Comparison of Flexural Strength between Fiber-Reinforced Polymer and High-Impact Strength Resin

Transcription

1 MILITARY MEDICINE, 173, 10:1023, 2008 Comparison of Flexural Strength between Fiber-Reinforced Polymer and High-Impact Strength Resin Denis Vojvodic, DDS*; Franjo Matejicek, PhD ; Ante Loncar, PhD ; Domagoj Zabarovic, DDS*; Dragutin Komar, DDS*; Ketij Mehulic, DDS* ABSTRACT Fractures of polymer material are one of the most frequent reasons for the repair of removable dental prostheses. Therefore, there is a constant endeavor to strengthen them, and polymer materials with high resistance to fracture are being developed. The aim of this study was to determine the flexural strength of polymer materials and their reinforcements and thus give preference to their clinical use. Specimens with dimensions mm were tested after polymerization, immersion in water at a temperature 37 C for 28 days, and thermocycling by using the short-beam method to determine the flexural strength. Microscopic examination was performed to determine the quality of bonding between the glass fibers and matrix. Common polymer materials (control group) demonstrated the lowest flexural strength, although, when reinforced with fibers they showed higher flexural strength, matching that of the tested high-impact strength resin. Thermocycled specimens had the highest flexural strength, whereas there was no difference (p 0.05) between specimens tested after polymerization and immersion in water. INTRODUCTION Methyl metacrylate has been the most reliable denture base material ever, since its initial application in the early 1940s. Despite many advantages, methyl methacrylate is prone to fracturing. Fractures of the denture base polymer material are one of the most frequent (64%) reasons for denture repair. 1,2 Theoretically, an edentulous patient should not be able to fracture a denture because of the reasonably high static strength of denture construction and weak masticatory forces developed during the use of removable dental prostheses. 2 4 With the ever-increasing use of implants, as also for the anchorage of removable dental prostheses, occlusal forces that occur on the denture base may significantly increase. 5,6 Also, the influence of material fatigue on flexural strength is decisive, which is the reason for denture fractures. 7,8 The prime and most frequent example of material fatigue is the median line fracture of the maxillary removable dental prosthesis. 9 As opposed to the maxillary complete removable dental prosthesis, fracture of the mandibular complete removable dental prosthesis is not caused by fatigue, but most frequently occurs as a result of an impact, usually a fall on a hard surface. 2 Acrylic removable dental prostheses may be strengthened by modification of the polymer material or by incorporation of various reinforcements into the polymer, *Department of Prosthetic Dentistry, School of Dental Medicine, University of Zagreb, Av. G. Suska 6, Zagreb, Croatia. School of Engineering in Slavonski Brod, University of Osijek, Osijek, Croatia. Private practice, Health Center Crnomerec, Prilaz baruna Filipovica 11, Zagreb, Croatia. This work was presented at the World Dental Federation s Annual World Dental Congress, September 22 25, 2006, Shenzhen. This manuscript was received for review in August The revised manuscript was accepted for publication in July Reprint & Copyright by Association of Military Surgeons of U.S., because such reinforcements enhance flexural strength and resistance to impact. 10 Initially, reinforcement of removable dental prostheses was achieved by embedment of metal wires or nets, but that approach resulted only in partial improvement of flexural and impact strength. Subsequently, physical and mechanical properties of acrylic removable dental prostheses were enhanced by integration of different fibers with different fiber architectures into the denture base polymer. 