage, due to their varied flute pitch and sharpness along the length of the instruments. 5. Safe function of these files in curved canals is also gained through the use of nickel-titanium, available under the trademark Nitinol, an alloy which exhibits extreme flexibility and elastic memory. The purpose of endodontic shaping procedures is to create a continuously tapering preparation which is narrowest at the end of the canal, and widest near the crown of the tooth. By using files that vary in their tapers, root canals may often be prepared with a single shaping file instead of the 16 to 18 files required by conventional ISO-tapered instruments. The present system permits ideal root canal shaping results with less training and experience. All that is required is that a shaping instrument of appropriate taper be worked to the full length of the canal. In addition to the greatly improved ease and simplicity of shaping canals with a single instrument, this provides, for the first time, a pre-defined shape throughout the full length of the canal. One of the most important advantages provided by pre-defined root canal preparations is the resultant ability to optimize cleaning and filling procedures in root canal systems. Since the files of the present system involve different tapers, the tips of the shaping files are not used to cut a path in the canal as is the case with files with standard tapers. The tip diameters of a set of shaping files of the present system may, if desired, be the same between files of different tapers. There may also be different sets of these shaping files which differ in their tip diameters; i.e., one set of shaping files with tapers of .04, .06, .08, .10, and .12 mm/mm and the same 0.2 mm tip diameters; another set of shaping files with the same range of tapers but with 0.35 mm tip diameters, etc. The present system also includes sets of shaping files with similar non-ISO tapers which vary by evenly or proportionally increasing tip diameters. Shaping files of the present system for use with rotary cutting motions provide for longitudinally varying the pitch and relative sharpness of cutting flutes. This overcomes the problems of potential for breakage and slowness of cutting by varying the flute pitch from an in-line reamer-like angle at the shank end of the instrument to a more perpendicular K-type flute angle at the file tip. Instead of, or in addition to the above, the relative sharpness of the cutting flutes is varied along the length of the files, being sharpest at the strong shank end to allow for aggressive cutting by the wider flutes, and dullest near the smaller, more fragile tip of the file so these flutes can easily release from the canal wall during rotation, thereby preventing file breakage which can occur when the tip binds. Functional safety when using tapered shaping files in curved canals is enhanced through the use of the alloy nickel-titanium (Nitinol). This unusual metal exhibits stress-induced phase-transformation of its crystal lattice structure resulting in characteristics of superelasticity, allowing large-diameter Nitinol files to work around root canal curvatures that would be impossible with stainless steel. Nitinol's elastic memory also allows, in one aspect of the present system, curvatures to be preset in the file during the manufacturing process. By heating and cooling files made of this metal, different curvatures can be present in the metal so that the desired shape is "remembered", allowing the file to straighten in the relatively uncurved coronal portion of canals and recurve itself when the file tip moves deeper and encounters apical root canal curvatures. When added to a modified Hedstrom-fluted taper file, a preset file bend directed toward its safe edge causes the Nitinol version of this shaping instrument to automatically seek the inside of canal curvatures with its non-cutting side, thereby effortlessly preventing lateral perforation. In the accompanying drawings: FIG. 1 schematically represents a root canal in a tooth; FIG. 1A shows a conventional K-type file; FIG. 2 schematically represents the final intrusion of the tips of each of eleven conventional files into the root canal of FIG. 1, illustrating the conventional step-back technique; FIG. 3 is a schematic view of the conventional file of FIG. 1A being used to clean and shape the root canal, and specifically illustrates both the perforating and ledging of the root canal; FIG. 3A is a view like FIG. 3 except that it illustrates the damage which may result from misuse of a Gates-Glidden bur; FIG. 4 is a schematic view showing one of a series of variably-tapered implements in accordance with the present invention; FIGS. 4A and 4B are schematic views illustrating two different flute-to-shank transitional configurations of the implement of FIG. 4 designed to eliminate gouging of curved canal walls by the large shank-end cutting flutes; FIGS. 4C through 4E are schematic views illustrating different cross-sectional flute designs of the implement of FIG. 4 which vary the sharpness and cutting efficiency from the tip (dullest) to the shank (sharpest) in order to decrease the chances of breaking the tips of files when the flute near the tip grabs the canal wall; FIG. 5 (views C, D and E) are schematic views of a set of negotiating files of different tip diameters, different tapers and equal flute lengths; FIG. 1 shows an incisor tooth 10 located in the bone 12 of a jaw and having an opening 14 cut on the side of the tooth 10 in the interior side of the jaw (not shown). The tooth 10 has a nerve