Abstract:
An endodontic instrument for drilling root canals, in particular a flexible drilling instrument which comprises a working part and a securing end fitting intended for mounting the instrument in a mandrel of a contra-angle-type support provided with an electric motor used to drive an instrument clockwise. The casing of the working part is conical along its length and ends in a tip. The working part is provided with a spiral flute forming a ridge having a cutting edge. The drill bit, relief and cutting angles defined by the cutting edge can be varied along the length of the working part so that the instrument is particularly sharp in the area near the tip and so that the profile of the cutting ridge becomes progressively more blunt along the length of the instrument.

Description:
[0001]    This application is a National Stage completion of PCT/CH2012/000057 filed Mar. 13, 2012, which claims priority to Swiss patent application serial no. 465/11 filed Mar. 18, 2011. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention concerns an endodontic instrument for drilling root canals, particularly a flexible drilling instrument, the instrument having a longitudinal rotational axis and comprising a portion called the working portion, terminating in a tip at one extremity and at the other extremity in a securing end fitting that can be attached to a mandrel driven by an electric motor, the working portion comprising at least one spiral flute defining at least one ridge comprising an edge with a peak and two sides, exterior and interior, respectively, located on either side of the edge, the edge peak being situated on a circle whose center is located on the instrument&#39;s longitudinal axis of rotation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Cleaning and shaping root canals for receiving filling substance is accomplished using drilling instruments which have a conical active portion called the working portion that comprises several cutting edges arranged in a spiral along the working portion. 
         [0004]    An example of this type of dental instrument is illustrated by International Publication No. WO 2007/016278 A1 which describes a generally conical endodontic instrument comprising several spiral flutes with variable spaces between the spirals according to the sectors of the active portion. These instruments, called endodontic files, are designed for manual or mechanized use. When the endodontic files are driven by an electric motor they rotate at a slow speed (for example between 150 and 600 rotations per minute) and progress through the canal opening in such a way that their active area can cut or scrape the walls of the root canal. Areas that were reduced in length at the beginning of the intervention extend farther and farther as the files advance through the canals. The drive torque that must be applied to the files in order to overcome the increasing force of friction and to make the files turn at the desired speed as they advance increases more and more. Progression through the canal further increases the risk of the file becoming blocked or tighten. When this occurs the torque applied to the file increases strongly and there is a risk of the file breaking. File breakage inside the root canal is an outcome the dentist dreads, since the broken point is generally irretrievable. 
         [0005]    The endodontic instrument which is the object of Publication No. EP 1 752 109 comprises several, specifically three, spiral flutes defining the concave surfaces whose extremities are located on a circumscribed circle. These extremities constitute the peaks of the instrument&#39;s working angles and at least one side of these angles is constituted by the concave flute surfaces. It happens that these angles are all negative or neutral so as to form scraping angles, but never cutting angles, since only positive angles are capable of performing cutting operations. In order to be positive, the tangents to the sides of the angles must be situated on the same side of the radius of the circumscribed circle that corresponds to the peak of the angle. Therefore, this instrument cannot be used as an instrument that cuts sufficiently to form a root canal. 
         [0006]    Furthermore, the instruments currently available on the market for implementing the new technique of using mechanically driven files are derived directly from traditional spiral shaped instruments for manual use. This spiral formation is often the source of the tightening and blocking phenomenon that can cause instrument breakage. 
         [0007]    Another problem that may occur is that the instrument, instead of following the original shape of the root canal within the sharply curved areas, may have a tendency to carve out its own path or deform the original canal walls when the flute edges are too sharp. One solution consists of blunting the cutting edges of the instrument to a greater or lesser extent in order to prevent them from cutting the material and boring a path that does not correspond to the natural shape of the canal. This can be accomplished by forming an enlarged flat radial area on an endodontic instrument in which there is a large separation between the flute edges. This geometry allows the edges to be blunted, but it has the drawback of increasing working friction and heating the instrument, running the risk of breakage. In addition, enlarging the flat radial area makes the instrument more rigid, resulting in diminished flexibility and an increased risk of uncontrolled deviation within the curved areas of the root canals. 
