- Email: [email protected]
Altered corrosion resistance from casting to stainless steel posts
Altered posts
corrosion
resistance
from casting
John A. Sorensen, D.M.D.,* Michael J. Engelman, D.D.S.,** D.D.S.,*** and Angelo A. Caputo, Ph.D.**** University of California, School of Dentistry, Los Angeles,Calif.
to stainless
steel
Tony Daher,
Heat treatment during the burnout procedure may cause corrosion of the stainless steel post. This study was undertaken to determine whether alteration of the corrosion resistance of stainless steel posts occurs as a result of various simulated burnout procedures. Stainless steel posts were divided into five groups of five posts: group 1, control; 2, gypsum-bonded investment, bench cooled; 3, gypsum-bonded investment, quenched; 4, phosphate-bonded investment, bench cooled; and 5, phosphate-bonded investment, quenched. The posts were placed in individual containers of Ringer’s solution and observed at 30, 180, and 600 days. Scanning electron microscopy, energy dispersive x-ray spectroscopy, optical emission spectroscopy, and optical microscopy were used to evaluate the posts qualitatively and quantitatively. Investment and heat treatment altered the metallic structure of stainless steel posts. Prefabricated posts submitted to simulated burnout procedures had a noticeable reduction in corrosion resistance. Direct casting to stainless steel posts is contraindicated.(J PROSTHETDENT 1990;63:630-7.)
W hen minimal
coronal tooth structure remains in a tooth that has received endodontic treatment, a post and core is often used to provide adequate retention and resistance form for a crown. Casting a core to a prefabricated post of dissimilar metal for this procedure has been advocated.lM5One technique involves casting a core directly to a prefabricated stainless steel post.5s Clinically, casting of a core to a prefabricated post offers four advantages, (1) the stainless steel post has high strength and superior mechanical properties, g-11 (2) the prefabricated post device provides a more accurate reproduction of the canal because a plastic pattern may distort or bend;lO* l1 (3) less time is required to fit and seat the post and core because only the expansion of the core is a consideration;5 and (4) the possibility of casting inclusions in the post is eliminated. There is concern regarding the alteration of corrosion resistance of stainless steel posts from heat treatment during the burnout procedure.5y 11*l2 Several authors have implicated the corrosion products of such heat-treated posts as a cause of root fracture.13-l6 This study evaluated the alteration of corrosion resistance of stainless steel posts resulting from various simulated burnout procedures.
Presented before the Pacific Coast Society of Prosthodontists meeting, Napa, Calif. This study was partially supported by Whaledent International, New York, N.Y. *Assistant Professor,Director, Graduate Prosthodontics. **Assistant Professor,Section of Removable Prosthodontics. ***Lecturer, ****Professor,
10/l/18541
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Section of Removable Chairman, Section
Prosthodontics. of Biomaterials.
MATERIAL
AND
METHODS
Stainless steel parallel-sided serrated posts (Para-post System, Whaledent International, New York, N.Y.) were divided into five groups of five posts. Group 1 was untreated and acted as the control stainless steel post. Groups 2 through 5 were invested, heat treated, and cooled according to the guidelines in Table I. In each of the treatment groups, five posts were sprued in a circle to the sprue-former. The posts were positioned so that an equal amount of investment surrounded each post. Groups 2 and 3 were invested with a gypsum-bonded investment (Beauty Cast, Whip Mix Corp., Louisville, KY.). The manufacturer’s recommendations were followed for water/powder ratios, burnout times, and temperatures. The burnout temperature was programmed for a slow high rise to 600” C and held for 30 minutes. Group 2 was removed and allowed to air cool. Group 3 was removed, air cooled for 1 minute, and then quenched in running water. Groups 4 and 5 were invested with a phosphate-bonded investment (Ceramigold 2, Whip Mix Corp.). The manufacturer’s recommendations were followed for water/powder ratio, burnout times, and temperatures. The burnout temperature was programmed to 300’ C for 30 minutes followed by a 30-minute rise to 700’ C and held to that temperature for 30 minutes. Group 4 was removed from the oven and air cooled. Group 5 was allowed to air cool for 1 minute, then quenched in running water. The posts in groups 2 through 5 were divested and sandblasted with aluminum oxide under 2.8 kg/cm2 pressure for 1 minute per post. All investment residues and dark colored spots were abraded away until a uniform metallic color was present. The five posts from each group were immersed separately in 10 ml of Ringer’s solution for 30 days. The samples were ranked in relative order of quantity of JUNE
1990
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Fig. 1. Experimental groups of stainless steel posts in Ringer’s solution after 180 days.
