Method for repairing of veneered dental cast restoration

There is provided a method for indirect bonding of chemically or light curable resin based restoratives especially acrylates and methacrylates, to the metal base of dental cast restorations. The invention is particularly useful in cases of in situ repairs of damaged or worn porcelain or polymeric veneers on cast restorations made of precious and non-precious metals. According to the invention, the exposed metal of the restoration is covered with a layer of primer consisting essentially of a copolymer of polymethacrylic or polyacrylic acid followed by the application of the polymerizable restorative material.

FIELD OF THE INVENTION 
This invention concerns the use of a water insoluble copolymer of 
polyacrylic or polymethacrylic acid as a primer for enhancing bonding 
strength between the metal base of the restoration and the polymerizable 
restorative material, used for re-veneering of the damaged or worn parts 
of the restoration. 
The primer is preferably in the form of a solution of the film forming 
polyacrylic or polymethacrylic acid based resin in a volatile solvent. The 
restorative material is preferably in the form of a polymerizable 
methacrylic or acrylic resin or blends thereof containing one to four 
methacrylate groups per molecule, tooth colored and containing a fine 
vitreous filler. The restorative may be of the chemically or light (UV or 
visible) initiated curing system type. The primer acts as a bridge between 
the metal base, usually made of gold, palladium or nickel alloys, and the 
restorative material. It provides good bonding strength between these 
materials allowing for successful and durable in situ repairs of veneers 
on cast restorations. 
BACKGROUND OF THE INVENTION 
Progress made in the field of prosthetic denistry during the last century, 
combined with constantly increasing longevity of the population and the 
rise in the standard of living, have contributed greatly to the popularity 
and increased utilization of permanent dental prothesis. With this came 
the need for a cost effective, esthetically acceptable and reliable method 
for restoring fractured or worn veneers made of porcelain or acrylic.type 
resins. These materials are more fragile and/or softer than the durable 
metal base of the cast restoration. Therefore, they are more susceptible 
to fracture and/or wear. 
The in situ repair of damaged or worn restorations was, up to now, 
virtually impossible. applying a layer consisting of a copolymer of 
acrylic or methacrylic acid and a copolymerizable monomer over the metal 
base of the restoration, followed by the application of a conventional 
resih based dental restorative. The intermediate layer provides good 
bonding strength, both to metal and restoratives, thus assuring durability 
of the restoration. Illustrative resin based dental restoratives are set 
forth in Lee U.S. Pat. No. 4,107,845, Waknine U.S. Pat. No. 4,544,359, 
Waknine U.S. Pat. No. 4,547,531, and Bowen U.S. Pat. No. 3,066,112. The 
entire disclosures of these patents are hereby incorporated by reference 
and relied upon. 
By copolymerizing acrylic or methacrylic acid with another monomer, 
insolubility in oral fluids is achieved, while preserving the good 
adhesive properties of polyacids. The presence of ethylenically 
unsaturated groups in the primer, combined with its limited chemical 
resistance to acrylic or methacrylic resins (present in the restorative) 
assures a good bond between the primer and the restorative, enhanced by 
the copolymerization in the interface layer. 
SUMMARY OF THE INVENTION 
Conventional permanent dental prothestic restorations, such as crowns and 
bridges, are most commonly composed of a cast metal base and porcelain or 
acrylic veneer. Precious and non-precious metals, used for making cast 
restorations, include gold, palladium, silver and nickel alloys, stainless 
steel, etc. The porcelain veneers are relatively resistant to wear but 
susceptible to mechanical damage, because of the brittleness of the 
material. The acrylic veneers are less likely to chip or break, but are 
likely to wear. 
The need for an effective and durable repair method is well recognized by 
the dental profession. For economic, as well as for convenience reasons, 
such repairs should be made in situ; without removing the restoration from 
the mouth. 
