High Palladium Alloy

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Effects of three soldering techniques on the strength of high-palladium alloy solder joints Marisol Chaves, DDS, MS,a Stanley G. Vermilyea, DMD, MS,b Efstratios Papazoglou, DDS, MS,c and William A. Brantley, PhDd College of Dentistry, The Ohio State University, Columbus, Ohio Statement of problem. Little information is available on the optimum technique for soldering highpalladium alloys, which have gained considerable popularity for prosthodontic applications.

Purpose. The objective of this study was to compare the flexural stress at the proportional limit of four noble dental alloy specimens soldered with torch, oven, and infrared techniques.

Materials and methods. The high-palladium alloys studied were Legacy XT (Jelenko), Freedom Plus (Jelenko), and IS 85 (Williams/Ivoclar). A gold-palladium alloy, Olympia (Jelenko), served as the control. Thirty round bars, 18 × 3 mm, were cast from each alloy, cut in half, aligned, and joined using Olympia Pre solder (Jelenko) for the gas-oxygen torch and the infrared technique and Alboro LF solder (Jelenko) for the oven technique. Each soldered bar was subjected to three-point bending, and the maximum elastic stress or strength of the solder joint was calculated at the proportional limit. Data were analyzed by twoway ANOVA and the Ryan-Einot-Gabriel-Welsch (REGW) multiple range test at the 0.05 level of significance. Results. There was no significant difference between torch and oven-soldering, but both were significantly different from the infrared technique. ANOVA showed a significant difference between alloys, but this difference could not be detected with the REGW test. SEM examination of the fracture surfaces revealed grooves associated with the path of crack propagation. X-ray energy-dispersive spectroscopic analysis failed to detect copper in the solders, and there were no significant changes in the solder compositions after the melting procedures. Conclusions. All three techniques can yield satisfactory solder joints in high-palladium alloys. These joints should be well-polished to achieve optimal strength. (J Prosthet Dent 1998;79:677-84.)

CLINICAL IMPLICATIONS The results of this investigation indicate that the strengths of high-palladium alloy joints properly prepared by the torch, oven, and infrared soldering techniques are acceptable for clinical use and have generally comparable values. The current results also demonstrate the importance of having solder joints that are properly polished. The apparent notch-sensitivity of solder joints in high-palladium alloy castings that contain finishing grooves suggests that there should be some concern when cyclic loads are applied over a clinically appropriate period of time.

This article is based on a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Graduate School of The Ohio State University, 1997. Presented at the AADR meeting in Orlando, Florida as a finalist for the Arthur R. Frechette award competition, March 1997. Presented at the Carl O. Boucher Conference, April 1997. Supported by the Greater New York Academy of Prosthodontics and by NIDR Research Grant DE10147. a Senior Resident, Advanced Education Program in Prosthodontics. b Associate Professor and Chair, Section of Primary Care. c Clinical Assistant Professor, Section of Restorative Dentistry, Prosthodontics and Endodontics; and Doctoral Graduate Student in Oral Biology. d Professor, Section of Restorative Dentistry, Prosthodontics and Endodontics; and Director, Graduate Program in Dental Materials. JUNE 1998

H

igh-palladium alloys have become popular for the fabrication of metal ceramic restorations and implant superstructures. Among the reasons for the great interest in these alloys are their cost, excellent mechanical properties, and good bonding with dental porcelain.1 The alloys and the investments used to fabricate cast restorations are selected carefully to compensate for dimensional changes that occur during the fabrication process. For three-unit prostheses, the marginal adaptation, described by Ziebert et al.,2 was the same for soldered restorations or one-piece castings. However, Gegauff and Rosenstiel3 found that for three-unit fixed partial denTHE JOURNAL OF PROSTHETIC DENTISTRY

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Table I. Nominal compositions (wt%) of alloys* Alloy

