WO1999037237A1

WO1999037237A1 - Orthodontic bracket

Info

Publication number: WO1999037237A1
Application number: PCT/US1999/001702
Authority: WO
Inventors: David A. Cheselske
Original assignee: R & D Molded Products, Inc.
Priority date: 1998-01-27
Filing date: 1999-01-27
Publication date: 1999-07-29
Also published as: AU2564099A

Abstract

An improved orthodontic bracket is provided that includes a bracket base having undercut surfaces which receive adhesive so that once cured, the adhesive provides a mechanical gripping force to hold the bracket against the patient's tooth. An adhesive can be selected for its ability to bond to the patient's tooth with little regard to the adhesive's ability to bond to the bracket base. According to this design, even if the bond between the adhesive and the bracket base fails, the mechanical gripping force provided between the undercut surfaces of the bracket base and the cured adhesive, which is securely bonded to the tooth's surface, prevents the bracket from being dislodged from the tooth's surface. The undercuts can be provided in a number of different forms. In one form, undercuts are provided in an insert that is provided inside the bracket base. In another form, undercuts are provided directly in apertures provided in the base. In yet another form, pairs of angled apertures cooperate to form channels in the bracket base which provide the mechanical gripping undercut surfaces.

Description

ORTHODONTIC BRACKET

FIELD OF THE INVENTION

This invention is directed to an orthodontic bracket with characteristics that improve its ability to securely bond to a patient's tooth.

BACKGROUND OF THE INVENTION The present industry method for holding an orthodontic bracket to a tooth is based on adhesion between a base surface of the bracket and the tooth. Strong adhesion is essential as significant stresses are placed on a patient's teeth during orthodontic treatment. If a bond fails, the orthodontic treatment can be set back significantly.

One problem in obtaining a reliable bond between an orthodontic bracket and a tooth surface is that the two surfaces to be bonded to one another generally have different physical properties. Therefore, an adhesive that bonds well to the plastic or metal surfaces of a typical orthodontic bracket may not necessarily bond well to the surface of a tooth. Likewise, an adhesive selected for its ability to bond well with the surface of a tooth may not bond well to the orthodontic bracket. In order to improve adhesion between the bracket and tooth, some orthodontic brackets employ a roughened or textured dental bonding surface. Other orthodontic brackets have been designed with porous dental bonding surfaces which also help to prevent bond failure. However, such brackets can be complicated and expensive to produce in large quantities. Therefore, while such brackets have improved bonding characteristics and reduce the incidence of bond failure, there is still room for improving the bonding reliability between orthodontic brackets and tooth surfaces.

SUMMARY OF THE INVENTION

According to the present invention, an improved orthodontic bracket is provided which, due to its design, takes advantage of both adhesion and mechanical forces between the adhesive and the orthodontic bracket to reliably and durably bond the bracket to a patient's tooth. According to this design, should the adhesion between the tooth and the orthodontic bracket fail, the bracket will still be held to the tooth by mechanical forces between the adhesive and the orthodontic bracket. According to the invention, a conventional arch wire support bracket is provided on an orthodontic bracket base. The bracket base is adapted to be bonded to the surface of a patient's tooth. The bracket base includes one or more orifices with undercut surfaces into which the adhesive can flow during installation so that once cured, in addition to providing a conventional bond between the tooth and bracket, the cured adhesive provides a mechanical gripping force between the tooth and the bracket base. More specifically, the undercut surface of the orifice provides a ridge or lip with an inner surface that faces away from the dental bonding surface of the bracket base to form an interlocking grip that mechanically holds the bracket base to the tooth.

The orifices provided in a bracket base can take a number of forms. In one embodiment, the orifice penetrates the dental bonding surface of the bracket base with a circular hole of a first diameter which expands to a second diameter larger than the first further within the bracket base. The expanding hole forms an undercut surface or a lip which provides a mechanical grip between the tooth and the bracket base when filled with adhesive. In another embodiment, a number of ridges are provided along an aperture, each forming a lip for providing a mechanical bonding surface within the aperture. In another embodiment, the apertures are provided as preformed inserts made of plastic or metal that are molded into the bracket base during its manufacture. In yet another embodiment, threaded apertures which provide undercut surfaces are provided directly in the bracket base. Such apertures can be either machined into a pre-molded bracket base, or can be molded into a bracket base during the molding step by the use of a retractable threaded pin. In still other embodiments, pairs of apertures are provided through the dental bonding surface of a bracket base to intersect and form a channel which defines the undercut surface. Such channels can be molded into the bracket base using retractable pins during the molding step, or can be machined into a pre-molded orthodontic bracket.

In yet another embodiment, a reinforcing insert is provided inside a plastic orthodontic bracket. The reinforcing insert includes a base from which a pair of walls extend on opposite ends of the base. Distant from the base, each wall includes a center arch wire slot and a pair of ears, one on each side of the slot. When the reinforcing insert is molded into an orthodontic bracket, the ears reinforce the tie wings to improve the strength of the bracket. Regardless of the particular embodiment, according to the present invention, an undercut surface is provided for mechanically gripping the bracket base to the tooth with cured adhesive. The undercut surface defines a surface that opposes the dental bonding surface of the bracket base to provide the requisite mechanical grip. This also permits the use of materials such as nylon which have not generally been accepted by the industry for use in manufacturing orthodontic brackets due to poor adhesion compared with other materials of construction.