Especially difficult to repair are cases where, due to fracture or wear, a 
substantial area of metal is exposed. The success of the restoration 
procedure depends on, among other factors, a bonding force between a 
restorative repair material and a metal base. For reasons of esthetics and 
wear resistance the material of choice for making such repairs should be a 
filled acrylic or methacrylic resin of a suitable consistency. Materials 
of this type are commonly used in denistry for filling cavities and for 
resurfacing deficient enamel; they are known under the name of composite 
restoratives. Illustrative examples of such restorative materials are set 
forth in the four above-mentioned U.S. patents. 
Unfortunately, composite restoratives do not exhibit good adhesive 
properties when applied on metals in general, and on glues and its alloys 
in particular. Attempts to increase the bonding strength by roughening the 
exposed surface of the metal did not bring improvement significant enough 
to assure durable restorations. The epoxy type resins, used as metal 
primers, did not provide adequate bonding strength; moreover, these resins 
are susceptible to deterioration in moist environments. Cyanoacrylate 
glues have also been proven ineffective for the reason of susceptability 
to deterioration in oral conditions. Some manufacturers have recommended 
the use of organofunctional silane primers for repairing damaged porcelain 
veneers. These primers, while improving bonding to porcelain, did not 
contribute to bonding to metal. Therefore, they are virtually useless in 
clinical situations where relatively large surfaces of metal are exposed. 
The method of repairing veneers, according to this invention, employs a 
novel primer consisting of, or consisting essentially of, a copolymer of 
acrylic or methacrylic acid, preferably with a methacrylate or acrylate 
ester. Such primer has been found to provide a strong bond to metal bases 
as well as to restoratives. The primer is preferably applied on metal in 
the form of a solution in a volatile solvent. The solvent is allowed to 
evaporate, leaving a layer of the polymeric primer on the surface. The 
primer itself may be formulated in the following forms: 
1. As a polymerizable light or self-cured mixture of acrylic, methacrylic, 
polyacrylic, or polymethacrylic acids with an acrylate or methacrylate 
ester. 
2. As a copolymer of acrylic or methacrylic acid with an acrylate or 
methacrylate ester. 
3. As a polymerizable mixture of a copolymer or copolymers described in 2 
above with an acrylate or methacrylate ester. 
The acrylate and/or methacrylate ester portion of the polymerizable mixture 
can be 1 to 50%, preferably 5 to 25% of the total of (1) acrylate and/or 
methacrylate ester and (2) acrylic acid and/or methacrylic acid. When a 
cross-linking agent is present, e.g., a diacrylate or methacrylate, it 
should be present preferably in an amount of not over 10%. 
The acid number of the mixture (1), (2), or (3) should at least 20, 
preferably above 65, most desirable range being 80 to 120. 
Unless otherwise indicated, all parts and percentages are by weight. 
The method can comprise, consist essentially of, or consist of the stated 
steps with the recited materials. 
In order to facilitate application, it is desirable that a volatile solvent 
is used as a carrier for the above-described resins The solvent should 
have the following characteristics: 
1. It should exhibit a fast rate of evaporation at body temperature. 
2. It should be biologically acceptable. 
3. It should not interfere with the adhesive properties of the primer. 
After evaporation of the solvent, the polymeric layer may be further 
polymerized, by chemical means, or by ultra-violet or visible light. 
Methacrylic and acrylic acids or their homo- and copolymers, used in the 
primer formulations according to this invention, have molecular weights, 
prior to curing, of no more than 1,000,000, preferably no more than 
400,000. 