         [0008]    Finally, the walls of a root canal require different treatments along the length of the canal so that the dental surgeon must adapt the instruments according to the desired result. Current instruments do not allow variable treatment sequences to take place throughout the length of the root canal, but require the use of a series of instruments that vary in shape from one instrument to the next. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention proposes to improve the disadvantages described above and to furnish a means for ensuring effective root canal preparation by placing at the practitioner&#39;s disposal an instrument that is sharp enough in the desired areas to form the appropriate root canal, yet blunt enough in the desired areas to avoid cutting the material and comply with the original shape, and flexible enough to follow the sharply curved areas in the canal. 
         [0010]    This goal is attained by the instrument of the invention as described below and characterized in that at different levels along the length of the working portion, the cutting angle at the peak of the at least one flute, defined as being the angle formed by the tangents to the peak on the sides of the at least one ridge with the radius of the circle formed by the peak, varies in width in at least one predetermined area on the working portion and is either negative when the tangent to the exterior side of the ridge is located on the side opposite the tangent to the interior side opposite the ridge relative to the radius of the circle, or nil when the tangent to the exterior side of the ridge coincides with the radius of the circle, or positive when the tangent to the exterior side of the ridge is located on the same side as the tangent to the interior side opposite the ridge relative to the radius of the circle, to define along the ridge of the at least one flute either a scraping area or a cutting area, that are either more or less active. 
         [0011]    According to a particularly advantageous embodiment in which the instrument comprises three flutes defining three ridges each comprising an edge having a peak and two sides, interior and exterior, respectively, located on either side of the edge at different levels along the length of the working portion, the cutting angles on the respective peaks defined by the tangents to the peaks of the interior sides of the ridges with the respective radii of the circle corresponding to the peaks, can vary in size independently of one another on at least one predetermined zone of the working portion; and are either negative when the tangents to the interior sides of the ridges are located on the side opposite the tangents to the exterior sides opposite the ridges relative to the radii of the circle, or nil when the tangents to the interior sides of the ridges coincide with the radii of the circle, or positive when the tangents to the interior sides of the ridges are located on the same side as the tangents to the exterior sides opposite the ridges relative to the radii of the circle, to define along the edges of the same ridge either a scraping area or a cutting area, that are either more or less active. 
         [0012]    According to a preferred form of embodiment, the endodontic instrument comprises at least one zone in which the cutting angle at the respective peak increases progressively from the tip towards the instrument support fitting. 
         [0013]    According to another preferred embodiment, the endodontic instrument comprises at least one zone wherein the cutting angle at the respective peak decreases progressively from the tip towards the instrument support fitting. 
         [0014]    The endodontic instrument may advantageously comprise several zones distributed along the working portion in which the relief angle at a peak, which is the angle formed by the tangent to the peak on the side of the corresponding ridge with the tangent to this same peak on the circle, the drill bit angle formed by the tangent with the tangent to the side of the same ridge, and the cutting angle are different from one zone to another. 
         [0015]    The endodontic instrument may comprise on its working portion at least one zone called the cutting zone. 
         [0016]    The endodontic instrument may comprise on its working portion at least one zone called the scraping zone. 
         [0017]    The endodontic instrument may comprise on its working portion at least one zone called the neutral zone. 
         [0018]    According to other embodiments, the instrument may comprise two or four flutes which define two or four ridges, each comprising an edge supporting a peak, and two sides respectively located on either side of the edge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention and its principal advantages will be more readily apparent from the description of different embodiments, with reference to the attached drawings, in which: 
           [0020]      FIG. 1  is a schematic cross-section illustrating a transverse section of a first specific embodiment of the instrument according to the invention; 
           [0021]      FIG. 2  is a schematic cross-section illustrating a transverse section of a second specific embodiment of the instrument according to the invention. 
           [0022]      FIG. 3  is a schematic cross-section illustrating a transverse section of a third specific embodiment of the instrument according to the invention. 
           [0023]      FIG. 4  is a schematic cross-section illustrating a transverse section of a fourth specific embodiment of the instrument according to the invention. 
           [0024]      FIG. 5  is an overall view of one embodiment of the instrument according to the invention. 
           [0025]      FIGS. 5A ,  5 B,  5 C,  5 D and  5 E represent transverse cross-sections of the instrument of  FIG. 5  at different levels on the working portion. 
           [0026]    FIG.  5 AA is an enlarged view of  FIG. 5A . 