11 For that reason, graphite, glass, and organic fibers, such as aramide and polyethylene fibers, were used to improve the flexural and impact strength Most commonly used fibers today are glass fibers, because of their acceptable esthetics 13,16 20 and good bonding with polymers via silane coupling agents Another approach to flexural strength improvement is the usage of rubber phase in polymer pearls, materials known as high-impact strength resins As outlined by one of the manufacturers of high-impact strength resin, their SR-IVOCAP System (Ivoclar Vivadent, Schaan, Liechtenstein) also solves the problem of dimensional changes, 24 which is another great disadvantage of polymer materials, because of polymerization contraction. Materials with such characteristics should ensure perfect reproduction of the modeled object (denture) in wax to enable better contact of the denture base and bearing tissue Therefore, this procedure should be especially suitable for complete removable dental prostheses, partial removable dental prostheses, occlusal devices, and orthodontic devices, as well as for relining of removable dental prostheses. 24,27 29 Different strength and/or quality enhancers of polymer materials are already described both in dental and technical literature. Results are often contradictory, since recommendations and explanations of manufacturers that produce denture base materials are usually biased and only rarely comply with the results of objective investigations. Because of this MILITARY MEDICINE, Vol. 173, October

2 reason, the main goal of this study is assessment of flexural strength of commonly used polymers reinforced with glass fibers, and their comparison with the flexural strength of high-impact strength resins after performing aging treatments. Obtained results could give the preference of which material to use with regard to investment:benefit ratio. MATERIALS AND METHODS Two hundred and ten quadratic specimens with smooth surfaces and dimensions of mm were made of Meliodent Heat Cure and Meliodent Rapid (Heraeus Kulzer, Hanau, Germany) polymer, the aforementioned polymers reinforced with glass unidirectional fibers and net (StickTech Ltd., Turku, Finland) and high-impact strength resin Ivocap Plus High Impact (Ivoclar, Schaan, Liechtenstein). Specimens were split across seven different groups with 30 specimens each. To obtain uniform specimens with glass fibers, an accurately placed special metal cuvette, with two thick polished metal parts on the sides and two thin metal parts in the middle, was constructed. The middle metal parts had 10 quadratic perforations of the size of a specimen (18 10 mm). One thin metal part was 1-mm thick, whereas the other thin metal part was 2-mm thick, and placed together (3-mm thick) they also served as a placeholder for proper glass fiber alignment (1 mm from one side and 2 mm from the other side of a specimen). All metal parts of the cuvette were covered twice with a thin layer of Ivoclar Separating Fluid (Ivoclar Vivadent, Schaan, Liechtenstein). Preimpregnated glass fibers (StickTech Ltd., Turku, Finland), unidirectional (Stick) or net shaped (Stick Net), were impregnated with Meliodent Heat Cure polymer syrup (weight ratio polymer/monomer, 10:8) for 8 minutes. Pressure-heat polymerization polymer (Meliodent Heat Cure) was mixed according to the manufacturer s instructions and placed in both halves (one thick part plus one thin middle part) of the special metal cuvette, after which the impregnated glass fibers, unidirectional (Stick) or net shaped (Stick Net), were placed in-between. The unidirectional glass fibers were laid along the specimens, so that they were orthogonal to the force to be applied, whereas the net-shaped glass fibers were positioned at an angle of 45 o. The cuvette was closed and put in a hydraulic press (Zlatarne, Celje, Slovenia) under 200 bar. The cuvette was subsequently moved to a manual bench vice and the polymerization was performed in a polymerizing apparatus (type 5518; KaVo EWL, Biberach, Germany) according to the manufacturer s instructions. A similar procedure was followed for the Meliodent Rapid autopolymerizing material, the only difference being a shorter impregnation time of glass fibers 2 minutes in Meliodent Rapid polymer syrup because of the autopolymerizing character of the material. Given that the Ivocap Plus High-Impact material (Ivoclar) required its own special flask and apparatus (SR-IVOCAP System; Ivoclar), wax patterns with the aforementioned dimensions were modeled and mounted on a wax profile (3-mm thick) providing the injection method. The original Ivoclar flask was filled