FIELD OF THE INVENTION

This invention relates to endodontic files.

BACKGROUND OF THE INVENTION

Preparation of root canals is normally done using an integrated series of handheld endodontic files made of stainless steel or Nickel Titanium (NiTi). In use, the root canal is gradually widened using one file after the other. The smallest files typically have a tip diameter in the range between 0.06 to 0.30 mm and a core diameter about 0.60 mm. NiTi files begin at a higher tip diameter of 0.15 mm owing to limitations in processing the material, which causes warming and molecular change of the NiTi and gives rise to the limitation in minimum diameter. This is also why today most NiTi manufacturers add an electro-polishing step after the final grinding in order to free part of the stress that cause micro cracks built up in the material during manufacture and to avoid the effect of cracks. Continued expansion of the root canal is achieved using a motorized handpiece with NiTi files whose core diameter starts at 1.2 mm.

Endodontic files are constructed similar to a high speed twist drill having a shank that tapers to a cone. Spiral flutes are cut in the shank forming edges that are sharpened to create a cutting edge, whereby clockwise rotation of the file into the root canal creates the effect of a screw pulling towards the apical constriction. The difference between the various files is material thickness, the angle and depth of grinding, method of edge cutting, and different rake angles. Conventional endodontic files of the kind described above are subject to a number of inherent drawbacks. First, in order to achieve maximum flexibility that will allow access to curved root canals, the diameter of the cutting edge at the tip must be reduced as much as possible. In other words, precisely that part of the file which does the hard work of penetrating the root canal during twisting is subject to localized concentration of stress in the spiral, which in any case is narrow. The result is a high percentage of breakages close to the narrower apical constriction.

To reduce load in the region of a curved root canal, it is known to first expand the upper ¾ of the root canal using a series of files. This commonly destroys the curvature of the root canal and damages the dentine layer, thereby weakening the canal and the tooth.

Motorized files also typically require initial widening of the root channel to a diameter of a minimum of 0.20 mm, using both manual files and a series of three small diameter Path Finder NiTi files of successively larger diameters of 0.13, 0.16 and 0.19 mm. These files are exposed to increased torque and fracture stresses, their cores are formed of thin material and they also have a high potential for perforation owing to their high penetration derived and no resistance feedback to the user due to high torque and the pulled in effect from the hardness of the material and the very narrow tip.

Manual endodontic files made from stainless steel of diameter 0:06 to 0.15 mm are subject to enhanced torques in narrow curved root canals with hard dentin layers and sometimes break. Motorized handpieces for rotating endodontic files are optimized to work at low speeds in the order of 150-300 rpm with the capability to produce the high torque required for effective cutting sharp edges. They also require a torque and speed control system which gives rise to increased costs not only of the files but also of the handpiece.

Cutting requires relatively high power and the entire load is ultimately conveyed to the outer cutting edge of the file. As a result there are formed varying torsion and shear stresses that in addition to material fatigue may give rise to fracture of the cutting tool.

The file has a tendency to snag the inner wall surface of the root canal if pushed too fast by the operator and/or if self-propelled owing to the rotating screw-thread in the narrow end of the root canal. Protection mechanisms are known that automatically change the direction of rotation but these also increase the effect on the NiTi file molecular changes, which can cut in one direction only, usually clockwise depending on the direction of the spiral cutting edge.

The working surfaces or leading edges of conventional endodontic files are manufactured with active or sharp cutting edges along the entire length of the working surface. This configuration can predispose the instrument to great amounts of torque leading to premature fatigue and breakage. This drawback can be mitigated by varying the taper and the length of the working cutting surface. Likewise the number of flutes may be reduced in order to decrease the cutting surface area and thereby the torque particularly in larger diameter instruments, albeit at the price of inhibiting the ability of the instrument to continue to haul debris coronally. As a result, the instrument can become easily clogged creating unnecessary drag on the instrument.

Based on these considerations, US Patent No. 7,955,078 discloses a set of endodontic instruments, each having a conical working portion that has an effective contact area defined by the exposed surface area of the working portion's respective cone-like shape. The endodontic instrument retains the flutes along the entire length of the working surface to maintain hauling action. The leading edge or the working surface, however, is modified such that only a portion of the working surface cuts. This modification is brought about by blunting or rolling the edge of flutes both at the tip and shank ends of the instrument, leaving only the central portion of the cutting surface active. According to the patent, the rolling edges will prevent the instrument from over- enlarging or tearing the foramen of the ECS distally and mitigate drag and pre-mature fatigue proximally.

