Dentistry – Orthodontics – Bracket
Reexamination Certificate
2000-07-18
2002-03-19
Lucchesi, Nicholas D. (Department: 3732)
Dentistry
Orthodontics
Bracket
C433S020000
Reexamination Certificate
active
06358043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an orthodontic appliance, particularly relates to an orthodontic bracket made of plastic, and an archwire engaged to the bracket.
2. Description of the Related Art
So far, as the orthodontic bracket as an essential part of an orthodontic appliance, various materials such as metal, ceramics, plastics and the like have been employed. The orthodontic bracket has respective merits and demerits depending upon the material (raw material) of which it is made.
For example, the metal orthodontic bracket has a mechanical strength required in the orthodontic treatment, but has a demerit that the metal color will attract attention when a mouth is opened, giving an aesthetically undesirable impression.
The ceramic orthodontic bracket is aesthetically more excellent than the metal one, but has a demerit, for example, that the ceramics is materially harder than tooth enamel. This will often abrade occluded teeth and cause the archwire to be abraded as well. Further, since the ceramic orthodontic bracket is materially fragile, the bracket will be often cracked when it is applied with torque by an angular wire. Further, there may be a trouble that the tie wings supporting the ligature will be broken when the archwire is tied to the bracket.
Further, the ceramic orthodontic bracket has a high friction resistance compared with the metal one and will detract the slide property of archwire. Namely, the ceramic bracket tied onto the archwire will not smoothly slide along the archwire. This will prevent the effective movement of tooth and cause the orthodontic treatment to take more time.
The plastic orthodontic bracket is superior in moldability (processability), but has a demerit, for example, that aesthetic property will be lost due to the progress of discoloration and deterioration by chemical components and pigments contained in foods. Further, the plastic orthodontic bracket is inferior in mechanical strength compared with the metal one.
Especially the recent orthodontic bracket has been required to have a special aesthetic property in addition to the fundamental function required in orthodontics.
Therefore, the ceramic orthodontic bracket and plastic orthodontic bracket have come to be generally employed.
As to the ceramic orthodontic bracket and plastic one, the slot portion is, for example, contrived as is replaced by metal or other material to reinforce the slot portion and simultaneously decrease its friction resistance, and further the metal is provided as a core material (reinforcing member) inside of the bracket (U.S. Pat. No. 5,595,484).
Recently, resins of low water absorption have been developed, which will not be discolored, for example, by coffee having a relatively strong pigment.
Under these circumstances, demand for the plastic orthodontic bracket has been increased. The plastic orthodontic bracket is generally made of polycarbonate(PC) because the polycarbonate is superior in transparency, chemical stability, impact resistance, dimensional precision when molding in addition to non-poisonous property, thereby to heighten the adaptation for the orthodontic bracket.
Thus, the plastic orthodontic bracket is superior in aesthetic effect and has come to be generally employed. However, the plastic orthodontic bracket has some problems remaining unsolved.
First of all, the problem is the solvent crack. Generally, the orthodontic bracket is directly adhered to a tooth by a dental adhesive. Therefore, in case the plastic bracket has an inner stress therein at the time of being molded (inner stress due to distortion generated when molding), the plastic bracket will often react to an organic solvent contained in the adhesive to produce the solvent crack at the bracket base and wings. As a result, the plastic bracket reduces the strength thereof to about {fraction (1/10)} of the standard strength rendering the plastic bracket to be useless.
In this respect, there has been an inconvenience that a specific adhesive, which will produce no possible problem, has to be selected in consideration of the compatibility between the plastic material for the orthodontic bracket and the solvent contained in the adhesive.
A second problem is lowered strength and aesthetic property due to the hydrolysis of synthetic resin.
Namely, it may be considered that the hydrolysis of polycarbonate is encouraged by the cut of ester coupling because carbonyl group is easily coupled to water molecule. Thus, the strength is reduced to a half of the original strength in about one year in the mouth of human body temperature. Further, the polycarbonate will easily be hydrolyzed if it has a high percentage of water contained before being molded. As results, it comes to invite the degradation of quality including bubbles created in the molded member, lack of surface luster, uneven fluidity, and lack of tensile strength and durability. Further, it is generally known that the plastics susceptible to hydrolysis will easily propagate microbes which produce the hydrolysis enzyme (enzyme for catalyzing hydrolysis) to accelerate the hydrolysis.
A third problem is adhesion strength.
Namely, it is preferable that the plastic orthodontic bracket is made of a resin of low water absorption which will produce no hydrolysis for a long period of time to maintain a desirable aesthetic property. However, the synthetic resin of low water absorption lacks the compatibility to the adhesive for obtaining sufficient adhesion strength to fix the resin to the tooth.
The plastic bracket having a reinforcing structure will be further specifically explained.
For example, the following plastic brackets having the reinforcing structure have been known. Namely, as disclosed in U.S. Pat. No. 4,299,569 and JP-A-9-98988, a metallic slot liner is provided in a mediodistal direction. As disclosed in U.S. Pat. Nos. 5,595,484, 5,813,852 and 5,597,302, a metal member having U-shaped cross section is insert-molded in the direction perpendicular to the slot.
In the metal slot liner structure and the metal insert structure, during the injection molding process, the liner and the insert member is disposed in an injection molding die prior to molding. Then, the resin encloses the liner and the insert member by the injection molding to form the plastic bracket. In this case, the resin for use is transparent or translucent and has high aesthetic property, such as polycarbonate, acryle, polyuretane and polyacetal, which are adhesive and water-resistant. However, in the molding process, during solidification of the resin by cooling, large residual stress occurs on the inside of the resin due to difference of expansion coefficients between the metallic liner/the insert member and the matrix resin.
Thus, in the structure in which the metallic liner and the insert member are integrally molded, comparatively large residual stress exists therein. Consequently, as described above, the plastic bracket senses to react with the adhesive solvent so as to cause the solvent crack when the bracket adheres to the tooth surface. Further, due to the existent residual stress, some kinds of resins accelerate the hydrolysis as described above. Thus, the tie wing or the like is broken before the generally required orthodontic treatment period (18 to 24 months).
Further, it has been generally performed that silane-coupling-treated inorganic filler, which is glass filler, is mixed to the matrix resin in an amount of 10 wt % to 50 wt %, to thereby enhance the mechanical strength. For example, as disclosed in U.S. Pat. Nos. 4,717,341 and 5,254,002, in the orthodontic appliance, the physical property and the durability of the bracket and the archwire are enhanced within such range as not to lower aesthetic property.
Glass filler is dispersed into the plastic bracket so as to enhance the cracking resistance property and the rigidity. Simultaneously, micro inner stress occurs. That is, a matrix resin having high inner stress exists around glass filler because of the difference of the thermal expansion coeffici
Kowata Masumi
Mottate Mikio
Shiga Kiyoshi
Lucchesi Nicholas D.
Tomy Incorporated
LandOfFree
Orthodontic appliance does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Orthodontic appliance, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Orthodontic appliance will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2852444