Plastic orthodontic bracket

Abstract

An all plastic orthodontic bracket is disclosed which is adapted to be attached directly to the labial surface of a tooth. The bracket has a quasi triangular shaped body as viewed from the facial view to minimize forces being applied to the bracket during mastication and occlusion. The apical section of the triangular shaped body is adapted to be attached to the incisal section of the tooth and the base of the triangular shaped body is adapted to be attached to the gingival section of the tooth. All surfaces of the bracket body extending from a ligature undercut over the front surface of the bracket body are smooth and convex with the exception of those surfaces defined by an archwire slot and a second slot which is perpendicular to the archwire slot. Smooth convex bracket body surfaces substantially lessen plastic fatigue commonly found in prior art plastic orthodontic brackets having sharp corners. The ligature undercut is of a depth which is substantially filled by a small gauge archwire or an elastic ring. The relatively shallow depth of the ligature undercut insures that maximum bracket bulk is distributed between the archwire slot and the ligature undercut to lessen the tendency for bracket body fatigue between these points.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to plastic orthodontic brackets of the type which are adapted to be directly cemented to the labial surface of a tooth. More particularly, the invention is directed to the shape of the bracket body which lessens the tendency of plastic fatigue found in prior art plastic brackets.

2. Description of the Prior Art

Prior art plastic orthodontic brackets are geometrically configurated to have the same shape as their all metal counterparts. The history of use of plastic orthodontic brackets as a result of this type of design has been replete with frustration and disappointment caused by bracket distortion and breakage. Bracket distortion and breakage manifest themselves in increased clinical time consumption on a per patient basis. As a result of the clinical time expended in replacing broken and distorted plastic brackets, many orthodontists insist upon using all metal brackets even in light of the unaesthetic appearance of the "all metal look".

The following United States patents disclose plastic orthodontic brackets which do not have a design which minimizes breakage and distortion:

3,303,565 2,971,258 3,052,027 3,469,314 3,345,745 3,464,112 3,521,355 3,496,637 3,504,438 3,765,091 3,773,850

None of the aforementioned patents embodies a plastic orthodontic bracket which is concerned with eliminating bracket breakage or distortion by utilizing a quasi triangular shaped bracket body, more effective material bulk distribution between the archwire slot and a ligature undercut by lessening the depth of the ligature undercut and the use of a smooth convex shape of the bracket body which eliminates sharp angles except in the archwire slot and in a vertical slot disposed perpendicular to the archwire slot which is used for applying mesio-distal rotational forces to the tooth to which the bracket is attached.

SUMMARY OF THE INVENTION

In light of the disadvantages of plastic bracket breakage and distortion, the present invention represents a substantial improvement in minimizing the aforementioned drawbacks by utilizing a design which incorporates the inherent structural properties of plastic as the criteria for shaping the bracket instead of the structural properties of metal.

One aspect of the present invention is the quasi triangular shape of the bracket body as seen from the facial view. When the bracket base is attached to the labial surface of a tooth, the narrower apical section is oriented closest to the incisal section while the wider base portion of the triangular bracket body is oriented closest to the gingival section. The quasi triangular shape of the bracket functions to minimize the forces applied to the bracket during mastication and occlusion by minimizing the surface area of the bracket which contacts food or other teeth when the jaws are about to close during mastication or occlusion.

Another aspect of the invention is the provision of a relatively shallow ligature undercut which extends at least partially around the periphery of the side of the bracket body. The ligature undercut is substantially filed when a small gauge ligature wire or elastic ring is disposed therein. The preferred depth is approximately 0.030-0.035 inch. The use of the shallow ligature undercut insures that the maximum amount of bulk bracket material is located between the ligature undercut and an archwire slot cut in the front surface of the bracket body to lessen the chance of plastic fatigue. The base of the bracket extends away from the ligature undercut a distance sufficient to prevent the adhesive used for attaching the back surface of base portion of the bracket to the labial surface of the tooth from occluding the ligature undercut.

A final aspect of the invention is that all surfaces on the bracket body as defined from the ligature undercut extending to the archwire slot and to a slot disposed perpendicular to the archwire slot which is used for rotating the teeth mesio-distally are smooth and convex to lessen internal stress in the bracket body which is caused by surfaces meeting in sharp corners.

