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Acrylic resin denture repair with adhesive resin and metal wires: Effects on strength parameters
Acrylic resin denture repair with adhesive wires: Effects on strength parameters Gregory L. Polyzois, DDS, Dr Dent, MSCD,~ Andreas G. Andreopoulos, Chem Eng, Dr Eng,b and Panagiotis E. Lagouvardos, DDS, Dr Dent? University of Athens, School of Dentistry, and National Technical University Greece
resin and metal
of Athens, Athens,
The fracture of acrylic resin dentures is an unresolved problem in prosthodontics. In this study one brand of denture base acrylic resin was used to make specimens in the form of strips and maxillary denture bases. The specimens were cut and repaired with one type of an autopolymerizing adhesive resin and metal wires. The mechanical properties of the repaired specimens were measured, and the efficiency of each method was evaluated. The statistical results of this study revealed that geometric characteristics of a maxillary denture combined with the shape and pretreatment of reinforcement were the controlling factors for the overall mechanical behavior. Furthermore this study revealed that data with clinical significance can only be obtained by testing specimens similar to the original items used in dental practice. (J
DENT 1996;75:381-7.)
PROWHET
T
he fracture of acrylic resin dentures is an unresolved problem in prosthodontics. Midline fracture was the most common problem in a recent denture survey conducted in the United Kingdom, where 29% of the total denture repairs were made for this type of fracture.l Of these midline fractures 71% were seen in maxillary complete dentures, and 29% were seen in mandibular dentures. Denture fracture occurs both outside and inside the mouth. Outside the mouth failure occurs through impact as a result of dropping the denture. Inside the mouth excessive biting force causes fracture. In function, however, midline fracture is the result of flexural fatigue failure caused by cyclic deformation of the base and is more likely to occur because flexure of the denture base occurs along the midline. Regardless of the reason for the fracture or the method of repair, satisfactory repairs must have adequate strength, be easily and rapidly completed, match the original color of the material, and retain its dimensional accuracy. However, these criteria cannot always be achieved. A denture repaired with heat-cured resin exhibits approximately 85% of its original strength, and an autopolymerizing repair has only 55% to 65% of the original heat-cured denture strength.2 Several investigators have studied the repair strength of denture base resins with heat-curing acrylic resin,3, 4 autopolymerizing acrylic resin,5-10 cyano-
=Assistant Professor, Department of Prosthodontics, Maxillofacial Prosthetics Service, School of Dentistry, University of Athens. bAssociate Professor, Department of Chemical Engineering, National Technical University of Athens. “Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Athens. Copyright 0 1996 by The Editorial Council of THE JOURNAL OF PROSTHETIC
DENTISTRY.
0022-3913/96/$5.00
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+ 0.
10/l/69037
acrylates,ll and visible light-cured resins such as premixed, gel, and powder-liquid.12-14 Many different approaches to solving problems have been associated with broken dentures, such as modifying the denture material itself (high-impact resins) or reinforcing it. The most. common reinforcing technique is the use of metal wires embedded in the prostheses, although their value remains questionable.l, 15-17 It is well known that acrylic resin exhibits a low potential for adhesion to nonacrylic materials. Different methods exist for improving the adhesion between metal and acrylic resin, namely, sandblasting of the surface of the wire,ls, I9 silanization of the surface of the metal with different techniques,20l 21 or the use of a metalbonding resin.22-24 In 1985 an autopolymerizing adhesive resin (Meta Fast, Sun Medical Co., Ltd., Kyoto, Japan) with various applications in prosthetic dentistry was first marketed.25 The adhesive resin consisted of a powder (methyl methacrylateethyl methacrylate copolymer, MMA-EMA copolymer), a liquid (MMA), and a bonding liner (MMA-, 4-META). The bonding liner, which contained the metal adhesive monomer 4-metacryloxyethyl trimellitate anhydride (4META), is the most important component of the kit. The various applications of the bonding liner for removable prosthodontics are (1) rebasing of a metallic denture, (2) repairing a metallic denture or the addition of artificial teeth, (3) reinforcement with cobalt-chromium or nickelchromium wires when working and repairing a resin denture plate, (4) repair of resin fracture at cast wire clasp, and (5) coating for clasp leg or wire clasp. This study examined the strength of the heat-cured acrylic strips and denture bases repaired with Meta Fast metal adhesive resin alone or in combination with metal wires.
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~~~IS~Y
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AND ~GO~~S
Fig. 1. {A) Halves of acrylic resin strips placed in openended stone molds IB) 3 mm apart (C).
Fig. 2. Acrylic resin strips and metal wire placed into central recess with 3 mm gap.
