Abstract:
An improved rotary endodontic file includes a multi-planar pre-curved apical portion having a first and a second curvature. The pre-curved apical portion begins at about one-third to one-quarter of the working length as measured from the tip end of the endodontic instrument. The first and second curvature permit the file to continually search and engage the walls of a tooth root canal in a forced inside-forced outside manner. As the file works its way down, it breaks free of any ledges encountered, probes lateral- and delta-type openings, and does not allow the file to stray from the curvature of the canal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention generally relates to endodontic files designed to be used in a rotary hand piece and whose working portion is made of a shape-memory material such as nickel-titanium alloy. More particularly, this invention relates to a pre-curved rotary endodontic file. 
         [0002]    Endodontic files intended for use as manual hand files and made out of stainless steel are almost always pre-curved. However, this type of file could not be used in a hand piece because the file would either cut into the root canal wall or break. When rotated in a direction opposite the cutting direction, the file would tend to deform and straighten. 
         [0003]    Endodontic files intended for use in a rotary hand piece and made out of a shape memory material such as nickel-titanium alloy do not make use of a pre-curvature. This type of file is designed to be “true” or straight along its entire active or working length relative to the longitudinal axis of the file. The shape-memory characteristic of the file allows it to combine high strength with high flexibility, allowing the file to traverse the curves of a tooth root canal. However, the file continually wants to straighten itself as it traverses those curves and therefore always urges the working portion of the file against the outside curvature of the root canal wall at the expense of the inside curvature. Torsion forces experienced by the file, as well as friction and heat generated by the file, increase as the amount of surface area of the working length of the file contacts the root canal wall. Additionally, because the curvature of the root canal tends to be irregular, the file can encounter a ledge and bind up or cut into the wall, deviate from the curve, and begin to make its own path. Short radius curvatures tend to more problematic with respect to the above than are longer radius curvatures. 
         [0004]    Shape-memory eliminated the breakage problems experienced by stainless steel files, and stainless steel files, because they were not flexible, had to be pre-curved. (However, the pre-curved stainless steel files tended to irreversibly straightened when becoming compressed as they traversed straight portions of the root canal.) To intentionally form a curve in a shape-memory endodontic file runs counter to the very reason that a shape-memory material is preferred for use in the first place. However, U.S. Pat. No. 5,842,861 discloses a set of files made from shape-memory material and having a pre-curvature or “hook” in the last one-third of the file. The hook at the end “makes it easier to direct the file down into the apical region of the root canal, particularly where the root is rather tortuously curved or twisted.” Each pre-curved file in the set of files can have a different degree of curvature so that a desired shape of the apical region of the root canal can be achieved. 
       SUMMARY OF THE INVENTION 
       [0005]    An improved rotary endodontic instrument or file made according to this invention has a working length made of a shape memory material but includes a pre-curved apical portion having more than one curve. The pre-curved apical portion begins at about one-third to one-quarter of the working length as measured from the tip end of the endodontic instrument. Two or more curvatures are provided in the apical portion, with at least two of the curves lying in a different plane. 
         [0006]    Objects of this invention are to provide an improved rotary endodontic file which continually searches the walls of a tooth root canal and, as it works its way down into the canal, dramatically reduces the torque exerted on the file while at the same time maintaining cutting and scraping efficiency, searches the curvature of the canal in addition to lateral- and delta-type openings in the wall of the canal, breaks free of any ledges encountered, and does not allow the file to stray from the curvature of the canal and create new (undesired) openings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a view of a pre-curved rotary endodontic file made according to this invention and having a first curvature of between 60° to 75° along the apical portion of the file (that is, starting at about the last one-third to one-quarter of the working length). 
           [0008]      FIG. 2  is a view of another preferred embodiment of a pre-curved rotary endodontic file made according to this invention. The file has a first pre-curvature in a range of about 30° to 45° along the apical portion of the file. 
           [0009]      FIG. 3  is a view of the file of  FIG. 2  taken along section line  3 - 3  of  FIG. 2 . The file includes a first curvature in a range of about 30° to 45° (and could be as high as 75° as shown in  FIG. 1 ) and a second pre-curved portion lying in a plane located about 90° relative to a plane containing the first curvature. 
           [0010]      FIG. 4  is view of the file of  FIG. 2  as it rotates within a root canal and works its way downward. The pre-curvature of the file prevents a significant portion of the working length of the file from engaging the root canal wall and allows the tip end of the file to continually search the wall. As the file rotates, the tip end of the file makes a circular motion. 
           [0011]      FIG. 5  is a view of a file of  FIG. 2  after rotating about 90° from its position as shown in  FIG. 4 . 
