An elastomeric orthodontic ligature for engagement with a tooth-mounted orthodontic appliance such as a bracket having conventional tie wings. In one application, the ligature is engaged behind and between upper and lower tie wings of the bracket, and extends over the bracket surface to hold an archwire in a slot of the bracket. The ligature differs from known elastic ligatures of flat torus shape in that it is arched or curved to simplify and speed initial engagement behind one of the bracket tie wings.

BACKGROUND OF THE INVENTION
 Orthodontic treatment of improperly positioned teeth involves the
 application of mechanical forces to urge the teeth of one or both dental
 arches into an alignment which provides correct occlusion and is
 cosmetically attractive. Most techniques use so-called orthodontic
 brackets which are small slotted metal or ceramic bodies shaped for direct
 cemented attachment to the front or rear surfaces of teeth, or
 alternatively for attachment to bands which are fitted over and cemented
 to the teeth.
 Most orthodontic brackets in current use are of an "edgewise" style as
 invented by Edward Angle in the 1920's. An edgewise bracket has a
 generally mesiodistally extending slot which opens away from the tooth
 surface on which the bracket is mounted, and is typically rectangular in
 cross section. A resilient curved archwire is seated in the bracket slot,
 and the wire is bent or twisted before installation so the resulting
 restoring force exerted by the seated archwire tends to shift, rotate or
 tip the associated tooth into a corrected position.
 The archwire must be somehow secured in the bracket slot to resist
 dislodging forces as imposed, for example, during brushing of teeth or
 chewing of food, or by the restoring force of the archwire itself. The
 bracket is formed with oppositely extending tie wings, around which some
 form of ligature can be fastened to extend over the seated archwire to
 hold it against movement out of the slot. Historically, small
 stainless-steel tie-wire ligatures have been used, and the installation
 and anchoring twisting of these tie wires is time consuming, sometimes
 uncomfortable for the patient, and requires considerable skill.
 An important improvement in ligation was made in the 1960's by Drs.
 Anderson and Klein, and is described in U.S. Pat. No. 3,530,583, the
 disclosure of which is incorporated herein by reference. The improvement
 is a torus or doughnut-shaped flat ring of circular cross section, and
 made of an elastomeric polymer such as polyurethane which is compatible
 with the environment of the mouth. The ring is stretched over the opposed
 tie wings, and extends over and against the seated archwire. The
 elastomeric ligature is generally easier and quicker to install than a
 wire ligature, and twisting of the wire ends (along with the risk of
 long-term tissue irritation) is of course eliminated. Flat toroidal rings
 of this type, and of noncircular cross section, have also been proposed
 (e.g., U.S. Pat. No. 3,758,947).
 Elastic ligatures nevertheless remain a challenge to install, as they must
 be angled upwardly or downwardly to be hooked over and behind the first
 tie wing (or an elastic hook if the bracket is so equipped) and similarly
 maneuvered to fit over and seat behind the opposed tie wing. Just as with
 wire ligatures, installation on posterior brackets is particularly
 difficult, and even the anterior brackets are awkward to engage due to
 interference with the patient's lips or gum tissue by tweezers or forceps
 which grip the ring during installation.
 The problem is compounded by a periodic need to remove and reinstall the
 ligatures when an archwire is changed or requires adjustment, or when the
 ligatures lose elasticity and restoring force, or become discolored.
 Elastic ligatures have also been made available in different colors which
 are appealing to younger patients, and sometimes ligatures are changed for
 "vanity" reasons when a child wants different colors. Every ligature
 change, for whatever reason, includes significant chair time, possible
 patient discomfort, and the time and attention of the orthodontist and
 assistants. It is to the solution of these ongoing problems that the
 present invention is directed.
 SUMMARY OF THE INVENTION
 The orthodontic ligature of this invention is an arched non-planar ring or
 loop with a central opening, and which is made of an elastomeric polymer
 such as polyurethane which is molded in the arched shape. In one form, the
 ring is folded in a V-shape when viewed from the side. In an alternative
 form, the ring cross section is substantially constant and circular, and
 the ring has inner and outer side surfaces which are generally C-shaped
 and are substantially concentric. Importantly, the arch or fold of the
 ligature is in the ring itself to avoid projections or tabs which can
 cause tissue irritation and other problems. de
 BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a perspective view of a arched elastomeric orthodontic ligature
 according to the invention;
 FIG. 2 is a front elevation of the ligature;
 FIG. 3 is a side view of the ligature;
 FIG. 4 is a sectional view on lines 4-4 of FIG. 2;
 FIG. 5 is a side view similar to FIG. 3, but showing an alternative
 outwardly flared center opening;
 FIG. 6 is similar to FIG. 5, but showing an inwardly flared center opening;
 FIG. 7 is a front elevation of a double ligature for applying tension force
 between two anchor points;
 FIG. 8 is a sectional view on lines 8-8 of FIG. 7;
 FIG. 9 is a perspective view of an orthodontic bracket on tooth, and
 showing a ligature of the invention positioned for initial engagement; and
 FIG. 10 is a sectional side view of the bracket shown in FIG. 9 with the
 ligature fully engaged.