MATERIAL
reinforce the specimens with wire, a grinding stone disk (2.5 mm thick) was used to create a central channel, 2.5 mm x 65 mm, perpendicular to the butt joint for placement of wires. The open-ended stone molds were then used so that the surfaces to be prepared could be moved apart to leave a 3 mm gap. The metal wires were half-round hard (0.75 mm x 1.5 mm in diameter) and round spring hard wires (1.5 mm in diameter) (Remanium, Dentaurum, Pforzheim, Germany) that were sandblasted with 50 pm aluminum oxide (Al2031 delivered by air pressure applied at 5.8 x lo2 KPa (84 psi) for 1 minute. After handling, all wires were ultrasonically cleaned in alcohol for 5 minutes and then treated with a thin layer of Meta Fast bonding liner before they were placed in the central recess. The application of Meta Fast adhesive resin for the repair (Fig. 2) was built up into the central recess and the butt joint with the brush-on procedure as recommended by the manufacturer. When wire reinforcement was used, the buildup of layers of Meta Fast adhesive resin was completed within 3 minutes after the application of the bonding liner. After the polymerization process was completed at room temperature (21 1 1” C), the repaired strips were trimmed to their original dimensions with 600~grit paper and kept in distilled water at 37” C for 48 hours before testing was performed (Fig. 3). All the denture bases were processed on stone models (Titanit, Renfert GmbH Co, Singen, Germany) derived from a silicone mold (Silastic, Dow Corning Ltd., Glamorgan, U. K.) of a medium-sized maxillary stone master model with no undercuts. On each stone cast a vacuumformed 3 mm thick baseplate (Bioplast, Scheu-Den~l, Iserlohn, Germany) was adapted to ensure uniformity of denture base thickness. Casts and base plates were invested with dental stone in metal denture flasks. The vacuum-formed baseplates were discarded, and heat-curing resin was packed and processed. After processing was performed, each denture base was lightly finished to remove
AND METHODS
Two types of specimens, acrylic resin strips and maxillary denture bases, were used. The maxillary denture base with its three-dimensional form approximates the manner in which the denture resin is used clinically. The bulk and shape gives the acrylic resin denture base unique physical properties that may not reflect those of standard acrylic resin strips. For example, the frenum notch in a maxillary denture provides a stress riser from which a midline fracture originates. A heat-curing acrylic resin (Meliodent, Bayer Dental, Newbury, Berkshire, U. K.) was mixed in a ratio of 23.4 gm powder to 10 ml monomer as recommended by the manufacturer and was polymerized in a dry heat oven at 75” C for 12 hours. This denture resin was used to prepare all specimens, strips, and denture bases. Four groups of strips and dentures were prepared. Group A was the control group and had no repair. Group B used Meta Fast adhesive resin treatment only on the fracture. Group C used Meta Fast adhesive resin treated with round wires, and group D used Meta Fast adhesive resin treated with half-round wires. Acrylic resin strips were produced in molds made by investing pattern blocks, 3.5 mm x 10.5 mm x 65 mm, of Perspex (ICI Ltd., Wehvyn, Garden City, U. K.) in dental stone in the conventional dent~e-easing procedure. After the stone set, the blocks were removed carefully, and the acrylic resin was mixed, packed, and then processed. The size of the finished strip specimen was 3 mm x 10 mm x 65 mm (20.05 mm). Specimens were stored in a distilled water bath for 1 month at 37” C to ensure complete satiation. After 1 month of storage the specimens were cut into halves with a diamond saw to simulate fracture. The cut surfaces were ground off to a butt profile by use of 240-g& silicon carbide paper under water. The halves of the strips were placed in open-ended stone molds of the same dimensions as the intact strips (Fig. 1). The openended molds were prepared before the strips were cut. To
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Fig. 3. Acrylic resin strips repaired with Meta Fast adhesive resin and reinforcing wire.
the flash to the vestibular roll portion and was lightly pumiced. To cheek for uniformity the thickness of each denture base was measured in the ~d~palat~ portion with a caliper that could be read to 0.05 mm. All denture bases were immersed in distilled water at 37” C for 1 month. After the saturation in water was performed, dental stone models were poured into the bases and allowed to set to make new casts. For a sample to receive the reinforcing wire, a 70 mm x 2.5 mm x 2.5 mm horseshoe shape recess was created approximately 3 to 4 mm below and palatal to the corresponding crest of the ridge. Each denture base was removed from its stone cast and sectioned with a cutting disk in a sag&al direction along the midline to simulate fracture. The cut edges were prepared to a butt profile by grinding under water on 240-g& SiC paper to create a 3 mm gap between the two sections of the base (Fig. 4). The sections of the bases were placed on their respective casts, and the repair procedure was followed. The same materials used for the repair procedures and groups as acrylic resin strips were used for the denture bases. After polymerization was performed, the excess of the repair resin was removed, and the bases were placed in distilled water at 37” C for 48 hours before testing (Fig. 5). Twelve bases were constructed and repaired for each group. All the tests were performed in air at 21” 2 1” C with the Monsanto universal testing machine (Model TlO, Monsanto Ltd., Swindon, Wiltshire, U. K.), which operated at a crosshead speed of 5 mm/minute.
Testing
acrylic
resin
strips
After being stored in water, unrepaired and repaired strips were placed in the Monsanto bending jig (TG 168) (Fig. 6) and fractured under three-point loading with a distance of 50 mm between the jig supports. The load-deflection behavior was recorded automatically by the chart recorder of the testing machine. The failing loads of the
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OF PROS~TIC
D~~ISTRY
Fig. 4. ~~llary denture base with horseshoe metal wire in repaired groove and 3 mm gap along midline.
Fig. 5. Maxillary denture base reinforced with metal wire and repaired with Meta Fast adhesive resin.
acrylic resin matrix of strips were obtained from the first apex or “blip” on the machine curve. The deflection at fracture was obtained from the same curves with a precision of t8 pm.
Testing
denture
bases
Af%terbeing stored in water, each denture base was placed in the compression cage (Model TG 175, Monsanto Ltd.), and acompressive load was applied through a 10 mm ball attachment (Fig. 7). The application of the downward load along the midline of the tissues surface of the denture was designed to be equivalent to an upward load on both sides combined with an unyielding support in the center of the palate. The nontissue side of each denture base was secured on the lower plate ofthe compression cage, and the load applicator was positioned on the tissue side of the base at the midline height of the convexity in a position to correspond to the second premolar and first molar regions. To facilitate the placement of the load applicator in a repro-
THE JOURNAL
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POLYZOIS,
ANDREOPOULOS,
AND LAGOW’ARDOS
t
Fig. 6. Diagram of bending jig.
Fig. 7. Compression cage shows location of applied force.
ducible manner for each denture base, the midline and a line that corrected the left and right buccal frenum notches were drawn on the tissue side. The tip of the load applicator was placed each time at the intersection of the two lines (Fig. 8). The load to failure of the intact base and repaired joints was recorded by the chart recorder of the testing machine.
RESULTS The fracture load of denture bases and the fracture load and deflection at break of the acrylic resin strips are presented in Table I. A Kruskal-Wallis, nonparametric oneway analysis of variance (ANOVA) followed by a multiple comparisons test according to Siegel and Castellan26 were used to analyze the data. A summary of statistical analyses is presented in Table II. From Table I it appears that fracture load and deflection of strips followed the same pattern, namely, the specimens repaired with Meta Fast acthesiveresinonly(B)exhibitedthelowestvalues,whereas specimens repaired with Meta Fast adhesive resin and round wires exhibited the greatest (C). Intact strips (A) and
Fig. tor.
