CASTING

TITANIUM

REMOVABLE

PARTIAL

DENTURES

considerable variations within the ranges of measured results. Cross arch lmean measurements were not proportional. Shrinkages in the premolar region (1% mean) were less than those in the molar region (2.6% mean). Dimensional changes in cast titanium RPD frameworks can be expected to be similar to those found with commonly used RPD base metal alloys. Methods for best controlling factors influencing dimensional changes have yet to be fully investigated. To provide a comprehensive view of pure titanium as an alternative RPD metal, further study of technical procedures and materials is needed. REFERENCES 1. Barice WJ. Titanium net shape technologies. Warrendale: The Metallurgical Society of .4IME, 1984:179-90. 2. Moser JB, Lin JHC, Taira M, Greener EH. Development of dental PdTi alloys. Dent Mater 1985;1:37-40. 3. Ida K, Togaya T, Tsutsumi S, Takeuchi M. Effects of magnesia investments in the dental casting of pure titanium or titanium alloys. Dent Mater J 1982;1:8-21. 4. Kevin TA, Kay JF. Cook DC, Jarcho M. The effect of surface macrotexture and hydror.ylapatite coating on the mechanical strengths and histologic profiles of titanium implant materials. J Biomed Mater Res 1987;21:1395-414. 5. Taira M, Moser JB, Greener EH. Studies of Ti alloys for dental castings. Dent Mater 1989;5:45-50. 6. Riesgo 0, Greener EH, Moser JB. Titanium alloys with dental porcelain [Abstract]. J Dent Res 1984;63:319.

7. Togaya T, Suzuki M, Tsutsumi S, Ida K. An application of pure titanium to the metal porcelain system. Dent Mater J 1983;2:210-9. 8. Barice WJ. Titanium net shape technologies. Warrendale: The Metallurgical Society of AIME, 1984:155-73. 9. Pulskamp FE. A comparison of the casting accuracy of base metal and gold alloys. J PROSTHET DENT 1979;41:272-6. 10. Shanley 35, Ancowitz SJ, Fen&r RK, Pelleu GB. A comparative study of the centrifugal and vacuum-pressure techniques of casting removable partial denture frameworks. J PROSTHET DENT 1981;45:18-23. 11. Szurgot KC, Marker BC, Msser JB, Greener EH. The casting of titanium for removable partial dentures. Quint Dent Technol Yearbook 1988;12:171.

12. Carter TJ, Kidd NJ. The precision casting of cobalt-chromium alloy. Part 1. The influence of casting variables on dimensions and finish. Br Dent J 1965;118:383-90. 13. Carter TJ, Kidd NJ. The precision casting of cobalt-chromium alloy. Part 2. The influence of casting variables on microstructure and mechanical properties. Br Dent J 1965;118:431-6. 14. Roydhouse RH, Skinner EW. The accuracy of large castings. J Dent Res 1961;40:1057-78. 15. Rantanen TEE. Accuracy of the palatal plate of removable partial den-

tures, and influence of laboratory handling of the investment on the accuracy. Dent Mater 1986;2:28. 16. Firtell DN, Muncheryan AM, Green AJ. Laboratory accuracy in casting removable partial denture frameworks. J PROSTHET DENT 1985;54:856-62.

Reprint requests to: DR. RONALD BLACKMAN DENTAL SCHOOL UNIVEFWT~ OF TEXAS HEALTH SCIJZNCE CENTER 7703 FLOYD CURL DR. SAN ANTONIO, TX 78284-7890

Bond strength and failure analysis resins bonded to denture teeth James M. S. Clancy, D.D.S., M.S.,* L. Foster Hawkins,** Ph.D.,*** and Dan B. Boyer, D.D.S., Ph.D.**** University

of’ Iowa, College of Dentistry,

of light-cured

denture

John C. Keller,

Iowa City, Iowa

This study examined the tensile bond strength and failure sites of one heat-cured (Microlon) and two visible light-cured (Triad and Extoral) denture resins to two types of denture teeth. The resins were processed into cylinders against denture teeth milled to the same size. Half of the specimens were thermocycled. After tensile testing, statistical analysis (p 0.06) showed that the strongest bond was achieved with the heat-cured resin Microlon, whereas Triad resin displayed the stronger bond between the two light-cured resins. Scanning electron microscopic analysis sbowed that the nature and location of the fracture sites was different among the three resins tested. (J F%OSTHET DENT 1991;65:316-24.)

v*

isible light-cured (VLC) denture resins have become popular for many prosthodontic applications since the introduction of Triad VLC resin (Dentsply Interna-

*Assistant Professor, Department of Prosthodontics. **Senior dental student. ***Associate Professor, Dows Institute for Dental Research. ****Professor, Department of Operative Dentistry.

