Dental article containing light-curable paste

Abstract: A dental tape is disclosed comprising a light-curable, non-toxic paste layered between two cover sheets, at least one of which releasably adheres to the paste, and a dental article is disclosed comprising an orthodontic bracket having a base and a light-curable, non-toxic paste sandwiched between the base and a releasably adhering substrate.

Description: The present invention relates to an orthodontic bracket having a layer of curable paste thereon acting as an adhesive. Curable pastes useful as restorative materials and adhesives for orthodontic brackets are known. However, use of such pastes often requires painstaking measurement of components and mixing by the dental practitioner. Further, the practitioner must carefully determine the minimum amount of paste necessary for bonding in order to avoid excess adhesive around the bracket, which would require further time consuming removal in order to eliminate an undesirable site for plaque accumulation. Furthermore, many known pastes have the disadvantage of having to be activated outside the mouth, before application, thereby reducing available working time inside the mouth; once the paste was activated, the practitioner had little time to work the paste, e.g., into a suitable restoration element. U. S. Patent No. 4,204,325 discloses an orthodontic bracket having a curable paste layer applied thereto, which is covered by a releasable film. After removing the film, and prior to placement of the bracket on the tooth, the paste layer is activated by surface application of a curing agent. Since activation occurs outside the mouth, the practitioner is given little time to position the bracket at the precise location desired on the tooth surface. Accordingly, the present invention is a dental article comprising an orthodontic bracket having a base adapted to adhere to a tooth surface and a non-toxic paste containing a photo-initiator, said paste being sandwiched between the base and a releasably adhering substrate. Fig 1 is an end elevational view of the adhesive-coated orthodontic bracket of the present invention. Light-curable, non-toxic pastes useful in accordance with the present invention contain a photoinitiator and a filler dispersed in a resin. Useful resins are hardenable, organic compounds having sufficient strength, hydrolytic stability, and non-toxicity to render them suitable for use in the mouth. Examples of such resins include acrylate, methacrylate, urethane, and epoxy resins, such as disclosed in U.S. Pat. Nos. 3,066,112, 3,539,533, 3,629,187, 3,709,866, 3,751,399, 3,766,132, 3,860,556, 4,002,669, 4,115,346, 4,259,117, 4,292,029, 4,306,190, 4,327,014, 4,379,695, 4,387,240, and 4,404,150. Mixtures and derivatives of such resins are also useful. Preferably, the resin is a mixture of diglycidylmethacrylate of Bisphenol A ("Bis-GMA") and triethyleneglycol dimethacrylate ("TEGDMA"). Useful photoinitiators are non-toxic compounds, such as ketone or alpha-diketone compounds, alone or in combination with suitable amines, peroxides, sulfur compounds, phosphorus compounds, or other compounds capable of reacting with or being sensitized by the ketone or alpha-diketone compounds to effect polymerization of the resin. Preferred photoinitiators include a mixture of dl-camphorquinone and dimethylaminophenethanol ("DMAPE"), and photoinitiator systems such as disclosed in European Pat. Application No. 0290133. Suitable fillers for oral use are well known, such as disclosed in U.S. Pat. Nos. 4,695,251 and 4,435,160. Preferred fillers include quartz, radiopaque glass, and non-vitreous microparticles as disclosed in U.S. Pat. No. 4,503,169. The microparticles are particularly advantageous in pastes intended for anterior restorations since they provide a cured composite paste having a low visual opacity. Other preferred fillers include low hardness minerals such as kaolinite, mica, pyrophyllite, and talc. The paste is preferably prepared in the absence of light by first combining the resin and photoinitiator and then adding the filler. The absence of occluded air from the paste is desirable. Accordingly, the paste is preferably mixed under partial vacuum as well. The paste contains a filler and optional pigment to produce varying degrees of opacity as desired. Optionally, the paste is fiber or fiber scrim reinforced. Useful fiber types include glasses, such as electrical grade glass, and ceramics. Useful scrims have various yarn types, identified by the number of filaments per yarn, the number of yards per 2.54 cms in both warp (machine) and fill (transverse) directions, and the weave style. Optionally, the paste includes suitable adjuvants such as accelerators, inhibitors, stabilizers, pigments, dyes, viscosity modifiers, extending or reinforcing fillers, surface tension depressants and wetting aids, or anti-oxidants. Particularly useful optional ingredients include colloidal silica thickeners, which help provide an optimum viscosity for the paste, and may permit higher amounts of filler loading needed for particular applications. Amounts of components used in the paste vary depending on particular ingredients used as well as the intended application. Generally the resin/filler ratio is adjusted to provide a sufficient consistency to the paste to permit easy handling and prevent slump. Generally, more heavily loaded pastes provide more visually opaque cured products. The amount of photoinitiator used is generally an amount sufficient to cure