8. Intersection point for placement of load applica-
those repaired with Meta Fast adhesive resin and halfround wire (D) demonstrated intermediate values. Statistical analysis revealed a significant (p c 0.05) reduction of fracture load of specimen repaired with Meta Fast adhesive resin (82%) and Meta Fast adhesive resin reinforced with half-round wire (34%) compared with the intact ones. Also, a si~i~c~t increase (47%) was found when round wire was used. The use of round and half-round wire significantly increased the repair fracture load. The postrepair fracture loads were found to be 697% for one-half round wire compared with 260% for the repair with Meta Fast adhesive resin alone. The deflection at break of repaired strips with wires increased si~i~cantly (~ < 0.05) from 27% to 40% compared with intact specimens and from 489% to 548% compared with repairs with Meta Fast adhesive resin alone. The use of Meta Fast adhesive resin alone revealed a significant 78% (p < 0.05) reduction of the deflection at break compared with intact specimens. The compression test of maxillary denture bases revealed a significant increase (p < 0.05) offracture load with denture bases repaired with round wires and compared with bases repaired with intact (24%) and Meta Fast adhesive resin (27%). Intact, Meta Fast adhesive resin and half-round wire repaired bases demons~ated no si~i~cant differences (p c 0.05) when the fracture load was compared. During the testing procedures of the failure mode of repairs, all the repaired strips fractured completely at the interface between the heat-cured denture base material and the repair resin. When wires were used, the two pieces of fractured strips were held together (Fig. 9). Denture bases repaired only with the Meta Fast adhesive resin fractured completely along the midline at the interface of the denture base repair resin (Fig. 10). When wires were used to reinforce the repaired denture bases, the fracture line followed the midline at the interface of repair and denture base resin that initiated from the posterior border and terminated fairly abruptly a few millimeters before
POLYZOIS,
ANDREOPOULOS,
Table
Results
I.
AND LAGOUVARDOS
of bending
THE JOURNAL
tests of acrylic
strips
and maxillary
denture
OF PROSTHETIC
DENTISTRY
bases (n = 12)
Repair method A Fracture load of strips (N) Fracture load of dentures (N) Deflection at break of strips (mm)
75.17% 71.67 726.67 638.8 4.27 3.87
A, Control; B, repair with Meta Fast; *Mean value. *SD. :95% confidence limits of the mean.
C, repair
+ 5.36t - 78.57$ + 138.32 - 814.6 i 0.64 4.68 with
Meta
Fast
B
C
D
13.87 I 2.24 12.44 - 15.29 710.83 k 125.88 630.8 - 790.8 0.92 -t 0.17 0.81 1.03
110.58 k 9.15 104.77 - 116.4 902.5 i 193.21 779.7 - 1025.3 5.96 k 0.33 5.75 6.18
49.96 + 5.22 46.64 - 53.28 707.51 i- 115.85 633.9 - 781.1 5.42 i 0.61 5.04 5.81
and round
the reinforcing wire without complete fracture and separation of broken pieces. This was the fracture pattern for most of the denture bases.
DISCUSSION Meta Fast autopolymerizing resin repair material for the acrylic resin strips exhibited strength only 18% that of the intact heat-cured strips. This finding is in agreement with previous studies that reported repair strength that ranged from 35% to 75% when autopolymerizing resin was used,2,
(1.5 mm);
Table
D, repair
II.
with
Summary
Fracture load of strips Fracture load of dentures Deflection at break of strips
Meta
Fast and half-round
of statistical KruskalWallis KruskalWallis KruskalWallis
wire
(0.75 x 1.5 mm).
analyses H = 44.106, df = 3, p = 0.000 BDAC H = 9.122, df = 3,~ = 0.0277 DBAC H = 38.576, df = 3,~ = 0.000 BADC
Letters connected by lines are not different Code letters as in Table I.
at the 95% significance
level.
4, 5, 7,27
The combination of Meta Fast adhesive resin and metal wires that were placed perpendicular to the repaired butt joint of acrylic resin strips dramatically increased the fracture load and deflection of specimens. It appeared that the incorporation of metal wires played a dominant role in the overall mechanical behavior of the repaired strips and had an overriding effect on the repair strength over the mechanical and adhesive properties of repair and denture resin, respectively. The repair strength depends on the flexural characteristics of the repair resin and its adhesive properties to the denture base, whereas deflection is controlled by both the nature of the repair joint and flexibility of the strip. The incorporation of metal wires allows greater deflection of acrylic resin strips before the fracture of the repair and combined with the increased fracture loads gives greater values of toughness, which means that more energy is necessary for breaking the denture. In the case of round wire the increase of fracture load and deflection was significant (p < 0.05) when compared with that of the intact strips. The same finding was also observed with the fracture load (24%) of denture bases repaired with the round wire. From these findings the combination of Meta Fast adhesive resin and round wire (1.5 mm) revealed the most favorable effect. The compression test of maxillary denture bases is not widely reported in the dental literature, and few studies on intact and unrepaired denture
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1996
bases are available.28-32Also, available literature is limited in terms of studies of denture base repair that incorporate metal wires during denture repair; therefore direct comparisons cannot be made. However, an estimate can be attempted with studies concerning the reinforcing effect of metal wires on the fracture resistance of intact denture resins. Carroll and von Fraunhofer,15 Rufflno,16 and Vallittu and Lassilals, lg reported that round and half-round metal wires thicker than 1 mm had a significant reinforcing effect on denture resins when compared with unreinforced ones. This finding is in agreement with the results of this study on the significant reinforcing effect of 1.5 mm round wire on the repair strength of denture resins when compared with intact and unreinforced ones. It is interesting to mention the effect of placing a metal wire in the anterior part of a maxillary denture on the fracture process. Several authors have reported that the anterior palatal area is the region of maximum tensile stress in a maxillary denture and that midline fracture starts from the weak points, namely, the incisal notch, the area behind central incisors, and then propagates along the palate to the posterior border of the denture.l, 28, 33-35 It is evident from the results of this study that the reinforcing wires, independent of fracture load, seemed to change the area of stress concentration and crack initiation from the anterior region to the posterior border of the maxillary denture. This finding refers only to the maxillary
385
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POLYZOIS,
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Fig. 9. Adhesive failure of acrylic res!sinstrips at butt joint. A, Wire holds two pieces of fractured strip together. B, No wire present, fractured strip separates.
dentures repaired along the midline and wire reinforced in the anterior region. Addi.tional factors such as resin-wire mterfacial characteristics should be taken into consideration. In this study focus was placed on wire cleaning and pretreatment. However, this procedure is critical in reinforcing plastics and deserves special attention.