Table I. Experimental treatment of stainless steel posts
Group
1 2 3 4 5
Burnout
technique
Min
“C
Investment
None Gypsum-bonded Gypsum-bonded Phosphate-bonded
Cooling technique
None
30 30 30 30 Phosphate-bonded 30 30
None
600 600 300 700 300 700
Air cool Quench Air cool Quench
Table II. Optical emission spectroscopic results of untreated stainless steel Parapost Element
Content
(% ) *
C
* * *
Mn P S Si Cr Ni MO
0.27 17.5 11.4
0.50 Remainder
Fe *Less than 0.05%.
precipitate. Samples were reevaluated at 180 days to monitor corrosion progress. After 600 days, the surface of one of the remaining posts from each group was examined with a scanning electron microscope @EM) at x500 magnification @R/50, International Scientific Instruments, Inc., Milpitas, Calif.). Energy dispersive x-ray spectroscopy (EDS) (Kevex Delta Class IV with Quantum Detector, Foster City, Calif.) was used to determine the elemental constituents of the surface to a depth of 1 micron. The sample was exposed to a high-energy electron beam and gave off X rays characTHE
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1
2 3 4 5
Y
--
--
-
Fig. 2. Stainless steel posts after 180 days’ immersion in Ringer’s solution.
teristic of the element of origin. This was determined by comparing the energy emitted with known x-ray emissions for each element. Optical emission spectroscopy (OES) was used to determine the constituents of the control post following the American Society for Testing and Materials standard method for optical emission spectrometric analysis of stainless type 18-8 steels by the point-to-plane technique.17 An electrical arc was produced between the sample and a graphite electrode. The radiant energy emitted was characteristic of the element of origin. A portion of one post of each of the five groups was embedded in epoxy and treated according to the standard practice for detecting susceptibility to intergranular attack in austenitic stainless steels. ls Practice A oxalic acid test for classification of etch structures for austenitic stainless steels was followed. The embedded posts were polished with 120 gumgrind, 15 pm grind, 6 Mm coarse polish, and 0.05 pm alumina final polish. All samples were electrolytically etched with 10% oxalic acid at room temperature for 1.5 631
SORENSENETAL
3. SEM photomicrograph of stainless post surfaces after 600 days’ immersion in Ringer’s solution (original magnification x500). A, Control. B, Gypsum-bonded investment, air-cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
Fig.
minutes at 1 A/cm?+.The posts were examined and photographed at ~700 magnification with an optical microscope (OM) (Leitz Orthoplan, E. Leitz, Inc., Rockleigh, N.J.). The samples were then gold sputter-coated to a 20 nm thickness to provide a conductive surface for SEM examination. The resulting photographs were made at ~700 magnification and compared.
632
RESULTS During devesting procedures with aluminum oxide abrasion, the investment residue of group 3 was most difficult to clean. The condition of the stainless steel posts, inspected at 30 and 180 days, confirmed that surface alteration had occurred. After 180 days, examination of the five sample bottles
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Fig. 4. Energy dispersive x-ray spectrograms of stainless steel post surfaces after 600 days’ immersion in Ringer’s solution. A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
showed obvious corrosion products in samples 2 through 5 (Fig. 1). The control group 1 had no precipitate. More precipitate was present in gypsum-bonded investment samples (groups 2 and 3). The posts in samples 2 through 5 showed surface alteration and corrosion products (Fig. 2). On metallographic examination at X40 magnification, the control group showed no surface changes. Gypsum-bonded posts (groups 2 and 3) showed the greatest surface corro-
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sion. Phosphate-bonded posts (groups 4 and 5) showed moderate surface corrosion. After 600 days a metallic film coated the inside of the glass containers of groups 2 and 3. Surface examination of the five samples with SEM at X500 revealed no apparent degradation of the control group, but groups 2 through 5 exhibited irregular corroded surfaces (Fig. 3). Energy dispersive x-ray spectroscopy (EDS) of the five
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Fig. 6. Optical photomicrographs of stainless steel post cross sections after 600 days’ immersion in Ringer’s solution and treatment with ASTM practice A-oxalic acid test (original magnification ~‘700). A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched. groups was similar and showed the major constituents of the surface to be iron, chromium, and nickel (Fig. 4). Impurities were also noted, including silicon, sodium, aluminum, phosphorus, calcium, and oxygen. All impurities were not present on all samples but no significant differences were found in samples 2 through 5 (Fig. 4, B through E). Group RRA
1 had only chlorine and silicon as impurities (Fig. 4, A). Optical emission spectroscopy (OES) identified iron (70%), nickel (17.5%), and chromium (11.4%) as the major elements contained in the control sample (Table II). Molybdenum, silicon, sulfur, carbon, phosphorus, and manganese were present in small amounts.
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Fig. 6. SEM micrographs of stainless steel post cross sections after 600 days’ immersion in Ringer’s solution and treatment with ASTM practice A oxalic acid test (original magnification x700). A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsumbonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
Optical microscopic (OM) analysis of group 1 after treatment with oxalic acid showed a uniform austenitic microstructure (Fig. 5, A). Groups 2 through 5 exhibited precipitation of carbides (Fig. 5, B through E). Examination of groups 2 and 4 revealed an austenitic microstructure with dark areas or “ditches” where precipitated carbides
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werelocated (Fig. 5, B and D). Groups 3 and 5 had a combined austenitic and martensitic microstructure with dark areas representing precipitated carbides (Fig. 5, C and E). SEM analyses of the groups were similar to the optical analysis. Group 1 had few carbides in an austenitic microstructure (Fig. 6, A). Groups 2 and 4 showed grain bound-
635
ary ditching from carbide precipitation in austenitic stainless steel (Fig. 6, B and D). Groups 3 and 5 revealed a combined austenitic and martensitic microstructure with more carbide formation than groups 2 and 4 (Fig. 6, C and E).
