t of fixed partial
dentures
joined by infrared
soldering
Gerard Byrne, BDS, MSD,aLeon W. Laub, PhD,b Jenq-Yong Hu, BDS, MS,“and Martin F. Land, DDS, MSDd Loyola University of Chicago,School of Dentistry, Chicago,Ill. This study determined the accuracy of fit of three-unit fixed partial dentures joined by an infrared soldering technique compared with one-piece fixed partial denture castings and individually cast crowns. Wax patterns of prepared Ivorine teeth, maxillary left central incisor and maxillary left canine, were injection molded; a plastic rod was used as a pontic. One group of patterns was cast as one-piece dentures; the other group was sectioned in the connector area, cast individually, and then joined by infrared soldering. Castings were seated on their respective dies, embedded in epoxy resin, and sectioned. Gap distances between the casting and the die were measured at specified marginal sites with a profile projector. Results showed that the At of infrared-soldered fixed partial dentures was significantly better than that of one-piece castings and was comparable with the fit of single crowns. The gap openings measured in all castings were within the range of clinical acceptability. (J PROSTRET DENT 1992;68:591-6.)
ental soldering is a traditional method for joining the components of fixed partial dentures (FPDs). Several soldering techniques are available, including a recently introduced infrared technique. An alternative to soldering is to cast the wax components of the FPD in one piece. A general desire for improved efficiency and cost effectiveness has led to a gradual increase in the number of FPDs that are cast as one-piece restorations, thus bypassing soldering procedures. However, soldering is preferred by many operators as the optimal method of joining FPD units. In addition, soldering is sometimes necessary for a specific procedure-for example, the joining of cast gold to metal/ceramic units. LITERATURE
REVIEW
Several investigators1-5 have recommended one-piece casting as superior to soldering for joining FPD units. One-piece casting eliminates the soldering step, maximizes the strength of connectors,6 and produces more accurate fitting prostheses.3 Others7-r1 favor soldering over onepiece casting. They cite improved fit with a soldering technique because of reduced interabutment distortion. Fusayama et al3 compared the accuracy of multiple-unit FPDs made by the one-piece casting technique and various soldering techniques. They concluded that cast FPDs were more accurate than soldered FPDs. However, the average
aAssistantProfessor,Departmentof Fixed Prosthodontics,Loyola University of Chicago. bProfessor,Department of Dental Materials, Loyola University of Chicago. CPrivatepractice, Hsinchu, Taiwan, Republic of China. dAssociateProfessorand Chairman, Department of Fixed Prosthodontics, Loyola University of Chicago. 10/l/36188
THE JOURNAL OF PROSTHETIC DENTISTRY
marginal opening reported for a four-unit FPD was 0.20 mm (200 pm). Bruce4 evaluated multiple-unit castings with a two-abutment expandable die system. He concluded that FPDs up to 15.5 mm in length could be cast with reasonable accuracy. Ziebert et al.6 compared the accuracy of FPDs of varying lengths fabricated as one-piece castings with those joined by soldering. They observed that the fit of all the three-unit FPDs, whether cast or soldered, was similar. Reported mean marginal gap widths ranged from 32 pm for preceramic soldering to 42 Frn for one-piece castings, and these widths increased as span length increased. The distal margin of the posterior abutment and the mesial margin of the anterior abutment had the largest marginal discrepancy. These investigators suggested that FPDs exceeding four units be soldered. Schiffeleger et a1.12 compared the marginal discrepancies of three-, four-, and five-unit one-piece castings and reported mean values for marginal gap widths of 54, 92, and 105 pm, respectively. An important factor thought to cause poor fit of onepiece FPD castings is the inadequate retainer-to-retainer expansion of investment, I1 followed by cooling of the cast structure. The interabutment distortion results in a marginal error that can compromise the simultaneous optimal fit of the retainers. Garlapo et a1.i3 assesseddistortion of cast FPDs, and concluded that a four-unit FPD could be cast in one piece without significant vertical warping. Other investigators”, i2 have reported improved marginal fit after sectioning and soldering the components of cast FPDs. Proposed solutions for minimizing distortion include the use of an all-wax spruing system in preference to plastic sprues,14 the use of larger rather than smaller casting rings,15 and the use of oval-shaped casting ringslz Other technique aspects of soldering-that is, specifying a gap width and an indexing method-are important for
591
BYRNE
Fig. tic.
1. Standardized
FPD wax pattern
with plastic pon-
Fig.
3. Measurement
ET AL
sites at a chamfer margin.
Nicholls31 compared accuracy of the indexing media and concluded that zinc oxide-eugenol (ZOE) bite registration paste was the most accurate. Moon et a1.l’ found that the most accurate results could be obtained with a plaster nonremoval technique. This study evaluated the fit of infrared-soldered FPDs versus one-piece cast FPDs.
MATERIAL
Fig.