with Moldano plaster (Heraeus Kulzer, Hanau, Germany) and wax patterns were placed at least 1 cm from its margin. After the plaster had hardened, the flask was opened and wax was rinsed in the rinsing machine (type 5522; KaVo EWL). The plaster surfaces were isolated twice in a thin layer using Ivoclar separating fluid. The injection method was executed in a manner in which the capsule containing Ivocap Plus High-Impact material was initially prepared. The monomer from the bottle was poured into the capsule with polymer powder and then shaken for 5 minutes in a Cap vibrator (Ivoclar). Both parts of the flask were joined and the system of pressurized air (6 bar) was connected for 5 minutes. Later, the flask was immersed in boiling water in a polymerization bath (Ivoclar) for 35 minutes, after which it was held in cold water for 20 minutes. Throughout this entire cooling process the flask with samples was still subjected to the 6-bar pressure. After polymerization and cooling. the cuvettes used for both methods were opened and the specimens were detached. Possible polymer excess on all of the specimens was removed with a carbide bur (Ivomill; Ivoclar Vivadent). The margins were finished using sandpaper (Sianor 7/0B; Sianor, Frauenfeld, Switzerland). The specimens of the stated dimensions were checked with calipers (Dentarium ; Dentarium, Ispringen, Germany), with the maximum allowed deviation of 0.05 mm. The specimens of all seven groups were further subdivided into three subgroups of 10 specimens each that were further tested by the short-beam method (Fig. 1). 30 The moving speed of the blade was set to 1.5 mm/minute to determine the samples flexural strength after (1) polymerization of the specimens, (2) immersion in distilled water with the temperature at 37 C (Btuj thermostat; Btuj, Poznan, Poland) for 28 days, and (3) thermocycling of the specimens according to Hansson s method. 31 Fiber-reinforced specimens were placed in a testing holder, in a position wherein the fiber reinforcements were closer to the posts (1 mm away from the posts and 2 mm from the blade). The force causing breakdown was noted and the flexural strength was calculated according to the formula: max F max l 6 2 N 4 b h mm 2 MPa where F max is the measured force of the loader (N), l is the distance between posts (here 15 mm), b is the width of the specimen (here 10 mm); and h is the height of the specimen (here 3 mm). Numerical results of the flexural strength were analyzed with a SPSS statistical package (SPSS Inc., Chicago, Illinois). Statistical analysis was performed using descriptive statistics, one-way analysis, and univariate analysis of variance. The statistical significance of difference between flexural strength values of the specimens was calculated using the Scheffe test. To determine the quality of bonding between fibers and matrix, glass fiber-reinforced specimens were randomly cho MILITARY MEDICINE, Vol. 173, October 2008

3 FIGURE 1. Specimen loading: scheme and dimensions. sen, sealed in Durofix material (Struers, Rodovre, Denmark), ground, and polished according to the routine procedure, 32 to obtain a smooth surface suitable for microscopic examination, which was performed with an Olympus BH2-UMA light microscope, (Olympus Optical, Tokyo, Japan). Characteristic images were photographed through the microscope ocular using a camera Sony SSC-DC 18p (Sony, Osaka, Japan). RESULTS Common Meliodent Heat Cure and autopolymerizing Meliodent Rapid polymer specimens demonstrated the lowest flexural strength in their groups, whereas the specimens reinforced with fibers showed higher flexural strength values, in addition to tested high-impact strength resin (Fig. 2). Scheffe s test applied across seven investigated groups of specimens revealed a statistically significant difference (p 0.05) between some of these groups, as shown in Table I. The test between subject effects revealed that the type of polymer (pressure-heat polymerizing, autopolymerizing, high-impact strength resin), type of glass fibers (unidirectional or net), and aging procedures (immersion in distilled water at 37 C for 28 days, thermocycling) had a significant influence (p 0.05) on the achieved flexural strength values (Table II). Autopolymerizing polymer material was the weakest (p 0.05), whereas there was no statistically significant difference (p 0.05) between the pressure-heat polymerizing polymer material and the high-impact strength resin (Table III). Unidirectional glass fibers positioned perpendicularly across the applied force, on the specimen, strengthen polymer material more successfully than net-shaped glass fibers positioned at an angle of 45 o. Specimens reinforced with unidirectional fibers also exhibit higher flexural strength than high-impact strength resin specimens (p 0.05; Table III). Thermocycled specimens had the highest flexural strength (p 0.05), whereas FIGURE 2. Arithmetic means of flexural strength. MILITARY MEDICINE, Vol. 173, October

4 TABLE I. Descriptive Statistics and Scheffe Test between the Groups of Specimens a 95% Confidence Interval for Mean Scheffe Test between Groups Group n Mean (MPa) SD SE Lower Bound Upper Bound Minimum Maximum , Total a 1 Meliodent Rapid polymer plus Stick fibers (net); 2 Meliodent Rapid polymer plus Stick fibers (unidirectional); 3 Meliodent Rapid polymer (control); 4 Meliodent Heat Cure polymer (control); 5 Meliodent Heat Cure polymer plus Stick fibers (net); 6 Meliodent Heat Cure polymer plus Stick fibers (unidirectional); 7 Ivocap. TABLE II. Test of between Subject Effects on Flexural Strength Source Type III Sum of Squares df Mean Square F Significance Corrected model 84, (a) 14 6, Intercept 2,845, ,845, , Type of fibers 52, , Polymer 10, , Aging procedure 6, , Type of fiber polymer 4, , Type of fiber-aging procedure Polymer-aging procedure Type of fiber polymer-aging procedure 5, , Error 43, Total 3,107, Corrected total 127, Subjects: type of fibers unidirectional or net; polymer Meliodent Rapid, Maliodent Heat Cure, or Ivocap; aging procedure after polymerization, after immersion for 28 days in distilled water at 37 C, after thermocycling. there was no statistically significant difference (p 0.05) between specimens tested after polymerization and after immersion in distilled water for 28 days (Table III). Microscope image analysis showed the existence of voids between glass fibers and polymer matrix and only partial bonding between fibers and polymer material (Figs. 3, 4, 5, 6). After a short beam test was performed, the specimens revealed mostly adhesive type of breakdown between fiber reinforcements and polymer matrix, resulting in pullouts of fibers from the matrix. DISCUSSION Flexural strength tests are considered appropriate for testing polymer dental materials In comparison with flexural strength tests, direct measurements of tensile strength are technically more demanding, as they do not enable testing of the selected specimen surface and do not consider the flexural deformation that occurs during masticatory load. Compressive strength is not directly, although in a very complex way correlated with specific fractures that occur during flexural and tensile load. Measurement of diametric tensile strength requires that the tested material does not demonstrate the characteristic of the plastic flow, which is not inherent in most dental polymer materials. To determine the longevity of the mechanical properties of materials in the demanding environment of the oral cavity, complex procedures of artificial aging, such as underwater storage in thermally controlled conditions and cyclic temperature changes, were developed. Water entry in polymer material is primarily caused by diffusion and partly by the polarity of the polymer chains that is caused by unsaturated molecules and unbalanced intermolecular forces. Molecules of water that penetrate into the areas between the polymer chains remain there and act like wedges between these chains. That may cause softening of the denture base, since absorbed water can act as a poly(methyl methacrylate) (PMMA) plasticizer, a fact that emanates from the interaction with the polymer structure. It diminishes the mechanical properties of the material, resulting in lower flexural strength and lower modulus of elasticity MILITARY MEDICINE, Vol. 173, October 2008