DISCUSSION According to the manufacturer, the prefabricated Paraposts posts are essentially an 18-8 stainless steel. This class of stainless steel is an alloy of iron and carbon that contains 18 % chromium and 8 % nickel.g Optical emission spectroscopy (OES) showed the control sample consisted of 17.5% chromium and 11.4% nickel, which is within the established range of 18-8 stainless steel alloys.17 The untreated posts revealed no change whereas the heat-treated posts showed obvious corrosion when examined without magnification and with the SEM. Considering the characteristics of stainless steel, altered corrosion resistance of the posta was predictable. It has been demonstrated that heating 18-8 stainless steel between 400’ to 900’ C may negate its resistance to corrosion.g* lge21This corrosive tendency was attributed to the precipitation of chromium carbide at the grain boundaries at elevated temperatures.“* 21For the present study, the standard test for detecting susceptibility to intergranular attack in austenitic stainless steels revealed precipitation of carbides in groups 2 through 5, the same groups with extensive corrosion. The presence of chromium at the boundaries indicated a decreased chromium grain content and reduced corrosion resistance. Greater carbide precipitation was evident in the quenched samples (groups 3 and 5) (Fig. 6, C and E) compared with the air-cooled samples (groups 2 and 4) (Fig. 6, B and D). Varying the cooling method resulted in different amounts of carbide precipitation. The high temperatures and cooling of the stainless steel samples also resulted in a change of microstructure. The control (group 1) and air-cooled samples (groups 2 and 4) revealed only austenitic microstructure. The quenched groups (groups 3 and 5) showed a combined austenitic and martensitic microstructure. The martensitic structure is less resistant to corrosion,21 which may contribute to the difference in corrosion resistance. Corrosion of stainless steel after heat treatment has been confirmed by research on orthodontic bands. Maijer and Smith22 showed that the corrosion of orthodontic bands is associated with surfaces that have been subjected to the high temperatures of welding or soldering. Bu~hman~~ evaluated the effects of heating and recycling of previously used orthodontic bands. Heating bands to temperatures greater than 450’ C changed the metallic structure of the bands, rendering them susceptible to metallic intergranular corrosion. Corrosion of post and core materials has been shown before. EspevikU demonstrated that base metal alloys deteriorated when placed in artificial saliva. Pameijer et al.% tested stainless steel, gold-plated stainless steel, and cast gold pins in vitro and in vivo. An analysis of the constitu636
ents of the pins revealed a change in composition of all of the pins. Derand% investigated screw posts composed primarily of zinc and copper in vivo. A 10 % incidence of post corrosion was found, with zinc and copper ions detected in the saline solution. Several retrospective studies of fractured teeth implicated corrosion products as a cause of root fracture.13-16 Conclusive evidence establishing an unequivocal relationship between corrosion products and root fracture is not available. The migration of metallic ions from the post into dentin and surrounding tissues is another important consideration of corrosion products.16* 27-29Studies have observed discoloration of tooth structure restored with metal posts. Arvidson and Wroblewski29 observed the presence of components of the posts in dark gingival discolorations adjacent to restorations. The health and esthetic impact of corrosion product ion movement has yet to be delineated. Further study is needed to determine whether various luting agents cause surface degradation of prefabricated posts. Additional research should also evaluate the interaction of dissimilar metals and their effect on corrosion resistance.
CONCLUSIONS 1. Investment and heat treatment alters the metallic structure of stainless steel posts. 2. Heat treatment in gypsum- or phosphate-bonded investments negatively alters the corrosion resistance of stainless steel posts. 3. Direct casting to stainless steel posts is contraindicated. We thank PhotoMetrics Micrcanalytical Laboratories, Huntington Beach, Calif. for technical assistance. REFERENCES 1. Harty FJ, Leggett LJ. A post crown technique using a nickel-cobaltchromium post. Br Dent J 1972;132:394-9. 2. Sheets DE. Dowel and core foundations. J PROSTHFX DENT 1970;23:5865. 3. Christy JM, Pipko DJ. Fabrication of a dual-post veneer crown. J Am Dent Assoc 1967;75:1419-25. 4. Youdelis R, Morrison K. Full coverage restoration of puipless anterior and bicuspid teeth. J Can Dent Assoc 1966;32:516-21. 5. Miller AW. Post and core systems: which one is best? J PROSTHET DENT 1982;4&27-38. 6. Zarb GA, Bergman B, Ciayton JA, MacKay HF. Preparing patients for the fixed or removable prosthdontic service. In: Prosthodontic treatment for partially edentuious patients. St Louis: CV Mosby Co, 1978136-7. 7. Pipko DJ, Hadeed GJ. Fabrication of drop-cast aluminum cores. J PROSI’BET DENT 1985;53:501-4. 8. Spector MR. A cast core system with interlocking posts. J PROSTHET DENT 1986,65:16-g. 9. Craig RG. Cast and wrought base metal alloys. In: Restorative dental matmiale. 7th ed. St Louis: CV Mosby Co, 1985;384-410. 10. Charlton G. A prefabricated post and core for porcelain jacket crowns. Br Dent J 1985;119:452-6. 11. Stokes AN. Post crowns: a review. Int J Endodont 1987;20:1-7. 12. Caputo AA, Standlee JP. Restoration of endodonticaiiy treated teeth.
