Abstract:
A dental appliance comprising at least two plies of thermoformable material, wherein a first ply of a first material, having a first set of characteristics properties, provides a first primary surface of the dental appliance and wherein a second ply of a material different from the first ply, having a second set of characteristic properties, provides an opposing second primary surface of the dental appliance.

Description:
The present invention is generally directed to dental appliances and more particularly to thermoformed dental appliances. 
     BACKGROUND 
     Orthodontic and other dental laboratories provide services for the custom-fabrication of dental appliances for an individual patient according to a treatment plan and a prescription provided by an attending orthodontist or dentist. A poured and cured stone replica of a patient&#39;s mouth structure, such as an initial malocclusion for example, is typically provided by the doctor for the laboratory&#39;s use. 
     The dental technicians at the lab first modify the stone model by cutting the mal-positioned teeth free of the model and the adjacent teeth. Next, a technician repositions the teeth on the model semi-rigidly into desired, ideal positions as specified by the doctor and as determined by the doctor&#39;s diagnosis and subsequent treatment plan. After the stone model has been modified or “corrected” in this manner, the model will be positioned within an appliance-forming machine where a sheet of thermo-formable elastomeric material is typically “sucked-down” or pressed over the stone model(s). 
     The use of thin sheets of various thermo-formable plastics including vinyl and olefin-type materials has been adopted for many current appliances. This, along with an efficient forming process involving the rapid use of pressure, vacuum and heat, has replaced prior molding and casting processes. The use of these materials, including polypropylene in sheet form and in thicknesses of about 1 mm (before thermoforming), eliminates the time-consuming steps of mixing, catalyzing and curing and optionally heat-curing in a pressure flask, as was required by natural rubber, medical-grade urethane, silicone and vinyl silicone series materials. 
     Polypropylene (PP), for example, is often used in making dental appliances because it is non-reactive chemically and thoroughly biocompatible. However, PP alone may exhibit plastic creep over time, resulting in minor, but undesirable changes in the appliance&#39;s shape and it may also lack the level of clarity desired by some patients. Likewise, PP&#39;s characteristics for making a dental appliance by thermoforming is adequate, but not ideal for consistent, accurate products. This may prevent cost effective production of consistent, high-quality appliances demanded by dentists and their patients. 
     Better thermoforming characteristics are found in medical grade copolyester sheets, which are also used to make dental appliances. However, dental appliances made from copolyester sheets alone have a tendency to become brittle over time. As a result, a patient wearing such an appliance who has a tendency to grind his teeth may grind into or through the appliance, reducing or eliminating the appliance&#39;s effectiveness. 
     SUMMARY OF THE INVENTION 
     It has been discovered that a multiple ply sheet of different materials can be used to manufacture a dental appliance that has the desired mechanical properties for a dental appliance while also exhibiting the type of thermo-forming characteristics that permit dental appliances to be consistently produced with high quality. 
     According to an exemplary embodiment of the invention, a dental appliance comprises a thermoformable material having at least two plies, in which each of the two plies are of different materials. The material of the first ply has a first set of characteristic properties and forms a first surface of the appliance and the material of the second ply has a second set of characteristic properties and forms an opposing second surface of the appliance. Each ply has two major surfaces; the first and second plies have a common interface, with one ply having a major surface facing away from the teeth and the other ply having a major surface facing toward the teeth. 
     According to another exemplary embodiment of the invention, a method of making a dental appliance is also provided. The method includes providing a sheet of thermoplastic material having at least two plies as described above, preferably by co-extruding, and thereafter thermoforming the sheet to form a dental appliance. 
     One advantage of exemplary embodiments of the invention include providing a thermoformed dental appliance made from a multi-ply sheet of different materials that has better combined mechanical and thermoforming properties than a sheet of a single material. 
     Another advantage of certain exemplary embodiments of the invention is that the thickness of the multi-ply sheet used in the dental appliance is comparable with that of single ply materials used in thermoformed dental appliances. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a dental appliance according to one exemplary embodiment of the invention. 
         FIG. 1   a  is a cross-sectional view of the dental appliance of  FIG. 1 . 
         FIG. 2  illustrates a coextruded sheet for use in forming a dental appliance according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     As used herein, “dental appliance” means any device capable of being formed for use in dental applications with or without additional components such as wires and includes, by way of example only, aligners, positioners, night guards, retainers, splints, bleaching trays, and anterior bridges.  FIG. 1  illustrates an exemplary dental appliance  5  according to one embodiment of the invention. The appliance  5  has a first, or outer, surface  15  that, when worn, is exposed to the patient&#39;s mouth tissue, as well as any food or drink entering the patient&#39;s mouth. A second, inner surface  25  of the dental appliance  5  that is arranged to face the surface of one or more of the patient&#39;s teeth. 
