Patent Publication Number: US-2010129764-A1

Title: Ceramic orthodontic bracket with improved debonding characteristics

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a ceramic bracket that is used during orthodontic treatment to move a patient&#39;s teeth to desired positions. More particularly, the present invention relates to a ceramic orthodontic bracket having structure for facilitating removal of the bracket at the conclusion of treatment. 
     2. Description of the Related Art 
     Orthodontic treatment is directed to movement of malpositioned teeth to improved positions in the oral cavity. Orthodontic treatment can greatly enhance the patient&#39;s facial appearance, especially in areas near the front of the patient&#39;s mouth. Orthodontic treatment can also improve the patient&#39;s occlusion so that the teeth function better with each other during mastication. 
     One type of orthodontic treatment involves the use of a set of appliances and archwires that are commonly known collectively as “braces”. During treatment, tiny slotted appliances known as brackets are affixed to the patient&#39;s anterior, cuspid and bicuspid teeth, and an archwire is placed in the slot of each bracket. The archwire forms a track to guide movement of the teeth to orthodontically correct positions. Ends of the archwire are often received in the passages of small appliances known as buccal tubes that are affixed to the patient&#39;s molar teeth. 
     In the past, orthodontic brackets were commonly welded or brazed to bands that were placed around the teeth. Today, orthodontic brackets are often bonded directly to the enamel surface of the teeth by an adhesive. Once treatment has been completed, the archwire is removed from the slots of the brackets and each bracket is then removed from the associated tooth. 
     Orthodontic brackets are typically made of metal, ceramic or plastic. Improved ceramic brackets are described in U.S. Pat. Nos. 5,439,379 and 5,366,372. The ceramic brackets described in those patents have two sections that are spaced apart from each other by a channel. In some of the embodiments described in those patents, the bracket sections are connected to each other by a line of weakness that comprises a thin web of material that lies along the bottom of the channel. The web is constructed to fracture during debonding of the bracket from the tooth. 
     The brackets that are shown in certain embodiments described in U.S. Pat. Nos. 5,439,379 and 5,366,372 are debonded at the conclusion of treatment by using a hand instrument to urge the sections in directions toward each other until the thin web of material fractures. Hand instruments that are especially useful for debonding such brackets are described in those patents as well as in U.S. Pat. No. 6,474,988 and published U.S. patent application No. 2006/0127835. The hand instruments include wall portions for engaging the sides of the bracket so that the sections of the bracket pivot toward each other and away from the tooth surface when handles of the hand instrument are squeezed together. 
     However, orthodontic treatment may lead to increased tooth sensitivity, especially if the course of treatment has extended for a period of several months. The removal of orthodontic brackets at the conclusion of treatment often results in the application of a force on the adjacent tooth surface. This force may be relatively small but still considered painful by some patients with sensitive teeth. As can be appreciated, any improvement in reducing pain associated with debonding of orthodontic brackets would be considered an advantage. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a ceramic orthodontic bracket having structure that facilitates detachment of the bracket at the conclusion of treatment. The ceramic bracket includes mesial and distal sections that are detached from the tooth by pivotal movement, and the force needed to initiate pivotal movement is significantly less than the force needed to initiate pivotal movement of mesial and distal sections of ceramic brackets previously known in the art. Yet, the ceramic orthodontic bracket of the present invention can safely withstand the forces normally encountered during the course of orthodontic treatment without substantial risk of inadvertent detachment from the patient&#39;s tooth before treatment has been concluded. 
     In more detail, the present invention is directed toward a ceramic orthodontic bracket that comprises a mesial section and a distal section. The mesial section and the distal section each include an external surface, and the external surface of the mesial section and the distal section together present a base for bonding the bracket to a tooth. The bracket also includes an archwire slot extending across the mesial section and the distal section in a generally mesial-distal direction and an elongated channel that extends in a generally occlusal-gingival direction between the mesial section and the distal section. The channel has a depth in a lingual direction that is greater than the lingual depth of the archwire slot. The bracket further includes a line of weakness extending between the mesial section and the distal section lingually of the channel and extending along the length of the channel. The line of weakness enables the bracket to be debonded from a tooth by pivoting the mesial section and the distal section about a reference axis extending generally parallel to the longitudinal axis of the channel. The bracket additionally includes a recess that extends between the external surface of the mesial section and the distal section in a direction generally parallel to the channel. The recess has a bottom surface that is spaced in a facial direction from adjacent regions of the external surface of the mesial section and the distal section. The base has an overall width in a mesial-distal direction and an overall height in an occlusal-gingival direction, and the ratio of the overall width to the overall height is less than about 0.95. 