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13. 14. 15. 16.
17.
18.
19. 20.
21.
CORROSION
RESISTANCE
22. Maijer R, Smith DC. Corrosion of orthodontic bracket bases. Am J Orthod 1982;81:43-8. 23. Buchman D. Effects of recycling in metallic direct-bond orthodontic brackets. Am J Orthod 1980;77:654-88. 24. Espevik S. Corrosion of base metal alloys in vitro. Acta Odontol Stand 1978;36:113-6. 25. Pameijer CH, Giants P, Mobasherat MA. On clinical corrosion of pins. Swed Dent J 1983;7:161-7. 26. Derand T. Corrosion of screwposts. Odont Revy 1971;22:371-8. 27. Soremark R, Ingela 0, Plett H, Samsabl LK. Inthmnce of some dental restorations on the concentrations of inorganic constituents of the teeth. Acta Odontol &and 1962;20:215-24. 28. Bergenholtz A, Hedegard B, Soremark R. Studies of the transport of metal ions from gold inlays into environmental tissues. Acta Odontol &and 1965;23:135-46. 29. Arvidson K, Wroblewski R. Migration of metallic ions from screwposts into dentin and surrounding tissues. Stand J Dent Ras 1978;86:209-5.
In: Biomechanics in clinical dentistry. Chicago: Quintessence Pub1 Co Inc, 1987;185-203. Angmar-Mansson B, Omnell K, Rud J. Root fractures due to corrosion: 1. metallurgical aspects. Odontologisk Revy 1969;20:245-65. Petersen KB. Longitudinal root fracture due to corrosion of an endodontic post. J Can Dent Assn 19’71;2:66-8. Rud J, Omnell K. Root fractures due to corrosion: diagnostic aspects. Stand J Dent Res 1970;78:397-403. Silness J, Gustavsen F, Hunsbeth J. Distribution of corrosion products in teeth restored with metal crowns retained by stainless steel posts. Acta Odontol Stand 1979;37:317-21. American Society for Testing and Materials. Standard method for optical emission spectrometric analysis of stainless type 18-8 steels by the point-to-plane technique (Designation: E327-68). In: Priemonn-Storer RA, ed. Annual book of ASTM standards. Easton, Md: ASTM, 1985;3.06:150-3. American Society for Tasting and Materials. Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels (Designation A262-84). In: Priemon-Storer RA, ed. Annual book of ASTM standards. Easton, Md: ASTM, 1985;1.05:81-98. Brick RM, Gordon RB, Arthur A. Stainless steels. In: Structure and properties of alloys, New York: McGraw-Hill Book Co, lQ65;339-58. Lyman T. Metals handbook: atlas of microstructure of industrial alloys. 8th ad. Mehl RF, ed. Metals Park, Ohio: American Society for Metals, 1972;7:82. Phillips RW. Wrought base metal alloys. In: Skinner’s science of dental materials. 7th ed. Philadelphia: WB Saunders Co, 1983;641-56.
Using
a tooth-reduction
Reprint requests to: DR. JOHNA.SORENSEN CHS 33-041 SCH~~LOFDENTISTRY UNIVERSITyOFCALIFORNJs4 LosANGELEs.CA~OOZ~
guide for modifying
L. Kirk Gardner, D.D.S.,* Arthur 0. Rahn, D.D.S.,** Gregory R. Parr, D.D.S.,** and David W. Richardson, Medical
College
of Georgia,
School
of Dentistry,
Augusta,
natural
teeth
D.D.S.***
Ga.
A simplified method of transferring diagnostic odontoplastic information from the cast to the patient is described. This technique can be used successfully when treating fixed, removable, or combination prosthodontics patients. (J F’ROSTIIET DENT
1990;63:637-9.)
T
ransferring tooth modifications accurately from altered diagnostic casts to the mouth can be difficult and tedious. A technique for fabrication of a treatment template is described.
PROCEDURE 1. Perfect the occlusal plane on diagnostic casts by using a template or any other desired method with a sharp knife, stones, or burs (Fig. 1).lm4 2. Paint the cast with a tinfoil substitute and allow it to dry. 3. Adapt autopolymerizing acrylic resin (dough stage) to the facial surface of the modified teeth on the cast and cut to the occlusal height of the altered teeth.
*Associate **Professor, ***Assistant
Professor, Department of Prosthodontics. Department of Prosthodontics. Professor, Department of Prosthodontics.
