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Comparison of acidulated phosphate fluoride gel and hydrofluoric acid etchants for porcelain-composite repair
3a and Gregory I?. Stewart, School of Dental Medicine, Southern Illinois University, Aiton, Ill.
b
orcelain to produce a porous surface scanning ~~~ect~~~ microscopy when compared to an acidulated gel. Some ~~ve~t~~at~~~ have suggested the greater porosity of greater composite-to-porcelain bond. This two common fluoride etchants. The g eBectron microscopy to ensure that chants yielded bond strengths that ps 1s suggested that the intraoral use of hydr dangerous acidulated phosphate Buo DENT ~~~4;~~:~2~-~:~
he longevity of intraoral porcelain repair has been a problem in dentistry because of an inability to bond repair material to porcelain successfully long-term. An initial attempt to address this problem involved the use of a course diamond to roughen the fractured porcelain surface and increase the micromechanical retention of the repair material to the porcelain1 Cyanoacrylates, acrylic resins, or composite resins were used to repair metal ceramic resto,rations, but were only partially successful because of inherent deficiencies.2 Recent advances have had promising results to solve this problem. Silane coupling agents appear to enhance bond strength by promoting a chemical bond between composite resin and porcelain. 3,4 In addition, various porcelain et&ants appear to improve bond strengths by increasing the rn~~rorne~h~~ca~ retention5, 6 Other factors that influence bond strength include the type and brand of porcelain,7, 8 type and brand of et~hant,e-~~concentration of etchant,5-7 time of etching,57,~ hydration before repair,1s-15 brand of silane coupler,*, is, J.4thermocycling of repair,13 and aging of repair.l* The factor most often cited as the most significant was etching of the porcelain surface with hydrofluoric (HE’) acid 4; E,9
aAssistant E’rofe~sor~ Department of Restorative Dentistry. SProfessor, Department of estorative Dentistry. Copyright a 1934 by The Editorial Council of THE JOURNAL OF
visi ~~~s~b~~e &.xori the by~r~~~~~~c ~~v~5t~~at~o~ tested
Some studies focused on the bond bc;ween HF acidetched porcelain and composite resin and ~e~~~t~~ the bond strength to be stronger than the c~41esivestren the ~~d~~~~~~materials5, s bond strengths of HF acid to a (OF) gels, another porcelain etchantSxO*i found that both MF acid and AR? gel ~~~~~~~~~cohesive failure if used in conjunction with a s&me coupling agent." The study also showed that the use of silane coupling agent was a more significant factor than an WF acid e&3 in improving bond strength, which ~o~t~a~~~:~~ate~ the titerature.*,a The other study also found. that an etch produced bond strengths comparable to an BF a&% etched control.‘” 1x1this study, a IO-minute etch the highest bond strengths for the A.P.Fgel. T’he c etched 1 minute with 10% WF acid. Howler, the e%cacy of a l-minute etch for 10% HI? acid has not yet been studied. The questimonstill remains: Does a.~~~~ere~ce exis$ between tbe bond strengths by these two et&ants’! HF acid is extremely caustic to soft tissue and ~~g~~~~~ fos clinical use.la However, many ~~~~?~fa~~~~~rs ~~~t~~~~ to promote it for intraoral use, It seems ~~~r~~~~~t.~to reinvestigate APF gel as an etchant beca;xse of the reduced risk it presents. Tbis study was composed of two parts. The first part compared ~ho~~omicrogra~~s of the etch created APF gel and HF acid to those ~~b~~s~~~iIn earlier This was done to verify that the etchiq ~~~c~~~~~sused in this study produced results consistent with those published. The second part attempted to compare bond strengths of the two etehan’ts by me ni two dental purcelains.
THE
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DENTISTRY
Fig.
MATERIAL Part
AND
9.5% HF
1. Diagrammatic
representation
METHODS
I
Will-Ceram (Williams Gold and Refining, Buffalo, N. Y.) and Biobond (Dentsply, Dentsply International Inc., York, Penn.) porcelains were used to generate samples for the scanning electron microscope (SEM). Six samples of each product were made with body porcelain. The porcelain was condensed in a stainless steel mold with an internal diameter of 4 mm and a height of 3 mm. The cylinders were placed on a platinum foil sagger tray and fired in a calibrated porcelain oven (Ney III-Modular, J. M. Ney Company, Bloomfield, Conn.). The cylinders were fired at conventional sintering temperatures, and the manufacturer’s directions were followed accordingly. The cylinders were prepared with a medium-grit diamond disk (Brasseler USA Inc., Savanna, Ga.) to produce a flat, uniformly textured surface and then thoroughly rinsed with tap water and air-dried. The two porcelain groups were then divided. Half of each group was etched with 1.23% APF gel (Sultan, Sultan, Inc., Englewood, Calif.). The other half was etched with 9.5% HF acid (Ceram-etch, Cresco Products Inc., Stafford, Tex.). This produced four subgroups for scanning electron microscopic (SEM) investigation: (1) Will-Ceram/APF, (2) Will-Cream/HF, (3) Biobond/APF and (4) Biobond/HF. A lo-minute etch was used for the APF gel. The HF acid was applied for 5 minutes according to the manufacturer’s recommendations. After etching, the samples were thoroughly rinsed with tap water for 1 minute and air-dried. 122
STEWART
Biobond n=68
Wil-ceram n=68
1.23% APF
AND
1.23% APF
of sample development
9.5% HF
and treatment.