5 TABLE III. Scheffe Test for the Significance (Sig.) between Different Factors (Polymer, Type of Fibers, Aging Procedure) Influencing Bond Strength Mean Difference 95% Confidence Interval Factor 1 Factor 2 (Factor 1 Factor 2) SE Sig. Lower Bound Upper Bound Meliodent Rapid Meliodent Heat Cure a Ivocap a Meliodent Heat Cure Meliodent Rapid a Ivocap Ivocap Meliodent Rapid a Meliodent Heat Cure Unidirectional fibers Net a Ivocap a Net Unidirectional fibers a Ivocap a Ivocap Unidirectional fibers a Net a After polymerisation 28 days in distilled water After thermocycling procedure a days in distilled water After polymerisation After thermocycling procedure a After thermocycling procedure After polymerisation a days in distilled water a a The mean difference is significant at the 0.05 level. FIGURE 3. Microscopic image of a specimen section: Meliodent Heat Cure polymer reinforced with unidirectional glass fibers (original magnification, 100). FIGURE 4. Microscopic image of a specimen section: Meliodent Heat Cure polymer reinforced with glass fibers net (original magnification, 100). Different authors prefer different periods of underwater storage and different temperatures room temperature or 37 C. It is essential to stress that the important decrease of flexural strength value occurs during the first 4 weeks of immersion, whereas even a longer period does not present a statistically significant decrease. 38,39 For that reason, immersion in water at 37 C during 28 days was used. However, a decrease of flexural strength values after 4 weeks of immersion in water was beyond the scope of the present study. There was no statistically significant difference (p 0.05) between specimens tested after polymerization and after immersion in distilled water for 28 days. On the contrary, the flexural strength values remained the same or were slightly increased. It could be inferred that water immersion of polymer material caused relaxation of the stress that occurred during polymerization shrinkage, a fact proven to be the cause for the increase of flexural strength values for the tested polymer materials. 40,41 The effects of thermocycling that simulate ingestion of cold and hot food/beverages can have a significant impact on mechanical properties of polymer materials, 42,43 as well as on the color, surface smoothness, and resistance to abrasion. 43 The thermocycling procedure performed in this investigation resulted in an increase in flexural strength values (especially in the control group of Meliodent Heat Cure specimens where flexural strength increased even for 35%), and it was found that the thermocycled specimens had the highest flexural strength (p 0.05). It appears as if the increase in temper- MILITARY MEDICINE, Vol. 173, October

6 FIGURE 5. Microscopic image of a specimen section: Meliodent Rapid Repair polymer reinforced with unidirectional glass fibers (original magnification, 100). FIGURE 6. Microscopic image of a specimen section: Meliodent Rapid Repair polymer reinforced with glass fibers net (original magnification, 100). ature in these thermocycled specimen subgroups resulted in the effect of prolonged polymerization, which could in turn result in the decrease of the residual monomer volume, enhancing the mechanical properties of the material and thus increase the flexural strength. After thermocycling, the flexural strength values in the specimen subgroup that was produced by an Ivocap injection procedure increased only slightly (for 4%) in comparison to the Ivocap specimens tested after immersion in water. This fact could be attributed to the already low residual monomer volume in these specimens, which could not be significantly lowered with prolonged polymerization during the thermocycling procedure. Also, lower polymerization shrinkage of the Ivocap material means less porosity and such a polymer is more resistant to water absorption, with the total net effect of directly influencing the mechanical properties. Results of this study revealed that the flexural strength values of the Ivocap material remained stable despite applied artificial aging and were higher compared to the values of the two control groups made from classical pressure-heat polymerizing material and autopolymerizing material. It must be singled out that out of the two usual techniques for removable dental prosthesis production injection and pressure-heat techniques 44,45 the injection technique results in lower polymerization shrinkage 44 and therefore enables greater precision in removable