- Email: [email protected]
Evaluation of the electrolytic etching depth of a nickel-chromium base alloy used in resin-bonded cast restorations
LOBENZANA
exception was found in specimens made with PTM-88 alloy, which was consistent with results from the previous studies.4 Since the PTM-88 alloy had a significantly different composition than the other Pd rich alloys, it is possible that the weaker bond strength was due to a less-adherent metal oxide. This explanation was supported by the surface roughness evaluation. The surface roughness of 0.94 for PTM-88 alloy specimens indicated that the debonded surface was smoother than the surface not exposed to ceramic. The rationale for this result was that the ceramic-metal failure occurred at the metal-oxide interface. It seems that the surface roughness ratio was a fairly accurate predictor of the ceramic-metal bond strength. The good correlation of the BRS and R&M values provided evidence that roughness measurements might be a reliable indicator of bond strengths. The surface roughness test method would be beneficial for discriminating between ceramic-metal bond strengths of alloys that are not well characterized or are without elastic modulus.
SUMMARY
ET AL
However, when the flexure strength loads were normalized by the modulus of elasticity to compensate for different stiffness of the alloys, only specimens of one Pd rich alloy, Option, had a significantly greater bond strength. This indicated the importance of knowing the relative stiffness of the different test alloys in order to achieve an accurate assessment of the bond strength that reflects the clinical situation. REFERENCES 1. Anusavice
KJ, DeHoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13. 2. Schaffer SP. An approach to determining the bond strength of ceramometal systems. J PROSTHET DENT 1982;48:282-4. 3. Van Vlack LH. Elements of materials science and engineering. 3rd ed. Reading, Mass: Addition-Wesley Pub1 Co, 1975;177-80. 4. Lorensana RE, Chambless LA, Marker VA, Staffanou RS. Ceramometal bond strengths of four palladium-rich alloys. [Abstract]. J Dent Res 1985;54:585. Reprint
requests
to:
DR. ROBERT S. STAFFANOU BAYLOR COLLEGE OF DENTISTRY 3302 GASTON AVE. DALLAS, TX 75246
The loads measured from the flexure bond strength test and the shear bond strength test produced similar results.
Evaluation of the electrolytic nickel-chromium base alloy restorations Herbert University Wattens,
SchBffer, of Innsbruck, Austria
Dr.Med.,* School
etching depth of a used in resin-bonded cast
and Anton Piffer**
of Dentistry,
Innsbruck,
Austria;
and
Swarovski
D. & Co.,
This study examined the relation of the electrolytic etching depth of a nickelchromium base alloy to the etching time. Cast samples (22 mm long, 4 mm wide, 1.6 mm thick) were etched in the Eltrokor etching unit, using Korolyt A as the etching solution with a constant current density of 400 mA/cm2. On each sample an approximately 3 mm long area was etched for 5,7,9, and 11 minutes. Between each etched area, a field of approximately 2 mm long remained unetched. The surface of each sample was scanned with a protllometer. The distance of the compensation plane of the scanned etched profile to the unetched areas was then measured for each etching time. The average etched depth with an etching time of 5 minutes is 30.4 pm (SD 2.39); with a time of 7 minutes the depth is 44 pm (SD 3.49); with a time of 9 minutes the depth is 59.8 gm (SD 4.10); and with an etching time of 11 minutes the depth is 72.9 gm (SD 2.94). Taking the average of all etching depths per etching time, the average loss of substance is 6.409 pm per minute etching time. (J PROSTHET DENT 1990;64:680-3.)
*Assistant Professor, Department of Restorative Dentistry, University of Innsbruck, School of Dentistry. **Physicist, Department of Research and Development, Swarovski D. & Co. 10/l/22012 680
A
fter 15 years of research and development, resinbonded fixed prosthesis are established as a treatment of partial edentulism. The components in the bonding system (metal, composite resin, and enamel) have been examined in various studies.le4 The crucial criteria for long-term sucDECEMBER
1990
VOLUME
64
NUMBER
6
ELECTROLYTIC
ETCHING
DEPTH
OF BASE ALLOY
Fig.
