WilPam R. Larson, DDS, MS,” Donna L. Dixon, DMD,b Steven A. Aquiline, DDS, MS,e am3 James M. S. Clancy, DDS, MSb Iiniversity of Minnesota, School of Dentistry, Minneapolis, Minn., and University of Iowa, College of Dentistry, Iowa City, Iowa This study compared the moduli of elasticity of three provisional crown partial denture resins. Eighteen specimens of each resin were reinforced carbon graphite Abers and 18 resin specimens from each group contained Six specimens of each resin, with and without fibers, were tested within fabrication, after 30 days, and after 90 days of water storage. The mean elasticity of ali the resins increased significantly with 5ber incorporation. storage did not have a signiffcant effect on the moduli of elasticity of the tested. (J F%OSTAET DENT 1991;66:816-20.)

F.

axed partial dentures have become a well-established treatment modality for many partially edentulous patients.* Because these restorations are made indirectly in a dental laboratory, several days or weeks are usually required for completion. During this period, the patient must wear a provisional restoration that provides pulpal protection, positional stability, maintenance of occlusal function. ease of cleansibility, strength, retention, and esthetics for the prepared teeth.” Provisional restorations are most often made directly or indirectly using an autopolymerizing acrylic resin material. While using these resins provides many advantages, disadvantages such as liquid sorption over time3-5 and inadequate strength to resist strong occlusal forces6 are also exhibited by these materials. It has been determined that the resistance of provisional acrylic resins to propagation of cracks ultimately determines the strength and serviceability of provisional fixed partial dentures. A fracture failure is usually related to the initiation of a crack and its propagation until the restoration is displaced.7 Some investigators have attempted to increase the fracture strength of these materials by decreasing the size and number of internal porosities through pressure curing.8 Increases in fracture strength were noted wit.h this method, but not to a significant degree. Other methods have been used to improve the mechanical properties of acrylic resins. These methods include: application of maximum bulk of material to heavily stressed “Assistant Clinical Professor, Department

of Prosthodontics, University of Minnesota, School of Dentistry. “Assistant Professor, Department of Prosthodontics, University of Iowa, College of Dentistry. Associate Professor, Department of Prosthodontics, University of lowa, College of Dentistry. 1 O/I/27543

and fixed with no 5bers. 6 hours of moduli of Water resins

regions of the resin; copolymerization and crosslinking; and reinforcement with glass fibers, nylon fibers, aluminum and sapphire whiskers, polycarbonates, and metal strengtheners.g However, most of these methods, have had little or no success in increasing resin strength. In 1986 Ruyter et al. lo also recognized the inadequate strength to resist occlusal forces exhibited by resin materials, and studied poly (methylmethacrylate) reinforced with carbon graphite fibers. These investigators concluded that the incorporation of these fibers into resin greatly increased the fracture stress and flexural modulus of these materials. Of course, such an increase in strength would be desirable in long-span provisional restorations. This study investigated the effect of carbon graphite fiber reinforcement on the moduli of elasticity of three commonly used provisional fixed partial denture resins after storage in water over time. MATERIAL

AND

METHODS

A stainless steel mold was used to make resin specimens measuring 65 x 10 x 3 mm. l1 Eighteen specimens each were made from a poly (methylmethacrylate) resin (Jet Temporary Crown and Bridge Resin, Lang Dental Manufacturing Co., Chicago, Ill.), a poly (ethylmethacrylate) resin (Splintline Tooth Shaded Acrylic, Lang Dental Manufacturing Co.), and a polyvinyl (ethylmethacrylate) resin (Trim, Henry J. Bosworth Co., Skokie, Ill.). During a pilot study, a 2.51 polymer-to-monomer ratio provided the best viscosity during stainless steel mold placement for the Jet acrylic resin. A 2:l ratio was found to be appropriate for Splintline resin, and a 3:l ratio was best for Trim resin. These ratios were consistently used through this investigation. Each acrylic resin was prepared by the same investigator, was placed in the mold, and was allowed to polymerize under 20 psi pressure for 10 minutes.