-2- BRIEF DESCRIPTION OF THE DRAWINGS

Other features, aspects, and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims, and accompanying drawings where:

FIG. 1 is a front elevation view in section of one embodiment of an orthodontic bracket of the present invention;

FIGS. 2a-f are front elevation views in section of variations on inserts for use with the orthodontic bracket of the type illustrated in FIG. 1 ;

FIGS. 3, 4 and 5a are front elevation views in section of other embodiments of the orthodontic bracket of the present invention;

FIG 5b is a bottom plan view of the embodiment of FIG. 5a; FIGS. 6 and 7 are bottom plan views of still further embodiments of the orthodontic bracket of the present invention;

FIG. 8 is a bottom plan view of yet another embodiment of the orthodontic bracket of the present invention;

FIG. 9 is a front elevation view in section of the orthodontic bracket of FIG. 8 taken along line 9—9; FIG. 10 is a side elevation view in section of a portion of the orthodontic bracket of

FIG. 8 taken along line 10-10;

FIG. 11 and 12 are front elevation views in section of the equipment used in molding the orthodontic bracket of FIG. 1 ;

FIGS. 13 and 14 are front elevation views in section of equipment used for molding the orthodontic bracket of FIG. 4;

FIG. 15 is a front elevation view in section of the equipment used in molding the orthodontic bracket of FIG. 9;

FIG. 16 is a perspective view of a reinforcing insert for an orthodontic bracket; and FIG. 17 is a top plan view of a reinforcing insert blank used for making the reinforcing insert of FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 , one embodiment of the improved orthodontic bracket of the present invention is illustrated. The bracket 20 includes a bracket base 22 upon which is provided an arch wire support bracket 24. The arch wire support bracket illustrated is of a traditional design including a pair of tie wings 26. However, any type of arch wire support bracket may be used. For example, tubes of the type known as buccal tubes which are often provided on the brackets adhered to the patient's rearmost molars can be used as the arch

-3- wire support bracket in place of the illustrated tie wings.

The bracket base preferably includes a concave dental bonding surface 28 which is of a shape that approximates the convex outer surface of the tooth to which the bracket is to be bonded. Of course, different shapes for the dental bonding surfaces may be provided so that an appropriate bracket can be selected for a particular tooth. Moreover, while concave outer surfaces are illustrated, brackets may also be provided with convex dental bonding surfaces for bonding to the inner surfaces of certain teeth. According to the embodiment of FIG. 1. a hollow insert 32 is provided in the bracket base to improve the adhesion characteristics between the bracket and the tooth's surface. The insert is of a generally cylindrical shape with an inner wall that defines an internal surface 34 defining ridges of a saw-toothed cross section. This arrangement of the internal surface defines a plurality of lips 36 with undercut surfaces that oppose the dental bonding surface of the bracket base. These lips serve as anchor points for the adhesive used to bond the bracket to the patient's tooth, thereby mechanically holding the bracket in place.

The insert is preferably molded or cast into place in the bracket base. Any of the generally used materials of construction for orthodontic brackets can be used for both the bracket base and the insert of the present invention. Examples include metals such as stainless steel or titanium, ceramics or polymeric materials. Combinations of materials can also be used. For example, a polymeric bracket can be molded around a stainless steel insert. Presently, polymeric materials are preferred for the brackets as they are easier to manufacture.

In order to anchor the insert firmly within the bracket base, an outer surface 42 is provided on the insert that is preferably textured. Examples of appropriate textures include roughened or knurled surfaces. The insert further includes a flange 44 which helps to anchor the insert within the bracket base to prevent the insert from being pulled from the bracket base during use.

Referring to FIGS. 2a-2f, alternative designs for inserts are illustrated. In FIG. 2a, a hollow cylindrical insert is provided with an internal surface 46 which includes a plurality of scalloped ridges 47 which run circumferentially around the inside of the insert. These ridges define a plurality of undercut surfaces 48 that work with the adhesive to provide a mechanical grip between the adhesive and the bracket base. According to FIG. 2b, a hollow cylindrical insert is illustrated with an inner surface 51 having a plurality of circumferential grooves 52 of trapezoidal cross section. The grooves form undercut surfaces 53 which oppose the outer surface of the base to mechanically assist in anchoring the bracket to the tooth.

According to FIG. 2c, a cylindrical insert is provided with a threaded interior surface 55. The thread of the interior surface defines a spiral undercut surface 56 for anchoring the

-4- bracket to the tooth. In FIG. 2d. a cylindrical insert is illustrated with an internal surface 57 that defines a plurality of square internal grooves 58. As with the other embodiments, these grooves form a plurality of undercut surfaces 59 for improving adhesion of the bracket to the tooth. In FIG. 2e, yet another cylindrical insert is provided. This insert includes a beveled internal surface 61 which defines the requisite opposing lip surface 62. A chamfered edge 63 is provided to simplify the molding of an orthodontic bracket around the insert. In FIG. 2f. still another cylindrical insert is provided, this insert including a lower inwardly facing lip 64 which defines the lip surface 65 for improving the adhesion of the bracket to the tooth surface. As in the previous embodiment, a chamfered edge 66 is provided to simplify the molding of an orthodontic bracket around the insert.