     The appliance  5  is formed from a multi-ply sheet  10  as shown in  FIG. 2 . The sheet  10  includes a first ply  100  of a first material and a second ply  200  of a second material different from the first material. The sheet  10  has a first surface  11  that is also a first surface of the first ply  100  and an opposing second surface  12  that is also a first surface of the second ply  200 . The first surface  11  of the sheet  10  may be positioned during the dental appliance manufacturing process to form either one of the outer surface  15  or the inner surface  25  of the dental appliance  5 , such that the sheet&#39;s opposing second surface  12  forms the opposite surface  25  of the dental appliance  5 . 
     The first ply  100  in the multi-ply sheet  10  can be any thermoplastic or elastomeric material that is permanently deformable under heat or pressure and that has properties suitable for use in a dental appliance. Thermosetting materials may also be used, which, for example, may be cured during the thermoforming process. By “permanently deformable” is meant that the materials can be formed to a desired shape under conditions of elevated pressure and/or temperature and which will substantially retain that shape under normal conditions experienced in the mouth, but does not necessarily mean that subjecting the materials to elevated pressures and temperatures would not cause permanent deformation to a new, different shape. Properties which are desirable for use in a dental appliance include transparency, toughness, and biocompatibility by way of example only. Copolyesters have the characteristic of providing consistently accurate thermoformed dental appliances and may be particularly suitable materials for use as the first ply  100 . Other exemplary materials for use as the first ply  100  include ethyl vinyl acetate (EVA), polypropylene (PP), polyvinylchloride (PVC), low density polyethylene (LDPE), and high density polyethylene (HDPE). 
     The second ply  200  in the multi-ply sheet  10  can be any thermoplastic or elastomeric material permanently deformable under heat or pressure that is a different composition from the first ply material. The second ply material should also have suitable properties for use in a dental appliance and should have a coefficient of thermal expansion comparable or equal to that of the first ply material. Otherwise, the subsequent thermoforming process to make the dental appliance  5  may cause one ply to shrink disproportionately with respect to the other, which may result in undesirable crimping or puckering that could lead to a poor quality or unusable dental appliance. One suitable material for the second ply  200  include copolyester ethers, although copolyesters of a composition different from the first ply material may also be used. Other exemplary materials for use as the second ply  200  include ethyl vinyl acetate (EVA), polypropylene (PP), polyvinylchloride (PVC), low density polyethylene (LDPE), and high density polyethylene (HDPE), and may also include blends of polymeric materials, with or without a rubberizing additive. 
     Because certain copolyesters may have excellent thermoforming properties but less desirable mechanical characteristics, the material selected for the second ply  200  may be selected for mechanical properties to provide a multi-ply sheet  10  having a combination of desirable properties. Conversely, if the material selected for the second ply  200  has good mechanical properties, it may, but does not necessarily need to have comparably good thermoforming properties if the material for the first ply  100  is selected for that characteristic. 
     In the preferred embodiments in which the multiple ply sheet  10  is manufactured by coextrusion, the materials of the two plies  100 ,  200  should also be compatible with one another for the coextrusion operation or be capable of being made compatible through the addition of a compatibility agent as are known to those of ordinary skill in the art. 
     Because the sheet  10  is multi-layered and because the plies  100 ,  200  may have different characteristic properties, the materials selected for each ply  100 ,  200  may be selected depending on whether the particular ply will primarily be in contact with the tooth surface, i.e. the ply providing the inner surface  25  of the dental appliance  5 , versus the ply primarily in contact the soft tissue of mouth, i.e. the ply providing the outer surface  15  of the dental appliance  5 . It will be appreciated that the different materials may be of different polymeric classes (e.g., copolyesters versus copolyester ethers) or may be of the same class, but have different compositions (e.g., copolyester A versus copolyester B). 
     According to a presently preferred embodiment, the sheet  10  is a copolyester/copolyester ether coextruded sheet and more specifically is a coextruded sheet having plies of Medstar and Ecdel. “Medstar” is a trade designation for a copolyester commercially available from Eastman Chemical Co. of Kingsport, Tenn. “Ecdel” is a trade designation for a copolymer of cyclohexane dicarboxylic acid, ethylene glycol, and cyclohexane-1,4-dimethanol that is also commercially available from Eastman Chemical. 
