Soldering of removable partial dentures

James S. Brudvik, D.D.S.,* and Jack I. Nicholls, Ph.D.** University of Washington, School of Dentistry, Seattle, Wash.

l-1 emovable partial dentures are not normally sec- tioned, related, and soldered when they do not fit, as is a common practice for fixed partial dentures. Work authorization requests for framework soldering are most often related to fractures, additions of clasps, and retention for resin.

Clinical dental literature gives only a few references to the soldering of chrome-based alloys for removable partial dentures. In the most significant of these, MacEntee et al.’ found that they could create weld joints at least as strong as the parent metal when welding Vitallium with a 0.25 mm gap between the welded parts.

A number of studies on the strength of solder joints relative to the solder gap have shown that wider gaps produce stronger joints.2s3

Strength of joint is not the sole factor in the selection of gap size. Willis and Nicholls4 found that a minimum gap distance without contact gave the least distortion. A compromise between these two factors, strength and distortion, appears indicated.

Dental soldering, brazing, and welding can be accomplished with a variety of heat sources. The most common are electrosoldering and torch soldering.

Electrosoldering, as it is called in dentistry, is more properly named resistance brazing. It is applicable to joints that have a relatively simple configuration.

Gold solder, 0.800 fine (Tripple Thick, Ticonium Co., Albany, N.Y.), has been used for many years with excellent results by the author on nickel-chromium alloys. The high cost of this solder has encouraged the search for other solders or brazing alloys that would give equal results. At present, a variety of brazing alloys are available. Some of the brazing alloys are nickel-chrome in the 70-16 range and similar to Ticonium. Ticonium can be used with the electrosol-

Presented at the Academy of Denture Prosthetics, Boston, Mass.

Supported by Biomedical Research Support Grant No. RR-05346. *Associate Professor, Department of Prosthodontics.

**Professor, Department of Restorative Dentistry.

dering device to weld itself, resulting in a strong joint.*

The high heat generated in these brazing operations raises the possibility of embrittlement. These deleteri- ous effects on the strength of the solder joint can be minimized by rapid soldering at low soldering temper- atures.

The purpose of this investigation was to obtain a ranking of predictable joint strengths obtainable with materials and techniques common in the dental labora- tory field that would aid the dentist in the repair or assembly of removable partial dentures.

MATERIAL AND METHODS

Wax tensile bar specimens were formed by injection molding (Fig. 1). The specimens were placed in two groups (40 in each) and invested and cast according to the instructions of the alloy manufacturer. One group was cast in Ticonium 100 (Ticonium Co.) (low-heat group) and the other in Vitallium (Howmedica, Inc., Dental Division, Chicago, Ill.) (high-heat group). The castings were sandblasted and cut from the sprue leads. Each tensile bar was finished with disks and a coarse stone to remove any imperfections resulting from the casting process.

The tensile bars were invested in small blocks of Complete (J. F. Jelenko Co., Armonk, N.Y.) with a recess in the center of the bar. The investment blocks were dried, and then the bars were sectioned with a thin 0.33 mm disk (Veri-Thin Disc, National Keystone Products, Philadelphia, Pa.). The resulting cut was measured optically with a magnified grid (Edmund Scientific Co., Barrington, N. J.) and the opening found to be 0.4 mm. This gap was considered to be the smallest gap obtainable with cutoff disks currently in laboratory use.

The invested and cut tensile bars were divided into groups of 10. Each group was subjected to two different soldering/brazing operations.

*Mercer, R.: Personal communication, 1981.

762 JUNE 1983 VOLUME 49 NUMBER 6

SOLDERING OF REMOVABLE PARTIAL DENTURES

TICONIUM VfTALLIUM

Fig. 1. Injection mold and wax tensile bar specimens Fig. 2. Tensile strengths of solder joints. ‘T = Torch; (developed by Dr. Harold Morris, V.A. Hospital, Allen E = electric; TS = torch solder; 300 = 0.800 solder; and Park, Mich.). AG-CU = silver-copper solder.