In all of the above embodiments, the inserts are described as being of generally cylindrical shapes. Cylindrical shapes are generally preferred as they tend to be easier to manufacture by either molding or machining processes. However, it is clear that shapes other than cylindrical shapes, such as square, triangular or irregular prism shapes, or any number of other shapes, can similarly be used so long as an inner lip or undercut surface is included so as to provide the requisite mechanical bonding surface.

According to FIG. 3, another type of insert is illustrated. As with the previous embodiments, this orthodontic bracket includes a bracket base 72 and an arch wire support bracket 73. However, rather than including a hollow insert molded into the bracket base, a solid cylindrical insert 74 is provided to extend from a cavity 75 in the bracket base. Such an insert can be molded or cast into the bracket. The insert illustrated includes an outer surface 76 with a saw-toothed cross section. The teeth define a plurality of undercut surfaces 77 which oppose the outer surface of the bracket base. As with the undercut surfaces of the previous embodiments, these surfaces provide the surface area for mechanically holding the bracket to the tooth's surface using cured adhesive. While just one configuration for the lip surfaces is described, it is apparent that other lip surfaces can be used. Just a few examples include a threaded outer surface for the insert, a square or trapezoidal ridged outer surface, or a beveled outer surface.

Referring to FIG. 4, yet another bracket of the present invention is provided. This bracket includes a bracket base 82 and an arch wire support bracket 83. The bracket base includes an internal threaded aperture 84 similar to that of the insert illustrated in FIG. 2c. The thread defines a spiral undercut surface 85 which provides an anchoring surface that opposes the outer surface of the bracket base to assist in anchoring the bracket to the tooth.

While other inner surfaces can be provided, a threaded aperture is preferred for ease of manufacturing. Such a threaded aperture can be molded into the bracket base using conventional molding techniques in which a threaded rod is used to form the threaded

-5- aperture during the molding process.

Still another bracket design is illustrated in FIGS. 5a and b. This bracket includes a bracket base 92 and arch wire support bracket 93. It also includes a threaded aperture 94.

However, for this bracket, an annular threaded aperture is provided with not only a threaded outer surface 95. but a threaded inner surface 96 as well. Such a bracket base can be molded using a rod with a threaded outer surface and a threaded internal opening extending along a portion of its axis. By selecting the appropriate thread pitches for the inner and outer threads of the rod, the rod can be withdrawn from the bracket while the mold is being separated without binding in the bracket base. Such a bracket provides undercut surfaces on the inner and outer surfaces of the aperture.

Several of the embodiments described above employ either threaded inserts or threaded apertures. While the undercut surfaces provided in all embodiments provide a mechanical gripping force which helps to prevent the bracket from being pulled from the tooth's surface, even if the bond between the adhesive and the bracket fails, there is some risk that any rotational force being applied to such a bracket may cause the bracket to unthread from the cured adhesive. Therefore, as illustrated in FIG. 5b, the dental bonding surface of such a bracket base 92 includes a pair of channels 97, arranged peφendicular to one another and extending across the center portion of the bracket base. Once the bracket is bonded to the patient's tooth, the cured adhesive in the channels helps to prevent the bracket base from rotating with respect to the surface of the tooth. Referring to FIG. 6, a similar design is provided for a bracket base with a threaded aperture 103 similar to that of the bracket of FIG. 4. Here, a pair of perpendicular channels 102 are provided on the dental bonding surface 101 of the bracket base. According to FIG. 7, rather than a pair of channels arranged peφendicular to one another, a bracket base 105 includes a pair of parallel channels 106 which similarly help prevent the threaded aperture 103 from unthreading from the cured adhesive. Even if the aperture does not include a threaded surface, such channels can prevent rotation of the bracket with respect to the tooth. As is apparent, the present invention is directed to an improved bracket base that is intended to improve the adhesion between a bracket base and a patient's tooth. According to the invention, even if the adhesion between the adhesive and the bracket fails, the mechanical forces of the cured adhesive acting on the undercut surfaces of the bracket base hold the bracket in place on the patient's tooth. However, to still further improve the adhesion between the bracket base and the patient's tooth, a textured surface 107 can be provided on the outer surface of the bracket base. It is generally known in the art that a textured bracket base will improve adhesion between the bracket base and the adhesive. Any number of different textures can be used. Examples include a roughened surface, a knurled surface or a

-6- grid surface. Preferably, for brackets made by injection molding, such textured surfaces are directly molded into the bracket base during the molding process. This avoids the need for multiple step manufacturing techniques which can be both costly and time consuming.

For ease of illustration, in the previously described embodiments, just a single aperture is described for a particular orthodontic bracket. However, it is clear that a plurality of apertures, whether molded or machined into the bracket base or provided as inserts, can be provided on a single orthodontic bracket. The use of multiple apertures will further help to prevent the bracket from unthreading from, or rotating with respect to the tooth during use.

However, since orthodontic brackets are fairly small, for ease of manufacture it is generally preferred that just a single aperture be provided for the above embodiments.