           [0012]      FIG. 6  is a view of the longitudinal centerline of a file made according to this invention relative to the X plane. 
           [0013]      FIG. 7  is a view of the longitudinal centerline of the file of  FIG. 6  taken along section line  7 - 7  of  FIG. 6  (relative to the Y plane). 
           [0014]      FIG. 8  is a top view of the longitudinal centerline of the file of  FIG. 6  taken along section line  8 - 8  of  FIG. 7 . 
       
    
    
     ELEMENTS AND ELEMENT NUMBERING USED IN THE DRAWINGS 
       [0000]    
       
         Pre-curved endodontic instrument  10   
         Chuck  11   
         Depth calibration grooves  13   
         Active portion or working length  15   
         Flutes  17   
         Cutting edge  19   
         Central longitudinal axis of  10   21   
         Apical portion  23   
         Tip end of  15  and end of  41 , 61   25   
         Engaged portion of  15   27   
         Unengaged portion of  15   29   
         Center line of  15   31   
         First curved portion or curvature  41   
         Start of  41 , 51   43   
         First plane  45   
         Envelope of motion  47   
         Second curved portion or curvature  51   
         Rising or falling sloping portion  52   
         End of  51 , start of  61   53   
         Second plane  55   
         Envelope of motion  57   
         Apex of  51   59   
         Third curved portion or curvature  61   
         Rising or falling sloping portion  62   
         Envelope of motion  67   
         Apex of  61   69   
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    Referring to the figures, a multi-planar pre-curved endodontic instrument  10  made according to this invention includes a chuck  11  configured for insertion into a rotary hand piece, a set of depth calibration grooves  13 , and an active portion or working length  15 . Working length  15  is typically tapered along its length and includes a plurality of spaced-apart flutes  17  that meet to form either a landed or landless cutting edge  19 . The cutting edge  19  provides a desired cutting or scraping effect depending upon the helical angle of the flutes  17 , the rake angle at the cutting point of edge  19 , and the direction in which instrument  10  is rotated. The length of working length  15  may vary, but it is typically about 16 mm in length. The mass of the working length  15  is sufficient so that instrument  10  may be used in continuous forward or reverse rotation without “winding up” on itself as flute surface or edge  19  cuts or scrapes at a root canal wall. 
         [0042]    Endodontic instrument  10  may be made of a nickel-titanium alloy or other type of super-elastic material typically used for endodontic instruments. Unlike prior art endodontic instruments made of super-elastic material, the centerline  31  of working length  15  is not straight in its entirety with respect to the central longitudinal axis  21  (or axis of rotation) of the instrument  10 . Rather, the instrument  10  includes two or more curvatures  41 ,  51 ,  61  located along the apical portion  23  of the working length  15 , with at least two of the curvatures  41 ,  51  or  61  lying in a different plane  45 ,  55  than the other. The apical portion  23  extends from about one-third to one-quarter of the working length  15  to the tip end  25 . For example, the apical portion  23  of a 16 mm working length  15  extends to about 4 to 5 mm above the tip end  25  of the instrument  10 . 
         [0043]    The first curvature  41  is located in a first plane  45  and places tip end  25  a distance Δ 1X  from the central longitudinal axis  21  of instrument  10  (see  FIGS. 9 &amp; 11 ). For purpose of description, first plane  45  can be a horizontal or X-plane. In one preferred embodiment, endodontic instrument  10  is designed for slight to medium root canal curvatures with curvature  41  being formed by a radius “R” which allows the curve  41  to sweep away from axis  21  at an angle α in a range of about 30-45° (see e.g.,  FIG. 2 ). In another preferred embodiment, endodontic instrument  10  is designed for severe root canal curvatures and radius “R” places allows curve  41  to sweep away from axis  21  at an angle α in a range of about 60-75° (see e.g.,  FIG. 1 ). 
         [0044]    Because the central longitudinal axis  21  serves as the central axis of rotation for instrument  10 , the unrestrained envelope of motion  47  provided by curvature  41  at tip  25  is defined by a circle having a diameter 2Δ 1X  (see  FIG. 8 ). When the first curvature  41  is in a restrained condition, that is, when working length  15  is working its way down a root canal, the envelope of motion  47  may be less than it is in the unrestrained state but still greater than one provided by a typical endodontic instrument having a straight centerline along the apical portion. 
         [0045]    Up until the start  43  of curvature  41 , the central longitudinal axis  21  and the centerline of working length  15  are common to one another (see  FIG. 6 ). The location of the starting point  43  along the apical portion  23  is determined by the radius “R” and desired angle α, with a larger radius R placing the starting point  43  of curve  41  closer to one-third of the working length  15  and a smaller radius R placing the starting point  43  closer to tip end  25 . The starting point  43  of the first curvature  41  may also correspond to the starting point of the second curvature  51 . 