Table I. Tensile strength comparison between alloys

Alloy comparison Ticonium (kg/mm*) Vitallium (kg/mm’) Statistical significance

As cast Oxyacetylene torch and torch

solder Oxyacetylene torch and 0.800

solder Electm and 0.800 solder

Electm and silver-copper solder

120.08 + 8.09 117.32 it 7.63 .Nonc 64.35 f 16.17 81.94 + 13.32 Signific~ant at 98%,

68.53 i 10.38 90.42 5 12.47 Sigmficmt at 99%

98.99 + 11.70 100.22 + 16.57 N o n e

48.39 rf 10.25 55.60 k 16.30 None

Table II. Tensile strength comparison (Vitallium)

Table III. Tensile strength comparison (Ticonium)

Alloy comparison

Tensile strength

(knlmm’) Statistical

significance

As cast versus

electric and 0.800 solder

Electric and 0.800

solder versus torch and torch

solder

Electric and 0.800 solder versus

torch and 0.800 solder

Torch and 0.800

solder versus torch and torch

solder

117.32 + 7.63 Significant at 98%

100.22 rt 16.57

100.22 + 16.57 Significant at 98%

81.94 k 13.32

100.22 r 16.57 None

90.42 + 12.47

90.42 t 12.47 None 81.94 r 13.32

Alloy Tensile strength Statistical

comparison (kg/nun’) sigificunce

As cast versus 120.08 f 8.09 Significant at 99% electric and 0.800 98.99 t 11.70

solder Electric and 0.800 98.99 + 11.70 Significant at 99%

solder versus 64.35 + 16.17

torch and torch

solder Electric and 0.800 98.99 + 11.70 Significant at 99.9%

solder versus 68.53 2 10.53

torch and 0.800 solder

Torch and 0.800 68.53 k 10.53 None

solder versus 64.35 + 16.17

torch and torch solder

__-_.----. _--_.-__-

First, each alloy was soldered using 0.800 fine Tripple Thick solder (Howmedica, Inc., Dental Divi- sion) with both electrosoldering (Electrosoldering Machine, Yates & Bird, Chicago, Ill.) and torch soldering (Little Torch, Tescom, Minneapolis, Minn.).

Then, the soldering operation was repeated using Jelenko Torch Solder (J. F. Jelenko Co.) and an oxyacetylene torch and also Sure Bond silver solder (W. Zorovich, Inc., Dayton, Ohio), using electrosolder- ing for both alloys. After soldering, the specimens were

THE JOURNAL OF PROSTHETIC DENTISTRY 763

BRUDVIK AND NICHOLLS

Fig. 3. Lingual plate that has been sectioned and seated in mouth. Because both sections show an acceptable fit, soldering is indicated. Soldering lingual plate is technically more complex than soldering lingual bar (Fig. 4).

bench cooled and recovered. These materials, equip- ment, and techniques are representative of current laboratory approaches to the soldering of removable partial dentures.

The soldered specimens were turned to a measured diameter in the area of the solder joint and evaluated with the Instron Universal Testing Machine (Instron Corp., Canton, Mass.).

RESULTS

After fracture from loading in the testing machine, each solder joint was examined under magnification, and those that were incomplete were replaced with new specimens. Seventeen of 80 specimens were defective. The defects appeared to be flux inclusions, probably caused by using a flux that was too thick.

The results are as shown in Fig. 2 and Tables I to III. Means and standard deviations were calculated and a number of comparisons made using the Student’s t-test for significance. The assumption that the lower heat alloy would result in stronger solder joints was not substantiated.

The strongest joints in both alloys were made with 0.800 fine solder using the electrosoldering machine. These joints were significantly stronger than the other solder combinations in both alloys; but when the two alloys were compared, there was no difference between them. In fact, their tensile strength mean values were almost exactly the same (Ticonium, 98.99 kg/mm2; Vitallium, 100.22 kg/mm2).

When compared to the as-cast specimens, electric soldering with 0.800 fine solder gave values that were 85% as strong as those of the cast alloy.

Other combinations of solders and heat sources gave

Fig. 4. Framework that will allow sectioning and soldering of any element if needed. This framework has been joined with torch solder and an oxyacetylene torch and is ready for finishing.

greater reductions in tensile strength, with the silver- copper solder less than half as strong as the as-cast alloys.

The heat source was significant with the low-heat alloy (Ticonium) but not with the high-heat (Vital- lium).

DISCUSSION

While tensile strength testing gives some indication of the clinical performance of solder joints in removable partial denture frameworks, there are other considera- tions. Partial dentures are subjected to stress-relaxation forces, and the toughness of the joint may be as important as its ultimate tensile strength.

Distortion from the various soldering procedures was not evaluated in this study but is obviously of great concern. It is of no value to create a strong joint if it is distorted beyond the level of clinical acceptance.

