Abstract:
An aesthetic ceramic material is combined with a resilient metal to obtain an aesthetic self-ligating appliance with an easily operable latch mechanism. The appliance uses a door assembly that includes a ceramic ligating cover coupled to a resilient clip pivotable about a hinge mechanism. The clip functions as a latch to reversibly secure the clip and ligating cover to the appliance and retain an archwire in an archwire slot, and is capable of providing active ligation whereby the clip elastically deflects while directly or indirectly providing a continuous force to the archwire. Advantageously, the clip has a configuration that allows an orthodontic practitioner to pivot open the door assembly with minimal force while preventing the inadvertent opening of the door assembly as a result of normal forces applied by the archwire against the door assembly during the course of treatment.

Description:
FIELD OF THE INVENTION 
       [0001]    Provided are appliances useful for orthodontic treatment and methods related thereof. In particular, the provided appliances and methods relate to self-ligating orthodontic appliances. 
       BACKGROUND 
       [0002]    Orthodontic appliances are devices used in the professional supervision, guidance and correction of a patient&#39;s malpositioned teeth. The many benefits of orthodontic treatment include the attaining and maintaining of a proper bite function, enhancement of facial aesthetics, and easier maintenance of dental hygiene. Orthodontic appliances are placed in mechanical engagement with the patient&#39;s teeth and apply gentle mechanical forces that gradually move the teeth toward corrected positions to achieve a proper bite (or occlusion). 
         [0003]    A very common type of orthodontic treatment uses tiny slotted appliances called orthodontic brackets, which are adhesively attached to either the front or back surfaces of the patient&#39;s teeth. To move the teeth within an upper or lower arch, a resilient arch-shape wire (“archwire”) is mechanically engaged, or “ligated,” into the slot of each bracket. The ends of the archwire are generally captured in appliances called molar tubes, which are bonded to the patient&#39;s molar teeth. As the archwire slowly returns to its original shape, it acts as a track that guides the movement of teeth toward their desired positions. The brackets, tubes, and archwire are collectively known as “braces.” 
         [0004]    Conventional brackets are ligated to the archwire with the help of opposing tiewings, which are cleat-like projections on the bracket body. After the archwire is placed in the archwire slot, either a tiny elastomeric “O”-ring ligature or a metal ligature wire is looped over the archwire and beneath the undercut portions of tiewings located on opposite sides of the archwire slot. By tightly encircling the undercut portions of the tiewings, the ligature (or ligature wire) can secure the archwire within the archwire slot of each bracket, while still allowing the archwire to slide longitudinally along the slot. Depending on the relative sizes and shapes of the archwire and the slot, it is possible to achieve a precise mechanical coupling between the two bodies. This enables the practitioner to control the position and orientation of each individual tooth in the arch. 
         [0005]    Both of the ligating mechanisms above have certain drawbacks. For example, the frictional contact between O-ring ligatures and the archwire can increase resistance to archwire sliding within the slot. Moreover, the elastic properties of these ligatures can degrade over time, resulting in unpredictable sliding mechanics. While these ligatures can be made from translucent polymers for aesthetic treatment, these same ligatures also tend to stain in the presence of dark-colored foods and liquids. Ligature wire poses its own problems, since the process of tying and trimming the wire can be cumbersome and time-consuming for the orthodontic professional. Being made of metal, ligature wire is also considered non-aesthetic. 
         [0006]    Self-ligating brackets present a solution to at least some of the above problems. These appliances generally use a clip, spring member, door, shutter, bail, or other ligation mechanism built into the bracket itself to retain the archwire in the slot, thereby obviating use of a separate ligature. Several advantages can derive from the use of these ligation mechanisms. For example, these appliances can decrease friction between the archwire and the bracket compared with appliances ligated with elastomeric ligatures, potentially providing faster leveling and aligning of teeth in early stages of treatment. Depending on the mechanism, these appliances can also simplify the installation and removal of an archwire, significantly reducing chair time for the treating professional. Finally, self-ligating brackets can provide better hygiene than conventional brackets, which use elastomeric ligatures and ligature wires that can trap food and plaque. 
       SUMMARY 
       [0007]    Technical challenges stand in the way of achieving an aesthetic self-ligating bracket that is both easy to use and reasonably capable of being manufactured. For one, the materials used in a clip, spring member, door, bail, or other ligation mechanism are typically metallic, and strongly contrast with the natural color of teeth. While polymeric materials are aesthetic and can be configured for this function, polymers are generally soft, vulnerable to mechanical wear and fatigue, and stain easily during the course of treatment. Ceramic materials have long been known to provide good strength, resistance to staining, and excellent aesthetics. However, these materials tend to be brittle, can be difficult to machine and assemble, and do not have the resiliency needed for most ligation mechanisms. Molded ceramic parts are subject to further limitations, since such parts generally need to be readily removable from the mold. 