Referring now to FIGS. 8-10, yet another embodiment of an orthodontic bracket of the present invention is illustrated. FIG. 8 represents a bottom plan view of a bracket base 1 1 1 which includes six angled apertures 112 provided in three pairs. The individual apertures of each pair of apertures are angled toward one another to intersect and form a continuous channel. The channels act as the undercuts which help to anchor the bracket base to the patient's tooth. The intersection of the pairs of apertures is further illustrated in FIGS. 9 and 10 which are sectional elevation views of the bracket of FIG. 8 taken from the side and front. respectively. The angle with which the apertures penetrate the dental bonding surface of the bracket base is preferably between about 30 and 60 degrees with an angle of about 45 degrees being most preferred.

Turning to FIGS. 1 1 and 12, the equipment used in molding an orthodontic bracket with an insert of the type shown in FIG. 1 is illustrated. The combination of right and left ejector half slides 122 and a top mold half 123 together define the shape for the top of the bracket to be molded. A bottom mold half 124 defines the shape of the bottom of the bracket and imparts any desired texture to the tooth bonding surface of the bracket base. The bottom mold half also includes tracks (not shown) along which the right and left ejector half slides are permitted to slide. An aperture 125 provided through the bottom mold half receives a pin 126 having a tip 127 for holding the insert 32 in place during the molding of the bracket. A pair of cam blocks 128 are keyed to the top mold half and operate to slide the right and left ejector half slides along the tracks and away from the molded bracket when the top mold half is lifted.

According to FIG. 13, the equipment used for molding an orthodontic bracket of the type shown in FIG. 4 is illustrated. Like the equipment of FIG. 1 1 , a top mold half 132 and a bottom mold half 133 are provided as are right and left ejector half slides 134. An aperture 135 provided through the bottom mold half receives a removable pin 136 with a threaded tip 138 for forming the threaded aperture of the molded bracket base. Once the bracket is

-7- molded, a pair of cam blocks 139 slide the top mold half and right and left ejector half slides from the molded bracket which remains attached to the threaded pin. The pin can then be manually unwound from the bracket leaving a threaded aperture in the bracket base.

According to FIG. 14. the equipment of FIG. 13 is modified somewhat to permit the automatic unwinding of the pin from the bracket after it is molded. Like the equipment of FIG. 13. top and bottom mold halves 132. 133, right and left ejector half slides 134 and cam blocks 139 are provided. However, for this embodiment, a pin 141 with a threaded tip 142 is provided that is linked to an unwind gear 144 for permitting the automatic unwinding of the threaded pin from the bracket once molded. The unwind gear is linked to the pin by a shaft 145 mounted within a guide bushing 146 to allow rotation of the pin and unwind gear. A gear rack 148 engages with the unwind gear such that when it is moved laterally, it causes rotation of the shaft. A lower thread 149 on the rotatable shaft includes a thread pitch matched to that of the threaded shaft. The threaded end of the rotatable shaft is mounted in a threaded aperture 151 at the lower end of the molding equipment.

In order to use the automatic unwinding function, after the bracket is molded, the top mold half is lifted, engaging the cam blocks to retract the mold halves from the molded bracket. Simultaneously, preferably using a mechanical linkage (not shown) between the gear rack and the cam blocks, the gear rack is moved laterally to cause rotation of the shaft which winds downward, retracting into the threaded aperture and automatically retracting the threaded pin from the molded bracket base.

While the molding equipment for the orthodontic bracket of FIG. 5a is not illustrated, the equipment described above, either using a manual pin unwind or automatic pin unwind, can be modified to produce such a bracket in that the threaded pin can include a threaded aperture for forming the threaded post within the threaded aperture of the bracket base. Of course, the thread pitches for the threaded post and the threaded aperture must match to simplify the withdrawal of the pin once the bracket is molded.

Referring to FIG. 15, the equipment for molding the bracket of FIGS. 9 and 10 is illustrated. Such a bracket is preferably molded by the use of a mold with top and bottom mold halves 153, 154, right and left ejector half slides 155 and cam blocks 156 similar to those described above. Each of the right and left half slides includes three retractable pins 158 which protrude into the molded volume through apertures in the bottom mold half. As the mold is separated after the bracket has been formed, the pins are automatically withdrawn from the molded bracket as the right and left half slides withdraw from the bracket. For smooth operation, each set of three pins should extend in a direction generally parallel to the path of its respective ejector half slide. Upon their removal, the six pins leave the three pairs of intersecting apertures which improve the adherence of the finished bracket to a patient's

-8- tooth.