2. Measurement
sites at a shoulder margin.
successful soldering. The width of the gap between the two pieces to be joined is probably the most controversial parameter in soldering. It is considered to affect the distortion of the system as well as the strength of the joint. Suggested gap distances range from tight contact,16 0.1 rnrn,17, ls 0.15 rnrn,ls21 0.2 mm,22 0.25 mm,23,24 0.3 mm,25-2g to 0.5 mm.g, l2 Shillingburg et a1.30 reported that the opposing surfaces on either side of the solder joint should be parallel to each other; there was more likelihood of distortion if the space between units was not of uniform width. Harper and
592
AND
METHODS
Standard metal/ceramic preparations, with 1.0 mm facial shoulder and 0.3 mm lingual chamfer, were cut on Ivorine (Columbia Dentoform Corp., New York, N. Y.) teeth, the maxillary left central incisor and maxillary left canine. The teeth were attached to an acrylic base, with a coronal separation of 6.5 mm32 representing the mesiodistal width of the maxillary left lateral incisor. Eleven replicas were produced with a silicone mold (RTV 630, General Electric Co., Waterford, N.Y.) and a high strength dental stone (Silky Rock, Whip-Mix Corp., Louisville, KY.). Standardized FPD wax patterns were fabricated using an injection molding technique and a hard inlay wax (Casting Wax, Whip-Mix Corp.). A plastic sprue bar (No. 10 gauge) was used instead of a wax pontic in FPD specimens to ensure a standard pontic size and configuration (Fig. 1). This procedure greatly simplified the creation of a standardized soldering area. A stone jig was fabricated on a surveyor for the purpose of sectioning the plastic bar at a position 0.5 mm distal from the central incisor with a 0.15 mm thick diamond disk. This technique produced identical soldering gaps. Three groups of patterns were fabricated. Group I consisted of two individual retainers, a maxillary left central incisor and canine, and was used as a control for casting fit; no pontic was used. Group II consisted of five FPDs
OCTOBER
1992
VOLUME
68
NUMBER
4
FPDS
JOINED
BY INFRARED
SOLDERING
I. Marginal gap measurements (in microns) at each tooth site
Table
Group MLCI
Mesial Mean SD
Distal Mean SD Labial Mean SD
Lingual Mean SD
Group
I* MLC
IIT
MLCI
MLC
Group
Table III. Marginal gap measurements (in microns) at external and internal sites Group
III?
MLCI
1*
IIt
111t
14.8 1.8
65.2 17.2
18.9 5.1
14.0 2.1
24.1 13.7
18.8 6.3
MLC
13.5 1.3
13.5 2.6
70.1 13.4
27.8 16.0
17.2 5.3
16.3 4.2
14.5 1.7
16.0 1.4
20.3 10.0
60.2 19.3
21.4 7.2
20.6 4.4
10.5 1.7
6.5 1.3
19.2 3.0
22.0 9.9
16.0 4.2
15.4 6.1
15.5 2.6
17.3 2.2
29.8 8.1
23.1 1.2
27.1 7.0
19.0 6.3
External Mean SD
Internal Mean SD *Mean
of eight measurements.
fMean of 40 measurements.
MUX, Maxillary left central incisor; MLC, maxillary left canine. *Mean of four measurements. tMean of 20 measurements.
Table II. Combined marginal gap measurements (in microns) for each tooth Group 1*
Maxillary
III?
34.8 22.9
20.4 7.4
left central incisor
Mean 13.5 SD 2.6 Maxillary left canine Mean 13.3 SD 4.6 Maxillary Mean SD
IIt
33.3 17.8 20.9 5.8 left central incisor and maxillary left canine 13.4 34.0 19.1 3.7 21.9 6.7
*Mean of 16 measurements for each tooth. TMean of 80 measurements for each tooth.
prepared as one-piece castings; a plastic sprue simulated a pontic. Group III consisted of five FPDs, in which the simulated pontic was sectioned and the retainers were prepared individually. The marginal 3 mm region of the wax patterns was readapted and the patterns were invested in a phosphatebonded investment (Hi-Temp 2, Whip-Mix Corp.). A powder-liquid ratio was previously determined33 to produce consistent, adequately expanded castings that fit passively on the dies, exhibiting neither excessive looseness nor tightness. After staged burnout to 1400° F, castings were made with a high-palladium content alloy (Option, J.M. Ney Co., Bloomfield, Conn.), melted with a natural gas-oxygen mixture. Castings were cleaned in hydrofluoric acid, examined under a binocular microscope at 20 power magnification for small nodules, and if satisfactory, were seated on their respective dies. Individual components of group III
TNE
JOURNAL
OF PROSTHETIC
DENTISTRY
4. Location of gap sites between casting and die around the maxillary left canine. A, Labial; B, mesial; C, lingual; and D, distal.
Fig.
were positioned on their respective dies and were placed on the surveyor used to section the simulated pontic. The diamond disk used initially to create the gap distance was passed through the joint area again to ensure a standardized gap width of 0.15 mm. Occlusal indices were fabricated using the method of Moon et al.rs Components were soldered using an infrared apparatus (J.M. Ney Co.), following the manufacturer’s instructions. All castings were then seated on their original dies and were embedded in epoxy resin. Each FPD was sectioned, one cut through each retainer labiolingually and one cut mesiodistally through both retainers. Each FPD was then metallurgically polished to a high luster with 5 pm aluminum oxide slurry on a felt pad. The gap between the casting and die on the maxillary left central incisor and the maxillary left canine was measured at predetermined marginal sites using a profile projector (V-12, Nikon, Inc., Garden City, N.Y.). Measurements were taken at 150, 300, 450, and 600 ,um from the margin for shoulder preparations (Fig. 2). Similar
593
BYRNE
ET AL
Fig. 5. Marginal gaps between casting and die at sites around the maxillary left canine of a one-piece cast FPD. A, Labial; B, mesial; C, lingual; D, distal. (Original magnification x50.)
measurements were made at 50,100,150, and 200 pm from the margin for chamfer preparations (Fig. 3). Four measurements were made by one investigator at each mesial, distal, labial, and ligual site on the central incisor and canine. Group I (two individual retainers) consisted of four measurements at each site on the central incisor and canine; groups II and III (five FPDs, respectively) consisted of 20 measurements at each site on the central incisor and canine. Marginal gaps at each measurement site on the central incisor and canine were viewed on a scanning electron microscope (SEM) at 50 power magnification.