CARBON

GRAPHITE

FIBER

REINFORCEMENT

-

Jet with carbon graphite fibers

0 - - - - - - - -8

Jet without carbon graphite fibers

-

Splintline with carbon graphite fibers

t3- - - - - - --E) M

Splintline without carbon graphite fibers Trim with carbon graphite fibers

B - - - - -- - -A

Trim without carbon graphite fibers

4-

4 0

3-

II 8

0 0

I

I

30

60

Time (Days) Fig. 1. Mean moduli of elasticity of resin groups over time.

Table

I.

Descriptive statistics for resin groups

Acrylic

resin

Sample No.

Jet (no fibers) Splintline (no fibers) Trim (no fibers) Jet (fibers) Splintline (fibers) Trim (fibers)

18 18 18 18 18 18

Mean modulus of elasticity WW

1.950 1.317 0.557 3.647 2.961 1.501

SD

0.106

0.068 0.144 0.746 0.623 0.501

Standard error of mean

0.025 0.016 0.034 0.176 0.147 0.118

cv

5.430 5.124 25.777 20.451 21.040 33.371

CV. Coefficient of variation.

Using the same polymer-monomer ratios described, 18 additional specimens were made from each resin with the incorporation of carbon graphite fibers (Hewlett-Packard, Kiska, Sweden). These fibers were supplied in braided strands that were cut into lengths of 63 m m and soaked in the appropriate monomer before placing them between two poured layers of resin in the mold compartments After fabrication, all specimens were placed in deionized water at 37O C. Immediately or very shortly thereafter (within 6 hours of fabrication), six specimens of each resin, with and without carbon fibers, were placed on an Instron Universal (Instron Corp., Canton, Mass.) testing machine. Using a three-point testing rig similar to the design described by Stafford and Handley12 and a 50 kg compres-

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sive load cell, the modulus of elasticity was determined for each specimen. All testing was accomplished at a crosshead speed of 0.5 cm/min, with a chart speed of 3.0 cm/min. The testing procedure described was also accomplished after 30 and 60 days’ storage in water. The accumulated data were used to determine the moduli of elasticity for the specimens according to the following formulas: d = y (Vh/ Vc), where d = sample deflection, y = distance on chart, Vh = crosshead speed, Vc = chart speed; and E = P1/4dbt, where E = elastic modulus, P = load, 1 = length, d = sample deflection, b = width, and t = thickness. The widths and thicknesses used were mean values of measurements made at three points on each specimen with a dial measuring caliper. 817

LARSON

ET AL

without carbon graphite fibers 3.647 (0.176)

with carbon graphite fibers

3.6 .:.:::::::::::::::::::::::::::::::::: .~.~.~.~,~.~_~_~_~.~.~,~.~.~,~.~,~.~. .:.:.:::::::::::::::::::::::::::::::: ..‘.‘ . .. :::::::::::::::::::::::::::::::::::: .~.‘.‘.‘.‘.‘.~.~.‘.~.‘.~.~.~.~.~.~.~. ::::::j::::::::::::::::::::::::::::. .:.~:.:.:.~:.~:.:.:.:.:.:.:.:.:.: :.:.:.:.:.:.:.:.?:.:.:.:.:.:.:..... .~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~:~ .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: :.:.~:.:.:.:.~~:.~:.~.:.:.:.:.:. :::::::::::::::::::::::::::::::::::: .........._...................... ....~...‘ ...‘.~.‘.........~.......... :::::::::::::::::::::::::i:::::::::: .~.~.~.~.~_~.~.~.~.~.~.~.~.~.~.~.~.~ .‘.‘.‘.‘.‘.‘.~.‘.~.‘.‘.~.~.~.‘.~.’.~ ::::::::::::::::::::j::::::::::::::: ~.‘ .‘.‘.‘.~.‘.‘.~.~.‘.~.~.~.~.~.’.~ ‘.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.’ ::::::::::::::::::::::::::::::: ::::::j:::::::::::: 1. . , . . . .j:::::::: .,.,.,,.... >>>>:.:,:.>>:.: ........... :::::::::::::::::::::::::::::::: .~.~.‘.~.~.‘.‘.~.~.~.‘.‘.~.~.‘.~ ::j::::;::::::::::::::::::::::