Yet another aspect of the invention is that a one-piece bracket reinforcing insert can be provided within the bracket to add further strength. Such reinforced brackets are preferred by some orthodontists as they can improve the strength of a plastic bracket. Referring to FIG. 16, a bracket reinforcing insert 171 of the present invention is illustrated. The reinforcing insert includes a base portion 173 from which two walls 174 extend peφendicularly. Four ears 175 are provided, two extending outwardly from each wall. Each wall also includes an arch wire slot 177 opposite the base. Such a reinforcing insert can be molded into an orthodontic bracket such that the ears reinforce the tie wings. While some prior art reinforcing inserts are known, the geometry of the presently described reinforcing insert has certain advantages. One type of prior art reinforcing insert includes a pair of walls folded up from a flat sheet of material whereby the walls are parallel to the arch wire. The reinforcing bracket of the present invention provides a channel for the arch wire that is much more square and of a more consistent shape than such prior art reinforcing inserts. Furthermore, the reinforcing insert of the present invention is generally stronger than such prior art brackets in that the forces transferred to the reinforcing insert from the arch wire during orthodontic treatment are parallel to the planes defined by the walls of the reinforcing insert rather than peφendicular to the walls. A force applied peφendicular to walls that have been folded up from a flat sheet of material tend to cause a bending of the walls at the fold, thus imparting an undesirable flex to the reinforcing insert.

While not shown, it is also apparent that the base portion of the reinforcing insert can be modified to include undercut surfaces such as those illustrated in FIGS. 1 -3. As an alternative, the base portion can be formed or machined with an aperture or apertures to permit the molding of an orthodontic bracket around the reinforcing insert with apertures that form the desired undercut surfaces of the present invention.

Such bracket reinforcing inserts can be manufactured by a number of different methods. Referring to FIG. 17, one method is to stamp a flat reinforcing insert blank 179 out of sheet metal. The base 173, walls 174, ears 175 and arch wire slots 177 can be formed by such a process. The insert is held in place on a metal carrier strip. The insert is produced in a progressive die and run in a punch press. Once formed, the walls of the blank can be folded upward along the dashed lines 181 to produce a finished reinforcing insert. Preferably, such blanks are stamped from a 0.010 to 0.012 inch thick sheet of 301 stainless steel ^lA to ^lA hard. One possible detriment to stamping blanks which are then folded or bent into finished reinforcing inserts is that the bending step can introduce stress and cracking into the metal. This stress can then be imparted to the plastic bracket during molding. Such cracking is undesirable as it weakens the bracket.

-9- A second manufacturing method is by the injection powdered metal molding process. One advantage is that the various parts of the one-piece reinforcing insert can be made with varying wall thicknesses so that certain sections of the plastic bracket can be strengthened where needed. This can also help to reduce the amount of material used while maintaining strength and durability. In contrast, the stamping process only permits a single thickness for the reinforcing insert. Another advantage of the injection powdered metal molding process over the stamping process is that there is no bending of the reinforcing insert during manufacture. The reinforcing insert is molded in the exact shape as needed. Any stress introduced in the injection powdered metal molding process is relieved during sintering and heat treating steps. Still another advantage is that it permits the use of a wider range of materials for the reinforcing insert. While the stamping process is generally limited to materials such as stainless steel, the injection powdered metal molding process permits the use of other materials such as titanium. One possible problem with stainless steel is that it contains nickel, a metal which can cause allergic reactions to certain patients. The use of the injection powdered metal molding process permits the use of metals other than stainless steel such as titanium. Titanium is the presently preferred material for a reinforcing bracket made by the injection powdered metal molding process. While the stamping process for making a reinforcing insert is generally preferred for its ease of manufacturing, the injection powdered metal molding process is generally preferred for the flexibility it provides in selection of materials.

Because the orthodontic brackets of the present invention are adhered to the patient's teeth by a mechanical grip more than adhesion between the adhesive and the bracket, the selection of an appropriate material for the bracket is simplified, making a broader range of materials available for such orthodontic brackets.

While any number of different molding machines can be used for making the orthodontic brackets of the present invention, the presently preferred machine for making injection molded plastic or polymeric orthodontic brackets is a Boy Model 22 S injection molding machine sold by Boy Machine Inc. of Exton, Pennsylvania. Similar machines can be used for injected powdered metal molding or injected powdered ceramic molding. Furthermore, while direct molding of the brackets of the present invention is generally preferred, the same equipment can also be used to make sacrificial patterns for use in investment casting of brackets. Moreover, while direct molding of apertures as described above into a bracket base is preferred, apertures could instead be machined into a pre-formed bracket by various methods. One example of such machining equipment is an Okuma machining center of the type distributed by Okuma America Coφoration of Charlotte, North Carolina.

-10- In any of the embodiments and manufacturing processes illustrated above, the orthodontic bracket^'s torque angle can be manufactured in the slot or the base during manufacture. When injection molding a plastic bracket or ceramic bracket with a metal insert, torque in the slot or the base can be imparted during molding. For ease of illustration, the brackets are shown with zero torque angle. If a torque angle is manufactured into the base, the angle of the axis for the aperture having the undercut surfaces can be peφendicular to the base, peφendicular to the slot, or at some other angle. However, in the preferred embodiment, the angle for the aperture is peφendicular to the base.

When injection molding an orthodontic bracket, the desired surface finish for the dental bonding surface of the bracket base can be incoφorated in the surface of the mold or tool during manufacturing. Thus, a textured, frosted or some other surface finish can be imparted to the bracket during the molding process rather than as a later step as is often described in the prior art. The secondary processing steps of such prior art methods make such orthodontic brackets more costly and difficult to make.