         [0008]    Aesthetic self-ligating appliances, particularly those made from ceramic materials, are also generally “passive” ligation devices. In passive ligation, the archwire is held captive within the slot but allowed to “float” freely within the archwire slot. Such a configuration can provide low friction between archwire and appliance but the freedom of movement within the archwire slot can compromise control. By contrast, in “active ligation,” the appliance imparts a continuous force urging the archwire toward the bottom wall or side wall of the slot. Active ligation can be desirable in some stages of treatment, particularly when using square and rectangular archwires, because “actively” seating these wires into the bracket slot can improve transmission of torque and rotational forces to the teeth. 
         [0009]    The aforementioned limitations can be addressed by combining different materials, each having certain preferred mechanical properties, to obtain an active self-ligating appliance with an easily operable ligation mechanism and superior aesthetics. The appliance uses a door assembly that includes a ceramic ligating cover coupled to a resilient clip pivotable about a hinge mechanism. The clip functions as a latch to reversibly secure the clip and ligating cover to the appliance and retain an archwire in an archwire slot. Alone or in combination with associated components of the door assembly, the clip can further provide for active ligation whereby the clip elastically deflects while imparting a continuous, positive force to the archwire. Advantageously, the clip can display force characteristics allowing an orthodontic practitioner to pivot open the door assembly easily while also preventing the inadvertent opening of the door assembly as a result of normal forces applied by the archwire against the door assembly during the course of treatment. 
         [0010]    In one aspect, an orthodontic appliance is provided. The orthodontic appliance comprises: a base having a outer surface adapted for bonding to a tooth; a body extending outwardly from the base in a direction away from the outer surface of the base and having an elongated archwire slot therein extending along a generally mesial-distal direction, the archwire slot having a bottom wall and a pair of sidewalls; a hinge coupled to the body and having a hinge axis extending along a generally mesial-distal direction; and a door assembly comprising: a ceramic ligating cover; and a resilient clip coupled to one or both of the ligating cover and the hinge, wherein the door assembly is pivotable along the hinge axis between an open position allowing access to the archwire slot and a closed position obstructing access to the archwire slot, the ligating cover substantially obscuring the clip when the appliance is in its closed position. 
         [0011]    As an option, the orthodontic appliance further comprises an air gap extending between the ligating cover and the clip, the air gap providing space for the clip to elastically deform in a direction away from the bottom wall of the archwire slot in active ligation. 
         [0012]    In another aspect, a method of activating an archwire in an orthodontic appliance is provided, the orthodontic appliance having a body with an elongated archwire slot having a bottom wall and pair of side walls therein and a latched door assembly including a resilient clip provided alongside a ligating cover presenting an air gap therebetween, and a hinge interconnecting the body and door assembly. The method comprises: placing the archwire in the archwire slot; and pivoting the door assembly about the hinge until the clip latches to the body, the clip resiliently deflecting into the air gap while the clip applies a compressive force urging the archwire towards the bottom of the archwire slot. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of an orthodontic assembly including an appliance and archwire segment according to one embodiment, looking toward its facial, mesial, and gingival sides; 
           [0014]      FIG. 2  is a plan view of the assembly of  FIG. 1 , looking toward its facial side; 
           [0015]      FIG. 3  is a gingival view of the assembly of  FIGS. 1-2 , looking toward its gingival side; 
           [0016]      FIG. 4  is an occlusal view of the assembly of  FIGS. 1-3 , looking toward its occlusal side; 
           [0017]      FIG. 5  is a mesial view of the assembly of  FIGS. 1-4 , looking toward its mesial side, with some internal components and features shown in phantom; 
           [0018]      FIG. 6  is a perspective view of the assembly of  FIGS. 1-5 , showing the appliance in an opened configuration and looking toward its facial, mesial, and gingival sides; 
           [0019]      FIG. 7  is a perspective view of an orthodontic assembly including an appliance engaged to an archwire segment according to another embodiment, looking toward its facial, mesial, and gingival sides; 
           [0020]      FIG. 8  is a plan view of the assembly of  FIG. 7 , looking toward its facial side; 
           [0021]      FIG. 9  is a cross-sectional view of the assembly of  FIGS. 7-8 , looking toward its mesial side; 
           [0022]      FIG. 10  is an occlusal view of the assembly of  FIGS. 7-9 , looking toward its occlusal side; 
           [0023]      FIG. 11  is a perspective view of the assembly of  FIGS. 7-10 , showing the appliance in an opened configuration and looking toward its facial, mesial, and gingival sides; 
           [0024]      FIG. 12  is a perspective view of an orthodontic assembly including an appliance engaged to an archwire segment according to still another embodiment, looking toward its facial, mesial, and gingival sides; 
           [0025]      FIG. 13  is a perspective view of the assembly of  FIG. 12 , showing the appliance in an opened configuration, and looking toward its facial, mesial, and gingival sides. 
       
