Abstract:
An endodontic instrument including an elongate member having a longitudinal axis, a proximal end, a distal end and a working length between the proximal and distal ends. The working length is formed with a plurality of surface portions cut from the outer surface thereof along paths extending along at least partially around the longitudinal axis. The cut surface portions are then physically twisted to form helical cutting and/or debris removal edges extending around the longitudinal axis. The instrument may be a file or reamer used in root canal procedures and may have three, four or more longitudinally extending surface portions and corresponding edges. The cutting edges define grind angles that vary along the working length. Flexibility of the instrument may also be easily varied according to the invention. Methods of manufacturing the instrument may include grinding flats on the outer surface of a wire blank, with the flats partially twisting around the longitudinal axis. The blank with the partially twisting flats is then physically twisted to form the helical cutting and/or debris removal edges.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to endodontic instruments, such as files and reamers and, more specifically, to those instruments especially useful in root canal procedures. 
     BACKGROUND OF THE INVENTION 
     Endodontists use various types of instruments for cleaning and enlarging the root canals of the teeth. In a typical root canal procedure, an endodontist first makes an opening in the surface of the tooth to provide access to the interior. The endodontist then utilizes small instruments, such as hand held files and reamers, to clean and enlarge the narrow, tapered root canals. In a conventional procedure, the endodontist fills the prepared root canals with gutta percha, which is a rubber-like substance, and then seals the tooth with protective cement. The endodontists may sometimes apply a crown to the tooth as a final step. 
     Typically, the endodontist uses a series of delicate, flexible files to clean out and shape the root canals. Each file includes a proximal and typically including a handle to be gripped between the fingers of the endodontist and a distal end or tip. A working length with helical flutes and cutting edges is located between the proximal and distal ends. The endodontist uses files of increasingly larger diameter to sequentially increase the diameter of the root canal and achieve the desired diameter and shape. 
     Endodontic root canal files and reamers have been formed from twisted blanks in generally three different configurations. One type is formed by twisting a ground blank having a square cross section to create four helical cutting edges per revolution. Another type consists of a twisted blank of triangular cross section having three cutting edges per revolution. The third type, often referred to as a K-flex type, is formed from a blank having a parallelogram-shaped cross section, such as a rhomboid-shaped cross section. After twisting this type of blank, two cutting edges and two debris removal edges will be formed per revolution. All three of these types of instruments have a tapered major diameter or cross-sectional dimension and a tapered minor diameter or cross-sectional dimension in which the taper angles are generally the same. Also, the angles formed between the surfaces that define the cutting and debris removal edges are constant along the length of the instrument. In other words, a given grind angle of an edge on the instrument remains the same along the entire working length of that instrument. 
     Existing endodontic files and reamers formed from twisted blanks are designed in such a manner that the minor diameter is purely a function of the major diameter. The undesirable consequences of this type of design become significant for instruments that have a greater taper along the working length. In particular, these instruments become much stiffer toward the proximal end or handle of the instrument. This can cause the instrument to be difficult to maneuver within curved root canals because the instrument may not flex enough to conform to the shape of the canal. Although certain helically fluted endodontic instruments have been formed completely by grinding to achieve more constant flexibility along the length, these instruments have significant drawbacks. First, instruments formed completely by grinding are more costly to manufacture. Also, twisted instruments may be formed in a wide variety of cross-sectional shapes, depending on the shape of a initially ground wire blank. 
     In view of problems in this field, including those problems noted above, it would be desirable to provide an endodontic instrument, such as a file or reamer formed from a twisted blank, in which the size of one diameter or cross-sectional dimension is formed independent of the other to optimize flexibility, strength and other operating characteristics of the instrument. In this manner, instruments of greater taper may be formed with greater flexibility for maneuvering within curved root canals, while also retaining sufficient strength to resist breakage during use. 