10/l/22666

THE JOURNAL

OF 1PROSTHETIC

DENTISTRY

tional, Inc., York, Pa.) These applications include complete and removable partial denture relines, provisional fixed partial dentures, denture repairs, record bases, and custom impression trays. Ease of handling and reduced laboratory time are primary reasons for their popularity. The physical properties and biocompatibility of Triad resin have been the subject of several investigations. Ogle et a1.l concluded that most physical properties of Triad compared favorably with those of heat-cured and auto-

315

CLANCY

ET AL

Fig. 1. Milled denture teeth before flasking.

filmrnl jernrn q-1, I I

I

-t

2mm

jl2mm I

I

lomm

Fig. 2. Diagram illustrating pattern.

Fig. 4. Tooth-resin specimens in mold before bonding with test resin.

’

I 5mm

dimensions of first master ____Fig. 5. Completed specimen.

Fig. 3. Tooth-resin specimens before bonding with test resin. polymerizing denture base resins. The staining characteristics, transverse strength, and microhardness of Triad resin were examined by Khan et a1.,2who reported superior transverse strength and microhardness compared with conventional denture base resins. However, the Triad resin showed significantly more staining than the conventional resin. More recently, three independent studies examined the shear bond strength of VLC resins to denture base resins. Razavi et al3 found the bond strength of Triad resin bonded to four different resins to be as strong as the Triad resin itself. Curtis et a1.4examined the bond strength of a VLC resin bonded to heat-cured and autopolymerizing

316

resins after thermocycling and reported significantly lower values than that of heat-cured resin bonded to autopolymerizing resin. Conners et al5 concluded that use of Triad bonding agent and/or methylmethacrylate monomer with some form of surface roughening was necessary to achieve an acceptable bond to heat-cured resin. Similar results were reported in an investigation by Bolouri et al6 where blocks of heat-cured resin were joined with either autopolymerizing resin or visible light-cured resin and subjected to a three-point bend test. The VLC resin test values were found to be significantly lower than those for the autopolymerizing resin. The physical properties of Astron VLC resin (Astron Dental Corporation, Portland, Ore.) and Triad resin were compared with those of autopolymerizing resin by Tulachka and Moser.7 Under SEM analysis, the surface texture of Astron resin contained numerous voids and resembled that of autopolymerizing resin. Unlike Triad material, which was totally dependent on visible light for polymerization, the Astron resin achieved an initial set through selfactivation before final curing with visible light. Bonding of the VLC resins to denture teeth has been investigated by several authors. Ishigama et al.* found that

FEBRUARY

1991

VOLUME

65

NUMBER

2

BOND

STRENGTH

AND

FAILURE

ANALYSIS

Denture Tooth Location f

llrnrnf

$6mm

~--+---j-j

[ 12rnrn

yz~z~z~ I

I

5mm

Fig. 6. Diagram illustrating

dimensions of completed specimen.

the fracture streqlth of denture teeth bonded to a block of Triad resin was greater than that of teeth bonded to a block of heat-cured resin with Triad as a repair medium. The bond strength of three types of denture teeth bonded to heat-cured, autopolymerized, and VLC denture base resins was examined by Kawara et a1.gusing a four-point bend test. VLC resin bonded poorly to all three types of teeth with no significant differences in bond strength. Similar results were reported by Clancy and Boyer.lO Their investigation compared the tensile bond strength of two types of denture teeth bonded to heat-cured, autopolymerized, and Triad VLC resin. They also reported poor bonding values with the Triad resin bonded to both types of teeth. This investigat.ion was done to test the tensile bond strength of two visible light-cured denture resins bonded to two types of resin denture teeth after thermocycling. The fracture sites were examined under a scanning electron microscope (SEM) to establish the nature of the bond failure.

