FIELD OF THE INVENTION

The present invention relates to endodontic devices and more particularly to a rotary endodontic file with a frictional elastomer grip to be utilized with a standard dental handpiece.

BACKGROUND OF THE INVENTION

Modern endodontic treatment involves removal of the damaged pulp, followed by cleaning and shaping of the root canal space, irrigating, and filling with biocompatible material such as gutta-percha and a root canal sealer.

Shaping of the canal is performed either manually, using a series of stainless steel hand-held files, or mechanically, using rotary nickel -titanium files adapted to fit a standard or dedicated dental handpiece.

The manual process relies on the experience of the dentist, is physically difficult to perform and is time-consuming. The use of files requires repeated changing between numerous different rat-tail files, progressively increasing from the smallest to the largest. Its reach to the apical constriction for educate widening and removal of the smear layer is limited. The reason for such limitations relates to the moderate elasticity of the steel files and the risk of fracture when excessive force is applied.

Although the field of elastic rotary nickel -titanium endodontic files has developed greatly over the last few years, fracture (separation) of nickel -titanium rotary files is still often reported, and hence the use of such is restricted. In order to overcome the fracture problem, a large range of dedicated handpiece systems, powered by slow speed transaction equipped with torque and speed control limitation circuits, have been developed, requiring a high level of professional skill for their operation. As a result, the cost of files and the related equipment per endodontic treatment became significantly higher and are typically operated only by experts in the endodontic field, while ordinary dentists use standard handpiece in a risky procedure. Additionally the round cross-section of the file prevents adequate cleaning when the canal cross-section is oval or narrow in shape. In such cases, enlargement of the canal cross-section to accept the round cross-section of the file, may weaken or perforate the wall or the apical portion of the canal. The same risk of perforation applies in case of a curved root canal, where the file tends to follow the path of least resistance, acting rigorously on the convex walls within the reach of the file, and insufficiently over the concave portions.

US20070099149 entitled "Endodontic device and method of utilizing and manufacturing same" corresponding to IL 171705 in the name of the present applicant discloses an endodontic device for cleaning, filing or reaming root canals. The device includes one or more metallic, flexible strands having an edge, a working section, a connecting section and a coupling head connected thereto. The strands are coated along the working section with a thin layer of a binder having abrasive particles embedded therein. The flexible, longitudinal strands are so constructed that their outer surfaces serve as active filing surfaces.

In operation the strand section of the endodontic file is inserted into the root canal to the apex, and the strand or strands are mechanically rotated by a handpiece to a high speed. The thin, flexible portion of the device is centrifugally thrown against the canal wall following the natural curvature and complex cross-section of the root canal in exact manner. The whole contents of US20070099149 are incorporated herein by reference.

WQ2008/102352 entitled "Endodontic file" corresponding to IL 181439 in the name of the present applicant and likewise incorporated herein by reference, it is further suggested to use an endodontic file comprising at least one cord and a wire wound around at least a major portion of the cord, the wire having an abrasive outer surface.

However, in both the above-mentioned references, the file is. firmly gripped and forcibly rotated by a standard or dedicated high speed handpiece, normally having no means or even technical ability, while in such high rotation speed, to control the delivered torque due to the high inertia of the motor itself. As a result although intended for high speed operation, the file may break if for some reason sticking or locking of the cord in the root canal occurs during operation. Consequently, there exists a need for limiting the transmitted torque in order to prevent breakage of the wire endodontic file in case of sticking. Such an additional safety measure in operation will encourage use of a high-speed rotary endodontic file by those dentists who currently refrain from using such devices for fear of damaging the dentine layer or perforating the root canal.

SUMMARY OF THE INVENTION

In accordance with the invention there is therefore provided an endodontic file comprising at least a central longitudinal cord, a helically wound wire at least partially surrounding the cord and an elastomeric grip partially covering the helically wound wire near a first end thereof and having an outer diameter that is slightly wider than an internal diameter of a barrel of a dental instrument whereby on insertion into the barrel it is supported therein only by friction.

Advantageously, the elastomer grip is cylindrical with a conical extension tapering to a very thin cover towards an active portion of the helically wound wire. Optionally longitudinal fins of the same elastomer material are formed in a circular array around the cylindrical portion of the grip.

The invention further provides a method for safely limiting the torque imparted to an endodontic file according to the invention utilized with a dental instrument, the method comprising:

inserting the grip of said file into the mounting barrel of a dental instrument; and imparting rotational movement to the file , based only on friction, wherein the imparted torque is limited by the percentage of squeeze of said elastomer grip during insertion to the mounting barrel and by the coefficient of friction between the grip elastomer material and the metal barrel of the handpiece, such that if the shear force between the file and the root canal increases beyond a predetermined value, the grip will slip in the handpiece barrel and no breakage damage will occur to the file, nor any undesirable damage to the dentine layer.

