Glass orthodontic bracket

Abstract: An orthodontic bracket is made of a partially crystallised glass material. The orthodontic bracket has a thin compressive outer crystalline layer (38). A method of making the bracket comprises forming the bracket from the glass material and subjecting it to a heat treatment to cause crystallisation of the outer layer.

Description: In the field of orthodontics there has been an increasing demand for brackets that are visually aesthetic. In response to this need, various materials have been suggested for use as orthodontic brackets. Plastics materials have been found to be unsatisfactory due to their inability to provide the appropriate strength necessary for an orthodontic bracket. Various ceramic materials have been suggested in the prior art. However, prior art brackets made from these materials are quite brittle and are subject to fracture. There has also been suggested orthodontic brackets made of single crystalline alumina or cubic zirconia. While these have been found to provide the strength necessary for use as an orthodontic bracket, they are relatively expensive materials, and are relatively difficult to machine due to their hardness. They are also inclined to fracture due to their extreme hardness. It has also been suggested to make orthodontic brackets from an ion exchange strengthened glass, such as described in US-A-4,784,606. This patent discloses strengthening the glass by subjecting it to an ion exchange reaction. The ion exchange treatment is carried out by immersing the glass bracket in a bath of molten salt at elevated temperatures above the strain point and below the softening point of the glass. The treatment time can vary from 2 to 24 hours. After it is removed from the bath, the bracket is washed clean of excessive salt. This of course adds substantial manufacturing cost. Additionally, the ion layer formed is very thin, and as a result is subject to early wear which can result in failure of the part due to stress risers that can form. It has further been suggested in the prior art to produce orthodontic brackets of a glass ceramic having dual crystalline structure, such as illustrated in US-A-4789649. This patent discloses a bracket structure having relatively large crystals covered by a layer of smaller, flat crystals. While this structure may provide increased strength, its central crystalline structure is relatively difficult to machine and is subject to fracture in the same manner as other crystalline type brackets of the prior art. EP-A-0296384 describes a ceramic orthodontic bracket which has a silica-containing layer on its tooth-facing base. A silanation layer is also applied to the base. The orthodontic bracket of the present invention is made of a relatively inexpensive glass which can provide the desired strength, is easy to manufacture, relatively resistant to fracture, and does not require bath solutions or cleaning operations in order to enhance the strength of the material. In one aspect of the present invention there is provided an orthodontic bracket including a base portion having a tooth contacting surface and a body member including walls defining an archwire slot, the bracket being characterised by being made of an amorphous glass material and having an outer crystalline strengthening layer. In another aspect of the present invention, there is provided a method of making a glass orthodontic bracket, characterised by the steps of: (a) forming a bracket from a glass material which is crystallisable when subjected to a heat treatment process; and (b) subjecting the bracket to a heat treatment wherein an outer crystalline strengthening layer is formed; whereby a bracket made of an amorphous glass material and having an outer crystalline strengthening layer is made. In order that the invention may be fully understood, a preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an orthodontic bracket made in accordance with the present invention; Figure 2 is a perspective view of a portion of a glass plate from which the orthodontic bracket of Figure 1 can be made; Figure 3 is a top plan view of Figure 2 illustrating, in solid line, the outline of an orthodontic bracket blank to be cut therefrom; Figure 4 is a perspective view of the orthodontic bracket blank cut from the plate illustrated in Figure 3; Figure 5 is a perspective view of the bracket blank of Figure 4, after the slot has been machined; Figure 6 is a top plan view of the orthodontic bracket after machining the notching illustrated by broken lines in Figure 5; Figure 7 is a bottom perspective view of the bracket of Figure 1; and Figure 8 is a greatly enlarged cross-sectional view of a portion of the bracket illustrated in Figure 1 taken along the lines 8-8 of Figure 1. Figure 1 shows an orthodontic bracket 10 made in accordance with the present invention. Bracket 10 comprises a pair of tie wings 12 and 14 respectively, which are supported by a base 20 having a tooth contact surface 31 for attachment to the tooth and a connecting portion 15 which connects tie wings 12, 14. The tie wings 12, 14 and connecting portion 15 define a pair of side walls 18 which form an aligned archwire slot 20 for receiving an orthodontic archwire (not shown) as is typically done in the prior art. A substantially flat bottom wall 19 connects side walls 18 and forms the bottom of slot 20. It should be understood that the bracket 10 may be of any desired configuration used in the art, with the one shown in Figure 1 being for the purpose of illustration only. The bracket 10 of the present invention is manufactured from a sheet of glass 26 provided in sheet form as illustrated in Figure 2. Referring to Figure 3, there is illustrated, in solid line, the outline of an orthodontic bracket blank 30 to be cut from the plate 26. In the preferred form of the present invention, ultrasonic machining techniques are used to cut a plurality of individual bracket blanks 30 from plate 26. The bracket blanks 30 cut therefrom, see Figure 4, are then subjected to a plurality of further machining operations so as to form bracket 10. The