3.2 2.8 2.4

1.6

.~.‘.~.‘.‘.~.‘.‘.~.~.~.‘.~.~.~.~ :.:.:.~:.:.:.:.:.:.:.:.:.:.:.:.. ::::::::::::::::::::::::::::::::. :::::::::::::::::::::::::::::: .:.:.~.:.:.:.:.:.:.:.:.:.:.:.:.:

0.8

Jet

2.961 (0.147) .‘.‘.‘.‘.~.‘.~.‘.~.‘.~.~.‘.~.‘.~.~.~ .~.~_~.~.~.~.~.~_~_~.~.~.~.~.~.~.~.~ .~.~.‘.~,~_~,~_~.~.~.~.~.~.~.~.~.~.~ ::::::::::::::::::::::::::::::::::: .~.~.~.~.~.~.~.~_~_~.~.~.~...~.~.~.. .‘.‘.‘.~.~.~.~.~.~.‘.~.~.‘.~.‘.~.~.’ .~.~.~.~.~.~.~_~.~.~.~.~.~.~.~.~.~.~ .~.~.~_~.~.~.~.~.~,~.~.~.~.~.~.~.~.~ _~_~.~.~,~.~.~,~.~,~.~.~,~.~,~.~,~.~ .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: :.:.:.:.:.:.:.:.:.:.:.:.:,:.~:.:.:. ~.~.~.~.~.~.~,~.~.~.~.~,~.~.~.~.~.~ ~.~.~.~,~.~.~,~.~.~.~.~,~.~.~.....~ .‘.‘.‘.‘.~.‘.‘.~.~.~.‘.‘.~.~.~.~.~.~ ~.~.~_~,~_~..,~.~,~_~.~,~.~,~.~.~.~ :::::::::::::::::::::::::::::::::::: .‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.~.‘.~.’.~ . . . . . . . . . . . . . . . . . ~.‘.‘.‘.‘.‘.‘.‘.~.‘.~.~.~.~.~.~.~.~. ~.~.~.~,~_~_~_~_~.~_~.~.~.~.~.~...~. ~~:.~:.:.:.:.:.:.:.:.:.:.:.:.:.:. :::::::::::::::::::::::::::::::::::: . . . . ..I........... :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. . . . . . . . . . . . ‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.~.~.~. ~_~.~.~.~.~.~.~,~.~.~.~.~.~ ‘.‘.‘.‘.:.:.:.:.:.:.:.:.:.:.:.:.~.:. . .. . . .. . :.:.:.:.:.:.:.:.:.:.:.:.:.:,:.:.:.:. . . . . . . . . . . . . ~.~.‘.‘.‘.‘.~.~.~.~.~.~.~.~.~.~.~.~. ~.~.~.‘,~.‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.~.~.~. :::j::::::::::::::::::::::::::::::: :.:.:.:.:.:.:.~:.:.~:.:.~~:.:.:. /:.:.:_:_:_:_:_:.:.:.~.~.~,~~.~,~.: ‘.~.~.‘.‘.‘.‘.‘.‘.‘.‘.‘.~.‘.‘.~.~.~. ~.‘.‘.~.‘.~.‘.‘.‘.‘.‘.~.~.~.~.~.~.~. :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:. ,‘.‘.‘.‘.~.‘.‘.‘.‘.‘.‘.~,~.‘,~.~,~.~ ,~.~.~.~*~.~.~.~.~_~.~.~,~.~,~.~,~.~ ,‘.~.‘.~.~.~.~,~.~.~,~.~,~.~,~.~,~.~ .~.~.~.~_~.~.~.~.~.~,~.~,~.~.~.~ . . . . . . . . . . .‘.‘.~.~.‘.~.~.~.~.~.~.~.~.~.~,~.~. :::::::::::::i::::::::::::::::::: ,..‘.‘.~.~.~.~.~.~,~.~.~.~.~.~,~.~, .‘.~.‘.‘.‘.‘.‘.~.‘.‘.‘.‘.‘.‘.~.~.~. . . . . . . . . . . . . . :::::::::::::::::::::::::::::::::::

1 so1 10.1181

. . . . . ,........ . . . . . .

. .

.... . .

. . . .

Splintline

Trim

iWy#ic Resins Numbers In parentheses are standard error of the means

Fig. 2. Comparison of mean moduli of elasticity of resins with and without carbon graphite fibers.