METHODS

AND MATERIAL

Trubyte Bioform plastic teeth and Trubyte Bioform interpenetrating polymer network (IPN) abrasion-resistant teeth (Dentsply International Inc., York, Pa.) were chosen for bonding to three types of base material: Microlon L-W heat-cured denture resin (Hygenic Corp., Akron, Ohio), Triad VLC resin (Dentsply International, Inc.), and Extoral VLC resin {Pro-Den Systems, Inc., Portland, Ore.). Posterior teeth at least 9 mm in width were embedded in sticky wax and milled on a pantograph engraving machine (Model PL-3, Gorton Machine Corp., Racine, Wis.) into uniform cylinders 8 mm in diameter. The center of each ridge-lap section was further milled into a smaller cylinder 6 mm in diameter and 2 mm in length (Fig. 1). Two perpendicular grooves were placed on the occlusal surface of the teeth with a No. 33 bur for mechanical retention. A master pattern was made from a block of methylmethacrylate resin (Fig. 2) and duplicated in a mold made from vinyl polysiloxane bite registration material (Regisil, L.D. Caulk Company, Milford, Del.) backed by yellow

THE

JOURNAL

OF .PROSTHETIC

DENTISTRY

7. Specimen mounted in testing device before tensile loading.

Fig.

stone within an upper denture flask (Teledyne Dental Products, Hanau Division, Buffalo, N. Y.). Ten duplicate dies were produced and used to make a new master mold of similar design. A second master pattern of different dimensions was milled from a block of methylmethacrylate in the same manner as the first. Five duplicates of this pattern were used to make a second master mold within an upper denture flask. Microlon heat-cured resin was mixed according to the manufacturer’s directions. The milled tooth specimens were placed in the first master mold and the acrylic resin was packed against the occlusal surface of the teeth into the retention grooves. After removal of all excess resin, the flask was placed in a curing tank for 9 hours at 165’ F. After curing, the specimens were carefully removed and the ridge lap surfaces of the teeth were flattened with No. 220 grit sandpaper in a custom jig. This process was repeated

317

CLANCY

ET AL

Denture Base Resin Tooth

Thermocycling

Duncan Results

Standard Acrylic Standard Acrylic IPN IPN

Thermocycled Non-thermocycled Thermocycled Non-thermocycled

Microlon>Triad>Extoral Microlon>Extoral>Triad Microlon>Triad>Extoral Microlon>Triad>Extoral

Tooth

Denture Base Resin

Duncan Results

Standard Acrylic IPN

Extoral Extoral

Non-thermocycled>Thermocycled Non-thermocycled>Thermocycled

Thermocycling

Denture Tooth Denture Base Resin

Thermocycling

Duncan Results

Microlon Microlon Extoral

Thermocycled Non-thermocycled Non-thermocycled

Standard Acrylic>lPN Standard Acrylic>lPN Standard Acrylic>lPN

Fig. 8. Statistically significant results as shown by Duncan’s multiple range test.

Table I. Mean tensile bond strengths in test groups (in megapascals) IPN

abrasionresistant Group Thermocycled

Nonthermocycled

Base resin

Standard acrylic resin

x

SD

x

SD

23.2 11.8* l.O* 25.3

2.8 6.3 2.5 2.8

9.1

2.6

Microlon

19.1

3.9

Triad Extoral

11.7

3.7

5.0t

2.1

Microlon

17.4*

4.4

Triad

10.6

2.6

Extoral

7.4*

1.8

12.0

2.7

N = 10 except as noted. *N = 9 because of fracture during processing. TN = 8 because of fracture during processing.

for 60 IPN and 60 plastic tooth-resin specimens (Fig. 3). Twenty IPN and 20 plastic tooth-resin specimens were systematically placed in the second master mold (Fig. 4). Microlon heat-cured resin was mixed according to the manufacturer’s directions and a small amount of monomer was brushed against the ridge-lap surface. The resin was packed against this surface in a standard air compress and cured at 165” F for 9 hours. After removal from the mold the specimens were examined for discrepancies. All acrylic resin flash was carefully removed with a sandpaper disk (Figs. 5 and 6). The tooth-resin specimens to be used with the VLC resins were managed in a slightly different manner. The lower half of the master mold for these specimens contained a clear liner that was removable. The Triad material was