2. Stylus of profilometer at start of measuring pro-
cess.
etched in Korolyt A etchant for 7 minutes at a current density of 400 mA/cm2. (Original magnification ~500.)
Fig.
1. Wiron 88 alloy surface electrolytically
cess seem to result from the numerous variable treatment parameters. Mircomechanic retention by electrolytic etching of the metal surface seems to be the most successful of the various retention mechanisms. It was found that for each type of alloy the etching procedure must be made with a specific etching solution under special electrolytic conditions (Fig. 1). For the fixing process, specially modified self-curing composite resins were developed. These composite resins show a more favorable flow and an extended working time than filling composite resins. Apart from a michromechanic retention, an additional chemical bonding seems possible with certain metal-resin combinations.5 The same result can be achieved by a silicone-coated metal surface.6, 7 Exact preparation when obtaining a retention and a resistance form is seen as the crucial factor for longterm success.*The preparation of grooves, guiding surfaces, and rest seats must be planned accurately and executed as carefully as possible.g A fine preparation of the entire retention wing in the enamel increases the bonding with the fixing composite resin. lo, I1 The optimal solution is a complete integration of the metal framework into the tooth.12 A problematic overcontouring and a possible periodontal irritation can thus be avoided.i3 Furthermore, the exact fit of the metal framework is of special importance. The aim is a minimal marginal gap, as is required in the usual fixed restorations. An exact fit and a minimal marginal gap lead to the required composite resin film thickness, which should be as thin as possible. Thus the negative effects on the adhesive and cohesive strength of the bonding system, such as thermal dimensional alterations and swelling caused by water absorption of the composite resin, can be reduced. These requirements can only be achieved by the complete reproduction of all THE
JOURNAL
OF PROSTHETIC
DENTISTRY
preparation details in the metal framework, using the model casting technique. A final factor that influences the fit of the metal framework is the electrolytic etching of the internal surface of the metal before the cementation. This study examined the loss of substance of a nickelchromium base alloy during the etching procedure.
MATERIAL
AND
METHODS
Ten samples, 22 mm long, 4 mm wide, and 1.5 mm thick, made of Wiron 88 (Bremer Goldschliigerei, Bremen, West Germany) alloy were cast according to the manufacturer’s directions. The samples were polished using a grinding machine (Planopo12, Struers, Copenhagen, Denmark) and a 2400 mesh abrasive paper. In order to simulate the effects of temperature on the metal during the porcelain bake cycles, the samples were put in the ceramic oven four times under the following conditions: opaque bake 960’ C, vacuum, holding time 2 minutes; dentin bake 930” C, vacuum, holding time 2 minutes; correction bake 920” C, vacuum, holding time 2 minutes; and glaze bake 930“ C, holding time 2 minutes. Afterward the oxide film on the surface was removed by means of continous sand blasting with a 25 pm grit aluminium oxide at 1.5 bar. Electrolytic etching was done with Korolyt A etching solution (Bremer GoldSchliigerei) at a constant current density of 400 mA/cm2 in the Eltrokor etching unit (Bremer Goldschliigerei). A fresh solution was used for each sample. A 3 mm long surface was etched on each sample for 5, 7, 9, and 11 minutes. Before each etching step, those surfaces that should remain unetched were covered with wax. Between each etched surface, regions of approximately 2 mm long remained unetched. After etching, the wax was removed with a hot water jet and the samples were ultrasonically cleaned in ethyl acetate for approximately 10 minutes. The surface of each sample was scanned twice with a profilometer (Form-Talysurf, Rank Taylor Hobson, Leicester, Great Britain) with a distance of approximately 1 mm (Fig. 2), and the results were printed out, vertically magnified 500 times (Fig. 3). Due to exact waxing, a 681
SCHiiFFER
WlFmN 88 400mA&m2 UOROWT A ETTME zw 7
6’
s,7,9,11
AND
PIFFER
min 7’
Fig. 3. Typical profilometric record.
