METAL FOIL FOR PREPARING A DENTAL COPING AND CROWN FOR A CERAMO-METAL RESTORATION

Abstract: A dental coping (10) for use in preparing a ceramo-metal dental restoration upon which a ceramic veneer is applied and fired comprising a metal laminated structure having multiple layers of precious metal characterized by an intermediate layer (12) of palladium and at least one gold layer (14, 16) on each side thereof.

Description: This invention relates to the field of dental restorations and more particularly to an improved metal coping for a ceramo-metal dental restoration. Background of Invention A new technique for constructing a porcelain to metal crown having a fracture resistance comparable to or greater than the fracture resistance to impact forces of the veneer cast metal crown and which overcomes many of the shortcomings of the conventional porcelain jacket crown is disclosed in U.S. Pat.Nos. 4,273,580 (EP-B-0 016 315) and 4,459,112 (EP-B-0 104 320), respectively. In accordance with U.S. Pat. No. 4,273,580, a precious metal foil, preferably a laminate of several precious metal layers, is swaged about a prepared die of a tooth to form a metal matrix upon which a veneering material such as porcelain is fired. However, unlike the conventional porcelain jacket crown, the metal matrix is not removed or separated from the veneering material but is instead retained as a metal coping for the finished porcelain jacket crown. The metal coping is employed as an understructure in the conventional porcelain to metal cast crown. The physical strength of the metal coping may be substantially enhanced and the ease of preparing the restoration greatly simplified by converting the metal foil starting material into a preformed coping of predetermined geometry as taught and described in U.S. Pat. No. 4,459,112 referred to above. The metal foil starting material is cut into a circular segment and folded to form multiple folds which are uniformly spaced apart and preferably extend radially from a central unfolded area. This multiple fold geometry makes it easy to adapt the preformed coping to the die without the need for superior skill and craftsmanship and even more importantly increases the strength of the coping. Although the preformed coping as above described has certain advantages, it is not essential to the practice of the present invention. In fact, any preformed shape or method of construction may be used. A metal coping should function to both protect the tooth abutment and as a structural support for the crown or bridge. In the latter respect, the coping supports the veneer material and provides structural strength and rigidity for the dental restoration. An ideal coping will act as an extension of the vital abutment tooth to protect the tooth against fracture and to resist distortion and displacement from the forces applied during mastication. The strength of the metal coping after it is swaged and removed from the die is dependent upon its hardness and rigidity. These characteristics may be satisfied using a precious metal which is known to be hard and relatively rigid such as platinum. Rigidity is basically controlled by thickness. Conversely, the ability to adapt and swage the preformed coping to the die so as to assure a proper adaptation with accurate marginal fit requires the coping to be highly workable, i.e., it should be soft and flexible. To be flexible the material should be thin. A dental coping should accordingly be of a material composition which is soft and flexible when it is adapted to the die and yet is hard and rigid after adaptation so as to provide the required structural support for the restoration. These apparent contradictory requirements are met by the coping and crown construction of the present invention. The hardness or softness of a metal is determined by measuring its resistance to permanent indentation. A hardness number is assigned to the material using any one of several conventional hardness tests such as the Vickers hardness test, which uses a diamond pyramid indenter. Summary of the Invention The metal coping of the present invention includes a layer which consists substantially of palladium of a palladium gold alloy, which is a soft readily workable material having a low Vickers hardness number, symmetrically disposed between layers of gold. It was discovered that this combination of materials will function before a sintering as a soft material and after sintering will convert to a harder and more rigid material. It was further discovered that using gold layers symmetrically disposed about the palladium layer increases the fracture resistance of the coping and minimizes distortion from differences in thermal expansion of the metal during heat treatment. In accordance with the present invention, it was discovered that the layer of palladium can be of substantially pure palladium or a palladium alloy preferably of palladium and gold containing from about 50% palladium to 100% palladium and that the metal coping may be formed from a relatively thick single layer of palladium or a palladium gold alloy having a high fusing temperature which is symmetrically disposed between at least two layers preferably equal in thickness and with each composed substantially or entirely of gold. In one embodiment of the present invention, the dental coping comprises a high fusing temperature component including a single layer of a precious metal alloy containing from about 50 to 100% palladium and two precious metal layers composed substantially or entirely of gold symmetrically disposed about the palladium alloy. In another embodiment of the present invention the dental coping comprises a high fusing