Table IT. ANOVA of mean moduli of elasticity for each resin group (with and without carbon graphite fibers) over time Variable

(source)

Model Error Total Group Carbon Time Group carbon Group . time Carbon . time Group . carbon . time *Significant

Degree of freedom

Sum of squares

17 so 107 2 1

2 2 4 2 4

117.569 19.153 136.722 57.551 55.056 0.260 3.177 0.965 0.110 0.450

Mean square

6.916 0.213

F value

PR>F*

32.50

0.0001

135.21 258.70 0.61 7.46 1.13 0.26 0.53

0.0001 0.0001

0.5457 0.0010 0.3458 0.7724 0.7151

at p < 0.05.

Finally, the calculated values for the moduli of elasticity for the specimens (in gigapascals [ GPa] ) were recorded and were subjected to statistical analysis (Table I). RESULTS The mean modulus of elasticity for each resin group is graphically displayed in Figs. 1 and 2. A three-way facto-

rial analysis of variance (ANOVA) (p < 0.05) fixed effects model (group by carbon by time) was used. The group and carbon main effects as well as the group by carbon interaetion were significant. Time, however, was not found to be significant (Table II). Ducan’s multiple range test was performed to compare all the resins without the time factor. The results from this analysis are shown in Table III. The

CARBON

GRAPHITE

FIBER

REINFORCEMENT

Jet acrylic resin with carbon graphite fibers exhibited a significantly higher mean modulus of elasticity than the other resins, and the Trim resin without carbon graphite fibers was shown to have a significantly lower mean modulus of elasticity than all the other specimens. The mean moduli of elasticity of the acrylic resins were all statistically different except for that of the Trim with carbon graphite fibers and Splintline without carbon graphite fibers, which were statistically similar.

DISCUSSION Because previous studies have demonstrated a significant effect on the moduli of elasticity for denture base resins with and without carbon graphite fiber reinforcement when stored in water over time,5, 6,lo this same effect was expected in this investigation. Surprisingly, however, no significant change in this property occurred over the go-day storage period. Perhaps this lack of influence may have been due to the differences in the chemical nature and processing techniques between the crown and fixed partial denture resins tested and the previously tested denture base resins. Denture base acrylic resins are pressure-packed, heatcured, and are cross-linked. The pressure packing decreases the number of microporosities; heat curing eliminates excessmonomer; and the cross-linking may make the acrylic resin less polar, thus decreasing the water sorption rate. All of these factors may have had a tendency to slow the rate of water sorption, which would have allowed a noticeable decrease in the moduli of elasticity over time. The crown and fixed partial denture acrylic resins used in this investigation were not pressure-packed, heat-cured, nor were they cross-linked to a significant degree. As a result, these resins may have had an increase in the number of microporosities, monomer retention, and a large polarity that would enhance the rate of water sorption. Theoretically, these acrylic resins may have already reached their maximum water sorption within a matter of a few hours as a result of the monomer retention and the high polarity. For these reasons, the strength of the acrylic resins may have remained uniform. It was not unexpected that carbon graphite fiber incorporation caused a significant mean increase in the moduli of elasticity for all the resins. The Splintline and Trim resins, a poly (ethylmethacrylate) and a polyvinyl (ethylmethacrylate), respectively, exhibited a much greater percent increase in their moduli of elasticity than the Jet resin, a poly (methylmethacrylate). This increase may have been due to the fact that these two materials were weaker than the Jet resin before reinforcement. Thus the magnitude of any increase in strength would appear greater for Splintline and Trim resins. Duncan’s multiple range test separated the materials, both reinforced and nonreinforced, into five significantly different groups. Only Splintline resin with fibers exhibited

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III. Duncan’s multiple range test for comparison of resins with and without carbon graphite fibers

Table

No.