    
    
     DEFINITIONS 
       [0026]    As used herein: 
         [0027]    “Mesial” means in a direction toward the center of the patient&#39;s curved dental arch. 
         [0028]    “Distal” means in a direction away from the center of the patient&#39;s curved dental arch. 
         [0029]    “Occlusal” means in a direction toward the outer tips of the patient&#39;s teeth. 
         [0030]    “Gingival” means in a direction toward the patient&#39;s gums or gingiva. 
         [0031]    “Facial” means in a direction toward the patient&#39;s lips or cheeks. 
         [0032]    “Lingual” means in a direction toward the patient&#39;s tongue. 
       DETAILED DESCRIPTION 
       [0033]    The sections below describe illustrative embodiments directed to self-ligating orthodontic appliances and methods related thereto. These embodiments are exemplary and accordingly should not be construed to unduly limit the invention. For example, it is to be understood that one of ordinary skill can adapt the disclosed appliances and methods for attachment to either the labial or lingual surfaces of teeth, to different teeth within the same dental arch (for example, corresponding appliances on mesial and distal halves of the dental arch), or to teeth located on either the upper or lower dental arches. 
         [0034]    The appliances and methods described herein may optionally be customized to the individual patient undergoing treatment. Material and dimensional specifications could also vary from those disclosed herein without departing from the scope of the claimed invention. Unless otherwise specified, the provided appliances and components could be constructed of any of a variety of metal, ceramic, polymeric, and composite materials known to those skilled in the art. Further, unless otherwise indicated, dimensions associated with the appliances and their components are not critical and the accompanying drawings are not necessarily drawn to scale. 
         [0035]    An aesthetic orthodontic bracket according to one embodiment is shown in  FIGS. 1-6  and designated by the numeral  100 . In each of these figures, the appliance  100  is shown engaged to a segment of an exemplary archwire  50 . Referring to  FIG. 1 , the appliance  100  has a base  102  having an outer surface  104  adapted for adhesive bonding to a patient&#39;s tooth. Preferably and as shown, the outer surface  104  is concave and substantially conforms to the convex outer surface of the tooth. 
         [0036]    In preferred embodiments, the outer surface  104  includes metal mesh, holes, bumps, recesses, undercuts, a microetched surface, glass grit, bonded particles, an organo-silane treated surface, or any other known mechanical or chemical modification to enhance adhesive bonding between the base  102  and the underlying tooth. Alternatively, the base  102  could also have a banded configuration in which the base  102  fully encircles the tooth to provide an even stronger bond. 
         [0037]    Extending outwardly from the base  102  in a direction away from the outer surface  104  of the base  102  is a body  106 . Optionally and as shown, the base  102  and body  106  are integral components made from an aesthetic material. For example, the base  102  and body  106  could be machined or molded from a polymeric material as disclosed in U.S. Pat. No. 4,536,154 (Garton, et al.), a ceramic material such as a fine-grained polycrystalline alumina as disclosed in U.S. Pat. No. 6,648,638 (Castro, et al.), or a polymer-ceramic composite such as glass-fiber reinforced polymeric composites as disclosed in U.S. Pat. No. 5,078,596 (Carberry, et al.) and U.S. Pat. No. 5,254,002 (Reher, et al.). 
         [0038]    The body  106  has a facial surface  108  and an elongated archwire slot  110  located extending in a generally mesial-distal direction across the facial surface  108 . Referring now to the mesial view in  FIG. 5 , the archwire slot  110  has a bottom wall  112  along with occlusal and gingival side walls  114 . As shown, the archwire  50  is received in the archwire slot  110  and has a generally rectangular cross-section that substantially corresponds with walls  112 ,  114  of the archwire slot  110 . Filling the archwire slot, as is shown here, can provide for a precise coupling between the archwire  50  and appliance  100  and give the treating practitioner a high degree of control over the movement of teeth. Of course, other archwire geometries can be used. 
         [0039]    As shown in  FIGS. 1 ,  5 , and  6 , a hinge  116  is coupled to the body  106 , and a door assembly  119  is coupled to the hinge  116  whereby the archwire  50  is held captive in the archwire slot  110 . In the configuration shown, the archwire  50  is securely ligated to the appliance  100  such that the archwire  50  will not become accidently dislodged as a result normal chewing and brushing activity that occurs in a patient&#39;s mouth. However, the archwire  50  can, and should, be capable of sliding along the length of the archwire slot  110 , thereby allowing the archwire  50  to function as a track that guides the movement of maloccluded teeth. Such sliding is especially important as the teeth unravel during the leveling and aligning stages of treatment. 
         [0040]    The appliance  100  has a configuration that provides for traditional methods of ligation. As shown in  FIG. 1 , a gingival undercut  132  and occlusal undercut  133  are located on respective gingival and occlusal sides of the body  106 . Undercuts  132 ,  133  provide areas where an elastic “O”-ring ligature, powerchain, or ligature wire can be secured to retain the archwire  50  in the archwire slot  110 . Although not critical for treatment, independent ligation can be useful, for example, when closing gaps (e.g. using a powerchain) or intentionally creating friction (e.g. using elastic ligatures) during the finishing stage of treatment. 