     As will be explained in more detail below, the relatively small aspect ratio of the bracket (i.e., the ratio of its overall width to its overall height) increases the likelihood that the configuration of the bracket&#39;s base will precisely match the configuration of the patient&#39;s tooth surface for a large number of patients. The matching configurations of the bracket base and the tooth surface may help to improve the strength of the adhesive bond between the bracket and the tooth. As a result, the bracket is less likely to unintentionally detach from the tooth during the course of treatment as might otherwise occur, for example, when the patient bites into a relatively hard item of food. 
     Further details of the invention are defined in the features of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a ceramic orthodontic bracket according to one embodiment of the invention, looking at the bracket toward its facial, occlusal and distal sides; 
         FIG. 2  is a front elevational view of the bracket shown in  FIG. 1 , looking at the bracket toward its facial side; 
         FIG. 3  is a side elevational view of the bracket shown in  FIGS. 1 and 2 , looking at the bracket toward its mesial side; 
         FIG. 4  is a bottom view of the bracket shown in  FIGS. 1-3 , looking at the bracket toward its occlusal side; 
         FIG. 5  is a rear elevational view of the bracket shown in  FIGS. 1-4 , looking at the bracket toward its lingual side; 
         FIG. 6  is a reduced, fragmentary bottom view of an exemplary hand instrument along with the bracket shown in  FIGS. 1-5 , wherein jaws of the hand instrument have been placed on opposite sides of the bracket as might occur during a debonding procedure; and 
         FIG. 7  is a rear elevational view of a prior art ceramic orthodontic bracket, looking in a direction toward its lingual side. 
     
    
    
     DEFINITIONS 
     
         
         
           
             “Mesial” means in a direction toward the center of the patient&#39;s curved dental arch. 
             “Distal” means in a direction away from the center of the patient&#39;s curved dental arch. 
             “Occlusal” means in a direction toward the outer tips of the patient&#39;s teeth. 
             “Gingival” means in a direction toward the patient&#39;s gums or gingiva. 
             “Facial” means in a direction toward the patient&#39;s cheeks or lips. 
             “Lingual” means in a direction toward the patient&#39;s tongue. 
           
         
       
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A ceramic orthodontic bracket constructed in accordance with one embodiment of the present invention is illustrated in  FIGS. 1-6  and is broadly designated by the numeral  10 . The bracket  10  includes a mesial section  12  and a distal section  14  that is spaced from the mesial section  12 . The mesial section  12  includes an external surface  16  and the distal section  14  includes an external surface  18  as shown, for example, in  FIGS. 4 and 5 . 
     The external surfaces  16 ,  18  together present a base  20  that faces in a lingual direction for bonding the bracket  10  to the enamel surface of a patient&#39;s tooth. Preferably, the base  20  has a concave configuration with a compound contour that presents a curved appearance in a reference plane perpendicular to a central mesial-distal reference axis of the bracket  10  as well as in a reference plane perpendicular to a central occlusal-gingival reference axis of the bracket  10 . Preferably, the concave compound contour of the base  20  precisely matches the convex compound contour of the adjacent portion of the tooth surface to which the bracket  10  is bonded during the course of treatment. 
     Optionally, the base  20  includes a number of particles for enhancing the adhesive bond between the base  20  and the enamel surface of the patient&#39;s tooth. Suitable particles include tiny shards of aluminum oxide, such as No. 150 grit size or No. 220 grit size aluminum oxide particles designated “38 Alundum” Norton brand abrasives from Saint-Gobian Abrasives, Inc. These particles preferably protrude outwardly at various angles to present undercut regions. The undercut regions provide pockets where the adhesive establishes a mechanical interlock with the base  20  once the adhesive has hardened. 
     Optionally, the particles are omitted from all or a portion of the perimeter of the base  20 . As one example, the particles may be omitted from a 0.25 mm wide edge portion that extends around the entire perimeter of the base  20 . As another example, the particles may be omitted from edge portions in an asymmetric pattern as described in U.S. Pat. No. 5,746,594 (Jordan et al.). For instance, the particles may be omitted along a mesial edge of the mesial section  12  and a distal edge of the distal section  14 . 