318

placed in a mold and trial packed under the air compress. After separation of the upper half of the mold and removal of excess flash, the specimens were placed under visible light in the Triad curing unit for 12 minutes. Next the specimens and the clear liner were removed intact from the mold and replaced under the curing light for an additional 12 minutes. This was to ensure a complete cure within the Triad material. The specimens were then separated from the liner and trimmed of excessflash with a sandpaper disk. Twenty plastic and 20 IPN specimens were made with the Triad resin. The Extoral VLC resin was provided as a powder-liquid system. After the two components were mixed together according to the manufacturer’s directions, the resin was placed in the same master mold used with the Triad material. The remainder of the fabrication procedures were identical to those used for Triad resin. Twenty plastic and 20 IPN specimens were made. All specimens were stored in a dry condition at room temperature for 1 week. Prior to tensile testing, half of the specimens in each test group were thermocycled between 5” C and 55” C in 50-second cycles for 24 hours (approximately 1000 cycles). Tensile testing was performed on an Instron

Universal

testing machine

(Instron

Corporation,

Canton, Mass.) by use of a 200 kg load cell with a crosshead speed of 0.05 cm/min and chart speed of 2 cm/min. Specimens were placed in a custom alignment device and subjected to tensile force until failure (Fig. 7).

RESULTS The results of testing are summarized

in Table I. Group

means were compared by use of the Duncan multiple range

FEBRUARY

1991

VOLUME

66

NUMBER

2

BOND

STRENGTH

AND

FAILURE

ANALYSIS

30

0

25

20

g

a.5 ::y a 0.0. 5.. :::: ::>

15

10

5

i

Standard Denture

Fig.

9. Comparison

JOURNAL

OF PROSTHETIC

~ Extoral

R :::: :::: 0.0. :::: :::: 0.0.

Mlcrolon

Triad

Extoral

IPN Abrasion-Resistant Denture Teeth

of tensile bond strengths

10. SEM view of heat-cured

DENTISTRY

::::

Acrylic Teeth

test at the 5% level of significance (Fig. 8). Strongest bonding occurred between the heat-cured resin Microlon to both types of teeth. The actual bond strength of the material was not determined because the typical fracture was within the tooth itself or the resin cylinder. The VLC resin Triad displayed stronger bonding than the VLC resin, Ex-

THE

l .*. :::: :*::

.*.a .‘... . l .-. :::: :::: x 0.0. :::: f ‘. :::: 5% 1.:.

Triad

Microlon

Fig.

Non-themmcyckd

q Thermocycled

T

resin fracture

of tooth-resin

combinations.

site (within

resin cylinder).

toral, to both kinds of teeth, except in the nonthermocycled, plastic tooth group. Thermocycling had no effect on the bond strength of Microlon or Triad resins but decreased the bond strength of Extoral resin to both types of teeth. Bonding to the standard plastic denture teeth was significantly stronger than bonding to the IPN abrasion-

319

CLANCY

Fig. 11. SEM view of denture tooth-Triad section).

resistant teeth within the Microlon resin and nonthermocycled Extraoral resin groups. However, there was no statistical difference between the IPN and the standard teeth among the Triad resin and thermocycled Extoral resin groups (Fig. 9).

SEM analysis After the interfacial bond testing, representative specimens were chosen from each test group for evaluation of their fracture interfaces with SEM. Specimens were mounted on aluminum SEM stubs and coated with Au-Pd. Examination was performed with an Amray 1820-D SEM/ Edax (Amray Inc., Bedford, Mass.) at magnifications of x25 to x250. The heat-cured specimens generally fractured cohesively within the denture tooth or the resin cylinder. SEM analysis was consistent with the normal appearance of poly ~methylmethacrylate) resin at high m~ni~cation (Fig. 10). The visible light-cured resins typically failed at the interface between the tooth and the test resin. SEM analysis revealed a mixed adhesive-cohesive fracture between the tooth and resin, and within the VLC resins themselves (Figs. 11 and 12). Interpretation of the exact composition of the VLC fracture sites was limited by the ability to distinguish specific structures and/or materials under SEM analysis. Therefore, several additional tooth-resin specimens were made to facilitate these interpre~tions. The appearance of a typically prepared tooth surface with and without Triad bond-