CLIMCAL
SIGNIFICANCE
From this study it is evident that results with clinical significance can be obtained only by testing of specimens designed as close as to the original items used in dental practice. This process is sometimes difficult regarding the testing procedure and conditions. In those cases the data obtained by the use of other kinds of specimens (for example, bars) can only be of indicative and comparative value. Therefore the results must be interpreted with caution. Special care should be devoted to any indi~du~ case of reinforcing with metal wire, However, under the conditions of this study it seemed that certain types of metal wires may considerably enhance the repair of one kind of denture acrylic resin independent of the type of specimen.
1. Darbar UR, Huggett R, Harrison A. Denture fracture: a survey. Br Dent J 1994;176:342-5. 2. Anderson JN. Applied dental materials. 5th ed. Oxford: Blackwell Scientific Publ, 1976269-74. 3. Ware AL, Docking AR. The strength of acrylic repairs. Aust J Dent 1950$X27-32. 4. Shen C, Colaizzi FA, Birns B. Strength of denture repairs as influenced by surface treatment. J PROSTHET DENT 1984;52:844-8. 5. Beyli MS, von Fraunhofer JA. Repair of fracture acrylic resin. J PROSTHET DENT 1980;44:497-50.
6. Harrison A, Belton EL, Meades K. Do self-curing gain strength with age? J Dent 1977,5:334-8. 7, Berge M. Bending strength of iutact and repaired Acta Odont &and 198~41:187-91,
386
acrylic
resin repairs
denture
base resins.
~~OPO~OS,
AND BOWLS
10. Typical fractures of maxi1lar-y denture bases repaired with Meta Fast adhesive resin.
Fig.
8. Stipho HD, Stipho AS. Effectiveness and durability of repaired acrylic resin joints, J PROSTHET DENT 1987;58:249-53. 9. Grajower R, Goultschin J. The transverse strength of acrylic resin strips and of repaired acrylic samples. J Oral Rehabill984,11:237-47. 10. Ward +JE, Moon PC, Levine RA, Behrendt CL. Effect of repair surface design, repair materials and processing method on the transverse strength of repair acrylic denture resin. J PROSTHET DENT 1992;67:81520. 11. Demetriou FP, PoIyzois CL, Andreopoulos AG. Reinforced cyanoaeryIates as repair materials. J Mater Sci Lett 1988;7:13-4. 12. Ishigami K, Shirane M, Aoyama Y, et al. Basic studies on visible lightcuring resin as a denture base. Part 4: Its strength in the repair of fractured parts of heat-curing denture base resin. J Nihon Univ Sch Dent 1986;28:356-9. 13. Andreopoulos AG, Polyzois GL, Demetriou PP. Repairs with visible light-curing denture base materials. Quintessence Int 1991;22: 703-6. 14. Andreopoulos AG, Polyzois GL. Repair of denture base resins using visible light cured (VLC) materials. J PROSTHET DENT 1994;7!2:462-8. 15. Carroll CE, von Fraunhofer JA. Wire reinforcement of acrylic resin prostheses. J PROSTHET DENT 1984;52:639-41. 16. Ruffino AR. Effect of steel strengthener on fracture resistance of the acrylic resin complete denture base. J PROSTHET DENT 1985;54:75-8. 17. Jennigs RE, Wuebbenhorst AM. The effect of metal reinforcements on the transverse strength of arnylic denture base materlal. J Dent Child 1960;27:162-8. 18. Vallittu PK, Lassila VP. Effect of metal strengthener’s surface roughness on fracture resistance of acrylic denture base material. J Oral Rehabil 1992;19:385-91. 19. Vallittu PK, Lassila VP. Reinforcement of acrylic resin denture base materialwithmetalorfibrestrengtheners. JOralRehabil1992;19:22530. 20. Hero H, Ruyter IE; Waarli ML, Hultq~st. Adhesion of resins to Ag-Pd alloys by means ofthe silicoating~hnique. J Dent Res 198~6~1380-5. 21. Vallittu PK. Effect of some properties of metal strengtheners on the fracture resistance of acrylic denture base material construction. J Oral Rehabil 1993;20:241-8. 22. Gaw DA, Mahood M. Micromechanical and chemical techniques for attaching acrylic resin to cobalt-chromium alloy [Abstract]. J Dent Res 1989;68:590. 23. Tanaka T, Nagata K, Takeyama M, Atsuta M, Nakabayashi N, Masuhara E. C-META opaque resin. A new resin strongly adhesive to nickel-chromium alloy. J Dent Res 1981;60:1697-706. 24. Matsumura H, Nakabayashi N. Adhesive 4-META’MMA-TTB opaque resin with poly(methy1 methacrylate)-coated titanium dioxide. J Dent Res 1988;67:29-32. 25. Yasuda N, S&o R, Iida M, Yatabe M, Ai M. META FAST self-curing metal bonding resin [In Japanese]. Quint Dent Teohnol1985; lO:l28592.
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26. Siegel S, Caste&n NG. Nonparametric statistics for the behavioral sciences. 2nd ed. New York: MC Graw Hill Book Company, 1989: 213. 27. Leong A, Grant AA. The transverse strength of repairs in polymethylmethacrylate. Austr Dent J 1971;16:232-4. 28. Smith DC. The acrylic denture. Mechanical evaluation of mid-line fracture. Br Dent J 1961;100:257-67. 29. Peyton FA, Anthony DH. Evaluation of denture processed by different techniques. J PROSTHET DENT 1963; 13:269-82. 30. Morris JC, Khan 2, van Fraunhofer JA. Palatal shape and flexural strength of maxillary denture bases. J PROSTHET DENT 1985;53:670-3. 31. Hayden WJ. Flexural strength of microwave-cured denture baseplates. Gen Dent 1986;34:367-71. 32. Zissis AJ, Polyzois GL. Fracture energy of denture bases: the effect of activation mode of polymerization. Quint Dent Technol1993;16:154-8.
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AND LAGOUVARDOS
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DENTISTRY
33. Matthews E, Wain EF. Stresses in denture bases. Br Dent J 1956; 100:167-71. 34. Glantz P-O, Stafford GD. Clinical deformation of maxillary complete dentures. J Dent 1983;11:224-30. 35. Darbar UR, Huggett R, Harrison A. Stress analysis techniques in complete dentures. J Dent 1994;22:259-64.