The invention will be readily appreciated by reference to the detailed description when considered in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures and wherein:

FIG. I is a perspective view of a prior art plastic orthodontic bracket;

FIG. II is a front view of a plastic orthodontic bracket constructed according to the invention;

FIG. III is a side view of a plastic orthodontic bracket constructed according to the invention; and

FIG. IV is a top view of a plastic orthodontic bracket constructed according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a typical prior art plastic orthodontic bracket 10 is shown therein. The bracket 10 is comprised of a base 12 and a body 14. Archwire slot 16 is cut horizontally through the center of the bracket. The upper and lower surfaces 18 and 20 are of equal width. The bracket 10 may either be cemented to a metal band which encircles a tooth to provide a strong bond therewith or cemented directly to a tooth. As may be seen from inspection of FIG. I, the top and bottom surfaces 18 and 20 of bracket 10 each have a substantial surface area which is contacted by food or other teeth during mastication and occlusion. This substantial surface area causes the body of the bracket 14 to be subjected to considerable force during mastication or occlusion with the attendant likelihood of stress fatigue in the plastic body 14 of the bracket. A ligature undercut 22 is provided partially around the periphery of the bracket body 14. Ligature undercut 22 is cut so deep that a small gauge ligature wire or adhesive band would not substantially fill the ligature undercut 22. The deep ligature design concept substantially lessens the material bulk distribution between archwire slot 16 and ligature undercut 22 with the attendant likelihood of an increase in stress fatigue between these points. Reference numerals 24, 26 and 28 identify sharp edges defined by the meeting of relatively flat surfaces. Each of these sharp edges represents an internal stress point of the plastic bracket which may fatigue under continued stress.

Referring to FIGS. II-IV, a plastic orthodontic bracket which is constructed according to the present invention is shown as it would be attached to a maxillary lateral tooth. Bracket 10 has a horizontally disposed archwire slot 30 cut in the front surface of the bracket body 32. The rear surface of base 34 of bracket 10 is concavely contoured mesio-distally and gingivo-incisally to conform to the labial contour of the tooth to which the bracket is adapted to be attached. The base contour of brackets which are used on different types of teeth are chosen to conform to the different labial contours of those teeth to which the bracket is to be attached. The preferred bracket shape is quasi triangular with top section 36 comprising the base of a triangle while bottom section 38 forms the apical section of a triangle. However, other tapered geometrical shapes could also be used. Vertical slot 40 is provided to allow an archwire to be tied in utilizing a mesial or distal rotational force. The preferred position is over the clinical crown of the tooth. Ligature undercut 42 is purposely cut shallow enough to be substantially filled by a small gauge ligature wire or elastic ring. Preferably, the depth of ligature undercut 42 is approximately 0.035 inch or less. This shallow undercut provides maximum material bulk between the ligature undercut 42 and archwire slot 30 to lessen the chance of plastic fatigue between these points. The ligature undercut 42 is smooth and concave in shape. Extending from ligature undercut 42 over the front surface of the bracket body 32, all surfaces are smooth and convex except the archwire and vertical slots 30 and 40 to lessen internal stress which normally occurs in plastic brackets which have sharp corners. The base 34 of the bracket 10 is extended, as identified by reference numeral 44, from the ligature undercut 42 a distance sufficient to prevent the adhesive used for attaching the back surface of the base 34 to the labial surface of the tooth from occluding the ligature undercut 42. The lip section 46 of the bracket body 32 located next to the ligature undercut 42 is relieved incisally and gingivally to the archwire to facilitate easy placement of the ligature.

Referring specifically to FIG. III, it may be seen that archwire slot 30 is not perpendicular to the plane of the base 34. The archwire slot forms an acute angle which is measured between a perpendicular line from the base of the archwire slot and the base of the bracket which is parallel to the labial surface of the tooth. This acute angulation produces a built in torque in the bracket which requires less torque to be placed in the archwire. This configuration produces less destructive wear of the plastic material where it is in contact with the sharp corners of the metal wire.

The following table gives the preferred bracket dimensions and angulations which are to be used with different types of teeth.

TABLE 1 __________________________________________________________________________ bracket slot base bracket base width angulation curvature height thickness mm mm mm __________________________________________________________________________ maxillary central 5.8 6.3 0 21.degree. 0.5 maxillary lateral 4.8 5.3 0 14.degree. 1.0 maxillary cuspid 5.8 6.3 + 7.degree. 0.5 mandibular central 4.8 5.3 0 17.degree. 1.0 mandibular lateral 4.3 6.3 0 17.degree. 1.0 mandibular cuspid 5.8 6.3 + 4.degree. 0.5 __________________________________________________________________________

To use the bracket, base 34 is cemented to the labial surface of the tooth so that the narrow apex section 38 of the bracket body is positioned closest to the incisal edge of a tooth to which the bracket base is attached. This configuration minimizes the forces applied to the bracket body during mastication and occlusion. While many types of dental adhesives may be used to cement the bracket directly to a tooth, the preferred type of adhesive is disclosed in United States patent application Ser. No. 386,416 filed Aug. 7, 1973.