RESULTS Marginal gap openings at each measurement site-mesial, distal, labial, and lingual-are reported for group I (individual cast crowns), group II (one-piece cast FPDs), and group III (infrared soldered FPDs) in Table I. One-way analysis of variance was used to determine whether differences in test results were significant. Tukey’s Student range (HSD) test was applied to detect differences among means. Marginal gaps in group II were significantly larger (p I 0.05) than in group III for the central incisor at mesial (70.1 versus 17.2 pm) and labial (19.2 versus 16.0 pm) sites, and for the canine at mesial(27.8 versus 16.3 pm), distal (60.2 versus 20.6 pm), and labial (22.0 versus 15.4 pm) sites. 594
Within group II significantly different marginal gap openings 0, 5 0.05) were determined between the central incisor and canine at mesial, distal, and lingual locations. Within group III, marginal gap openings at the lingual of the central incisor were significantly larger than those at the lingual of the canine. All marginal measurements taken on the central incisor and canine at mesial, distal, labial, and lingual locations were combined for each group. The overall marginal gap opening for each group was: Group I (n = 32): 13.4 + 3.7 Km; group II (n = 160): 34.0 I: 21.9 pm; group III (n = 160): 19.1 1? 6.7 pm (see Table II). The gap opening for group II was significantly larger (p I 0.05) than that of either group I or group III. The largest sample-to-sample difference in marginal gap opening occurred among group II castings, as indicated by the large standard deviation. There was no significant difference between groups I and III. As an indicator of distortion during fabrication of threeunit FPDs, marginal gap openings were determined for external and internal sites. External marginal sites were the sum of the mesial gap of the central incisor and the distal gap of the canine. Internal marginal sites were the sum of the distal gap of the central incisor and the mesial gap of the canine. At external sites the marginal gap opening for group II (65.2 pm) was significantly larger (p 5 0.05) than that of either group I (14.8 pm) or group III (18.9 pm) (see OCTOBER
1992
VOLUME
68
NUMBER
4
FPDS
JOINED
BY INFRARED
SOLDERING
Fig. 6. Marginal gaps between casting and die at sites around the maxillary left canine of a soldered FPD. A, Labial; B, mesial; C, lingual; D, distal. (Original magnification x50.)
Table III). No significant difference was determined at external sites between groups I and III. At internal sites, the marginal gap opening for group II (24.1 +m) was significantly larger (p 5 0.05) than that for group I (14.0 pm). No significant difference was determined at internal sites between groups I and III, or between groups II and III. External and internal marginal gaps were compared within each group. Significant differences (p 5 0.05) were only determined within group II. Marginal gaps at labial, mesial, lingual, and distal aspects on sectioned specimens of the maxillary left canine are shown in Figs. 4 through 6 for seated one-piece cast and soldered FPDs. In these SEM microphotographs (50 power magnification), no overhanging or short margins were seen; the fit of one-piece cast and soldered FPDs appeared to be clinically acceptable. DISCUSSION The fit of one-piece cast FPDs has been shown in this study to be poorer than that of either infrared soldered FPDs or single crowns. Fit invol.ves the ability to seat an appliance completely on its die. Variables such as seating force, preparation geometry, and use of a luting agent will affect fit. A large range of seating forces has been used in previous investigations of fit,7, 11,12,14,I5 but an optimal value has not been estabTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
lished. In this study, light finger pressure was applied to seat castings on their dies; the epoxy embedding material did not interfere with seating. In this study, labial marginal gaps were consistently smaller than gaps at other measurement sites for each casting. This result may have been a result of the slight labial tipping caused by the preparation geometry when the casting was seated on its die. Often, when a luting agent is used to cement an FPD, lift-up may occur, which contributes to the gap at the margin. In this study, no luting agent was used. Several factors may account for the poorer fit of onepiece cast compared with soldered FPDs.s l2 One is the difficulty in removing the wax pattern from its die after reflowing the margins, particularly when the abutment preparations are parallel. During removal, distortion of the pattern can occur. A second factor is the effect of the setting expansion of the investment on a sprued wax pattern, which can result in an undersized casting.14 Mesiodistal distortion of the wax pattern can lead to increased marginal gaps. Because a plastic pontic was used in the wax FPD patterns in this study, gap errors may have been introduced as a result of the difference in the coefficient of thermal expansion and the melting range of the two materials. A third factor is nonuniform shrinkage of a larger volume of molten alloy during solidification. Although the fit of one-piece cast FPDs was determined 595
BYRNE
to be poorer than that of the other groups studied, the overall marginal gap was 34.0 pm. This value is lower than marginal gaps for one-piece cast FPDs reported in the literature: 49.1 pm,2 42.0 pm,6 and 54.0 wrn.12 Given the number of steps involved in soldering that can lead to distortion, an important result of this study is that the fit of soldered FPDs and single crowns is statistically the same. In this study, care was taken when soldering to standardize block dimensions, gap configuration, and heating cycle to minimize distortion. Marginal gap width does differ at lingual measurement sites of the central incisor and canine (Table I, group III); however, from a comparison of external and internal sites (Table III, group III), no mesial-distal distortion was determined.