Samples were gold-coated and viewed at Xl000 and ~3000 magnification with a scanning electron microscope (JSM 35F, JEOL, Ltd., Tokyo, Japan) to evaluate depth and character of etch. A representative region of each sample was then photographed at both magnifications. Photomicrographs were compared with photomicrographs published in earlier studies to verify that the procedures used produced the surface morphology characteristic of the etchants.
Part
II
The torsion test is intended to subject a cylindrical rod with a constant cross-sectional area to a torsional load. Tempo material (J. M. Ney Co.) was used for fabrication of the cylindrical metal substructures of the porcelain samples. Will-Ceram and Biobond porcelains were used to generate porcelain-fused-to-metal samples (Fig. 1). Opaque and body porcelain were baked on the cylindrical metal bases according to the manufactures’ recommendations, The opaque layer was subjected to a single firing cycle. The body porcelain was subjected to two firing cycles. Each sample was formed by use of a diamond disk of medium grit (Brasseler USA Inc.) to create a flat surface perpendicular to the sample’s axis of rotation. The porcelain was then machined to produce a cylinder with an axis of rotation identical to the cylindrical metal substructure. During this procedure, the porcelain cylinders were adjusted to produce bonding pairs in which diameters were matched to l/100 mm. Initial cylinder diameters averaged 3.45 mm, but subsequent cylinder diameters were reduced to an average of 2.64 mm. VOLUME
72
NUMBER
2
s 2. Torsional testing device with bonded sample in chuck. Bonding pairs were made for each commercial porcelain assigned to one of two subgroups. Each subgroup was then etched with either the 1.23% APF gel or 9.5% HI? acid by following the identical protocol previously described. The bonding pairs were then visually aligned along their rotational axes with a paralleling device. A silane coupling agent (Prilane, Cresco Products Inc., Stafford, Tex.) was applied according to the manufacturer’s directions. This was immediately followed by the application of an u&lied resin (Command bonding resin, Kerr/ Sybron, Romulus, &/Ii&.), which was blown thin with compressed air. Al~%~rnent was again visually confirmed and the samples were separated 2 mm apart. The bonding pairs were tbec Med. together with a filled composite resin ~~o~~an~ Ultraflne, Kerr/Sybron). Concentricity of the bonding pairs was verified, and the bonded samples were stored for 1 week before torsional testing in deionized water at 37” C. The torsional testing device (Fig. 2) consisted of a chuck att~achedalong a shaft to a disk 3 inches in diameter. The shaft was held by two doubie ball bearing-lined supports for rotation and alignment of its long axis. The supports were secured to a l-inch thick aluminum plate. The cylindried metal end of tbe sample was locked into the jaws of the chuck and the other end was positioned in the open end of a movable hollow housing. The base of the housing was then secured to the aluminum plate. The opening of the housing face was closed with modeling clay and filled with iow-fusing alloy (Fusible Metal, Wm. Dixon, Carlstadt, N. J.) through a top opening, which locked the specimen 4rmly without prestressing.17 A torsional load -wasapplied to each specimen at the rate of 1.0 newton-meter per minute until failure occurred. The shear strength was calculated by the following equation: yo = Tc!J, where T is the applied torque, c is the radius of the rod, and J is the polar moment of inertia of the rod. For a circular rod this reduced to the following: yo = 2T/c3. The tensile stress, 7’: is the numeric equivalent to the value of and randomly
the shear stress, yo, but is inc!ined
81, G &gmss
to the axis
of *he Y bar.?& Torsional testing of k&tie joizxs has serieisl ahir antages. For instance, because the tensile and shear stress ha-/e equal magnitudes, torsional testing readly ~~~~~~~ni~~~s which is greater, the shear bond stsengh or the tensile strength of the materials. If the shear bond slrengtb of the bond is greater, a helical, cohesive fracture occurs through the material (Fig. 3). If the tensile stren$~~ of the material joined is greater, a clean, a hesive break o~:cilrsat i.he joint. Another advantage of the torsinnaB method is that all stresses are accounted for, unlike push-pall shear testing, which does not account for the bending :&asses developed All data were analyzed by a t-m-way anai.vsis ai” variance 123
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AND
STEWART
Fig. 4. Will-Ceram porcelain etched with 1.23 % APF gel. (Original magnification x3000.)