           [0027]      FIG. 6  represents a view of another embodiment of the instrument according to the invention. 
           [0028]      FIGS. 6A ,  6 B,  6 C,  6 D and  6 E represent transverse cross-sections of the instrument of  FIG. 6  at different levels on the working portion. 
           [0029]    FIG.  6 AA is an enlarged view of  FIG. 6A . 
           [0030]      FIG. 7  represents a view of another embodiment of the instrument according to the invention. 
           [0031]      FIGS. 7A ,  7 B,  7 C,  7 D,  7 E and  7 F represent transverse cross-sections of the instrument of  FIG. 7  at different levels on the working portion. 
           [0032]    FIG.  7 AA is an enlarged view of  FIG. 7A . 
           [0033]      FIG. 8  represents a view of a fourth embodiment of the instrument according to the invention. 
           [0034]      FIGS. 8A ,  8 B,  8 C,  8 D,  8 E and  8 F represent transverse cross-sections of the instrument of  FIG. 8  at different levels on the working portion. 
           [0035]    FIG.  8 AA is an enlarged view of  FIG. 8A . 
           [0036]      FIG. 9  represents a view of a fifth embodiment of the instrument of the invention. 
           [0037]      FIGS. 9A ,  9 B,  9 C,  9 D,  9 E,  9 F and  9 G represent transverse cross-sections of the instrument of  FIG. 9  at different levels on the working portion. 
           [0038]    FIG.  9 AA is an enlarged view of  FIG. 9A . 
           [0039]      FIG. 10  represents a view of a sixth embodiment of the instrument of the invention. 
           [0040]      FIGS. 10A ,  10 B,  10 C,  10 D,  10 E,  10 F and  10 G represent transverse cross-sections of the instrument of  FIG. 10  at different levels on the working portion. 
           [0041]    FIG.  10 AA is an enlarged view of  FIG. 10A . 
           [0042]      FIG. 11  represents a view of a seventh embodiment of the instrument of the invention. 
           [0043]      FIGS. 11A ,  11 B,  11 C,  11 D,  11 E,  11 F and  11 G represent transverse cross-sections of the instrument of  FIG. 11  at different levels on the working portion. 
           [0044]    FIG.  11 AA is an enlarged view of  FIG. 11A . 
           [0045]      FIG. 12  represents a view of an eighth embodiment of the instrument of the invention. 
           [0046]      FIGS. 12A ,  12 B,  12 C,  12 D,  12 E,  12 F and  12 G represent transverse cross-sections of the instrument of  FIG. 12  at different levels on the working portion. 
           [0047]    FIG.  12 AA is an enlarged view of  FIG. 12A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]      FIG. 1  represents a cross-section view along a transverse plane perpendicular to the longitudinal rotational axis of an instrument or endodontic file  10  like the one shown in  FIGS. 9 and 10 . In this case instrument  10  comprises three spiral flutes  14 ,  15 ,  16  defining three ridges  20 , the edges  24 ,  25 ,  26  of which support peaks A, B and C and three sides  10   a,    10   b  and  10   c,  interior and exterior, alternately, located on either side of the edges. The transverse cross-section defines a generally triangular portion of peaks A, B and C which are located on a circle E whose center O is located on the longitudinal rotational axis of the instrument. Edges  24 ,  25 ,  26  constitute the geometric locations of peaks A, B and C, respectively. The rotational direction of instrument  10  is represented as an arc by arrow R. 
         [0049]    Angle α A , called the relief angle at peak A, is defined as being the angle formed by tangent AX in A to exterior side  10   b  of ridge  20 , the edge  24  of which supports this peak A, with tangent AS at A, to circumscribed circle E. Similarly, relief angle α B  on peak B can be defined as being the angle formed by tangent BX at B to exterior side  10   c  of ridge  20 , the edge  25  of which supports this summit B, with tangent BS at B to circumscribed circle E; and relief angle α C  on peak C as being the angle formed by tangent CX at C to exterior side  10   a  of ridge  20 , edge  26  of which supports the peak C, with tangent CS at peak C to circumscribed circle E. 