     SUMMARY OF THE INVENTION 
     The present invention provides an endodontic instrument, such as a file or reamer or other cutting, shaping or cleaning instrument, comprising an elongate member with optimal flexibility, strength and other operating characteristics. The elongate member includes a longitudinal axis, which is preferably straight when not in use, and a proximal end, a distal end and a working length generally between the proximal and distal ends. The working length is formed with an outer surface comprising a plurality of twisted or curved surface portions defining at least one cutting edge formed at a junction between adjacent surface portions. The adjacent surface portions are preferably both cut from the outer surface along paths extending along and at least partially around the longitudinal axis. There may be only one twisting surface portion or multiple twisting surface portions on a blank ground according to the invention. These cut surface portions are further physically twisted to form helical cutting edges extending around the longitudinal axis. 
     The adjacent surface portions define a cutting edge or debris removal edge having an angle which, in accordance with one aspect of the invention, varies along the working length. Preferably, the angle defined at each edge decreases in a direction from the distal end to the proximal end. This inventive aspect benefits the instrument in several ways. For example, even at larger tapers, the instrument will be more flexible than conventional instruments of the same taper. Also, one or more cutting edges will be sharper at the proximal end of the working length because of the reduced angle between adjacent surface portions at this location. This allows the proximal end to be a more efficient cutter at that portion of the tooth having the greatest amount of material needing removal. Finally, this inventive aspect also provides an increased area for debris removal at the proximal end. 
     The invention also enables the minor and major diameters of the instrument to be sized independent of one another. This aspect allows the minor diameter or cross-sectional dimension to be maintained substantially constant along the working length, while the major diameter or cross-sectional dimension includes a taper. Thus, a twisted instrument according to this aspect of the invention will have a more constant flexibility along the working length notwithstanding a significant taper existing along the major diameter. Other embodiments of this general aspect are also possible and include forming the minor and major diameters with different tapers. 
     Endodontic instruments of this invention may be formed with many different cross sectional shapes. Typically, the elongate member of the finished instrument will have three or four longitudinally extending surface portions and a corresponding number of longitudinal, helically-shaped edges. The elongate member may be formed from materials having superelastic properties and/or other materials, such as titanium, carbon steel or stainless steel. 
     A preferred method of making endodontic instruments according to the invention includes removing material from an outer surface of a wire blank in at least a first path extending along and twisting at least partially around the longitudinal axis of the wire blank, and then twisting one end of the wire blank with respect to the other and about the longitudinal axis to form at least one helical cutting edge along the working length. Preferably, at least two adjacent surface portions are cut or ground from the outer surface of the wire blank in first and second paths extending along and twisting at least partially around the longitudinal axis of the wire blank. For example, a three-sided endodontic instrument may be formed from a blank ground to have two partially twisting outer surface portions and one straight or axially extending outer surface portion. As another example, a four-sided instrument may be formed with four longitudinally extending, partially twisting ground flats. It will be appreciated that one or more of the ground surface portions may be flats or may have other cross-sectional shapes, such as concave shapes. In the preferred embodiments, the twisting surface portions of the blank will be initially ground such that they twist from about 2° to about 60° about the longitudinal axis along the working length. The working length may be formed in various lengths depending on the intended use of the instrument. 