60
Fig. 4. Mean etching depth and standard deviation in microns (wm).
distinctly marked separation between etched and unetched metal surfaces was visible. The distance of the compensation plane of the scanned profile to the unetched surfaces was then measured for each etching time. The results of the first and the second measurements were added separately for each etching time and the arithmetic mean and the standard deviation were calculated.
RESULTS The etching depths of the individual samples at different etching times (5, 7, 9, and 11 minutes) are shown in Table I. The average etching depths and the standard deviations of all samples are shown in Fig. 4. The evaluation of the etching depth per minute with an etching time of 5 minutes results in an average loss of substance of 6.08 Mm; with an etching time of 7 minutes the loss is 6.285 pm; with an etching time of 9 minutes the loss is 6.644 pm; and with an etching time of 11 minutes the loss is 6.627 I.cm.When adding up all etching depths per etching time, an average loss of substance of 6.409 pm per minute of etching time was recorded. 682
DISCUSSION The aim of electrolytic etching is to achieve a maximal microretentive surface morphology of the metal. The loss of substance occurring during the etching of the metal must not be overlooked. If the metal loss on the internal surface of the metal framework occurs evenly, a deterioration of the marginal adaptation of a three-unit resin-bonded prosthesis on the opposite approximal surfaces of the abutment teeth would be the result. The loss of substance at the vertical marginal surfaces of the metal wings could cause gaps on the orally located marginal regions. The importance of this loss as related to clinical usage depends on the extent of the metal loss during the etching. Various studies dealing with inlay, onlay, and crown techniques showed that the cement film thickness will not allow a complete seating of castings.14A wider marginal gap is found after cementation than at the precementation tryin.r5 The use of a die spacer results in a significant improvement.16 It creates a space of approximately 25 pm for the cement on the internal surface of the cast, with the exception of a 1 mm wide marginal surface. We can only assume that these facts can be applied to adhesive techniques, since no comparable studies exist. However, the DECEMBER
1990
VOLUME
64
NUMBER
6
ELECTROLYTICETCAINGDEPTHOFBASEALLOY
Table
Etching depth and standard deviation in microns (em)
I.
1st Measurement Test
sample
No.
1 2 3 4 5 6 7 8 9 10 Mean
etching
depth
Standard deviation
etching
time
2nd Measurement
etching
time
(min)
5
7
9
11
5
7
9
11
30 32 28 32 26 32 30
42 48 40 44 38 46 42
52 58 64 62 54 64 58
48 44 46
60 62 60
30 36 30 32 28 30 30 32 32 28
40 50 42 50 42 44 40 48 42 44
50 62 66 62 60 64 58 62 60 58
70 78 76 70 74 78 70
34 28 28
70 76 76 70 70 76 72 70 74 72
74 72 70
30
43.8 3.33
59.4 4.00
72.6 2.67
30.8 2.35
44.2 3.82
60.2 4.37
73.2 3.29
2.49
film thickness of most fixing resins is approximately 35 prn.17,la An earlier study reported an optimal microretentive surface morphology of Wiron 88 ahoy with an etching time of 7 minutes at 400 mA/cm2.1g The loss of substance with an etching time of 7 minutes is approximately 44 pm. This loss could lead to the conclusion that the metal loss during the etching process creates a space for the fixing cement similar to that of the die spacer technique in fixed prosthetic dentistry. This space could result in an improved seating of the etched metal parts. It is not certain whether the aim of etching a microretentive surface structure is to produce a shallow etching depth. Although the etching process seems to have negative effects on an originally good fit of the metal framework, the clinical cementing process could lead to an improvement of the marginal adaption. Should these considerations be confirmed by further studies, the etching depth of an optimal retentive surface morphology of the metal must be adjusted to the film thickness of the fixing resin.
CONCLUSIONS The electrolytic etching of a nickel-chromium base alloy (Wiron 88) at a constant current density of 400 mA/cm2 shows different etching depths depending on the etching time. The average etching depth with an etching time of 5 minutes is 30.4 Fm; with a 7-minute etching time, the depth is 44 pm; with a 9-minute etching time the depth is 59.8 km; and with an 11-minute etching time the depth is 72.9 km. The average loss of metal substance per minute of etching time is 6.409 pm.
