Low Fusing Alloy

Use of low fusing alloy in dentistry Alvin G. Wee, BDS, MS,a Robert L. Schneider, DDS, MS,b and Steven A. Aquilino, DDS, MSc College of Dentistry, University of Iowa, Iowa City, Iowa Statement of problem. Low fusing alloy has been used in dentistry for remount procedures in both fixed and removable prosthodontics, in implant prosthodontics for the fabrication of solid implant casts, in maxillofacial prosthetics as oral radiation shields, and in dental research for its unique properties. Previously, the use of low fusing alloy was thought to offer a high degree of dimensional accuracy. However, multiple in vitro studies have shown that its presumed dimensional accuracy may be questionable. Purpose. This article reviews the physical properties, metallurgical considerations of low fusing alloy, its applications in dentistry, and a safe, simple method of using low fusing alloy. (J Prosthet Dent 1998;80: 540-5.)

CLINICAL IMPLICATIONS Although many uses of low fusing alloy have been described previously in the dental literature, the clinician/researcher must be aware of the accuracy of the material and potential health risks associated with its use.

L

ow fusing alloy has been used in a variety of dental applications for at least the last 36 years. Its first use was documented by Lucia1 in 1961 for the purpose of remounting crowns and fixed partial dentures. Since then, the use of low fusing alloy has been documented in fixed prosthodontics,1-7 removable prosthodontics,8 and implant prosthodontics.9 In maxillofacial prosthetics, low fusing alloy is used to fabricate oral radiation shield prostheses.9-16 Dental researchers who are familiar with the properties of low fusing alloy have also used it creatively within their research methods.17-19 Many low fusing alloys are available commercially (Table I), vary in composition, and display different desired properties. As its name implies, low fusing alloy melts at a low temperature, within the range of 117°F to 338°F.20 Low fusing alloy is easily cast into molds and ready for use after a rapid solidification of less than 5 minutes. The alloy may be easily recovered and recycled for reuse any number of times. This article presents a literature review that examines the physical properties and metallurgical considerations of low fusing alloy and its applications within prostho-

Supported in part by a Rotary International Foundation Multi-Year Ambassadorial Scholarship 1994-96. The authors have no connection with or conduct research for any company that is associated with low fusing alloy. aAssistant Professor, Sections of Restorative Dentistry, Prosthodontics and Endodontics, Department of Prosthodontics. bClinical Director, Oral and Maxillofacial Implant Center, Associate Professor, Department of Prosthodontics. cDirector, Graduate Prosthodontics, Professor, Department of Prosthodontics. 540 THE JOURNAL OF PROSTHETIC DENTISTRY

dontics and dental research. A safe, precise, and efficient method to use low fusing alloy is also presented.

PHYSICAL PROPERTIES AND METALLURGICAL CONSIDERATIONS The chief elemental components of low fusing alloy are bismuth, lead, tin, and occasionally indium (Table I). Most low fusing alloys are composed of 3 or more metals. Alloy systems that contain more than 2 metals have not been developed to the extent of binary diagrams because of the difficult preparation of the alloy systems.21 Published binary phase diagrams22 of bismuth, lead, tin, and indium illustrate some eutectic alloys in their microstructures. A mixture is identified as eutectic when the compositional metals are miscible in the liquid state but separate into 2 phases in the solid state. The 2 phases often precipitate as fine layers of one phase over the other.23 The partial eutectic microstructure in the composition of low fusing alloy can account for its physical properties. Eutectic alloys are relatively brittle as the presence of insoluble phases inhibits slip in the alloy. At times, the strength and hardness of these alloys may exceed their primary components, due to the composite structure of the alloy. In contrast, alloys composed of low fusing metals with partial eutectic microstructure typically retain their expected high ductility.21In a study by Toreskog et al.,24 a low fusing alloy (Cerrolow 136, Marmon Group, Inc., Belleforte, Pa.) was shown to be relatively soft. The alloy demonstrated a Knoop hardness of 9 but was too soft to be measured for Brinell hardness. However, these researchers reported a negligible relationship between Knoop hardness and abraVOLUME 80 NUMBER 5

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Table I. List of low fusing alloys used in the dental literature

Alloy

Fusible metal Plumbers solder Metallomat alloy B2OE2 alloy Melotte’s metal Belmont alloy 2491 Indalloy 136 Cerrolow 136

Lipowitz’s metal Belmont alloy 2503 Ostalloy 158 Cerrobend

Composition*

Melting temperature

Bi

Pb

Sn

In

Ag

Williams Dixon, Inc., Carstadt, N.J. NA Ivoclar, Schwann, Liechtenstein Alpha Metals Inc., Jersey City, N.J.