dental prosthesis production when compared to the pressure-heat polymerization procedure Common Meliodent Heat Cure and autopolymerizing Meliodent Rapid polymer revealed the lowest values of flexural strength in their groups, whereas specimens reinforced with fibers showed higher flexural strength values (p 0.05; Fig. 2 and Table I). Glass fibers strengthened the polymer, because testing load was shared between polymer matrix and the fibers as a result of the established bond. Autopolymerizing material was the weakest (p 0.05) regardless of the details of fiber reinforcements. Between the pressure-heat polymerizing material and high-impact strength resin, there was no statistically significant difference (p 0.05) because of the strengthening effect of glass fiber reinforcements used in the pressure-heat polymerizing material. Unidirectional glass fibers strengthen polymer material more successfully than net-shaped glass fibers and the corresponding specimens also have higher flexural strength than high impact strength resin specimens (p 0.05). Partial bonding between glass fibers and polymer matrix with the existence of voids is the result of established dental prosthetic laboratory routine production, without idealization of procedure conditions (vacuum mixing of the polymer material, etc.). The main intent of the present study is generation of reproducible results comparable to the general dental prosthetic laboratory practice. But even such imperfect reinforcements are good enough to significantly increase the flexural strength of the investigated glass fiberreinforced specimens to match that of the high-impact strength resin. CONCLUSIONS On the basis of the results of this study, it can be concluded that Ivocap Plus High Impact injection material constantly had high values of flexural strength, regardless of the artificial aging procedures. Reinforcements of polymers using glass fibers increased their flexural strength, which was then comparable to that of the tested high-impact strength resin. Because of relatively high costs of the SR-Ivocap System equipment (and relatively low costs of the material), this procedure could be recommended for larger dental prosthetic laboratories with mass production of removable dental prostheses, whereas for smaller laboratories, usage of fiber reinforcements (relatively expensive) to strengthen common polymer materials would be more cost effective. Unidirectional glass fibers placed perpendicular to the load direction would 1028 MILITARY MEDICINE, Vol. 173, October 2008

7 strengthen polymer material more successfully than netshaped glass fibers. Contrary to expectations, thermocycled specimens had the highest flexural strength. Therefore, prolonged polymerization of the polymer material should be proposed, disregarding the manufacturer s instructions on the relatively short duration of polymerization. Such an approach, in the authors opinion, had a direct impact on the better mechanical properties of the used material in addition to the extension of clinical duration of the prosthetic device. ACKNOWLEDGMENTS Presented results originate from the scientific project Investigation of Materials and Clinical Procedures in Prosthetic Dentistry supported by the Ministry of Science, Education, and Sports of the Republic of Croatia, Grant REFERENCES 1. Vallittu PK, Lassila VP, Lappalainen R: Evaluation of damage to removable dentures in two cities in Finland. Acta Odontol Scand 1993; 51: Narva KK, Vallittu PK, Helenius H, Yli-Urpo A: Clinical survey of acrylic resin removable denture repairs with glass-fiber reinforcement. Int J Prosthodont 2001; 14: Schneider RL: Diagnosing functional complete denture fractures. J Prosthet Dent 1985; 54: Lassila V, Holmlund I, Koivumaa KK: Bite force and its correlations in different denture types. Acta Odontol Scand 1985; 43: Rengert BR, Sullivan RM, Jemt TM: Load factor control for implants in the posterior partially edentulous segment. Int J Oral Maxillofac Implants 1997; 12: Gunne J, Rangert BR, Glantz PO, Sversson A: Functional loads on free-standing and connected implants in three-unit mandibular prostheses opposing complete dentures: an in vivo study. Int J Oral Maxillofac Implants 1997; 12: Smith DC: The acrylic denture. Mechanical evaluation, mid-line