Patent Publication Number: US-2021177543-A1

Title: Dental file

Description:
TECHNICAL FIELD 
     The present invention relates to a dental file used for expanding/cleaning a root canal in dental treatment. 
     BACKGROUND ART 
     There is a reamer or a file as a dental root canal cutting tool used for expanding or cleaning a root canal in dental treatment (see Patent Literature 1). The reamer is mainly for cutting the inside of the root canal by rotation, and the file is for cutting the inside of the root canal by rotation or back-forth movement in an axial direction. 
     A typical dental root canal cutting tool is made of stainless steel. However, in recent years, a highly-elastic nickel titanium product suitable for treatment of a root canal curved in a complicated shape has been sometimes used. Further, not only a manual type but also an electric type connected to a dental hand piece (an engine) upon use have been increasingly used as power. The electric type allows quick treatment. 
       FIG. 6  is a plan view of a general dental file. The dental file  100  described herein is connected to a dental hand piece upon use. The dental file  100  has a working portion  101  including a spiral cutting blade in a shape narrowing toward a tip end, a shaft  102  formed continuously to a back end of the working portion  101 , and a gripping portion  103  formed continuously to a back end of the shaft  102  and attached to the dental hand piece. 
     In the general method for manufacturing the dental file described above, processing is first performed such that a portion to be the working portion is formed from a thin wire rod with a certain taper rate (e.g., 6/100) by a centerless processing machine. Thereafter, a groove to be the cutting blade of the working portion is formed by a blade groove grinding machine. If processing is performed using a thick wire rod, the tapered portion to be the working portion and the groove to be the cutting blade can be technically formed at the same time. However, due to a great grinding amount and easy damage of a grinding stone, a method in which a thin wire rod is ground at two stages that the tapered portion is first formed and the cutting blade is subsequently formed is often employed. In the case of a nickel titanium dental file, the grinding stone is particularly vulnerable, and for this reason, such a dental file may be manufactured by the method in which the thin wire rod is ground at two stages. 
     For the dental file, performance allowing cutting while following a root canal curved in a complicated shape is required. That is, even when high cutting performance is exhibited, if flexibility is poor, the dental file cannot curve along a curved line of the root canal, but cuts the root canal in a straight-ahead direction. This might lead to a disadvantage that an uncut portion remains in the vicinity of a tip end of the root canal. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 4247346 
     SUMMARY OF INVENTION 
     Problems to be Solved By Invention 
     In view of the above-described situation, the present invention is intended to provide a dental file having excellent flexibility while holding a moderate level of cutting performance so that high root canal following performance can be exhibited. 
     Solution to Problems 
     In order to achieve the object described above, a dental file according to the present invention includes a working portion in a spiral shape, in which an outer edge of a sectional shape of the working portion at any position includes a single arc and three or more lines, one end of the arc forms a first cutting blade, and the three or more lines are positioned inside a virtual circle partially including the arc. Further, it is preferred that a length of the arc is equal to or greater than 1/12 and equal to or less than ⅓ of a circumferential length of the virtual circle. 
     Further, a second cutting blade may be provided inside the virtual circle, or the spiral shape of the working portion may be configured such that a spiral pitch narrows toward a tip end. 
     Effects of Invention 
     The dental file of the present invention exhibits high performance and has high flexibility, and therefore, provides an effect that the dental file can cut even a root canal curved in a complicated shape while following the root canal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a plan view of a dental file, and  FIG. 1B  is an enlarged A-A sectional view. 
         FIG. 2A  illustrates the sectional shape of a working portion of a typical product and  FIG. 2B  illustrates the sectional shape of a working portion of the present invention. 
         FIGS. 3A-3C  illustrate views for describing the steps of manufacturing the dental file,  FIG. 3A  being a plan view of a wire rod,  FIG. 3B  being a plan view of a tapered member, and  FIG. 3C  being a plan view of the dental file. 
         FIGS. 4A-4C  illustrate views for describing the sectional shape of the dental file,  FIG. 4A  being a view of a core contacting three sides,  FIG. 4B  being a view of the core contacting one side, and  FIG. 4C  being a view of the core contacting two sides. 
