Patent Publication Number: US-10772708-B2

Title: Aesthetic orthodontic bracket and method of making same

Description:
CROSS REFERENCE TO RELATED CASES 
     This application is a continuation of application Ser. No. 14/039,348, filed Sep. 27, 2013, which is a continuation of application Ser. No. 12/540,627, filed Aug. 13, 2009, now U.S. Pat. No. 8,585,398, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/088,519 filed Aug. 13, 2008, and the benefit of U.S. Provisional Patent Application Ser. No. 61/106,358 filed Oct. 17, 2008; the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to orthodontic brackets and, more particularly, to aesthetic orthodontic brackets having movable closure members such as slides or latches. 
     BACKGROUND 
     Orthodontic brackets represent a principal component of corrective orthodontic treatments devoted to improving a patient&#39;s occlusion. In conventional orthodontic treatments, an orthodontist or an assistant affixes brackets to the patient&#39;s teeth and engages an archwire into a slot of each bracket. The archwire applies corrective forces that coerce the teeth to move into orthodontically correct positions. Traditional ligatures, such as small elastomeric O-rings or fine metal wires, are employed to retain the archwire within each bracket slot. Due to difficulties encountered in applying an individual ligature to each bracket, self-ligating orthodontic brackets have been developed that eliminate the need for ligatures by relying on a movable latch or slide for captivating the archwire within the bracket slot. 
     Conventional orthodontic brackets are ordinarily formed from stainless steel, which is strong, nonabsorbent, weldable, and relatively easy to form and machine. Patients undergoing orthodontic treatment using metal orthodontic brackets, however, may be embarrassed by the visibility of metal, which is not cosmetically pleasing. To improve the cosmetic appearance, certain conventional orthodontic brackets incorporate a bracket body of a transparent or translucent non-metallic material, such as a polymer resin or a ceramic, that assumes or mimics the color or shade of the underlying tooth. Such orthodontic brackets may rely on a metallic insert lining the archwire slot for strengthening and reinforcing the bracket body in the vicinity of the archwire slot. As a result, the appearance of metal in the patient&#39;s mouth, while still present to some degree, is less noticeable in ordinary view and, therefore, brackets characterized by a non-metallic bracket body are more aesthetically pleasing. Forming bracket bodies from transparent/translucent material, for example ceramic materials, has become desirable due to the improved aesthetics. However, ceramic materials are brittle and subject to a greater likelihood of fracture in use. Consequently, there is a need for ceramic brackets that are resistant to pressures needed to move teeth to their orthodontically correct positions. 
     While forming traditional, non self-ligating bracket bodies from transparent or translucent materials has generally improved the aesthetics of these brackets, improved aesthetics for self-ligating brackets has heretofore remained problematic. By way of example, current aesthetic self-ligating orthodontic brackets may use a transparent or translucent bracket body, but continue to utilize a closure member (e.g., ligating slide) made out of metal. One such example of this arrangement is disclosed in U.S. Patent Publication No. 2004/0072117, the disclosure of which is incorporated by reference herein in its entirety. These metal closure members may visibly detract from the aesthetic appearance desired by most patients, especially for the brackets attached to incisors and canines located in the anterior of the oral cavity. These self-ligating brackets have maintained the use of metal closure members generally because of the strength, ductility, and toughness required of such members. Thus, the aesthetics of self-ligating brackets has yet to be fully realized. 
     Consequently, there is a need for an improved, more fully aesthetic self-ligating orthodontic bracket that overcomes this and other deficiencies of conventional self-ligating orthodontic brackets. 
     SUMMARY OF INVENTION 
     To these ends, an orthodontic bracket for coupling an archwire with a tooth comprises a bracket body configured to be mounted to the tooth. The bracket body includes an archwire slot adapted to receive the archwire therein and a movable member engaged with the bracket body. The movable member is movable relative to the body between an opened position in which the archwire is insertable into the archwire slot and a closed position in which the movable member retains the archwire in the archwire slot. The bracket body and the movable member are made from a transparent or translucent ceramic material. 
     In one embodiment, the orthodontic bracket comprises a retention mechanism for limiting the movement of the movable member toward the open position, and a first stop feature that is separate from the retention mechanism for limiting movement of the movable member toward the closed position. The retention mechanism includes an apparent contact area between the retention mechanism and one of the movable member and bracket body. The first stop feature includes a first contact area between the movable member and bracket body that is greater than the apparent contact area. 
     In one embodiment, the bracket body includes a support surface that at least in part defines a slide engagement track. The movable member engages with the slide engagement track. At least a portion of the support surface is positioned lingually of a labial edge of the archwire slot. 
     In one embodiment, the bracket body comprises a polycrystalline ceramic having a grain size distribution characterized by an average grain size in the range of larger than 3.4 μm to about 6 μm. In one embodiment, the orthodontic bracket further comprises a ligating slide comprising the polycrystalline ceramic. 
     In yet another embodiment, an orthodontic bracket comprises a bracket body configured to be mounted to the tooth, including an archwire slot configured to receive the archwire therein, the bracket body comprising a polycrystalline ceramic having a grain size distribution characterized by an average grain size in the range of about 3.5 μm to about 5 μm, by having up to about 50% of the grains being less than about 3 μm in size, by having up to about 90% of the grains being less than about 10 μm in size, and by having grains larger than 10 μm in size occupying up to about 50% of the volume of the bracket body. The polycrystalline ceramic has a fracture toughness of at least 4.0 MPa·m 1/2 . 
     In still another embodiment of the present invention, a method of making an orthodontic bracket comprises molding a bracket body from a ceramic powder and sintering the molded body to form a sintered body having a grain size distribution characterized by an average grain size in the range of larger than 3.4 μm to about 6 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the general description given above and the detailed description given below, serve to explain various aspects of the invention. 
         FIG. 1  is a perspective view of a self-ligating orthodontic bracket in accordance with one embodiment of the invention with a ligating slide shown in an opened position; 
         FIG. 2  is a perspective view of the self-ligating orthodontic bracket shown in  FIG. 1  with the ligating slide shown in a closed position; 
         FIG. 3  is a front elevation view of the ligating slide shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of the self-ligating orthodontic bracket shown in  FIG. 1  with the slide removed; 
         FIG. 5  is a rear elevation view of the ligating slide shown in  FIG. 1 ; 
         FIG. 6  is a cross-sectional view of the self-ligating orthodontic bracket shown in  FIG. 2  taken generally along line  6 - 6 ; 
         FIG. 7  is a cross-sectional view of the self-ligating orthodontic bracket shown in  FIG. 1  taken generally along line  7 - 7 ; 
         FIG. 8  is a cross-sectional view of the self-ligating orthodontic bracket shown in  FIG. 2  taken generally along line  8 - 8 ; 
         FIG. 9  is a side elevation view of the ligating slide shown in  FIG. 1 ; 
         FIG. 10  is a rear elevation view of the self-ligating orthodontic bracket shown in  FIG. 1 ; 
         FIG. 11  is an enlarged view of encircled portion  11  shown in  FIG. 6 ; 
         FIG. 12  is a rear perspective view of a self-ligating orthodontic bracket in accordance with an alternative embodiment; 
         FIG. 13  is a front elevation view of the self-ligating orthodontic bracket shown in  FIG. 1 ; 
         FIG. 14  is a cross-sectional view similar to  FIG. 7  of the orthodontic bracket shown in  FIG. 1  taken generally along line  7 - 7 ; 
         FIG. 15  is a perspective view of a self-ligating orthodontic bracket in accordance with another embodiment of the invention; 
         FIG. 16  is a perspective view of a self-ligating orthodontic bracket in accordance with one embodiment of the invention with a ligating slide shown in a closed position; 
         FIG. 17  is a perspective view of the self-ligating orthodontic bracket shown in  FIG. 16  with the ligating slide shown disassembled from the bracket body; 
         FIG. 18  is a cross-sectional view of the self-ligating orthodontic bracket shown in  FIG. 16  taken generally along line  18 - 18 ; 
         FIG. 19  is a rear elevation view of the ligating slide shown in  FIGS. 16 and 17 ; 
         FIG. 19A  is a cross-sectional view of the ligating slide shown in  FIG. 19  taken along line  19 A- 19 A; 
         FIG. 20  is a cross-sectional view of the orthodontic bracket shown in  FIG. 16  taken generally along line  20 - 20 ; 
         FIG. 21  is a perspective view of a self-ligating orthodontic bracket in accordance with one embodiment of the invention with a ligating slide shown in a closed position; 
         FIG. 22  is a perspective view of the self-ligating orthodontic bracket shown in  FIG. 21  with the ligating slide shown disassembled from the bracket body; 
         FIG. 23  is a cross-sectional view of the self-ligating orthodontic bracket shown in  FIG. 21  taken generally along line  23 - 23   
         FIG. 24  is a rear elevation view of the ligating slide shown in  FIGS. 21 and 22 ; 
         FIG. 24A  is a cross-section view of the ligating slide shown in  FIG. 24  taken along line  24 A- 24 A; 
         FIG. 25  is a graph illustrating the effect of surface flaws on the flexural strength of polycrystalline alumina; 
         FIGS. 26A, 26B, and 26C  are micrographs of polycrystalline alumina bracket material taken at a magnification of 440× in accordance with embodiments of the invention; 
         FIG. 26D  is a micrograph of polycrystalline alumina bracket material taken at a magnification of 110×; 
         FIGS. 27A, 27B, 27C, and 27D  are micrographs of polycrystalline alumina bracket material taken at a magnification of 440× in accordance with embodiments of the invention; 
         FIGS. 28A, 28B, 28C, and 28D  are graphs depicting grain size distributions of the microstructures depicted in  FIGS. 27A, 27B, 27C, and 27D , respectively; 
         FIG. 29  is a graph of the calculated volume fraction for three grain size ranges for the microstructure depicted in  FIG. 27B  in accordance with one embodiment of the invention; and 
         FIG. 30  is a graph of the calculated volume fraction for three grain size ranges for the microstructure depicted in  FIG. 27C  in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Although the invention will be described next in connection with certain embodiments, the invention is not limited to practice in any one specific type of orthodontic bracket. The description of the embodiments of the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. In particular, those skilled in the art will recognize that the components of the embodiments of the invention described herein could be arranged in multiple different ways. 
     Referring now to the drawings, and to  FIGS. 1 and 2  in particular, an orthodontic bracket  10  includes a bracket body  12  and a movable closure member coupled to the bracket body  12 . In one embodiment, the movable closure member may include a ligating slide  14  slidably coupled with the bracket body  12 . The bracket body  12  includes an archwire slot  16  formed therein adapted to receive an archwire  18  (shown in phantom) for applying corrective forces to the teeth. The ligating slide  14  is movable between an opened position ( FIG. 1 ) in which the archwire  18  is insertable into the archwire slot  16 , and a closed position ( FIG. 2 ) in which the archwire  18  is retained within the archwire slot  16 . The bracket body  12  and ligating slide  14  collectively form a self-ligating orthodontic bracket  10  for use in corrective orthodontic treatments. Moreover, while the movable closure member is described herein as a ligating slide, the invention is not so limited as the movable closure member may include other movable structures (e.g., latch, spring clip, door, etc.) that are capable of moving in any appropriate manner between an opened and closed position. 
     The orthodontic bracket  10 , unless otherwise indicated, is described herein using a reference frame with the bracket  10  attached to a labial surface of a tooth on the upper jaw. Consequently, as used herein, terms such as labial, lingual, mesial, distal, occlusal, and gingival used to describe bracket  10  are relative to the chosen reference frame. The embodiments of the invention, however, are not limited to the chosen reference frame and descriptive terms, as the orthodontic bracket  10  may be used on other teeth and in other orientations within the oral cavity. For example, the bracket  10  may also be coupled to the lingual surface of the tooth or be located on the lower jaw and be within the scope of the invention. Those of ordinary skill in the art will recognize that the descriptive terms used herein may not directly apply when there is a change in reference frame. Nevertheless, the invention is intended to be independent of location and orientation within the oral cavity and the relative terms used to describe embodiments of the orthodontic bracket are to merely provide a clear description of the examples in the drawings. As such, the relative terms labial, lingual, mesial, distal, occlusal, and gingival are in no way limiting the invention to a particular location or orientation. 
     When mounted to the labial surface of a tooth carried on the patient&#39;s upper jaw, the bracket body  12  has a lingual side  20 , an occlusal side  22 , a gingival side  24 , a mesial side,  26 , a distal side  28  and a labial side  30 . The lingual side  20  of the bracket body  12  is configured to be secured to the tooth in any conventional manner, such as for example, by an appropriate orthodontic cement or adhesive or by a band around an adjacent tooth (not shown). The lingual side  20  may further be provided with a pad  32  that defines a bonding base  33  adapted to be secured to the surface of the tooth. The pad  32  may be coupled to the bracket body  12  as a separate piece or element, or alternatively, the pad  32  may be integrally formed with the bracket body  12 . The bracket body  12  includes a base surface  34  and a pair of opposed slot surfaces  36 ,  38  projecting labially from the base surface  34  that collectively define the archwire slot  16  extending in a mesial-distal direction from mesial side  26  to distal side  28 . The slot surfaces  36 ,  38  and base surface  34  are substantially encapsulated or embedded within the material of the bracket body  12 . The archwire slot  16  of the bracket body  12  may be designed to receive the orthodontic archwire  18  in any suitable manner. 
     In reference to  FIG. 4 , the bracket body  12  further includes a support surface  40  extending in a generally gingival-occlusal direction from slot surface  38 . A pair of opposed guides  42 ,  44  are carried by support surface  40  and are positioned on respective mesial and distal sides  26 ,  28  thereof. The guides  42 ,  44  are generally L-shaped and each includes a first leg  42   a ,  44   a  projecting from support surface  40  in the labial direction. Guide  42  has a second leg  42   b  or ear projecting in the distal direction while guide  44  has a second leg  44   b  or ear projecting in the mesial direction so that collectively guides  42 ,  44  partially overlie support surface  40 . Support surface  40  and guides  42 ,  44  collectively define a slide engagement track  46  for supporting and guiding ligating slide  14  within bracket body  12 . 