         [0050]    Angle β A  at peak A or angle XAY are called the drill bit angle, the angle formed by tangent AX at A to exterior side  10   b  of ridge  20 , edge  24  of which supports peak A, with tangent AY at A to interior side  10   a  on the same ridge, edge  24  of which supports peak A. Similarly, the angle formed by tangent BX at B to exterior side  10   c  of ridge  20 , edge  25  of which supports peak B, with tangent BY at B on interior side  10   b  of the same ridge, edge  25  of which supports peak B will be called angle β B  or angle XBY; and the angle formed by tangent CX at C on exterior side  10   a  of ridge  20 , edge  26  of which supports peak C, with tangent CY at C to interior side  10   c  of the same ridge  20 , interior edge  26  of which supports peak C, is called angle β c  or angle XCY. 
         [0051]    The angle formed by tangent AY at peak A on interior side  10   a  of ridge  20 , edge  24  of which supports peak A, with radius AO of circumscribed circle E corresponding to peak A is called cutting angle            A  at A. Similarly, cutting angle            B  at B is the angle formed by tangent BY at peak B on interior side  10   b  of ridge  20 , edge  25  of which supports peak B with radius BO corresponding to peak B, and cutting angle            C  at C is the angle formed by tangent CY at peak C on interior side  10   c  of ridge  20 , edge  26  of which supports peak C, with radius CO corresponding to peak C. 
         [0052]    A cutting angle           is called positive when the tangent to the interior attack side of the ridge  20  concerned is located on the same side as the tangent to the exterior side opposite the ridge relative to the corresponding radius. In the case of angles            A ,            B ,            C , or YAO, YBO, YCO, respective tangents AY, BY and CY are effectively situated on the same side as tangents AX, BX and CX relative to respective radii AO, BO and CO. The cutting angles called positive and the instrument has an essentially cutting characteristic in the area concerned. 
         [0053]      FIG. 2  is a view similar to that of  FIG. 1 , but simplified for purposes of clarity. Instrument  10  comprises, as before, three spiral flutes  14 ,  15 ,  16 . As before, the transverse cross-section defines a generally triangular section of peaks A, B, C which are inscribed in a circumscribed circle E. Flutes  14 ,  15 ,  16  define ridges  20 , the edges of which are the geometric locations of peaks A, B and C, respectively. 
         [0054]    Angle α A , called the relief angle at peak A, is defined as being the angle formed by tangent AX at A to exterior side  10   b  of ridge  20 , the edge of which supports the summit A, with tangent AS at A, to circumscribed circle E. Similarly, relief angle α B  at peak B may be defined as being the angle formed by tangent BX at B to exterior side  10   c  of ridge  20 , the edge of which supports peak B, with tangent BS at B to circumscribed circle E; and angle relief angle α C  at peak C as being the angle formed by tangent CX at C to exterior side  10   a  of ridge  20 , the edge of which supports peak C, with tangent CS at peak C to circumscribed circle E. 
         [0055]    Angle β A  or angle XAY, the angle formed by tangent AX at A to exterior side  10   b  of ridge  20 , the edge of which supports peak A with tangent AY at A to interior side  10   a  of the same ridge  20 , the edge of which supports peak A, is called the drill bit angle at peak A. Similarly, the angle formed by tangent BX at B to exterior side  10   c  of ridge  20 , the edge of which supports peak B, with tangent B, with tangent BY at B to interior side  10   b  of the same ridge, the edge of which supports peak B will be called angle β B  or angle XBY; and the angle formed by tangent CX at C to exterior side  10   a  of ridge  20 , the edge of which supports peak C, with tangent CY at C to interior side  10   c  of the same ridge  20 , the edge of which supports peak C, will be called angle β C  or angle XCY 
         [0056]    The angle formed by tangent AY at the peak of interior side  10   a  of ridge  20 , the edge of which supports peak A, with radius AO corresponding to peak A is called cutting angle            A  at A. Similarly, the angle formed by tangent BY at peak B to interior side  10   b  of ridge  20 , the edge of which supports peak B, with radius BO corresponding to peak B is called cutting angle            B  at B; the angle formed by tangent CY at peak C to interior side  10   c  of ridge  20 , the edge of which supports peak C with radius CO corresponding to peak C is called cutting angle            C  at C. 
         [0057]    In this case cutting angle            A  which defines the orientation of the ridge corresponding to peak A is nil. The edge of the ridge is called neutral. Tangent AY coincides with radius AO. 