broken line 27 indicates the outline of the slot 20 to be machined into bracket blank 30. In the present invention the slot 20 is preferably formed by diamond wheel grinding. The broken lines 29 in Figure 5 illustrate the next machining step to be conducted on bracket blank 30. In this next machining operation, the tie wings 12, 14 and connecting portions 15 are formed. In a preferred form of the present invention, diamond wheel cutting is used for this operation. Thereafter, ultrasonic machining is conducted on the bottom of the bracket blank 30 to finally form bracket 10 to form contact surface 31 for attachment to the tooth. In the preferred form of the present invention, ultrasonic machining techniques are used for cutting blanks 30 from plate 26 and forming contact surface 31. It is to be understood that various other machining techniques may alternatively be used in order to form the orthodontic bracket. After the bracket 10 has been finally formed, it is then subjected to a heat treatment process wherein a thin compressive crystalline outer layer 38 is formed on the surface of the bracket 10 as illustrated in Figure 8. A glass composition that is useful in providing an outer crystalline layer consists essentially of the following oxides in the approximate weight percentages as set forth below, exclusive of minor impurities: In the particular embodiment illustrated, a glass made from the following oxides in the appropriate weight percentages was used: The bracket 10 using the appropriate glass composition provides an inner amorphous portion 39 within the outer crystalline layer 38. The thin outer layer 38 does not substantially affect the clarity of the glass, thus leaving the bracket substantially transparent or translucent. The thin crystalline layer 38 has a thickness t in the range of 0.0005 to 0.005 inches (0.0127 - 0.127 mm), preferably between about 0.003 to 0.005 inches (0.0762 - 0.127 mm). The outer crystalline layer 38 provides a compressive force on the surface, resulting in a substantial increase in the strength and fracture toughness of the bracket 10. The thickness t of layer 38 should not be too thick in relationship to the size of the product, as this can cause too great a compressive force to be produced which can result in failure of the part. The thickness t of layer 38 is preferably no greater than about 25% of the cross-sectional thickness of the product at its thinnest point. It is important in order appropriately to initiate the crystalline growth at the surface during the heat treatment process, that the surface should have an appropriate roughness. The applicants have found that the surface, after the machining operation, should be no less than 4 RMS (root means squared) and generally no greater than about 250 RMS. It is important that the crystallization formed during the heat treatment should be initiated on the surface and not in the interior of the bracket 10, because if this happens it will have the opposite result, that is, weakening of the bracket. If the surface finish is too smooth, crystalline growth may initiate internally of the product. Preferably the surface roughness is between 30 to 125 RMS. Applicants have found that the particular orthodontic bracket 10 in Figure 1, having the composition set forth in Table II, should be heated to a temperature in the range of 600-800°C. It is important that the bracket 10 be heat treated under the optimum conditions to initiate crystallization on the surface. Applicants have found that the time and temperature required is sensitive to the particular composition being used. Thus, a little experimentation may be necessary for any particular composition being used to determine the appropriate time and temperature necessary for crystallization. Generally, the higher the temperature, the shorter the time period necessary to obtain the thin outer crystalline layer 30. In the particular embodiment illustrated, the applicants have found that a heat treatment conducted at a temperature of about 700°C for 1 to 2 hours provides the desired results. The applicants have also found that the environment in which the brackets are heat treated can have a significant affect. For example, the orthodontic brackets are typically placed in a boat, which is placed into an appropriate oven wherein they are heat treated. The applicants have also found that the material of the boat and whether the boat is covered or not can have an effect upon the ultimate fracture toughness. In an evaluation of fracture toughness in accordance with various heat treatment conditions, the fracture toughness was tested for specimens made out of the material from Table II, and tested for fracture toughness in accordance with ASTM-procedure. A control test specimen, identified as C, was made but not subjected to any heat treatment. A first test specimen (Sample 1) was made and subjected to a heat treatment process of 700°C for about 1 hour with it being heated in an aluminium oxide boat which was uncovered. Sample 2, a second test specimen, was heat treated for approximately 35 minutes in an aluminium oxide tray which was uncovered. Sample 3, a third test specimen, was heat treated at 690°C for approximately 1 hour in an aluminium oxide tray which was uncovered. A fourth test specimen (Sample 4) was heat treated at 700°C for approximately an hour in a stainless steel boat which was placed on bricks on its removal from the oven to be cooled. All No. 4 Samples were fractured. It is important to place the bracket in a boat or tray having a thermal conductivity substantially similar to that of the material of the bracket. The following Table III sets forth a comparison of the fracture toughness of the control and heat-treated specimens 1, 2 and 3 set forth above. As can be seen from the foregoing, the first test specimen (Sample No 1), i.e. which was heat treated for 60 minutes in an uncovered aluminium oxide tray, showed the greatest fracture toughness. Fracture toughness increased by a factor of approximately 4, as compared with a product not heat treated. Thus, heat treating the bracket