Non-limiting examples of mono-acrylate or mono-methacrylate copolymerizing 
monomers (or polymers thereof) used in formulations according to this 
invention are: phenyl-, cyclohexyl-, lower alkyl (C.sub.1 -C.sub.6)-, 
glycidyl-, hydroxyalkyl (C.sub.2 -C.sub.4)-, chloroalkyl (C.sub.1 
-C.sub.6)-, alkoxy-alkyl (C.sub.1 -C.sub.4)-, furfuryl, tetrahydrofurfuryl 
and vinyl acrylates or methacrylates. Specific esters are phenyl acrylate, 
phenyl methacrylate, methyl acrylate, methyl methacrylate, ethyl acrylate, 
ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, 
butyl methacrylate, isobutyl methacrylate, hexyl acrylate, hexyl 
methacrylate, glycidyl acrylate, glycidyl methacrylate, hydroxyethyl 
acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl 
methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 
chloroethyl acrylate, chloroethyl methacrylate, methoxyethyl acrylate, 
methoxyethyl methacrylate, methoxybutyl acrylate, methoxybutyl acrylate, 
ethoxyethyl acrylate, ethoxyethyl methacrylate, furfuryl acrylate, 
furfuryl methacrylate, tetrahydrofuryl acrylate, tetrahydrofurfuryl 
methacrylate, vinyl acrylate, and vinyl methacrylate. 
Non-limiting examples of di-, or polymethacrylates and acrylates used in 
formulations according to this invention are: ethylene, diethylene, 
polyethylene and other mono-, di-, or higher polyalkylene glycol 
dimethacrylates and diacrylates; C.sub.2 to C.sub.12 alkylene 
dimethacrylates and acrylates; trimethylolopropane trimethacrylate and 
acrylate; pentaerythritol trimethacrylate and acrylate; pentaerythritol 
tetramethacrylate and acrylate; 2,2-bis[4',4"-(3'"-methacroyl (or 
acroyl)-2'"-hydroxy-propoxy)phenyl.multidot. propane (commonly called 
Bis-GMA), and its urethane derivatives, 2,2-bis[4',4"-(2'"-methacroyl (or 
acroyl)-ethoxy) phenyl] propane (commonly called EBA), 
2,2-bis[4',4"-(methacroyl)-phenyl]-propane (commonly called BADM), etc. 
Specific diacrylates and methacrylates, in addition to those just 
mentioned, are ethylene glycol dimethacrylate, ethylene glycol diacrylate, 
diethylene glycol diacrylate, diethylene glycol dimethacrylate, 
triethylene glycol diacrylate, triethylene glycol dimethacrylate, 
tetraethylene glycol diacrylate, tetraethylene glycol dimethacylate, 
propylene diacrylate, propylene dimethacrylate, trimethylene diacrylate, 
trimethylene dimethacrylate, tetramethylene diacrylate, tetramethylene 
dimethacrylate, hexamethylene diacrylate, hexamethylene dimethacrylate, 
decamethylene diacrylate, decamethylene dimethacrylate, dodecamethylene 
diacrylate, and dodecamethylene dimethacrylate. 
Non-limiting examples of solvents, used individually or in mixtures, 
suitable for use in formulations according to this invention are: Water, 
C.sub.1 -C.sub.5 alcohols, ethylene chloride, dichloromethane, 
trichloroethylene, chloroform, C.sub.1 -C.sub.6 alkylacetates, propionates 
and butyrates, acetone and other C.sub.1 -C.sub.4 alkyl ketones, lower 
(C.sub.1 -C.sub.4) ethers, etc. Specific examples of such solvents, in 
addition to those mentioned above, are methyl alcohol ethyl alcohol, 
propyl alcohol, isopropyl alcohol, butyl alcohol, sec. butyl alcohol, amyl 
alcohol, methyl acetate, ethyl acetate propyl acetate, isopropyl acetate, 
butyl acetate, amyl acetate, hexyl acetate, methyl propionate, ethyl 
propionate, butyl propionate, methyl butyrate, ethyl butyrate, methylethyl 
ketone, diethyl ketone, methyl butyl ketone, dimethyl ether, diethyl 
ether, methyl ethyl ether, dipropyl ether, dibutyl ether, ethyl butyl 
ether. 
Any chemical dental composite restorative may be used in conjunction with 
the above-described primers. The restorative may be chemically or light 
cured, the preference of one over another depending on the particular 
formulation of the primer used, and especially on its curing system. For 
best results, the light cured primer should be used with light cured 
restoratives, and the chemically cured primers with chemically cured 
restoratives. 