         FIGS. 5A and 5B  illustrate views for comparison among different spiral pitches of the working portion,  FIG. 5A  illustrating a dental file with equal pitches and  FIG. 5B  illustrating a dental file with pitches narrowing toward a tip end. 
         FIG. 6  is a plan view of a general dental file. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the attached drawings. 
       FIG. 1A  is a plan view of a dental file of the present invention, and  FIG. 1B  is an enlarged A-A sectional view (a cross section taken across a longitudinal axis of the working portion). 
     The dental file  10  has a working portion  11  configured to cut a root canal, a shaft  12  formed continuously to a back end of the working portion  11 , and a gripping portion (not shown) formed continuously to a back end of the shaft  12  and attached to a dental hand piece. The material of the dental file  10  may be highly-elastic nickel titanium suitable for treatment of the root canal curved in a complicated shape. The working portion  11  described herein is in a tapered shape of which section becomes smaller toward a tip end, and is configured such that similar sections are connected to each other in a spiral shape. The sectional shape of the working portion  11  at any position has a single arc  40  (hereinafter referred to as a “land”). 
     The land  40  is in the arc shape, and therefore, a virtual (i.e., imaginary) circle  50  partially including the land  40  is present. That is, the land  40  is the arc on the circumference of the virtual circle  50 , and the entirety of the working portion  11  other than the land  40  is within the virtual circle  50 . For example, in a case where an outer edge of the sectional shape of the working portion  11  is in a substantially quadrangular shape formed by the land  40  and three lines  41 ,  42 ,  43 , all of these three lines  41 ,  42 ,  43  other than the land  40  are inside the virtual circle  50 . Note that the sectional shape inside the virtual circle  50  is basically the substantially quadrangular shape, but is not specifically limited. The sectional shape may be a shape other than the substantially quadrangular shape. That is, it may only be required that the outer edge of the sectional shape of the working portion  11  includes the single land  40  (the arc) and three or more lines. The shapes of the lines  41 ,  42 ,  43  are basically curved lines close to straight lines, but are not specifically limited. These shapes may be curved lines or straight lines. Note that in a case where the sectional shape of the working portion  11  is too small, strength against bending and rupture is insufficient, and for this reason, the sectional shape needs to be such a sectional shape that a certain level of strength can be ensured. 
     When the length L of the land  40  is too long, the area of contact with the root canal is large, and therefore, a resistance is high and the section is large. This might lead to a problem in flexibility. Conversely, when the land length L is too short, the sectional shape is small, and therefore, the strength is insufficient. For these reasons, the land length L may be a length of 1/12 to ⅓ of the circumferential length of the virtual circle  50 . That is, when the radius of the virtual circle is R, the land length L is 2πR× 1/12≤L≤2πR×⅓. In other words, the center angle θ of the arc of the land  40  is equal to or greater than 30° and equal to or less than 120°. Note that the flexibility is very important, and therefore, the land length L may be preferably equal to or less than 2/7 of the circumferential length of the virtual circle  50 . That is, the center angle θ is approximately equal to or less than 100°. 
     One end of the land  40  forms a first cutting blade  15  configured to cut the inside of the root canal. Moreover, a second cutting blade  16  may be, in addition to the first cutting blade  15 , provided inside the virtual circle  50 . The first cutting blade  15  described herein is outside the second cutting blade  16 . Thus, the first cutting blade  15  greatly contributes to cutting performance of the dental file  10 , and the second cutting blade  16  plays an auxiliary role. In an example of the section illustrated in FIG.  1 B, an intersection between the lines  42 ,  43  forms the second cutting blade  16 . Note that in some cases, no cutting blade is provided inside the virtual circle  50 , and an intersection between the line  41  and the line  42  or the intersection between the line  42  and the line  43  inside the virtual circle  50  is rounded. 