The invention still further provides a method of a root canal of a tooth during an endodontic treatment using the endodontic file according to the invention, the method comprising: opening the root canal using a hand-held file of the smallest applicable thickness;

frictionally attaching the endodontic file to a standard or dedicated handpiece; inserting the working section of the file into the root canal;

setting the handpiece to rotate at speed of between 6000 to 30000 rpm;

providing irrigation';

using resultant centrifugal forces acting about the slender working section of the file, which is thrown against the internal dentine layer of the canal wall following the natural curvature and complex cross-section of the root canal in exact manner, thereby providing complete canal preparation in a single time-saving procedure.

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:

Fig. 1 is a perspective view of an assembled endodontic file according to an embodiment of the present invention;

Fig. 2 is a cross-sectional perspective view of the endodontic file of Fig. 1;

Figs. 3a, 3b, 3c are plan views of the different components of the endodontic file of Fig. 1 ;

Figs. 4a, 4b are pictorial representations of the file in operation under different working conditions;

Fig. 5 is a pictorial representation showing a first mode of operation of the file in a root canal of a tooth;

Fig. 6 is a pictorial representation showing a second mode of operation of the file in a root canal of a tooth;

Fig. 7 is an enlarged view of an end portion of the file shown in Fig. 6; and

Fig. 8 is a perspective view of the file prior to insertion into a handpiece head. 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.

Figs. 1 and 2 show an endodontic file 10 to be utilized with a dental instrument, such as a handpiece 52 (Fig. 8) to which an elastomer grip 16 of the file 10 is attachable and supported only by friction, being slightly wider than the internal diameter of the mounting barrel of the dental instrument. The file 10 permits complete canal preparation in a single, time-saving procedure after the first canal opening is made, typically using the smallest applicable hand-held file thickness which is normally the 0.15 size. The file 10 is composed of at least three basic elements, a central longitudinal cord 12, a helically wound wire 14 at least partially surrounding the cord 12 and an elastomer grip 16 partially covering the helically wound wire 14 near a first end thereof .

With reference to Figs. 1 to 3c, the central longitudinal cord 12 may be made of twisted thin stainless-steel wires, welded together at a first end by laser or other heating process, to form a hemispherical globule 18 preferably having the same diameter as the cord 12. A typical cord 12 is between 0.2mm to 0.3mm in diameter and is twisted of 15 to 25 wires. The cord 12 is firmly secured at a second end thereof to the helically wound wire 14 by tight coiling of few turns 20 of the helically wound wire 14 around the cord 12. Optionally, the cord 12 can be secured by soldering, brazing, welding or gluing to the helically wound wire 14. The cord 12 is longer than the surrounding helically wound wire 14, leaving an exposed portion 22, near the globule 18 which is coated with a circular, helical or longitudinal pattern of an abrasive layer 24 (Fig. 3c). The globule 18 has a preferably smooth finish and is not coated with abrasive layer. In one embodiment, the exposed portion 22 of the cord 12 is coated with a single circular pattern of an abrasive layer of approximately 1mm width which is spaced apart by approximately 1mm from the globule 18. The exposed portion 22 is of 3mm to 6mm in length.

The surrounding helically wound wire 14 may be made of stainless-steel having a diameter of 0.2 to 0.4 mm. The helically wound wire 14 is divided along its length into two portions, an inactive portion 26 (Fig. 3b) mostly covered by the elastomer grip 16 and an active portion 30 typically between 8mm to 15mm in length. At least part of the outer surface of the active portion 30 is coated with an abrasive layer 32 (Fig 3c). In a second embodiment, at least part of the outer surface of the active portion 30 is coated with intermittent coating of an abrasive layer 32. As specifically shown in Fig. 3c, intermittent spiral coating is used so as to cover alternate coils of the helically wound wire 14. The abrasive layers 32 and 24 of the active portion 30 and the exposed portion 22 respectively, consists of abrasive particles embedded in a binder, such as thin nickel layer or elastomer binder. Common abrasives are selected from the group including aluminum oxide, silicon carbide, zirconium or diamond powder. The overall length of the active portion 30 and the exposed portion 22 constitute a working section 38 as will be further described below, typically between 12mm to 18mm in length. The pitch 33 (Fig. 3c) of the helically wound wire 14 particularly in the active portion 30 is larger than the wire diameter such that adjacent coils are spaced apart by a certain amount, providing additional free space for debris capture. Optionally varying pitch may be applied, such that different portions of the helically wound wire 14 accept different pitch values. At least part of the active portion 30 of the helically wound wire 14 is advantageously tapered 34 by a grinding, polishing or etching process so as to form a thinner tail 36 close in diameter to the cord 12. In another embodiment, only the tapered part 34 of the active portion 30 is at least partially coated with an abrasive layer. It will be noted however, that only the few turns 20 are tightly coiled over the cord 12, the majority by length of the helically wound wire 14 is loosely wounded over the cord 12 including the tapered portion 34 (Fig. 3b), such that an annular gap of between 0.2mm to 0.8mm exists between the cord 12 and the helically wound wire 14 along at least the entire length of the active portion 30. The free radial play of the cord 12 in the helically wound wire 14 contributes in preventing high strain of the cord 12 when introduced into a curved canal and to an independent operation of the cord when the helically wound wire 14 is restricted in motion as will be explained below.