Mean modulus of elasticity (GPa)

Duncan

Acrylic resin Jet (fibers) Splintline (fibers) Jet (no fibers) Trim (fibers) Splintline (no fibers) Trim (no fibers)

18 18 18 18 18 18

3.647 2.961 1.950 1.501 1.317 0.557

A B C D D E

group

a mean modulus of elasticity that was in the range of Jet acrylic resin, It was evident from this experimental data, therefore, that a poly (methylmethacrylate) may be the material of choice for making provisional restorations compared with poly (ethylmethacrylates) or polyvinyl (ethylmethacrylates). CLINICAL

SIGNIFICANCE

In the posterior regions intraorally where esthetics are not always of the utmost importance, carbon graphite fibers can be incorporated successfully into long-span provisional fixed partial dentures. By increasing the moduli of elasticity of the acrylic resins, these provisional restorations can endure the greater stresses created during mastication. The incorporation of carbon graphite fibers may therefore prevent the inconvenience of provisional restoration fracture, especially when the patient must wear the provisional fixed partial denture for an extended period of time. From the data gathered in this investigation and in another,13 the acrylic resin found to possess the greatest strength is a poly (methylmethacrylate). Jet resin without carbon graphite fiber reinforcement was significantly stronger than Trim resin with and without fiber reinforcement and Splintline resin without fiber reinforcement. Of the three resins, Jet was considered the material of choice. The use of carbon graphite fibers is promising for the reinforcement of long-span provisional fixed partial dentures, since the incorporation of the fibers would not require a significant amount of extra time or cost. Esthetics, however, present a legitimate limitation to the use of these fibers. Until the black of the carbon can adequately be concealed with an opaque medium, these provisional fixed restorations would be limited to the posterior regions intraorally. CONCLUSIONS The findings of this investigation suggest the following conclusions: 1. The moduli of elasticity of Jet, Trim, and Splintline resins, with and without carbon graphite fiber reinforce-

819

LARSON ET AL

y t%i%rent. The order of strengths med., are r&8 a=- as follows: G&K Sp&ntine< Trim. tion of carbon graphite fibers signifi2. Thein ca&y increases the modulus of elasticity of the three resins tested. 3. Storage in water over time does not have a significant effect on themodulusof elasticity of the three resins tested. REFiFRENCES 1. Baum L, Porte E. Fabrication of an immediate temporary bridge. J Corm State Dent Assoc 1981;55:91-2. 2. Shillingburg WT, Hobo S, Wbitaett LD. Fundamentals of tied prostbodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1982161. 3. -Miller SD. The ante&or fixed provisional restoration: a direct method. J PROSTHET IX&T 1983;50:516-9. 4. Beyh MS, vcmF&@mfer JA. An an&.&of causesof fracture of acrylic resin dentums. $-PRQSTHET DENT 1981;46:238-41. 5. Bjciik N,~Ek&zand KG, Ruyter IE. Carbon/graphite fiber reinforced p&m& impb@ bridge prosthesis. Swed Dent J 1985;28:77-84. 6. Smith ISA, Sauer JA- Sorbed water and mechanical behavior of poly (methyl methacrylate)I Pi& Rubber Process Appl1986;6:57-65. 7. Gegauff AC, Pryor HC. Fracture toughness of provisional resins for tIxed prosthudontics. J PR-THET DENT 198’7;58:23-9. 8.~Donovan TE, Hurst RG, Campsgni WV. Physical properties of acrylic

resin polymerized by four different techniques. J PROSTHET DENT 1986;54:622-4.

9. Yasadanie N, Mahood M. Carbon fiber acrylic resin composite: an investigation of transverse strength. J PROSTHFTT DENT 1986;54: 543-7. 10. Ruyter IE, Ekstrand KG, Bjork N. Development of carbon/graphite fiber reinforced poly (methyl methacrylate) suitable for implant-fixed dental bridges. Dent Mater 198@2:6-9. 11. Dixon DL, Ekstrand KG, Breeding LC. The transverse strengths of three denture base resins. J PROSTHET DENT 1991;66510-3. 12. Stafford GD, HandIey RW. Transverse bend testing of denture base polymers. J Dent 1975;3:251-5. 13. Wang RL, Moore BK, Goodacre CJ, Swarm ML, Andrec CJ. A comparison of resins for fabricating provisional fixed restorations. Int J Prosthodont 1989;2:173-84. Reprint requests to: DR. DONNA L. DIXON COLLEGE OF DENTISTRY UNIVERSTY OF IOWA IOWA CITY, IA 52242

Contributing

author

Karl G. Ekstrand, DDS, PhD, Assistant Clinical Professor, Department of Prosthodontics, University of Iowa, College of Dentistry, Iowa City, Iowa.