       FIG. 2A  illustrates the sectional shape of the working portions of a typical product, and  FIG. 2B  illustrates the sectional shape of the working portions of the present invention. The working portions in a substantially rectangular shape (the substantially quadrangular shape) as the most basic sectional shape are compared to each other. The sectional shape of the working portion  111  of the typical product is the substantially rectangular shape formed by substantially parallel lines  141 ,  143  and opposing lands  140 ,  142 , and cutting blades  115  are formed at two locations on a virtual circle  150 . On the other hand, the sectional shape of the working portion  11  according to the present invention is formed by the single land  40  and three lines  41 ,  42 ,  43 , the first cutting blade  15  is only at a single location on the virtual circle  50 , and the second cutting blade  16  is provided inside the virtual circle  50 . Needless to say, the lines  41 ,  42 ,  43  may be straight lines. 
     As a result of comparison between the typical product and the present invention, a dental file  110  as the typical product configured such that the cutting blades  115  function at two locations basically exhibits higher cutting performance. However, the typical product has a large section and exhibits high cutting performance, and therefore, it is concerned that when a root canal curved in a complicated shape is cut, cutting is performed in a straight-ahead direction without following a root canal shape. On the other hand, in the dental file  10  of the present invention, the second cutting blade  16  is added to the first cutting blade  15 , and therefore, a certain level of cutting performance is ensured while the section is smaller than that of the typical product. That is, the dental file  10  does not lead to excessive drilling and has excellent flexibility, and therefore, is configured to easily follow even the root canal curved in the complicated shape. 
     If the position of the second cutting blade  16  is too close to the virtual circle  50 , the cutting performance is high. If the position of the second cutting blade  16  is too close to the center O of the virtual circle  50 , almost no cutting performance is exhibited, and for this reason, there is no point in providing the cutting blade. For these reasons, it is assumed that the position of the second cutting blade  16  is preferably a position with a distance of 0.6 times to 0.8 times as great as the radius R of the virtual circle  50  from the center O of the virtual circle  50 . Note that in a case where the first cutting blade  15  exhibits sufficient cutting performance, no second cutting blade is provided in some cases. 
       FIGS. 3A-3C  illustrate views for describing the steps of manufacturing the dental file,  FIG. 3A  being a plan view of a wire rod,  FIG. 3B  being a plan view of a tapered member, and  FIG. 3C  being a plan view of the dental file. The working portion  11  of the dental file  10  is formed by the following manufacturing steps. 
     First, the wire rod  30  as a nickel titanium product is prepared. The section of the wire rod  30  is in a circular shape, and the length thereof is determined based on the length of the dental file  10  as a finished product. The nickel titanium product is used because such a product is a material rich in elasticity and easily follows even the root canal curved in the complicated shape. 
     Next, the wire rod  30  is ground, and in this manner, the tapered member  31  having a tapered portion  32  narrowing toward a tip end at a constant rate is formed. The tapered portion  32  is a portion to be the working portion  11  of the finished product, and therefore, is in a conical shape with a length and a taper rate corresponding to those of the working portion  11 . Note that the sectional shape of the conical shape is the virtual circle  50 . Assuming that the radius of the section of the tapered portion  32  at any position is R, the rate of change in a diameter  2 R in an axial direction is the taper rate, and such a taper rate can be a predetermined value of 4/100 to 6/100 (an inclination on one side is 2/100 to 3/100), for example. 
     Finally, the tapered portion  32  is further ground such that the sectional shapes of  FIG. 1B  are connected to each other in the spiral shape. In this manner, the working portion  11  is formed. In the sectional shape of the working portion  11  as described herein, the land  40  is not ground such that a surface of the tapered portion  32  directly forms the land  40 , and only a portion inside the virtual circle  50  is ground. 
       FIGS. 4A-4C  illustrate views for describing the sectional shape of the dental file,  FIG. 4A  being a view of a core contacting three sides,  FIG. 4B  being a view of the core contacting one side, and  FIG. 4C  being a view of the core contacting two sides. 