NA NA NA NA

NA 0 53 52

50 0 32

50 45 16

0 0 0

0 2 0

Belmont Metals Inc., Brooklyn, N.Y. Indium Corp., Utica, N.Y. Marmon Group Inc., Belleforte, Pa.

136°F 136°F 136°F

49 49

Belmont Metals Inc. Arconium Corp. of America, Providence, R.I. Cerro Metal Products, Belleforte, Pa.

158°F 158°F 158°F

50 50 50

Manufacturer’s name

Cd

18 12 21 0 18 12 21 0 Likely to be similar to Belmont alloy 2491 and Indalloy 136 26.7 26.7 26.7

13.3 13.3 13.3

0 0 0

0 0 0

10 10 10

*Reported as percentage. NA: Not available.

Table II. Percentage expansion of low fusing alloy Study

Toreskog et al.24 Yoon et al.26

Alloy

Reported expansion* (time)

Actual expansion* (time)

Cerrlow 136 (Marmon Group Inc.) Indalloy 136 (Indium Corp.)

NA

Occlusal = +0.42 Cervical = +0.24 (24 h) 0.00 (0 h) –0.05 (0.5 h) 0.12 (336 h) 0.00 (0 h) –0.40 (0.5 h) –0.43 (336 h) 0.00 (0 h) 0.01 (0.5 h) 0.08 (336 h) 0.00 (0 h) 0.28 (0.5 h) 0.024 (336 h) 0 - High SD (at least 24 h)

Plumber’s solder (NA)

Metallomat alloy (Ivoclar) B2OE2 alloy (Alpha Metals Inc.) Wee et al.27

Belmont alloy 2491 (Belmont Metals Inc.)

0 (1 h)

NA

+0.10

NA

0.0 (initially) –0.02 (5 h)

*Reported as percentage. NA: Not available.

sive resistance for low fusing alloy.24 This differs from the “apparent” positive correlation between hardness and abrasion resistance associated with the majority of the other materials tested in their study.24 The presence of partial eutectic microstructures in a low fusing alloy also contributes to its low melting point. Solidification of eutectic alloys present similar curves to their pure metals, with the exception that solidification temperature is lower than that of the pure metals. These eutectic alloys possess a melting point at the eutectic composition, rather than a melting range. Any other possible combinations of the pure metals in the alloy system will have a higher fusion temperature than the melting point of the eutectic mixture.23 Thus, NOVEMBER 1998

eutectic alloys have been used to lower the fusion temperature of alloys if desired.21 In a eutectic alloy, a flow or creep may occur even at room temperature if the recrystallization temperature of the matrix metal (lead) is low.21 As one of the chief elements of low fusing alloy, lead causes the alloy to slowly expand over time (Table II). It is unusual for products with different trade names to demonstrate similar composition and melting temperatures (Table I). Alloys that demonstrate such similarities most likely are provided by the same supplier but marketed under different trade names. The often referred to “Melotte’s metal” in the dental literature is actually Belmont alloy 2491 (Belmont Metals Inc.), 541

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Fig. 1. Low fusing alloy used in remount cast for multiple posterior single crowns in mandibular arch.

Fig. 2. Low fusing alloy used in remount cast for mandibular RPD.

which has been clarified by Pameijer.5 Examination of the composition and melting temperatures of Indalloy 136 (Indium Corp., Utica, N.Y.), Cerrolow 136 (Marmon Group Inc.) and Belmont Alloy 2491 (Belmont Metals Inc.) suggests that these alloys are similar (Table I). Lipowitz’s metal is also marketed under different trade names, including Ostalloy 158 (Arconium Corp. of America), Belmont Alloy 2503 (Belmont Metals Inc.), and Cerrobend (Cerro Metal Products).7 Some manufacturers claim that low fusing alloy has little or no dimensional change when passing from the liquid to the solid state.20 However, the properties and use of low fusing alloy and its dimensional accuracy are questioned.24-27 It is not surprising, given the compositional variation of these alloys, that low fusing alloys produced by different manufacturers have a wide range of dimensional accuracy. Low fusing alloy, as examined by Toreskog et al.,24 was found to be completely compatible with all the impression materials studied at that time, including polysulfide (Permlastic, Kerr Mfg. Co., Detroit, Mich.). However, it was found that dies produced by the alloy frequently exhibited rounded corners and pits or nodules as a result of the collapse of voids in the impression.