FIG. 9A of US Patent No. 7,955,078 shows an instrument 902 having a reduced working portion. FIG. 9B shows a working portion 904 of the instrument 902. FIG. 9C shows a non-cutting tip portion 906 of the instrument 902. FIG. 9D shows a non-cutting shank-end portion 908 of the instrument 902. FIGS. 9E-9H show an implementation in which the instrument 902 tapers from shank to tip.

In summary, US Patent No. 7,955,078 proposes a set of tools each one of which has a designated cutting area that varies longitudinally along the tool's axis for working on a different section along the axis of the root canal. The remaining edges of the flutes in each tool are blunted since they are not intended to cut and thus serve no useful function. Blunting the unused sections of each tool reduces the tool's diameter and avoids contact with the inner surface of the root canal, thereby reducing torque with its attendant problems described above. However, it should be noted that the tools are cutting tools similar to high speed twist drills or masonry drills whose purpose is to create the dental cavity. This is quite different from an endodontic file whose purpose is to enlarge the root canal. Thus, endodontic files do not require a cutting edge, the flutes serving principally for withdrawing debris and having abrasive surfaces. There is no suggestion in US Patent No. 7,955,078 to render the surfaces of the tools abrasive. Furthermore, in order to enlarge the root canal using the tools described in US Patent No. 7,955,078, a complete series of tools is required since each is configured to operate over only a fractional depth of the root canal. This requires the dentist to constantly change tools.

US Patent No. 4,850,867 discloses an endodontic instrument having a non- cutting pilot segment, a short cutting segment, and a flexible shaft segment, which can have a handle at its distal end for manual manipulation, or an adapter for attachment to a mechanical handpiece. The non-cutting pilot, the short length of the cutting segment, and the flexibility of the shaft combine to allow the instrument to be used in curved root canals without causing undue change in the natural root canal contours.

US Patent No. 5,836,764 discloses an endodontic file for use in shaping a root canal having a shank and a flexible working portion having cutting edges which may be dulled at along one side of the file. The working portion is formed of nickel titanium alloy and may be pre -curved.

The pitch and relative sharpness of the cutting flutes are longitudinally varied. It is further stated that the potential for breakage and slowness of cutting is overcome 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 allegedly preventing file breakage which can occur when the tip binds. Nevertheless, it is to be noted that the flutes are not entirely rendered blunt and they maintain their cutting facility.

SUMMARY OF THE INVENTION

The invention provides a fluted endodontic file having an abrasive working surface for path finding and enlarging the root canal, which has reduced friction and which cleans the root canal using normally only a single file without the need to change tools. A single file according to the invention, owing to its improved flexibility, is typically capable of reaching the full depth of the root canal while expanding its diameter to 0.20-0.25 mm corresponding to the diameter at the tip of the file. Although such a file may be operated using a rotary handpiece, it is usually more convenient and quicker to use a single manual file, which replaces the series of 2-5 manual files conventionally required. Thereafter, further treatment can continue using any type of rotary file.

In many cases, the invention allows a prepared root canal to be cleaned using only a single endodontic file owing to the frictional resistance between the fluted edges of the file and the internal wall of the root canal being significantly less than that of a corresponding cutting edge. As a result, most of the effort applied manually is translated to vertical force, the wasted horizontal component of applied force being low. For the same reason, the endodontic file according to the invention may be operated by a motorized handpiece rotating at high speeds thereby allowing the procedure to be completed more quickly. However, there may be cases such as very narrow root canals where two or even three endodontic files may be required.

In accordance with the invention there is thus provided an endodontic file having a solid shank at least part of which supports an abrasive fluted working section having at least one spiral flute dimensioned for working within a root canal to a full depth of the root canal, an edge of each spiral flute being blunt along a substantially complete length of the flute.