         [0058]    The area of the instrument represented in  FIG. 1  is a tool for cutting the material and the area of the instrument represented in  FIG. 2  is a tool for scraping the material.  FIG. 3  represents a view similar to that of  FIG. 2 . Instrument  10  comprises, as before, three spiral flutes  14 ,  15 ,  16 . As before, the transverse cross-section defines a generally triangular section of peaks A, B and C which are inscribed in a circumscribed circle E. Flutes  14 ,  15   16  define ridges  20 , the edges of which are the geometric locations of peaks A, B and C, respectively. 
         [0059]    The drill bit angle, the relief angle and the cutting angle have the same definitions as previously. In this case cutting angle            A  which defines the orientation of the edge corresponding to peak A is negative. The interior tangent to the surface called the attack surface of the ridge  20  concerned is located on the side opposite the tangent to the surface opposite the ridge  20  concerned relative to the corresponding radius AO. 
         [0060]    The area of the instrument represented in  FIG. 3  is a tool for scraping the material and not a cutting tool. 
         [0061]      FIG. 4  represents a view similar to those of  FIGS. 2 and 3 . Instrument  10  comprises as before three spiral flutes  14 ,  15  and  16 . As before, the transverse cross-section defines a generally triangular section of peaks A, B and C which are inscribed in a circumscribed circle E. The flutes define ridges, the edges of which are the geometric locations of peaks A, B and C, respectively. Angle α A , called the relief angle at peak A, is defined as being the angle formed by tangent AX to peak A on the exterior surface of side  10   a  with the perpendicular to radius AO of circumscribed circle E, or the tangent AS at peak A relative to circumscribed circle E. 
         [0062]    The drill bit angle, the relief angle and the cutting angle have the same definitions as before. In this case, cutting angle            A  which defines the orientation of the edge corresponding to peak A is positive. Tangent AX to the surface called the attack surface on the ridge  20  concerned is situated on the same side as tangent AY to opposing surface  10   bde  of the ridge  20  concerned relative to corresponding radius AO. In light of the orientation of the attack surface relative to the direction of rotation R, the area of the instrument shown in transverse cross-section is an instrument for scraping. 
         [0063]    With reference to  FIG. 5 , endodontic instrument or file  10  comprises a portion called the working portion  11  and a securing end fitting  12  for mounting the instrument in a supporting counter-angle type mandrel equipped with an electric motor driving it clockwise. In this example, the entire length of the casing of working portion  11  is conical and terminates in a tip  13 , the securing end fitting  12  being cylindrical to ensure that it remains inside the mandrel (not shown). In this embodiment, working portion  11  is equipped with a single spiral flute  14  forming a ridge  20  with an edge, the cutting edge  24 . 
         [0064]    In this embodiment cross-sectional views  5 A through  5 E taken along the cross-sections respectively designated by  5 A- 5 A;  5 B- 5 B;  5 C- 5 C;  5 D- 5 D and  5 E- 5 E, as well as the enlarged view in FIG.  5 AA, show the disposition of edge  24 . Apart from the fact that instrument  10  comprises only a single flute  14 , this embodiment corresponds to the instrument described with reference to  FIG. 1 . The instrument is a cutting tool with a positive cutting angle           in the portion near tip  13 , but which may vary along the length of working portion  11 . 
         [0065]    It should be noted that drill bit angle β, relief angle α and cutting angle           may vary along the entire length of working portion  11 . In the present case, the instrument is particularly sharp in the area near tip  13  and the profile of cutting edge  24  becomes progressively more blunt along the length of instrument  10 . 
         [0066]    In the following description, identical elements bear the same reference numerals. Instrument  10  shown in  FIG. 6  comprises, as before, a working portion  11  with a generally parallelepiped section, and a securing end fitting  12  for mounting the instrument in a mandrel driven by an electric motor. In this embodiment, as in the case shown in  FIG. 5 , working portion  11  is equipped with a single spiral flute  14  forming an edge, the cutting edge  24 . 