Reprint
requests
to:
DR. GREGORY L. POLYZOIS DEPARTMENT OF PROSTHODONTICS SCHOOL OF DENTISTRY UNIVERSITY OF ATHENS 2 THIVON ST. 11527 ATHENS GREECE
387
resin and metal
of Athens, Athens,
The fracture of acrylic resin dentures is an unresolved problem in prosthodontics. In this study one brand of denture base acrylic resin was used to make specimens in the form of strips and maxillary denture bases. The specimens were cut and repaired with one type of an autopolymerizing adhesive resin and metal wires. The mechanical properties of the repaired specimens were measured, and the efficiency of each method was evaluated. The statistical results of this study revealed that geometric characteristics of a maxillary denture combined with the shape and pretreatment of reinforcement were the controlling factors for the overall mechanical behavior. Furthermore this study revealed that data with clinical significance can only be obtained by testing specimens similar to the original items used in dental practice. (J
DENT 1996;75:381-7.)
PROWHET
T
he fracture of acrylic resin dentures is an unresolved problem in prosthodontics. Midline fracture was the most common problem in a recent denture survey conducted in the United Kingdom, where 29% of the total denture repairs were made for this type of fracture.l Of these midline fractures 71% were seen in maxillary complete dentures, and 29% were seen in mandibular dentures. Denture fracture occurs both outside and inside the mouth. Outside the mouth failure occurs through impact as a result of dropping the denture. Inside the mouth excessive biting force causes fracture. In function, however, midline fracture is the result of flexural fatigue failure caused by cyclic deformation of the base and is more likely to occur because flexure of the denture base occurs along the midline. Regardless of the reason for the fracture or the method of repair, satisfactory repairs must have adequate strength, be easily and rapidly completed, match the original color of the material, and retain its dimensional accuracy. However, these criteria cannot always be achieved. A denture repaired with heat-cured resin exhibits approximately 85% of its original strength, and an autopolymerizing repair has only 55% to 65% of the original heat-cured denture strength.2 Several investigators have studied the repair strength of denture base resins with heat-curing acrylic resin,3, 4 autopolymerizing acrylic resin,5-10 cyano-
=Assistant Professor, Department of Prosthodontics, Maxillofacial Prosthetics Service, School of Dentistry, University of Athens. bAssociate Professor, Department of Chemical Engineering, National Technical University of Athens. “Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Athens. Copyright 0 1996 by The Editorial Council of THE JOURNAL OF PROSTHETIC
DENTISTRY.
0022-3913/96/$5.00
APRIL1996
+ 0.
10/l/69037
acrylates,ll and visible light-cured resins such as premixed, gel, and powder-liquid.12-14 Many different approaches to solving problems have been associated with broken dentures, such as modifying the denture material itself (high-impact resins) or reinforcing it. The most. common reinforcing technique is the use of metal wires embedded in the prostheses, although their value remains questionable.l, 15-17 It is well known that acrylic resin exhibits a low potential for adhesion to nonacrylic materials. Different methods exist for improving the adhesion between metal and acrylic resin, namely, sandblasting of the surface of the wire,ls, I9 silanization of the surface of the metal with different techniques,20l 21 or the use of a metalbonding resin.22-24 In 1985 an autopolymerizing adhesive resin (Meta Fast, Sun Medical Co., Ltd., Kyoto, Japan) with various applications in prosthetic dentistry was first marketed.25 The adhesive resin consisted of a powder (methyl methacrylateethyl methacrylate copolymer, MMA-EMA copolymer), a liquid (MMA), and a bonding liner (MMA-, 4-META). The bonding liner, which contained the metal adhesive monomer 4-metacryloxyethyl trimellitate anhydride (4META), is the most important component of the kit. The various applications of the bonding liner for removable prosthodontics are (1) rebasing of a metallic denture, (2) repairing a metallic denture or the addition of artificial teeth, (3) reinforcement with cobalt-chromium or nickelchromium wires when working and repairing a resin denture plate, (4) repair of resin fracture at cast wire clasp, and (5) coating for clasp leg or wire clasp. This study examined the strength of the heat-cured acrylic strips and denture bases repaired with Meta Fast metal adhesive resin alone or in combination with metal wires.
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THE JOURNAL
OF PROSAIC
~~~IS~Y
POLYZOIS,
~~OPO~OS,
AND ~GO~~S
Fig. 1. {A) Halves of acrylic resin strips placed in openended stone molds IB) 3 mm apart (C).
Fig. 2. Acrylic resin strips and metal wire placed into central recess with 3 mm gap.
MATERIAL
reinforce the specimens with wire, a grinding stone disk (2.5 mm thick) was used to create a central channel, 2.5 mm x 65 mm, perpendicular to the butt joint for placement of wires. The open-ended stone molds were then used so that the surfaces to be prepared could be moved apart to leave a 3 mm gap. The metal wires were half-round hard (0.75 mm x 1.5 mm in diameter) and round spring hard wires (1.5 mm in diameter) (Remanium, Dentaurum, Pforzheim, Germany) that were sandblasted with 50 pm aluminum oxide (Al2031 delivered by air pressure applied at 5.8 x lo2 KPa (84 psi) for 1 minute. After handling, all wires were ultrasonically cleaned in alcohol for 5 minutes and then treated with a thin layer of Meta Fast bonding liner before they were placed in the central recess. The application of Meta Fast adhesive resin for the repair (Fig. 2) was built up into the central recess and the butt joint with the brush-on procedure as recommended by the manufacturer. When wire reinforcement was used, the buildup of layers of Meta Fast adhesive resin was completed within 3 minutes after the application of the bonding liner. After the polymerization process was completed at room temperature (21 1 1” C), the repaired strips were trimmed to their original dimensions with 600~grit paper and kept in distilled water at 37” C for 48 hours before testing was performed (Fig. 3). All the denture bases were processed on stone models (Titanit, Renfert GmbH Co, Singen, Germany) derived from a silicone mold (Silastic, Dow Corning Ltd., Glamorgan, U. K.) of a medium-sized maxillary stone master model with no undercuts. On each stone cast a vacuumformed 3 mm thick baseplate (Bioplast, Scheu-Den~l, Iserlohn, Germany) was adapted to ensure uniformity of denture base thickness. Casts and base plates were invested with dental stone in metal denture flasks. The vacuum-formed baseplates were discarded, and heat-curing resin was packed and processed. After processing was performed, each denture base was lightly finished to remove
AND METHODS
Two types of specimens, acrylic resin strips and maxillary denture bases, were used. The maxillary denture base with its three-dimensional form approximates the manner in which the denture resin is used clinically. The bulk and shape gives the acrylic resin denture base unique physical properties that may not reflect those of standard acrylic resin strips. For example, the frenum notch in a maxillary denture provides a stress riser from which a midline fracture originates. A heat-curing acrylic resin (Meliodent, Bayer Dental, Newbury, Berkshire, U. K.) was mixed in a ratio of 23.4 gm powder to 10 ml monomer as recommended by the manufacturer and was polymerized in a dry heat oven at 75” C for 12 hours. This denture resin was used to prepare all specimens, strips, and denture bases. Four groups of strips and dentures were prepared. Group A was the control group and had no repair. Group B used Meta Fast adhesive resin treatment only on the fracture. Group C used Meta Fast adhesive resin treated with round wires, and group D used Meta Fast adhesive resin treated with half-round wires. Acrylic resin strips were produced in molds made by investing pattern blocks, 3.5 mm x 10.5 mm x 65 mm, of Perspex (ICI Ltd., Wehvyn, Garden City, U. K.) in dental stone in the conventional dent~e-easing procedure. After the stone set, the blocks were removed carefully, and the acrylic resin was mixed, packed, and then processed. The size of the finished strip specimen was 3 mm x 10 mm x 65 mm (20.05 mm). Specimens were stored in a distilled water bath for 1 month at 37” C to ensure complete satiation. After 1 month of storage the specimens were cut into halves with a diamond saw to simulate fracture. The cut surfaces were ground off to a butt profile by use of 240-g& silicon carbide paper under water. The halves of the strips were placed in open-ended stone molds of the same dimensions as the intact strips (Fig. 1). The openended molds were prepared before the strips were cut. To
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Fig. 3. Acrylic resin strips repaired with Meta Fast adhesive resin and reinforcing wire.