320

ET AL

VLC resin fracture site at the interface (tooth

ing liquid was examined (Fig. 13). In addition, several specimens were made with Triad and Extoral resins respectively to cover half of a prepared tooth surface. These specimens were cured, then subjected to tensile force until fracture. Analysis of these specimens allowed interpretation of VLC fracture sites to be more specific by providing a visual comparison of unprepared tooth surface, bonding liquid alone on a prepared tooth surface, and representative VLC resin-tooth fracture interfaces (Figs. 14 through 16). The following observations were made: Triad resin specimens appeared to fail primarily in an adhesive fashion between the tooth and the bonding liquid. However, a smaller portion of the bond sites failed cohesively within the Triad resin or the bonding liquid itself. The Extoral resin specimens primarily failed cohesively within the resin, but a smaller portion failed adhesively between the resin and denture tooth. The results of the fracture site observations are listed in Table II.

DISCUSSION The results of this investigation were in general agreement with those of previous bonding studies involving visible light-cured denture resin.s-lo The bond strength of the VLC resins was lower than that of the heat-cured resin bond to both types of denture teeth. Several variables that were thought to have influenced the results of an earlier investigationlO were carefully controlled in this study. This included (1) limiting the mechanical stress to the speci-

FEBRUARY

1991

VOLUME

66

NUMBER

Z

BOND

STRENGTH

AND

FAILURE

ANALYSIS

IFig. 12. SEM view of denture tooth-Extoral ((tooth section).

VLC resin fracture site at the interface

Fig. 13. SEM view of prepared denture tooth surface with Triad VLC bonding liquid on left half of specimen. mens Iduring fabrication, (2) achieving adequate depth of cure fc)r all VLC specimens, and (3) use of a self-centering device during tensile testing. The Triad resin displayed higher bond strength values than those of the previous in-

THE

JO WRNAL

OF F’ROSTHETIC

DENTISTRY

vestigation but still were significantly lower than the hkeatcured values. Triad VLC resin displayed stronger bonding than the Extoral VLC resin for all groups except those in the 321

CLANCY

ET AL

Fig. 14. SEM view of denture tooth-Triad VLC resin fracture site at the interface (tooth section). Portion designated A displays adhesive failure; portion B, cohesive failure within resin; and portion C, unbonded tooth surface.

Fig.

15. SEM view at higher magnification of surface A seen in Fig. 13.

nonth lermocycled, plastic tooth group. Extoral material is prima lrily marketed as a chairside reline resin. It is possible th at the material may not be as effective when bonded to de]nture teeth because of the highly cross-linked resin used jin denture teeth.

322

Thermocycling had no effect on the bond strength of ‘the heat-cured resin, Microlon, or the Triad VLC resin. H owever, it significantly decreased the strength of the Extl oral VLC resin bond to both types of teeth. This finding shemid be investigated further.

FEBRUARY

1991

VOLUME

65

NUMBER

2

BOND

STRENGTH

AND

FAILURE

ANALYSIS

Fig.

16. SEM view at higher magnification of area B seen in Fig. 13.

Bonding to the standard acrylic resin denture teeth was greater or equal to bonding to the IPN abrasion-resistant teeth, dependent on the base used. The bond was greater for the Microlon resin and the nonthermocycled Extoral resin groups, but no different for the Triad resin and thermocycled Extraoral resin groups. IPN denture teeth have a higher degree of cross-linking than standard plastic teeth that should result in lower bond strength because of less availability of unlinked polymer chains. In this study there was no difference within the Triad resin groups, in agreement with the fi:ndings of Kawara et al.g These findings should also receive further investigation. SEM analysis was useful in determining the nature of tensile bond failure with the two VLC resins. The Triad resin appeared to fail primarily at the interface between the bonding liquid and the tooth. Although remnants of bonding liquid and/or VLC resin were visible on the tooth surface, most of the area was similar in appearance to the prepared tooth surface before bonding. The Extoral resin was opposite in appearance, having mostly VLC resin remaining on the denture tooth surface. This implies that the Extoral resin bond was stronger than the resin itself under tensile load. However, the strength of the material itself may be less than other VLC resins and should be investigated further. The clinical significance of the results of this study is difficult to evaluate. Use of the VLC denture resins in short-term prosthodontic applications would seem to be acceptable, but caution should be exercised in planning their use in a long-term or high-stress situation. Further investigation into the physicalproperties of these materi-

THE

JOURNAL

OF F’ROSTAETIC

DENTISTRY

Table II.