     In the sectional view of the working portion  11 , the land  40  is not ground such that the surface of the tapered portion  32  directly forms the land  40 , and only portions corresponding to three lines  41 ,  42 ,  43  are ground and formed. The line  41  and the line  42  are perpendicular to each other, and the line  42  and the line  43  are perpendicular to each other. That is, the line  41  and the line  43  are parallel with each other, and the line  42  is perpendicular to any of the line  41  and the line  43 . Moreover, an intersection between the line  41  and the land  40  forms the cutting blade  15 , and therefore, for forming the cutting blade  15  with a great rake angle (a sharp rake angle), the line  41  is a slightly-curved line in some cases. Moreover, since the working portion  11  is in the spiral shape, it is difficult to grind the line  42  and the line  43  into perfect straight lines, and it is assumed that the line  42  and the line  43  are curved lines close to straight lines. Thus, in some cases, the line  41  and the line  43  are not completely parallel with each other, and an angle between the line  42  and the line  41  and an angle between the line  42  and the line  43  are not 90°. Note that these lines are gently-curved lines, and therefore, it may be assumed that the outline of the sectional shape of the working portion  11  is the substantially rectangular shape. Thus, the lines  41 ,  42 ,  43  will be described below as straight lines. 
     It is assumed that the axial center position of the tapered member  31  is O and the radius of the tapered portion  32  in the section at any position is R. Moreover, it is assumed that in the dental file  10  as the finished product, a circle with the radius R about the axial center position O is the virtual circle  50 . That is, the land  40  is part of the virtual circle  50 , and therefore, points at both ends of the land  40  are on the virtual circle  50 . Moreover, the axial center position O is the center position O of the virtual circle  50 , and needless to say, the center position O is positioned inside the substantially rectangular shape as the sectional shape of the working portion  11 . 
     A distance from the center position O of the virtual circle  50  to each of three lines  41 ,  42 ,  43  is the length of a line extending perpendicularly to each of three lines  41 ,  42 ,  43  from the center position O. Assuming that the length of a shortest one of these perpendicular lines is r 0 , a circular portion with the radius r 0  about the center position O is a portion formed continuously across the entire length of the working portion  11  in the spiral shape, and is the core  51  of the dental file  10 . Note that the radius r 0  decreases toward a tip end of the dental file  10 , and therefore, the core  51  is in a conical shape narrowing toward a tip end. 
       FIG. 4A  is the view in a case where the lengths of the lines perpendicular to the lines  41 ,  42 ,  43  are equal to each other. In other words,  FIG. 4A  is the view in a case where the core  51  contacts the lines  41 ,  42 ,  43  as three sides of the section of the working portion  11 . Thus, the section of the working portion  11  is formed in such a manner that the circular section of the tapered portion  32  is ground from three directions by a depth of R-r 0 . As described above, if the lengths of the perpendicular lines are equal to each other, grinding amounts are also equal to each other, therefore, it is only required that the same grinding program is repeated three times with the rotation by 90° before the subsequent grinding programs. Thus, manufacturing can be simplified. Such manufacturing simplification can contribute to mass production and quality stability. Note that description has been made based on the case where the lengths of the perpendicular lines are equal to each other, but the lengths of the perpendicular lines can be different values. 
       FIG. 4B  is the view in a case where the length r 0  of the line perpendicular to the line  42  is shortest. In other words,  FIG. 4B  is the view in a case where the core  51  contacts only the line  42  as one side of the section of the working portion  11 . 
     Assuming that the lengths of the perpendicular lines other than r 0  are r 1  and r 2 , r 0 &lt;r 1  and r 0 &lt;r 2  are satisfied, and in some cases, r 1 =r 2  is satisfied. Note that although not shown in the figure, the line that the core  51  contacts is the line  41  or the line  43  in some cases, and in this case, the position of the shortest perpendicular line varies. Moreover, the section of the working portion  11  is formed in such a manner that the circular section of the tapered portion  32  is ground from three directions by depths of R −r 0 , R-r 1 , and R-r 2 . 
       FIG. 4C  is the view in a case where the lengths r 0  of the lines perpendicular to the lines  41 ,  43  are shortest. In other words,  FIG. 4C  is the view in a case where the core  51  contacts the lines  41 ,  43  as two sides of the section of the working portion  11 . Note that although not shown in the figure, two sides that the core  51  contacts may be the line  41  and the line  42  or the line  42  and the line  43 . Moreover, the section of the working portion  11  is formed in such a manner that the circular section of the tapered portion  32  is ground from three directions by depths of R-r 0 , R-r 0 , and R-r 3 . 