Fig. 5. Biobond porcelain etched with 1.23% APF gel. (Original magnification x3000.)
(ANOVA) and computed with a statistical software package (StatView Student, Abacus Concepts Inc., Berkeley, Calif.). Significance for all analyses was defined asp < 0.05. RESULTS Part I The photomicrographs in this study resembled others published earlier, which documented surface alterations of other commercial porcelains treated with these etchants.ll, l2 Figs. 4 through 7 depict the resultant etch patterns of Will-Ceram and Biobond porcelain treated with 1.23% APF gel or 9.5% HF acid. Photomicrographs of the porcelain samples etched with HF acid revealed a three-dimensional lattice of voids and channels. There 124
were subtle differences in etching characteristics between the two porcelains, but they were similar in regard to depth of penetration and potential for microretention. The photomicrographs of the porcelains etched with the APF gel revealed a relatively smooth surface compared with the HF acid etch. Part
II
During testing of the initial group of bonded samples, a problem developed with the testing apparatus. In 16 of the 34 samples, the chuck that secured one end of the sample failed because of the high torsional loads that were applied to the samples. Attempts to remove these samples from the testing apparatus resulted in the cohesive destruction of all
VOLUME
72
NUMBER
2
. 7. Biobond porcelain etched with 9.5% HF acid. {Original rna~~~~cat~o~~~3000.:
16 samples. As a result, the Riobond specimen total was reduced to 18, with eight in the HF group and 10 in the APF gr0Lp.
To facilitate testing oftbe second group, the diameter of the Will-Ceram porcelain cylinders was reduced before etching.c This was inconsequential as long as the diameter of the bonding pairs was approximately equal. This reduced the load necessary to cause failure without affecting the stress. After modification, all samples in this group were successfully tested. The mean torsional loads required to fracture the samples in all groups are shown in Fig. 8. Statistical analysis of the data is Eisted in Table I. The lack of statistical significance reported does not represent an acceptance of the
null hypothesis,
I- A/ j fs a* ai&‘erence 2nd thus means that ‘ALL.
between the bored strengths of the t-w0 _I et‘-hants r Becawe all of the specimens fractured cohesiveBy,the data obtained do not represent bond strengths of the Jyamxdninbond to
THEJOURNALOFPROSTHETICDENTISTRY
Biobond/HF
TYLKAANDSTEWART
Wit-ceram/APF
Wil-ceram/HF
Biobond/APF
Groups Fig.
8. Mean shear bond strengths and standard deviations.
composite interface, but the tensile limits of the parent materials, the porcelain/composite resin assembly. As such, the strengths compared in this study were of the parent materials, not the bond, and because all of the samples were uniformly generated, no difference would be expected. DISCUSSION Torsional loading as applied in this study allowed the generation of a uniform state of stress along the sample. Loading of the sample to catastrophic failure readily identified the weaker of the two bonds involved, cohesive or adhesive. All fractures in this study were spiral in nature and thus cohesive. This means that the shear bond strength of the porcelain-to-composite resin interface was superior to the cohesive strength of the porcelain and composite resin materials as tested. Visual comparison of the etched porcelain surface morphology showed remarkable differences between the etchants. The 1.23 % APF gel is routinely used for intraoral fluoride application and creates a smooth, homogenous surface of the exposed porcelain, whereas the HF acid produces a porous, amorphous surface.g, l2 As such, some investigators have used surface morphology to select the method of porcelain preparation when the effects of various bonding conditions on shear bond strength of composite resin to etched porcelain are studied.5v1g If the more convoluted etch of the HF acid truly increases the micromechanical retention, it would be expected to produce a stronger bond strength than the APF group. However, this common assumption that equates the etched surface irregularity with bond strength has not been substantiated. It is possible that there were differences in the bond strengths produced by the two etchants in this study. However, any 126
difference that may have existed was undetectable because of the cohesive failure of the samples. Organosilanes are commonly used to enhance the porcelain-to-composite-resin bond by producing a chemical instead of mechanical bond at the porcelain-to-compositeresin interface. This could help account for the unexpected optimal bond strengths found with both etchants, even though the microretentiveness of the APF gel appeared remarkably inferior. Since it appears that an optimal bond can be achieved with either etchant, in conjunction with an organosilane, the intraoral use of dangerous HF acid should be seriously questioned. A greater concern now lies in the investigation of the durability of this bond. Research is needed to determine what effect prolonged hydration and thermocycling have on the bond strength of the two etchants. CLINICAL
IMPLICATIONS
APF gel is a topical fluoride gel commonly present in dental offices. Hydrofluoric acid is a poisonous and extremely caustic substance. The widely accepted theory that HF acid enhances the composite-resin-to-porcelain bond more than an APF gel was not substantiated by this investigation. As such, the intraoral use of HF acid appears unwarranted at this time. CONCLUSIONS Scanning electron microscopy was used to evaluate the surface irregularity of porcelain etched with 1.23 % APF gel or 9.5 % HF acid. Strength of the porcelain-to-compositeresin bond was’tested for each etchant on Will-Ceram and Biobond dental porcelains. The following conclusions were made.