The preferred materials which can be used for making the orthodontic brackets of the present invention include ceramics, metals and plastics. Ceramics are generally of silica and can have either a clear or opaque appearance. Metals include titanium and stainless steel. Plastics include nylon, polycarbonates, or any number of other plastics used for orthodontic brackets. Nylon and polycarbonates are the preferred plastics.

In the past, the orthodontic industry has used the criteria of adhesion, performance and clarity and appearance in selecting a suitable plastic for plastic orthodontic brackets. Materials that fit these criteria are acrylic, polycarbonate and polysulfone. Glass-filled polycarbonate has generally been deemed the preferred material. Advantages of glass- filled polycarbonates include their good impact resistance, stiffness, high clarity, resistance to creep, good adhesion properties and minimal moisture absorption. Disadvantages include poor scratch and chemical resistance and brittleness making brackets made from glass-filled polycarbonates subject to stress cracking. Orthodontists have also expressed certain functional problems associated with glass-filled polycarbonate brackets.

Orthodontists often recommend that patients avoid certain foods which can cause staining of the brackets. The tie wings of such brackets can often exhibit wear, sometimes to the point of failure. Also, when removing such brackets from the patient's teeth, parts of the base of the bracket can break off and remain bonded to the teeth. This makes it difficult to clean the tooth when treatment is complete, or if a new bracket needs to be applied.

It has been discovered that according to the present invention, nylon is a preferred material for a plastic orthodontic bracket. Advantages of nylon include high strength. stiffness, wear and abrasion resistance, outstanding resistance to fatigue and repeated impact, a low coefficient of friction, barrier properties and outstanding chemical resistance. Nylon also has outstanding transparency and outstanding processability. especially its high flow properties in thin sections. Nylon also exhibits high resistance to stress cracking.

Compared to polycarbonates, nylon has excellent chemical resistance and barrier properties. Thus there is less need to avoid certain foods which might stain a polycarbonate orthodontic bracket. Nylon also has good wear and abrasion resistance plus outstanding resistance to fatigue and repeated impact and high resistance to stress cracking. Nylon also exhibits better tie wing wear than polycarbonate brackets.

One negative aspect of nylon is that it has limited adhesion using traditional adhesives. However, this disadvantage is overcome by the bracket designs of the present invention. According to the present invention, adhesion to the bracket is of secondary importance as the bracket is primarily held in place by the cured adhesive forming a matrix taking on the shape of the apertures in the bracket base. Such a matrix mechanically grips the bracket base to the tooth by the undercut surfaces provided in the apertures of the bracket base. The inclusion of an aperture in the dental bonding surface of an orthodontic bracket provides a further benefit in that it permits a more custom fit to the tooth's surface, During bonding, the adhesive will flow to match identically the surface of the tooth.

For prior art brackets, the orthodontist has to consider the adhesion between the adhesive and the bracket as well as between the adhesive and the tooth to ensure a durable bond. Consequently, according to the present invention, the selection of an appropriate adhesive for bonding the bracket to a tooth is simplified. When using one of these brackets, an adhesive can be selected based primarily on its ability to adhere to the tooth^'s surface

Yet another advantage of nylon is its processability. With lower material and mold temperatures, manufacturing of the bracket is made easier, safer and faster, especially when placing any insert or inserts in the mold. This does not hold true for polycarbonate and other materials.

The nylon used in making orthodontic brackets of the present invention can be either glass-filled or unfilled. While glass-filled nylon is generally stronger and seems to adhere better than unfilled nylon, unfilled nylon has better clarity. Glass-filled nylon is generally opaque. The presently preferred nylon is Nylon 12. The preferred unfilled nylon is one sold under the name Grilamid® TR55 LX, made by EMS-Chemie AG of Zurich,

Switzerland and distributed by EMS-American Grilon Inc. of Sumter, South Carolina. The preferred glass-filled nylon is one sold under the name Grivory® GV, which is also made by EMS-Chemie AG. The most preferred glass-filled nylon is Grivory® GV-6H, a 60% glass-

-12- reinforced grade with maximum stiffness. Such nylon materials are knowτι for their strength and have been proposed as metal substitutes.

EXAMPLES

A number of sample orthodontic brackets were injection molded without any internal undercuts. Some of the test brackets were molded of Grilamid® TR55 LX unfilled nylon (tests similar to those set forth below were also performed with test brackets made of Grilamid® TR55 instead of Grilamid® TR55 LX and similar results were achieved) while others were molded of Grivory® GV-6H glass-filled nylon. The test brackets included a base with zero torque angle and a 0.5 inch concave radius of curvature in both directions. The bracket base was about 0.145 inches by 0.145 inches. The surface finish of the dental bonding surface was RMS 70.7949. Rather than including tie wings, the test brackets were formed with a shank opposite the dental bonding surface.

All tests were conducted using bovine teeth which are generally recognized as having properties similar to human teeth. All bond strengths were measured with a Tri-Coastal electronic scale. For the bond strength testing, a metal hook was threaded into an aperture in the shank of each test bracket. A pulling force was exerted on the hook in a direction peφendicular to the plane between the tooth and bracket base. The test brackets were de- bonded using a combination of tensile, shear and peel. This de-bonding method was the result of the mounting of each test bracket approximately 0.062 inches off the center line of the test apparatus. Being off center exerted more force to one side of the bracket than the other while still pulling in a peφendicular direction. The tooth with the test bracket was suspended and weight was added to the scale until failure. The amount of force necessary to cause failure was recorded as the bond strength.

