Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION:
Field of the Invention
The invention relates to plastic orthodontic `
plastic 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 plaætic brackets.
Description of the Prior Art
Prior art plastic orthodontic brackets are geo-
metrically 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 distor-
tion 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 plas-
tic orthodontic brackets which do not have a design which mini-
mizes breakage and distortion:
3,303,565 ~ 2,971,258 3,052,027
3,469j314 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 orthodon-
tic bracket which is concerned with eliminating bracket break-
age or distortion by utilizing a quasi triangular shaped
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bracket body, more effective material bulk distribution
between the archwire slot and a ligature undercut by lessen-
ing 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 meso-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.
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One aspect of the device disclosed herein 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 tri-
angular shape of the bracket functions to minimize the
forces applied to the bracket during mastication and occlu-
sion 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 device disclosed herein 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 sub-
stantially filled when a small gauge ligature wire or elastic
ring is disposed therein. The preferred depth is approxi-
mately .015 mm. 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 device disclosed herein is
that all surfaces on the bracket body as defined from the
ligature undercut extending to the archwire slot and to a
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slot disposed perpendicular to the archwise 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.
Accordingly, this invention provides 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 single quasi-
triangular shaped bracket body extending from the bracket
base, 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.
The invention will be readily appreciated by refer-
ence to the detailed description when considered in conjunc-
tion with the accompanying drawings in which like reference
numerals designate like parts throughout the figures and
wherein:
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Figure I is a perspective view of a prior art
plastic orthodontic bracket;
Figure II is a front view of a plastic orthodontic
bracket constructed according to the invention;
Figure III is a side view of a plastic orthoaontic
bracket constructed according to the invention; and
Figure IV is a top view of a plastic orthodontic
bracket constructed according to the invention.
Detailed Description of the Invention
Referring to figure I, a typical prior art plastlc
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 axe 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 figure 1, 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 parti-
ally 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
30 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
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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 figures II-IV, a plastic orthodontic
bracket which is constructed according to the present in-
vention is shown as it would be attached to a maxillary
lateral tooth. Bracket lO has a horizontally disposed arch-
wire slot 30 cut in the front surface of the bracket body
32. The rear surface of base 34 of bracket lO 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 f~rms 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 clini-
cal 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 .015 mm 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 arch-
wire and vertical slots 30 and 40 to lessen internal stress
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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 ligatur:e ~nder-
cut 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 extension 44 is preferably 0.5 mm long. The lip
section 46 of the bracket body 32 located next to the liga-
ture undercut 42 is relie,ved incisally and gingivally to
the archwire to facilitate easy placement of the ligature.
. .
Referring specifically to Figure III, it may be
seen that archwire slot 30 is not perpendicular to the plane
of the base 34. The archwire sl~ot 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
parallQl 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.
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TABLE 1
p, I_ p, ~ It
~q o u
E .4 ~ ~ ~
~ O
maxillary central 5.8 6.3 0 21 0.5
maxillary lateral 4.8 5.3 0 14 1.0
maxillary cuspid 5.8 6.3 + 7 0.5
mandibular central 4.8 5.3 0 17 1.0
mandibular lateral 4.3 6.3 0 17 1.0
mandibular cuspid 5.8 6.3 + 4 0.5
To use the bracket, base 34 is cemented to the labial sur-
face 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 config-
uration 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 adheslve is disclosed
in United States patent application 386,416 filed August
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 poly-
carbonate is chosen, the shear streng*h should range from
10,000 to 16,000 p.s.i. Deformation at 70 F under a 4000 psi
load is less than .1%. If an unfilled polycarbonate is chosen,
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the shear strength should range from 6000 to 8500 psi.
Deformation at 70F under a 4000 psi load is from .1 to .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.
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