the resin after a brief exposure to a curing light. For general applications, the paste preferably comprises 10-40% resin, 60-90% filler, and 0.1-5.0%, more preferably 0.1-1.0%, photoinitiator. The thickness of the paste layer is adjustable by the skilled artisan depending on specific applications, with preferred thickness varying between 0.2 and 1.3 mm. Removably adhering cover sheets are preferably "hand hard" (i.e., resilient) films made of polyester, fluorinated polymers such as polytetrafluoroethylene, or olefin polymers such as polypropylene or polyethylene. More preferably, the removably adhering cover sheet has a low-adhesion backsize coating applied on the surface contacting the paste for ease of removal. Such low-adhesion backsize coatings include silicone coatings and polytetrafluoroethylene coatings, which are well known to those skilled in the art. Preferably, the thickness of the removably adhering cover sheet is between 0.00635 and 0.0127 mm. For an orthodontic bracket adhesive, disposed on an orthodontic bracket according to the present invention, the paste preferably contains 10-70% resin and 30-90% filler by weight of the paste. Precise amounts, determinable by the skilled artisan, vary depending on the particular components used as well as the bracket composition. The filler preferably has a Mohs hardness less than 4, more preferably less than 3. These preferred values for Mohs hardness ensure a filler that is less hard than either natural tooth enamel or the typical abrasive materials used for adhesive removal, thus facilitating removal of the adhesive after the orthodontic bracket is removed at the termination of treatment while minimizing enamel damage. When used as a bracket adhesive, the consistency of the paste is preferably adjusted to provide the desired mechanical retention of the paste on the bracket as well as to allow good mesh penetration and flow around the screen wires of both 60 and 100 mesh screen bases of orthodontic brackets. In the present invention, an orthodontic bracket is provided with a layer of light-curable adhesive sandwiched between the base of the bracket and a releasably adhering substrate. Preferably, the substrate is a flexible cover sheet, such as the removably adhering cover sheets described hereinabove. Referring to Fig. 1, paste layer 4 is provided between base 5 of bracket 6 and cover sheet 8. To obtain the embodiment shown in Fig. 1, for example, after removing a releasably adhering cover sheet (not shown in Fig. 1), the cover sheet-backed adhesive is pressed onto the base between press platens or nip rolls with predetermined pressure sufficient to provide penetration of mesh screens or perforations, where present. Preferably, a tape is is first cut and shaped to correspond to the size of the bracket base before application thereto. Alternatively, the paste can also be applied to the bracket base and then covered by a removably adhering cover sheet. To bond the bracket 6 to a tooth, the cover sheet 8 is then removed from the paste and the paste-covered bracket base applied to the tooth. A light source is then directed around the periphery of the bracket base, through the bracket itself if transparent to the curing light, or through the lingual side of the tooth for a sufficient time to cure the paste. The present invention frees the practitioner from the need to dispense and apply an adhesive to an orthodontic bracket. Furthermore, adequate penetration of the adhesive into the bracket screen or perforations can be predetermined and ensured under manufacturing conditions, freeing the practitioner of this uncertainty. In addition, excess adhesive is minimized or eliminated, reducing the time normally required for clean-up. Useful orthodontic brackets are well known in the art and are made from materials such as stainless steel, ceramics (e..g., polycrystalline alumina and sapphire), and transparent plastics. Typically, the bracket has bracket posts adapted for fastening arch wires that are attached to a base adapted to conform to the buccal or labial surface of the tooth. The bracket base may have an irregular surface adapted to receive the paste in such a way that, when cured, the paste anchors the bracket to the tooth surface. Examples of modified bracket bases are well known and include wire mesh screens fixed to the base as well as perforated and undercut bases. Most preferably, metal bracket bases are equipped with a wire mesh screen having, e.g., a 60 or 100 mesh size. To anchor a metal bracket securely to the tooth, the paste preferably penetrates the screen, flowing around the individual mesh wires. The paste layer provided on the bracket in accordance with the present invention has a preferable thickness between 0.245 and 0.508 mm. Visual opacity of the paste is controlled by matching the refractive indices of the resin and the filler. The closer the refractive indices, the more translucent the cured paste. Of course, opacity is also affected by the thickness of the cured paste. That is, a cured paste may be translucent at a given thickness, but become progressively more opaque as thickness increases. Furthermore, visual opacity is reduced by removal of occluded air from the paste, e.,g., by mixing the paste under partial vacuum. The paste is preferably radiopaque (i.e., opaque to x-rays) or radiolucent (i.e., translucent to x-rays). The amount of radiopacity can vary and is measurable by known methods, such as disclosed in the aforesaid U.S. Pat. No. 4,503,169. Optionally, the