compared with the process of remounting on the articulator. The use of low fusing alloy to fabricate remount casts for multiple fixed units has been recommended (Fig. 1).1-6 After fitting the castings intraorally for proximal contacts, marginal fit, and contours, it has been recommended that the fixed restorations be stabilized with a mixture of temporary cement and petroleum jelly,6 or a multiform impression paste (Lactona Corp., Philadelphia, Pa.).4 After an interocclusal record and face-bow transfer are made, an impression is made over the seated castings. Several techniques have been described to ensure the “absolute” accuracy of the remount impression.4-6 Before pouring the remount impression, internal surfaces of the castings are painted with a separating medium (petroleum jelly,4 Rubbersep, or Mucolube5). All exposed external portions of the castings are covered with either melted baseplate wax, reversible, or irreversible hydrocolloid material. A low fusing alloy is melted and poured into the impression containing the castings to cover and include the occlusal one third of the dental alveolar ridges. Walker4 used a low fusing alloy from Dentalloy Inc. (Stanton, Calif.), whereas Pameijer5 recommended using a low fusing alloy called “Melotte’s metal/Belmont alloy 2491” (Belmont Metals Inc.). Retentive components, such as pins or paper clips, are heated and placed into the cooled alloy, and the remainder of the cast is poured in die stone. The low fusing alloy allows easy and repeated removal of the castings from the remount cast.5,6 If gypsum is used, abrasion and possible fracture of the die5 can occur. Low fusing alloy has also been used to fabricate a remount cast for removable partial dentures (RPDs) (Fig. 2). Reitz8 recommended making an interocclusal record and a face-bow transfer before making an intraoral irreversible hydrocolloid impression of the seated

CLINICAL APPLICATIONS IN PROSTHODONTICS Remount procedures in fixed and removable prosthodontics Although remounting fixed and removable prostheses involves a number of additional procedures, the remounting technique permits the clinician to refine the occlusion in a more controlled environment than can be experienced intraorally. The decision to carry out a clinical remount is made by assessing the difficulties associated with refining the occlusion intraorally, as 542

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RPD. A low fusing alloy (Melotte’s metal/Belmont alloy 2491) was then poured into the natural teeth portion of the impression. A retentive component was heated and embedded into the low fusing alloy within the impression, and the rest of the cast was poured with plaster. This method allows the removal of the RPD after the remount cast is made, so that any type of interocclusal record can be taken. Teeth chipping or wear are less likely to occur during the process of equilibrating the prosthesis.

Implant solid cast fabrication Recently, low fusing alloy has also been recommended as a material for the fabrication of solid implant casts. Schneider and Wee9 described the use of a low fusing alloy in the fabrication of a solid multi-implant cast. The low fusing alloy (Belmont 2491) has a melting point of 136°F and a linear expansion after solidification of 0.05%.20 The melted alloy was poured into the implant impression around abutment replicas. Once the alloy solidified, retentive components were placed in the alloy and the rest of the impression was poured in either ADA type IV or V dental stone (Fig. 3).

Applications in dental research Dental researchers knowledgeable of the properties of low fusing alloy have used the alloy successfully in their research methods. Particularly useful properties include the castability of the alloy at low temperatures17 and the nonbrittle nature of the alloy.18,19 Low fusing alloy has been used to measure the accuracy of die materials and impressions. Stackhouse et al.17 used low fusing alloy (Indalloy 136, Indium Corp.) to create a counter die to evaluate the accuracy of various die/impression material combinations. Wang et al.18 developed an accurate and reproducible intraoral method of measuring the distance between 2 teeth by using casts fabricated from low fusing alloy (Cerrolow-136). Zwetchkenbaum et al.19 used low fusing alloy in a study comparing the sizes of the marginal gaps of nonrelined, relined corrected, and paint-on corrected acrylic resin provisional crowns after thermocycling and occlusal loading.