     Other features, objects and advantages of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a tooth and an endodontic instrument in accordance with the invention shown in use within a root canal; 
     FIG. 2 is an enlarged elevational view of a portion of the endodontic instrument shown in FIG. 1; 
     FIG. 3A is an end view of an initial step in a grinding process used to form the endodontic instrument of FIGS. 1 and 2; 
     FIG. 3B is a view similar to FIG. 3A, but showing the wire blank rotated during the grinding process; 
     FIG. 3C is a view similar to FIGS. 3A and 3B, but showing the grinding process of a second surface portion; 
     FIG. 3D is a view similar to FIGS. 3A-3C, but showing a grinding process for a third surface portion; 
     FIG. 3E is a view similar to FIGS. 3A-3D, but showing the grinding process for a fourth surface portion; 
     FIG. 3F is a side elevational view taken along line  3 F— 3 F of FIG. 3E; 
     FIG. 4 is a fragmented side elevational view showing the minor diameter of a blank ground in accordance with FIGS. 3A-3E; 
     FIG. 5 is a fragmented side elevational view showing the major diameter of the blank shown in FIG. 4; 
     FIG. 6 is an end view of the blank ground in accordance with FIGS. 3A-3E; 
     FIG. 7 is a perspective view of the blank ground in accordance with FIGS. 3A-3E; 
     FIG. 8A is an end view of an initial step in a grinding process used to form a blank for a three-sided endodontic instrument constructed in accordance with the invention; 
     FIG. 8B is a view similar to FIG. 8A, but showing the blank rotated during the grinding process; 
     FIG. 8C is a view similar to FIGS. 8A and 8B, but showing the grinding process of a second, partially twisting surface portion; 
     FIG. 8D is a view similar to FIGS. 8A-8C, but showing a grinding process for a third, straight surface portion; 
     FIG. 9 is an end view of the three-sided blank ground in accordance with the invention; 
     FIG. 10 is a fragmented side elevational view taken along line  10 — 10  of FIG.  9  and showing the minor diameter of the blank; 
     FIG. 11 is a fragmented side elevational view taken along line  11 — 11  of FIG.  10  and showing the major diameter of the blank; 
     FIG. 12 is a perspective view of the three-sided blank ground in accordance with FIGS. 8A-8D; 
     FIG. 13 is a fragmented, elevational view of a twisted instrument formed from the three-sided blank of FIG. 12; 
     FIG. 14 is a fragmented, elevational view of an alternative endodontic instrument twisted from a three-sided blank; 
     FIG. 15 is a cross-sectional view taken along line  15 — 15  of FIG. 14; and 
     FIG. 16 is a cross sectional view taken along lines  16 — 16  of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, an endodontic instrument  10  constructed in accordance with a preferred embodiment of the invention is shown being used during a root canal procedure on a tooth  12 . Tooth  12  includes root canals  14 ,  16  and an upper interior portion  18  which has been initially opened using another instrument, such as a drill (not shown). Instrument  10  includes a handle  20  for manual gripping by, for example, an endodontist and a working length  22  having helical flutes, as will be discussed in more detail below. Although these instruments are typically manipulated manually, the invention may be adapted to power-operated instruments as well. In a conventional manner, instrument  10  may be rotated in the direction of arrows “A” and reciprocated in the direction of arrow “B” by the endodontist to clean out and enlarge root canal  16 . 
     As shown in the enlarged view of working length  22  in FIG. 2, respective flutes are formed by twisted surface portions  24 ,  26 ,  28 ,  30 . These surface portions  24 ,  26 ,  28 ,  30  are defined between respective edges  32 ,  34 ,  36 ,  38 . As further shown in FIG. 2, and explained in more detail below, a minor diameter or cross-sectional dimension “d” and a major diameter or cross-sectional dimension “D” are evident along the working length  22 . Minor diameter “d” preferably remains substantially constant along working length  22 , while major diameter “D” becomes progressively larger in a direction extending from distal end  40  to proximal end  42  of working length  22 . Due to the substantially constant minor diameter “d” extending along the working length  22 , the flexibility of working length  22  is maintained generally constant along working length  22 . As will also be discussed below, minor diameter “d” may also have a taper so as to increase slightly in diameter from distal end  40  toward proximal end  42 . However, the rate of taper is preferably substantially less than the rate of taper of major diameter “D”. For example, the rate of taper for minor diameter “d” may be in the range of 0 to about 0.06, while the rate of taper for major diameter “D” may be in the range of about 0.02 to about 0.14. 