     Note that the case of the sectional shape of the dental file illustrated as an example in  FIGS. 4( a ), 4( b ), and 4( c )  has the following features. First, in a case where the section is cut along a line connecting the center position O of the virtual circle  50  and the center of the land  40 , cut portions are symmetrical to each other. That is, the lengths of diagonal lines of the sectional shapes are equal to each other. Moreover, the lines  41 ,  43  adjacent to the land  40  are parallel with each other, and the lines  41 ,  43  adjacent to the land  40  and the line  42  facing the land  40  cross perpendicularly to each other. A virtual line connecting end portions of the land  40  are parallel with the line  42  facing the land  40 . 
     The position G of the center of gravity of the section of the working portion  11  shifts from the center position  0  of the virtual circle  50 . Such a state is called off-center. The dental file  10  of the present invention is off-centered, and therefore, is characterized in that the dental file  10  has excellent flexibility and fracture resistance and exhibits favorable cutting-piece discharging performance to smoothly cut the root canal. 
     As a result of a root canal followability test for the typical product  110  and the dental file  10  of the present invention having the sectional shapes illustrated in  FIG. 2B , it has been found that the dental file  10  of the present invention can more equally cut a root canal wall as compared to the dental file  110  as the typical product, and therefore, the dental file  10  of the present invention has better root canal followability. It is assumed that as reasons for such a result, the section of the working portion  11  according to the present invention has higher flexibility due to a smaller second moment of area than that of the typical product and there is a greater clearance between the working portion and the root canal wall as compared to the typical product due to the land at the single location and the off-centered section. Thus, it is assumed that the dental file  10  of the present invention has such a structure that the dental file  10  easily follows even the root canal shape curved in, e.g., an S-shape. Further, the dental file  10  of the present invention has the land with a moderate length, and can moderately cut the root canal by the first cutting blade  15  on the virtual circle without excessively drilling the root canal. In addition, at a curved portion of the root canal, the second cutting blade  16  functions as a cutting blade configured to softly contact the root canal wall to gently drill the root canal wall. Thus, it is assumed that even the wall of the root canal curved in, e.g., the S-shape can be equally cut. 
     For obtaining a dental file with much better performance, performance comparison was made among different spiral pitches of the working portion.  FIGS. 5A and 5B  illustrate views for comparison among the different spiral pitches of the working portion,  FIG. 5A  illustrating a dental file with equal pitches and  FIG. 5B  illustrating a dental file with pitches (hereinafter referred to as a “front-narrower pitch”) narrowing toward a tip end. Note that for clarifying the pitch, visible portions of the land  40  are illustrated as black. The dental file with the equal pitches is configured such that all pitches are the same as each other, but the dental file with the front-narrower pitches is configured such that the pitch narrows toward the tip end. Note that contrary to  FIG. 5B , a product configured such that a pitch expands toward a tip end is also conceivable. However, in this case, strength in the vicinity of the tip end is weak and deformation is easily caused, and for this reason, such a product is excluded from comparison targets. 
     As a comparison test, a screwing test for estimating the force of moving the working portion  11  to a root apex and comparison of a lifting load when the working portion  11  is pulled out of the root canal were conducted. As a result, the force of moving the working portion  11  to the root apex was at the substantially same level even with different pitches, but the lifting load was lower in the case of the front-narrower pitch. It is assumed that this is because the root-side pitch of the working portion  11  is greater in the case of the front-narrower pitch and therefore, the rake angle of the cutting blade  15  decreases and is obtuse with respect to the root canal wall and resistance decreases. 
     Thus, it has been found that for further enhancing the performance of the dental file  10 , it is effective to decrease the spiral pitch toward the tip end. 
     As described above, the dental file of the present invention exhibits sufficient cutting performance and has high flexibility, and therefore, can exhibit performance with excellent root canal followability. Moreover, the dental file also exhibits favorable cutting-piece discharging performance. Further, the spiral pitch is changed so that the dental file with higher performance can be provided. 
     LIST OF REFERENCE SIGNS 
     
         
           10  dental file 
           11  working portion 
           12  shaft 
           15  first cutting blade 
           16  second cutting blade 
           30  wire rod 
           31  tapered member 
           32  tapered portion 
           40  arc (land) 
           50  virtual circle 
         R virtual circle radius 
         L land length