     The support surface  40  includes a mesial portion  48 , a distal portion  50 , and a central portion  52  intermediate the mesial and distal portions  48 ,  50 . Guides  42 ,  44  are configured to overlie, but be spaced from, mesial and distal portions  48 ,  50 , respectively, so as to receive the ligating slide  14 . The central portion  52  includes a raised boss  54  that projects generally in the labial direction, the purpose of which is discussed in more detail below. Such a configuration essentially defines gingivally-occlusally directed tracks or grooves  56 ,  58  in the support surface  40 . In addition, the central portion  52  of support surface  40  includes a recess or cutout  60  at an occlusal end thereof that defines a stop surface. As explained in more detail below, the stop surface is configured to cooperate with the ligating slide  14  to accommodate imposed forces (e.g., mastication forces) on the bracket  10  in an improved manner. 
     As noted above, to improve the aesthetics of the orthodontic bracket  10 , the bracket body  12  is formed from a translucent or transparent non-metallic material. For example, the bracket body  12  may be formed from a transparent or translucent ceramic material. Additionally, the bracket body  12  may be tooth colored. In one embodiment, the ceramic material may be a polycrystalline alumina or aluminum oxide. However, by way of example and not limitation, other polycrystalline ceramic materials may be used, such as polycrystalline zirconia or zirconium oxide. Accordingly, in one embodiment, the bracket body  12  may be formed by ceramic injection molding (CIM) followed by sintering and/or hot isostatic pressing (HIPing). 
     In yet another embodiment, a portion of the bracket body  12  or the entire surface thereof may be treated to increase the torque strength of the bracket body  12 . By way of example, the bracket body  12  may have a coating deposited or otherwise formed thereon. For example, the coating may be deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) with a thickness of up to about 15 μm. Further, the coating may be amorphous, nanocrystalline, or have a microstructure that contains grains that are finer than the grains of the bracket body  12 . In one embodiment, rather than grinding and/or polishing the surface, a portion or all of the surfaces of the bracket body  12  may be ion milled or acid etched to remove surface imperfections and generate compressive surface stresses therein, thereby strengthening the bracket body  12 . In addition, or alternatively, the surfaces may be metal ion bombarded, mixed metal ion bombarded, or laser melted to improve fracture toughness. It will be appreciated that a combination of one or more surface treatments may be utilized to increase the torque strength of the bracket body  12 . 
     As shown in  FIG. 3 , the ligating slide  14  includes a mesial portion  62 , a distal portion  64 , and a central portion  66  intermediate the mesial and distal portions  62 ,  64 . Guides  42 ,  44  are configured to overlie mesial and distal portions  62 ,  64 , respectively, and central portion  66  may be configured such that the labial side of central portion  66  is substantially flush with the labial side of the guides  42 ,  44  ( FIG. 2 ). Such a configuration essentially defines gingivally-occlusally directed tracks or grooves  68 ,  70  in the labial side of the ligating slide  14  which move along guides  42 ,  44  as the ligating slide  14  is moved between the opened and closed positions. The mesial and distal portions  62 ,  64  do not extend the full gingival-occlusal extent of the ligating slide  14 , but instead stop short of the gingival side  72  to define two, generally planar platform surfaces  74 ,  76 , respectively. As shown in  FIG. 2 , and discussed in more detail below, when the ligating slide  14  is in the closed position, the platform surfaces  74 ,  76  may be adjacent or form a portion of the archwire slot  16 , and more particularly, form a portion of the slot surface  38  that bounds a side of archwire  18 . 
     In contrast to many aesthetic self-ligating brackets, the ligating slide  14  may also be formed from a translucent or transparent material. For example, the ligating slide  14  may be formed from a transparent or translucent ceramic material. In one embodiment, the ceramic material may be the same as that used to form the bracket body  12 . Those of ordinary skill in the art will recognize, however, that there are other suitable materials which would provide an aesthetic ligating slide  14 . In addition, to improve the strength and aesthetics, the ligating slide  14  may be surface treated in a manner similar to that described above for bracket body  12 . Advantageously, forming both the bracket body  12  and ligating slide  14  from a transparent or translucent material will reduce the visibility of the orthodontic bracket when it is secured to a patient&#39;s tooth. 
     As shown in  FIGS. 4 and 5 , the orthodontic bracket  10  includes a securing mechanism that secures the ligating slide  14  in at least the closed position. To this end, the securing mechanism includes a projecting portion in one of the bracket body  12  or ligating slide  14  and a receiving portion in the other of the bracket body  12  or ligating slide  14  that cooperate to keep the ligating slide  14  in at least the closed position. The securing mechanism may further prevent the ligating slide  14  from detaching from the bracket body  12 . In one exemplary embodiment, the securing mechanism includes a resilient engagement member, such as a generally elongate, cylindrical, tubular spring pin  78  ( FIG. 4 ), coupled to the bracket body  12 , and a retaining slot  80  ( FIG. 5 ) formed in the ligating slide  14 . Although this embodiment is described with the spring pin  78  associated with the bracket body  12  and the retaining slot  80  associated with the ligating slide  14 , those of ordinary skill in the art will recognize that the invention is not so limited. For example, a spring pin may be coupled to the ligating slide and a suitable retaining slot may be formed in the bracket body. 
     As shown in  FIGS. 4, 6, and 7 , spring pin  78  includes a first portion and a second portion. The first portion of spring pin  78  is configured to be received within a bore  82  formed in support surface  40 . The second portion of spring pin  78  projects away from support surface  40  in a general labial direction so as to extend into slide engagement track  46 . The spring pin  78  includes a cutout or slit  84 , the purpose of which is described below, formed in the sidewall thereof and extends along at least a portion of the length of the spring pin  78 . The spring pin  78  may be formed, for example, through a rolling process so as to define the slit  84 , or alternatively, may be formed by cutting a tubular member to form slit  84 . Additionally, the spring pin  78  may be formed from materials including stainless steel, titanium alloys, NiTi-type superelastic materials, or other suitable materials. 
     The bore  82  is positioned occlusally of the archwire slot  16  and extends from the lingual side  20  to the support surface  40  of bracket body  12 . Additionally, the bore  82  may be formed in the raised boss  54  of central portion  52 . Locating spring pin  78  within raised boss  54  provides additional support to spring pin  78  and prevents or reduces cantilevered bending or flexing of spring pin  78 . The bore  82  includes a first portion  86  open to support surface  40  and adapted to receive the spring pin  78  therein, and a second portion  88  open at the lingual side  20  of bracket body  12  and adapted to receive an occluding member configured to secure the spring pin  78  within bore  82 . In one embodiment, the occluding member may be a ball  90 . The occluding member, however, is not so limited, as other occluding members may also be used to secure spring pin  78  within bore  82 . A generally tapered transition region  92  may be disposed between the first and second bore portions  86 ,  88 . 
     During assembly, the spring pin  78  is inserted into the bore  82  from the lingual side  20  (and with the ligating slide  14  engaged with bracket body  12 ) so as to be positioned within first bore portion  86 . The ball  90  is inserted into the second bore portion  88  and secured to the bore  82 . By way of example, the ball  90  may be adhesively coupled with bore  82 . Those of ordinary skill in the art may recognize other ways to secure the ball  90  within bore  82 . The cross dimension of second bore portion  88  may be greater than a cross dimension of the first bore portion so as to facilitate insertion of the spring pin  78  within bore  82 . In particular, the tapered transition region  92  may facilitate insertion of the spring pin  78  within bore  82  during the assembly process. In one embodiment, ball  90  may be formed of zirconia, but other suitable materials, such as PMMA, polycarbonate, glass, and the like, may also be used to form ball  90 . Although the spring pin  78  may be formed from metal, the spring pin  78  (and ball  90  should it be made from a non-aesthetic material) is positioned relatively deep within the orthodontic bracket  10  to minimize any impact on the aesthetics of the bracket  10 . 
     As shown in  FIG. 5 , the lingual side  94  of ligating slide  14  includes a cavity  96  having a base surface  98 , a gingival end  100 , and an occlusal end  102 . The gingival end  100  is open so as to receive the raised boss  54  therein as the ligating slide  14  is moved between the opened and closed positions. The occlusal end  102  of cavity  96  is closed off by a tab member  104  extending outwardly (e.g., lingually) from lingual side  94  and having a contacting surface  106  confronting the cavity  96 , the purpose of which is discussed in more detail below. 
     The retaining slot  80  is formed in the base surface  98  of the cavity  96  and extends generally in the gingival-occlusal direction (e.g., in the direction of movement of ligating slide  14 ). The retaining slot  80  may be formed so as to extend completely through the slide  14  in the labial-lingual direction (not shown), or so as to extend only partially through the slide  14 , and therefore not be visible from a labial side  108  of the slide  14  (e.g., a blind slot), as shown in  FIGS. 3 and 5 . Such a blind slot configuration reduces the sites on the labial side of the bracket  10  where food or other material from the oral cavity could collect, thereby improving overall hygiene. In one embodiment, the retaining slot  80  has an enlarged portion  110  at an occlusal end  112  of the slot  80  in communication with a straight segment portion  114  having a closed gingival end  116 . The enlarged portion  110  may be circular, as shown, or have other suitable shapes. The cross dimension of the circular portion  110  is larger than the cross dimension of the straight segment portion  114  to define a pair of opposed protrusions  118  at the transition therebetween. 
     When the ligating slide  14  is coupled to the bracket body  12 , the spring pin  78  is received in retaining slot  80 , which moves relative to the spring pin  78  as the ligating slide  14  is moved between the opened and closed positions. The spring pin/retaining slot configuration provides for securing the ligating slide  14  in at least the closed position. To this end, the slit  84  in the spring pin  78  allows the spring pin  78  to be generally radially flexed or elastically deformed. Thus, the spring pin  78  is capable of radially expanding and contracting depending on the radial force being imposed thereon. To this end, the slit  84  in the spring pin  78  allows at least the slit portion to be generally radially flexed or elastically deformed relative to its central axis  69 . As used herein, radially flexed includes not only uniform radial changes, but also includes non-uniform or partial radial changes, such as that which occurs during squeezing of a resilient C clip. In other words, at least a portion of spring pin  78  has a first effective diameter or radius of curvature (such as in an unbiased state) but is capable of being flexed, such as by squeezing the spring pin  78 , so as to have a second effective diameter or radius of curvature smaller than the first effective diameter or radius of curvature. While the slit  84  in spring pin  78  allows for radial contraction/expansion, such movement may be achieved in other ways. 
     In operation, when the ligating slide  14  is in the closed position ( FIG. 2 ), the spring pin  78  is disposed in the enlarged portion  110  of retaining slot  80  and is permitted to radially expand such that the spring pin  78  engages the wall of enlarged portion  110 . Those of ordinary skill in the art will recognize that the spring pin  78  does not have to engage the wall of enlarged portion  110 , but must at least have a cross dimension (e.g., diameter) when radially expanded that is larger than the cross dimension of the straight segment portion  114 . When so disposed in the enlarged portion  110 , the protrusions  118  provide a threshold level of resistance to any movement of the ligating slide  14  away from the closed position and toward the opened position. However, if a sufficiently large opening force is applied to the ligating slide  14  in the occlusal direction, for example, the interaction between the retaining slot  80  and spring pin  78  causes the pin  78  to radially contract (due to the squeezing imposed by the slot  80 ) so that the spring pin  78  moves past the protrusions  118  and into the straight segment portion  114  of the retaining slot  80 . 
     Once positioned in the straight segment portion  114 , the spring pin  78  bears against the sides thereof such that a threshold sliding force, which is less than, and perhaps significantly less than the opening force, must be imposed to overcome the drag and move the ligating slide  14  relative to the bracket body  12  as spring pin  78  traverses straight segment portion  114 . Thus, once opened, the ligating slide  14  does not just freely slide or drop to the fully opened position, but must be purposefully moved toward the opened position. If the ligating slide  14  is only partially opened, the slide  14  may be configured to maintain its position relative to the bracket body  12  (due to, for example, the friction forces between the spring pin  78  and the slide  14 ) until the threshold sliding force is imposed to continue moving the slide  14  toward the opened position. Such a configuration reduces the likelihood of unintentionally closing the slide  14  during, for example, an orthodontic treatment. When the ligating slide  14  is moved toward the closed position, the spring pin  78  recovers or snaps back to its radially expanded position as the spring pin  78  enters the enlarged portion  110  to once again secure the ligating slide  14  in the closed position. 
     As introduced above, the raised boss  54  supports the spring pin  78  during the opening and closing of the ligating slide  14 . In this regard, when the ligating slide  14  is moved toward the open position, any drag forces from contact of the spring pin  78  with the retaining slot  80  may create a shear-type force on the pin  78 . The raised boss  54  reduces the likelihood that the pin  78  will plastically deform or fracture when the ligating slide  14  moves past the pin  78  because it supports the length of the pin  78  that extends from the surface  40 . By contrast, without the raised boss  54 , and where a length of the pin  78  projects labially from the support surface  40 , it is more likely that the pin  78  would experience a cantilever-type or torque force sufficient to bend or break the pin  78  about the edge of the bore  82  when the slide  14  moves in either direction. 
     The securing mechanism, including spring pin  78  and retaining slot  80 , is more fully disclosed in pending U.S. patent application Ser. No. 12/147,877 (the &#39;877 application), the disclosure of which is incorporated by reference herein in its entirety. Additionally, the other securing mechanisms disclosed in the &#39;877 application may also be used for the aesthetic orthodontic bracket  10  as disclosed herein. Thus, the securing mechanism is not limited to the spring pin/retaining slot configuration shown in the figures and described above. 