3. Barrack
G. Recent
Dm
19&1;52:619-26.
1. Livaditis
GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 198&47:52-a. 2. Thompson VP, Castillo ED, Livaditis GJ. Resin-bonded retainers. Part I. Resin bond to electrolytically etched nonprecious alloys. J PROSTHET DENT 1983;50:771-9.
DENTISTRY
advances
in etched
cast restorations.
J PROSTHET
4. Winkler SH, Morris HF, Monteiro JM. Changes in mechanical properties and microstructure following heat treatment of a nickel-chromium base alloy. J PROSTHJXT DENT 1984;52:821-7. 5. Marx R, Dziadeck K. Wasserbestidigkeit des NiCr-PMMA-Klebeverbundes. Zahnarztl Welt Ref 1986;95:521-2. 6. Re GJ, Kaiser DA, Malone WFP, Garcia-Godoy F. Shear bond strengths and scanning electron microscope evaluation of three different retentive methods for resin-bonded retainers. J PROSTHZF DENT 1988;59:568-73. 7. Laufer B-Z, Nicholls JI, Townsend JD. SiO-C coating: a composite-tometal bonding mechanism. J PROSTWET DENT 1988,60:320-7. 8. Pegoraro LF, Barrack G. A comparison of bond strengths of adhesive cast restorations using different design, bonding agents, and luting resins. J PROSTHET DENT 1987;57:133-8. 9. Eshleman JR, Janus CE, Jones CR. Tooth preparation designs for resin-bonded fixed partial dentures related to enamel thickness. J PROSTHET DENT 1988;60:18-22. 10. Aker DA, Aker JR, Sorensen SE. Effect of methods of tooth enamel preparation on the retentive strength of acid-etch composite resins. J Am Dent Assoc 1979;99:185-9. 11. Olsen RA, Duke ESt, Norling BK. Enamel reduction and the bond strength of resin-bonded retainers. J PROSTHET DENT 1988;60:32-5. 12. Scbiiffer H, Ruech W, Dumfahrt H. Erfahrungen mit der S&ire-Ata-Technik in der Kronen-Brticken-Prothetik. Z Stomatol 1987;&1:433-9. 13. Creugers NHJ, Snoek PA, Vogel8 ALM. Overcontouring in resinbonded prostheses: plaque accumulation and gingival health. J PROSTIiET DENT 1988;59:17-21. 14. Eames WB, O’Neal SJ, Monteiro J, Miller C, Roan JD Jr, Cohen KS. Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-7. 15. Molvar MP, Gores M. Seating of cast gold inlays and onlays with and without margin bevels. Oper Dent 1988;13:138-43. 16. Antonaon DE, Fischlschweiger W. Scanning electron microscopy in clinical dental research. Dent J Fla 1983;54:15-8. 17. Forbes JF, Horn JS. Characterization of bonding composites for two types of metal retainers [Abstract]. J Dent Res 19&4;63:320. 18. Kullmann W. Die Filmdicken van Befestigungskunstostoffen. Zabnarztl Welt Ref 1985;94:943-53. 19. Schiiffer H. Das Klebeverbundsystem Wiron 8&SuperbondZahnschmelz, Dentin. Z Stomatol 1989;86:505-18. Reprint
REFERENCES
THEJOURNALOFPROSTHETIC
(min)
requests
to:
DR. HERBERT SCHAFER SCHOOL OF DENTISTRY UNIVERSITY OF INNSBRUCK ANICH~TRABE 35 A-6020 INNSBRUCK AUSTRIA
683
exception was found in specimens made with PTM-88 alloy, which was consistent with results from the previous studies.4 Since the PTM-88 alloy had a significantly different composition than the other Pd rich alloys, it is possible that the weaker bond strength was due to a less-adherent metal oxide. This explanation was supported by the surface roughness evaluation. The surface roughness of 0.94 for PTM-88 alloy specimens indicated that the debonded surface was smoother than the surface not exposed to ceramic. The rationale for this result was that the ceramic-metal failure occurred at the metal-oxide interface. It seems that the surface roughness ratio was a fairly accurate predictor of the ceramic-metal bond strength. The good correlation of the BRS and R&M values provided evidence that roughness measurements might be a reliable indicator of bond strengths. The surface roughness test method would be beneficial for discriminating between ceramic-metal bond strengths of alloys that are not well characterized or are without elastic modulus.