     In accordance with the presently preferred embodiment, the multi-ply sheet  10  is produced by coextrusion, although chemical and/or mechanical bonding techniques may also be used to form the multi-ply sheet  10 , such as using adhesive, surface tension, a joining element, or by imparting a surface finish, such as a dovetail, that permits one ply to capture another, by way of example only. 
     Techniques for coextruding sheets of two or more plies of different material is well known to those of ordinary skill in the art. In accordance with exemplary embodiments of the invention, the die(s) for use in the coextrusion process is sized and dimensioned to produce a multi-ply sheet  10  having a total thickness (before thermoforming) of about 0.25 mm to about 5 mm, more preferably less than about 1 mm and still more preferably less than about 0.875 mm. Each ply within the multi-ply sheet is typically equal in thickness, although the relative thicknesses can be varied, and may be as high as about 70% to about 90% of one and about 10% to about 30% of the other. 
     Referring to  FIG. 2 , an opposing second surface  120  of the first ply  100  is positioned to face toward an opposing second surface  220  of the second ply  200 . It will be appreciated that the sheet  10  may be extruded such that the plies  100 ,  200  are in direct contact with one another at an interface  150 . Alternatively, a binder or adhesive may be intermediate the two plies  100 ,  200  to chemically or mechanically enhance the plies&#39; adherence to one another to produce a single sheet  10 . 
     It will further be appreciated that more than two plies can be employed, although the overall sheet thickness should remain as described above. The material selected for any subsequent ply can be the same or different than the material used in any other ply, although at least one ply of a different material is typically intermediate any two plies of the same material. In some embodiments, it may even be possible to use an intermediate ply of a material, as for example, a bonding layer, that is not biocompatible but which has particularly desirable other characteristics that is sandwiched between two outer plies of materials which are biocompatible, thereby negating the bio-incompatibility of the intermediate material. 
     Once the multiple ply sheet  10  has been produced, it may be used in dental appliance  5  manufacture such as by thermoforming it to a final shape in a manner known to those of ordinary skill in the art. Generally, thermoforming is accomplished by subjecting the multiple ply sheet  10  to a combination of pressure, heat and/or vacuum while positioned over a stone model which results in the sheet  10  generally conforming to the shape of the model to form the dental appliance  5 . Custom appliances are created by providing a model of a particular patient&#39;s mouth in which the dental appliance will be used; the stone model may be a model of the patient&#39;s actual mouth or, in the case of an aligner, for example, the stone model may be a slightly modified version of the patient&#39;s mouth created by a laboratory technician to which the actual mouth is expected to conform over time. 
     Because the multi-ply sheet  10  will typically have two surfaces  11 ,  12  with different characteristics from one another, when the sheet  10  is aligned with the stone in preparation for thermoforming, the ply to be primarily in contact with the teeth should be arranged to face the stone so that it forms the inner surface  25  of the dental appliance  5 . That is, whether the first surface  11  of the multi-ply sheet  10  (i.e., the outer surface  110  of the first ply  100 ) corresponds to the outer surface  15  or the inner surface  25  of the dental appliance  5  depends on whether the material selected for the first ply  100  is better suited for facing toward or away from the surface of the patient&#39;s tooth. For example, in accordance with the preferred embodiment illustrated in  FIG. 1   a  in which the multiple ply sheet  10  has a first ply  100  of Medstar and a second ply  200  of Ecdel, the sheet  10  is positioned and the manufacturing process is conducted so that the Medstar material (i.e., the first ply  100 ) forms the inner surface  25  of the dental appliance  5 , while the Ecdel material (i.e., the second ply  200 ) forms the outer surface  15  of the dental appliance  5 . 
     In some cases, it may be economical to produce the multi-ply sheet  10  in dimensions larger than would ordinarily be used for making any single dental appliance  5 . Thus, the multi-ply sheet  10  may be provided in large rolls or sheet form which can be cut into smaller pieces by a laboratory technician. Alternatively, the multi-ply sheet  10  may be cut down into individual sized pieces as part of the manufacturing and packaging process for use in one or more specific models of thermoforming devices. To keep the smaller multi-ply sheets  10  from sticking together and prevent damage during handling, a protective film may be removably adhered to each surface  11 ,  12  of the sheet  10 . 
     While the foregoing specification illustrates and describes exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.