Pd

Cu

Ga

In

Au

Ag

Ru

Freedom Plus** Legacy XT** IS 85*** Olympia**

78 75.5 82 38.5

8.0 — — —

5.0 6.0 6.0 1.5

6.0 6.0 3.5 8.5

2.0 2.0 2.5 51.5

— 10.0 2.5 —

<1.0 <1.0 — <1.0

Sn

— — 3.5 —

*Values were obtained from the product information literature. **J.F. Jelenko and Co., Armonk, N.Y. ***Williams Division/Ivoclar North America, Amherst, N.Y.

tures (FPDs), the best adaptation was produced after soldering. Several studies have shown that, as the length of the prosthesis increases or involves the curvature of the arch, the potential for a poorly fitting prosthesis increases.4-6 With the recent development of implant-retained prostheses, which may often be long and may need the use of increased amount of casting alloy, fabricating a one-piece casting incorporating machined parts and having these machined surfaces accurately and passively fit the implant superstructure becomes even more difficult. As a result, the practitioner must have an effective method of compensating for poorly fitting castings. Soldering provides a means of improving the fit of a prosthesis and, at the same time, joining the components rigidly.7 The strength of solder joints is of utmost importance for clinical success of the prostheses. Many studies have reported measurements of tensile strength for solder joints, with mixed results. Some investigators have found that preceramic solder joints are stronger than postceramic solder joints.8 Monday and Asgar9 and Lorenzana et al.10 reported that there was no significant difference in the ultimate tensile strength of postceramic and preceramic solder joints. However, other authors11-13 indicated that the ultimate tensile strength of postceramic solder joints was greater than that of preceramic solder joints. Although tensile tests are useful in the evaluation of soldering effectiveness, dental prostheses are subjected mainly to flexural loading during clinical use.14 A popular method to obtain the flexural strength of a material is three-point bending, where a concentrated load is applied to the center of a uniform beam that is supported near each end. The flexural strength is often termed the modulus of rupture or transverse strength. In this study, torch, oven, and infrared soldering techniques were compared for three high-palladium dental alloys to determine the best method to join these metals. A well-known gold-palladium alloy served as a control for the experiments. A literature review indicated that there are no published studies comparing these soldering techniques for the high-palladium alloys.

MATERIAL AND METHODS The specimens were fabricated from polystyrene plastic patterns used for the fabrication of tensile test speci678

mens according to ANSI/ADA Specification No. 5.15 The ends of the plastic patterns were cut with a diamond disk, leaving round bars 3 mm in diameter and 20 mm in length. The specimens were then sprued with wax (10 bars per casting ring) and invested with a carbon-free phosphate-bonded investment (High Span II, J.F. Jelenko and Co., Armonk, N.Y.). The manufacturer’s recommendations were followed for the special liquidto-powder ratio and burnout schedule for the investment. The high-palladium alloys used to cast the specimens were Freedom Plus, Legacy XT (Jelenko) and IS 85 (Williams/Ivoclar, Amherst, N.Y.). A gold-palladium alloy (Olympia, Jelenko) served as the control. The alloy compositions (wt%) are provided in Table I. Melting was performed with a multiorifice gas-oxygen torch, and the alloys were cast with a broken arm centrifugal casting machine. The castings were permitted to bench cool before devestment. After air abrasion with 50 µm aluminum oxide, castings were numbered at each end and randomly assigned to treatment groups. Specimens were then sectioned at their midpoint with a low-speed diamond saw (VR/50, Leco Corp., St. Joseph, Mich.) and water coolant. After sectioning, the bars were ultrasonically cleaned with acetone and dried. Corresponding halves of each casting were placed in a lathe for proper alignment. A thickness gauge was used to provide a 0.5 mm gap. Autopolymerizing resin (GC Corporation, Tokyo, Japan) was used to unite the halves of each bar. Each bar was then invested in a silicone mold (Coltene, Whaledent Inc., Mahwah, N.J.) with HiHeat soldering investment (Whip Mix, Louisville, Ky.). The area to be soldered was left exposed during the investing procedure, and an airway was created under the soldering area to provide uniform heating of the joint. The invested blocks were preheated according to the alloy manufacturer’s instructions or those of the soldering equipment manufacturer (infrared technique). Olympia Pre Solder (Jelenko) was chosen for the torch and infrared-soldering groups, and Alboro LF solder (Jelenko) for the oven technique groups. For the torch-soldering, the investment blocks were preheated to 1100° F, and a gas-oxygen torch with a No. 2 orifice point was kept moving at an oblique angle to the preheated investment surface. A strip of approxiVOLUME 79 NUMBER 6