canal 16 extending to the tip of the tooth 10 which is embedded in the bone 12. FIG. 1A shows a conventional K-type file 20, having a handle 22 supporting the file 20. The file 20 has a flute length X, a shank diameter Y, and a tip diameter Z. In a standard file of this type, X = 16 mm, Y = 0.32 mm and Z varies with the size of the file, beginning with 0.06 mm for the smallest file and increasing to 1.4 mm for the largest files. The file 20 has a sharp tip 24, which is needed since each successive file in a series has a larger diameter at the tip. FIG. 2 shows the file 20 inserted into the root canal 16, which is enlarged from the view shown in FIG. 1. With conventional files, the step-back technique discussed above is used, with each progressively larger file being inserted shallower and shallower into the root canal 16. The numbers along the root canal 16 near the tip end of the root canal 16 represent the maximum extent to which different size files are inserted, with file sizes from 10 to 60 (representing tip diameters from 0.10 mm to 0.60 mm) being used. A minimum of nine to eleven files are required, with 15 to 17 instruments being common. FIG. 3 shows the file 20 inserted into a root canal 30 in a molar tooth 32. A hole 34 to admit the file 20 is present in the biting surface of the tooth 32. Since the file 20 was too large and not curved enough, it has perforated the curved root canal 30 at the location indicated by the reference numeral 36. Perforation can also occur when the file 20 is not pulled against the outside curve of the root canal 30 by using the proper anti-curvature motion. Also in FIG. 3 the tip 24 of the file 20 has left the root canal 30 and cut a ledge along the outside curvature of the root canal 30. Succeeding files may well become trapped in the ledge also, and will not properly clean and shape the root canal 30. The present invention eliminates both perforation and ledging of a root canal, and in addition makes the root canal operation quicker and easier to perform. FIG. 3A depicts a molar tooth 32 with a root canal 30 accessed by a provision of a hole 34. Instead of the file 20, a Gates-Glidden bur 37 is shown working in the root canal 30, driven by a power head 38. The bur 37 has perforated the curved root canal 30 at the location indicated by the reference numeral 36. FIG. 4 shows a tapered shaping file 50 of my invention having a handle 52 in which is affixed a shank 54 terminating in a working portion 56. The handle 52 has a hole 58 extending through it near the proximal end for the insertion of a safety retraction loop. The shank 54 has a diameter A which is slightly less than the maximum diameter B of the first flute 62, with transitional lands 66 extending between the first flute 62 and the smaller diameter file shank 54. The working portion 56 comprises a plurality of flutes 60, each being provided with cutting edges about the periphery, diminishing in diameter and sharpness and increasing in pitch with distance from the first flute 62. The tip 64 is smooth, without any cutting edge, and has a diameter C which is slightly smaller than that of the last flute 60 which transitions by the land 65 to the tip 64 (FIG. 4F). While any or all of the shaping instruments described herein may be mounted in a latch-grip shank 81, as seen in FIG. 4H, and operated in a dental handpiece, FIG. 4 shows the handle 52 of my preferred embodiment, formed in the shape shown between two opposed planar surfaces 76 and 84. These planar surfaces are 2 mm in diameter. From the boundary of the surface 76, the handle enlarges through a smooth curved region 78 to its maximum diameter of 6 mm at the point 80. In a section 82 between the maximum diameter 80 and the proximal end 84, the handle tapers linearly and is scored all around with a plurality of tapered grooves 83. The transverse plane at the maximum diameter 80 is spaced 2 mm from the planar surface 76 and the distance between the plane of the maximum diameter 80 and the proximal end 84 is 8 mm, the overall length of the handle 52 being 10 mm. The grooves 83 and the shape of the scored, tapered section 82 provide an enhanced gripping area for the user's thumb and fingers to facilitate applying torque to the file as it is moved into the root canal toward the apical region. The shape of this handle 52 is to be contrasted with the shape of the handle 22 in the conventional file 20 of FIG. 1A which is designed for push-pull cutting and 1/4 turn-pull cutting motions, rather than apically-directed counterclockwise rotary cutting action. The embodiment shown in FIG. 4 has a diameter B equal to 1.0 mm, whereas the dimension A, the diameter of the shank 54, is 0.95 mm. By providing the shank with a slightly smaller diameter A than the diameter B of the first flute 62, binding of the file shank in a root canal is avoided. However, if the diameter of the shank 54 is too much smaller than the diameter of the first flute 62, the file may be weakened at that transitional point, and the largest shank-end flute may transport the original path of the canal, as seen in FIG. 6A. The transitional lands 66 seen in FIGS. 4A and 6B extending from the first flute 62 to the shank 54 help to guide the file in the root canal and limit divergence of the first flute from the preparation path. FIG. 4B shows an alternate embodiment with a shank guide 68 which is separate from the cutting flutes of the file. This shank guide embodiment is separated from the largest flute by a distance 69 which allows a chip space for tooth dust to collect after it is cut from the canal