         [0046]    The second curvature  51  is located along a length of the first curvature  41  but lies in a second plane  55  which is different than that of first plane  45  (see  FIG. 7 ). Preferably, the first and second planes  45 ,  55  are orthogonal planes and, for the purpose of description, second plane  55  is a vertical or Y-plane. Unlike the hook-shaped first curvature  41 , which is formed by radius R, the second curvature  51  is a wave-like shape formed by rising and falling sloping portions  52 . Each sloping portion  52  may be substantially the same length as the other and arranged at substantially the same angle β relative to the central longitudinal axis  21  (see  FIG. 7 ). 
         [0047]    The apex  59  of the second curvature  51  places the corresponding point of centerline  31  at a distance Δ 2Y  from the central longitudinal axis  21 . Because the central longitudinal axis  21  serves as the central axis of rotation for instrument  10 , the unrestrained envelope of motion  57  of the instrument  10  provided by second curvature  51  has a diameter equal to that of the diameter of working length  15  at apex  59  plus Δ 2Y or 2X  (see  FIGS. 7 &amp; 8 ). When the second curvature  51  is in a restrained condition, that is, when working length  15  is working its way down a root canal, the envelope of motion  57  may be less than it is in the unrestrained state but still greater than one provided by a typical endodontic instrument having a straight centerline along the apical portion. 
         [0048]    The third curvature  61  preferably starts at the point  53  where second curvature  51  ends and lies on an opposite side of central longitudinal axis  21  (see  FIG. 7 ). The third curvature  61 , which has a wave-like shape similar to that of the second curvature  51 , preferably lies in the same plane  55  as the second curvature  51 . Similar to second curvature  51 , third curvature  61  is a wave-like shape formed by rising and falling sloping portions  62 . Each sloping portion  62  may be substantially the same length and arranged at substantially the same angle γ relative to the central longitudinal axis  21  (see  FIG. 7 ). 
         [0049]    The apex  69  of the third curvature  61  places the corresponding point of centerline  31  at a distance Δ 3Y  from the central longitudinal axis  21  (see  FIG. 8 ). Because the central longitudinal axis  21  serves as the central axis of rotation for instrument  10 , the unrestrained envelope of motion  67  of the instrument  10  provided by third curvature  61  produces an annulus having an outer diameter equal to that of envelope  47  and an inner diameter equal to that of envelope  57 . 
         [0050]    Generally speaking, any cross-sectional area of the working length  15  which resides within an interior space of a root canal presents a potential contact area for engaging a respective opposing wall of the root canal. The total amount of contact area for the working length  15  is equal to the number of cutting edges  19  in communication with the wall of the root canal multiplied by the contact area of each cutting edge  19 . Therefore, the cutting edges  19 , which could be landed edges, create drag that reduces the flexibility of file  10  and requires increased torque to overcome. As the amount of required torque increases, so does the probability of file breakage in the root canal. 
         [0051]    Because of its design, endodontic instrument  10  reduces the total amount of contact area between the cutting edges  19  and the root canal as the working length  15  rotates within the root canal and works its way down the root canal. For example, first curvature  41  allows tip end  25  to make a circular motion and search the walls of the root canal (see  FIGS. 4 &amp; 5 ) and enter lateral- and delta-type openings located along the main wall. When the tip end  25  encounters a ledge or irregularity in the wall, the end  25  breaks free of it rather than becoming entrapped or causing the working length  15  to deviate from the curvature of the canal. As tip end  25  continues its search of the wall, the curvatures  41 ,  51 ,  61  allow the working length  15  to search the curvature of the canal and place pressure on the working length  15 , forcing a portion  27  of the working length  15  toward an inside of the root canal and another portion  29  toward an outside of the canal (see  FIGS. 7 &amp; 12 ). In other words, whatever portion of the root canal wall that tip end  25  encounters, portion  29  is forced to the opposite wall. 
         [0052]    This forced-inside, forced-outside action provides for cleaning and enlarging of the root canal as working length  15  works its way down the root canal. A straight file, even if formed from a superelastic material, always follows the outside of the curve. Additionally, whenever tip end  25  encounters a short radius curve, the tip end  25  and working length  15  easily enter the curve. When a straight file encounters the same curve, it has a tendency to transport the curve. Last, despite having two or more curvatures  41 ,  51 ,  61 , the apical portion  23  of the working length  15  can straighten when being compressed by tight, straight passageways of the root canal and then reform to its curved shape after emerging from those passageways. 
         [0053]    While preferred embodiments of an endodontic file made according to this invention have been disclosed with a certain degree detail, the scope of the invention is limited by the following claims.