     Methods for affixing ceramic particles to ceramic bodies are described in U.S. Pat. No. 5,108,285 (Tuneberg). The particles may be affixed to the sections  12 ,  14  by embedding the particles in a glass frit. Other types of particles and methods for affixing particles to ceramic bodies are described in U.S. Pat. Nos. 5,071,344 (Wong et al.) and 5,295,823 (Farzin-Nia). 
     An elongated archwire slot  22  extends across the mesial section  12  and the distal section  14  in a generally mesial-distal direction. The archwire slot  22  is located between an occlusal tiewing  24  and a gingival tiewing  26  of the mesial section  12  and an occlusal tiewing  28  and a gingival tiewing  30  of the distal section  14 . Optionally, and as depicted in the drawings, a hook  32  having an enlarged, rounded head is integrally connected to the outer end of the gingival tiewing  30  of the distal section  14  and extends in a gingival direction away from the archwire slot  22 . 
     An elongated channel  34  extends in a generally occlusal-gingival direction between the mesial section  12  and the distal section  14 . The channel  34  has a depth in a lingual direction that is greater than the depth of the archwire slot  22  in a lingual direction. As such, the distance between the bottom or lingual side of the channel  34  and the base  20  is less than the distance between the bottom or lingual side of the archwire slot  22  and the base  20 . The bottom of the channel  34  has a radius of curvature that is in the range of about 0.1 mm to about 0.15 mm and more preferably is about 0.13 mm. As illustrated in  FIG. 2 , the channel  34  has a center portion as well as two end portions that extend at a slight angle relative to the center portion. 
     Preferably, and as shown in  FIGS. 1-6 , the archwire slot  22  is defined on its occlusal, lingual and gingival sides by an archwire slot liner  36 . The archwire slot liner  36  is preferably made of a metallic material such as austenitic stainless steel. An example of a suitable material is Type 316L stainless steel having a thickness of about 0.05 mm. 
     The archwire slot liner  36  is preferably bonded to the mesial and distal sections  12 ,  14  by a braze material such as an alloy comprising silver, copper and at least 1.5% by weight titanium, based on the entire weight of the brazing alloy. Suitable brazing alloys are described in pending U.S. patent application Ser. No. 11/536,724 entitled “ORTHODONTIC BRACKET WITH BRAZED ARCHWIRE SLOT LINER” (Wyllie II, et al.). Other methods of attaching the archwire slot liner  36  to the sections  12 ,  14  are also possible, including the use of an adhesive, a mechanical interlock, an interference fit or any combination of the foregoing. 
     The bracket  10  also includes a mesial post  38  and a distal post  40  that are integrally connected to the mesial section  12  and the distal section  14  respectively. The posts  38 ,  40  extend outwardly in opposite directions away from each other. Preferably, and as shown, each post  38 ,  40  extends along a reference axis that is parallel to the longitudinal axis of the archwire slot  22 . As illustrated for example in  FIGS. 3 and 4 , the posts  38 ,  40  are located in a lingual direction relative to the archwire slot  22 . 
     Each of the posts  38 ,  40  includes an outermost head and a neck that integrally interconnects the head and the adjacent section  12 ,  14 . The archwire slot liner  36  includes a mesial extension that extends across the facial side of the mesial post  38  and a distal extension that extends across the facial side of the distal post  40 . The mesial post  38  extends through a mesial spring clip  42  and the distal post  40  extends through a distal spring clip  44 , and each clip  42 ,  44  has an opening that faces in a facial direction. 
     The clips  42 ,  44  comprise a latch for releasably retaining an archwire (not shown) in the archwire slot  22 . The clips  42 ,  44  are shown in their normal, relaxed orientation in the drawings, but have opposed arm portions that are movable away from each other in order to admit an archwire into the archwire slot  22  when desired. To this end, the arm portions include outer, curved edges that extend along the facial side of the sections  12 ,  14 . The opening of the clips  42 ,  44  enlarges to admit an archwire into the archwire slot  22  when an archwire is urged against the outer curved edges. 
     The clips  42 ,  44  are made of a resilient material such as alloys of nitinol or beta-titanium. The clips  42 ,  44  have inherent resiliency to enable the arm portions of the clips  42 ,  44  to move apart from each other a sufficient distance to permit passage of the archwire into the archwire slot  22 . Subsequently, this inherent resiliency enables the arm portions to spring back toward each other and toward their normal, relaxed configuration as shown in the drawings in order to thereafter retain the archwire in the archwire slot  22 . 