The preferred type of plastic used in constructing the bracket is chosen from the polycarbonate family. It has the following physical properties: If a filled polycarbonate is chosen, the shear strength should range from 10,000 to 16,000 p.s.i. Deformation at 70.degree.F under a 4000 psi load is less than 0.1%. If an unfilled polycarbonate is chosen, the shear strength should range from 6000 to 8500 psi. Deformation at 70.degree.F under a 4000 psi load is from 0.1 to 0.3%.

It should be understood that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications which do not constitute departures from the spirit and scope of the invention.

Claims

What is claimed is:

1. A plastic orthodontic bracket comprising a base which is adapted to be attached to the labial surface of a tooth between the gingival and the incisal sections and a body which is joined to said base, said body having a quasi-triangular shaped front surface, an archwire slot cut in the front surface of said body, a ligature undercut extending completely around the periphery of the front surface of said body, and a smooth and convex surface extending from said ligature undercut to said archwire slot.

2. In an orthodontic bracket as recited in claim 1 wherein said plastic is a filled polycarbonate having the following physical properties:

shear strength ranging from 10,000 to 16,000 p.s.i. and deformation at 70.degree.F under a 4,000 p.s.i. load being less than 0.1%.

3. In an orthodontic bracket as recited in claim 1 wherein said plastic is an unfilled polycarbonate having the following physical properties:

shear strength ranging from 6,000 to 8,500 p.s.i. and deformation at 70.degree.F under 4,000 p.s.i. load being from 0.1% to 0.3%.

4. A plastic orthodontic bracket as recited in claim 1 further comprising means cut in the front surface of said bracket body for use in applying mesial or dital rotational forces to a tooth to which the bracket is attached.

5. In an orthodontic bracket as recited in claim 4 wherein said means comprises a slot cut in the front surface of said bracket body, said slot being perpendicular to said archwire slot.

6. In an orthodontic bracket as recited in claim 1 wherein said ligature undercut is smooth and concave and has a maximum depth of 0.015mm or less.

7. A plastic orthodontic bracket comprising a base portion which is adapted to be attached to the labial surface of a tooth between the gingival and incisal sections and a body which is joined to said base, said body having a front surface which is quasi-triangular shaped for minimizing the forces exerted on said bracket during mastication and occlusion when said bracket is attached to said tooth so that the apex of said quasi-triangular shaped front surface is positioned closest to the incisal section of a tooth and the base of said quasi-triangular shaped front surface is positioned closest to the gingival section of the tooth.

8. In an orthodontic bracket as recited in claim 7 further comprising an archwire slot cut in the front surface of said bracket body and a ligature undercut extending completely around the periphery of said front surface, said ligature undercut being smooth and concave and having a depth of 0.015mm or less.

9. An orthodontic bracket as recited in claim 7 wherein said bracket body further comprises an archwire slot, means cut in the front surface of said bracket body for use in applying mesial or distal rotational forces to a tooth to which the bracket is attached, said slot being cut into the front surface of said bracket body, and a ligature undercut extending completely around the periphery of said bracket body, and a smooth and convex surface extending from said ligature undercut to said archwire slot.

10. In an orthodontic bracket as recited in claim 8 wherein said means comprises a slot cut in the front surface of said bracket body, said slot being perpendicular to said archwire slot.

11. An orthodontic bracket as defined in claim 3 wherein said base portion of said bracket extends away from the ligature undercut a distance sufficient to prevent adhesive used for attaching the base of the bracket to the labial surface of the tooth from occluding the ligature undercut.

12. An orthodontic bracket as recited in claim 5 wherein the base is extended away from the ligature undercut at least for 0.015 inch.

13. In an orthodontic bracket as recited in claim 7 wherein said plastic is a filled polycarbonate having the following physical properties:

shear strength ranging from 10,000 to 16,000 p.s.i. and deformation at 70.degree.F under a 4,000 p.s.i. load being less than 0.1%.

14. In an orthodontic bracket as recited in claim 7 wherein said plastic is an unfilled polycarbonate having the following physical properties:

shear strength ranging from 6,000 to 8,500 p.s.i. and deformation at 70.degree.F under 4,000 p.s.i. load being from 0.1% to 0.3%.