Bonding was done using either 3M Concise Orthodontic Bonding System No. 1960 adhesive, a product distributed by 3M Unitek Coφoration of Monrovia, California, or Light- Cure Patient Cartridge Adhesion System with Fluoride, Kit Catalog No. A 10-510, a product distributed by Lancer Orthodontics, Inc. of San Marcos, California. The manufacturer's directions were generally followed for bonding.

For the Unitek bonding system, the etching solution was applied to each tooth. The two-part bond resin was mixed and applied to the tooth. This was allowed to dry for 1 ^lΛ to 2 minutes. The two-part adhesive was then mixed and applied to the base of the test bracket and the test bracket was adhered to the tooth. A fixture was used to hold the bracket firmly to the tooth during cure. The test brackets were allowed to sit for 5-10 minutes before testing. For the Lancers bonding system, the etching solution was applied to each tooth. The light-cure resin was then applied to the tooth and cured under a UV lamp for about 30

-13- seconds. The light cure adhesive was thinned slightly using a small amount of light cure resin and applied to the base of the test bracket. The test bracket was then adhered to the tooth and cured for approximately 30 seconds using a UV lamp. A fixture was used to hold the brackets firmly to the teeth during cure. The test brackets were allowed to sit for 5-10 minutes before testing.

EXAMPLE 1 Ten of the unfilled test brackets were bonded to bovine teeth using the Unitek bonding system as set forth above. Ten of the unfilled test brackets were bonded to bovine teeth using the Lancers bonding system as set forth above. The mean bond strength using the Unitek bonding system was 0.58 pounds. The mean bond strength using the Lancers bonding system was 0.52 pounds. All bond failures were the result of adhesive failure rather than failure of the bracket itself.

EXAMPLE 2

Twenty of the unfilled test brackets were machined to include a threaded aperture in the dental bonding surface similar to that illustrated in FIG. 4. The aperture was machined with a 0.025 inch pitch and major and minor diameters of 0.1 14 and 0.089 inches, respectively. The aperture was machined to a depth of 0.040 inches. Ten of these test brackets were bonded to bovine teeth using the Unitek bonding system. The remaining ten were bonded to bovine teeth using Lancers bonding system. Each test bracket was then tested for bond strength. According to the tests, the mean bond strength using the Unitek bonding system was

17.02 pounds. The mean bond strength using the Lancers bonding system was 15.80 pounds. All bond failures were the result of adhesive failure rather than failure of the bracket itself. Observation under a microscope of the test brackets revealed that the internal shape of the bracket base exhibited no deformation, stress cracking or breakage. Comparing the results of Example 2 to Example 1 , a significant improvement in bond strength was realized by the addition of undercut surfaces according to the present invention.

EXAMPLE 3 Ten of the glass-filled test brackets (without apertures or undercut surfaces) were bonded to bovine teeth using the Unitek bonding system as set forth above. Ten more of the glass-filled test brackets (without apertures or undercut surfaces) were bonded to bovine teeth using the Lancers bonding system as set forth above. Each test bracket was

-14- then tested for bond strength.

According to the tests, the mean bond strength using the Unitek bonding system was 9.02 pounds. The mean bond strength using the Lancers bonding system was 8.70 pounds.

All bond failures were the result of adhesive failure rather than failure of the bracket itself.

EXAMPLE 4

Twenty of the glass-filled test brackets were machined as described for Example 2. Ten of these test brackets were bonded to bovine teeth using the Unitek bonding system.

The remaining ten were bonded to bovine teeth using Lancers bonding system. Each test bracket was then tested for bond strength.

According to the tests, the mean bond strength using the Unitek bonding system was 20.72 pounds. The mean bond strength using the Lancers bonding system was 18.60 pounds. All bond failures were the result of adhesive failure rather than failure of the bracket itself. Comparing the results of Example 4 to Example 3, a significant improvement in bond strength was realized by the addition of undercut surfaces according to the present invention. Observation under a microscope of the test brackets revealed that the internal shape of the bracket base exhibited no deformation, stress cracking or breakage.

EXAMPLE 5

Ten of the unfilled test brackets and ten of the glass-filled test brackets were machined as described for Example 2. Rather than adhering each of these brackets to a bovine tooth, a machine screw of the same size and pitch of the threaded aperture was threaded into the aperture. Each test bracket was then suspended by the machine screw and the strength of the test bracket was determined by the method set forth above. The puφose of this test was to determine the theoretical strength of the test bracket based on the undercut surfaces without regard to the strength of the adhesive. According to this test, the mean failure force for the unfilled test brackets was 32.30 pounds. The mean failure force for the glass-filled test brackets was 61.54 pounds. For each of these test brackets, failure resulted by the "stripping" of the thread of the aperture.

The design of the orthodontic brackets of the present invention allow for improved bonding without the many processing steps of prior art methods. For the embodiments described above, a plastic orthodontic bracket with an undercut surface having a lip at least about 0.030 inches in depth is adequate to provide the necessary mechanical adhesion.