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mately 5 mm of solder was placed over the joint, and the solder was allowed to flow through the gap. The oven-soldering experimental groups were subjected to the Vita Omega cycle (Vident, Baldwin Park, Calif.) under vacuum for porcelain application (two opaque bakes, two body bakes and a low glaze bake), before the bar was cut with the diamond saw. After the blocks were preheated in the oven, a 5 mm strip of fluxed solder was placed in contact with the parts to be soldered. Then the temperature in the oven was raised to the melting temperature of the solder, allowing it to flow through the joint space. The soldering procedure was performed using the vacuum available with the porcelain furnace. For the infrared-soldering groups, the preheated blocks were placed on the platform of the infrared soldering machine (Ney Infrared Unit, J.M. Ney Co., Bloomfield, Conn.). The invested specimen and the solder strip were fluxed, and this solder strip was placed in contact with the parts to be soldered. The joint area was placed 1 mm below the alignment rod, and the infrared lamp was activated and held at maximum level until the solder flowed. The soldered specimens were permitted to bench cool before devesting. The specimens were cleaned of residual investment, and the solder joints were finished flush with the remainder of the bar. Mounted stones and rubber wheels were used with a low-speed dental handpiece, and the bars and the handpiece were attached to the lathe. The diameter of each soldered joint was measured, and external defects for each specimen were noted. The specimens were subjected to a three-point flexural load in a screw-driven mechanical testing machine (model 4204, Instron Corp., Canton, Mass.) at a crosshead speed of 0.25 mm per minute. The load and position of the crosshead were recorded by means of a Pentium personal computer, with the software program Labview for Windows (National Instruments, Austin, Texas) and a data acquisition card with a frequency of 1 Hz. The raw data from each test bar were stored in the computer as a text file. The software program Excel (Microsoft Corporation, Redmond, Wash.) was used to plot and manipulate the data. The midspan flexural stress was calculated at the proportional limit (just before the point of initial nonlinear deformation as determined from a graphic representation of load versus crosshead movement) or at the point of failure, whichever was lower. The equation used for the calculation of the maximum elastic flexural stress (σmax) developed at the midpoint of a centrally loaded round beam was as follows: σmax = 8Fl/πd3 where F = load, l = distance between support points (test span length of 12.5 mm) and d = diameter. This equation was developed from the well-known elastic flexure formula σmax = Mc/I, where M is the maximum bendJUNE 1998

ing moment (Fl/4 for three-point bending), I is the moment of inertia (πd4/64 for a circular cross-section), and c is the radius of the bar.16 Ten test specimens were prepared for each combination of alloy and soldering technique for a total of 120 specimens. A power analysis17 performed on the experimental data, assuming α = 0.05 and a power of 80% (β = 0.20), indicated that a sample size of 10 would show significant differences of 185 MPa in the mean proportional limit or fracture stress between sample groups. Bartlett’s test17 was performed to determine the homogeneity of the variances for all of the specimen groups, and the mean values of flexural stress (proportional limit or fracture stress) values for all the groups were ranked and subjected to two-way analysis of variance (ANOVA) to examine the overall effects of alloy and soldering technique, as well as their interactions. The mean values for the specimen groups were ranked, and two-way ANOVA on the ranks18,19 showed that there were highly significant differences between techniques. The Ryan-Einot-Gabriel-Welsch (REGW) multiple range test20 with a significance level of α = 0.05 was used on the ranks to determine which specific specimen groups were significantly different from each other. The REGW test is considered to have less likelihood of Type II statistical errors than the well-known Tukey multiple range test. In addition, representative fracture surfaces were randomly selected from each group and examined with a SEM (JSM-820, Jeol Ltd., Tokyo, Japan). The two solders were also melted individually using the three techniques, embedded in epoxy resin, ground with 400 and 600 grit metallographic paper, and polished with a series of alumina slurries (from 15 to 0.05 µm particle size). Quantitative elemental composition information was provided by x-ray energy-dispersive spectroscopic analyses (EDS), using a Link eXL microanalysis system with a PentaFET detector and an ultrathin window (Oxford Instruments Group, High Wycombe, England) coupled to the SEM. A similar procedure was performed for the asreceived unmelted solder, and the changes in the compositions of the solders after the melting procedures were determined.