     In addition to sufficiently securing the ligating slide  14  in at least the closed position, the securing mechanism may also operate as a retention mechanism to prevent or reduce accidental or unintentional detachment of the ligating slide  14  from the bracket body  12  during use, such as when the ligating slide  14  is in the opened position. To this end, the length of the retaining slot  80  may limit the gingival-occlusal travel of ligating slide  14  relative to the bracket body  12 . For example, the spring pin  78  may abut the gingival end  116  of the retaining slot  80  when the ligating slide  14  is in the fully opened position. Because the gingival end  116  closes the retaining slot  80 , further movement of the ligating slide  14  in an occlusal direction relative to bracket body  12  is prohibited, and ligating slide  14  cannot become separated or detached from bracket body  12 . 
     Similarly, in the fully closed position of the ligating slide  14 , the spring pin  78  is positioned in the enlarged portion  110  at the occlusal end  112  of the retaining slot  80 , which may prohibit further movement of the ligating slide  14  in the gingival direction relative to the bracket body  12 . As discussed in more detail below, the orthodontic bracket  10  may include other features that, in lieu of or in addition to the securing mechanism, prevent movement of the ligating slide  14  in the gingival direction relative to the bracket body  12 . 
     In this regard, designs that limit or stop gingival movement of the ligating slide  14  relative to bracket body  12  using only the spring pin  78  may be susceptible to premature failure of the securing mechanism. In these designs, all the forces that are imposed on the ligating slide  14 , such as during chewing, are transmitted to the bracket body  12  via the spring pin  78 . Due to the relatively small contact area between the spring pin  78  and ligating slide  14 , the forces transmitted to the bracket body  12  through spring pin  78  may be sufficient to shear, notch, or otherwise damage the spring pin  78 . Thus, as is described more fully below, it may be desirable to limit the gingival movement of the ligating slide  14  using stop surfaces with increased areas of contact between the ligating slide  14  and bracket body  12 . The increased contact area will essentially spread the imposed loads on the ligating slide  14  across a greater area of the bracket body  12 . Such a configuration will prevent or reduce the risks of premature failure of the securing mechanism. In this regard, the stop surfaces may reduce or prevent the spring pin  78  from bottoming out on the occlusal end  112  of the retaining slot  80 . Additionally, when the ligating slide  14  is in the closed position, the stop surfaces define contact areas that are greater than an apparent contact area between the spring pin  78  and retaining slot  80  in the absence of the stop surfaces. 
     To this end, when the ligating slide  14  is in the closed position, the gingival side  72  of the ligating slide  14  may be configured to engage a first contacting or stop surface of the bracket body  12 . In this regard, as shown in  FIGS. 6 and 7 , the orthodontic bracket  10  may include an overshoot feature configured as a cutout  120  formed in the labial side  30  of the bracket body  12  adjacent slot surface  36 . The cutout  120  defines a ledge  122  which extends above slot surface  36  and is configured to engage or be adjacent the lingual side  94  of ligating slide  14  when the slide  14  is in the closed position. Providing such on overshoot eases the acceptable tolerances in the coupling of the ligating slide  14  and bracket body  12  so as to cover the archwire slot  16  when in the closed position. 
     Cutout  120  also defines a gingival wall  124  that overhangs the gingival side  72  of the ligating slide  14  when in the closed position, as shown in  FIG. 6 . This overhang prevents or reduces food or other material in the oral cavity from contacting the gingival side  72  of the slide  14  and inadvertently moving the slide  14  toward the opened position. The gingival wall  124  of cutout  120  may also provide the first contacting surface. In this regard, when the ligating slide  14  is in the closed position, the gingival side  72  of the slide  14  may be configured to engage the gingival wall  124  of the cutout  120  prior to the spring pin  78  bottoming out on the occlusal end  112  of retaining slot  80 . The contact between the gingival side  72  of the ligating slide  14  and gingival wall  124  of bracket body  12  increases the area over which forces imposed on the ligating slide  14  are transmitted to the bracket body  12 . Furthermore, when the ligating slide  14  is in the closed position, a portion of the retaining slot  80  may be exposed to the archwire slot  16 , as shown in  FIG. 6 . 
     As shown in the figures, orthodontic bracket  10  may include a tool receptacle  126  that cooperates with a tool (not shown) for moving the ligating slide  14  away from the closed position and toward the opened position. With reference to  FIGS. 4, 6 and 7 , the labial side  30  of bracket body  12  includes tool receptacle  126  defining a gingival wall  128 , a mesial wall  130 , a distal wall  132 , and a labial wall  134 . The receptacle  126 , however, is open along an occlusal end thereof so as to be accessible to at least a portion of ligating slide  14 . For example, the tool receptacle  126  may be open to the gingival side  72  of ligating slide  14 . Various tools and methods for using tool receptacle  126  are more fully disclosed in U.S. patent application Ser. No. 12/147,854, the disclosure of which is incorporated by reference herein in its entirety. As illustrated in these figures, the intersection of the tool receptacle  126  and cutout  120  diminishes the area of the first contacting surface between the gingival side  72  of ligating slide  14  and the gingival wall  124  of cutout  120 . For example, with tool receptacle  126  formed in bracket body  12 , the first contacting surface may be strips along the occlusal edges of mesial, distal, and labial walls  130 ,  132 , and  134 , respectively. 
     Orthodontic bracket  10  may further include a second contacting or stop surface between the ligating slide  14  and bracket body  12  that limits or stops gingival movement of the ligating slide  14  relative to bracket body  12 . The second contacting surface may operate alone or in conjunction with the first contacting surface as described above. In this regard, and as shown in  FIGS. 6 and 7 , when the ligating slide  14  is in the closed position, the contacting surface  106  of tab member  104  may be configured to engage bracket body  12  prior to the spring pin  78  bottoming out on the occlusal end  112  of retaining slot  80 . More particularly, the occlusal side  22  of bracket body  12  may provide the second contacting surface. To this end, the contacting surface  106  of tab member  104  includes a first engaging portion  136  and a second engaging portion  138  that respectively engage a first engaging portion  140  and second engaging portion  142  of bracket body  12 . The contact between the tab member  104  of the ligating slide  14  and bracket body  12  increases the area over which forces imposed on the ligating slide  14  are transmitted to the bracket body  12 . Thus, the likelihood of premature failure of the securing mechanism may be reduced. 
     The relationship between the tab member  104  and bracket body  12  may have additional benefits. For example, when the ligating slide  14  is in the closed position, the tab member  104  is positioned in recess  60  in support surface  40  and at least partially fills a void between the guide members  42 ,  44  adjacent the occlusal side  22  of bracket body  12 . Filling such a void with the tab member  104  reduces the sites available for plaque and/or food buildup. Moreover, as shown in  FIGS. 6 and 7 , the second engaging portions  138 ,  142  may be angled or chamfered relative to first engaging portions  136 ,  140 . The chamfered configuration on the ligating slide  14  allows for an increased wall thickness in portions of slide  14 . For example, as shown in  FIG. 6 , the thickness t 1  of the ligating slide  14  in a gingival-occlusal direction, and the thickness t 2  of the ligating slide  14  in the labial-lingual direction may be increased relative to more traditional non-chamfered configurations. The increased thicknesses provide additional strength and rigidity to the ligating slide  14 . 
     In addition to the various features described above, orthodontic bracket  10  may include several other features that provide benefits to the design of the bracket and/or to the implementation of the bracket during orthodontic treatments. By way of example, one feature is directed to the position of the ligating slide  14  relative to the bracket body  12 , and more particularly, relative to the archwire slot  16 . In this regard, conventional self-ligating brackets typically have bracket bodies that form an archwire slot and bound the archwire on all but one side thereof. The unbounded side of the archwire slot is then closed off by the movable closure member. A substantial portion, if not all, of the closure member is generally positioned on the labial side of the archwire slot and as a result the bracket body must include structure extending labially of the archwire slot to accommodate the closure member, such as during movement between the opened and closed position. As a consequence, the labial-lingual width of the self-ligating orthodontic bracket is generally relatively large. The increased labial-lingual width not only makes the brackets more noticeable, and thus less aesthetically pleasing, but also may decrease comfort for the patient. Additionally, relatively large labial-lingual widths increase the occurrence of bond failure due to, for example, mastication forces. 
     To address such shortcomings of conventional self-ligating brackets, orthodontic bracket  10  is designed to provide a self-ligating feature with a decrease in the labial-lingual width, thus improving the aesthetics and comfort to the patient. In this regard, the ligating slide  14  has been moved lingually within the bracket body  12  relative to conventional self-ligating bracket designs. Thus, as shown in  FIG. 8 , the support surface  40  that defines slide engagement track  46  ( FIG. 4 ) is no longer labial of the archwire slot  16 , but includes a portion of which is now positioned lingual of a labial edge  143  of the archwire slot  16 . In one embodiment, the slide engagement track  46  intersects the slot surface  38  at an intermediate location thereof, as shown in  FIG. 8  (i.e., between base surface  34  and labial edge  143 ). Alternatively, however, the slide engagement track  46  may be partially closed off from the archwire slot  16  by an intervening wall (shown in  FIG. 16  and described below) but still be positioned lingually of the labial edge  143  of archwire slot  16 . 
     Moving the slide engagement track  46  lingually within the bracket body  12  results in a number of design features in ligating slide  14  to accommodate such movement. As illustrated in  FIGS. 2 and 9 , the ligating slide  14  includes a bracket-engaging portion  144  that confronts the bracket body  12 , and a slot-covering portion  146  that confronts the archwire slot  16  when in the closed position. The transition between the two portions occurs adjacent the platform surfaces  74 ,  76 . In reference to  FIG. 9 , in addition to stopping the mesial and distal portions  62 ,  64  short of the gingival side  72  to form platform surfaces  74 ,  76  ( FIGS. 3 and 5 ), the lingual side  94   a  of the slot covering portion  146  is offset from the lingual side  94   b  of the bracket-engaging portion  144  in, for example, a generally labial direction. The offset, generally shown at  148 , provides for movement of the ligating slide  14  along slide engagement track  46  that has been moved lingually within bracket body  12  and allows the slot-covering portion  146  to cover the archwire slot  16  so as not to interfere with the archwire  18 , which it might otherwise do but for the offset  148 . Furthermore, the corner  149  formed between the lingual side  94   a  of slot-covering portion  146  and platform surfaces  74 ,  76  is not sharp, but instead is curved or rounded so as to reduce the stress concentrations on the ligating slide  14  at the corner  149 . By way of example, the corner  149  may have a radius of curvature of greater than approximately 0.003 inch. 
     In the particular embodiment shown, when in the closed position, the ligating slide  14  confronts and bounds the archwire  18  in the generally labial direction (e.g., one side of archwire  18  as shown in  FIG. 6 ). Additionally, and as a result of the lingual movement of the ligating slide  14  within bracket body  12 , the platform surfaces  74 ,  76  also confront the archwire  18  and may, for example, also bound the archwire by forming a portion of slot surface  38  (e.g., a second side of archwire  18  as shown in  FIG. 8 ). In this regard, the platform surfaces  74 ,  76  may be configured to be flush with the slot surface  38  or may be slightly below (e.g., spaced occlusally of) slot surface  38 . Locating the platform surfaces  74 ,  76  slightly below slot surface  38  may reduce the frictional engagement between the archwire  18  and archwire slot  16 . 
     Moving the ligating slide  14  lingually within the bracket body  12  of orthodontic bracket  10  reduces the labial-lingual width of bracket  10 . The reduced width in this direction improves the aesthetics by making the brackets less noticeable, improves the comfort of the brackets to the patient, and may further reduce the occurrence of bond failure. 
     In another aspect, functionality and use of orthodontic bracket  10  may be enhanced by the inclusion of additional features on the bonding base  33  of the pad  32 . In this regard, there are several shortcomings of conventional brackets relative to the bonding process to the tooth. For example, it is not uncommon for excessive adhesive, used to bond the bracket to the tooth, to leak beyond the periphery of the bonding base of the bracket, thus requiring monitoring and cleanup. In some bracket designs, the bracket fails to contain the adhesive to relevant areas during the bonding process, thus also requiring monitoring and cleanup. Additionally, in many conventional brackets, the bonding base of the bracket may not be designed for relatively easy removal of the bracket from the tooth. In this regard, the bracket may not include a convenient feature that cooperates with a de-bonding tool for removing the bracket from the tooth. Furthermore, some designs fail to include any features that facilitate the bond between the bracket and tooth so as to result in a more reliable bond therebetween. 
       FIGS. 6, 7, and 10  illustrate the bonding base  33  of the orthodontic bracket  10  designed to address these and other shortcomings of conventional brackets. In one aspect, the bonding base  33  includes a lingually-extending lip  150  along at least a portion of the periphery  152  of the bonding base  33 . In one embodiment, the lip  150  extends along substantially the entire periphery of the bonding base  33 . As shown in  FIGS. 6 and 7 , the lip  150  defines the inner side walls  154  of an open well or cavity  156 . Bonding adhesive is adapted to be disposed within the cavity  156  when the bracket  10  is to be bonded to the tooth. The side walls  154  bound the adhesive and prevent or reduce the likelihood of the adhesive from escaping beyond the periphery  152  of the bonding base  33 . Thus, the time, expense and aggravation of cleaning up the adhesive is eliminated or reduced. 