     The clips  42 ,  44  are sufficiently stiff to retain the archwire in the archwire slot  22  during the course of treatment so long as the forces exerted by the archwire on the bracket  10  are below a certain minimum value in a generally facial direction (more particularly in a direction opposite to the direction of insertion of the archwire into the archwire slot  22 ). However, whenever the forces exerted by the archwire on the bracket  10  in the same direction are greater than the minimum value, as might occur when unexpectedly high forces are encountered, the arm portions move apart from each other to open the clips  42 ,  44  and release the archwire from the archwire slot  22 . 
     The latch, comprising the spring clips  42 ,  44 , preferably releases the archwire from the archwire slot  22  in a generally facial direction whenever the archwire exerts a force in the same direction on the bracket  10  that is in the range of about 0.1 kg to about 5 kg, more preferably in the range of about 0.2 kg to about 2.5 kg, and most preferably in the range of about 0.34 kg to about 1.4 kg. Preferably, the minimum value is sufficiently high to prevent the archwire from unintentionally releasing from the archwire slot  22  during the normal course of orthodontic treatment. As such, the archwire can exert forces on the bracket  10  that are sufficient to carry out the treatment program and move the associated teeth as desired. 
     Preferably, the minimum value for self-release (i.e., self-opening) of the latch is substantially less than the force required in the same direction to debond the bracket  10  from the associated tooth. The minimum value for self-release of the latch is preferably less than about one-half of the force required in the same direction to debond the bracket from the associated tooth. For example, if the expected bond strength of the adhesive bond between the bracket  10  and the associated tooth is 7.2 kg in a facial direction, the latch is constructed to self-release the archwire whenever the archwire exerts a force in the same facial direction on the bracket  10  that is somewhat greater than about 3.6 kg. 
     To determine the force to release the latch, a section of archwire is selected having an area in longitudinally transverse sections that is complemental to (i.e., substantially fills) the cross-sectional area of the archwire slot  22 . Next, a sling is constructed and is connected to the archwire section at locations closely adjacent, but not in contact with, the heads of the posts  38 ,  40 . Optionally, the sling is welded or brazed to the archwire section. Next, the sling is pulled away from the bracket  10  while the bracket  10  is held in a stationary position, taking care to ensure that the longitudinal axis of the archwire section does not tip relative to the longitudinal axis of the archwire slot  22 . The force to release the latch may be determined by the use of an Instron testing apparatus connected to the sling, using a crosshead speed of 0.5 in/min (1.3 cm/min). Alternatively, a shaker apparatus (such as Model 300 from APS Dynamics of Carlsbad, Calif.) may be used along with a force transducer (such as Model 208C01 from PCB of Buffalo, N.Y.) to measure the force. 
     Other aspects of the clips  42 ,  44 , including methods of manufacture, are described in published U.S. patent application No. 2006/0147868 (Lai et al.). Optional aspects of the clips  42 ,  44  are described in U.S. Pat. No. 7,014,460 (Lai et al.). Additional aspects of the posts  38 ,  40 , including other aspects are related to the construction of the clips  42 ,  44 , are described in published U.S. patent application No. 2006/0024635 (Lai). Other examples of clips and latches are described in U.S. Pat. No. 6,302,688 (Jordan et al.) and 6,582,226 (Jordan et al.) as well as in published U.S. patent application No. 2006/0172249 (Lai et al.). 
     A line of weakness extends beneath lingual portions of the mesial and distal sections  12 ,  14 , and lies in a lingual direction relative to the channel  34 . In the illustrated embodiment, the line of weakness comprises a relatively thin frangible web  46  that integrally connects a lingual portion of the mesial section  12  to a lingual portion of the distal section  14 . The web  46  extends in a generally occlusal-gingival direction that preferably is parallel to the longitudinal axis of the center portion of the channel  34  and is directly adjacent the bottom of the channel  34 . 
     Other lines of weakness are also possible. For example, the mesial section and the distal section may be manufactured as initially separate components and the line of weakness may comprise a section of adhesive material that bonds the mesial and distal sections together. The line of weakness may also comprise an interference-fit coupling between the mesial and distal sections that bends or fractures during a debonding procedure. 