         [0041]    In more detail, the door assembly  119  includes a ligating cover  120  and a resilient clip  122 , each independently coupled to the hinge  116 . The hinge  116  is provided by a simple cylindrical hinge pin  117 , operatively coupled to both the clip  122  and the body  106 . The hinge pin  116  also has a longitudinal hinge axis  118  that extends along a generally mesial-distal direction, allowing relative rotation of the body  106  about the hinge axis  118  relative to the ligating cover  120  and the clip  122 . It is not necessary that the hinge  116  use the hinge pin  117 . Alternatively, for example, the body  106  and door assembly  119  could be connected to each other by a flexible polymeric membrane. 
         [0042]    The ligating cover  120  is made from a non-staining ceramic material that is optionally the same material used to construct the base  102  and body  106 . The ligating cover  120  has a facial surface  123  that has a generally rectangular shape, similar to that of the appliance  100  as a whole when viewed from the facial direction. Optionally and as shown, the facial surface  123  has a vertical alignment groove  124  extending across the facial surface in a generally occlusal-gingival direction. Advantageously, the alignment groove  124  can assist the practitioner in positioning the appliance  100  on the tooth during a bonding procedure. 
         [0043]    Referring now to  FIG. 5 , the clip  122  comprises a shaft portion  126  having an eyelet  128  on its occlusal end and a hook portion  130  on its gingival end, resulting in the clip  122  having a generally “J”-shaped configuration. The eyelet  128  has an aperture  129 , allowing the hinge pin  117  to extend through the clip  122 . The hook portion  130  functions as a latch by engaging the gingival undercut  132  on the body  106  when the door assembly  119  is in its closed position. In this position, the hook portion  130  is retained by an interference fit with the gingival undercut  132 . 
         [0044]    The clip  122  is preferably made from a resilient metal alloy, such as stainless steel, titanium, cobalt-chromium alloy (such as manufactured by Elgiloy Specialty Metals, Elgin, Ill.), or a shape-memory alloy such as an alloy of nickel and titanium (e.g. Nitinol). Preferably, the clip  122  is sufficiently resilient so that the shape of the clip  122  when relaxed does not significantly change during the course of treatment. As another option, the clip  122  could be made from any other resilient material known to one skilled in the art, such as a flexible polymer or composite material. 
         [0045]    Preferably, the ligating cover  120  and clip  122  interconnected. Moveover, these components may be either adhesively or mechanically coupled to each other. The latter approach is shown in phantom in  FIG. 5 , where the clip  122  further includes a tab  136  extending outwardly, in a generally facial direction, from the shaft portion  126 , and a set pin  138  extends through both the ligating cover  120  and the tab  136 . With the ligating cover  120  and the clip  122  mutually fastened by the set pin  138  on one end and the hinge pin  117  on the other end, these components are fixed relative to each other and, as a result, jointly rotate about the hinge axis  118  when operating the appliance  100 . 
         [0046]      FIG. 4  shows another view of the mechanism of the hinge  116 . As shown in phantom, the hinge pin  117  comprises a central section  140  and a pair of end sections  142 , the central section  140  extending through the door assembly  119  and the end sections  142  extending through the body  106 . More particularly, the central section  140  of the hinge pin  117  has three subsections—a central subsection  144  and a pair of end subsections  146 , the central subsection  144  extending through the clip  122  and the end subsections  146  extending through the ligating cover  120 . This is merely an exemplary configuration, however, and others are also possible. For example, as an alternative, the central subsection could extends through the ligating cover  120  and the end subsections  146  could extend through the clip  122 . 
         [0047]    Two additional characteristics of the appliance  100  warrant mention. 
         [0048]    First, the directionality of the hinge mechanism, as shown in the figures, can help minimize the chance of accidentally opening the door assembly  119  during mastication, since the door assembly  119  opens towards a direction away from the occlusal teeth surfaces. It should be understood, however, that the occlusal and gingival directions could easily be reversed, if desired, without affecting the operation of the appliance  100 . 
         [0049]    Second, the aesthetics of the appliance  100  is greatly enhanced by virtue of the ligating cover  120 . The ligating cover  120  has a mesial-distal width that is at least that of the clip  122 , and thus substantially obscures the clip  122  when the door assembly  119  is in its closed position. As illustrated in  FIG. 2 , for example, the ligating cover  120  extends over the facial surfaces of the clip  122 , causing the clip  122  to be obscured when viewed from the facial direction. 