Within the context of the description and the appended claims it is to be understood that the term "substantially" allows for a very small part of the endodontic file extending from the tip to a distance of up 2.5 mm may be untreated and thus retain its cutting edge. The spiral flutes serve the double purpose of withdrawing debris by screw action and affording an abrasive surface of reduced area compared with a file having no flutes. The cutting edges of the flutes are removed throughout the complete length of the working section of the file. BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. 1 and 2 are pictorial views showing conventional fluted endodontic files having a cutting edge along its working surface;

Fig. 3 is a pictorial view showing a fluted endodontic file according to the invention having a blunt edge along the whole of its working surface;

Fig. 4a is an enlarged view of the conventional fluted file shown in Fig. 2;

Fig. 4b is a cross-section of the file shown in Fig. 4a taken along the line A-A; Fig. 5a is an enlarged view of the fluted file shown in Fig. 3 according to a first embodiment the invention;

Fig. 5b is a cross-section of the file shown in Fig. 5a taken along the line A-A;

Fig. 6a is an enlarged view of the fluted file shown in Fig. 3 according to a second embodiment the invention;

Fig. 6b is a cross-section of the file shown in Fig. 6a taken along the line A-A; and

Fig. 7 is a pictorial view showing a fluted endodontic file according to a further embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description of some embodiments, identical components that appear in more than one figure or that share similar functionality will be referenced by identical reference symbols.

In order to highlight the distinction between the invention and the prior art, reference is made to Figs. 1 and 2 showing details of a conventional fluted endodontic file 10 having a shank 11 that may be fastened to the handle (not shown) of a manual file or terminate in a D-type coupler 12 for coupling to a motorized rotary handpiece. Likewise, the file may be configured for coupling to a motorized handpiece adapted for to and fro reciprocal motion. The shank 11 is fluted along its working length and the flutes 14 have a cutting edge along their working surface as seen in Fig. 2. The flutes 14 cut the inner surface of the wall of the root canal during rotation of the file, while extracting debris owing to the screwing action of the file.

Figs. 3 and 4 show details of a fluted endodontic file 15 according to the invention having a NiTi or stainless steel shank 11 that may be fastened to the handle (not shown) of a manual file or terminate in a D-type coupler 12 for coupling to a motorized rotary handpiece. Alternatively, the shank may end in a friction grip for engaging the chuck of a motorized rotary handpiece and that acts as a safety clutch that slips in the chuck in the event that the torque exceeds a safe value as disclosed in WO 2011/104705. The shank 11 is fluted along the complete working length 16 (constituting a working section of the shank) with spiral flutes 14 whose edges are completely rendered blunt. This may be done by clipping or trimming the corners of the flutes or grinding the edges to form a semi-circular contour as seen in Fig. 4.

The surface of the file along the complete working length 16 is then roughened by scraping or by spraying under pressure metal particles or other material that form pits in the surface of the file including the flutes and render it abrasive. At the end of this process, the NiTi or stainless steel tool changes from being a drill-like cutting tool to a file that easily penetrates the prepared root canal by manual rotation or, where speed is not an issue, by rotation of the handpiece. Although no longer effective as cutting edges, the helical flutes are retained partly in order to improve the flexibility of the file and to assist in removal of the debris, as explained above and as marked 16 in Fig. 3. However, owing to the significant reduction in frictional torque and shear forces applied to the flutes, the number of flutes as well as their pitch can be the same as in conventional files tools. Consequently, the flutes remain effective for the removal of dentine debris.

The dentin removed from the wall of the root canal is similar to a runny paste comprising the small grains that are removed and mixed with the rinsing liquid. This paste easily sticks to the abrasive surface of the spiral flutes and does not remain inside the root canal. In contrast thereto, the particles that are removed from the sharp corners of a conventional spiral are in effect chips, some of which are compressed into the corners of the root canal and are therefore not caught by the spiral and so are not properly vacated from the root canal. This highlights another benefit of the endodontic file according to the invention over conventional endodontic files. Yet a further benefit is that the small grains that are removed by abrasion are easily rinsed clear of the root canal, which is not always true in the case with the chips of dentin removed by conventional files where the removal may be incomplete and the unremoved residue may cause problems.