Having described the invention with respect to its preferred embodiments, it will be

-15- clear to one of ordinary skill in the art that numerous variations of the invention can be made without departing from the spirit of the invention. Therefore, the invention is to be defined by the following claims rather than by the preferred embodiments.

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Claims

1. An orthodontic bracket of the type to be bonded directly to a tooth's surface, the orthodontic bracket comprising: an arch wire support bracket; and a bracket base, the bracket base comprising: a dental bonding surface approximating the surface of the tooth; and at least one insert provided in the bracket base, the insert having at least one undercut surface.

2. The orthodontic bracket of claim 1 wherein the insert comprises: a hollow body portion having inside and outside surfaces; the inside surface defining the at least one undercut surface; and the outer surface adapted to hold the insert within the bracket base.

3. The orthodontic bracket of claim 2 wherein the outer surface is roughened and the outer surface further defines a flange.

4. The orthodontic bracket of claim 2 wherein the at least one undercut surface of the inner surface of the insert comprises a plurality of ridges.

5. The orthodontic bracket of claim 2 wherein the at least one undercut surface comprises a continuous thread.

6. The orthodontic bracket of claim 2 wherein the at least one undercut surface comprises at least one lip.

7. The orthodontic bracket of claim 1 wherein the insert is located within an aperture in the bracket base and the at least one undercut surface of the insert is located on an outside surface of the insert.

8. The orthodontic bracket of claim 7 wherein the outside surface of the insert comprises a plurality of ridges.

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9. The orthodontic bracket of claim 7 wherein the outside surface of the insert comprises a continuous thread.

10. The orthodontic bracket of claim 1 further comprising at least one groove on the dental bonding surface of the bracket base.

1 1. The orthodontic bracket of claim 1 wherein the bracket is made of nylon.

12. The orthodontic bracket of claim 1 wherein the insert further comprises a reinforcing base and a reinforcing arch wire support bracket.

13. An orthodontic bracket of the type to be bonded directly to a tooth's surface, the orthodontic bracket comprising: an arch wire support bracket; and a bracket base, the bracket base comprising: a dental bonding surface approximating the surface of the tooth: and at least one aperture extending through the dental bonding surface and into the base, the aperture including at least one preformed undercut surface.

14. The orthodontic bracket of claim 13 wherein the at least one preformed undercut surface comprises a threaded wall.

15. The orthodontic bracket of claim 13 wherein the at least one preformed undercut surface comprises an outer threaded wall and an inner threaded wall.

16. The orthodontic bracket of claim 13 wherein the at least one aperture is defined by an insert provided in the bracket base.

17. The orthodontic bracket of claim 13 wherein the at least one aperture is integral to the bracket base.

18. The orthodontic bracket of claim 13 further comprising at least one groove in the dental bonding surface of the bracket base.

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19. The orthodontic bracket of claim 13 wherein the bracket is made of nylon.

20. The orthodontic bracket of claim 13 further comprising an insert including a reinforcing base and a reinforcing arch wire support bracket.

21. The orthodontic bracket of claim 20 wherein the insert is made of titanium.

22. An orthodontic bracket of the type to be bonded directly to a tooth^'s surface, the orthodontic bracket comprising: an arch wire support bracket; and a bracket base, the bracket base comprising: a dental bonding surface approximating the surface of the tooth: and at least one pair of apertures extending through the dental bonding surface and into the base, the at least one pair of apertures intersecting to define a channel with a surface opposing the dental bonding surface.

23. The orthodontic bracket of claim 22 comprising three pairs of apertures which define three channels.

24. The orthodontic bracket of claim 22 wherein each of the apertures extends through the dental bonding surface of the bracket base at an angle of between about 30 and about 60 degrees from the dental bonding surface.

25. The orthodontic bracket of claim 22 wherein each of the apertures extends through the dental bonding surface of the bracket base at an angle of about 45 degrees from the dental bonding surface.

26. The orthodontic bracket of claim 22 wherein the bracket is made of nylon.

27. The orthodontic bracket of claim 22 further comprising an insert including a reinforcing base and a reinforcing arch wire support bracket.

28. The orthodontic bracket of claim 27 wherein the insert is made of titanium.

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29. An orthodontic bracket of the type to be bonded directly to a tooth^'s surface, the orthodontic bracket comprising: an arch wire support bracket; a base; and an embedded titanium reinforcing insert, the insert comprising a reinforcing base and a reinforcing arch wire support bracket.

30. The orthodontic bracket of claim 29 wherein the reinforcing insert is molded from metal.

31. The orthodontic bracket of claim 29 wherein the reinforcing base of the reinforcing insert further comprises an aperture defining at least one undercut surface.

32. An orthodontic bracket of the type to be bonded directly to a tooth's surface, the orthodontic bracket comprising: an arch wire support bracket; and an integral base, the arch wire support bracket and base being made of nylon.

33. The orthodontic bracket of claim 32 further comprising a dental bonding surface approximating the surface of the tooth and at least one aperture extending through the dental bonding surface and into the base, the aperture including at least one preformed undercut surface.

34. The orthodontic bracket of claim 13 wherein the at least one preformed undercut surface comprises a threaded wall.

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