SUMMARY
ET AL
However, when the flexure strength loads were normalized by the modulus of elasticity to compensate for different stiffness of the alloys, only specimens of one Pd rich alloy, Option, had a significantly greater bond strength. This indicated the importance of knowing the relative stiffness of the different test alloys in order to achieve an accurate assessment of the bond strength that reflects the clinical situation. REFERENCES 1. Anusavice
KJ, DeHoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13. 2. Schaffer SP. An approach to determining the bond strength of ceramometal systems. J PROSTHET DENT 1982;48:282-4. 3. Van Vlack LH. Elements of materials science and engineering. 3rd ed. Reading, Mass: Addition-Wesley Pub1 Co, 1975;177-80. 4. Lorensana RE, Chambless LA, Marker VA, Staffanou RS. Ceramometal bond strengths of four palladium-rich alloys. [Abstract]. J Dent Res 1985;54:585. Reprint
requests
to:
DR. ROBERT S. STAFFANOU BAYLOR COLLEGE OF DENTISTRY 3302 GASTON AVE. DALLAS, TX 75246
The loads measured from the flexure bond strength test and the shear bond strength test produced similar results.
Evaluation of the electrolytic nickel-chromium base alloy restorations Herbert University Wattens,
SchBffer, of Innsbruck, Austria
Dr.Med.,* School
etching depth of a used in resin-bonded cast
and Anton Piffer**
of Dentistry,
Innsbruck,
Austria;
and
Swarovski
D. & Co.,
This study examined the relation of the electrolytic etching depth of a nickelchromium base alloy to the etching time. Cast samples (22 mm long, 4 mm wide, 1.6 mm thick) were etched in the Eltrokor etching unit, using Korolyt A as the etching solution with a constant current density of 400 mA/cm2. On each sample an approximately 3 mm long area was etched for 5,7,9, and 11 minutes. Between each etched area, a field of approximately 2 mm long remained unetched. The surface of each sample was scanned with a protllometer. The distance of the compensation plane of the scanned etched profile to the unetched areas was then measured for each etching time. The average etched depth with an etching time of 5 minutes is 30.4 pm (SD 2.39); with a time of 7 minutes the depth is 44 pm (SD 3.49); with a time of 9 minutes the depth is 59.8 gm (SD 4.10); and with an etching time of 11 minutes the depth is 72.9 gm (SD 2.94). Taking the average of all etching depths per etching time, the average loss of substance is 6.409 pm per minute etching time. (J PROSTHET DENT 1990;64:680-3.)
*Assistant Professor, Department of Restorative Dentistry, University of Innsbruck, School of Dentistry. **Physicist, Department of Research and Development, Swarovski D. & Co. 10/l/22012 680
A
fter 15 years of research and development, resinbonded fixed prosthesis are established as a treatment of partial edentulism. The components in the bonding system (metal, composite resin, and enamel) have been examined in various studies.le4 The crucial criteria for long-term sucDECEMBER
1990
VOLUME
64
NUMBER
6
ELECTROLYTIC
ETCHING
DEPTH
OF BASE ALLOY
Fig.
2. Stylus of profilometer at start of measuring pro-
cess.
etched in Korolyt A etchant for 7 minutes at a current density of 400 mA/cm2. (Original magnification ~500.)
Fig.