RESULTS The mean values with standard deviations of the proportional limit for ductile solder joints or the fracture strength of brittle solder joints for the various specimen groups are summarized in Figure 1. Bartlett’s test showed that the variances were significantly different from each other. For this reason, the technique of ranking the observations was used. Two-way ANOVA on the ranks showed that there were highly significant differences between techniques and a significant difference between alloys (Table II). There was no significant interaction between the alloys and soldering techniques. The sensitive REGW multiple range test showed that, even though 679

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Fig. 1. Values of proportional limit or fracture strength for soldered joints of high-palladium alloys obtained with three-point bending test.

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Fig. 3. SEM photomicrograph of fracture surface of IS 85 alloy for oven-soldered specimen shows ductile fracture characteristics of this alloy. (Bar = 10 µm.)

Fig. 4. SEM photomicrograph of torch-soldered Olympia specimen. (Bar = 1 mm.) Fig. 2. SEM photomicrograph of fracture surface for oven-soldered IS 85 specimen. (Bar = 1 µm.)

there were differences in the mean values for the four alloys that ANOVA (Table II) indicated were significant (p = 0.048), these differences were not statistically significant when the results for the three soldering techniques were pooled (Table III). The REGW test was also used to analyze differences between the soldering techniques used in this study where the results for the four alloys were pooled. It showed no statistically significant differences between the torch-soldering and oven-soldering (Table IV). However, these two soldering techniques were significantly different from the infrared-soldering technique. No specimen was rejected from the statistical analysis after being fractured, regardless of the amount of porosity. 680

The specimens soldered with the oven technique separated completely after being subjected to the three-point bending test. No sign of bending or lines of fracture were visually evident. The fracture path appeared to be completely flat, giving the impression that an adhesive fracture had occurred between the parent metal and solder. Flux was used on the oven-soldered specimens (as recommended by the manufacturer) and on the infrared-soldered specimens because of the impossibility for the solder to flow without the flux. The results showed that the flux did not perceptibly affect the solderability of the alloys, but further quantitative analysis of the effect of the flux would be useful. Figure 2 is a photomicrograph of the fracture surface for an oven-soldered IS 85 specimen. SEM examination revealed that the specimen fractured cohesively through the solder, even though visual examination suggested VOLUME 79 NUMBER 6

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Table II. Two-way ANOVA results based on ranking the observations Source