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Fig. 1. Occlusal plane adjusted by use of 20-degree template. 4. Lubricate acrylic resin surfaces in the modified locations with petroleum jelly and apply a second mix of acrylic resin to make the lingual portion of the guide. 5. After the acrylic resin is cured, remove the guide in two pieces and smooth any rough surfaces (Figs. 2 and 3). 637
corrosion
resistance
from casting
John A. Sorensen, D.M.D.,* Michael J. Engelman, D.D.S.,** D.D.S.,*** and Angelo A. Caputo, Ph.D.**** University of California, School of Dentistry, Los Angeles,Calif.
to stainless
steel
Tony Daher,
Heat treatment during the burnout procedure may cause corrosion of the stainless steel post. This study was undertaken to determine whether alteration of the corrosion resistance of stainless steel posts occurs as a result of various simulated burnout procedures. Stainless steel posts were divided into five groups of five posts: group 1, control; 2, gypsum-bonded investment, bench cooled; 3, gypsum-bonded investment, quenched; 4, phosphate-bonded investment, bench cooled; and 5, phosphate-bonded investment, quenched. The posts were placed in individual containers of Ringer’s solution and observed at 30, 180, and 600 days. Scanning electron microscopy, energy dispersive x-ray spectroscopy, optical emission spectroscopy, and optical microscopy were used to evaluate the posts qualitatively and quantitatively. Investment and heat treatment altered the metallic structure of stainless steel posts. Prefabricated posts submitted to simulated burnout procedures had a noticeable reduction in corrosion resistance. Direct casting to stainless steel posts is contraindicated.(J PROSTHETDENT 1990;63:630-7.)
W hen minimal
coronal tooth structure remains in a tooth that has received endodontic treatment, a post and core is often used to provide adequate retention and resistance form for a crown. Casting a core to a prefabricated post of dissimilar metal for this procedure has been advocated.lM5One technique involves casting a core directly to a prefabricated stainless steel post.5s Clinically, casting of a core to a prefabricated post offers four advantages, (1) the stainless steel post has high strength and superior mechanical properties, g-11 (2) the prefabricated post device provides a more accurate reproduction of the canal because a plastic pattern may distort or bend;lO* l1 (3) less time is required to fit and seat the post and core because only the expansion of the core is a consideration;5 and (4) the possibility of casting inclusions in the post is eliminated. There is concern regarding the alteration of corrosion resistance of stainless steel posts from heat treatment during the burnout procedure.5y 11*l2 Several authors have implicated the corrosion products of such heat-treated posts as a cause of root fracture.13-l6 This study evaluated the alteration of corrosion resistance of stainless steel posts resulting from various simulated burnout procedures.
Presented before the Pacific Coast Society of Prosthodontists meeting, Napa, Calif. This study was partially supported by Whaledent International, New York, N.Y. *Assistant Professor,Director, Graduate Prosthodontics. **Assistant Professor,Section of Removable Prosthodontics. ***Lecturer, ****Professor,
10/l/18541
630
Section of Removable Chairman, Section
Prosthodontics. of Biomaterials.
MATERIAL
AND
METHODS
Stainless steel parallel-sided serrated posts (Para-post System, Whaledent International, New York, N.Y.) were divided into five groups of five posts. Group 1 was untreated and acted as the control stainless steel post. Groups 2 through 5 were invested, heat treated, and cooled according to the guidelines in Table I. In each of the treatment groups, five posts were sprued in a circle to the sprue-former. The posts were positioned so that an equal amount of investment surrounded each post. Groups 2 and 3 were invested with a gypsum-bonded investment (Beauty Cast, Whip Mix Corp., Louisville, KY.). The manufacturer’s recommendations were followed for water/powder ratios, burnout times, and temperatures. The burnout temperature was programmed for a slow high rise to 600” C and held for 30 minutes. Group 2 was removed and allowed to air cool. Group 3 was removed, air cooled for 1 minute, and then quenched in running water. Groups 4 and 5 were invested with a phosphate-bonded investment (Ceramigold 2, Whip Mix Corp.). The manufacturer’s recommendations were followed for water/powder ratio, burnout times, and temperatures. The burnout temperature was programmed to 300’ C for 30 minutes followed by a 30-minute rise to 700’ C and held to that temperature for 30 minutes. Group 4 was removed from the oven and air cooled. Group 5 was allowed to air cool for 1 minute, then quenched in running water. The posts in groups 2 through 5 were divested and sandblasted with aluminum oxide under 2.8 kg/cm2 pressure for 1 minute per post. All investment residues and dark colored spots were abraded away until a uniform metallic color was present. The five posts from each group were immersed separately in 10 ml of Ringer’s solution for 30 days. The samples were ranked in relative order of quantity of JUNE
1990
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Fig. 1. Experimental groups of stainless steel posts in Ringer’s solution after 180 days.