SUMMARY
AND
CONCLUSIONS
The fit of three-unit infrared-soldered FPDs was evaluated and compared with that of FPDs cast in one-piece and single unit castings. Wax FPD patterns were injection molded, and standardized techniques were used to produce castings. Castings were sectioned on their respective dies, and gap openings were measured at predetermined marginal sites. Eased on marginal measurements, the following conclusions were made: 1. Infrared soldering produced FPDs that fit significantly better than cast one-piece FPDs. 2. The fit of infrared-soldered FPDs was comparable to that of single crowns. REFERENCES 1. 2.
3.
I. 8. 0". 10.
Penzer V. Fixed bridges without soldering. J PROSTHETDENT 1953; 3:718-20. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering, 0~casting on one piece. J Dent Res 1977;56:128-34. Fusayama T, Wakumoto S, Hosoda H. Accuracy of fixed partial dentures made by various soldering techniques and one-piece casting. J PR~STWETDENT 1964;14:334-42. Bruce RW. Evaluation of multiple unit castings for fixed partial dentures. J PROSTHETDENT 1964;14:939-43. Rubin JG, Sabella AA. One-piece castings for fixed bridgework. J PROSTHETDEXT 1955;5:843-7. Ziebert GJ, Hurtado A> Glapa C, Schiffleger BE. Accuracy of one-piece castings, preceramic and postceramic soldering. J PROSTHETDENT 1986;55:312-7. Stackhouse JA Jr. Assembly of dental units by soldering. J PROSTHET DENT 1967;18:131-9. Hollenback GM, Shell JS. The accuracy of dental appliances assembled by soldering. J Calif Dent Assoc 1965;41:207-10. Sloan RM, Reisbick MH, Preston JD. Post-ceramic soldering ofvarious alloys. J PROSTHETDENT 1982;48:686-9. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 1st ed. St. Louis: CV Mosby, 1988:438.
596
ET AL
11. Gegauff AG, Rosenstiel SF. The seating of one-piece and soldered fixed partial dentures. J PROSTHETDENT 1989;62:292-7. 12. Schiffeleger BE, Ziebert GJ, Dhuru VB, Brantley WA, Sigaroudi K. Comparison of accuracy of multi-unit one-piece castings. J PROSTHET DENT 1985;54:770-6. 13. Garlapo DA, Lee S, Choung CK, Sorensen SE. Spatial changes occurring in fixed partial dentures made as one piece castings. J PROSTHET DENT 1983;49:781-5. 14. Hinman RW, Tesk JA, Parry EE, Eden GT. Improving the casting accuracy of fixed partial dentures. J PROSTHETDENT 1985;53:466-‘71. 15. Sass FA, Eames WB. Fit of unit-cast fixed partial dentures related to casting ring size, and shape. J PROSTHETDENT 1980;43:163-7. 16. Steinman RR. Warpage produced by soldering with dental solders and gold alloys. J PROSTHETDENT 1954;4:384-95. 17. Pazzini NA, Pazzini LI, deAraujo PA, Lopes ES. The accuracy of soldering investment. J PROSTHETDENT 1979;42:530-3. 18. Moon PC, Eshleman JR, Douglas HB Jr, Garrett SC. Comparison of accuracy of soldering indices for fixed prostheses. J PROSTHETDENT 1978;40:35-8. 19. Monday JJL, Asgar K. Tensile strength comparison of presoldered and postsoldered joints. J PROSTHETDENT 1986;55:23-7. 20. Nicholls JI, Lemm RW. Tensile strength of presoldered and postsoldared joints. J PROSTHETDENT 1985;53:476-82. 21. Willis LM, Nicholls JI. Distortion in dental soldering as affected by gap distance. J PROSTHETDENT 1980;43:272-8. 22. Smith DL, Burnett AP, Gordon TE Jr. Laser welding of gold allovs. J Dent Res 1972;51:161-7. 23. Rasmussen EJ, Goodkind RJ, Gerberich WW. An investigation of tensile strength of dental solder joints. J PROSTHETDENT 1979;41:418-23. 24. Beck DA, Moon PC, Janus CE. A quantitative study of pre-porcelain soldered connector strength with palladium-based porcelain bonding alloys. J PROSTHETDENT 1986;56:301-6. 25. Stade EH, Reisbick MH, Preston JD. Preceramic and postceramic solder joints. J PROSTHETDENT 1974;34:527-32. 26. Staffanou RS, Radke RA, Jendresen MD. Strength properties of soldered joints from various ceramic metal combinations. J PROSTHET DENT 1980;43:31-9. 27. Anusavice KJ, Shafagh I. Inert gas presoldering of nickel-chromium alloys. J PROSTHETDENT 1986;55:317-23. 28. Ullo CA, Lyman S, Shiu A. Soldering index for minimal coverage retainers. J PROSTHETDENT 1984;51:847-8. 29. Saxton PL. Post-soldering of nonprecious alloys. J PROSTHETDENT 1980;43:592-5. 30. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence, 1978~306-7. 31. Harper RJ, Nicholls JI. Distortions in indexing methods and investing media for soldering and remount procedures. J PROSTHETDENT 1979;42:172-9. 32. Renner RP. An introduction to dental anatomy and esthetics. 1st ed. Chicago: Quintessence, 1985:49. 33. Byrne G, Goodacre CJ, Dykema RW, Moore BK. Casting accuracy of high-palladium alloys. J PROSTHETDENT 1986:55:297-301. Reprint requests to: GERARDBYRNE,BDS, MSD DEPARTMENTOFFIXED PROSTHODONTICS LOYOLAUNIVERSITYOF CHICAGO SCHOOLOFDENTISTRY 2160 SOUTHFIRST AVE. MAYWOOD,IL 60153
OCTOBER
1992
VOLUME
68
NUMBER
4
dentures
joined by infrared
soldering
Gerard Byrne, BDS, MSD,aLeon W. Laub, PhD,b Jenq-Yong Hu, BDS, MS,“and Martin F. Land, DDS, MSDd Loyola University of Chicago,School of Dentistry, Chicago,Ill. This study determined the accuracy of fit of three-unit fixed partial dentures joined by an infrared soldering technique compared with one-piece fixed partial denture castings and individually cast crowns. Wax patterns of prepared Ivorine teeth, maxillary left central incisor and maxillary left canine, were injection molded; a plastic rod was used as a pontic. One group of patterns was cast as one-piece dentures; the other group was sectioned in the connector area, cast individually, and then joined by infrared soldering. Castings were seated on their respective dies, embedded in epoxy resin, and sectioned. Gap distances between the casting and the die were measured at specified marginal sites with a profile projector. Results showed that the At of infrared-soldered fixed partial dentures was significantly better than that of one-piece castings and was comparable with the fit of single crowns. The gap openings measured in all castings were within the range of clinical acceptability. (J PROSTRET DENT 1992;68:591-6.)