As indicated above, the restoratives are normally acrylic or methacrylic 
resins. Usually, they are based on polyethylenically unsaturated acrylates 
such as Bis-GMA and/or triethylene glycol dimethacrylate (TEGDM). There 
can be used any of the polyunsaturated acrylates or methacrylates 
mentioned above in the restorative or any of those set forth in the 
aforementioned Bowen, Lee, and Waknine patents. The restorative can also 
contain any of the conventional fillers, e.g., silica, glass, e.g., 
borosilicate glass, barium silicate, calcium silicate. 
A typical procedure for restoring deficient veneers on cast restorations, 
according to this invention, involves the following steps: 
1. (Optional): The damaged or worn out surface, e.g, on a crown, bridge, or 
denture, is roughened with a dental burr or abrasive disks or cones, to 
increase surface contact with the primer. The veneer surrounding the 
exposed metal is ground to a feather edge. 
2. The area to be restored is coated with a thin layer of the primer having 
a chemical composition as described above. If the primer formulation 
contains a volatile solvent, the solvent is allowed to evaporate. The rate 
of evaporation of the solvent may be accelerated by blowing (preferably 
warm) air over the surface. If the primer is of the chemically cured type, 
sufficient time should be allowed to accomplish its cure. If it is of a 
light cured type, it may require irradiation with an appropriate 
instrument or it may be cured simultaneously with the light cured 
restorative applied thereafter. 
3. A restorative material, perferably a composite restorative having a 
fluid or semi-fluid consistency, is applied over the primer, smoothed with 
a brush or spatula and/or covered with a thin foil (transparent or 
transluscent if the material is of the light cured type). If the 
restorative is chemically cured, it should be allowed enough time to 
achieve virtually full cure before finishing (usually three to five 
minutes). If it is light cured, it should be irriatiated for a period of 
time specified for the given material and light source (usually 10 seconds 
to 2 minutes). 
4. (Optional): The restoration, after removing the foil (if used), is 
finished using conventional dental finishing tools and materials, such as 
carbide, corundum or diamond burrs, disks and cones, corundum and diamond 
polishing pastes, etc. 
There can be employed conventional chemical curing agents and 
polymerization activators with the primers of the present invention. Thus, 
there can be employed a visible light activated polymerization initiators 
the .alpha.-.beta. diketone type such as benzil, dl-camphoroquinone 
(2,3-bornanedione), and the like or benzoin and its derivatives (for 
example, benzoin alkyl ethers) in the case of ultraviolet cured 
formulations. Polymerization accelerators are employed in conjunction with 
such initiators. These accelerators are generally tertiary amines such as 
diethylaminoethyl acrylate, diethylamino ethylmethacrylate (commonly 
referred to as "DEAEMA"), dimethylamino ethylmethacrylate, and the like. 
Other accelerators include methacroyl or acroyl alkyl-dialkyl or 
dihydroxyalkyl amines and trialkylamines, having, preferably, one to ten 
carbon atoms in the alkyl moieties. Examples of such amines are 
tributylamine, tripropylamine, tridecylamine, butyl diethanoloamine, 
methyl, ethyl, propyl or butyl diethanoloamine, butyl dimethylamine, 
methacroylethyldimethylamine and cyclohexyl diethanoloamine. 
A preferred class of polymerization initiators or activators for self cured 
primer formulations includes tertiary N-dialkyl substituted aromatic 
amines and aromatic peroxides (e.g., benzoyl peroxide).

The invention is further illustrated by examples. These examples are 
provided solely for better understanding the nature of this invention and 
should not be interpreted as limiting its scope. 
DETAILED DESCRIPTION 
Example 1 
A chipped porcelain veneer on Degussa G Ceramic Gold had about nine square 
millimeters area of metal exposed. The veneer around the metal was ground 
down to a feature edge with a burr. The same burr was used to roughen the 
exposed surface. The area to be restored was thoroughly washed and dryed. 
