Comparison of compressive strength of solid and hollow pontic designs for ceramometal fixed partial dentures

REINFORCED

13.

Moskowitz hydrocolloid immediate

PROVISIONAL

RESTORATIONS

ME, Loft GH, Reynolds to evaluate preparations provisional restorations.

JM. Using irreversible and fabricate temporary J PROSTHET DENT

1984;51:330-3.

14.

Morgan DW, Comella MC, Staffanou RS. A diagnostic wax-up technique. J PROSTHET DENT 1975;33:169-77. Calagna LJ. A comprehensive treatment rationale combining prosthodontics and periodontics. J PROSTHET DENT 1973;

15.

Reprmt

requests

DR. CATHERINE

to:

BINKLEY

DEI\TAL CLINIC AMBULATORY CARE BLDG. UNIVERSITY OF LOUISVILLE LOUISVILLE,

KY 40292

30:781-K

Comparison of compressive strength of solid and hollow pontic designs for ceramometal fixed partial dentures H. E. Rosenstein, D.M.D.,* M. L. Myers, and R. H. Jarvis, D.D.S., M.S.**** Eastman Dental Center, Rochester, N.Y.

D.M.D.,**

T

he design of the metal framework influences the strength of a ceramometal restoration. Many designs have been advocated and generally emphasize framework rigidity, uniform porcelain thickness, and support of the porcelain through design features such as interproximal struts, lingual shoulders, and trestle configurations.le4 Conventional framework designs incorporate solid metal pontics and external surfaces that are convex, in an effort to minimize tensile stresses within the porcelain. Recently, Shoher and Whitman’ developed a reinforced porcelain system (RPS) that involves the placement of concavities on the external surface of the framework. According to Shoher this design places the porcelain in compression during the firing cycle and results in greater strength and fracture resistance of the porcelain. Shoher’s pontic design consists of a hollow configuration with supporting belts on the buccal and lingual surfaces. The advantages of the hollow pontic design are: reduced casting weight (40%); reduced cost of metal; decreased porosity from solidification shrinkage; Semifinalist, John J. Sharry Research Award competition, College of Prosthodontists. Second place, Stanley D Tylman Research competition, Academy of Crown and Bridge Prosthodontics. *Postdoctoral student, Department of Prosthodontics. **Assistant Chairman, Department of Prosthodontics. ***Chairman, Department of Prosthodontics. ****Clinical Associate, Department of Prosthodontics.

THE

JOURNAL

OF PROSTHETIC

DENTISTRY

American American

G. N. Graser,

D.D.S.,

M.S.,***

elimination of a large heat sink, which aids soldering; and greater porcelain thickness for improved esthetics. This study compared the strength of hollow and solid pontic designs. A three-point load was placed on ceramometal specimens with both solid and hollow pontic designs to measure the fracture resistance of the porcelain. MATERIAL

AND

METHODS

Sixty metal frameworks were made for testing. The frameworks were divided into four experimental groups: solid pontics, hollow pontics, solid pontics with porcelain, and hollow pontics with porcelain. An RPS molar wax pattern (Williams Gold Refining Co., Inc., Buffalo, N.Y.) was used for the hollow pontic design. The solid pontic design was made from the same wax pattern by filling the hollow spaces with wax. The size of the framework was designed to simulate a three-unit fixed partial denture. The connectors for the pontic were 3 mm in diameter and 1 mm in length and were connected to spherical ends that were designed to seat on a test jig (Figs. 1 and 2). A stone matrix was used to position the pontic patterns so that all specimens could be made uniform. The patterns were invested using Hi-Temp investment (Whip-Mix Corp., Louisville, Ky.) according to the manufacturer’s directions. The frameworks were cast in Williams’ W-3 (Williams Gold, Buffalo, N.Y.) gold-palladium ceramic alloy. A second stone matrix was used for the porcelain

693

ROSENSTEIN

Fig. Fig. Fig. Fig. Fig.

1. 2. 3. 4. 5.

Metal specimen with hollow pontic design. Metal specimen with solid pontic design. Specimen with porcelain after glazing. Specimen in Instron testing machine before testing. Typical fracture of porcelain specimen.

This produced a uniform thickness of 1.5 mm. Will-Ceram Porcelain (Williams Gold) was condensed and fired in a conventional fashion using the manufacturer’s temperature schedules.In the hollow pontic, a mixture of 50% porcelain and 50% opaque was condensedinto the hollow areas. After firing of opaque to the remaining metal of the pontic, two bakes of body application.

694

ET AL

porcelain were applied to each specimento compensate for shrinkage. All specimens were glazed simulate a completed porcelain-fused-to-metal

in air to restora-

tion (Fig. 3). All 60 specimenswere testedon an Instron Universal testing machine (Instron Corp., Canton, Mass.). Each specimenwas placed in the testing jig and a vertical load JUNE

1987

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SOLID

AND

HOLLOW

PONTIC

Table I. Statistical testing

DESIGNS

analysis

of compressive

COMPRESSIVE

Test material

N

x(kg)

Hollow design Solid design Hollow design with porcelain Solid design with porcelain