VOLUME
72
NUMBER
2
?L” Gross differences in the pbotom~~rographs between by the APF gel and HF acid resembled the etch meat 2. All samples experienced cohesive failure of the porce?ain-to-composite-resin bond, This means tbe shear bond strength of the repair by use of HF acid or APF gel, as used in this study, is greater than the cohesive strength of the porcelain and composite materials. 3. The popular concept that has equated greater irreguiarities of the etched porcelain surface with higher bond strengths requires additional research,
1i. Jo&en DG, Gaputo AA. Composite resin repair of porcelain denture teeth. J PROSFHET 3mr
1977;38:673-9.
2. Phillips KW. Science of dental materials. 8th ed. Philadelphia: WB Saunders, 1982224-44. 3. Newburg R, Pameijer CH. Composite resins bonded to porcelain with siliane soiution.J Am Dent Assoc 1978;96:288-91. 4. Calamia JR, Simcmsen RJ. Effects of coupling agents on bonding strength of etched porcelain [Abstract]. J Dent Res 1984;63:179. 5. Simomsen RJ, Calamia JR. Tensile bond strength of etched porcelain. !Abstract]. J Den: Res 1983;62:297. 6. Stangel 1; Na~hanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;65:146G-5. 7. Calamia JR, Vaidyanathan J, Vaidyanathan TK, et al. Shear bond strength of eicked porcelains [Abstract]. J Dent Res 1985;64:296. 8. Yemme!man JO, Kelp PR. Silane bonding porcelain teeth to acrylic. J Am Dent Assoc 1968;16:69-73.
Availability
9. Senda 4, Suzuki M, Jordan F1E.The eifec~ :fi ii~c :d+: and ~yciroiiu oric acids on porcelain surfacea [A%tractj. J aeni &?a 198w%236. 10. Nelson E; Bar&i N. Effect of APF etching iirr,e ::^ r&n baeded ;wrcelain [Abstract]. J Dent Res 1989$%X271, 11. Lacy AM. LsLuz J, Watanabe LG, Deliingrs M. G&cr oi porcelain sw fsce treatment
13. Diaz~ArnoId .4Mz Aquiline SA. An evaiution of tie bond strength of four crganosilane materials in response io iherm9: +tros~. J Pwwrnr:* DENT 198962:2X-60. I,
14. Diaz-Arnold AM, Schneider RL, Aqui!ino SX, Pdxdng strenghhs of in tracral parcelair, materiais. 3 PRCIRTHW DENT L%~$X~OS-S. 15. Bailey J-II. Pcrceiain-to-composite bond atre3gtks using four orgwcsilane materials. j FROSTHET DENT 1989;61:~~1-7. 16. Berkow R. Merck Manual. 14th ed. Rahway. P&FM~*xk C Co, :982:869. 17. Stewart GP, Maroso DJ, DuEey MJ, Normarc 311. S, uxsisnd m&cd for the eva!uaticn of tensile properties of ceramic matesiaie: denra! porcelain. J Denr. M&er 498?;3:74-8. 1s. Popov EP. Mechanics of materials. 2nd ed. Fhg1ewm.i Cii& NJ: Prec tice-Haii, 1976:319-62. 19. Hsu CS, Stangei I, Nathanson D. Shear bond streiiaq:i- :;i r&u! to etched porcelain [Abstrz.ci]. J Dent Res 1985:6~:296.
Reprmtrequeststc;: DR. DANIEL F. TYLKA DEPARTMENT 0F Rmmc4Trv~ DENTI~RY SCHOOLOFDENTAL~~~UICIN~ SOUTHERN ILLINOIS ~JNIVERSITY, BLDG. 284 ALTOK,IL 62002-4798
of JOURNAL
nausea
As a service to our subscribers, copies of back issues of THE JOURNAL OF PROSTHI3rX for the preceding 5 years are maintained and are available for pzlrchase from the publisher, Mosby, at a cost of $8.00 per issue. The following quantity discounts arc: available: 25 % off on quantities of 12 to 23, and one third of7 on quantities of 24 or morn Please write to Mosby, Subscription Services, 11330 esthne Industrial Drive, St. Louis, MO 63146-3318, or call (800)453-4351 or (314)453-4351 for information on availabitity trf particular issues. If unavailable from the publisher, photocopies of complete issues are 300 N. Zeeb Rd., Ann Arbor, Xi available from University Microforms International, 48106, (313)761-4700.