Clinical experience and data from industrial sources indicate that the lowest possible heat and the shortest possible time will result in the least distortion. Electro- soldering with 0.800 fine solder would seem to fulfill this requirement.

Tarnish of the solder joint when exposed is also of concern. Again, clinical experience has shown that the 0.800 fine solder and the torch solder will not show significant change. The silver-copper solder has a yellow color and can be expected to show change after exposure to the oral fluids. In addition, this solder proved to be the weakest of those tested. It appears to be limited to attaching wire clasps to the framework when the region of attachment is totally within the acrylic resin denture base.

Because there was no significant difference between

764 JUNE 1983 VOLUME 49 NUMBER 6

SOLDERING OF REMOVABLE PARTIAL DENTURES

alloys as cast or when joined with 0.800 fine and electric soldering, both Vitallium and Ticonium can be sectioned and soldered with confidence. Clinical expe- rience indicates that a tensile strength 85% of the as-cast strength is sufficient.

The use of the oxyacetylene torch introduces a greater range of strengths. The skill of the investigator in the use of this heat source is a factor to be considered. The selection of the tip size and the mixture of the two gases is absolutely critical. The joint strengths reported by MacEntee et al.’ exceeded the torch welds in this study possibly due to these technical considerations.

When the use of torch soldering was discussed with commercial laboratory technicians, a variety of tech- niques and materials were identified. Many techni- cians spoke of the operation as “welding,” and, indeed, a number used alloys called welding rods by their manufacturers. Those laboratories using electrosolder- ing unanimously preferred 0.800 fine solder with a number of manufacturers supplying this solder.

The results obtained in this study should be consid- ered the “floor” of the possible range of tensile strengths. The very thin, absolute butt joint used in the test samples is theoretically the weakest possible joint design. The skill of the authors in soldering obviously affected the quality of the joint. One can expect that an experienced senior dental technician will create more uniform joints of greater strength.

Electrosoldering of both high- and low-heat remov- able partial denture alloys can give comparable results as this study indicates. The selection of the solder to be used will depend on both the cost and the required strength. Electrosoldering is both easier to learn and less destructive to adjacent denture resin. It appears to be the heat source of choice.

CLINICAL APPLICATION

If the clinician has confidence in the soldering operation, then discrepancies in the fit of a framework can often be corrected. Major connectors can be sec- tioned; and if the sections then fit satisfactorily, they can be reassembled (Fig. 3). Lingual bars will obvious- ly be easier to solder than palatal plates. Many times there will be one rest or clasp assembly that does not

fully seat while the remainder of the framework is satisfactory. The offending component is sectioned in the minor connector and related properly in the mouth. Matrices of plaster or acrylic resin can be used to establish the desired relationship for the laboratory. In these repair situations the design of the major and minor connectors has a definite influence on the ease of the soldering operation. When the linguai bar major connector design is used, sectioning and soldering is easier and therefore more dependable than with a lingual plate major connector (Fig. 4). The easier access to the minor connector makes the soldering quite simple with either torch or electrosoldering. In addi- tion, soldering in a variety of repair situations can be

recommended with confidence.

SUMMARY

Both high- and low-heat removable partial denture alloys were soldered with combinations of different solders and heat sources. The soldered specimens were subjected to tensile strength testing, and a ranking of joint strength was developed. Joint strengths that were 85% as strong as the as-cast metal were created in both alloy types when 0.800 fine solder was used with an electrosoldering device as a heat source. All other combinations tested resulted in greater reductions in joint strength.

REFERENCES

hlacEntee, M. I., Hawbolt, E. B., and %ahel. J I. l‘hr tensile

and shear swength or a base metal weld joint usrd in dentisq J Dent Res 60:154, 1981.

Stade. E. Ii., Rcisbick. M. I-i., and Preston, ,j il.: Prttcwtmir and postceramic solder ioints. J PRO’rTlll~ ! ’ I)t~‘r 34:527,

197;. Rasmussen, E. J., Goodkind, R. J., and Herhrwh. \V. W.: An

investigatmn of tensile strength 01’ drnul wldcr joints. J PROTHEY DENT 41:418, 1970

Willis, E. J., and Nicholls, J. 1.: Dtstortion in dental soldering

‘ts alfrctcd 11): hap distance. J PROSTH~ r 1)~ x.1‘ 4W72. IVRO.

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