The file is effective in both directions although when first introduced into the root canal for measurement of the depth of the apical constriction, it is important to rotate the file in the direction of the helical spiral (usually clockwise). After initial widening of the root canal, the file may be subject to up and down movement either together with or instead of rotation. By minimizing resistance between the file and the inner wall of the root canal that allows gradual shaping of the root canal, files of larger diameter may be employed [0:20 to 0:25 mm] also for initial insertion into the root canal for measuring depth of the apical constriction. As the file approaches within 2 mm of the apical constriction, it is also possible and advisable to rotate the file in a counterclockwise direction so as to avoid the tendency of the tip of the file from screwing into the apical constriction, which reduces the operator's sensitivity of the tool and thereby makes it difficult for the operator to control movement of the file. Unlike conventional files, in the file according to the invention, abrasion is identical in either direction of rotation of the file such that the process of enlarging the root canal is not impaired while manual control during movement of the file toward the apex will be more sensitive, more reliable and most importantly safer. When shaping curved and narrow root canals, the file can be manually inserted until it reaches the apical constriction and only then coupled to a handpiece for rotation. Torque resistance is negligible because the file reaches the apical constriction by abrading rather than cutting the dentine.

Although the file shown in Fig. 3 has only two flutes, it will be appreciated that the invention is equally well applicable to endodontic files having any number of flutes and any spline geometry.

Fig. 4a is an enlarged view of the conventional fluted file 10 shown in Figs. 2, wherein the edges of the flutes 14 are marked 20. Fig. 4b is a cross-section of the file 10 shown in Fig. 4a taken along the line A-A showing that the edges 20 are sharpened to a point so as to render the tool suitable for cutting the root canal.

Fig. 5 a is an enlarged view of the fluted file 15 shown in Fig. 3 according to a first embodiment the invention, wherein the edges of the flutes 14 are again marked 20. Fig. 5b is a cross-section of the file 15 shown in Fig. 5a taken along the line A-A showing that the edges 20 are rounded thus rendering the tool incapable of cutting the root canal. In this embodiment, the complete surface 21 of the file including the shank and the flutes are rendered abrasive such that the file performs a similar operation to electro-polish.

Fig. 6a is an enlarged view of the fluted file 15 shown in Fig. 3 according to a second embodiment the invention. In this embodiment, only the edges 22 of the flutes, which effect surface contact with the root canal are rendered abrasive, the remainder of the shank being plain. This makes it easier for the file to work the root canal and allows the file to be more easily withdrawn during rotation.

It will be noted that the direction of the flute in Fig. 5 is shown as clockwise while that in Fig. 6 is shown as anticlockwise. Since the file does not cut but only abrades the internal wall of the root canal, it is effective regardless of the direction of the flute. Thus, in either case, either the complete surface 21 of the file including the shank and the flutes or only the edges 22 of the flutes are rendered abrasive. Again, in both cases, provided that the file is rotated in the same direction as the flutes, debris is extracted along the flutes by the screw-effect.

Fig. 7 shows a further embodiment similar to the one shown in Fig. 5a in that the edges of the working section of the file are rendered blunt, except for up to 1.5 mm at the tip end which is not blunted. This allows the file more easily to self-direct toward the apical foramen. It has been found that since in any case the file is generally conical in cross-section and the tip is of only very small diameter it plays substantially no part in the cutting and therefore the fact that its cutting edges are left intact does not derogate from the overall benefit afforded by the majority of the flute having blunt edges, while providing the additional benefit of self-direction and easy penetration in the narrow part of the apical zone. In this embodiment also either the complete surface 21 of the file including the shank and the flutes or only the edges 22 of the flutes may be rendered abrasive. For the sake of absolute clarity, it is to be understood that the invention requires that the spiral flute of the working section of the file be blunt. If the flute is longer than necessary, the distal part of the flute which is supported by the handle may if desired be provided with sharp edges since they will not, in case, participate in the root canal treatment. Consequently, this part of the shank is not part of the working section as used herein and in the appended claims.

It is also to be understood that the principle of the invention is independent of the cross-section of the endodontic file, which may be round but does not need to be. For example, D-type files are known having a non-circular cross-section and these are equally well suited for use with the invention.

It should also be noted that features that are described with reference to one or more embodiments are described by way of example rather than by way of limitation to those embodiments. Thus, unless stated otherwise or unless particular combinations are clearly inadmissible, optional features that are described with reference to only some embodiments are assumed to be likewise applicable to all other embodiments also.