     FIGS. 3A-3F illustrate a preferred method of manufacturing instrument  10 . In this regard, a cylindrical wire  44  has distal end to initially ground to a sharp point. Wire  44  may be formed of any suitable material used for endodontic instruments of this type. As a few examples, such materials include superelastic materials such as NiTi, or other materials such as titanium, carbon steel or stainless steel. A grinding wheel  48  is used to sequentially grind four longitudinally extending, partially twisting surface portions along wire  44 . Specifically, as shown by the end view of wire  44  in FIGS. 3A and 3B, grinding wheel  48  is rotated as wire  44  translates with respect thereto along its center axis  44   a . Simultaneously, wire  44  is rotated through an angle α of, preferably, between about 2° and about 60°. This forms surface portion  30 . 
     In the preferred embodiment, for example, wire  44  may be ground along a working length of about 4 mm to about 23 mm. Wire  44  is translated along grinding wheel  48  at a rate of about 100 in./min depending on the material and the size of wire  44 . Simultaneously, wire  44  is rotated clockwise about its center axis  44   a  preferably at a constant rate through an angle of about 2-60° until a position is reached as shown in FIG. 3B and, in solid lines, in FIG. 3F at the proximal end of the working length. During each of the grinding operations, as wire  44  translates past grinding wheel  48 , grinding wheel  48  is moved away from the center axis  44   a  of wire  44  at a preferred rate of about 0.5 in./min. depending on the wire translation rate mentioned above and the desired taper. This rate may change for the different surface portions. The depth of cut may be about 0.005 inches depending on the instrument size and material and the initial wire diameter is preferably 0.041 inches. 
     To form a four-sided configuration, wire  44  is indexed by 90° and the identical procedure is used to form another surface portion  26  as shown in FIG.  3 C. As shown in FIG. 3C, however, wire  44  is rotated counterclockwise during grinding. As shown in FIG. 3D, a third surface portion  28  is formed after another 90° index and, as shown in FIG. 3E, a fourth surface portion  26  is identically formed after a final 90° indexing operation. In the above-described manner, a ground blank  60  as shown in FIGS. 4,  5  and  6 , is constructed and ready to be physically twisted by any conventional method into a final instrument  10  as shown in FIG.  2 . One suitable twisting method is disclosed in U.S. patent application Ser. No. 09/014,139, assigned to the assignee of the present invention and the disclosure of which is hereby fully incorporated by reference. 
     As further shown in FIGS. 4,  5  and  6 , ground blank  60  will have a minor diameter “d”, as shown in FIG. 4, which may be substantially constant or slightly tapered along working length  22 . A major diameter “D”, as shown in FIG. 5, tapers more significantly as shown by dimensions T 1 , T 2 , T 3 . As further shown in FIG. 7, each surface portion  24 ,  26 ,  28 ,  30  gradually widens from about distal end  40  toward proximal end  42  and partially twists about axis  44   a  of ground blank  60 . The cross section of ground blank  60 , in this embodiment, transforms from a relatively square cross section proximate distal end  40  to a rhomboid cross section at proximal end  42 . As further evidenced in FIG. 6, edges  34 ,  38  will be sharper at proximal end  42  than at distal end  40 . Distal end  40  may be of rhomboid cross section, however, the rhomboidal shape at distal end  40  will not be as exaggerated as at proximal end  42 . Once ground blank  60  have been formed, it may be twisted in any conventional manner, such as in the manner disclosed in U.S. patent application Ser. No. 09/014,139, assigned to the assignee of the present invention and the disclosure of which is hereby fully incorporated by reference. 
     FIGS. 8A-8D illustrate the grinding process used for an illustrative three-sided instrument manufactured according to the invention. Specifically, as shown in FIGS. 8A and 8B, a wire  100  has already been ground proximate a distal end  102  so as to form a sharp point. The wire is otherwise preferably cylindrical. Wire  100  is then positioned adjacent a grinding wheel  104  in a manner similar to the first embodiment. Grinding wheel  104  is used to sequentially grind three longitudinally extending surface portions  106 ,  108 ,  110 . In this embodiment, only surfaces  106  and  108  are ground to be partially twisting surface portions as described above with respect to the first embodiment. Surface portion  110  is a straight, tapered surface portion which does not twist about central axis  100   a  of wire  100 . More specifically, as shown by the end view of wire  100  in FIGS. 8A and 8B, grinding wheel  104  is rotated as wire  100  translates with respect thereto along center axis  100   a . Simultaneously, wire  100  is rotated through an angle θ of, preferably, between about 2° and about 60°. The other parameters, such as working length dimension, translation rate, and rotation rate of wire  100  may be as described above with respect to the first embodiment. The rate at which grinding wheel  104  is moved away from center axis  100   a  and the depth of cut may also be generally the same as described with respect to the first embodiment. 