Alloy Technique Alloy*technique Error

DF

ANOVA SS

Mean square

3 2 6 108

7784.65 22231.95 10776.10 103192.80

2594.88333 11115.97500 1796.01667 955.48889

that separation was between the solder and alloy. An intergranular fracture surface can be seen as well, with evident separation between some grains. This fracture surface was characteristic of the solder. There was no evidence of microvoids or the dimpled rupture structure indicative of ductile fracture.21 Oven-soldered Olympia alloy specimens were also randomly selected after fracture and observed at the same range of magnifications used for the IS 85 specimens. The fracture surfaces for these specimens were similar to Figure 2, which would be expected because the same solder was used (Alboro LF), and the fracture mode was again cohesive through the solder. An area near the edge of an oven-soldered IS 85 specimen is depicted in Figure 3. This photomicrograph indicates that cohesive ductile fracture also occurred through the IS 85 alloy, because of the characteristic dimpled rupture appearance resulting from the microvoids. Previously, Stewart et al.22 found that this alloy exhibited excellent ductility, as reported by the manufacturer. SEM photomicrographs of fractured solder joints for Legacy XT and Freedom Plus specimens that had been oven-soldered showed characteristics similar to those of the fractured solder joints for IS 85, which follows from the cohesive nature of the fracture process through the same low-fusing solder. The fracture of the torch-soldered and infrared-soldered specimens visually appeared to have initiated at the tension side and propagated through the solder. Lowpower SEM examination (Fig. 4) confirmed this observation. For some of these specimens, fracture also occurred through the parent metal, but no separation of the specimen halves was observed. In contrast, the ovensoldered specimen halves of the alloys separated completely during testing. The fracture surface for a torch-soldered IS 85 specimen is depicted in Figure 5. Substantial porosity and a rounded structure for the solder below a void are evident. Parallel grooves on the surface resulted from the experimental procedure of holding each soldered specimen in the lathe and polishing with stone followed by rubber wheel (with a handpiece). An association of the fracture boundary with the polishing grooves is apparent. This same pattern was observed on a representative fracture surface of an infrared-soldered Legacy XT specimen. Again, there was a close association between the polishing grooves and the path of fracture in the solder. JUNE 1998

F value

Prob. > F

2.72 11.63 1.88 —

0.04833 0.00003 0.09078 —

Table III. Ryan-Einot-Gabriel-Welsch test results for the alloys* Alloy

Freedom Plus Legacy XT IS 85 Olympia

Number of specimens

30 30 30 30

Mean (MPa) REGW grouping

902.40 834.70 803.44 800.64

A A A A

*Alloys with the same letter are not statistically different at the α =0.05 level.

Table IV. Ryan-Einot-Gabriel-Welsch test results for the soldering techniques* Technique

Torch Oven Infrared

Number of specimens

40 40 40

Mean (MPa)

REGW grouping

881.62 868.79 755.48

A A B

*Techniques with the same letter are not statistically different at the α = 0.05 level.

A low-magnification photomicrograph of an infraredsoldered specimen of Olympia after fracture is shown in Figure 6. This photomicrograph shows that the principal path of fracture was in the alloy below the solder (white band in the center of the photomicrograph), i.e., cohesive fracture through the alloy. Numerous voids in the solder are also visible in Figure 6. In the low-magnification photomicrograph (Fig. 4) of a fractured Olympia specimen that had been torch soldered, an indentation in the solder where the bending load was applied can be seen (center left edge of photomicrograph). The width of the solder joint appears to be extended in a vertical direction near the right edge of the photomicrograph (toward the tension side in bending), due in part to crack propagation. However, the indentation and this tapering suggest some ductility of the solder. These observations were also made for another fractured specimen that had been soldered by the infrared technique. X-ray EDS analyses were performed on the two solders to determine their compositions. It was found that there was no copper in the composition of either solder. The compositions of the solders before melting and after the melting procedures with the three techniques are summarized in Table V. Only slight changes were found in the compositions of the solders after the melting procedures. 681

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Fig. 5. SEM photomicrograph of torch-soldered IS 85 specimen demonstrates solder porosity. (Bar = 10 µm.)

Fig. 6. SEM photomicrograph of fracture path through parent metal of infrared-soldered Olympia specimen. (Bar = 100 µm.)

Table V. Energy-dispersive spectroscopic analysis (wt.%) for composition of solders Solder

Alboro LF* Olympia Pre* Alboro LF (oven)** Olympia Pre (infrared)** Olympia Pre (torch)**

Au

62.9 70.5 62.3 72.1 70.9

(0.1) (0.3) (0.6) (0.1) (0.8)

Ag

23.2 16.5 24.1 16.7 16.7

(0.5) (0.5) (0.6) (0.4) (0.5)

Zn

Pd

13.5 (0.7) 0.9 (0.2) 12.1 (0.5) — —

— 10.9 (0.4) 0.9 (0.4) 10.3 (0.6) 10.3 (0.2)

Sn

0.4 1.1 0.6 0.9 1.3

(0.1) (0.1) (0.2) (0.1) (0.4)

*Solder as received from the manufacturer. **Solder after melting by the indicated technique. Entries are mean values with standard deviations in parentheses. Five analyses were performed on each type of solder and condition.