ental soldering is a traditional method for joining the components of fixed partial dentures (FPDs). Several soldering techniques are available, including a recently introduced infrared technique. An alternative to soldering is to cast the wax components of the FPD in one piece. A general desire for improved efficiency and cost effectiveness has led to a gradual increase in the number of FPDs that are cast as one-piece restorations, thus bypassing soldering procedures. However, soldering is preferred by many operators as the optimal method of joining FPD units. In addition, soldering is sometimes necessary for a specific procedure-for example, the joining of cast gold to metal/ceramic units. LITERATURE
REVIEW
Several investigators1-5 have recommended one-piece casting as superior to soldering for joining FPD units. One-piece casting eliminates the soldering step, maximizes the strength of connectors,6 and produces more accurate fitting prostheses.3 Others7-r1 favor soldering over onepiece casting. They cite improved fit with a soldering technique because of reduced interabutment distortion. Fusayama et al3 compared the accuracy of multiple-unit FPDs made by the one-piece casting technique and various soldering techniques. They concluded that cast FPDs were more accurate than soldered FPDs. However, the average
aAssistantProfessor,Departmentof Fixed Prosthodontics,Loyola University of Chicago. bProfessor,Department of Dental Materials, Loyola University of Chicago. CPrivatepractice, Hsinchu, Taiwan, Republic of China. dAssociateProfessorand Chairman, Department of Fixed Prosthodontics, Loyola University of Chicago. 10/l/36188
THE JOURNAL OF PROSTHETIC DENTISTRY
marginal opening reported for a four-unit FPD was 0.20 mm (200 pm). Bruce4 evaluated multiple-unit castings with a two-abutment expandable die system. He concluded that FPDs up to 15.5 mm in length could be cast with reasonable accuracy. Ziebert et al.6 compared the accuracy of FPDs of varying lengths fabricated as one-piece castings with those joined by soldering. They observed that the fit of all the three-unit FPDs, whether cast or soldered, was similar. Reported mean marginal gap widths ranged from 32 pm for preceramic soldering to 42 Frn for one-piece castings, and these widths increased as span length increased. The distal margin of the posterior abutment and the mesial margin of the anterior abutment had the largest marginal discrepancy. These investigators suggested that FPDs exceeding four units be soldered. Schiffeleger et a1.12 compared the marginal discrepancies of three-, four-, and five-unit one-piece castings and reported mean values for marginal gap widths of 54, 92, and 105 pm, respectively. An important factor thought to cause poor fit of onepiece FPD castings is the inadequate retainer-to-retainer expansion of investment, I1 followed by cooling of the cast structure. The interabutment distortion results in a marginal error that can compromise the simultaneous optimal fit of the retainers. Garlapo et a1.i3 assesseddistortion of cast FPDs, and concluded that a four-unit FPD could be cast in one piece without significant vertical warping. Other investigators”, i2 have reported improved marginal fit after sectioning and soldering the components of cast FPDs. Proposed solutions for minimizing distortion include the use of an all-wax spruing system in preference to plastic sprues,14 the use of larger rather than smaller casting rings,15 and the use of oval-shaped casting ringslz Other technique aspects of soldering-that is, specifying a gap width and an indexing method-are important for
591
BYRNE
Fig. tic.
1. Standardized
FPD wax pattern
with plastic pon-
Fig.
3. Measurement
ET AL
sites at a chamfer margin.
Nicholls31 compared accuracy of the indexing media and concluded that zinc oxide-eugenol (ZOE) bite registration paste was the most accurate. Moon et a1.l’ found that the most accurate results could be obtained with a plaster nonremoval technique. This study evaluated the fit of infrared-soldered FPDs versus one-piece cast FPDs.
MATERIAL
Fig.