I?ENTISTRY
“i I9
b
orcelain to produce a porous surface scanning ~~~ect~~~ microscopy when compared to an acidulated gel. Some ~~ve~t~~at~~~ have suggested the greater porosity of greater composite-to-porcelain bond. This two common fluoride etchants. The g eBectron microscopy to ensure that chants yielded bond strengths that ps 1s suggested that the intraoral use of hydr dangerous acidulated phosphate Buo DENT ~~~4;~~:~2~-~:~
he longevity of intraoral porcelain repair has been a problem in dentistry because of an inability to bond repair material to porcelain successfully long-term. An initial attempt to address this problem involved the use of a course diamond to roughen the fractured porcelain surface and increase the micromechanical retention of the repair material to the porcelain1 Cyanoacrylates, acrylic resins, or composite resins were used to repair metal ceramic resto,rations, but were only partially successful because of inherent deficiencies.2 Recent advances have had promising results to solve this problem. Silane coupling agents appear to enhance bond strength by promoting a chemical bond between composite resin and porcelain. 3,4 In addition, various porcelain et&ants appear to improve bond strengths by increasing the rn~~rorne~h~~ca~ retention5, 6 Other factors that influence bond strength include the type and brand of porcelain,7, 8 type and brand of et~hant,e-~~concentration of etchant,5-7 time of etching,57,~ hydration before repair,1s-15 brand of silane coupler,*, is, J.4thermocycling of repair,13 and aging of repair.l* The factor most often cited as the most significant was etching of the porcelain surface with hydrofluoric (HE’) acid 4; E,9
aAssistant E’rofe~sor~ Department of Restorative Dentistry. SProfessor, Department of estorative Dentistry. Copyright a 1934 by The Editorial Council of THE JOURNAL OF
visi ~~~s~b~~e &.xori the by~r~~~~~~c ~~v~5t~~at~o~ tested
Some studies focused on the bond bc;ween HF acidetched porcelain and composite resin and ~e~~~t~~ the bond strength to be stronger than the c~41esivestren the ~~d~~~~~~materials5, s bond strengths of HF acid to a (OF) gels, another porcelain etchantSxO*i found that both MF acid and AR? gel ~~~~~~~~~cohesive failure if used in conjunction with a s&me coupling agent." The study also showed that the use of silane coupling agent was a more significant factor than an WF acid e&3 in improving bond strength, which ~o~t~a~~~:~~ate~ the titerature.*,a The other study also found. that an etch produced bond strengths comparable to an BF a&% etched control.‘” 1x1this study, a IO-minute etch the highest bond strengths for the A.P.Fgel. T’he c etched 1 minute with 10% WF acid. Howler, the e%cacy of a l-minute etch for 10% HI? acid has not yet been studied. The questimonstill remains: Does a.~~~~ere~ce exis$ between tbe bond strengths by these two et&ants’! HF acid is extremely caustic to soft tissue and ~~g~~~~~ fos clinical use.la However, many ~~~~?~fa~~~~~rs ~~~t~~~~ to promote it for intraoral use, It seems ~~~r~~~~~t.~to reinvestigate APF gel as an etchant beca;xse of the reduced risk it presents. Tbis study was composed of two parts. The first part compared ~ho~~omicrogra~~s of the etch created APF gel and HF acid to those ~~b~~s~~~iIn earlier This was done to verify that the etchiq ~~~c~~~~~sused in this study produced results consistent with those published. The second part attempted to compare bond strengths of the two etehan’ts by me ni two dental purcelains.
THE
JOURNAL
OF PROSTHETIC
TYLKA
DENTISTRY
Fig.