         [0050]    Beginning with the door assembly  119  in its closed position as shown in  FIGS. 1-5 , the appliance  100  can be operated by inserting the pointed tip of a hand instrument into the gingival undercut  132  next to the hook portion  130  of the clip  122 . Then, by engaging the lingual-facing edge of the ligating cover  120  and then applying a gentle force in the facial direction, the terminal end of the hook portion  130  will elastically deflect toward the gingival direction, releasing the clip  122  from its interference fit with the undercut  132 . With continued nudging with the hand instrument, the entire door assembly  119  can be easily pivoted about the hinge axis  118  until it reaches the configuration shown in  FIG. 6 . In this position, access to the archwire slot  110  is allowed whereby archwire  50  can be removed and/or replaced as the practitioner sees fit. Subsequently, reversing the forces on the appliance  100  above can cause the door assembly  119  to pivot back to its natural closed position to obstruct access to the archwire slot  110 . Conveniently, the door assembly  119  can be closed using the tip of a finger without need for a hand instrument. 
         [0051]    The door assembly  119  can also be opened by inserting into the undercut  132  a flat instrument, having a tip shaped similarly to that of a flat-head screwdriver, and then rotating the instrument along its longitudinal axis. The rotary motion advantageously allows the flat instrument to cam open the door assembly  319  while reducing the risk of hyperextending the same. 
         [0052]    Preferably, the force required to open the door assembly  119  is sufficiently low to enable easy operation by a practitioner but also sufficiently high such that the door assembly  119  does not spontaneously disengage during normal patient activity that occurs during treatment, such as chewing and toothbrushing. Preferably, the threshold amount of upward (facial) force applied at the gingival undercut  132  to open the door assembly is at least about 0.9 newtons (0.2 lbf), at least about 2.2 newtons (0.5 lbf), or at least about 4.4 newtons (1 lbf). The threshold force is preferably up to about 5.3 newtons (1.2 lbf), up to about 6.7 newtons (1.5 lbf), or up to about 8.9 newtons (2 lbf). 
         [0053]      FIG. 6  shows the appliance  100  with the door assembly  119  fully opened, revealing further aspects of the ligating cover  120 , clip  122 , and body  106  ordinarily hidden during treatment. As shown, upper and lower channels  150 ,  152  extend along occlusal-gingival directions on the lingual-facing surface of the ligating cover  120  and the facial-facing surface of the body  106 , respectively. With the door assembly  119  closed, the clip  122  is sandwiched between the ligating cover  120  and the body  106 , the clip  122  at least partially residing in one or both of the channels  150 ,  152 . Optionally and as shown, the side walls of the channels  150 ,  152  closely conform to the mesial and distal sides of the clip  122 , and help prevent mesial or distal excursion of the clip  122  as the door assembly  119  is opened and closed. 
         [0054]    Significantly, the clip  122  does not abut against the upper channel  150  of the ligating cover  120  when in its relaxed configuration. Instead, as shown in  FIGS. 5 and 6 , the clip  122  is suspended alongside the ligating cover  120  such that a narrow air gap  154  extends along substantially all of the occlusal-gingival length of the channel  150  between the ligating cover  120  and the clip  122 . As shown in  FIG. 5 , when a sufficiently large archwire is received in the archwire slot  110 , the clip  122  can resiliently deflect away from the bottom wall  112  of the archwire slot  110  and at least partially into the air gap  154 . As a result of this elastic deformation, the ligation provided by the clip  122  becomes “active,” characterized by the clip  122  exerting a continuous force toward a generally lingual direction on the archwire  50  during the course of treatment. 
         [0055]    “Active ligation” (as opposed to “passive ligation”) occurs when a slotted orthodontic appliance imparts a continuous force urging the archwire toward the bottom wall (or sometimes side wall) of the slot. In later stages of treatment, when larger-sized square and rectangular archwires are typically used, “actively” seating these wires into the bracket slot can result in a more effective expression of the appliance prescription. In theory, active ligation can better transmit, for example, torque and rotational forces to the teeth. Another potential benefit of active ligation is the effect of storing some of the therapeutic force in the clip, as well as in the archwire. Some practitioners believe, in general terms, that a given wire will thus have its range of facial-lingual action increased and, therefore, produce more effective alignment than it would in a passively-ligated configuration. 
         [0056]    Preferably, the facial-gingival dimension of the archwire slot (with the door assembly  119  in its closed position) enables the appliance  100  to provide active ligation when the archwire  50  exceeds a certain pre-determined facial-lingual cross-sectional dimension. The facial-gingival dimension could also be based on enabling active ligation when there is at least some pre-determined degree of angular deviation between the archwire slot  110  and archwire  50 . In some embodiments, the archwire slot  110  has a facial-gingival clearance, as measured between opposing surfaces of the bottom wall  112  and the clip  122  when the door assembly  119  is closed, of at least about 640 micrometers (25 mil), at least about 660 micrometers (26 mil), or at least about 690 micrometers (27 mil). The facial-gingival clearance could be up to about 710 micrometers (28 mil), up to about 740 micrometers (29 mil), or up to about 840 micrometers (33 mil). 