1. Wiron 88 alloy surface electrolytically
cess seem to result from the numerous variable treatment parameters. Mircomechanic retention by electrolytic etching of the metal surface seems to be the most successful of the various retention mechanisms. It was found that for each type of alloy the etching procedure must be made with a specific etching solution under special electrolytic conditions (Fig. 1). For the fixing process, specially modified self-curing composite resins were developed. These composite resins show a more favorable flow and an extended working time than filling composite resins. Apart from a michromechanic retention, an additional chemical bonding seems possible with certain metal-resin combinations.5 The same result can be achieved by a silicone-coated metal surface.6, 7 Exact preparation when obtaining a retention and a resistance form is seen as the crucial factor for longterm success.*The preparation of grooves, guiding surfaces, and rest seats must be planned accurately and executed as carefully as possible.g A fine preparation of the entire retention wing in the enamel increases the bonding with the fixing composite resin. lo, I1 The optimal solution is a complete integration of the metal framework into the tooth.12 A problematic overcontouring and a possible periodontal irritation can thus be avoided.i3 Furthermore, the exact fit of the metal framework is of special importance. The aim is a minimal marginal gap, as is required in the usual fixed restorations. An exact fit and a minimal marginal gap lead to the required composite resin film thickness, which should be as thin as possible. Thus the negative effects on the adhesive and cohesive strength of the bonding system, such as thermal dimensional alterations and swelling caused by water absorption of the composite resin, can be reduced. These requirements can only be achieved by the complete reproduction of all THE
JOURNAL
OF PROSTHETIC
DENTISTRY
preparation details in the metal framework, using the model casting technique. A final factor that influences the fit of the metal framework is the electrolytic etching of the internal surface of the metal before the cementation. This study examined the loss of substance of a nickelchromium base alloy during the etching procedure.
MATERIAL
AND
METHODS
Ten samples, 22 mm long, 4 mm wide, and 1.5 mm thick, made of Wiron 88 (Bremer Goldschliigerei, Bremen, West Germany) alloy were cast according to the manufacturer’s directions. The samples were polished using a grinding machine (Planopo12, Struers, Copenhagen, Denmark) and a 2400 mesh abrasive paper. In order to simulate the effects of temperature on the metal during the porcelain bake cycles, the samples were put in the ceramic oven four times under the following conditions: opaque bake 960’ C, vacuum, holding time 2 minutes; dentin bake 930” C, vacuum, holding time 2 minutes; correction bake 920” C, vacuum, holding time 2 minutes; and glaze bake 930“ C, holding time 2 minutes. Afterward the oxide film on the surface was removed by means of continous sand blasting with a 25 pm grit aluminium oxide at 1.5 bar. Electrolytic etching was done with Korolyt A etching solution (Bremer GoldSchliigerei) at a constant current density of 400 mA/cm2 in the Eltrokor etching unit (Bremer Goldschliigerei). A fresh solution was used for each sample. A 3 mm long surface was etched on each sample for 5, 7, 9, and 11 minutes. Before each etching step, those surfaces that should remain unetched were covered with wax. Between each etched surface, regions of approximately 2 mm long remained unetched. After etching, the wax was removed with a hot water jet and the samples were ultrasonically cleaned in ethyl acetate for approximately 10 minutes. The surface of each sample was scanned twice with a profilometer (Form-Talysurf, Rank Taylor Hobson, Leicester, Great Britain) with a distance of approximately 1 mm (Fig. 2), and the results were printed out, vertically magnified 500 times (Fig. 3). Due to exact waxing, a 681
SCHiiFFER
WlFmN 88 400mA&m2 UOROWT A ETTME zw 7
6’
s,7,9,11
AND
PIFFER
min 7’
Fig. 3. Typical profilometric record.
60
Fig. 4. Mean etching depth and standard deviation in microns (wm).
distinctly marked separation between etched and unetched metal surfaces was visible. The distance of the compensation plane of the scanned profile to the unetched surfaces was then measured for each etching time. The results of the first and the second measurements were added separately for each etching time and the arithmetic mean and the standard deviation were calculated.