     The bracket  10  also includes an elongated recess  48  that extends between the external surface  16  of the mesial section  12  and the external surface  18  of the distal section  14 . The recess  48  has a bottom or facial-most surface that is spaced in a facial direction away from adjacent regions of the external surfaces  16 ,  18 . As one option, and as depicted in the drawings, the recess  48  has a generally semi-circular cross-sectional configuration in reference planes perpendicular to its longitudinal axis. An example of a suitable radius for the recess  48  in cross-sectional view is about 0.8 mm. However, other cross-sectional configurations are also possible, such as a generally “V”-shaped configuration. The recess  48  extends in a generally occlusal-gingival direction that preferably is parallel to the longitudinal axis of the web  46  and is directly adjacent the lingual side of the web  46 . 
     The web  46  has a thickness in a facial direction that is sufficiently small to enable the bracket  10  to be debonded from a tooth by pivoting at least one of the mesial and distal sections  12 ,  14  about a reference axis extending generally parallel to the longitudinal axis of the channel  34 . Examples of a suitable thickness of the web  46 , when measured in a facial direction adjacent the middle of the center portion of the channel  34 , include values in the range of about 0.36 mm to about 0.66 mm, depending upon other dimensions of the bracket  10  including the size of the archwire slot. 
     An exemplary hand instrument  60  for detaching the bracket  10  from a patient&#39;s tooth  10  is illustrated in  FIG. 6 . The hand instrument  60  includes a first jaw  62  and a second jaw  64  that is pivotally connected to the first jaw  62 . The hand instrument  60  also includes an elongated blade  70  that extends along a path between the jaws  62 ,  64 . 
     Only the outer tips of the jaws  62 ,  64  and the outer tip of the blade  70  are depicted in  FIG. 6 , and a more detailed description of the hand instrument  60  is set out in pending U.S. patent application Ser. No. 11/613,466 entitled “ORTHODONTIC HAND INSTRUMENT FOR DETACHING BRACKETS FROM TEETH” (Hart et al.). The outer tip of the first jaw  62  includes a first contact pad  66  and the outer tip of the second jaw  64  includes a contact pad  68 . Both of the contact pads  66 ,  68  are elongated and extend in directions parallel to each other and parallel to the pivot axis of the pivot (not shown) interconnecting the jaws  62 ,  64 . 
     When a practitioner desires to remove the bracket  10  from a tooth, the first contact pad  66  is placed in contact with the mesial side of the mesial clip  42  and the second contact pad  68  is placed in contact with the distal side of the distal clip  44 . Next, as handles of the hand instrument  60  are squeezed together, the contact pads  66 ,  68  are urged toward each other in a manner such that one or both of the sections  12 ,  14  pivot(s) toward each other in an arc and about a reference axis that is generally parallel to the longitudinal axis of the channel  34 . As one or both of the sections  12 ,  14  pivot away from the orientations illustrated in  FIG. 6 , the web  46  fractures and thereby enables one or both of the external surfaces  16 ,  18  to detach and move away from underlying areas of the patient&#39;s tooth. In instances where only one but not both of the sections  12 ,  14  detach from underlying areas of the tooth, the hand instrument  60  may be rocked by the user to detach the remaining section  12 ,  14 . 
     Additional aspects relating to ceramic brackets with lines of weakness such as frangible webs and other structure are described in U.S. Pat. Nos. 5,439,379 and 5,366,372. 
     The base  20  has an overall width in a mesial-distal direction and an overall height in an occlusal-gingival direction. In  FIG. 5 , the overall width of the base  20  in a mesial-distal direction is designated by the letter “x” while the overall height of the base  20  in an occlusal-gingival is designated by the letter “y”. The ratio of the overall width of the base  20  to the overall height of the base  20 , also known as the aspect ratio, is preferably less than about 0.95, more preferably less than about 0.90 and most preferably less than about 0.85. 
     Preferably, the overall width is less than about 3.6 mm. In general, brackets such as brackets  10  that are constructed for anterior teeth of the patient&#39;s lower dental arch may be narrower than other brackets due to the relatively narrow overall mesial-distal width of such teeth and as a result the aspect ratio of those brackets may be less than about 0.7. 
     The base also preferably has four smoothly-curved corners with a relatively large radius. An exemplary corner is designated by the numeral  50  in  FIG. 5 . Preferably, each of the four corners has a radius of curvature in the plane of the base  20  that is about 1.0 mm. 
     The base  20  has an overall area that is preferably in the range of about 7.5 mm 2  to about 11.5 mm 2 . The overall shape of the base  20  (including the aspect ratio, the area and the corners with a relatively large radius of curvature) combine to facilitate placement and bonding of the bracket  10  to the patient&#39;s tooth at the beginning of treatment as well as debonding of the bracket  10  from the patient&#39;s tooth at the conclusion of treatment. 