         [0057]    The facial-lingual width of the air gap  154  determines, in part, the range of archwire motion and/or size dimensions over which the active ligation is possible. It may be advantageous, in some cases, to use a larger width for the air gap  154  where it is desired to shift the balance between the amount of force provided by deflection of the clip  122  and the amount of force provided through deflection the archwire  50 . The air gap  154  can have, for example, a facial-lingual thickness of at least about 25 micrometers (1 mil), at least about 50 micrometers (2 mil), or at least about 80 micrometers (3 mil). The air gap  154  could also have a facial-lingual thickness of up to about 250 micrometers (10 mils), up to about 380 micrometers (15 mil), or up to about 510 micrometers (20 mil). 
         [0058]    The appliance  100  includes other optional advantageous features. For example, as shown in  FIGS. 4 and 6 , the appliance  100  also has a debonding groove  160  located on the facial surface  108  of the body and extending along a generally occlusal-gingival direction. The debonding groove  160  approximately bisects the appliance  100  into mesial and distal halves, and can be used to facilitate squeeze debonding of the appliance  100  from the tooth (by inducing a controlled fracture along the debonding groove  160 ) at the end of treatment. Optionally, such debonding could be carried out by opening the door assembly  119 , and then using a suitable instrument (such as How or Weingart pliers) to squeezes the mesial and distal sides of the body  106  toward each other. Further options and advantages of squeeze debonding of the appliance  100  can be found in issued U.S. Pat. No. 5,439,379 (Hansen). 
         [0059]    In another exemplary embodiment,  FIGS. 7-11  show an orthodontic appliance  200  engaged to an archwire  50 ′ and having many of the same features described with respect to appliance  100 . The appliance  200 , however, offers some added benefits as will be described below. 
         [0060]    Referring to  FIG. 7 , the appliance  200  has a base  202  and a body  206  similarly configured to those shown for the appliance  100  in  FIGS. 1-6 . Appliance  200  also has a door assembly  219  that includes a ligating cover  220  and resilient clip  222 , the door assembly  219  jointly pivoting about a hinge  216  using hinge pin  217 . The clip  222  differs significantly from the clip  122  in its overall size and shape, and manner of connecting to the ligating cover  220 . 
         [0061]    First, and as shown in the opened configuration shown in  FIG. 11 , the clip  222  has a generally “U”-shaped configuration, having two generally parallel shaft portions  226 , each of the shaft portions  226  terminating in an eyelet  228  fastened to the hinge pin  217 , as shown in  FIGS. 9 and 11 . On the opposite end of each shaft portion  226  is a hook portion  230  having characteristics similar to the earlier described clip  122 . Unlike the clip  122 , however, the clip  222  also has a generally straight connector portion  229  extending along a generally mesial-distal direction and interconnecting the terminal ends of the hook portions  230  as shown in  FIG. 11 . 
         [0062]    Second, the shaft portions  226  of the clip  222  are mechanically coupled to the ligating cover  220  by a set of flanges  270  that are located on the ligating cover  220  and extend along opposite-facing sides of the clip  222 . The depicted embodiment in  FIG. 11  shows four short flanges  270  on the lingual side of the clip  222  and two long flanges on the facial side of the clip  222 . By consequence of the interference fit between these components, the ligating cover  220  and the clip  222  jointly rotate about the hinge pin  217  during operation of the appliance  200 . Although not illustrated, other methods of coupling the ligating cover  220  and clip  222  are possible, including use of a set pin or adhesive. 
         [0063]    One of the advantages of using a non-planar clip, as embodied in the clip  222 , is increased mesial-distal length along which the archwire  50 ′ can contact the door assembly  219 . Because the door assembly  219  can engage the archwire  50 ′ at two locations that are spaced apart from each other along a mesial-distal direction, it is possible to reduce angular slop in the archwire  50 ′ and achieve greater rotation control than otherwise achievable by engaging the archwire  50 ′ at a single location. Like in the appliance  100 , the clip  222  of the appliance  200  can provide for active ligation when the archwire  50 ′ has a sufficiently large facial-lingual dimension. 
         [0064]    The cross-sectional view of  FIG. 9 , taken along the section  9 - 9  in  FIG. 8 , shows in greater detail the interaction between the appliance  200  and the archwire  50 ′. As shown, the terminal end of the hook portions  230 , along with the connector portion  229 , can precisely snap into an undercut  232  located on the gingival side of the body  206 . As a further option, the lingual underside of the clip  222  could include a shallow relief  280  (as shown) to tailor further the degree of space available for the archwire  50 ′. It is also possible, if desired, to adjust the spacing of the air gap  254  between the clip  222  and the facing surface of the ligating cover  220  to increase or decrease the degree of force that can be provided by the appliance  200  in an active ligation configuration. Similar benefits apply with respect to the appliance  100 . 
         [0065]    Another benefit, as shown in  FIG. 7 , is the creation of a generally rectangular recess  290 , located on the gingival side of the appliance  200 , to assist in operating the appliance  200 . The recess  290  is collectively defined by the hook portion  230 , connector portion  229 , and the ligating cover  220 . The recess  290  is sufficiently sized to accommodate the tip of a hand instrument for operating the door assembly  219 . The recess  290  is advantageously located at or near the mesial-distal midpoint of the gingival side of the appliance  200 , allowing forces imparted by a hand instrument to be distributed evenly to the hook portions  230  and shaft portions  226 . 