     In addition to containing the bonding adhesive within the periphery  152  of the bonding base  33 , the configuration of the lip  150  may provide other benefits. For example, as illustrated in  FIG. 11 , the corner  158  between an outer side wall  160  of bonding base  33  and the outer side wall  162  (e.g., occlusal, gingival, mesial, and/or distal sides) of the pad  32  may be configured to facilitate removal of the orthodontic bracket  10  from the tooth. In this regard, the corner  158  is radiused or chamfered so as to provide a small gap  164  between the tooth and bracket  10  along the periphery  152  of the bonding base  33 . In one embodiment, for example, the corner  158  is radiused with a radius of curvature between approximately 0.005 inch and approximately 0.010 inch. Those of ordinary skill in the art will appreciate that the radius of curvature may be smaller or larger than this range depending on the specific application. The gap  164  not only serves as a crack initiator during a de-bonding process, but also provides a purchase point for a tool, shown schematically at  166 , used during the de-bonding process. 
     In addition to the above, the inclusion of lip  150  on bonding base  33  may further provide for tailoring the de-bonding strength of the bracket  10  from the tooth. In this regard, the particular lip geometry may affect the bond strength in predictable ways, such that the lip geometry may be specifically configured to provide a desired debond strength. In particular, the height of the lip  150 , the thickness of the lip  150 , and/or the configuration of inner side wall  154  (e.g., smooth, wavy, grooved, etc.) may affect debond strength. Additionally, the configuration of the corner  158  (e.g., radius of curvature) may affect the amount of force an orthodontist applies to a tool for removing the bracket  10  from the tooth. 
     Furthermore, the bonding base  33  may include additional features for enhancing the bond between the bracket  10  and tooth. In this regard, and as shown in  FIG. 10 , the bonding base  33  may include a plurality of pegs or posts  168  thereon for improving bond strength. The posts  168  increase the contact area between the adhesive and the bracket and thereby increase bond strength. Additionally, the posts  168  may be further configured to increase the bond strength. For example, the posts  168  may be flattened or deformed at an outer end thereof so as to create undercuts (not shown). Adhesive fills the undercuts to, in essence, create a mechanical lock between the bonding base  33  and the adhesive. 
     In one embodiment, the posts  168  may be integrally formed with the bonding base  33 . For example, the orthodontic bracket  10  may be formed from a ceramic material, as discussed above, using a ceramic injection molding (CIM) process. One technique for forming such posts  168  includes laser shaping the bonding base  33  during the green or brown state of the CIM process. An exemplary laser shaping technique is more fully disclosed in U.S. Publication Nos. 2006/0163774 and 2006/0166159, the disclosures of which are incorporated by reference herein in their entirety. In addition to laser shaping the bonding base  33  to form posts  168 , those of ordinary skill in the art may recognize other techniques for forming posts  168 . 
     Other features in addition to, or in lieu of, posts  168  may be included to increase the bond strength between the bracket  10  and tooth via a suitable adhesive. Such features may include forming projections, recesses, undercuts, etc. in the bonding base  33 . For example, another technique includes ball basing the bonding base  33 . Ball basing uses a monolayer of small, generally spherical particles on the bonding base to effectively create undercuts. As more fully disclosed in U.S. Pat. No. 5,071,344, the disclosure of which is incorporated by reference herein in its entirety, a layer of adhesive is applied to the bonding base through, for example, a brushing or spraying technique. Thereafter, small, generally spherical particles (shown in phantom in  FIG. 12 ) are either sprinkled on the bonding base  33 , or the bracket  10  is tamped into a pile of particles, such that a relatively dense monolayer of particles is provided. The bracket is then heated in a furnace to diffusion bond the particles to the bonding base  33 . 
     As shown in  FIG. 12 , the bore  82  for receiving spring pin  78  is open to bonding base  33 . As noted above, during assembly, the spring pin  78  is inserted into bore  82  via this opening. Manufacturing processes will generally form the bond-enhancing features on bonding base  33  prior to insertion of spring pin  78  in bore  82 . Accordingly, it may be desirable to keep adhesive and other material, such as the particles used in a ball basing technique, from entering into bore  82 . In this regard, the bonding base  33  may include a lingually-extending lip  170  about the opening to bore  82 . For example, the lip  170  may be positioned along the periphery of the bore  82  or radially spaced therefrom by a relatively small amount. The lip  170  prevents or at least reduces the likelihood of adhesive and/or particles from entering bore  82  and perhaps making insertion of spring pin  78  within bore  82  problematic. 
     In still a further aspect, and as illustrated in  FIG. 13 , the orthodontic bracket  10  may include a single gingival tie wing  172  and two occlusal tie wings  182  that facilitate coupling of the bracket to other adjacent orthodontic devices using ligatures, elastic bands, or other connecting members known in the art. A single tie wing  172  may be desirable, compared to the more traditional two tie wing design, because it provides less surface area for food or other material in the oral cavity to bear on. As a result, bond reliability may be improved. The tie wing  172  may be centrally located on the bracket body  12  in the mesial-distal direction. As a result, mesial and distal sides  174 ,  176  of the wing  172  may have a sloped or scalloped configuration, and therefore be inclined in a gentle and smooth manner. Such shaping of the tie wing  172  enhances the comfort of orthodontic bracket  10 . 
     Further in this regard, manufacturers of orthodontic brackets continually seek improvements to bracket designs that provide greater comfort to the patient. For example, many conventional orthodontic brackets include labial sides that are irregular or discontinuous. In some situations, these irregularities may cause discomfort to the patient as, for example, soft oral tissue repeatedly engages the labial surface of the bracket. The orthodontic bracket  10  addresses such shortcomings by configuring the surface of the bracket  10  in a smooth and continuous manner. Thus, the edges or transitions between adjacent sides of the bracket may be characterized by one or more curves each having a generally large radius of curvature. 
     For example, the transitions, generally shown at  178 , between the labial side  30  and the mesial and distal sides  26 ,  28  may be generally arcuate and have a radius of curvature of between approximately 0.015 inch and approximately 0.025 inch. Additionally, the transition  180  between the labial side  30  and occlusal side  22  may also have a radius of curvature in the range provided above. Moreover, the edges of the tie wing  172  may also be smoothed by using a relatively large radius of curvature thereat. The smooth transitions between adjacent sides of the bracket result in an overall improvement to the comfort of the orthodontic bracket  10 . 
     As shown in  FIG. 14 , another feature includes configuring orthodontic bracket  10  such that ligating slide  14  moves along slide engagement track  46  at an angle □ relative to the base surface  34  of archwire slot  16 . In this regard, engagement track  46  extends generally along a translation plane  46   a  that is acutely angled relative to a base plane  34   a  associated with the base surface  34 . Such an angled feature was disclosed in U.S. Pat. No. 7,267,545 for molar self-ligating brackets. In molar applications, the angled feature helps avoid contact between the ligating member and the surrounding gingiva. As illustrated in  FIG. 14 , the angled nature of slide engagement track  46  in orthodontic bracket  10  provides for an increase in the wall thickness t 3  of the occlusal tie wings  182  (one shown) relative to the more traditional parallel engagement configuration (shown as a phantom line  184 ). Accordingly, the strength of the occlusal tie wings  182  is increased. Angling of the slide  14  to achieve an increase in tie wing thickness may be particularly relevant in high torque brackets. Moreover, in high torque applications, the angled nature of slide engagement track  46  also provides an increase in the clearance underneath the tie wing  182 . Thus, various connecting members (e.g., ligatures, O-rings, power chains, etc.) may be more securely coupled to bracket  10 . 
       FIG. 15 , in which like reference numerals refer to like features in  FIGS. 1-14 , illustrates an orthodontic bracket  210  in accordance with an alternative embodiment. Orthodontic bracket  210  is similar to orthodontic bracket  10  and only the differences will be discussed in detail. As an initial matter, the bracket  210 , as shown, is also configured and described from a reference frame of being applied to a tooth on the upper jaw. However, as discussed above, those of ordinary skill in the art will appreciate that the invention is not so limited. In many applications, it is desirable to include a hook with a dental bracket for coupling to an adjacent orthodontic device. Typically, the hook is a separate element that is permanently affixed to the bracket body. Alternatively, the hook may be an auxiliary device that is temporarily or releasably coupled to the bracket body. This may be achieved, for example, through the use of an auxiliary slot (e.g., vertical slot) formed in the bracket body that receives the shaft of a hook therein (not shown). 
     As shown in  FIG. 15 , in one embodiment, the orthodontic bracket  210  may include a hook, generally shown at  212 , integrally formed with the bracket body  12 . More particularly, in one embodiment, the hook  212  may include a stem  214  that extends generally in a gingival direction from the tie wing  172  and terminates in an enlarged head  216 . The combination tie wing/hook feature allows orthodontic bracket  210  to retain both of these capabilities but in a more efficient manner that obviates the need for separate components or auxiliary slots formed through the bracket body  12 . The invention, however, is not so limited as the hook  212  may be integrally formed with the bracket body  12  at locations other than tie wing  172  (not shown) depending on the specific application and/or desires of the orthodontist. For example, a hook  212  may be integrally formed with a bracket body that has no tie wings (e.g., molar brackets). 
       FIGS. 16 and 17 , in which like reference numerals refer to like features in  FIGS. 1-14 , illustrate an orthodontic bracket  220  in accordance with an alternative embodiment. Orthodontic bracket  220  is similar to orthodontic bracket  10  and only the differences will be discussed in detail. Like the brackets  10  and  210 , the orthodontic bracket  220  is also configured and described from a reference frame of being applied to a tooth on the upper jaw. As described above, bracket designs that limit or stop gingival movement of the ligating slide  14  relative to the bracket body  12  using only the pin  78  may be susceptible to premature failure, because much of the load imposed on the ligating slide  14  is transmitted to the bracket body  12  primarily through the pin  78 . Due to the small contact area between the pin  78  and the ligating slide  14 , the magnitude of these loads may be sufficient to shear, notch, or otherwise damage the pin  78 , the ligating slide  14 , and/or the bracket body  12 . By increasing the area of contact between the ligating slide  14  and bracket body  12 , the imposed loads are dispersed over a larger area. In other words, the imposed loads are distributed from the slide  14  directly to the body  12  rather than to the body  12  via the pin  78 . As described above with reference to the orthodontic bracket  10 , the area of contact may be increased by the contact of the tab  104  with the cutout  60 . In other embodiments, the area of contact between the ligating slide  14  and the bracket body  12  may be increased by additional or alternative features. 
     For example, and with reference to the exemplary embodiment shown in  FIGS. 16 and 17 , the orthodontic bracket  220  includes intervening walls  222 ,  224  integrally formed with the bracket body  12 . As shown best in  FIG. 17 , support surface  40  carries intervening walls  222 ,  224  which are positioned adjacent to the guides  42 ,  44  on the mesial and distal sides  26 ,  28  of surface  40 . Therefore, generally, the intervening walls  222 ,  224  partially close off mesial and distal sides of the slide engagement track  46  adjacent the archwire slot  16 . In particular, in the embodiment shown, the mesial intervening wall  222  projects distally from the first leg  42   a  of guide  42 , and the distal intervening wall  224  projects mesially from the first leg  44   a  of guide  44 . Thus, the intervening walls  222 ,  224  form contacting or stop surfaces in the slide engagement track  46 . The stop surfaces, like stop surfaces described above, limit movement of the ligating slide  14  in the gingival direction and increase the contact area between the ligating slide  14  and the bracket body  12  when the ligating slide  14  is in the closed position (as shown in  FIG. 16 ). In particular, the respective intervening walls  222 ,  224  form contacting surfaces or shoulders  226 ,  228  on the mesial and distal sides  26 ,  28  of the slide engagement track  46 . The shoulders  226 ,  228  abut a portion of the ligating slide  14  as described below. Furthermore, these contacting surfaces may operate alone or in conjunction with any of the contacting surfaces as set forth above, or below, to distribute loads on the slide  14  to the bracket body  12 . 
     Further, in regard to distribution of loads imposed by the archwire and with reference to  FIG. 17 , the intervening walls  222 ,  224  extend from the support surface  40  to the second leg  42   b ,  44   b  of the respective guide  42 ,  44 , or, in other words, the intervening walls  222 ,  224  form gussets that distribute loads imposed by the archwire on the guides  42 ,  44  to the body  12 . Specifically, the gussets reduce the torque experienced by the ears of each guide  42 ,  44 . This configuration improves the strength and rigidity of the respective guide  42 ,  44 . While the intervening walls  222 ,  224  are shown as extending the full mesial-distal width of the respective second legs  42   b ,  44   b , the walls  222 ,  224  need not extend the full mesial-distal width of the guides  42 ,  44  and still transmit loads between the slide  14  and the body  12 . For example, the mesial intervening wall  222  may project distally from the first leg  42   a  of the mesial guide  42  to a distance that is less than the full distal projection of the second leg  42   b  of the guide  42 . Similarly, the distal intervening wall  224  may project less than the full mesial-distal distance of the second leg  44   b  of guide  44 . 
     In one embodiment, as shown in  FIG. 17 , the intervening walls  222 ,  224  are adjacent to or form a portion of the archwire slot  16 , and, more particularly, form a portion of the slot surface  38 . The intervening walls  222 ,  224 , however, may be placed occlusally of the archwire slot  16 . In this regard, the intervening walls  222 ,  224  may be configured to be flush with the slot surface  38  or may be spaced occlusally of the slot surface  38 . 
     As a consequence, and as shown in  FIG. 17 , the intervening walls  222 ,  224  may complement or replace features found in brackets  10  and  210 . For example, the raised boss  54  (shown in  FIG. 4 , for example) may not be present. Those of ordinary skill in the art will observe, however, that the bracket body  12  may be configured with both the intervening walls  222 ,  224  and the raised boss  54 . In this embodiment, that is, without the boss  54 , the slide engagement track  46  may include a single groove  230  rather than the two groves  56 ,  58  separated by the boss  54 . The ligating slide  14  may then have complementary features to the single groove  230 , as described below. 