     More particularly, the relatively small aspect ratio of the bracket  10  of the present invention is an advantage during a debonding procedure because less force is needed to urge the sections  12 ,  14  in directions toward each other. For example, and with reference to  FIG. 6 , if a bracket were constructed similar to bracket  10 , and such hypothetical bracket had a base with an area identical to the area of bracket  10  but with a larger aspect ratio, the overall mesial-distal width of the base of such a bracket would be larger than the overall width of base  20 . As a result, a larger force (as can be represented by the larger resulting moment arm) is needed to overcome the forces of adhesion and pivot the mesial and distal sections than the force necessary to pivot the mesial and distal sections  12 ,  14  of the bracket  10  of the present invention. 
     With respect to the preferred embodiment illustrated in the drawings, the location of the clips  42 ,  44 , being outboard of the mesial and distal sections  12 ,  14  respectively, provides additional benefits. The thickness of the clips  42 ,  44  advantageously helps to increase the leverage provided by the jaws  62 ,  64  during a debonding procedure. Furthermore, the spacing between the posts  38 ,  40  enhances rotational control over movement of the associated tooth in instances when the posts  38 ,  40  are in contact with the archwire. 
       FIG. 7  is an illustration of a base  20  of an exemplary ceramic orthodontic bracket  100  known in the art. In  FIG. 7 , the overall width of the base  120  in a mesial-distal direction is designated by the letter “x′” while the overall height of the base  120  in an occlusal-gingival is designated by the letter “y′”. The aspect ratio of the prior art bracket base  120  illustrated in  FIG. 7  is approximately 1.3, while the aspect ratio of the base  20  shown in  FIG. 5  according to the present invention is about 0.85. Aspect ratios of other similar prior art brackets, but constructed for the relatively narrow anterior teeth of the lower dental arch, are known with aspect ratios of about 1.0. 
     In addition, the smaller aspect ratio of the brackets  10  of the present invention is an advantage due to the characteristics of the compound contour of typical human adult teeth. Such teeth usually have a radius of curvature that is greater in central reference planes perpendicular to a mesial-distal reference axis compared to the radius of curvature in central reference planes perpendicular to an occlusal-gingival reference axis. As a consequence, the risk of substantial misfit between the base and any given tooth that may be encountered in a particular instance is less than might otherwise occur in comparison to known prior art ceramic brackets. 
     Moreover, the smoothly curved corners  50  of the base  20  with a relatively large radius of curvature facilitate positioning of the bracket  10  on the patient&#39;s tooth. If, for example, the patient&#39;s tooth surface is less convex (i.e., has a flatter configuration) than expected, the base of the bracket will often contact the tooth surface at the four corners of the base. By providing corners  50  with a relatively large radius of curvature, it is less likely that the base  20  will tend to rock or “hang” on the corners. 
     Another advantage of the present invention is that removal of adhesive flash is facilitated. It has been observed that more flash tends to be extruded along occlusal and gingival edges of the base of brackets having a compound contoured base as compared to the amount of flash extruded from mesial and distal edges of the base of such brackets. The smaller aspect ratio of the brackets of the present invention leads to less, if any, projection or overhang of the tiewings  24 ,  26 ,  28 ,  30  past the perimeter of the base. As a result, it is easier to access the adhesive flash along the occlusal and gingival edges of the base with a hand instrument such as a scaler. 
     The bracket  10  (including the sections  12 ,  14 , the tiewings  24 ,  26 ,  28 ,  30 , the hook  32 , the posts  38 ,  40  and the web  46 ) are preferably made of a transparent or translucent ceramic material such as polycrystalline translucent aluminum oxide. Examples of suitable polycrystalline translucent ceramic materials are described in U.S. Pat. No. 6,648,638 (Castro et al.) and include aluminum oxide ceramic materials having an average grain size of no greater than 1.0 micron. Other ceramic materials are also possible, including the polycrystalline ceramic materials described in U.S. Pat. No. 4,954,080 (Kelly et al.) and single crystal ceramic materials (such as described in U.S. Pat. No. 4,681,538 to DeLuca et al.). 
     Those skilled in the art will recognize that other options, alternatives and additions are possible to the orthodontic brackets set out above without departing from the essence of our invention. Consequently, the invention should not be deemed limited to the specific embodiments described in detail above, but instead only by a fair scope of the claims that follow along with their equivalents.