Table I. Experimental treatment of stainless steel posts
Group
1 2 3 4 5
Burnout
technique
Min
“C
Investment
None Gypsum-bonded Gypsum-bonded Phosphate-bonded
Cooling technique
None
30 30 30 30 Phosphate-bonded 30 30
None
600 600 300 700 300 700
Air cool Quench Air cool Quench
Table II. Optical emission spectroscopic results of untreated stainless steel Parapost Element
Content
(% ) *
C
* * *
Mn P S Si Cr Ni MO
0.27 17.5 11.4
0.50 Remainder
Fe *Less than 0.05%.
precipitate. Samples were reevaluated at 180 days to monitor corrosion progress. After 600 days, the surface of one of the remaining posts from each group was examined with a scanning electron microscope @EM) at x500 magnification @R/50, International Scientific Instruments, Inc., Milpitas, Calif.). Energy dispersive x-ray spectroscopy (EDS) (Kevex Delta Class IV with Quantum Detector, Foster City, Calif.) was used to determine the elemental constituents of the surface to a depth of 1 micron. The sample was exposed to a high-energy electron beam and gave off X rays characTHE
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1
2 3 4 5
Y
--
--
-
Fig. 2. Stainless steel posts after 180 days’ immersion in Ringer’s solution.
teristic of the element of origin. This was determined by comparing the energy emitted with known x-ray emissions for each element. Optical emission spectroscopy (OES) was used to determine the constituents of the control post following the American Society for Testing and Materials standard method for optical emission spectrometric analysis of stainless type 18-8 steels by the point-to-plane technique.17 An electrical arc was produced between the sample and a graphite electrode. The radiant energy emitted was characteristic of the element of origin. A portion of one post of each of the five groups was embedded in epoxy and treated according to the standard practice for detecting susceptibility to intergranular attack in austenitic stainless steels. ls Practice A oxalic acid test for classification of etch structures for austenitic stainless steels was followed. The embedded posts were polished with 120 gumgrind, 15 pm grind, 6 Mm coarse polish, and 0.05 pm alumina final polish. All samples were electrolytically etched with 10% oxalic acid at room temperature for 1.5 631
SORENSENETAL
3. SEM photomicrograph of stainless post surfaces after 600 days’ immersion in Ringer’s solution (original magnification x500). A, Control. B, Gypsum-bonded investment, air-cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
Fig.
minutes at 1 A/cm?+.The posts were examined and photographed at ~700 magnification with an optical microscope (OM) (Leitz Orthoplan, E. Leitz, Inc., Rockleigh, N.J.). The samples were then gold sputter-coated to a 20 nm thickness to provide a conductive surface for SEM examination. The resulting photographs were made at ~700 magnification and compared.
632
RESULTS During devesting procedures with aluminum oxide abrasion, the investment residue of group 3 was most difficult to clean. The condition of the stainless steel posts, inspected at 30 and 180 days, confirmed that surface alteration had occurred. After 180 days, examination of the five sample bottles
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Fig. 4. Energy dispersive x-ray spectrograms of stainless steel post surfaces after 600 days’ immersion in Ringer’s solution. A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
showed obvious corrosion products in samples 2 through 5 (Fig. 1). The control group 1 had no precipitate. More precipitate was present in gypsum-bonded investment samples (groups 2 and 3). The posts in samples 2 through 5 showed surface alteration and corrosion products (Fig. 2). On metallographic examination at X40 magnification, the control group showed no surface changes. Gypsum-bonded posts (groups 2 and 3) showed the greatest surface corro-
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sion. Phosphate-bonded posts (groups 4 and 5) showed moderate surface corrosion. After 600 days a metallic film coated the inside of the glass containers of groups 2 and 3. Surface examination of the five samples with SEM at X500 revealed no apparent degradation of the control group, but groups 2 through 5 exhibited irregular corroded surfaces (Fig. 3). Energy dispersive x-ray spectroscopy (EDS) of the five
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Fig. 6. Optical photomicrographs of stainless steel post cross sections after 600 days’ immersion in Ringer’s solution and treatment with ASTM practice A-oxalic acid test (original magnification ~‘700). A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsum-bonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched. groups was similar and showed the major constituents of the surface to be iron, chromium, and nickel (Fig. 4). Impurities were also noted, including silicon, sodium, aluminum, phosphorus, calcium, and oxygen. All impurities were not present on all samples but no significant differences were found in samples 2 through 5 (Fig. 4, B through E). Group RRA
1 had only chlorine and silicon as impurities (Fig. 4, A). Optical emission spectroscopy (OES) identified iron (70%), nickel (17.5%), and chromium (11.4%) as the major elements contained in the control sample (Table II). Molybdenum, silicon, sulfur, carbon, phosphorus, and manganese were present in small amounts.
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Fig. 6. SEM micrographs of stainless steel post cross sections after 600 days’ immersion in Ringer’s solution and treatment with ASTM practice A oxalic acid test (original magnification x700). A, Control. B, Gypsum-bonded investment, air cooled. C, Gypsumbonded investment, quenched. D, Phosphate-bonded investment, air cooled. E, Phosphate-bonded investment, quenched.
Optical microscopic (OM) analysis of group 1 after treatment with oxalic acid showed a uniform austenitic microstructure (Fig. 5, A). Groups 2 through 5 exhibited precipitation of carbides (Fig. 5, B through E). Examination of groups 2 and 4 revealed an austenitic microstructure with dark areas or “ditches” where precipitated carbides
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werelocated (Fig. 5, B and D). Groups 3 and 5 had a combined austenitic and martensitic microstructure with dark areas representing precipitated carbides (Fig. 5, C and E). SEM analyses of the groups were similar to the optical analysis. Group 1 had few carbides in an austenitic microstructure (Fig. 6, A). Groups 2 and 4 showed grain bound-
635
ary ditching from carbide precipitation in austenitic stainless steel (Fig. 6, B and D). Groups 3 and 5 revealed a combined austenitic and martensitic microstructure with more carbide formation than groups 2 and 4 (Fig. 6, C and E).