RESULTS The etching depths of the individual samples at different etching times (5, 7, 9, and 11 minutes) are shown in Table I. The average etching depths and the standard deviations of all samples are shown in Fig. 4. The evaluation of the etching depth per minute with an etching time of 5 minutes results in an average loss of substance of 6.08 Mm; with an etching time of 7 minutes the loss is 6.285 pm; with an etching time of 9 minutes the loss is 6.644 pm; and with an etching time of 11 minutes the loss is 6.627 I.cm.When adding up all etching depths per etching time, an average loss of substance of 6.409 pm per minute of etching time was recorded. 682
DISCUSSION The aim of electrolytic etching is to achieve a maximal microretentive surface morphology of the metal. The loss of substance occurring during the etching of the metal must not be overlooked. If the metal loss on the internal surface of the metal framework occurs evenly, a deterioration of the marginal adaptation of a three-unit resin-bonded prosthesis on the opposite approximal surfaces of the abutment teeth would be the result. The loss of substance at the vertical marginal surfaces of the metal wings could cause gaps on the orally located marginal regions. The importance of this loss as related to clinical usage depends on the extent of the metal loss during the etching. Various studies dealing with inlay, onlay, and crown techniques showed that the cement film thickness will not allow a complete seating of castings.14A wider marginal gap is found after cementation than at the precementation tryin.r5 The use of a die spacer results in a significant improvement.16 It creates a space of approximately 25 pm for the cement on the internal surface of the cast, with the exception of a 1 mm wide marginal surface. We can only assume that these facts can be applied to adhesive techniques, since no comparable studies exist. However, the DECEMBER
1990
VOLUME
64
NUMBER
6
ELECTROLYTICETCAINGDEPTHOFBASEALLOY
Table
Etching depth and standard deviation in microns (em)
I.
1st Measurement Test
sample
No.
1 2 3 4 5 6 7 8 9 10 Mean
etching
depth
Standard deviation
etching
time
2nd Measurement
etching
time
(min)
5
7
9
11
5
7
9
11
30 32 28 32 26 32 30
42 48 40 44 38 46 42
52 58 64 62 54 64 58
48 44 46
60 62 60
30 36 30 32 28 30 30 32 32 28
40 50 42 50 42 44 40 48 42 44
50 62 66 62 60 64 58 62 60 58
70 78 76 70 74 78 70
34 28 28
70 76 76 70 70 76 72 70 74 72
74 72 70
30
43.8 3.33
59.4 4.00
72.6 2.67
30.8 2.35
44.2 3.82
60.2 4.37
73.2 3.29
2.49
film thickness of most fixing resins is approximately 35 prn.17,la An earlier study reported an optimal microretentive surface morphology of Wiron 88 ahoy with an etching time of 7 minutes at 400 mA/cm2.1g The loss of substance with an etching time of 7 minutes is approximately 44 pm. This loss could lead to the conclusion that the metal loss during the etching process creates a space for the fixing cement similar to that of the die spacer technique in fixed prosthetic dentistry. This space could result in an improved seating of the etched metal parts. It is not certain whether the aim of etching a microretentive surface structure is to produce a shallow etching depth. Although the etching process seems to have negative effects on an originally good fit of the metal framework, the clinical cementing process could lead to an improvement of the marginal adaption. Should these considerations be confirmed by further studies, the etching depth of an optimal retentive surface morphology of the metal must be adjusted to the film thickness of the fixing resin.
CONCLUSIONS The electrolytic etching of a nickel-chromium base alloy (Wiron 88) at a constant current density of 400 mA/cm2 shows different etching depths depending on the etching time. The average etching depth with an etching time of 5 minutes is 30.4 Fm; with a 7-minute etching time, the depth is 44 pm; with a 9-minute etching time the depth is 59.8 km; and with an 11-minute etching time the depth is 72.9 km. The average loss of metal substance per minute of etching time is 6.409 pm.
3. Barrack
G. Recent
Dm
19&1;52:619-26.
1. Livaditis
GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 198&47:52-a. 2. Thompson VP, Castillo ED, Livaditis GJ. Resin-bonded retainers. Part I. Resin bond to electrolytically etched nonprecious alloys. J PROSTHET DENT 1983;50:771-9.
DENTISTRY
advances
in etched
cast restorations.
J PROSTHET
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