Comparison

of fracture sites Frequency of fracture type

Cohesive

Mixed, primarily adhesive

Mixed, primarily cohesive

33 2 3

4 34 0

2 2 31

Microlon Triad Extoral x2 = 138.98; p c 0.005.

als and longitudinal clinical data are required before these materials are recommended as an alternative to methylmethacrylate resins.

CONCLUSIONS This investigation evaluated the tensile bond strengths of two visible light-cured denture resins and one heatcured denture resin bonded to standard plastic denture teeth and IPN abrasion-resistant denture teeth with and without thermocycling. The failure sites were then examined under scanning electron microscopy. The conclusions were as follows: 1. The strongest bonding was with the heat-cured resin Microlon bonded to Dentsply standard acrylic resin teeth and to Dentsply IPN abrasion-resistant denture teeth. The visible light-cured resin Triad displayed stronger bonding than the visible light-cured resin Extoral to both types of teeth except the standard acrylic resin teeth in the nonthermocycled group.

323

CLANCY

2. Thermocycling had no effect on the bond strength of Microlon and Triad resins, but decreased the bonding of Extoral resin to both types of teeth. 3. Bonding to the standard acrylic resin denture teeth was stronger or equal to bonding to the IPN abrasion-resistant denture teeth, dependent on the base used. The bond was greater with the Microlon and nonthermocycled Extoral resin groups, but no different for the Triad and thermocycled Extoral resin groups. 4. The nature and location of the fracture sites were different among the three resins tested.

4. Curtis D, Eggleston T, Watanabe L, Marshell S. Visible light cured resin as a reline material [Abstract]. J Dent Res 1989;68:337. 5. Conners M, Barzilay I, Myers ML, O’Connell BC, Graser GN. Bond of reline/repair materials to heat cured acrylic resin [Abstract]. J Dent Res 1988;67:136. 6. Bolouri A, Marker VA, Sarampote R. Comparison of a visible light curing and chemically activating repair resins [Abstract]. J Dent Res 1989;68:337. 7. Tulachka GJ, Moser JB. Viscoelastic properties of a new light cure reline material [Abstract]. J Dent Res 1989;68:336. 8. Ishigama K, Mashio T, Tsukui J, Umi T, Maeda M, Iwachi N, Ujhe Y, Satoh Y, Anzai M, Ohki K. Basic studies on visible light-curing resin as a denture base. J Nihon Univ Sch Dent 1987;29:35-41. 9. Kawara M, Carter JM, Ogle RE, Johnson RR, Sorensen SE. Bonding of plastic teeth to denture base resins [Abstract]. J Dent Res 1989;68: 924. 10. Clancy JMS, Boyer DB. Comparative bond strength of light-cured, heat-cured, and autopolymerizing denture resins to denture teeth. J

REFERENCES 1. Ogle RE, Sorenson SE, Lewis EA. A new visible light-cured resin system applied to removable prosthodontics. J PROSTH~T DENT 1986;56:497-6.

2. Khan Z, van Fraunhofer JA, Razavi R. The staining characteristics, transverse strength, and microhardness of a visible light-cure denture material. J PROSTHET DENT 1987;57:384-6. 3. Razavi R, Khan Z, von Fraunhofer JA. Bonding of Triad light cured denture reline material to denture base resins [Abstract]. J Dent Res 1988:67:136.

ET AL

PROSTHET DENT 1989;61:457-62.

Reprint requests to: DR. JAMES M. S. CLANCY COLLEGE OF DENTISTRY, S-417 UNIVERSITY OF IOWA IOWA CITY, IA 52242

Correction In the September issue of the JOURNAL OF PROSTHETIC DENTISTRY, on page 362, under the heading “Bonding of restorative materials to tooth structure,” the sentence “Other studies showed reduced bond strengths after storage of up to 9 years in distilled water.‘s@13~” should have read “Other studies showed reduced bond strengths after storage of up to 9 months and 1 year in distilled water.“.61”

324

FEBRUARY

1991

VOLUME

65

NUMBER

a