DISCUSSION The development of the strength for a solder joint involves melting, flowing, and wetting of the solder by capillary forces between the parent metal and the solder.23 All three techniques used in this study were considered to be acceptable for soldering the three representative high-palladium alloys. It is important to consider the expertise of the operator for achieving solder joints of similar characteristics. The problems that have been reported with the commonly used gas-oxygen torch for soldering metal ceramic alloys are gas inclusions, voids, improper melting of the solder, and excessive oxidation of the pieces to be soldered.24 The main advantages with this technique are the availability of the gasoxygen torch in the dental laboratories and the flexibility of this method for use with the preceramic and postceramic solders. It was initially expected that the torch soldering would exhibit the greatest variation in results because of the relatively poor control in the temperature during the soldering procedure. However, this result was not always found. For example, the flexural strength of the Olympia specimens that were torch soldered had one of the smaller standard deviations of all the groups (Fig. 1). 682

It has been previously noted that a power analysis showed that the sample size of 10 used in this study would be able to detect a significant difference (α = 0.05) between specimen groups of approximately 185 MPa with a power of 80% (β = 0.20). This difference was approximately 28% greater than the pooled standard deviation of 144 MPa for all 12 specimen groups. For example, sample sizes of 15 and 41 would have been required to show significant differences of 150 MPa and 90 MPa, respectively, at the same α and β levels, and would have necessitated much larger numbers of 180 and 492 soldered specimens for the entire study. Although a larger sample size might have revealed statistically significant differences between some additional sample groups compared with the results obtained with the current sample size, the general observations and clinical recommendations from this study would not be substantially altered. An alternative approach to the conventional torch or oven soldering is the use of an infrared heat source. In the commercially available unit (Ney) used in this study, infrared energy for soldering is supplied by a quartziodine-tungsten-filament lamp. Similar to the gas-oxygen torch, flux is required with the solder in this techVOLUME 79 NUMBER 6

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nique.25 Some of the reported advantages of infrared soldering are lack of gas inclusions, limitation of heating area and less working time.25,26 Cheng et al.25 found that solder joints of a cobalt-chromium base metal alloy produced by infrared soldering had higher ultimate tensile strength than those produced by the gas-oxygen torch. Other investigators reported little difference in joint integrity and strength, whether the thermal energy was delivered by a torch flame or infrared apparatus.27,28 Most of the infrared solder joints in this study fractured in a cohesive mode through the solder, but some showed failure through the parent metal (Fig. 6). This soldering technique requires meticulous control of the focal point because, if this is not achieved, the solder will melt unevenly and result in weaker joints, or not at all. Careful attention to voltage fluctuations is necessary when using the infrared unit, because such changes could alter the efficiency of the soldering procedure. The specimens soldered with the infrared technique were the weakest for each alloy (Fig. 1 and Table IV), but they were judged to be sufficiently strong for dental prostheses. Oven soldering (postceramic soldering) is the best choice when porcelain application has been performed. The maximum temperature can be controlled to allow solder flow without distorting the porcelain. Staffanou et al.11 observed that more consistent connectors, in strength and size, were obtained with oven soldering for a wide range of dental alloys, when compared with torch soldering. Stade et al.29 demonstrated that joints of similar or superior strength to the parent metal were obtained when oven soldering was used. However, oven soldering requires a well-calibrated oven to achieve the proper temperature for melting the solder, and the solder should contact both sides of the parent metal to be soldered. This technique can only be used for postceramic soldering, because most of the ovens do not achieve temperatures necessar y to melt the preceramic solder. In this study, all of the oven-soldered joints principally fractured cohesively through the solder and generally had few voids when observed with the SEM. Although all the alloys soldered well, producing joints that were judged strong enough to resist intraoral forces, SEM observations revealed that the fracture path of the solder joints followed microscopic grooves produced by the finishing procedures for the specimens. Another study should be performed to examine the relationships between finishing procedures and strength of the solder joints. The general composition of dental solders is reported23 to consist primarily of the elements gold, silver, and copper. However, as previously noted, no copper was detected in the two solders used in this study (Table V). Other components, such as zinc (found in both solders), tin, and phosphorus, are included to reduce the fusion temperature and improve flow. JUNE 1998