DISCUSSION According to the manufacturer, the prefabricated Paraposts posts are essentially an 18-8 stainless steel. This class of stainless steel is an alloy of iron and carbon that contains 18 % chromium and 8 % nickel.g Optical emission spectroscopy (OES) showed the control sample consisted of 17.5% chromium and 11.4% nickel, which is within the established range of 18-8 stainless steel alloys.17 The untreated posts revealed no change whereas the heat-treated posts showed obvious corrosion when examined without magnification and with the SEM. Considering the characteristics of stainless steel, altered corrosion resistance of the posta was predictable. It has been demonstrated that heating 18-8 stainless steel between 400’ to 900’ C may negate its resistance to corrosion.g* lge21This corrosive tendency was attributed to the precipitation of chromium carbide at the grain boundaries at elevated temperatures.“* 21For the present study, the standard test for detecting susceptibility to intergranular attack in austenitic stainless steels revealed precipitation of carbides in groups 2 through 5, the same groups with extensive corrosion. The presence of chromium at the boundaries indicated a decreased chromium grain content and reduced corrosion resistance. Greater carbide precipitation was evident in the quenched samples (groups 3 and 5) (Fig. 6, C and E) compared with the air-cooled samples (groups 2 and 4) (Fig. 6, B and D). Varying the cooling method resulted in different amounts of carbide precipitation. The high temperatures and cooling of the stainless steel samples also resulted in a change of microstructure. The control (group 1) and air-cooled samples (groups 2 and 4) revealed only austenitic microstructure. The quenched groups (groups 3 and 5) showed a combined austenitic and martensitic microstructure. The martensitic structure is less resistant to corrosion,21 which may contribute to the difference in corrosion resistance. Corrosion of stainless steel after heat treatment has been confirmed by research on orthodontic bands. Maijer and Smith22 showed that the corrosion of orthodontic bands is associated with surfaces that have been subjected to the high temperatures of welding or soldering. Bu~hman~~ evaluated the effects of heating and recycling of previously used orthodontic bands. Heating bands to temperatures greater than 450’ C changed the metallic structure of the bands, rendering them susceptible to metallic intergranular corrosion. Corrosion of post and core materials has been shown before. EspevikU demonstrated that base metal alloys deteriorated when placed in artificial saliva. Pameijer et al.% tested stainless steel, gold-plated stainless steel, and cast gold pins in vitro and in vivo. An analysis of the constitu636
ents of the pins revealed a change in composition of all of the pins. Derand% investigated screw posts composed primarily of zinc and copper in vivo. A 10 % incidence of post corrosion was found, with zinc and copper ions detected in the saline solution. Several retrospective studies of fractured teeth implicated corrosion products as a cause of root fracture.13-16 Conclusive evidence establishing an unequivocal relationship between corrosion products and root fracture is not available. The migration of metallic ions from the post into dentin and surrounding tissues is another important consideration of corrosion products.16* 27-29Studies have observed discoloration of tooth structure restored with metal posts. Arvidson and Wroblewski29 observed the presence of components of the posts in dark gingival discolorations adjacent to restorations. The health and esthetic impact of corrosion product ion movement has yet to be delineated. Further study is needed to determine whether various luting agents cause surface degradation of prefabricated posts. Additional research should also evaluate the interaction of dissimilar metals and their effect on corrosion resistance.
CONCLUSIONS 1. Investment and heat treatment alters the metallic structure of stainless steel posts. 2. Heat treatment in gypsum- or phosphate-bonded investments negatively alters the corrosion resistance of stainless steel posts. 3. Direct casting to stainless steel posts is contraindicated. We thank PhotoMetrics Micrcanalytical Laboratories, Huntington Beach, Calif. for technical assistance. REFERENCES 1. Harty FJ, Leggett LJ. A post crown technique using a nickel-cobaltchromium post. Br Dent J 1972;132:394-9. 2. Sheets DE. Dowel and core foundations. J PROSTHFX DENT 1970;23:5865. 3. Christy JM, Pipko DJ. Fabrication of a dual-post veneer crown. J Am Dent Assoc 1967;75:1419-25. 4. Youdelis R, Morrison K. Full coverage restoration of puipless anterior and bicuspid teeth. J Can Dent Assoc 1966;32:516-21. 5. Miller AW. Post and core systems: which one is best? J PROSTHET DENT 1982;4&27-38. 6. Zarb GA, Bergman B, Ciayton JA, MacKay HF. Preparing patients for the fixed or removable prosthdontic service. In: Prosthodontic treatment for partially edentuious patients. St Louis: CV Mosby Co, 1978136-7. 7. Pipko DJ, Hadeed GJ. Fabrication of drop-cast aluminum cores. J PROSI’BET DENT 1985;53:501-4. 8. Spector MR. A cast core system with interlocking posts. J PROSTHET DENT 1986,65:16-g. 9. Craig RG. Cast and wrought base metal alloys. In: Restorative dental matmiale. 7th ed. St Louis: CV Mosby Co, 1985;384-410. 10. Charlton G. A prefabricated post and core for porcelain jacket crowns. Br Dent J 1985;119:452-6. 11. Stokes AN. Post crowns: a review. Int J Endodont 1987;20:1-7. 12. Caputo AA, Standlee JP. Restoration of endodonticaiiy treated teeth.