     Similar to the absence of the raised boss  54 , the cutout  60 , also described above and shown in  FIG. 4 , may not be present. As shown in  FIG. 4 , the cutout  60  forms a stop surface in the bracket body  12 . The cutout  60  cooperates with the tab member  104  on the ligating slide  14  shown for example in  FIG. 5 . However, as shown in  FIG. 17 , where the body  12  includes intervening walls  222 ,  224 , the bracket body  12  may not include the cutout  60  since the intervening walls  222 ,  224  provide, at least in part, stop surfaces. Consequently, the ligating slide  14  may not include tab  104 , as described more fully below. 
     With reference to  FIG. 18 , in one embodiment, the bore  82  is moved away from the archwire slot  16  by elongating the body  12  by about 0.010 inch (shown, for example, by the distance between phantom line or pad  32 ′ of bracket  10  and pad  32  of the bracket  220 ) and providing tapered sides  230 ,  232  on occlusal side  22  as compared to occlusal side  22 ′ for the body  12  of bracket  10 . In particular, the bore  82  of the bracket  220  is moved occlusally away from the archwire slot  16 . The relative bore of the bracket  10  of  FIG. 1  is labeled  82 ′. Similarly, the relative locations of the occlusal side of the bracket  10  and bracket  220  are illustrated by comparing occlusal sides  22 ′ and  22 , respectively. Additionally, in the absence of the cutout  60 , the bracket body  12  has a single occlusal tie wing  234  rather than mesial and distal occlusal tie wings  182  spaced apart by the cutout  60 . The single occlusal tie wing  234  may extend nearly the full width of the bracket body  12 . This configuration may ease attachment of connecting members (not shown) therein, may increase the strength of the body  12 , as well as, reduce the sites available for plaque and/or food buildup. 
     Similar to the ligating slide  14  shown in  FIGS. 1 and 2 , the ligating slide  14  depicted in  FIGS. 16, 17, and 19  includes mesial and distal portions  62 ,  64  that do not extend the full gingival-occlusal extent of the ligating slide  14 . However, with reference to  FIG. 19 , the mesial and distal portions  62 ,  64  define shoulders  236 ,  238  configured to engage bracket body  12  prior to the spring pin  78  bottoming out on the occlusal end  112  of retaining slot  80 . Specifically, the shoulders  236 ,  238  of the mesial and distal portions  62 ,  64  abut a portion of the intervening walls  222 ,  224 , such as the shoulders  226 ,  228 , when the ligating slide  14  is moved to the closed position. 
     To this end, the shoulders  236 ,  238  or contacting surfaces of mesial and distal portions  62 ,  64  include a distal engaging portion  240  and a mesial engaging portion  242  that respectively engage engaging portions of the body  12 , and, in particular, engage portions of walls  222 ,  224 , like shoulders  226 ,  228 . As set forth above and with reference to  FIG. 17 , the surface contact between the engaging portions of the ligating slide  14  and the engaging portions of the bracket body  12  increase the area of contact between the body  12  and slide  14  thereby distributing more load directly between the slide  14  and body  12 . 
     Furthermore, while no portion of the mesial portion  62  or of the distal portion  64  may extend past intervening walls  222 ,  224  or form a portion of the slot surface  38  (as shown in  FIG. 16 ), a portion of one or both of the mesial and distal portions  62 ,  64  may be adjacent to or form a portion of the slot surface  38  similar to the platform surfaces  74 ,  76  described above and shown in  FIGS. 2 and 5 . For example, partial platform surfaces (not shown) may be present where the intervening walls  222 ,  224  extend along only part of the mesial-distal extent of the guides  42 ,  44 . This configuration would amount to a combination of the ligating slide having both shoulders  234 ,  236  and platform surfaces  74 ,  76  (shown in  FIG. 5 ). 
     With reference to the ligating slide  14  shown in  FIGS. 19 and 19A , the ligating slide  14  has a substantially planar lingual side  94   b  that slidably cooperates with the single groove  230  described above. Specifically, where the raised boss  54  is absent, the ligating slide  14  does not include the cavity  96  shown, for example, in  FIG. 5 . In other words, the retaining slot  80  may be formed directly in the lingual sides  94   a ,  94   b  rather than being formed within cavity  96 . 
     In addition and with reference to  FIGS. 18, 19 and 19A , where the bore  82  is moved nearer the occlusal side  22  of the body  12 , as described above, the retaining slot  80  may likewise be formed nearer to the occlusal side of the ligating slide  14 . Advantageously, rather than having the retaining slot  80  formed in the slot covering portion  146 , as shown in  FIG. 6 , the retaining slot  80  is formed entirely within the bracket engaging portion  144 . In one embodiment, the gingival end  116  of the retaining slot  80  is positioned occlusally of the corner  149 . Accordingly, mesial-distal cross sections taken along planes oriented generally in the lingual-labial direction through the slot covering portion  146  do not intersect the retaining slot  80 . Positioning of the retaining slot  80  away from the slot covering portion  146  may improve load carrying capability or rigidity of the slide  14  due to the gain in material in this portion of the slide  14 . Moreover, moving the retaining slot  80  into the bracket engaging portion  144  has additional advantages. For example, referring to  FIG. 20 , in this regard, the retaining slot  80  is not exposed to the archwire slot  16  when the ligating slide  14  is in the closed position. 
     In further regard to the exemplary ligating slide  14  shown in  FIGS. 19 and 19A , a central platform surface  244  is formed in the central portion  66  of the ligating slide  14 . For example, in the absence of the raised boss  54  and the cavity  96 , shown in  FIGS. 4 and 5 , respectively, the central platform surface  244  may include a surface that faces in the gingival direction and may be formed between the corner  149  and the lingual side  94   b . With reference to  FIG. 20 , when the ligating slide  14  is in the closed position, the central platform surface  244  may be adjacent to or form a portion of the archwire slot  16 , and more particularly, form a portion of the slot surface  38 . 
     In another exemplary embodiment, and with reference to  FIGS. 21 and 22  in which like reference numerals refer to like features in  FIGS. 1-14 , an orthodontic bracket  250  includes bracket body  12  having a bridge  252  connecting opposing ears or guides  42 ,  44 . Like the brackets  10 ,  210 , and  220 , the bracket  250  is also configured and described from a reference frame of being applied to a tooth on the upper jaw. As best shown in  FIG. 22 , in one embodiment, the bridge  252  joins the opposing guides  42 ,  44  such that support surface  40 ; guides  42 ,  44 ; and bridge  252  collectively define a D-shaped engagement track  254  for supporting and guiding the ligating slide  14  between open and closed positions. Generally, by extending between opposing mesial and distal portions of the body  12  labially of the slide engagement track  46 , the rigidity and/or strength of the body  12  may be improved. Furthermore, the bridge  252  may enclose or encapsulate at least a portion of the bracket engaging portion  144  of slide  14  and may, accordingly, separate a portion of that side (i.e., the labial side  108 ) of the ligating slide  14  from the buccal mucosa. 
     Not only is the lingual side  108  of the bracket engaging portion  144  separated from buccal mucosa surfaces, the lingual side  108  of the slot engaging portion  146  is also separated from the buccal mucosa. For example, and with reference to  FIG. 23 , when the slide  14  is in the closed position, the labial side  108  of the slide engaging portion  146  may be positioned lingually of the labial side  30  of the body  12  (e.g., closer to the pad  32 ). The relative difference in position between the two sides ( 30  and  108 ) creates an overhang  256 . The overhang  256  may also prevent or reduce contact between the labial side  108  of the slot covering portion  146  and the buccal mucosa or food which would tend to force the ligating slide  14  toward the open position. 
     For example, when the ligating slide  14  is in the closed position (as shown in  FIGS. 21 and 23 ), loads tending to pull an archwire (not shown) from archwire slot  16 , such as normal corrective loads necessary for tooth movement, push a portion of the bracket engaging portion  144  outward where it is captured by the bridge  252 . Since the bridge  252  extends between the guides  42 ,  44 , it distributes loads directly from the ligating slide  14  to the body  12 . 
     With reference to  FIGS. 22, 23, 24, and 24A , in one embodiment, the ligating slide  14  has a substantially D-shaped cross section or another cross section that is configured to slidably engage the engagement track  254  defined by the bridge  252 ; the guides  42 ,  44 ; and the support surface  40 . The lingual side  94   b  of the bracket engaging portion  144  is substantially planar and is configured to slidably cooperate with the support surface  40 . The lingual side  94   a  of the slot covering portion  146  is offset from the lingual side  94   b  by the corner  149 . The corner  149  extends the mesial-distal width of the ligating slide  14 , and thus may form an edge or a platform surface  256 , like central platform surface  240 . However, the platform surface  256  extends the mesial-distal width of the ligating slide  14 . Furthermore, the platform surface  256  may be adjacent or form a portion of the archwire slot  16 , and more particularly, form a portion of the slot surface  38 . 
     As with the ligating slide  14  illustrated with bracket  220 , the gingival end  116  of the retaining slot  80  may be formed fully within the bracket engagement portion  144 . For example, the gingival end  116  of the slot  80  may be formed occlusally of the slot covering portion  146 . Or, in another example, the gingival end  116  of the slot  80  as shown in  FIG. 24A  may be formed occlusally of the corner  149 . Consequently, mesial-distal cross sections taken along planes oriented generally in the lingual-labial direction through the slot covering portion  146  may not intersect the retaining slot  80 . As set forth above, where the retaining slot  80  is formed fully within the bracket engagement portion  144 , the slide  14  may exhibit improved strength. Furthermore, the slot  80  may not be exposed to the archwire slot  16  when the ligating slide  14  is in the closed position (shown in  FIG. 23 ). Thus, any food or other debris that finds its way into the archwire slot  16  will not become lodged in the retaining slot  80 , which would possibly obstruct movement of the slide  14  to the open position. 
     With reference generally to any of the orthodontic brackets shown in  FIGS. 1-22 , but specifically to the orthodontic bracket  10 , the bracket body  12  comprises a polycrystalline ceramic, for example alumina or aluminum oxide (Al 2 O 3 ). And, in another embodiment, the bracket body  12  and the ligating slide  14  comprise the polycrystalline ceramic. The bracket body  12 , and optionally the ligating slide  14 , comprising the polycrystalline ceramic more readily resists fracture when loaded with tensile and flexural stresses, such as those generated by engagement with the archwire  18  or those that normally occur during orthodontic treatment. By way of example, and not limitation, orthodontic brackets described in  FIGS. 1-22  can be formed from the polycrystalline ceramic described herein. It will be appreciated that while self-ligating brackets are shown and described herein, embodiments of the present invention are not limited to self-ligating brackets. 
     As is known in the art, ceramic brackets tend to be brittle and too often fail during orthodontic treatment. Of course, bracket failure is problematic. For instance, a fractured bracket renders tooth movement ineffective. A more troubling problem is that pieces of the fractured bracket may be ingested or inhaled if fracture occurs while the bracket is in the patient&#39;s mouth. As will be described more fully below in conjunction with the examples, the inventors have discovered that the polycrystalline ceramic having a grain size distribution described, in part, by an average grain size in the range of larger than 3.4 μm to about 6 μm has unexpectedly high fracture toughness. For example, the polycrystalline ceramic having an average grain size in the range of larger than 3.4 μm to about 6 μm has an average fracture toughness of at least about 3.85 MPa·m 1/2  and, in a further example, the polycrystalline ceramic having an average grain size between about 4 μm and about 4.3 μm has an average fracture toughness that exceeds about 5.0 MPa·m 1/2 . By comparison, the average fracture toughness of polycrystalline alumina having an average grain size of 43 μm is about 3.28 MPa·m 1/2 . 
     For example and with reference to  FIG. 1 , the orthodontic bracket  10  comprising the polycrystalline ceramic described herein advantageously reduces the risk of patient ingestion or inhalation of portions of a fractured bracket, and the patient endures fewer, if any, bracket replacements. Overall, the orthodontic bracket  10  comprising the polycrystalline ceramic permits orthodontic treatment to proceed more quickly. In addition, the orthodontic bracket  10  is translucent such that the patient is less self-conscious during treatment. 
     In one embodiment, the orthodontic bracket  10  is prepared by injection molding a ceramic powder and binder mixture to form a bracket body. The binder may then be removed from the injection molded body by heating the injection molded body to a temperature, for example, between 200° C. and 700° C. Following removal of the binder, the injection molded body may then be presintered followed by sintering. By way of example, the ceramic powder may be alumina powder. High purity alumina powder (about 99.95 wt. % alumina) may be presintered at temperatures of between 900° C. and 1,200° C. followed by sintering at between 1,400° C. and 1,800° C. In other embodiments, the presintered injection molded body may be hot-isostatically pressed (HIPed). For example, a presintered injection molded body of alumina may be hot-isostatically pressed (HIPed) at between about 1,300° C. and about 1,600° C. with an applied pressure of about 68 to about 207 MPa. 
     In other embodiments, following sintering or HIPing, the bracket body  12  is annealed, i.e., heated to a temperature and held for a time sufficient to further modify the grain size distribution. Modification of the grain size distribution may occur at temperatures of about 1300° C. or higher for alumina. However, higher or lower temperatures than 1300° C. may modify the grain size distribution depending on the time the bracket body is held at that specific temperature. By way of example, the bracket body may be held at about 1300° C. for about 1 hour. In addition, the bracket body  12  may be heated in a variety of atmospheres including, for example hydrogen (H 2 ), nitrogen (N 2 ), oxygen (O 2 ), or argon (Ar). 
     To assess the performance of a ceramic material, the flexural strength of the material may be measured with a three-point bend setup. Samples for three-point bend testing are generally in the form of a rectangular bar. In a three-point bend setup, a bar of the material is supported on one side at two locations along the bar&#39;s length. Each support location is near one of the bar&#39;s ends. The distance between the opposing supports is referred to as the support span. A load is applied to the bar on the surface opposite to and centered between the supports. The load is gradually increased until the bar fractures. This arrangement (i.e., two supports on one side and a load applied between the supports on the opposing side) produces tensile stresses in one surface of the bar. The flexural strength may be calculated based on the dimensions of the bar and the load at the time of fracture according to the well-known equation: 
             σ   =       3   ⁢           ⁢   PS       2   ⁢           ⁢     wt   2               
where σ is the flexural strength, P is the load at fracture, S is the support span, w is the bar width, and t is the bar thickness.
 