walls during filing procedures. The shank guide diameter is only slightly smaller than the largest flute diameter 62 of a given file, so that it may guide that largest cutting flute without binding the prepared canal wall as the instrument progresses deeper in the root. In FIG. 4 the angle of the flutes relative to the longitudinal axis of the shank varies along the length of the file as is apparent from a comparison of the angle α, indicated by the broken line 70, with the angle β, indicated by the broken line 72. Thus the flute angle near the larger diameter shank end is reamer-like, while the flute angle at the file tip is more like a K-file flute angle. This flute design answers two critical challenges encountered when using a single shaping instrument to create a canal shape previously imparted with from 15 to 18 conventional instruments. Optimal function of this type of instrument requires enhanced cutting efficiency and flexibility at the larger shank end, and maximal strength at the more fragile file tip. Fortunately, tapered canal shaping objectives require less dentin removal apically; therefore the optimal tapered shaping file design can sacrifice apical cutting efficiency for added strength. The K-type flute angle of the FIG. 4 file provides this. In this embodiment, the flute pitch is 0.5 flutes/mm (one flute in two millimeters) adjacent the largest diameter flute 62 and two flutes/mm or four flutes in two millimeters adjacent the tip 64. The ratio of flute pitch at the shank to flute pitch at the tip (the overall flute pitch ratio) is four in the FIG. 4 embodiment. The flute pitch ratio, defined as the flute pitch at any point along the working portion of the file relative to the flute pitch at the tip, diminishes linearly along the length of the working portion of the file. Thus, in the embodiments of FIGS. 4A-4E the flute pitch ratio diminishes linearly from four at the shank to one at the tip. Other ratios may be selected for other file sets, if desired, but a linear decrease in flute pitch ratio from shank to tip is preferred. Similarly, the sharpness of the cutting flutes may be decreased from shank to tip, decreasing the chances of a cutting flute near the tip catching in a canal wall which easily causes file breakage when the file is used in a rotary cutting motion. The result is increased cutting efficacy near the shank end flutes where more dentin removal is needed and where the file has its greatest diameter and strength. This variable dulling may be accomplished by a number of means, including an increasing "U" blade (see FIG. 4C), a progressive flute edge radius as seen in FIG. 4D, or a different flute rake angle as seen in FIG. 4E. In Fig. 5, views C, D, and E, show a set of negotiation files 140 which are given the designation numbers 12/03, 15/04 and 17/05. The first number corresponds to the tip diameter which increases from 0.125 mm for file 140C to 0.175 mm for file 140E in increments of 0.025 mm. Each of the files has a flute length of 16 mm. The maximum flute diameter increases proportionally from 0.605 mm for file 140C to 0.975 mm for file 140E. Tapers increase in 0.01 mm/mm increments from file 140C with a taper of 0.03 mm/mm to file 140E with a taper of 0.05 mm/mm.

Claims: An endodontic file (140 C,D,E)for use in preparing a root canal in a tooth, said file having a shank, a tip, and a tapered working portion, the tapered working portion having at least one helical cutting flute with a cutting edge having a predetermined cross-sectional profile, the working portion being formed of nickel-titanium alloy, characterised in that for forming a generally tapered root canal from the crown to the apex of the root canal, the taper of the working portion is in the range of 0.03 to 0.12 mm/mm of axial length. The file of claim 1 wherein the sharpness of at least one helical cutting flute varies along the length of the working portion, being sharpest at the end next to the shank and least sharp in the vicinity of the file tip to reduce the likelihood of file breakage from binding of the tip in the root canal. The file of claims 1 or 2 wherein the flute angle relative to the axis of the shank varies along the length of the file working portion. The file of claim 3 wherein the flute angle adjacent the shank is less than the flute angle in the vicinity of the tip. The file of any preceding claim wherein the predetermined cross-sectional profile is linear (Figs. 4C and 4D). The file of any of claims 1 to 4 wherein the predetermined cross-sectional profile is S-shaped (Fig. 4E). The file of claim 6 wherein the file working portion has three spiral cutting edges (62) throughout its length which are equally spaced about the periphery of the file working portion and wherein each surface between adjacent cutting edges has a general S-shape when viewed in cross section (Fig. 4E). The file of claim 5 wherein the file working portion has three spiral cutting edges throughout its length, the cross section of the file working portion being in the shape of an equilateral triangle with linear faces between adjacent cutting edges (Fig. 4D). The file of any one of claims 1 to 4 wherein the file working portion has three spiral cutting edges equally spaced about the periphery of the file working portion and wherein the surfaces between adjacent cutting edges are concave in cross section (Fig. 4C-B).

Inventors: BUCHANAN LEONARD STEPHEN, BUCHANAN, LEONARD STEPHEN
Applicants: BUCHANAN LEONARD STEPHEN, BUCHANAN, LEONARD STEPHEN
Application year: 1995
Main classification: A61C5
IPC codes: A61C5