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CONCLUSIONS Under the conditions of this study, the following conclusions were drawn. 1. All three techniques used in this study were judged to be adequate for soldering high-palladium alloys. 2. Infrared soldering yielded the lowest values of flexural strength among the three techniques used. 3. X-ray energy-dispersive spectroscopic analysis indicated that there is no copper in either of the two solders used. 4. Elemental energy-dispersive spectroscopic analysis also showed that there are only minor changes in the composition of each solder after the melting procedures. 5. Microscopic grooves on finished specimens were revealed by the scanning electron microscope to have a close association with the paths of crack propagation. This observation indicates that solder joints should be well-polished to achieve optimal strength. We thank John C. Mitchell, Senior Electron Microscopist, Department of Geological Sciences, for expert technical assistance in performing the SEM/EDS analyses.

REFERENCES 1. Carr AB, Brantley WA. New high-palladium casting alloys: part 1. Overview and initial studies. Int J Prosthodont 1991;4:265-75. 2. Ziebert GJ, Hurtado A, Glapa C, Schiffleger BE. Accuracy of one-piece castings, preceramic and postceramic soldering. J Prosthet Dent 1986;55:3127. 3. Gegauff A, Rosenstiel S. The seating of one-piece and soldered fixed partial dentures. J Prosthet Dent 1989:62:292-7. 4. Bruce RW. Evaluation of multiple unit castings for fixed partial dentures. J Prosthet Dent 1964;14:939-43. 5. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering, or casting in one piece. J Dent Res 1977;56:128-34. 6. Garlapo DA, Lee S-H, Choung CK, Sorensen SE. Spatial changes occurring in fixed partial dentures made as one-piece castings. J Prosthet Dent 1983;49:781-5. 7. Padilla MT, Bailey JH. Margin configuration, die spacers, fitting of retainers/ crowns, and soldering. Dent Clin N Am 1992;36:743-64. 8. Squire BE. The relative strength of high and low fusing solders. [MS thesis.] Indianapolis: Indiana University, College of Dentistry; 1971. 9. Monday JJL, Asgar K. Tensile strength comparison of presoldered and postsoldered joints. J Prosthet Dent 1986;55:23-7. 10. Lorenzana RE, Staffanou RS, Marker VA, Okabe T. Strength properties of soldered joints for a gold-palladium alloy and a palladium alloy. J Prosthet Dent 1987;57:450-4. 11. Staffanou RS, Radke RA, Jendresen MD. Strength properties of soldered joints from various ceramic-metal combinations. J Prosthet Dent 1980;43:31-9. 12. Rasmussen EJ, Goodkind RJ, Gerberich WW. An investigation of tensile strength of dental solder joints. J Prosthet Dent 1979;41:418-23. 13. Rosen H. Ceramic/metal solder connectors. J Prosthet Dent 1986;56:6717. 14. Anusavice KJ, Okabe T, Galloway SE, Hoyt DJ, Morse PK. Flexure test evaluation of presoldered base metal alloys. J Prosthet Dent 1985;54:507-17. 15. Council on Dental Materials, Instruments and Equipment. Revised American National Standard/American Dental Association Specification No. 5 for Dental Casting Alloys. Chicago: American Dental Association; 1988. 16. Popov EP. Introduction to mechanics of solids. Englewood Cliffs (NJ): Prentice-Hall; 1968. p. 25-7, 181-5, 186-8. 17. Sokal RR, Rohlf FJ. Biometry. 3rd ed. New York: WH Freeman; 1995. p. 260-5, 396-401. 18. Conover WJ. Practical nonparametric statistics. 2nd ed. New York: John Wiley; 1980. p. 294-338. 19. Conover WJ, Iman RL. Rank transformation as a bridge between parametric and nonparametric statistics. Am Stat 1981;35:124-9.