Accuracy of low fusing alloy A common presumption when low fusing alloy is used for fixed and removable prosthesis remount procedures, for solid implant cast fabrication, or in dental research has been its accuracy. The recommended use of this alloy had been based on empirical evidence for particular techniques, as described by various clinicians.1,3-5,9 Measurements of accuracy of low fusing alloy were not documented to accompany these technique investigations. Low fusing alloy properties have been investigated,25 and its accuracy has been investigated for use as transfer casts after indexing,25 as counter dies for meaNOVEMBER 1998

Fig. 3. Low fusing alloy used in solid implant master cast.

suring impression and die material accuracy,17 and as solid implant casts27 (Table II). Toreskog et al.24 evaluated the use of low fusing alloy (Cerrolow-136) as a die material. The alloy with a melting point of 136°F was heated to 145°F in a constant temperature oven. The die was then poured, and the alloy was allowed to solidify under 30 pounds of pressure. The authors found that the alloy expanded to 0.42% at the occlusal region and 0.24% at the cervical region of the die. Harper and Nicholls25 evaluated distortion in indexing methods and investing media for soldering and remount procedures. They tested an ADA type IV dental stone (Vel-mix, Kerr Mfg. Co., Emeryville, Calif.), an autopolymerizing acrylic resin, and a low fusing alloy (Fusible Metal, Williams Dixon, Inc., Carstadt, N.J.) as a transfer cast after indexing with either polyether or zinc oxide eugenol (ZOE) impression paste. A 3-dimensional analysis of distortion was conducted. For the remount procedure, low fusing alloy exhibited significantly more 3-dimensional distortion than the ADA type IV dental stone. Yoon et al.26 investigated the linear dimensional changes of 3 low fusing alloys, using the master die of ADA Specification No. 19.28 The samples were bench cooled at room temperature for a half hour, separated, then measured with a toolmaker’s microscope that had a micrometer divided to 2.5 µm. Subsequent measurements were made at 1 hour, 1.5 hours, and 2-week intervals after casting. The authors reported a high degree of a dimensional change with the B20E2 alloy (Alpha Metals Inc.) (+0.3%) and the plumbers solder (–0.4%). The Indalloy 136 alloy shrank 0.05% ± 0.02%, then slightly expanded to 0.12% after 2 weeks. They concluded that this alloy presented desirable features, including a low fusing temperature, small dimensional change, minimal surface bubbles, relatively satisfactory 543

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Fig. 4. Melted low fusing alloy in hot water bath with plastic syringe.

Fig. 5. Plastic syringe used as dispenser for melted low fusing alloy.

surface detail, and availability. The dimensional change of the tested low fusing alloys did not coincide with the manufacturers’ stated dimensional accuracy of the alloys. Such results are not uncommon when considering differences in method and measurement conditions. Wee et al.27 evaluated the accuracy of low fusing alloy solid implant casts compared with gypsum casts poured in a polyether impression. The materials evaluated included a low fusing alloy (Belmont alloy 2491), Velmix, Die Keen (Miles Dental Products, South Bend, Ind.), and Resin Rock resin impregnated gypsum hybrid alloy (Whip Mix Corp, Louisville, Ky.). A digital veneer caliper was used to evaluate the linear horizontal dimensional change of the most distal abutments of the master cast and experimental casts were made from the various tested materials. Although the low fusing alloy was found to produce the least horizontal linear dimensional change, it also exhibited the greatest standard deviation. The mesiodistal strain was also evaluated when a master framework was secured to the various experimental casts by prosthetic retaining screws torqued to 10 Ncm. A 1-way analysis of variance (ANOVA) (α=.05) revealed a statistically significant difference from the mean (absolute) strain values among the materials groups (P=.0261, power=0.99) Duncan’s multiple range test (α=.05) indicated that low fusing alloy did not differ significantly from the other materials. Casts made from Resin Rock material produced the least amount of mean microstrain on the framework. Overall, studies that have evaluated the dimensional accuracy of low fusing alloy have shown them not to be accurate, as compared with standard gypsum materials.