2. Measurement
sites at a shoulder margin.
successful soldering. The width of the gap between the two pieces to be joined is probably the most controversial parameter in soldering. It is considered to affect the distortion of the system as well as the strength of the joint. Suggested gap distances range from tight contact,16 0.1 rnrn,17, ls 0.15 rnrn,ls21 0.2 mm,22 0.25 mm,23,24 0.3 mm,25-2g to 0.5 mm.g, l2 Shillingburg et a1.30 reported that the opposing surfaces on either side of the solder joint should be parallel to each other; there was more likelihood of distortion if the space between units was not of uniform width. Harper and
592
AND
METHODS
Standard metal/ceramic preparations, with 1.0 mm facial shoulder and 0.3 mm lingual chamfer, were cut on Ivorine (Columbia Dentoform Corp., New York, N. Y.) teeth, the maxillary left central incisor and maxillary left canine. The teeth were attached to an acrylic base, with a coronal separation of 6.5 mm32 representing the mesiodistal width of the maxillary left lateral incisor. Eleven replicas were produced with a silicone mold (RTV 630, General Electric Co., Waterford, N.Y.) and a high strength dental stone (Silky Rock, Whip-Mix Corp., Louisville, KY.). Standardized FPD wax patterns were fabricated using an injection molding technique and a hard inlay wax (Casting Wax, Whip-Mix Corp.). A plastic sprue bar (No. 10 gauge) was used instead of a wax pontic in FPD specimens to ensure a standard pontic size and configuration (Fig. 1). This procedure greatly simplified the creation of a standardized soldering area. A stone jig was fabricated on a surveyor for the purpose of sectioning the plastic bar at a position 0.5 mm distal from the central incisor with a 0.15 mm thick diamond disk. This technique produced identical soldering gaps. Three groups of patterns were fabricated. Group I consisted of two individual retainers, a maxillary left central incisor and canine, and was used as a control for casting fit; no pontic was used. Group II consisted of five FPDs
OCTOBER
1992
VOLUME
68
NUMBER
4
FPDS
JOINED
BY INFRARED
SOLDERING
I. Marginal gap measurements (in microns) at each tooth site
Table
Group MLCI
Mesial Mean SD
Distal Mean SD Labial Mean SD
Lingual Mean SD
Group
I* MLC
IIT
MLCI
MLC
Group
Table III. Marginal gap measurements (in microns) at external and internal sites Group
III?
MLCI
1*
IIt
111t
14.8 1.8
65.2 17.2
18.9 5.1
14.0 2.1
24.1 13.7
18.8 6.3
MLC
13.5 1.3
13.5 2.6
70.1 13.4
27.8 16.0
17.2 5.3
16.3 4.2
14.5 1.7
16.0 1.4
20.3 10.0
60.2 19.3
21.4 7.2
20.6 4.4
10.5 1.7
6.5 1.3
19.2 3.0
22.0 9.9
16.0 4.2
15.4 6.1
15.5 2.6
17.3 2.2
29.8 8.1
23.1 1.2
27.1 7.0
19.0 6.3
External Mean SD
Internal Mean SD *Mean
of eight measurements.
fMean of 40 measurements.
MUX, Maxillary left central incisor; MLC, maxillary left canine. *Mean of four measurements. tMean of 20 measurements.
Table II. Combined marginal gap measurements (in microns) for each tooth Group 1*
Maxillary
III?
34.8 22.9
20.4 7.4
left central incisor
Mean 13.5 SD 2.6 Maxillary left canine Mean 13.3 SD 4.6 Maxillary Mean SD
IIt
33.3 17.8 20.9 5.8 left central incisor and maxillary left canine 13.4 34.0 19.1 3.7 21.9 6.7
*Mean of 16 measurements for each tooth. TMean of 80 measurements for each tooth.
prepared as one-piece castings; a plastic sprue simulated a pontic. Group III consisted of five FPDs, in which the simulated pontic was sectioned and the retainers were prepared individually. The marginal 3 mm region of the wax patterns was readapted and the patterns were invested in a phosphatebonded investment (Hi-Temp 2, Whip-Mix Corp.). A powder-liquid ratio was previously determined33 to produce consistent, adequately expanded castings that fit passively on the dies, exhibiting neither excessive looseness nor tightness. After staged burnout to 1400° F, castings were made with a high-palladium content alloy (Option, J.M. Ney Co., Bloomfield, Conn.), melted with a natural gas-oxygen mixture. Castings were cleaned in hydrofluoric acid, examined under a binocular microscope at 20 power magnification for small nodules, and if satisfactory, were seated on their respective dies. Individual components of group III
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4. Location of gap sites between casting and die around the maxillary left canine. A, Labial; B, mesial; C, lingual; and D, distal.
Fig.
were positioned on their respective dies and were placed on the surveyor used to section the simulated pontic. The diamond disk used initially to create the gap distance was passed through the joint area again to ensure a standardized gap width of 0.15 mm. Occlusal indices were fabricated using the method of Moon et al.rs Components were soldered using an infrared apparatus (J.M. Ney Co.), following the manufacturer’s instructions. All castings were then seated on their original dies and were embedded in epoxy resin. Each FPD was sectioned, one cut through each retainer labiolingually and one cut mesiodistally through both retainers. Each FPD was then metallurgically polished to a high luster with 5 pm aluminum oxide slurry on a felt pad. The gap between the casting and die on the maxillary left central incisor and the maxillary left canine was measured at predetermined marginal sites using a profile projector (V-12, Nikon, Inc., Garden City, N.Y.). Measurements were taken at 150, 300, 450, and 600 ,um from the margin for shoulder preparations (Fig. 2). Similar
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Fig. 5. Marginal gaps between casting and die at sites around the maxillary left canine of a one-piece cast FPD. A, Labial; B, mesial; C, lingual; D, distal. (Original magnification x50.)
measurements were made at 50,100,150, and 200 pm from the margin for chamfer preparations (Fig. 3). Four measurements were made by one investigator at each mesial, distal, labial, and ligual site on the central incisor and canine. Group I (two individual retainers) consisted of four measurements at each site on the central incisor and canine; groups II and III (five FPDs, respectively) consisted of 20 measurements at each site on the central incisor and canine. Marginal gaps at each measurement site on the central incisor and canine were viewed on a scanning electron microscope (SEM) at 50 power magnification.