temperature component including at least three layers in a laminated arrangement with one layer of a precious metal alloy containing from 50 to 100% palladium bounded on both sides by a substantially identical layer composed substantially or entirely of gold and a low fusing temperature component composed essentially or entirely of gold superimposed upon the high fusing temperature component on one or both sides thereof. The dental crown of the present invention comprises a metal coping as described above, i.e. including a layer of a high fusing temperature precious metal alloy containing from at least about 50% palladium to 100% palladium disposed between substantially equal precious metal layers substantially or entirely of gold and a ceramic veneer covering all or part of the metal coping. Objects and Brief Description of the Drawings It is the principal object of the present invention to provide a dental crown with increased strength and resistance to fracture. It is a further object of the present invention to provide a dental coping for a dental crown which is easily adapted to a die yet physically strong after swaging and dimensionally stable in response to heat treatment. Other objects and advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings of which: Figure 1 is an enlarged perspective of a rectangular segment of a metal foil starting material for forming the dental coping and jacket crown of the present invention; Figure 2 is an end view of the segment of Figure 1 showing a three layered lamination in an arrangement according to the present invention; Figure 3 is an end view of an alternate laminated arrangement for the dental coping according to the present invention; and Figure 4 is a perspective of a preferred dental coping formed from the starting material of Figure 1 or Figure 3. Detailed Description of the Invention The metal foil starting material 10 of Figures 1 to 3 is a multi-layer lamination including a high fusing temperature precious metal layer 12 sandwiched between two substantially equal low fusing temperature metal layers 14 and 16 composed essentially or entirely of gold. High fusing temperature for purposes of the present invention means a melting temperature of at least about 1250 to 1300°C whereas low fusing temperature means a melting temperature substantially equal to the melting temperature of gold. The high fusing temperature precious metal layer 12 is composed of at least about 50% palladium to 100% palladium with the remainder, if any, preferably composed primarily of gold with one or more of the following additional elements in combination: silver, copper, iridium, platinum with the element iridium present, if at all, in a small amount relative to the other elements. A preferred composition for a palladium gold alloy layer 12 contains about 54% palladium, 35% gold with the major portion of the remainder divided between copper and silver. A layer of essentially pure palladium or a palladium gold alloy is a relatively soft material. Upon heat treatment diffusion of palladium into the low fusing gold layers 14 and 16 occurs to cause an increase in hardness and strength in the composite coping. Where a substantially pure palladium layer is used, the palladium should be of as high a purity as possible. This maximizes the increase in hardness upon heat treatment. Heat treatment is recommended before any veneering material is applied to cause some melting of the gold layers 14 and 16 to fill voids and spaces and to form a composite coping structure. Heat treatment is also necessary to fire each layer of porcelain or other veneer composition. The location of the palladium layer 12 between two substantially identical gold layers 14 and 16 is essential to the invention. This arrangement establishes dimensional stability under heat treatment and reduces the likelihood of distortion from expansion and contraction before and after porcelain baking. It should be noted that in the embodiment of Figures 1 and 2 a single layer of palladium or a palladium gold alloy is required for the high fusing temperature component of the present invention. Moreover, the single layer of palladium or palladium and gold may be made relatively thick, e.g., between 15 and 20 microns with 25 to 35 microns being preferred. The use of a relatively thicker layer of palladium provides added body which renders the coping less fragile, easier to handle and increases its strength after adaptation. The gold layers may have a thickness of between 4 to 30 microns with 6 to 30 microns being preferred. When a thin gold layer is used for the inside layer 16 of less than 14 microns the outer layer 14 may be slightly thicker than the inner layer 16. An alternative multi-layered arrangement 10 is shown in Figure 3 comprising a low fusing gold layer 18 (i.e. composed substantially or entirely of gold) superimposed over a high fusing ternary lamination 20 including a palladium layer 21, equivalent in composition to layer 12 of Figure 2, bounded by gold based layers 22 and 23 respectively. The gold based layers 22 and 23 each contain from 50 to 95% gold and from 0 to 50% of one or more of the following elements in combination: silver, palladium, platinum, iridium, copper and aluminum. The palladium layer can be of substantially pure commercial grade palladium. The low fusing gold layer 18 may also be disposed on both sides of the high fusing lamination 20. The invention is also not limited to a laminated arrangement with only one palladium layer bounded on each side by a gold based layer. The same arrangement can readily be expanded to include two palladium layers provided each palladium layer is symmetrically bounded on each side