     To form second surface portion  108 , wire  100  is indexed by 120° and the identical procedures are used to grind surface portion  108  as shown in FIG.  8 C. As shown in FIG. 8C, wire  102  is rotated counterclockwise instead of clockwise during the grinding process. This forms cutting edge  112  defined between surface portions  106  and  108 , with cutting edge  112  preferably having the same general characteristics as cutting edges  34 ,  38  formed in the first embodiment. 
     As shown in FIG. 8D, third surface portion  110  is formed after another 120° indexing step. This surface portion  110 , however, is not a twisted surface portion and therefore wire  100  is not rotated as surface portion  110  is ground by wheel  104 . Instead, wire  100  is translated axially with respect to grinding wheel  104  as grinding wheel  104  is moved away from center axis  100   a  to create a desired taper away from distal end  102 . This final grinding operation therefore creates two debris removing edges  114 ,  116  respectively defined between surface portions  106 ,  110  and  108 ,  110 . 
     The resulting ground blank  120  is best illustrated in FIGS. 9-12. As will be best appreciated from a review of FIGS. 9 and 12, cutting edge  112  will become sharper moving in a direction from distal end  102  toward a more proximal portion  122  (FIG.  12 ). The triangular cross-sectional shape of ground blank  120  will also change from more of an equilateral triangular shape adjacent distal end  102  to a more isosceles triangle toward proximal portion  122 . Blank  120  is preferably twisted using conventional methods into an instrument  130  as, for example, shown in FIG.  13 . Due to the relatively large angle between respective surface portions  106 ,  110  and  108 ,  110 , edges  114 ,  116  will function more to remove debris than to cut tissue during, for example, a root canal procedure as generally illustrated in FIG.  1 . It will be appreciated that the embodiment of this invention shown in FIGS. 8-12 is only one alternative embodiment and that many additional alternatives are possible. These alternatives may, for example, include three-sided ground blanks with each of the three surface portions at least partially twisting about the axis of the blank or other multi-sided ground blanks which are subsequently physically twisted and which include at least one surface portion which has been ground to at least partially twist about the axis of the blank. 
     FIGS. 14-16 illustrate one possible alternative embodiment of a twisted instrument  140  constructed from a three-sided blank which may be formed by the method shown and described in connection with FIGS. 8A-8D. More specifically, instrument  140  includes two surfaces  142 ,  144  which are ground with at least a partial twist and a third surface  146  having a straight, tapered grind. The differences between this embodiment and the previous embodiment essentially involve physical characteristics of the three-sided blank. For example, it will be noted that the taper along the length of instrument  140  is more exaggerated in the embodiment of FIG. 14 as compared to FIG.  13 . Also, the number of twists is less in the embodiment of FIG.  14 . Although more exaggerated, it will again be appreciated that instrument  140  has a cross sectional shape at the distal end (FIG. 16) which is closer to an equilateral triangle than at the proximal end (FIG.  15 ), which is a more elongated triangular shape. Instrument  140  may have a different number of twists than the number shown in FIG.  14 . Like the second embodiment, instrument  140  includes a cutting edge  148  defined between surfaces  142 ,  144 . The remaining two edges  150 ,  152  will essentially function as debris removal edges. 
     While the present invention has been illustrated by a description of the preferred embodiment and while this embodiment has been described in some detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. This has been a description of the present invention, along with the preferred methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims, wherein