the flash to the vestibular roll portion and was lightly pumiced. To cheek for uniformity the thickness of each denture base was measured in the ~d~palat~ portion with a caliper that could be read to 0.05 mm. All denture bases were immersed in distilled water at 37” C for 1 month. After the saturation in water was performed, dental stone models were poured into the bases and allowed to set to make new casts. For a sample to receive the reinforcing wire, a 70 mm x 2.5 mm x 2.5 mm horseshoe shape recess was created approximately 3 to 4 mm below and palatal to the corresponding crest of the ridge. Each denture base was removed from its stone cast and sectioned with a cutting disk in a sag&al direction along the midline to simulate fracture. The cut edges were prepared to a butt profile by grinding under water on 240-g& SiC paper to create a 3 mm gap between the two sections of the base (Fig. 4). The sections of the bases were placed on their respective casts, and the repair procedure was followed. The same materials used for the repair procedures and groups as acrylic resin strips were used for the denture bases. After polymerization was performed, the excess of the repair resin was removed, and the bases were placed in distilled water at 37” C for 48 hours before testing (Fig. 5). Twelve bases were constructed and repaired for each group. All the tests were performed in air at 21” 2 1” C with the Monsanto universal testing machine (Model TlO, Monsanto Ltd., Swindon, Wiltshire, U. K.), which operated at a crosshead speed of 5 mm/minute.
Testing
acrylic
resin
strips
After being stored in water, unrepaired and repaired strips were placed in the Monsanto bending jig (TG 168) (Fig. 6) and fractured under three-point loading with a distance of 50 mm between the jig supports. The load-deflection behavior was recorded automatically by the chart recorder of the testing machine. The failing loads of the
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1996
OF PROS~TIC
D~~ISTRY
Fig. 4. ~~llary denture base with horseshoe metal wire in repaired groove and 3 mm gap along midline.
Fig. 5. Maxillary denture base reinforced with metal wire and repaired with Meta Fast adhesive resin.
acrylic resin matrix of strips were obtained from the first apex or “blip” on the machine curve. The deflection at fracture was obtained from the same curves with a precision of t8 pm.
Testing
denture
bases
Af%terbeing stored in water, each denture base was placed in the compression cage (Model TG 175, Monsanto Ltd.), and acompressive load was applied through a 10 mm ball attachment (Fig. 7). The application of the downward load along the midline of the tissues surface of the denture was designed to be equivalent to an upward load on both sides combined with an unyielding support in the center of the palate. The nontissue side of each denture base was secured on the lower plate ofthe compression cage, and the load applicator was positioned on the tissue side of the base at the midline height of the convexity in a position to correspond to the second premolar and first molar regions. To facilitate the placement of the load applicator in a repro-
THE JOURNAL
OF PROSTHETIC
DENTISTRY
POLYZOIS,
ANDREOPOULOS,
AND LAGOW’ARDOS
t
Fig. 6. Diagram of bending jig.
Fig. 7. Compression cage shows location of applied force.
ducible manner for each denture base, the midline and a line that corrected the left and right buccal frenum notches were drawn on the tissue side. The tip of the load applicator was placed each time at the intersection of the two lines (Fig. 8). The load to failure of the intact base and repaired joints was recorded by the chart recorder of the testing machine.
RESULTS The fracture load of denture bases and the fracture load and deflection at break of the acrylic resin strips are presented in Table I. A Kruskal-Wallis, nonparametric oneway analysis of variance (ANOVA) followed by a multiple comparisons test according to Siegel and Castellan26 were used to analyze the data. A summary of statistical analyses is presented in Table II. From Table I it appears that fracture load and deflection of strips followed the same pattern, namely, the specimens repaired with Meta Fast acthesiveresinonly(B)exhibitedthelowestvalues,whereas specimens repaired with Meta Fast adhesive resin and round wires exhibited the greatest (C). Intact strips (A) and
Fig. tor.