         [0066]      FIGS. 12 and 13  show an appliance  300  engaging an archwire  50 ″, according to still another embodiment, in respective open and closed configurations. The appliance  300  has many of the same features as the appliance  200 , including a base  302 , body  306  with an archwire slot  310 , and door assembly  319 . However, unlike the appliances  100 ,  200 , the appliance  300  includes a door assembly  319  having a pair of planar, generally “J”-shaped clips  322   a ,  322   b  disposed on the mesial and distal sides of a ligating cover  320 , respectively. As shown in  FIG. 13 , each clip  322   a ,  322   b  has a hook portion  330   a ,  330   b  for releasably engaging an undercut  332  on the gingival side of the body  306 . 
         [0067]    Certain potential benefits can be realized with a dual-clip configuration. For example, using a pair of planar clips  322   a ,  322   b  instead of a single integrated clip can help facilitate manufacturing. Further, the lack of a connector portion between the clips  322   a ,  322   b  provides for a slightly larger recess to accommodate a hand instrument for operating the door assembly  319 . Finally, implementing a pair of clips instead of a singular clip can also decrease the effective force required to open the door assembly  319 , since it is possible for clips  322   a ,  322   b  to disengage individually from corresponding undercut  332 . 
         [0068]    It is to be understood that many aspects of the appliances  200 ,  300  are analogous to those of the appliance  100  as previously described. Accordingly, corresponding options and features of the appliances  200 ,  300  will not be repeated. 
         [0069]    One of the unexpected advantages of the provided appliances  100 ,  200 ,  300  relates to the much lower labial force needed to open the door assembly  119 ,  219 ,  319  when applied at the terminal end of the clip  122 ,  222 ,  322 , compared with the force needed when applied at the archwire slot  110 ,  210 ,  310 . It was discovered that the clip  122 ,  222 ,  322 , when deflected into a curved shape by the archwire, adopts a configuration that substantially increases the force required to disengage the clip  122 ,  222 ,  322  from the corresponding undercut  132 ,  232 ,  332 . As a result of this deflection, the actual force required to open the door assembly  119 ,  219 ,  319  is substantially greater at the archwire slot  110 ,  210 ,  310  than would be predicted geometrically by treating the door assembly  119 ,  219 ,  319  as a simple lever arm. 
         [0070]    The forces needed to unlatch the door assembly  119 ,  219 ,  319  can be quantified as follows. Unlatching the door assembly  119 ,  219 ,  319  by pivoting it from its closed to its open position requires a first minimum force applied to the clip  122 ,  222 ,  322  at the archwire slot  110 ,  210 ,  310  in a direction substantially perpendicular to the bottom wall of the archwire slot  110 ,  210 ,  310 . Furthermore, the act of unlatching the door assembly  119 ,  219 ,  319  requires a second minimum force when applied to the clip  122 ,  222 ,  322  at its hook portion  130 ,  230 ,  330   a ,  330   b  (in the same direction). In some embodiments, the first minimum force exceeds the second minimum force by a factor of about 2.5, about 3, about 3.5, about 4, about 4.5, about 5, or about 6. 
         [0071]    For the foregoing reasons, the configuration of the clip  122 ,  222 ,  322  provides a novel answer to the technical challenge of allowing the practitioner to easily open the door assembly  119 ,  219 ,  319  using a low threshold force at the gingival end of the door assembly  119 ,  219 ,  319  (e.g. using the tip of a hand instrument at the recess  290 ), while avoiding spontaneous disengagement of the archwire by imposing a relatively high threshold force to open the door assembly  119 ,  219 ,  319  at the archwire slot  110 ,  210 ,  310 . 
         [0072]    The comparative forces above can be simulated using, for example, finite element analysis (“FEA”). In the exemplary appliance  100 , the threshold facial force required to open the door assembly  119  was simulated using ANSYS FEA software (ANSYS, Inc., Canonsburg, Pa.) to be about 4.9 newtons (1.1 lbf) at the set pin  138  of the clip  122 , compared to about 28 newtons (6.2 lbf) at the archwire slot  110 . This approximate 6:1 force ratio compares to a ratio of about 2:1 force ratio based on linear distance from the hinge axis  118 . 
         [0073]    Various additional embodiments A-Z are provided below: 
         [0000]    A. An orthodontic appliance having: a base having an outer surface adapted for bonding to a tooth; a body extending outwardly from the base in a direction away from the outer surface of the base and having an elongated archwire slot therein extending along a generally mesial-distal direction, the archwire slot having a bottom wall and a pair of sidewalls; a hinge coupled to the body and having a hinge axis extending along a generally mesial-distal direction; and a door assembly having: a ceramic ligating cover; and a resilient clip coupled to one or both of the ligating cover and the hinge, where the door assembly is pivotable along the hinge axis between an open position allowing access to the archwire slot and a closed position obstructing access to the archwire slot, the ligating cover substantially obscuring the clip when the door assembly is in its closed position.
 