by a gold based layer of substantially equal thickness. Also, in the embodiment of Figures 1 and 2, the gold layers 14 and 16 need not be single layers but each may instead represent multiple layers of gold of varying gold purity. For example, each layer 14 and 16 can represent a combination of two or three gold based layers with the outermost layer being a low fusing temperature layer of essentially pure gold and another of such layers being a higher fusing temperature gold based layer equivalent in composition to gold layers 22 and 23 of Figure 3. Multiple gold based layers can be used to control the dimensional stability of the composite under heat treatment and its coefficient of expansion. Regardless of the arrangement, it is preferable for the practice of the present invention to use symmetrical layers of gold on both sides of the palladium or palladium alloy composition. If multiple gold layers are used each side of the palladium layer should preferably be symmetrical in number of layers, thickness and composition. A preformed coping 25 as shown in Figure 4 may be formed using the laminated arrangement of Figure 2 or 3 or any other arrangement within the scope of the present invention. The preformed coping 25 may be formed following the procedure taught and described in U.S. Pat. No. 4,459,112 (EP-B-0 104 320). As described in the patent, the metal foil starting material 10 is cut into a blank of circular geometry and folded to form multiple pleats 27. The pleats 27 extend from a central unfolded area 28. Other construction techniques may likewise be used to fabricate a preformed coping which may have any desired shape. The preformed coping 25 is then placed over the die (not shown) to adapt the coping to the die using any conventional swaging device. Once the coping is adapted and removed from the die, it is heat treated by placing it over the flame of a Bunsen burner for a short time period based on flame temperature to allow the low fusing temperature gold layer 12 to flow to form a compact metal matrix without air pockets. This heat treatment may also be carried out in a furnace at a temperature of about 1020°C to 1150°C. After the heating step, porcelain or another veneering material can be directly applied in a conventional manner to form the dental crown of the invention. It is desirable but not essential to coat the outside surface of the metal foil before the porcelain is applied with a bonding composition to achieve an unbreakable bond between the porcelain layers and the metal coping. A preferred bonding composition is taught and described in U.S. Pat. No. 4,434,211 which includes a gold based precious metal composition in combination with a halide of a noble metal such as a gold or silver chloride. The bonding material should be sintered to the metal coping at a temperature above 1600°F (871°C)which can be achieved simultaneously with the firing of the required porcelain outer layers. The heat treatment of the porcelain is conventional and any typical firing schedule may be applied with a firing temperature generally between 1600°F and 1820°F (871°C and 993°C).

Claims: 1. A dental coping for use in preparing a ceramo-metal dental restoration defined by a metal laminated structure having multiple layers of precious metal characterized by at least one intermediate precious metal layer composed of from 50 to 100% palladium with the remainder, if any, substantially of gold and at least two precious metal layers composed substantially or entirely of gold symmetrically disposed about said palladium layer. 2. A dental coping as defined in claim 1 consisting of a single palladium layer with at least one gold layer on each side. 3. A dental coping as defined in claim 2 wherein each layer of gold is equal in thickness. 4. A dental coping as defined in claims 2 or 3 wherein said layer of said palladium has a thickness of between 15 and 50 microns and wherein each layer of gold has a thickness of between about 4 to 30 microns. 5. A dental coping as defined in claim 4 wherein said layer of said palladium has a thickness of 35 microns and wherein each layer of gold has a thickness of about 14 microns. 6. A dental coping as defined in claim 5 wherein said layer of palladium alloy comprises about 54% palladium and about 35% gold, with the remainder selected in combination from the group consisting of silver, copper, iridium and platinum with the element iridium present, if at all, in a small amount relative to the others. 7. A dental coping as defined in claim 1 comprising a high fusing temperature component represented by at least one layer of palladium bounded between a layer substantially of gold on each side thereof and further comprising a low fusing temperature component of gold superimposed on at least one side of said high fusing temperature component. 8. A dental crown comprising a metal coping defined by a metal laminated structure characterized by a layer of a high fusing temperature precious metal alloy composed substantially or entirely of palladium with a layer substantially of gold on each side thereof and with a ceramic veneer covering all or part of the metal coping. 9. A dental crown as claimed in claim 8 wherein said layer of palladium contains from about 50% to 100% palladium with the remainder, if any, substantially of gold. 10. Use of a dental coping as claimed in any of claims 1 to 7 for preparing a ceramo-metal dental restoration by applying upon said coping a ceramic veneer and firing the product in a furnace.

Inventors: SHOHER, ITZHAK, DR., WHITEMAN, AHARON
Applicants: SHOHER, ITZHAK, DR., WHITEMAN, AHARON
Application year: 1986
Main classification: A61C5
IPC codes: A61C5, A61K6, A61C13