8. Intersection point for placement of load applica-
those repaired with Meta Fast adhesive resin and halfround wire (D) demonstrated intermediate values. Statistical analysis revealed a significant (p c 0.05) reduction of fracture load of specimen repaired with Meta Fast adhesive resin (82%) and Meta Fast adhesive resin reinforced with half-round wire (34%) compared with the intact ones. Also, a si~i~c~t increase (47%) was found when round wire was used. The use of round and half-round wire significantly increased the repair fracture load. The postrepair fracture loads were found to be 697% for one-half round wire compared with 260% for the repair with Meta Fast adhesive resin alone. The deflection at break of repaired strips with wires increased si~i~cantly (~ < 0.05) from 27% to 40% compared with intact specimens and from 489% to 548% compared with repairs with Meta Fast adhesive resin alone. The use of Meta Fast adhesive resin alone revealed a significant 78% (p < 0.05) reduction of the deflection at break compared with intact specimens. The compression test of maxillary denture bases revealed a significant increase (p < 0.05) offracture load with denture bases repaired with round wires and compared with bases repaired with intact (24%) and Meta Fast adhesive resin (27%). Intact, Meta Fast adhesive resin and half-round wire repaired bases demons~ated no si~i~cant differences (p c 0.05) when the fracture load was compared. During the testing procedures of the failure mode of repairs, all the repaired strips fractured completely at the interface between the heat-cured denture base material and the repair resin. When wires were used, the two pieces of fractured strips were held together (Fig. 9). Denture bases repaired only with the Meta Fast adhesive resin fractured completely along the midline at the interface of the denture base repair resin (Fig. 10). When wires were used to reinforce the repaired denture bases, the fracture line followed the midline at the interface of repair and denture base resin that initiated from the posterior border and terminated fairly abruptly a few millimeters before
POLYZOIS,
ANDREOPOULOS,
Table
Results
I.
AND LAGOUVARDOS
of bending
THE JOURNAL
tests of acrylic
strips
and maxillary
denture
OF PROSTHETIC
DENTISTRY
bases (n = 12)
Repair method A Fracture load of strips (N) Fracture load of dentures (N) Deflection at break of strips (mm)
75.17% 71.67 726.67 638.8 4.27 3.87
A, Control; B, repair with Meta Fast; *Mean value. *SD. :95% confidence limits of the mean.
C, repair
+ 5.36t - 78.57$ + 138.32 - 814.6 i 0.64 4.68 with
Meta
Fast
B
C
D
13.87 I 2.24 12.44 - 15.29 710.83 k 125.88 630.8 - 790.8 0.92 -t 0.17 0.81 1.03
110.58 k 9.15 104.77 - 116.4 902.5 i 193.21 779.7 - 1025.3 5.96 k 0.33 5.75 6.18
49.96 + 5.22 46.64 - 53.28 707.51 i- 115.85 633.9 - 781.1 5.42 i 0.61 5.04 5.81
and round
the reinforcing wire without complete fracture and separation of broken pieces. This was the fracture pattern for most of the denture bases.
DISCUSSION Meta Fast autopolymerizing resin repair material for the acrylic resin strips exhibited strength only 18% that of the intact heat-cured strips. This finding is in agreement with previous studies that reported repair strength that ranged from 35% to 75% when autopolymerizing resin was used,2,
(1.5 mm);
Table
D, repair
II.
with
Summary
Fracture load of strips Fracture load of dentures Deflection at break of strips
Meta
Fast and half-round
of statistical KruskalWallis KruskalWallis KruskalWallis
wire
(0.75 x 1.5 mm).
analyses H = 44.106, df = 3, p = 0.000 BDAC H = 9.122, df = 3,~ = 0.0277 DBAC H = 38.576, df = 3,~ = 0.000 BADC
Letters connected by lines are not different Code letters as in Table I.
at the 95% significance
level.
4, 5, 7,27
The combination of Meta Fast adhesive resin and metal wires that were placed perpendicular to the repaired butt joint of acrylic resin strips dramatically increased the fracture load and deflection of specimens. It appeared that the incorporation of metal wires played a dominant role in the overall mechanical behavior of the repaired strips and had an overriding effect on the repair strength over the mechanical and adhesive properties of repair and denture resin, respectively. The repair strength depends on the flexural characteristics of the repair resin and its adhesive properties to the denture base, whereas deflection is controlled by both the nature of the repair joint and flexibility of the strip. The incorporation of metal wires allows greater deflection of acrylic resin strips before the fracture of the repair and combined with the increased fracture loads gives greater values of toughness, which means that more energy is necessary for breaking the denture. In the case of round wire the increase of fracture load and deflection was significant (p < 0.05) when compared with that of the intact strips. The same finding was also observed with the fracture load (24%) of denture bases repaired with the round wire. From these findings the combination of Meta Fast adhesive resin and round wire (1.5 mm) revealed the most favorable effect. The compression test of maxillary denture bases is not widely reported in the dental literature, and few studies on intact and unrepaired denture
APRIL
wire
1996
bases are available.28-32Also, available literature is limited in terms of studies of denture base repair that incorporate metal wires during denture repair; therefore direct comparisons cannot be made. However, an estimate can be attempted with studies concerning the reinforcing effect of metal wires on the fracture resistance of intact denture resins. Carroll and von Fraunhofer,15 Rufflno,16 and Vallittu and Lassilals, lg reported that round and half-round metal wires thicker than 1 mm had a significant reinforcing effect on denture resins when compared with unreinforced ones. This finding is in agreement with the results of this study on the significant reinforcing effect of 1.5 mm round wire on the repair strength of denture resins when compared with intact and unreinforced ones. It is interesting to mention the effect of placing a metal wire in the anterior part of a maxillary denture on the fracture process. Several authors have reported that the anterior palatal area is the region of maximum tensile stress in a maxillary denture and that midline fracture starts from the weak points, namely, the incisal notch, the area behind central incisors, and then propagates along the palate to the posterior border of the denture.l, 28, 33-35 It is evident from the results of this study that the reinforcing wires, independent of fracture load, seemed to change the area of stress concentration and crack initiation from the anterior region to the posterior border of the maxillary denture. This finding refers only to the maxillary
385
TIzIEZ JOURNAL
OF PRO~ETIC
POLYZOIS,
DAISY
Fig. 9. Adhesive failure of acrylic res!sinstrips at butt joint. A, Wire holds two pieces of fractured strip together. B, No wire present, fractured strip separates.
dentures repaired along the midline and wire reinforced in the anterior region. Addi.tional factors such as resin-wire mterfacial characteristics should be taken into consideration. In this study focus was placed on wire cleaning and pretreatment. However, this procedure is critical in reinforcing plastics and deserves special attention.