MATERIAL Part
AND
9.5% HF
1. Diagrammatic
representation
METHODS
I
Will-Ceram (Williams Gold and Refining, Buffalo, N. Y.) and Biobond (Dentsply, Dentsply International Inc., York, Penn.) porcelains were used to generate samples for the scanning electron microscope (SEM). Six samples of each product were made with body porcelain. The porcelain was condensed in a stainless steel mold with an internal diameter of 4 mm and a height of 3 mm. The cylinders were placed on a platinum foil sagger tray and fired in a calibrated porcelain oven (Ney III-Modular, J. M. Ney Company, Bloomfield, Conn.). The cylinders were fired at conventional sintering temperatures, and the manufacturer’s directions were followed accordingly. The cylinders were prepared with a medium-grit diamond disk (Brasseler USA Inc., Savanna, Ga.) to produce a flat, uniformly textured surface and then thoroughly rinsed with tap water and air-dried. The two porcelain groups were then divided. Half of each group was etched with 1.23% APF gel (Sultan, Sultan, Inc., Englewood, Calif.). The other half was etched with 9.5% HF acid (Ceram-etch, Cresco Products Inc., Stafford, Tex.). This produced four subgroups for scanning electron microscopic (SEM) investigation: (1) Will-Ceram/APF, (2) Will-Cream/HF, (3) Biobond/APF and (4) Biobond/HF. A lo-minute etch was used for the APF gel. The HF acid was applied for 5 minutes according to the manufacturer’s recommendations. After etching, the samples were thoroughly rinsed with tap water for 1 minute and air-dried. 122
STEWART
Biobond n=68
Wil-ceram n=68
1.23% APF
AND
1.23% APF
of sample development
9.5% HF
and treatment.
Samples were gold-coated and viewed at Xl000 and ~3000 magnification with a scanning electron microscope (JSM 35F, JEOL, Ltd., Tokyo, Japan) to evaluate depth and character of etch. A representative region of each sample was then photographed at both magnifications. Photomicrographs were compared with photomicrographs published in earlier studies to verify that the procedures used produced the surface morphology characteristic of the etchants.
Part
II
The torsion test is intended to subject a cylindrical rod with a constant cross-sectional area to a torsional load. Tempo material (J. M. Ney Co.) was used for fabrication of the cylindrical metal substructures of the porcelain samples. Will-Ceram and Biobond porcelains were used to generate porcelain-fused-to-metal samples (Fig. 1). Opaque and body porcelain were baked on the cylindrical metal bases according to the manufactures’ recommendations, The opaque layer was subjected to a single firing cycle. The body porcelain was subjected to two firing cycles. Each sample was formed by use of a diamond disk of medium grit (Brasseler USA Inc.) to create a flat surface perpendicular to the sample’s axis of rotation. The porcelain was then machined to produce a cylinder with an axis of rotation identical to the cylindrical metal substructure. During this procedure, the porcelain cylinders were adjusted to produce bonding pairs in which diameters were matched to l/100 mm. Initial cylinder diameters averaged 3.45 mm, but subsequent cylinder diameters were reduced to an average of 2.64 mm. VOLUME
72
NUMBER
2
s 2. Torsional testing device with bonded sample in chuck. Bonding pairs were made for each commercial porcelain assigned to one of two subgroups. Each subgroup was then etched with either the 1.23% APF gel or 9.5% HI? acid by following the identical protocol previously described. The bonding pairs were then visually aligned along their rotational axes with a paralleling device. A silane coupling agent (Prilane, Cresco Products Inc., Stafford, Tex.) was applied according to the manufacturer’s directions. This was immediately followed by the application of an u&lied resin (Command bonding resin, Kerr/ Sybron, Romulus, &/Ii&.), which was blown thin with compressed air. Al~%~rnent was again visually confirmed and the samples were separated 2 mm apart. The bonding pairs were tbec Med. together with a filled composite resin ~~o~~an~ Ultraflne, Kerr/Sybron). Concentricity of the bonding pairs was verified, and the bonded samples were stored for 1 week before torsional testing in deionized water at 37” C. The torsional testing device (Fig. 2) consisted of a chuck att~achedalong a shaft to a disk 3 inches in diameter. The shaft was held by two doubie ball bearing-lined supports for rotation and alignment of its long axis. The supports were secured to a l-inch thick aluminum plate. The cylindried metal end of tbe sample was locked into the jaws of the chuck and the other end was positioned in the open end of a movable hollow housing. The base of the housing was then secured to the aluminum plate. The opening of the housing face was closed with modeling clay and filled with iow-fusing alloy (Fusible Metal, Wm. Dixon, Carlstadt, N. J.) through a top opening, which locked the specimen 4rmly without prestressing.17 A torsional load -wasapplied to each specimen at the rate of 1.0 newton-meter per minute until failure occurred. The shear strength was calculated by the following equation: yo = Tc!J, where T is the applied torque, c is the radius of the rod, and J is the polar moment of inertia of the rod. For a circular rod this reduced to the following: yo = 2T/c3. The tensile stress, 7’: is the numeric equivalent to the value of and randomly
the shear stress, yo, but is inc!ined
81, G &gmss
to the axis
of *he Y bar.?& Torsional testing of k&tie joizxs has serieisl ahir antages. For instance, because the tensile and shear stress ha-/e equal magnitudes, torsional testing readly ~~~~~~~ni~~~s which is greater, the shear bond stsengh or the tensile strength of the materials. If the shear bond slrengtb of the bond is greater, a helical, cohesive fracture occurs through the material (Fig. 3). If the tensile stren$~~ of the material joined is greater, a clean, a hesive break o~:cilrsat i.he joint. Another advantage of the torsinnaB method is that all stresses are accounted for, unlike push-pall shear testing, which does not account for the bending :&asses developed All data were analyzed by a t-m-way anai.vsis ai” variance 123
THE
JOURNAL
OF PROSTHETIC
TYLKA
DENTISTRY
AND
STEWART
Fig. 4. Will-Ceram porcelain etched with 1.23 % APF gel. (Original magnification x3000.)