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20. Welsch RE. Stepwise multiple comparison procedures. J Am Stat Assoc 1977;72:566-75. 21. Reisbick MH, Brantley WA. Mechanical property and microstructural variations for recast low-gold alloy. Int J Prosthodont 1995;8:346-50. 22. Stewart RB, Gretz K, Brantley WA. A new high-palladium alloy for implantsupported prostheses. [Abstract no. 423.] J Dent Res 1992;71:158. 23. Craig RG, editor. Restorative dental materials. 9th ed. St Louis: Mosby; 1993. p. 402-7. 24. Carlberg T, Wictorin L. Soldering of dental alloys under vacuum by IR-heating. Dent Mater 1986;2:279-83. 25. Cheng AC, Chai JY, Gilbert J, Jameson LM. Investigation of stiffness and microstructure of joints soldered with gas-oxygen torch and infrared methods. J Prosthet Dent 1994;72:8-15. 26. Wictorin L, Fredriksson H. Microstructure of the solder-casting zone in bridges of dental gold alloys. Odont Rev 1976;27:187-96. 27. Tehini GE, Stein RS. Comparative analysis of two techniques for soldered connectors. J Prosthet Dent 1993;69:16-9. 28. Cattaneo G, Wagnild G, Marshall G, Watanabe L. Comparison of tensile strength of solder joints by infrared and conventional torch technique. J Prosthet Dent 1992;68:33-7.

Noteworthy Abstracts of the Current Literature

29. Stade EH, Reisbick MH, Preston JD. Preceramic and postceramic solder joints. J Prosthet Dent 1975;34:527-32. Reprint requests to: DR. WILLIAM A. BRANTLEY COLLEGE OF DENTISTRY THE OHIO STATE UNIVERSITY 305 WEST 12TH AVE. ROOM 3005-L POSTLE HALL COLUMBUS, OH 43210-1241 Copyright © 1998 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/98/$5.00 + 0. 10/1/88849

CONTRIBUTING AUTHOR William M. Johnston, PhD, Professor, Section of Restorative Dentistry, Prosthodontics and Endodontics, College of Dentistry, The Ohio State University.

Microleakage of dentin-bonded crowns placed with different luting materials Patel S, Saunders WP, Burke FJT. Am J Dent 1997;10:179-83.

Purpose. Microleakage at the margins of dentin-bonded crowns may be a cause of failure of these types of restorations. This in vitro study assessed the microleakage of dentin-bonded porcelain crowns placed with three luting materials. Material and Methods. Forty-five teeth were prepared by reducing the occlusal surface by 2 mm; removing the convexity from the mesial, buccal, distal, and lingual surfaces; and forming a knifeedge finish line. One half of the cervical margin was placed on dentin/cementum and the remainder of the margin was placed on enamel. Individual crowns were fabricated in feldspathic porcelain and then cemented to the teeth with three luting materials, according to manufacturers recommendations. The teeth were stored in water for 2 weeks at 37° C before evaluating the microleakage with a dye penetration technique. Teeth with crowns were immersed in a 2% solution of methylene blue for 48 hours at 37° C. Specimens were then thermocycled while in the dye for an additional 6 hours. The teeth were then sectioned in a bucco lingual plane and examined at ×10 magnification with a ranking criteria ranging from 0 = no leakage to 4 = dye penetration grater than two thirds of the length of the restoration-tooth interface. Results. All groups exhibited leakage at the dentin and enamel margins and at the restorationcement interface. There were significant differences in leakage at the enamel margins for the three groups but not at the margins of the finished dentin. Conclusions. The resin cement combinations were unable to prevent microleakage completely. Although there was no significant difference in leakage at the margin in dentin for all three materials, Mirage ABC/FLC did not perform as well at the enamel margin. 30 References. — ME RAZZOOG

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