     The inventors have noted that there are many variables that influence flexural strength of a sample of material. For example, the method of manufacturing, preparation, or handling of the samples for testing or a combination thereof can greatly influence flexural strength as each may create flaws in the surface of the sample. Surface flaws (e.g., microcracks, porosity, surface damage, abnormal grains or another localized microstructural heterogeneity, or foreign inclusions, among others) are known to concentrate or magnify stresses. Magnification of the stresses occurs at each flaw&#39;s tip. As such, the localized tensile stress at each flaw&#39;s tip is greater than the applied tensile load. When the stress concentrated at the tip of one flaw exceeds the theoretical strength of the material, a crack originating at that flaw will rapidly propagate through the material. Flexural strength measurements, therefore, are inherently influenced by the surface condition of the sample. For example, the inventors measured the flexural strength of polycrystalline alumina having an average grain size of 4.5 μm for different crack sizes which were artificially introduced with a diamond indenter. The sensitivity of flexural strength of polycrystalline alumina to increasing crack size is illustrated in  FIG. 25 . 
     In addition, the flexural strength data may also be influenced by any of a number of other factors including, for example, the configuration of the sample, the number of tests performed, and the stress state in the actual bracket compared to the stress state in the sample, among others. In sum, flexural strength data does not necessarily provide an accurate prediction of the performance of ceramic orthodontic brackets in the clinical environment. 
     Recognizing the fallibility of flexural strength measurements noted above, the fracture toughness of the polycrystalline ceramic described herein was also determined. Fracture toughness is a material property that indicates how a material containing surfaces flaws (e.g., the notch in the notched samples) will respond to tensile stresses, and, in particular, how the bulk of the material is resistant to extension of a crack from the surface flaw. Therefore, unlike flexural strength measurements described above, fracture toughness measurements are a measurement of how the bulk material will respond to stresses in the event of surface imperfections. Taking into account the known factors that influence fracture, fracture toughness measurements provide a more accurate indication of the performance of a polycrystalline ceramic bracket within a clinical environment. 
     The fracture toughness may be determined by at least two methods. Using the three-point bend setup used for flexural strength measurements, fracture toughness can be calculated from the load at fracture obtained from breaking a bar of the material that contains a flaw or crack of controlled or known size. The fracture toughness may be calculated from the load at fracture according to the equation: 
               K   IC     =       (     PS     wt     3   2         )     ⁢     {       3   2     ⁢         (     a   t     )       1   2       ·     Y   ⁡     (     a   t     )           }             
where K IC  is the fracture toughness of the sample under a tensile stress that is oriented perpendicular to a crack, P is the load at fracture, S is the support span, w is the bar width, t is the bar thickness, and
 