RESULTS Marginal gap openings at each measurement site-mesial, distal, labial, and lingual-are reported for group I (individual cast crowns), group II (one-piece cast FPDs), and group III (infrared soldered FPDs) in Table I. One-way analysis of variance was used to determine whether differences in test results were significant. Tukey’s Student range (HSD) test was applied to detect differences among means. Marginal gaps in group II were significantly larger (p I 0.05) than in group III for the central incisor at mesial (70.1 versus 17.2 pm) and labial (19.2 versus 16.0 pm) sites, and for the canine at mesial(27.8 versus 16.3 pm), distal (60.2 versus 20.6 pm), and labial (22.0 versus 15.4 pm) sites. 594
Within group II significantly different marginal gap openings 0, 5 0.05) were determined between the central incisor and canine at mesial, distal, and lingual locations. Within group III, marginal gap openings at the lingual of the central incisor were significantly larger than those at the lingual of the canine. All marginal measurements taken on the central incisor and canine at mesial, distal, labial, and lingual locations were combined for each group. The overall marginal gap opening for each group was: Group I (n = 32): 13.4 + 3.7 Km; group II (n = 160): 34.0 I: 21.9 pm; group III (n = 160): 19.1 1? 6.7 pm (see Table II). The gap opening for group II was significantly larger (p I 0.05) than that of either group I or group III. The largest sample-to-sample difference in marginal gap opening occurred among group II castings, as indicated by the large standard deviation. There was no significant difference between groups I and III. As an indicator of distortion during fabrication of threeunit FPDs, marginal gap openings were determined for external and internal sites. External marginal sites were the sum of the mesial gap of the central incisor and the distal gap of the canine. Internal marginal sites were the sum of the distal gap of the central incisor and the mesial gap of the canine. At external sites the marginal gap opening for group II (65.2 pm) was significantly larger (p 5 0.05) than that of either group I (14.8 pm) or group III (18.9 pm) (see OCTOBER
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FPDS
JOINED
BY INFRARED
SOLDERING
Fig. 6. Marginal gaps between casting and die at sites around the maxillary left canine of a soldered FPD. A, Labial; B, mesial; C, lingual; D, distal. (Original magnification x50.)
Table III). No significant difference was determined at external sites between groups I and III. At internal sites, the marginal gap opening for group II (24.1 +m) was significantly larger (p 5 0.05) than that for group I (14.0 pm). No significant difference was determined at internal sites between groups I and III, or between groups II and III. External and internal marginal gaps were compared within each group. Significant differences (p 5 0.05) were only determined within group II. Marginal gaps at labial, mesial, lingual, and distal aspects on sectioned specimens of the maxillary left canine are shown in Figs. 4 through 6 for seated one-piece cast and soldered FPDs. In these SEM microphotographs (50 power magnification), no overhanging or short margins were seen; the fit of one-piece cast and soldered FPDs appeared to be clinically acceptable. DISCUSSION The fit of one-piece cast FPDs has been shown in this study to be poorer than that of either infrared soldered FPDs or single crowns. Fit invol.ves the ability to seat an appliance completely on its die. Variables such as seating force, preparation geometry, and use of a luting agent will affect fit. A large range of seating forces has been used in previous investigations of fit,7, 11,12,14,I5 but an optimal value has not been estabTHE
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lished. In this study, light finger pressure was applied to seat castings on their dies; the epoxy embedding material did not interfere with seating. In this study, labial marginal gaps were consistently smaller than gaps at other measurement sites for each casting. This result may have been a result of the slight labial tipping caused by the preparation geometry when the casting was seated on its die. Often, when a luting agent is used to cement an FPD, lift-up may occur, which contributes to the gap at the margin. In this study, no luting agent was used. Several factors may account for the poorer fit of onepiece cast compared with soldered FPDs.s l2 One is the difficulty in removing the wax pattern from its die after reflowing the margins, particularly when the abutment preparations are parallel. During removal, distortion of the pattern can occur. A second factor is the effect of the setting expansion of the investment on a sprued wax pattern, which can result in an undersized casting.14 Mesiodistal distortion of the wax pattern can lead to increased marginal gaps. Because a plastic pontic was used in the wax FPD patterns in this study, gap errors may have been introduced as a result of the difference in the coefficient of thermal expansion and the melting range of the two materials. A third factor is nonuniform shrinkage of a larger volume of molten alloy during solidification. Although the fit of one-piece cast FPDs was determined 595
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to be poorer than that of the other groups studied, the overall marginal gap was 34.0 pm. This value is lower than marginal gaps for one-piece cast FPDs reported in the literature: 49.1 pm,2 42.0 pm,6 and 54.0 wrn.12 Given the number of steps involved in soldering that can lead to distortion, an important result of this study is that the fit of soldered FPDs and single crowns is statistically the same. In this study, care was taken when soldering to standardize block dimensions, gap configuration, and heating cycle to minimize distortion. Marginal gap width does differ at lingual measurement sites of the central incisor and canine (Table I, group III); however, from a comparison of external and internal sites (Table III, group III), no mesial-distal distortion was determined.