Fig. 5. Biobond porcelain etched with 1.23% APF gel. (Original magnification x3000.)
(ANOVA) and computed with a statistical software package (StatView Student, Abacus Concepts Inc., Berkeley, Calif.). Significance for all analyses was defined asp < 0.05. RESULTS Part I The photomicrographs in this study resembled others published earlier, which documented surface alterations of other commercial porcelains treated with these etchants.ll, l2 Figs. 4 through 7 depict the resultant etch patterns of Will-Ceram and Biobond porcelain treated with 1.23% APF gel or 9.5% HF acid. Photomicrographs of the porcelain samples etched with HF acid revealed a three-dimensional lattice of voids and channels. There 124
were subtle differences in etching characteristics between the two porcelains, but they were similar in regard to depth of penetration and potential for microretention. The photomicrographs of the porcelains etched with the APF gel revealed a relatively smooth surface compared with the HF acid etch. Part
II
During testing of the initial group of bonded samples, a problem developed with the testing apparatus. In 16 of the 34 samples, the chuck that secured one end of the sample failed because of the high torsional loads that were applied to the samples. Attempts to remove these samples from the testing apparatus resulted in the cohesive destruction of all
VOLUME
72
NUMBER
2
. 7. Biobond porcelain etched with 9.5% HF acid. {Original rna~~~~cat~o~~~3000.:
16 samples. As a result, the Riobond specimen total was reduced to 18, with eight in the HF group and 10 in the APF gr0Lp.
To facilitate testing oftbe second group, the diameter of the Will-Ceram porcelain cylinders was reduced before etching.c This was inconsequential as long as the diameter of the bonding pairs was approximately equal. This reduced the load necessary to cause failure without affecting the stress. After modification, all samples in this group were successfully tested. The mean torsional loads required to fracture the samples in all groups are shown in Fig. 8. Statistical analysis of the data is Eisted in Table I. The lack of statistical significance reported does not represent an acceptance of the
null hypothesis,
I- A/ j fs a* ai&‘erence 2nd thus means that ‘ALL.
between the bored strengths of the t-w0 _I et‘-hants r Becawe all of the specimens fractured cohesiveBy,the data obtained do not represent bond strengths of the Jyamxdninbond to
THEJOURNALOFPROSTHETICDENTISTRY
Biobond/HF
TYLKAANDSTEWART
Wit-ceram/APF
Wil-ceram/HF
Biobond/APF
Groups Fig.
8. Mean shear bond strengths and standard deviations.
composite interface, but the tensile limits of the parent materials, the porcelain/composite resin assembly. As such, the strengths compared in this study were of the parent materials, not the bond, and because all of the samples were uniformly generated, no difference would be expected. DISCUSSION Torsional loading as applied in this study allowed the generation of a uniform state of stress along the sample. Loading of the sample to catastrophic failure readily identified the weaker of the two bonds involved, cohesive or adhesive. All fractures in this study were spiral in nature and thus cohesive. This means that the shear bond strength of the porcelain-to-composite resin interface was superior to the cohesive strength of the porcelain and composite resin materials as tested. Visual comparison of the etched porcelain surface morphology showed remarkable differences between the etchants. The 1.23 % APF gel is routinely used for intraoral fluoride application and creates a smooth, homogenous surface of the exposed porcelain, whereas the HF acid produces a porous, amorphous surface.g, l2 As such, some investigators have used surface morphology to select the method of porcelain preparation when the effects of various bonding conditions on shear bond strength of composite resin to etched porcelain are studied.5v1g If the more convoluted etch of the HF acid truly increases the micromechanical retention, it would be expected to produce a stronger bond strength than the APF group. However, this common assumption that equates the etched surface irregularity with bond strength has not been substantiated. It is possible that there were differences in the bond strengths produced by the two etchants in this study. However, any 126
difference that may have existed was undetectable because of the cohesive failure of the samples. Organosilanes are commonly used to enhance the porcelain-to-composite-resin bond by producing a chemical instead of mechanical bond at the porcelain-to-compositeresin interface. This could help account for the unexpected optimal bond strengths found with both etchants, even though the microretentiveness of the APF gel appeared remarkably inferior. Since it appears that an optimal bond can be achieved with either etchant, in conjunction with an organosilane, the intraoral use of dangerous HF acid should be seriously questioned. A greater concern now lies in the investigation of the durability of this bond. Research is needed to determine what effect prolonged hydration and thermocycling have on the bond strength of the two etchants. CLINICAL
IMPLICATIONS
APF gel is a topical fluoride gel commonly present in dental offices. Hydrofluoric acid is a poisonous and extremely caustic substance. The widely accepted theory that HF acid enhances the composite-resin-to-porcelain bond more than an APF gel was not substantiated by this investigation. As such, the intraoral use of HF acid appears unwarranted at this time. CONCLUSIONS Scanning electron microscopy was used to evaluate the surface irregularity of porcelain etched with 1.23 % APF gel or 9.5 % HF acid. Strength of the porcelain-to-compositeresin bond was’tested for each etchant on Will-Ceram and Biobond dental porcelains. The following conclusions were made.