under appropriate conditions provided significant improvement in the fracture toughness of the material. As can be seen from Sample No 3, the 690°C temperature simply did not provide the desired crystalline depth outer layer. Thus, it would appear that with this particular composition the time and temperature used was insufficient to obtain the full fracture toughness capable of being obtained from the composition. The applicants have compared orthodontic brackets made from the glass material of the present invention and heat treated in accordance with the present invention with brackets made of sapphire, cubic zirconium and polycrystalline aluminium materials of the prior art. As can be seen from the following Table IV, the brackets made out of the glass and heat-treated according to the present invention exhibited strength values equal to or better than sapphire and cubic zirconia, and markedly improved values over polycrystalline materials of the prior art. While the single crystal zirconia and alumina brackets showed a wide range of strength, approximately 20% of the brackets broke at values below acceptable levels. The strength of the glass materials was found to be of a more consistently high value, thus providing more brackets in the acceptable strength range. Subjecting the glass bracket 10 to the heat treatment according to the present invention results in a complete change to the structure of the outer surface. The heat treatment provides a fresh new thin crystalline layer which has very few surface flaws, thus minimising or eliminating the effects of the machining conducted thereon. It is to be understood that various modifications may be made to the present invention without departing from the scope thereof. For example, various other compositions may be used so long as the appropriate compressive crystalline outer layer may be formed. Other additives or impurities may be present which do not affect the structure or performance of the bracket. Another example of a composition believed to provide adequate performance is set forth below in Table V.

Claims: An orthodontic bracket (10) including a base portion (20) having a tooth contacting surface (31) and a body member including walls (18,19) defining an archwire slot (20), the bracket being characterised by being made of an amorphous glass material (39) and having an outer crystalline strengthening layer (38). An orthodontic bracket according to claim 1, in which the glass has substantially the following composition: An orthodontic bracket according to claim 1, in which the glass has substantially the following composition: An orthodontic bracket according to claim 1, in which the glass has substantially the following composition: An orthodontic bracket according to claim 1, in which the glass has substantially the following composition: An orthodontic bracket according to any preceding claim, in which the outer crystalline layer (38) has a thickness (t) in the range of 0.0127 to 0.127mm (0.0005 to 0.005 inches). An orthodontic bracket according to claim 6, in which the outer crystalline layer (38) has a thickness (t) in the range of 0.0762 to 0.127mm (0.003 to 0.005 inches). An orthodontic bracket made out of a glass material which is crystallisable in response to a heat treatment process, the glass material consisting essentially of the following: An orthodontic bracket according to claim 8, wherein the glass material consists essentially of the following: An orthodontic bracket according to claim 8, wherein the glass material consists essentially of the following: A method of making a glass orthodontic bracket (10), characterised by the steps of: (a) forming a bracket from a glass material which is crystallisable when subjected to a heat treatment process; and (b) subjecting the bracket to a heat treatment wherein an outer crystalline strengthening layer is formed; whereby a bracket made of an amorphous glass material (39) and having an outer crystalline strengthening layer (38) is made. A method according to claim 11, wherein the glass has substantially the following composition: A method according to claim 11, wherein the glass has substantially the following composition: A method according to any of claims 11 to 13, wherein the outer surface of the bracket prior to being subjected to the heat treatment is provided with a surface finish sufficient to initiate crystallisation growth at the surface when subjected to the heat treatment. A method according to any of claims 11 to 14, wherein the heat treatment is at a temperature in the range of 600 to 800°C. A method according to claim 15, wherein the temperature is approximately 700°C. A method according to claim 16, wherein the heat treatment comprises subjecting the bracket to the said temperature for approximately 1 hour. A method according to any of claims 11 to 17, wherein the rate of heating or cooling the bracket is not greater than 200°C/min. A method according to any of claims 11 to 18, wherein the bracket is placed in a boat made of a material having a thermal conductivity similar to that of the bracket. A method according to any of claims 11 to 19, wherein the heat treatment step is carried out to produce a crystalline outer layer (38) having a thickness (t) in the range of 0.0127 to 0.127mm (0.0005 to 0.005 inches). A method according to claim 20, wherein the heat treatment step is carried out to produce a crystalline outer layer (38) having a thickness (t) in the range of 0.762 to 0.127mm (0.003 to 0.005 inches). A method according to any of claims 11 to 21, including the step of finishing the bracket to a surface finish no less than 4 RMS before subjecting it to the heat treatment step. A method according to any of claim 22, including the step of finishing the bracket to a surface finish between 30 and 125 RMS before subjecting it to the heat treatment step.

Inventors: FARZIN-NIA FARROKH, PETTICREW RICHARD WILLIAM, FARZIN-NIA, FARROKH, PETTICREW, RICHARD WILLIAM
Applicants: ORMCO CORP, ORMCO CORPORATION
Application year: 1990
Main classification: A61C7
IPC codes: C03C3, C03C10, A61C7, A61K6