               Y   ⁡     (     a   t     )       =     1.964   -     2.837   ⁢     (     a   t     )       +     13.711   ⁢       (     a   t     )     2       -     23.250   ⁢       (     a   t     )     3       +     24.129   ⁢       (     a   t     )     4                     a   =         a   1     +     a   2     +     a   3       3           
where a is the average of three crack length measurements, a 1 , a 2 , and a 3  or is the depth of a known flaw.
 
     According to another method, fracture toughness can be calculated from Vickers hardness measurements. In this case, the fracture toughness may be calculated according to the following equation, 
               K   c     =     0.018   ⁢       (     E   HV     )       1   2       ⁢     (     P     c     3   2         )             
where K c  is the fracture toughness, P is the pressing load, E is the modulus, HV is the measured Vickers hardness, and c is one-half of the average of crack length produced by the Vickers hardness indenter.
 
     In one embodiment, the polycrystalline ceramic has a grain size distribution described, in part, by an average grain size in the range of larger than 3.4 μm to about 6 μm. The average grain size may be determined according to the line intercept method. According to this method, a line of known length is drawn on a micrograph of a polished cross section of the material. Intersections between the line drawn and each grain boundary are counted. An average length of the grains is determined dividing the length of the line by the number of intersections counted. The average grain size is calculated according to the equation D=1.56(L), where L is the average length of the grains. 
     Without intending to be bound by theory, the polycrystalline ceramic&#39;s resistance to crack propagation, that is, its fracture toughness, may be influenced by its microstructure, though the effect of a polycrystalline microstructure on crack propagation is not completely predictable. Unlike bodies of a single-crystal ceramic, like sapphire, or isotropic materials, like glass, the fracture toughness of the polycrystalline ceramic may depend on a number of factors including, for example, grain size, grain size distribution, density, and others some of which are not present in single crystals or glass. 
     In particular, the presence of grain boundaries may affect the crack&#39;s propagation direction and/or the crack&#39;s mode of propagation. A change in direction and/or change in mode may consume comparatively more energy than the energy required to propagate a crack along a straight path. The mode of crack propagation in polycrystalline ceramics is either intergranular or transgranular or both. Intergranular crack propagation follows the grain boundaries (that is, between grains) while transgranular crack propagation is through the grains. Accordingly, when a propagating crack encounters a grain boundary or a grain, the crack may be forced to change direction, change its mode of propagation (that is, from transgranular to intergranular or vice versa) or change both direction and mode of propagation. By forcing a change in the direction and/or mode of crack propagation, the length of the crack pathway increases, which consumes more energy, and, accordingly, the fracture toughness may increase. 
     In one embodiment of the present invention, the polycrystalline ceramic, having a grain size distribution described by an average grain size in the range of larger than 3.4 μm to about 6 μm, may force a crack to travel along a path that is longer relative to the polycrystalline ceramics of the prior art. Thus, greater stresses may be required to propagate the crack through the polycrystalline ceramic, as described herein, such that an orthodontic bracket made of the polycrystalline ceramic is characterized by an unexpected resistance to fracture. 
     In another embodiment, the mixture of both large and small grains in combination with the average grain size may further lengthen the crack&#39;s path through the polycrystalline ceramic and further improve the fracture toughness of the polycrystalline ceramic. By way of example, the polycrystalline ceramic having a grain size distribution described by an average grain size in the range of greater than 3.4 μm to about 6 μm may further comprise grains larger than 6 μm in size and grains smaller than 3.4 μm in size. 
     In one embodiment, further improvement in the fracture toughness of the polycrystalline ceramic may be obtained by a grain size distribution that is not a lognormal distribution. By definition, a lognormal distribution is characterized by a random variable whose logarithm is normally distributed about a mean. As an example, the grain size distribution according to one embodiment is multimodal. In particular, the grain size distribution may be a bimodal distribution. 
     In one embodiment, a bimodal distribution has a first peak or mode between a grain size of about 1 μm and about 5 μm and a second peak or mode at a grain size larger than about 5 μm. By way of example, the second peak may be between about 5.5 μm and about 7 μm. However, it will be appreciated that the second peak or additional peaks may occur at grain sizes larger than 7 μm. It will also be appreciated that the bimodal grain size distribution does not describe a duplex microstructure. In one embodiment, the average fracture toughness for a polycrystalline ceramic having an average grain size in the range of larger than 3.4 μm to about 6 μm and at least a bimodal grain size distribution is greater than about 4.0 MPa·m 1/2 . 
     In addition, the inventors have identified that a grain size distribution characterized by having a particular ratio between grains smaller than about 3 μm and larger grains may further enhance resistance to crack propagation. By way of example, the polycrystalline ceramic may have a grain size distribution having up to about 50% of the total number of grains less than about 3 μm in size. By way of further example, the polycrystalline ceramic may have a grain size distribution having the number of grains less than 3 μm in size of at least 10% such that in one embodiment, the number of grains less than 3 μm in size is, for example, between about 10% and about 50% of the total number of grains. In yet another example, the polycrystalline ceramic may be characterized by a grain size distribution having up to about 90% of the total number of grains less than about 10 μm in size. In a further example, the total number of grains less than about 10 μm in size is at least 70%. Therefore, in one embodiment, the total number of grains less than about 10 μm in size is between about 70% and about 90% of the grains. 
     In terms of volume fraction, according to one embodiment, the polycrystalline ceramic is characterized by a grain size distribution in which grains larger than 10 μm in size may occupy up to 50% of the total volume. By way of example, in one embodiment the grains larger than 10 μm in size occupy at least 10% of the total volume, and in a further example, the grains larger than 10 μm in size may occupy from about 10% up to 50% of the total volume. The volume fraction of grains larger than about 10 μm can be calculated by determining the volume of the grains of a particular size range, multiplying that volume by the total number of grains in that size range, and then dividing by the total volume of all the grains. 
     With regard to the volume fraction occupied by grains greater than 10 μm, the inventors believe that crack propagation through embodiments of the orthodontic bracket comprising the polycrystalline ceramic may be mixed mode because of the volume fraction ratio described above. That is, if a crack propagates into the polycrystalline ceramic, the polycrystalline ceramic may force the crack to change its mode of propagation one time or many times as it proceeds through the polycrystalline ceramic depending upon what size grain it encounters. For example, the presence of grains less than 10 μm in size may foster intergranular crack propagation. However, a grain that is 10 μm in size or larger may force a crack to change to transgranular propagation. Thus, a mixture of grain sizes, as described, may force a crack to alternate between modes and may, therefore, further lengthen the propagation pathway. Accordingly, the volume fraction of grains described may increase fracture toughness of the polycrystalline ceramic. 
     As previously noted, the orthodontic bracket  10  is aesthetically pleasing and is, in that regard, translucent. As is known in the art, the translucency of a polycrystalline ceramic, like alumina, is affected by its microstructure. For example, the grain size distribution, the quantity and location of any porosity, and the purity of the starting powder may affect the translucency of the orthodontic bracket  10  as well as the color of the transmitted light. Generally, as the density, grain size, and purity of the polycrystalline ceramic increases, the translucency increases. Thus, a polycrystalline ceramic orthodontic bracket of 100% density, high purity, and large average grain size would permit more light to pass through such that the orthodontic bracket  10  blends in with the underlying tooth color. Translucency can be quantified by measuring the amount of light of a particular wavelength that is transmitted through the polycrystalline ceramic. In one embodiment of the present invention, it is expected that the orthodontic bracket  10  comprising polycrystalline alumina having an average grain size of about 3.5 μm or greater will have a transmittance greater than 45% but less than 85%. 
     EXAMPLES 
     In order to facilitate a more complete understanding of embodiments of the invention, the following non-limiting examples are provided. 
     Two different batches (designated Batch #1 and Batch #2) of specimens of polycrystalline alumina having dimensions of about 25.4 mm by about 38.1 mm by about 1 mm were purchased from Tosoh Corporation, Tokyo, Japan. Twenty-four samples in the form of thin plates approximating the thickness and width of a bracket body were prepared from the Batch #1 specimens by cutting the Batch #1 specimens to the desired dimensions. Similarly, eight samples from Batch #2 were prepared by cutting the Batch #2 specimens to the desired dimensions. Each sample cut from the Batch #1 and Batch #2 specimens had a thickness of about 1.00±0.1 mm, a width of about 3.00±0.01 mm, and a length of about 12.00±0.01 mm. 
     The Batch #1 samples were divided into three groups labeled Batch #1A, Batch #1B, and Batch #1C. The samples of Batch #1A and Batch #2 were not subject to further heat treatments. Exemplary micrographs for the Batch #1A and Batch #2 samples are shown in  FIGS. 26A and 26B , respectively. 
     The Batch #1B samples were further heat treated in argon to a temperature of about 1,400° C. and held at that temperature for about 1 hour to modify the grain size distribution. An exemplary micrograph of a Batch #1B sample is shown in  FIG. 26C . 
     The Batch #1C samples were further heat treated in argon to a temperature of about 1,800° C. and held at that temperature for about 1 hour to modify the grain size distribution. An exemplary micrograph of a Batch #1C sample is shown in  FIG. 26D . 
     Four samples of each group were polished for flexural strength testing, and four others were machined to form a notch therein across the width for fracture toughness testing. The notch was designed to mimic the geometry of the archwire slot in an orthodontic bracket, such as the archwire slot  16  in the orthodontic bracket  10  illustrated in  FIG. 1 . The dimensions of each notch were about 0.57 mm wide and between about 0.050 mm and about 0.100 mm deep. The machining produced a 0.08 mm radius along opposing edges at the bottom of the slot. The notch was made by machining the samples with a 240/320 mesh diamond bonded wheel. 
     As described above, the three-point bend setup was used to break the notched and polished samples in each group. The support span measured about 9 mm. A load was applied to each sample at a rate of about 1 mm per minute until the sample fractured. The load at fracture was used to calculate the flexural strength of the notched and unnotched or polished samples, and the fracture toughness was calculated from the load at fracture for the notched samples. For calculating the fracture toughness, the measured notch depth (from about 0.050 mm to about 0.100 mm as measured) was assumed to be the crack length or a and together with the three-point bend load at fracture or P were used to calculate K IC  according to the equation set forth above. Table 1 provides the calculated averages for each group. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Average 
                 Average 
                   