VOLUME
72
NUMBER
2
?L” Gross differences in the pbotom~~rographs between by the APF gel and HF acid resembled the etch meat 2. All samples experienced cohesive failure of the porce?ain-to-composite-resin bond, This means tbe shear bond strength of the repair by use of HF acid or APF gel, as used in this study, is greater than the cohesive strength of the porcelain and composite materials. 3. The popular concept that has equated greater irreguiarities of the etched porcelain surface with higher bond strengths requires additional research,
1i. Jo&en DG, Gaputo AA. Composite resin repair of porcelain denture teeth. J PROSFHET 3mr
1977;38:673-9.
2. Phillips KW. Science of dental materials. 8th ed. Philadelphia: WB Saunders, 1982224-44. 3. Newburg R, Pameijer CH. Composite resins bonded to porcelain with siliane soiution.J Am Dent Assoc 1978;96:288-91. 4. Calamia JR, Simcmsen RJ. Effects of coupling agents on bonding strength of etched porcelain [Abstract]. J Dent Res 1984;63:179. 5. Simomsen RJ, Calamia JR. Tensile bond strength of etched porcelain. !Abstract]. J Den: Res 1983;62:297. 6. Stangel 1; Na~hanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;65:146G-5. 7. Calamia JR, Vaidyanathan J, Vaidyanathan TK, et al. Shear bond strength of eicked porcelains [Abstract]. J Dent Res 1985;64:296. 8. Yemme!man JO, Kelp PR. Silane bonding porcelain teeth to acrylic. J Am Dent Assoc 1968;16:69-73.
Availability
9. Senda 4, Suzuki M, Jordan F1E.The eifec~ :fi ii~c :d+: and ~yciroiiu oric acids on porcelain surfacea [A%tractj. J aeni &?a 198w%236. 10. Nelson E; Bar&i N. Effect of APF etching iirr,e ::^ r&n baeded ;wrcelain [Abstract]. J Dent Res 1989$%X271, 11. Lacy AM. LsLuz J, Watanabe LG, Deliingrs M. G&cr oi porcelain sw fsce treatment
13. Diaz~ArnoId .4Mz Aquiline SA. An evaiution of tie bond strength of four crganosilane materials in response io iherm9: +tros~. J Pwwrnr:* DENT 198962:2X-60. I,
14. Diaz-Arnold AM, Schneider RL, Aqui!ino SX, Pdxdng strenghhs of in tracral parcelair, materiais. 3 PRCIRTHW DENT L%~$X~OS-S. 15. Bailey J-II. Pcrceiain-to-composite bond atre3gtks using four orgwcsilane materials. j FROSTHET DENT 1989;61:~~1-7. 16. Berkow R. Merck Manual. 14th ed. Rahway. P&FM~*xk C Co, :982:869. 17. Stewart GP, Maroso DJ, DuEey MJ, Normarc 311. S, uxsisnd m&cd for the eva!uaticn of tensile properties of ceramic matesiaie: denra! porcelain. J Denr. M&er 498?;3:74-8. 1s. Popov EP. Mechanics of materials. 2nd ed. Fhg1ewm.i Cii& NJ: Prec tice-Haii, 1976:319-62. 19. Hsu CS, Stangei I, Nathanson D. Shear bond streiiaq:i- :;i r&u! to etched porcelain [Abstrz.ci]. J Dent Res 1985:6~:296.
Reprmtrequeststc;: DR. DANIEL F. TYLKA DEPARTMENT 0F Rmmc4Trv~ DENTI~RY SCHOOLOFDENTAL~~~UICIN~ SOUTHERN ILLINOIS ~JNIVERSITY, BLDG. 284 ALTOK,IL 62002-4798
of JOURNAL
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