B. The appliance of embodiment A, further having an air gap extending between the ligating cover and the clip, the air gap providing space for the clip to elastically deform in a direction away from the bottom wall of the archwire slot in active ligation.
 
C. The appliance of embodiment B, where the air gap has a facial-lingual thickness ranging from about 25 micrometer to about 510 micrometers.
 
D. The appliance of embodiment C, where the air gap has a facial-lingual thickness ranging from about 50 micrometers to about 380 micrometers.
 
E. The appliance of embodiment D, where the air gap has a facial-lingual thickness ranging from about 80 micrometers to about 250 micrometers.
 
F. The appliance of any of embodiments A-E, where one or both of the base and body comprise a ceramic material.
 
G. The appliance of embodiment F, where the ceramic material includes polycrystalline alumina
 
H. The appliance of any of embodiments A-G where the clip includes a shape-memory alloy.
 
I. The appliance of any of embodiments A-H where the hinge includes a hinge pin operatively coupled to both the clip and the body, where the hinge axis is the longitudinal axis of the hinge pin.
 
J. The appliance of embodiment I, where the hinge pin includes a central section and a pair of opposing end sections, the central section extending through the door assembly and the end sections extending through the body.
 
K. The appliance of embodiment J, where the central section includes a central subsection and a pair of end subsections, the first subsection extending through the clip and the end subsections extending through the ligating cover.
 
L. The appliance of embodiment J, where the central section includes a central subsection and a pair of end subsections, where the central subsection extends through the ligating cover and the end subsections extend through the clip.
 
M. The appliance of any of embodiments A-L, where the clip includes a shaft portion and a hook portion joined at one end of the shaft portion, thereby providing a generally “J”-shaped configuration.
 
N. The appliance of embodiments M, where the clip further includes a tab extending outwardly from the shaft portion and further having a set pin extending through both the ligating cover and the tab to allow the ligating cover and the clip to jointly rotate about the hinge axis.
 
O. The appliance of embodiment M, further having an undercut on either an occlusal or gingival side of the body, the hook portion retained by an interference fit with the undercut when the door assembly is in its closed position.
 
P. The appliance of embodiment O, the door assembly further having a recess collectively defined by the hook portion and the ligating cover when the door assembly is in its closed position, where the recess is sufficiently sized to accommodate the tip of a hand instrument for operating the door assembly.
 
Q. The appliance of embodiment P, where toggling the door assembly from its closed to its open position requires a first minimum force when applied to the door assembly at the archwire slot and a second minimum force when applied to the door assembly at the recess, the first minimum force exceeding the second minimum force by a factor of about 2.5.
 
R. The appliance of embodiment Q, where the first minimum force exceeds the second minimum force by a factor of about 4.
 
S. The appliance of embodiment R, where the first minimum force exceeds the second minimum force by a factor of about 6.
 
T. The appliance of any of embodiments A-S, further having a plurality of flanges located on the ligating cover and extending along opposite-facing sides of the clip whereby the ligating cover and the clip jointly rotate about the hinge axis.
 
U. The appliance of any of embodiments A-T, where the ligating cover has a mesial-distal width of at least the mesial-distal width of the clip.
 
V. The appliance of any of embodiments A-U, where the archwire slot has a facial-lingual clearance ranging from about 640 to about 740 micrometers, as measured between opposing surfaces of the bottom wall and the clip, when the door assembly is in its closed position.
 
W. A method of activating an archwire in an orthodontic appliance having a body with an elongated archwire slot having a bottom wall and pair of side walls therein and a latched door assembly including a resilient clip provided alongside a ligating cover presenting an air gap therebetween, and a hinge interconnecting the body and door assembly, the method including: placing the archwire in the archwire slot; and pivoting the door assembly about the hinge until the clip latches to the body, the clip resiliently deflecting into the air gap while the clip applies a compressive force urging the archwire towards the bottom of the archwire slot.
 
X. The method of embodiment W, where unlatching the door assembly from its closed to its open position requires a first minimum force applied at the archwire slot in a direction substantially perpendicular to the bottom wall and a second minimum force applied at the terminal end of the clip also in a direction substantially perpendicular to the bottom wall, the first minimum force exceeding the second minimum force by a factor of about 2.5.
 
Y. The appliance of embodiment X, where the first minimum force exceeds the second minimum force by a factor of about 4.
 
Z. The appliance of embodiment Y, where the first minimum force exceeds the second minimum force by a factor of about 6.
 
         [0074]    All of the patents and patent applications mentioned above are hereby expressly incorporated into the present disclosure. The foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding. However, various alternatives, modifications, and equivalents may be used and the above description should not be taken as limiting in the scope of the invention which is defined by the following claims and their equivalents.