                   
               
               
                   
                 Flexural 
                 Flexural 
                 Average 
               
               
                   
                 Strength, 
                 Strength, 
                 Fracture 
                 Average 
               
               
                   
                 Un-notched 
                 Notched 
                 Toughness, K IC   
                 grain size 
               
               
                 Group 
                 (MPa) 
                 (MPa) 
                 (MPa · m 1/2 ) 
                 (μm) 
               
               
                   
               
             
            
               
                 Batch #1A 
                 614.8 ± 88.0 
                 220.2 ± 44.4 
                 2.93 ± 0.59 
                 3.4 ± 0.3 
               
               
                 Batch #2 
                  563.0 ± 106.2 
                 291.3 ± 32.3 
                 3.85 ± 0.43 
                 4.1 ± 0.5 
               
               
                 Batch #1B 
                 627.9 ± 64.7 
                 325.5 ± 39.5 
                  4.3 ± 0.54 
                 4.5 ± 0.4 
               
               
                 Batch #1C 
                 384.0 ± 66.7 
                 248.3 ± 39.5 
                 3.28 ± 0.42 
                 43.0 ± 11.0 
               
               
                   
               
            
           
         
       
     
     Table 1 also provides the average grain size for each group of samples, as set forth above. To measure grain size, samples were prepared by standard polishing and etching techniques known in the art. Five representative micrographs were taken of each sample at magnifications of between about 110× to about 440×. Ten 6-inch lines were drawn on each micrograph. The intersections between each line and the grain boundaries were counted. The length of each line was divided by the number of grain boundary intersections and adjusted for magnification to obtain an average length of the grains per line. According to the line intercept method described above, the grain size for each line was calculated by multiplying the average length of the grains by 1.56. The grain sizes from each line were in turn averaged to provide an average grain size per group, which is provided in Table 1. The standard deviations provided in the table represent one standard deviation. 
     With reference to Table 1, while both the average fracture toughness and the average flexural strength were expected to increase as average grain size is reduced, the fracture toughness was observed to drop considerably at approximately an average grain size of 3.4 μm and below. The largest fracture toughness was observed at about 4.5 μm, and at some size above that it begins to decrease. In other words, the average fracture toughness is believed to peak somewhere in the range above 3.4 μm and at or below about 6 μm, and most likely in the range of about 3.5 μm to about 5.0 μm. For example, the samples in Batch #1B (average grain size of 4.5 μm) have an average fracture toughness in excess of 4.0 MPa·m 1/2  compared to Batch #1A (average grain size of 3.4 μm) and Batch #1C (average grain size of 43.0 μm), which have an average fracture toughness of roughly 2.9 MPa·m 1/2  and 3.3 MPa·m 1/2 , respectively. That is, the Batch #1B samples exhibited an increase in average fracture toughness of more than approximately 30% over the Batch #1C samples. 
     The transmittance of a sample from each of Batch #1A and Batch #1B (average grain sizes of 3.4 μm and 4.5 μm, respectively) was measured with diffused visible light per ASTM E 1348-02. The transmittance was measured on a BYK-Gardner, TCS Plus, Model 8870 with D56 diffuse light taken at 10°. The samples were disks of 20 mm in diameter. The disks were prepared by grinding opposing sides of the disks to obtain a thickness of 1 mm, and, subsequently, polishing the ground sides of the disk with 600 grit paper then with 3 micron diamond paste followed by 1 micron diamond paste until no scratches were visible on the polished surfaces at 200×. The sample from Batch #1A (average grain size of 3.4 μm) had a transmittance of 45%, and the sample from Batch #1C (average grain size of 43.0 μm) had a transmittance above 50%. For reference purposes, the transmittance through single crystal alumina with diffuse visible light was 85%. 
     In addition, bracket bodies of polycrystalline alumina from two different molds (Mold A and Mold C) were obtained from Tosoh Corporation, Tokyo, Japan. Bracket bodies from both molds were of a self-ligating bracket design. 
     With reference to Table 2, below, the hardness and average grain size of five brackets from each of Mold A and Mold C were measured in the as-received condition. The “as-received” brackets are labeled Mold A(*) and Mold C(*) in Table 2. A representative micrograph of the as-received Mold C(*) microstructure (i.e., the as-received bracket) is depicted in  FIG. 27A . The remaining brackets of Mold C were divided into 3 groups, each group being subject to additional heat treatment to modify their microstructures, as described below. 
     Five brackets of Mold C were subject to an additional heat treatment at a temperature of about 1,300° C. for about 1 hour in argon (Ar). This group of brackets is labeled Mold C(Ar) in Table 2. A representative micrograph of the Mold C(Ar) microstructure is depicted in  FIG. 27B . 
     Five brackets were further heat treated at a temperature of about 1,300° C. for about 1 hour in hydrogen (H 2 ). This group of brackets is labeled Mold C(H 2 ) in Table 2. A representative micrograph of the Mold C(H 2 ) microstructure is depicted in  FIG. 27C . 
     Five brackets were further heat treated at a temperature of about 1,300° C. for about 1 hour in oxygen (O 2 ). This group of brackets is labeled Mold C(O 2 ) in Table 2. A representative micrograph of the Mold C(O 2 ) microstructure is depicted in  FIG. 27D . 
     The hardness and the average grain size of each bracket of each group were measured according to the procedures outlined above. The average fracture toughness was calculated based on the Vickers hardness measurement according to the equation set forth above. 
     Table 2 lists the average Vickers Hardness, average fracture toughness, and the average grain size of all of the Mold A and Mold C brackets segregated by heat treatments, described above. The deviation provided for each of the average fracture toughness and the average grain size represents one standard deviation. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Average Fracture 
                 Average 
               
               
                   
                   
                 Hardness 
                 Toughness, K IC   
                 grain size 
               
               
                   
                 Group 
                 (HV) 
                 (MPa · m 1/2 ) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
                 Mold C(*) 
                 1714 ± 92 
                 4.01 ± 0.56 
                 3.5 ± 0.4 
               
               
                   
                 Mold C(Ar) 
                  1721 ± 124 
                 4.81 ± 0.69 
                 3.6 ± 0.3 
               
               
                   
                 Mold C(O 2 ) 
                 1660 ± 30 
                 5.57 ± 0.16 
                 4.0 ± 0.5 
               
               
                   
                 Mold C(H 2 ) 
                 1734 ± 56 
                 5.35 ± 0.91 
                 4.3 ± 0.5 
               
               
                   
                 Mold A(*) 
                 1644 ± 54 
                 4.3 ± 1.0 
                 5.0 ± 0.7 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 2, the average fracture toughness of the brackets has a similar trend to that illustrated in the polycrystalline alumina samples of Table 1 where the highest hardness and fracture toughness is observed for an average grain size between about 4.0 μm and about 4.3 μm. By way of example, the average fracture toughness calculated according to the hardness method reaches an average of nearly 5.6 MPa·m 1/2  at an average grain size of about 4.0 μm (Mold C (O 2 )). 
     In addition and with reference to  FIGS. 27A-27D , the microstructure of the polycrystalline ceramic is a mixture of very small grains and large grains. The grain sizes of the distributions for each of the microstructures of  FIGS. 27A-27D  were measured with analySIS software available from Olympus America Inc., Center Valley, Pa., using the grain size module. The distributions of the grain sizes measured with the analySIS software for the microstructures depicted in  FIGS. 27A-27D  are depicted in the graphs of  FIGS. 28A-28D , respectively. 
     As shown in  FIGS. 28A-28D , the grain size distributions contain grain sizes smaller than 3.4 μm and grains larger than 6 μm. For example,  FIG. 28A  depicts a grain size distribution having an average grain size of about 3.5 μm and having about 31% of the total number of grains less than about 3 μm, and, in a further example,  FIG. 28B  depicts a grain size distribution having an average grain size of about 3.6 μm and having about 39% of the total number of grains less than about 3 μm. 
     With particular reference to  FIG. 28B , two curves have been provided to more clearly illustrate a bimodal grain size distribution. One of the modes is located at about 3.4 μm. The other or second mode is located at about 6.0 μm. 
       FIGS. 29 and 30  are graphs depicting volume fraction of grains based on calculations made from the grain size distributions depicted in  FIGS. 27B and 27C , respectively. With reference to  FIG. 29 , by way of example, about 37% of the total volume of the grains is occupied by grains that have a size larger than 10 μm. With reference to  FIG. 30 , by way of another example, grains larger than 10 μm in size occupy about 50% of the total volume. 
     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the inventors to restrict or in any way limit the scope of the appended claims to such detail. Thus, additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.