paste used in accordance with the present invention contains reinforcing fibers such as glass or ceramic fibers. Preferably, the fibers have an index of refraction closely matching the cured resin to maintain the desired translucency. In the orthodontic bracket of the present invention, a fiber scrim is preferably imbedded in the adhesive layer. To more fully describe the present invention, the following non-limiting examples are provided. All parts and percentages in the examples are by weight unless indicated otherwise. EXAMPLE 1 A series of light-curable pastes containing a resin system and a filler system are mixed in a light-excluding mixer (Double Planetary Ross Mixer Model LDM, Charles Ross & Son, N.Y.) and formed into tapes for orthodontic adhesives. The resin system is a 50/50 blend of Bis-GMA and TEGDMA, combined with 0.25% camphorquinone and 0.5% dimethylaminophenethanol, based on the weight of the resin. The filler system formulations along with pertinent test data are shown in Table 1. The colloidal silica used is a hydrophilic, amorphous, fumed colloidal silica having a 50 m²/g particle surface area (available from Degussa Corp. under the name Aerosil™ OX-50). The ZrO₂/SiO₂ filler is prepared according to Example 6 in the aforesaid U.S. Pat. No. 4,503,169. SAMPLE NUMBER FILLER SYSTEM % FILLER LOADING IN PASTE COMPRESSIVE STRENGTH (p.s.i.) DIAMETRAL TENSILE STRENGTH (p.s.i.) CONSISTENCY (mm) WEIGHT% COLLOIDAL SILICA WEIGHT % ZrO₂/SiO₂ FILLER 1 0 80.00 80.0 57120 11613 27.78 2 2.0 78.00 80.0 57365 11332 28.58 3 2.8 77.20 80.0 58262 11487 31.75 4 4.0 76.00 80.0 58042 12176 39.69 5 4.5 77.80 82.3 62101 12049 25.4 6 8.0 72.00 80.0 62000 12467 46.04 7 10.7 73.20 83.9 66246 13526 36.51 8 13.2 74.80 88.0 64394 13664 28.58 9 21.97 65.93 87.9 68538 14030 28.58 10 32.0 48.00 80.0 66132 13312 43.66 11 40.0 40.00 80.0 67179 12641 39.69 12 48.0 32.00 80.0 69234 12804 26.99 13 52.5 22.50 75.0 68576 10468 32.54 14 56.08 14.02 70.1 71148 9831 42.07 15 62.3 0 62.3 68357 9546 42.86 The adhesives are formed into tapes by rolling or pressing the pastes between low adhesion cover sheets. Two types of low adhesion cover sheets are used: (1) a low-density polyethylene-coated paper with a silicone release agent (Polyslick™, General Electric) applied to the polyethylene coat and a high density polyethylene coat on the backside of the paper to prevent curling, and (2) a clay-filled kraft paper coated with a silicone release agent (Silicone Premium, General Electric). The tapes are slit into narrow widths to correspond to the bracket width. The bracket used is an American Orthodontics, Bracket No. 095-007, 100 mesh, stainless steel brazed, central. Segments are then cut to fit the bracket screen, one cover sheet is removed and the adhesive is pressed onto the bracket screen. The remaining cover sheet is removed and the bracket is pressed onto an acid etched bovine tooth. To cure the paste, an ESPE "Elipar" dental curing light is directed around the periphery of the bracket for 20 seconds. A 0.3675 mm adhesive thickness provides good adhesion, requiring little or no clean-up of excess adhesive. To determine compressive strength and diametral tensile strength, a sample of the uncured paste is placed in a 4.06 mm I.D. glass tube capped with silicone plugs in each end of the tube. The tube is placed in a test rig pressurized with air at 0.28 MPa. The test rig contains two ESPE "Elipar" dental curing lights aimed at opposing sides of the tube and mounted on a turntable which enables the lights to be rotated around the tube in a 180 o arc. The light guide of each curing light is spaced 3 mm from the tube wall. While operating both curing lights simultaneously and oscillating them continuously around the tube, the tube is exposed to four 20 second curing cycles. The tube is then removed from the test rig, placed on a pair of spaced rollers, and rotated under a 110 watt "Ritter" dental operatory light at a distance of 0.6 meters for one hour. The cured sample is removed from the tube and sliced into cylinders with a diamond saw. For compressive strength testing, an 8.23 mm long cylinder is employed, and for diametral tensile strength testing, a 2.21 mm long cylinder is employed. The cylinders are stored in 37 o C distilled water for 24 hours, then tested according to ADA Specification Nos. 8 and 27. Consistency is measured as the spread of 0.5 ml of paste sandwiched between two 10.16 x 10.16 cm glass plates under a 907.2 g weight. A quantity of 0.50 ml paste is delivered onto the bottom plate, then the top plate and 907.2 g weight are added. The combined mass of the top plate and the 907.2 g weight = 1027 ± 10 g. After 2 min., the spread (diameter) of the paste is measured to the nearest 0.794 mm, and 3 readings are averaged.

Claims: A dental article comprising an orthodontic bracket (6) having a base (5) and a non-toxic paste (4) sandwiched between the base (5) and a releasably adhering substrate (8) characterized in that the non-toxic paste (4) contains a photoinitiator. The article of claim 1 wherein the substrate releasably adheres to the paste (4) through a low-adhesion backsize coating. The article of claim 1 or 2 wherein the bracket (6) is transparent to actinic radiation. The article of any one of claims 1, 2 or 3 wherein the substrate is a flexible cover sheet (8).

Inventors: RANDKLEV RONALD M C O MINNESOT, RANDKLEV, RONALD M., C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1990
Main classification: A61C7
IPC codes: A61C7, A61C5, A61C13