CLIMCAL
SIGNIFICANCE
From this study it is evident that results with clinical significance can be obtained only by testing of specimens designed as close as to the original items used in dental practice. This process is sometimes difficult regarding the testing procedure and conditions. In those cases the data obtained by the use of other kinds of specimens (for example, bars) can only be of indicative and comparative value. Therefore the results must be interpreted with caution. Special care should be devoted to any indi~du~ case of reinforcing with metal wire, However, under the conditions of this study it seemed that certain types of metal wires may considerably enhance the repair of one kind of denture acrylic resin independent of the type of specimen.
1. Darbar UR, Huggett R, Harrison A. Denture fracture: a survey. Br Dent J 1994;176:342-5. 2. Anderson JN. Applied dental materials. 5th ed. Oxford: Blackwell Scientific Publ, 1976269-74. 3. Ware AL, Docking AR. The strength of acrylic repairs. Aust J Dent 1950$X27-32. 4. Shen C, Colaizzi FA, Birns B. Strength of denture repairs as influenced by surface treatment. J PROSTHET DENT 1984;52:844-8. 5. Beyli MS, von Fraunhofer JA. Repair of fracture acrylic resin. J PROSTHET DENT 1980;44:497-50.
6. Harrison A, Belton EL, Meades K. Do self-curing gain strength with age? J Dent 1977,5:334-8. 7, Berge M. Bending strength of iutact and repaired Acta Odont &and 198~41:187-91,
386
acrylic
resin repairs
denture
base resins.
~~OPO~OS,
AND BOWLS
10. Typical fractures of maxi1lar-y denture bases repaired with Meta Fast adhesive resin.
Fig.
8. Stipho HD, Stipho AS. Effectiveness and durability of repaired acrylic resin joints, J PROSTHET DENT 1987;58:249-53. 9. Grajower R, Goultschin J. The transverse strength of acrylic resin strips and of repaired acrylic samples. J Oral Rehabill984,11:237-47. 10. Ward +JE, Moon PC, Levine RA, Behrendt CL. Effect of repair surface design, repair materials and processing method on the transverse strength of repair acrylic denture resin. J PROSTHET DENT 1992;67:81520. 11. Demetriou FP, PoIyzois CL, Andreopoulos AG. Reinforced cyanoaeryIates as repair materials. J Mater Sci Lett 1988;7:13-4. 12. Ishigami K, Shirane M, Aoyama Y, et al. Basic studies on visible lightcuring resin as a denture base. Part 4: Its strength in the repair of fractured parts of heat-curing denture base resin. J Nihon Univ Sch Dent 1986;28:356-9. 13. Andreopoulos AG, Polyzois GL, Demetriou PP. Repairs with visible light-curing denture base materials. Quintessence Int 1991;22: 703-6. 14. Andreopoulos AG, Polyzois GL. Repair of denture base resins using visible light cured (VLC) materials. J PROSTHET DENT 1994;7!2:462-8. 15. Carroll CE, von Fraunhofer JA. Wire reinforcement of acrylic resin prostheses. J PROSTHET DENT 1984;52:639-41. 16. Ruffino AR. Effect of steel strengthener on fracture resistance of the acrylic resin complete denture base. J PROSTHET DENT 1985;54:75-8. 17. Jennigs RE, Wuebbenhorst AM. The effect of metal reinforcements on the transverse strength of arnylic denture base materlal. J Dent Child 1960;27:162-8. 18. Vallittu PK, Lassila VP. Effect of metal strengthener’s surface roughness on fracture resistance of acrylic denture base material. J Oral Rehabil 1992;19:385-91. 19. Vallittu PK, Lassila VP. Reinforcement of acrylic resin denture base materialwithmetalorfibrestrengtheners. JOralRehabil1992;19:22530. 20. Hero H, Ruyter IE; Waarli ML, Hultq~st. Adhesion of resins to Ag-Pd alloys by means ofthe silicoating~hnique. J Dent Res 198~6~1380-5. 21. Vallittu PK. Effect of some properties of metal strengtheners on the fracture resistance of acrylic denture base material construction. J Oral Rehabil 1993;20:241-8. 22. Gaw DA, Mahood M. Micromechanical and chemical techniques for attaching acrylic resin to cobalt-chromium alloy [Abstract]. J Dent Res 1989;68:590. 23. Tanaka T, Nagata K, Takeyama M, Atsuta M, Nakabayashi N, Masuhara E. C-META opaque resin. A new resin strongly adhesive to nickel-chromium alloy. J Dent Res 1981;60:1697-706. 24. Matsumura H, Nakabayashi N. Adhesive 4-META’MMA-TTB opaque resin with poly(methy1 methacrylate)-coated titanium dioxide. J Dent Res 1988;67:29-32. 25. Yasuda N, S&o R, Iida M, Yatabe M, Ai M. META FAST self-curing metal bonding resin [In Japanese]. Quint Dent Teohnol1985; lO:l28592.
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ANDREOPOULOS,
26. Siegel S, Caste&n NG. Nonparametric statistics for the behavioral sciences. 2nd ed. New York: MC Graw Hill Book Company, 1989: 213. 27. Leong A, Grant AA. The transverse strength of repairs in polymethylmethacrylate. Austr Dent J 1971;16:232-4. 28. Smith DC. The acrylic denture. Mechanical evaluation of mid-line fracture. Br Dent J 1961;100:257-67. 29. Peyton FA, Anthony DH. Evaluation of denture processed by different techniques. J PROSTHET DENT 1963; 13:269-82. 30. Morris JC, Khan 2, van Fraunhofer JA. Palatal shape and flexural strength of maxillary denture bases. J PROSTHET DENT 1985;53:670-3. 31. Hayden WJ. Flexural strength of microwave-cured denture baseplates. Gen Dent 1986;34:367-71. 32. Zissis AJ, Polyzois GL. Fracture energy of denture bases: the effect of activation mode of polymerization. Quint Dent Technol1993;16:154-8.
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AND LAGOUVARDOS
OF PROSTHETIC
DENTISTRY
33. Matthews E, Wain EF. Stresses in denture bases. Br Dent J 1956; 100:167-71. 34. Glantz P-O, Stafford GD. Clinical deformation of maxillary complete dentures. J Dent 1983;11:224-30. 35. Darbar UR, Huggett R, Harrison A. Stress analysis techniques in complete dentures. J Dent 1994;22:259-64.
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DR. GREGORY L. POLYZOIS DEPARTMENT OF PROSTHODONTICS SCHOOL OF DENTISTRY UNIVERSITY OF ATHENS 2 THIVON ST. 11527 ATHENS GREECE
387