DETAILED DESCRIPTION OF THE INVENTION
 An orthodontic ligature 10 according to the invention is shown in FIGS.
 1-4. The ligature is an arched, folded continuous ring or loop with a
 central opening 11, and inner and outer side surfaces 12 and 13 which are
 non-planar. The ligature is preferably molded in the shape shown in an
 elastomeric polymer such as polyurethane which is compatible with the
 environment of the mouth. A number of suitable alternative materials are
 described in U.S. Pat. No. 5,317,074, the disclosure of which is
 incorporated herein by reference.
 In the embodiment shown in FIGS. 1-4, central opening 11 is oval, but shape
 variations can easily be made to accommodate different styles of
 orthodontic brackets or other anchoring devices. For example, FIG. 5
 illustrates a ligature 10A with a central opening 11A which decreases in
 diameter toward outer surface 13A, and FIG. 6 shows a ligature 10B with an
 oppositely tapered opening 11B. Because the ring is molded, the central
 opening may also be circular, or any other desired shape. The edges of the
 hole may also be beveled for easier installation on specific styles of
 brackets.
 In one form, the ligature has a constant circular cross section as shown in
 FIG. 4, but other or variable cross-sectional shapes can also be used to
 accommodate different bracket and tiewing shapes, to minimize stress
 concentrations when stretched, and to vary the elastic properties of the
 ligature. Similarly, the inner and outer surfaces of the ligature are
 typically generally parallel as shown in FIG. 3, but other curvatures or
 angled bends can also be used.
 FIGS. 7 and 8 show a double ligature 16 with a pair of spaced-apart arched
 continuous rings 17 generally as described above, and which are connected
 by a strand 18 which is integrally molded with the rings. In this
 embodiment, the rings are C-shaped in side view, and the inner and outer
 surfaces are substantially concentric. Apart from the arched rings,
 ligature 16 is similar in function to a corresponding double ligature
 shown in the aforementioned U.S. Pat. No. 3,530,583 for either inter-arch
 or intra-arch applications, and in use, strand 18 is tensioned to apply a
 force urging together spaced-apart brackets or similar anchor points
 engaged with the rings.
 FIG. 9 illustrates the initial installation position of a ligature 10
 behind an upper tie wing 21 of an orthodontic bracket 22 secured to a
 tooth 23. The bracket has a mesiodistal slot in which is seated a portion
 of an archwire 24 which extends around the patient's dental arch. The
 archwire is illustrated with a circular cross section, but rectangular or
 other cross-sectional shapes are in common use. The ligature is gripped by
 a forceps or tweezer 26 which is moved toward the tooth, and then slightly
 downwardly to position the leading edge of the ligature behind the tie
 wing. The gripped end of the ligature is next moved downwardly over the
 archwire, and then toward the tooth so the gripped end of the elongated
 and tensioned ligature can be snapped in place behind a lower tie wing 27
 of the bracket. A side view of the thus-installed ligature is shown in
 FIG. 10.
 Importantly, the arched shape of the ligature enables the gripping forceps
 to be moved toward the tooth without uncomfortable interference with the
 gum, lip or cheek tissue during initial engagement. Ligature installation
 is accordingly quicker and easier, and with a much lower risk of patient
 discomfort.
 The dimensions of the ligature can be varied as necessary to be compatible
 with brackets or other anchoring devices of varying shape and size. A
 typical ligature for tying a rectangular archwire to a single-wing or twin
 orthodontic bracket will have an outside diameter (as measured along a
 central horizontal axis of FIG. 2) in the general range of about 0.090to
 0.125 inch, and the diameter of central opening 11 is in the general range
 of about 0.030 to 0.050 inch. A typical cross-sectional diameter of a ring
 of this type is about 0.03 to 0.04 inch. The term "ring" is used herein in
 the sense of a continuous loop which is not necessarily circular, and is
 more typically noncircular when viewed in front elevation as in FIG. 2.
 Referring again to FIG. 2 which is an arched-toroid embodiment of the
 invention, inner and outer side surfaces 12 and 13 are generally parallel,
 and the ligature ends define generally equal angles 29 and 30 with respect
 to a central axis 31 of opening 11. In a typical form compatible with many
 brackets, angles 29 and 30 are each in the range of about 45 degrees to 75
 degrees.
 There has been described a new arched continuous-loop elastic ligature
 which is much easier and faster to install than conventional flat
 torus-shaped ligatures, and provides significantly improved patient
 comfort during installation.