A metal primer, having the following chemical composition, was applied 
with a brush: 
______________________________________ 
Compound Parts (Weight) 
______________________________________ 
Chloroform 100 
Ethyl Acetate 18 
Copolymer of acrylic acid with 
12 
ethylmethacrylate having a 
molecular weight of about 
200,000 and an acid No. of 
100 
BIS-GMA 3 
Triethylene Glycol Dimethacrylate 
2 
Camphoroquinone 0.015 
methacroylethyl-dimethylamine 
0.02 
______________________________________ 
The solvent was allowed to evaporate for two minutes. A paint-on type 
restorative material having a shade very similar to the shade of the 
surrounding veneer was applied over the area to be restored with a brush. 
The restorative material included 40 parts polymerizable material (70% bis 
GMA and 30% triethylene glycol dimethacrylate) and 60 parts of filler (50 
parts borosilicate glass and 10 parts silica). There was also included 
0.3% benzil, 0.6% methacroylethyl dimethylamine and 0.03% butylated 
hydroxy toluene based on the total of polymerizable material and filler. 
The restoration was covered with a transparent polyester film, and was 
cured with a dental curing light (Optilux by Demetron Corporation) for 20 
seconds. The foil was removed, and the restoration was finished with a 
diamond burr, followed by polishing with a composite polishing paste 
(Luster by Scientific Pharmaceuticals). The result was, esthetically and 
funtionally satisfactory. 
Example 2 
A chipped porcelain veneer on silver/palladium alloy was repaired using the 
same technique and primer as described in Example 1. As restorative, Silux 
(a dimethacrylate type filled resin manufactured by 3M Company) was used. 
It was opacified with titanium dioxide for better hiding of the metal 
background. The result was esthetically and functionally satisfactory. 
Example 3 
A worn acrylic veneer on Ceramco-brand nonprecious metal base was 
successfully restored using the same technique as described in Example 1, 
however, with the primer consisting of an alcohol/water solution of 3 
parts of polyacrylic acid having a molecular weight of 750,000 and one 
part of hydroxethylmethacrylate with 0.2% of benzil and 0.3% of 
methacryoylethyldimethylamine used as polymerization activators. There was 
used CuRay Fil Restorative System (manufactured by Scientific 
Pharmaceuticals) as a restorative. The result was esthetically and 
functionally satisfactory. 
Example 4 
The adhesive strength obtained with a primer formulation described in 
Example 1 was determined in in-vitro studies performed as follows: 
Alloys, listed in the Table below, were cast into a 1.times.1 cm square 
specimen 2 mm thick. The specimens were embedded in phenolic rings with an 
epoxy resin and placed in special holders, adapted to attachment to an 
adhesion tester. The alloy surface was roughened with 320 grit 
silicon-carbide abrasive to produce a uniform surface roughness reading of 
0.35.+-.3 mm on a Surftester III (Mitutoyo, Japan). After roughening, the 
specimen was washed with water and dried. The primer was applied with a 
brush over the roughened area and dried at 37.degree. C. for 2 minutes. 
Cylindrical specimen made of the same restorative as in Example 1 5 mm in 
diameter was cured directly over the primed metal surfaces, using a Teflon 
split mold. Five samples were prepared for each alloy. 
Adhesion was tested under sheer forces on a Richie Universal Tester, at a 
crosshead speed of 1 mm per minute. The results are presented in the Table 
below: 
______________________________________ 
Average Adhesive Strength 
Metal PSI MPA 
______________________________________ 
Degussa G- 867 5.8 
Ceramic Gold 
Ceramco-Non 785 5.3 
Precious Alloy 
Olympia - by 705 4.8 
Jelenko 
Albabond - Silver 
840 5.7 
Palladium Alloy 
______________________________________ 
The results obtained in parallel tests, with no primer or with functional 
organosilane primer based on methacroylpropyl trimethoxy silane 
(Scientific Pharmaceuticals' Porcelain Primer) have shown adhesive 
strength in the range of 100-300 psi on gold and gold alloys, and 250-550 
psi on Albabond Silver/Palladium Alloy and Ceramco Non-Precious Alloy.