15 15 15

154.7

15

SD

SE

83.5

22.6 47.4 13.1

5.8 12.2 3.4

87.9

14.6

4.0

89.3

P-value

TESTS

m

Hollow

m

Solid

Pontbc Pontlc

<.OOl = .41

was placed on the direct center of the pontic by means of a descending metal hemicylindrical insert (Fig. 4). The crosshead speed was 0.1 cm/min. The failure load in kilograms was graphically recorded by the machine. For the porcelain/metal specimens, the initial porcelain fracture was used as the yield point of the restoration (Fig. 5). RESULTS Results of the compressive testing were calculated from the graphic readout on the Instron testing machine. The mean values for the four groups can be seen in Table I and Fig. 6. Analysis of variance and a NewmanKeuls multiple comparison indicated that the solid pontic without porcelain, as would be expected, was significantly stronger than the other three experimental groups. There were no significant differences between those three groups. The Student t-test was performed to compare the differences between the hollow and solid pontics and the hollow and solid pontics with porcelain. For the pontics without porcelain, as expected, the solid design exhibited a significantly higher compressive strength (it < .OOl). For the two types of pontics with porcelain, however, there were no significant differences in compressive strengths of porcelain failure between the hollow and solid designs (p = .41). DISCUSSION The poor tensile strength of porcelain necessitates a rigid metal substructure to resist porcelain fracture, hence the ceramometal restoration. However, the results of this study indicate that the fracture resistance of the porcelain on a pontic does not depend on having a solid metal pontic. This is in agreement with another study that compared solid and hollow triangular beams before and after porcelain application.6 This finding may be a result of strengthening of the hollow pontic by filling it with porcelain, which has a high compressive strength. Another possible explanation is that the fracture resistance of the porcelain is so much less than the metal that it is insignificant whether the pontic is hollow or solid. Porosity from solidification shrinkage occurs most often in thicker sections of a casting, especially pontics. THE

JOURNAL

OF PROSTHETIC

DENTISTRY

Metal Framework

Metal FMln@UOrk With

Porcelain Fig. 6. Comparison designs.

of compressive

strength

of pontic

This porosity can result in decreased framework strength or bubbling of porcelain as a result of gas entrapment. Because of bulk, a pontic can also act as a heat sink and thus make soldering procedures more difficult. The use of a hollow pontic can reduce pontic porosity by minimizing thick areas in the casting. Another advantage is the decreased expense of metal frameworks because less metal is needed for the hollow pontic. A disadvantage of a hollow pontic is that it requires an additional firing to fill the hollow spaces with porcelain. Nevertheless, it appears from this study that the use of a hollow pontic would not compromise the strength of a fixed partial denture. It does not support Shoher’s finding that it results in greater strength and fracture resistance of the porcelain. Further studies with longer spans and clinical applications are needed to further evaluate this concept. SUMMARY A total of 60 ceramometal frameworks were subjected to compressive loading. Half of the specimens had a hollow pontic design. After the addition of porcelain, it was found that there was no significant difference in the fracture resistance of the porcelain whether the pontic was solid or hollow. 695

ROSENSTEIN

REFERENCES 1.

2. 3. 4. 5.

6.

Straussberg G, Katz G, Kuwata M. Design of gold supporting structures for fused porcelain restorations. J PROSTHET DENT 1966;16:928-36. Weinberg LA. A new design for porcelain-fused-to-metal prostheses. ,J PROSTHET DENT 1967;17:178-94. Riley EJ. Ceramo-metal restoration. State of the science. Dent Clin North Am 1977;21:669-82. Miller LL. Framework design in ceramo-metal restorations. Dent Clin North Am 1977;21:699-716. Shoher I, Whiteman AE. Reinforced porcelain system: a new concept in ceramometal restorations. J PROSTHET DENT 1983; 50:489-96.

Tensile

strength

analysis

Reprint

ET AL

Miller LL. Tooth preparation and the design of metal substructures. In: McLean JW, ed. Dental ceramics: proceedings of the first International Symposium on Ceramics. Chicago: Quintessence Publishing Co Inc, 1983;196-204. requests

to:

DR. MICHAEL L. MYERS EASTMAN DENTAL CENTER 625 ELMWOOD AVE. ROCHESTER, NY 14620

of midpontic

soldering

Jonathan L. Ferencz, D.D.S.* New

York University

College

of Dentistry,

New

York,

N.Y.

S

oldering metals in dentistry for joining retainers to pontics is consideredroutine and predictable. Nevertheless,when a solderjoint fractures after a restoration is in service, there are serioustreatment and financial implications. The dental literature contains many studies on the accuracy of dental solderingas it relatesto gap distance, indexing techniques, and soldering investment.‘-’ Some advocate casting fixed partial dentures in one piece to avoid some of the problems of soldering. Huling and Clark7 found that laser welding and one-piece castings produced significantly less distortion than soldering. Fusayama et aL8found that one-piececastingsproduced more accurate cervical fit than that of various soldering techniques. Other studiesdeal with the strength of dental solder joints.9-2’Of 13 studies,11 measuredtensile strength and two studied flexure strength.‘*%” All 13 studies used premeasured bars of some geometric design so that calculations of tensile strength, yield strength, modulus of elasticity, or proportional limit could be calculated. Fairhurst and Ryge’Ofound that tensile strength values increased with larger gap distances partly becauseof greater porosity in the narrower gaps. Stade et al.” also reported stronger joints with the use of wider gap distances and cited superior strength of postceramic solderedjoints. Presented at the Academy of Denture Prosthetics, Toronto, Canada. *Clinical Associate Professor, Department of Prosthodontics and Occlusion.

696

Fig. 1. Teeth prepared partial denture.

on typodont

for three-unit

fixed

Rasmussenet a1.12had similar findings, whereas Monday and Asgar13found no significant difference in ultimate tensile strength values for presoldered and postsolderedjoints with gold-palladium ceramic alloy. However, they did find that torch soldering yielded strongerjoints than vacuum-oven soldering. Staffanou et a1.14also found that satisfactory joints can be achieved with either presoldered or postsoldered methods although a high percentage of presolderedjoints failed before testing. Comparing various alloys, Nicholls and Lem” found that both presoldered and postsolderedOlympia (J.F. Jelenko Co., At-monk, N.Y.), a gold-palladium alloy had higher tensile strength values than Jelenko 0 (J.F. JUNE 1987

VOLUME

57

NUMBER

6