The grip 16 is made of an elastomer material such as rubber or thermoplastic elastomer, preferably injection or pressure molded directly over the cord 12 and helically wound wire 14. Optionally the elastomer grip can be elastically stretched, bonded or heat shrunk over the helically wound wire 14. The grip 16 is advantageously cylindrical 42 with conical extension 44 tapering to a very thin cover 46 towards the active portion 30 of the helically wound wire 14. The conical extension 44 of the grip 16 constituting a transitional flexibility zone between the gripping cylinder 42 and the active portion 30 of the helically wound wire 14. specific hardness properties of the grip 16 material are selected to provide effective stress relieving of the helically wound wire along the conical extension 44 of the elastomer grip 16 during operation. The cylindrical portion 42 of the grip 16 is frictionally inserted into the mounting barrel of a standard or dedicated handpiece 52 (Fig. 8) for imparting rotational movement to the file, no other locking means is used. Optionally longitudinal fins 48 of the same elastomer material are formed in a circular array around the cylindrical portion 42 further improving the retention force to the handpiece barrel. It will be understood that the imparted torque is limited by the percentage of squeeze of the cylinder 42 or fins 48 during insertion to the mounting barrel and by the coefficient of friction between the grip 16 elastomer material and the metal barrel of the handpiece. Accordingly the maximal transmitted torque can be calculated or otherwise experimentally observed for any specific combination of cylinder 42 or fins 48 external diameter and elastomer raw material. Furthermore, the desired maximal transmitted torque can be precisely controlled and constantly repeated in a mass production of files 10 made in accordance with the present invention.

The grip 16 is optionally provided with a short top section 50 of reduced diameter, on which a metal cap (not shown) can be conductively attached to the central cord 12 using a conductive binder such as solder, laser weld or metal filled epoxy. The metal cap may serve as an electrical reference point for an apex locator, as known per se.

With reference to Fi2S. 4a to 8 the method of usase of th ^p ..file. 10 is described. In operation, the file 10 is attached to a standard or dedicated handpiece 52 (Fig. 8) as described above and inserted to the root canal 54 (Fig, 5) of a tooth 56. Manual back and forth motion is provided while the handpiece is set to rotate at speed of between 6,000 to 30,000 rpm and irrigation is supplied. The slender, working section 38 of the file 10 is centrifugally thrown against the internal dentin layer 58 of the canal wall following the natural curvature and complex cross-section of the root canal in exact manner. The abrasive coating gently buffs and polishes the dentin layer 58, leaving a smooth and clean surface with no smear traces. Back and forth motion enables introduction of lubricant and rinse of debris with constant water irrigation supplied by the handpiece as known per se. The direction of rotation of the file as shown by arrows in Figs. 4a to 7 is in the sense that will further tighten the twist of both the cord 12 and the helically wound wire 14. It will be understood that the opposite direction that will tend to loose the twist of the cord 12 and the helically wound wire 14 is also possible providing better removal rate of debris.

The file 10 adopts two operation modes related to its position in the root canal, while canal penetration is in the upper portion or top two thirds, also known as Coronal third and Middle third (Fig. 5), the file will work in a first operating mode as a single flexible unit including the working portion 38 and the conical extension 44 of the grip 16 (Fig. 4b). A second operation mode is adopted as the file approaches the narrow Apical third (Fig. 6), or the lower portion of the root canal having a depth that is approximately half the depth of the upper portion wherein the motion of the helically wound wire 14 along the active portion 30 also narrows, allowing mainly the central longitudinal cord 12 to utilize its free radial play in the helically wound wire 14 and the centrifugal force to oscillate independently (Figs. 4a, 7), facilitating penetration of the file 10 to the apical constriction widen and clean it during the same single procedure. Upon reaching the apical constriction, the globule 18 at the tip of the cord 12 is limited in space, such that even if additional circular motion is impaired it will rotate about its own axis, thus preventing perforation of the apical tissue. No significant axial force is needed during operation as related to prior art nickel -titanium files.

Transfer of torque from the handpiece to the file 10 is based only on friction. The unique effect of this method turns the file grip 16 into a sort of safety coupling. This guarantees that when the shear force between the file 10 and the root canal increases, for any reason, beyond a certain value, the grip 16 will slip in the handpiece barrel and no breakage damage will occur to the file, nor any undesirable damage to the dentin layer. The standard locking latch mechanism of the handpiece is eliminated.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments, the scope of the invention being indicated by the appended claims.