         [0067]    In this embodiment the transverse cross-sectional views  6 A through  6 E taken along the cross-sections respectively designated by  6 A- 6 A;  6 B- 6 B;  6 C- 6 C;  6 D- 6 D and  6 E- 6 E, as well as the enlarged view in FIG.  6 AA, show the disposition of edge  24 . Apart from the fact that instrument  10  comprises only a single flute  14 , this embodiment corresponds to the instrument described with reference to  FIG. 1 . The instrument is a cutting instrument with a negative cutting angle           in the area near tip  13 , but the angle may vary along the length of working portion  11 . 
         [0068]    As in the preceding example, note that drill bit angle β, relief angle α and cutting angle           may vary along the entire length of working portion  11 . In the present case, the instrument is a scraper in the area near tip  13  and the profile of cutting edge  24  becomes progressively finer along the length of instrument  10  until it becomes a cutting edge in the vicinity of securing end fitting  12  where the instrument functions as a cutter. 
         [0069]    Instrument  10  shown in  FIG. 7  is more or less rectangular in section and comprises two spiral flutes  14  and  15  generally symmetrically disposed along the entire length of working portion  11 . It will be noted from observing the transverse lines all along working portion  11 , that instrument  10  is essentially a scraping tool in the area of tip  13  and is progressively transformed into a cutting tool approaching the securing end fitting  12 . Cutting edges  24  and  25  of flutes  14  and  15  are relatively blunt near tip  13  and become progressively sharper near securing end fitting  12 . 
         [0070]    Instrument  10  shown in  FIG. 8  is similar to the one shown in  FIG. 7  and is more or less rectangular in section, comprising two spiral flutes  14  and  15  generally symmetrically disposed along the entire length of working portion  11 . It will be noted from observing the transverse lines all along working portion  11  that the present instrument  10 , in contrast to the instrument of  FIG. 7 , is essentially a cutting tool near tip  13  and is progressively transformed into a scraping tool approaching the securing end fitting  12 . Cutting edges  24  and  25  of flutes  14  and  15  are relatively sharp near tip  13  and become progressively more blunt towards securing end fitting  12 . 
         [0071]      FIGS. 9 and 10  represent two embodiments of instrument  10  in which the portion called the working portion  11  comprises three spiral flutes  14 ,  15 ,  16  defining three edges  24 ,  25  and  26 . 
         [0072]    On instrument  10  shown in  FIG. 9 , edges  24 ,  25  and  26  are first, cutting edges and then become scraping edges; and on instrument  10  shown in  FIG. 10 , the edges are first, scraping edges and then become cutting edges as they are displaced from tip  13  towards securing end fitting  12 . The drill bit, relief and cutting angles may vary independently of one another and all along working portion  11 . The drill bit angle, which is the angle formed by the tangents to the two surfaces of a single edge, defines to some extent the fineness of the cutting operation and the cutting capacity of the instrument, but also the flexibility of the area of the instrument in question. In actuality, the finer the instrument, that is, the smaller its transverse section surface, the greater its flexibility. The possibility of modulating the drill bit, relief and cutting angles allows the instrument to be adapted to the practitioner&#39;s needs through areas distributed along the length of its working area. 
         [0073]      FIGS. 11 and 12  are views illustrating an instrument comprising four flutes  14 ,  15 ,  16  and  17  which define four ridges having respectively the four edges  24 ,  25 ,  26  and  27  of the transverse cross-sections represented by cross-sections  11 A through  11 G and  11 AA, as well as  12 A through  12 G and  12 AA. In the portion near tip  13  the instrument  10  of  FIG. 11  is a scraping tool that progressively becomes a cutting tool towards securing end fitting  12 . On instrument  10  in  FIG. 12 , the instrument is first a cutting tool near tip  13  and is progressively transformed into a scraping tool towards securing end fitting  12 . 
         [0074]    These variations offer the practitioner the opportunity of adapting the different instruments to the treatment sequences being performed, and during a sequence, to the geometry of the patient&#39;s root canal. 
         [0075]    The present invention is not limited to the preferred embodiments described, but may undergo different modifications or variations obvious to a person skilled in the art. In particular, on the same instrument, the geometry of the edges may be modulated by passing from a scraping and/or smoothing function to a cutting function, then returning to a scraping function, or conversely, along the entire length of the working portion. 
         [0076]    Furthermore, the instrument may comprise a larger number of flutes, specifically spiral flutes. This number is limited only by the transverse dimensions of the instrument.