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13. 14. 15. 16.
17.
18.
19. 20.
21.
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RESISTANCE
22. Maijer R, Smith DC. Corrosion of orthodontic bracket bases. Am J Orthod 1982;81:43-8. 23. Buchman D. Effects of recycling in metallic direct-bond orthodontic brackets. Am J Orthod 1980;77:654-88. 24. Espevik S. Corrosion of base metal alloys in vitro. Acta Odontol Stand 1978;36:113-6. 25. Pameijer CH, Giants P, Mobasherat MA. On clinical corrosion of pins. Swed Dent J 1983;7:161-7. 26. Derand T. Corrosion of screwposts. Odont Revy 1971;22:371-8. 27. Soremark R, Ingela 0, Plett H, Samsabl LK. Inthmnce of some dental restorations on the concentrations of inorganic constituents of the teeth. Acta Odontol &and 1962;20:215-24. 28. Bergenholtz A, Hedegard B, Soremark R. Studies of the transport of metal ions from gold inlays into environmental tissues. Acta Odontol &and 1965;23:135-46. 29. Arvidson K, Wroblewski R. Migration of metallic ions from screwposts into dentin and surrounding tissues. Stand J Dent Ras 1978;86:209-5.
In: Biomechanics in clinical dentistry. Chicago: Quintessence Pub1 Co Inc, 1987;185-203. Angmar-Mansson B, Omnell K, Rud J. Root fractures due to corrosion: 1. metallurgical aspects. Odontologisk Revy 1969;20:245-65. Petersen KB. Longitudinal root fracture due to corrosion of an endodontic post. J Can Dent Assn 19’71;2:66-8. Rud J, Omnell K. Root fractures due to corrosion: diagnostic aspects. Stand J Dent Res 1970;78:397-403. Silness J, Gustavsen F, Hunsbeth J. Distribution of corrosion products in teeth restored with metal crowns retained by stainless steel posts. Acta Odontol Stand 1979;37:317-21. American Society for Testing and Materials. Standard method for optical emission spectrometric analysis of stainless type 18-8 steels by the point-to-plane technique (Designation: E327-68). In: Priemonn-Storer RA, ed. Annual book of ASTM standards. Easton, Md: ASTM, 1985;3.06:150-3. American Society for Tasting and Materials. Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels (Designation A262-84). In: Priemon-Storer RA, ed. Annual book of ASTM standards. Easton, Md: ASTM, 1985;1.05:81-98. Brick RM, Gordon RB, Arthur A. Stainless steels. In: Structure and properties of alloys, New York: McGraw-Hill Book Co, lQ65;339-58. Lyman T. Metals handbook: atlas of microstructure of industrial alloys. 8th ad. Mehl RF, ed. Metals Park, Ohio: American Society for Metals, 1972;7:82. Phillips RW. Wrought base metal alloys. In: Skinner’s science of dental materials. 7th ed. Philadelphia: WB Saunders Co, 1983;641-56.
Using
a tooth-reduction
Reprint requests to: DR. JOHNA.SORENSEN CHS 33-041 SCH~~LOFDENTISTRY UNIVERSITyOFCALIFORNJs4 LosANGELEs.CA~OOZ~
guide for modifying
L. Kirk Gardner, D.D.S.,* Arthur 0. Rahn, D.D.S.,** Gregory R. Parr, D.D.S.,** and David W. Richardson, Medical
College
of Georgia,
School
of Dentistry,
Augusta,
natural
teeth
D.D.S.***
Ga.
A simplified method of transferring diagnostic odontoplastic information from the cast to the patient is described. This technique can be used successfully when treating fixed, removable, or combination prosthodontics patients. (J F’ROSTIIET DENT
1990;63:637-9.)
T
ransferring tooth modifications accurately from altered diagnostic casts to the mouth can be difficult and tedious. A technique for fabrication of a treatment template is described.
PROCEDURE 1. Perfect the occlusal plane on diagnostic casts by using a template or any other desired method with a sharp knife, stones, or burs (Fig. 1).lm4 2. Paint the cast with a tinfoil substitute and allow it to dry. 3. Adapt autopolymerizing acrylic resin (dough stage) to the facial surface of the modified teeth on the cast and cut to the occlusal height of the altered teeth.
*Associate **Professor, ***Assistant
Professor, Department of Prosthodontics. Department of Prosthodontics. Professor, Department of Prosthodontics.
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Fig. 1. Occlusal plane adjusted by use of 20-degree template. 4. Lubricate acrylic resin surfaces in the modified locations with petroleum jelly and apply a second mix of acrylic resin to make the lingual portion of the guide. 5. After the acrylic resin is cured, remove the guide in two pieces and smooth any rough surfaces (Figs. 2 and 3). 637