ation protection prostheses can be constructed as a 1-piece7,10,11,13,29 or a 2-piece16 oral radiation shield. Noncancerous oral structures in the field of the external radiation beam are protected from unnecessary sequelae of therapeutic radiation31 through the use of the shield. A radiation protection prosthesis can shield a portion of the radiation from the tongue,11 alveolar ridge, and/or the oral tissues opposite the radiation source.16 Low fusing alloy has also been used to obturate the palatal vault to decrease radiation exposure to noninvolved, previously irradiated tissues during brachytherapy of the palate.14 Shielding adjacent tissues of the nose from radiation to treat carcinoma of the nasal cavity and/or nasal vestibule has also been described.29 Although lead is the ideal material for maximum shielding of radiation,12 its melting temperature of more than 600°F melts or distorts the methyl methacrylate that would be used as its carrier. The popular shielding material of choice to fabricate an oral shield prosthesis is a low fusing alloy called Lipowitz metal. Various thicknesses of Lipowitz’s metal (Ostalloy 158, Belmont Alloy 2503, and Cerrobend) have been shown to prevent a percentage of the therapeutic radiation passing through the shield to noncancerous tissue.7,11,32 Lipowitz low fusing alloy, with a lead content of 26.7% and a melting temperature of 158°F, can be poured into a shell of methyl methacrylate without affecting that material.

CLINICAL APPLICATIONS IN MAXILLOFACIAL PROSTHETICS Low fusing alloy has been documented by many for use as oral radiation shield prostheses.7,11-13,29,30 Radi544

SAFETY RECOMMENDATIONS Health hazard information for low fusing alloy cautions that dust vapors and/or fumes from the low fusing alloy may be irritating to the respiratory system and digestive system when ingested, resulting in acute and/or potentially chronic body reactions with overexposure.20 Because exposure may also cause irritation to VOLUME 80 NUMBER 5

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the eyes and skin, a face shield and/or vented goggles should be worn with gloves before handling the low fusing alloy. Protective clothing should also be worn and properly laundered after use. The low fusing alloy from the manufacturer should be sectioned in strips19 and melted in a water bath 20°F above the melting temperature of the alloy (Fig. 4).5,27 This will prevent overheating and emission of toxic vapors20 and minimize alloy shrinkage.5 A plastic syringe19,24 or an “eye drop” dispenser5 can be used to transfer the low fusing alloy from the water bath (Fig. 5). Use of a plastic calibrated syringe allows a precise amount of alloy to be dispensed27; small increments should be dispensed to minimize the shrinkage of the alloy.5

CONCLUSION The popularity of low fusing alloy in the 1970s and 1980s as a remount material has declined, due in part to the associated health hazards when the alloy is not properly manipulated20 and its questionable accuracy.17,24,25,27 However, the use of low fusing alloy in selected prosthodontic procedures and in dental research is still applicable, with consideration of safe, and efficient methods of handling. REFERENCES 1. Lucia VO. Modern gnathological concepts. 1st ed. St Louis: CV Mosby; 1961. 2. Huffman RW, Regenos JW. Principles of occlusion. Volume VIII. London (OH): H & R Press; 1973. 3. Kornfeld M. Mouth rehabilitation—clinical and laboratory procedures. Volume II. St Louis: CV Mosby; 1974. p. 904-5, 988-90, 1027-9. 4. Walker PM. Remounting multiples casting prior to final cementation. J Prosthet Dent 1981;46:145-8. 5. Pameijer JH. Remounting procedures. Periodontal and occlusal factors in crown and bridge procedures. Holland: Dental Center for Postgraduate Course; 1985. p. 404-12. 6. Rhoads JE, Rudd KD, Morrow RM. Impression and cast. Dental laboratory procedures—fixed partial dentures. Volume II. Second edition. St Louis: CV Mosby; 1981. p. 45-8. 7. Bashiri H, Suen JY. Dental considerations. In: Myers EN, Suen JY, editors. Cancer of the head and neck. 3rd ed. Philadelphia: WB Saunders; 1996. p. 117-30. 8. Reitz PV. Technique for mounting removable partial dentures on an articulator. J Prosthet Dent 1969;22:490-4. 9. Schneider RL, Wee AG. Fabricating low-fusing metal casts for more accurate implant prosthodontics. J Prosthodont 1996;5:301-3. 10. Aramany MA, Drane JB. Radiation protection prostheses for edentulous patients. J Prosthet Dent 1972;27:292-6. 11. Fleming TJ, Rambach SC. A tongue shielding radiation stent. J Prosthet Dent 1983;49:389-92. 12. Rahn AO, Boucher LJ. Maxillofacial prosthetics—principles and concepts. Philadelphia: WB Saunders; 1970. p. 49-82. 13. Poole TS, Flaxman NA. Use of protective prostheses during radiation therapy. J Am Dent Assoc 1986;112:485-8.