SUMMARY
AND
CONCLUSIONS
The fit of three-unit infrared-soldered FPDs was evaluated and compared with that of FPDs cast in one-piece and single unit castings. Wax FPD patterns were injection molded, and standardized techniques were used to produce castings. Castings were sectioned on their respective dies, and gap openings were measured at predetermined marginal sites. Eased on marginal measurements, the following conclusions were made: 1. Infrared soldering produced FPDs that fit significantly better than cast one-piece FPDs. 2. The fit of infrared-soldered FPDs was comparable to that of single crowns. REFERENCES 1. 2.
3.
I. 8. 0". 10.
Penzer V. Fixed bridges without soldering. J PROSTHETDENT 1953; 3:718-20. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering, 0~casting on one piece. J Dent Res 1977;56:128-34. Fusayama T, Wakumoto S, Hosoda H. Accuracy of fixed partial dentures made by various soldering techniques and one-piece casting. J PR~STWETDENT 1964;14:334-42. Bruce RW. Evaluation of multiple unit castings for fixed partial dentures. J PROSTHETDENT 1964;14:939-43. Rubin JG, Sabella AA. One-piece castings for fixed bridgework. J PROSTHETDEXT 1955;5:843-7. Ziebert GJ, Hurtado A> Glapa C, Schiffleger BE. Accuracy of one-piece castings, preceramic and postceramic soldering. J PROSTHETDENT 1986;55:312-7. Stackhouse JA Jr. Assembly of dental units by soldering. J PROSTHET DENT 1967;18:131-9. Hollenback GM, Shell JS. The accuracy of dental appliances assembled by soldering. J Calif Dent Assoc 1965;41:207-10. Sloan RM, Reisbick MH, Preston JD. Post-ceramic soldering ofvarious alloys. J PROSTHETDENT 1982;48:686-9. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 1st ed. St. Louis: CV Mosby, 1988:438.
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11. Gegauff AG, Rosenstiel SF. The seating of one-piece and soldered fixed partial dentures. J PROSTHETDENT 1989;62:292-7. 12. Schiffeleger BE, Ziebert GJ, Dhuru VB, Brantley WA, Sigaroudi K. Comparison of accuracy of multi-unit one-piece castings. J PROSTHET DENT 1985;54:770-6. 13. Garlapo DA, Lee S, Choung CK, Sorensen SE. Spatial changes occurring in fixed partial dentures made as one piece castings. J PROSTHET DENT 1983;49:781-5. 14. Hinman RW, Tesk JA, Parry EE, Eden GT. Improving the casting accuracy of fixed partial dentures. J PROSTHETDENT 1985;53:466-‘71. 15. Sass FA, Eames WB. Fit of unit-cast fixed partial dentures related to casting ring size, and shape. J PROSTHETDENT 1980;43:163-7. 16. Steinman RR. Warpage produced by soldering with dental solders and gold alloys. J PROSTHETDENT 1954;4:384-95. 17. Pazzini NA, Pazzini LI, deAraujo PA, Lopes ES. The accuracy of soldering investment. J PROSTHETDENT 1979;42:530-3. 18. Moon PC, Eshleman JR, Douglas HB Jr, Garrett SC. Comparison of accuracy of soldering indices for fixed prostheses. J PROSTHETDENT 1978;40:35-8. 19. Monday JJL, Asgar K. Tensile strength comparison of presoldered and postsoldered joints. J PROSTHETDENT 1986;55:23-7. 20. Nicholls JI, Lemm RW. Tensile strength of presoldered and postsoldared joints. J PROSTHETDENT 1985;53:476-82. 21. Willis LM, Nicholls JI. Distortion in dental soldering as affected by gap distance. J PROSTHETDENT 1980;43:272-8. 22. Smith DL, Burnett AP, Gordon TE Jr. Laser welding of gold allovs. J Dent Res 1972;51:161-7. 23. Rasmussen EJ, Goodkind RJ, Gerberich WW. An investigation of tensile strength of dental solder joints. J PROSTHETDENT 1979;41:418-23. 24. Beck DA, Moon PC, Janus CE. A quantitative study of pre-porcelain soldered connector strength with palladium-based porcelain bonding alloys. J PROSTHETDENT 1986;56:301-6. 25. Stade EH, Reisbick MH, Preston JD. Preceramic and postceramic solder joints. J PROSTHETDENT 1974;34:527-32. 26. Staffanou RS, Radke RA, Jendresen MD. Strength properties of soldered joints from various ceramic metal combinations. J PROSTHET DENT 1980;43:31-9. 27. Anusavice KJ, Shafagh I. Inert gas presoldering of nickel-chromium alloys. J PROSTHETDENT 1986;55:317-23. 28. Ullo CA, Lyman S, Shiu A. Soldering index for minimal coverage retainers. J PROSTHETDENT 1984;51:847-8. 29. Saxton PL. Post-soldering of nonprecious alloys. J PROSTHETDENT 1980;43:592-5. 30. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence, 1978~306-7. 31. Harper RJ, Nicholls JI. Distortions in indexing methods and investing media for soldering and remount procedures. J PROSTHETDENT 1979;42:172-9. 32. Renner RP. An introduction to dental anatomy and esthetics. 1st ed. Chicago: Quintessence, 1985:49. 33. Byrne G, Goodacre CJ, Dykema RW, Moore BK. Casting accuracy of high-palladium alloys. J PROSTHETDENT 1986:55:297-301. Reprint requests to: GERARDBYRNE,BDS, MSD DEPARTMENTOFFIXED PROSTHODONTICS LOYOLAUNIVERSITYOF CHICAGO SCHOOLOFDENTISTRY 2160 SOUTHFIRST AVE. MAYWOOD,IL 60153
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