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14. Randall ME, Salisbury PL 3d, Schmidtke M. Afterloaded radiation carrier for carcinoma of the palate. A clinical report. J Prosthet Dent 1988;60:655-9. 15. Beumer J III, Curtis TA, Morrish RB Jr. Radiation complications in edentulous patients. J Prosthet Dent 1976;36:193-203. 16. Coleman AJ. A technique for shielding electron beams used in radiotherapeutic management of head and neck cancer. J Prosthodont 1996;5: 129-32. 17. Stackhouse JA Jr, Yoon W, Von Hagen S. Low-fusing counterdies for measuring accuracy of dies or impression materials. J Prosthet Dent 1994; 71:209-14. 18. Wang JC, Charbeneau GT, Gregory WA, Dennison JB. Quantitative evaluation of approximal contacts in Class 2 composite resin restorations: a clinical study. Oper Dent 1989;14:193-202. 19. Zwetchkenbaum S, Weiner S, Dastane A, Vaidyanathan TK. Effect of relining on long-term marginal stability of provisional crowns. J Prosthet Dent 1995;73:525-9. 20. Material safety data sheet: Low melting alloys. Brooklyn: Belmont Metals; 1992. 21. Anusavice JK, editor. Phillip’s science of dental materials. 10th ed. Philadelphia: WB Saunders; 1996. p. 336-40. 22. Hansen M, Anderko K. Constitution of binary alloys. 2nd ed. New York: McGraw-Hill; 1958. p. 313-4,324-6,336-9,1106-9. 23. Craig RG. Restorative dental materials. 10th ed. St Louis: CV Mosby; 1997. p. 111-7. 24. Toreskog S, Phillips RW, Schnell RL. Properties of die materials—a comparative study. J Prosthet Dent 1966;16:119-31. 25. Harper RJ, Nicholls JI. Distortion in indexing methods and investing media for soldering and remount procedures. J Prosthet Dent 1979; 42:172-9. 26. Yoon W, Wexler I, Stackhouse JA. Low fusing counterdies to test die/impression accuracy. J Dent Res 1985;64:906 [abstract]. 27. Wee AG, Schneider RL, Aquilino SA, Huff TL, Lindquist TJ, Williamson DL. Evaluation of the accuracy of solid implant cast. [Master’s thesis.] Iowa City: The University of Iowa; 1997. 28. Revised American Dental Association Specification No. 19 for non-aqueous, elastomeric dental impression materials. American Dental Association. J Am Dent Assoc 1977;94:733-41. 29. Ghalichebaf M, Chalian VA, Shidnia H. A shielded radium source carrier nasal stent for the treatment of primary carcinoma of the nasal cavity. J Prosthet Dent 1984;51:383-6. 30. Beumer J, Curtis TA, Nishimura R. Radiation therapy of head and neck tumors. In: Beumer J, Curtis TA, Marunick MT, editors. Maxillofacial rehabilitation—prosthodontic and surgical considerations. St Louis: Ishiyaku EuroAmerica; 1996. p. 43-111. 31. Arcuri MR, Schneider RL. The physiological effects of radiotherapy on oral tissue. J Prosthodont 1992;1:37-41. 32. Farahani M, Eichmiller FC, McLaughlin WL. New method for shielding electron beams used for head and neck cancer treatment. Med Phys 1993;20:1237-41.

Reprint requests to: DR ALVIN G. WEE SECTION OF RESTORATIVE DENTISTRY, PROSTHODONTICS AND ENDODONTICS COLLEGE OF DENTISTRY THE OHIO STATE UNIVERSITY POSTLE HALL 305 W 12TH AVE COLUMBUS, OH 43210-1241 Copyright © 1998 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/98/$5.00 + 0. 10/1/92204

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