fracture. Br Dent J 1961; 110: Vallittu PK: Fracture surface characteristics of damaged acrylic-resin based dentures as analysed by SEM-replica technique. J Oral Rehabil 1996; 23: Beyli MS, von Fraunhofer JA: An analysis of causes of fracture of acrylic resin dentures. J Prosthet Dent 1981; 46: Uzun G, Hersek N, Tincer T: Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. J Prosthet Dent 1999; 81: Chong KH, Chai J: Strength and mode of failure of unidirectional and bidirectional glass fiber-reinforced composite materials. Int J Prosthodont 2003; 16: John J, Ganfandhar SA, Shah I: Flexural strength of heat-polymerized polymethyl methacrylate denture resin reinforced with glass, aramid, or nylon fibers. J Prosthet Dent 2001; 86: Solnit GS: The effect of methyl methacrylate reinforcement with silanetreated and untreated glass fibers. J Prosthet Dent 1991; 66: Yazdanie N, Mahood M: Carbon fiber acrylic resin composite: an investigation of transverse strength. J Prosthet Dent 1985; 54: Gutteridge DL. The effect of including ultra-high-modulus polyethylene fiber on the impact strength of acrylic resin. Br Dent J 1988; 164: Vallittu PK: Comparison of the in vitro fatigue resistance of an acrylic resin removable partial denture reinforced with continuous glass fibers or metal wires. J Prosthodont 1996; 5: Vallittu PK: Glass fiber reinforcement in repaired acrylic resin removable dentures: preliminary results of a clinical study. Quintessence Int 1997; 28: Altieri JV, Burstone CJ, Goldberg AJ, Petel AP. Longitudinal clinical evaluation of fiber-reinforced composite fixed partial dentures: A pilot study. J Prosthet Dent 1994; 71: Freilich MA, Karmarker AC, Bustone CJ, Goldberg AJ: Development and clinical applications of light-polymerized fiber-reinforced composite. J Prosthet Dent 1998; 80: Freilich MA, Duncan JP, Meiers JC, Goldberg AJ: Preimpregnated, fiber-reinforced prostheses: Part I. Basic rationale and complete-coverage and intracoronal fixed partial denture designs. Quintessence Int 1998; 29: Rosen MR: From treating solution to filler surface and beyond: the life history of a silane coupling agent. J Coat Technol 1978; 50: Ellakwa AE, Shortall AC, Marquis PM: Influence of fiber type and wetting agent on the flexural properties of an indirect fiber reinforced composite. J Prosthet Dent 2002; 88: Soederholm JJ, Shang SW: Molecular orientation of silane at the surface of colloidal silica. J Dent Res 1993; 72: Ivoclar SR: Ivocap System: instructions for use. Schaan, Ivoclar AG, Neihart TS, Li SH, Flinton RJ: Measuring fracture toughness of high impact poly(methyl methacrylate) with short rod method. J Prosthet Dent 1988; 60: Rodford RA: Further development and evaluation of high impact strength denture base materials. J Dent 1990; 18: Strohaver RA: Comparison of changes in vertical dimension between compression and injection molded complete dentures. J Prosthet Dent 1989; 62: Anderson G, Schulte JK, Arnold TG: Dimensional stability of injection and conventional processing of denture base acrylic resin. J Prosthet Dent 1988; 60: Salim S, Sadamori S, Hamada T: The dimensional accuracy of rectangular acrylic resin specimens cured by three denture base processing methods. J Prosthet Dent 1992; 67: Berg CA, Tirosh J, Israeli M: Analysis of short beam bending of fiber reinforced composites: testing and design. Philadelphia, American Society for Testing and Materials; Hansson O. Strength of bond with Comspan Opaque to three silicoated alloys and titanium. Scand J Dent Res 1990; 98: Struers: Preparation of Composites. Rodovre, Struers Technology, Ledizesky NH, Cheng YY, Chow TW, Ward IM: Acrylic resin reinforced with chopped high performance polyethylene fiber: properties and denture construction. Dent Mater 1993; 9: Vallittu PK, Lassila VP, Lappalainen R: Transverse strength and fatigue of denture acrylic-glass fiber composite. Dent Mater 1994; 10: Vallittu PK: Flexural properties of acrylic resin polymers reinforced with unidirectional and woven glass fibers. J Prosthet Dent 1999; 81: Kanie T, Fujii K, Arikawa H, Inoue K: Flexural properties and impact strength of denture base polymer reinforced with woven glass fibers. Dent Mater 2000; 16: Craig RG, Powers JM: Restorative Dental Materials, Ed 11. St. Louis, Mosby, Vallittu PK, Ruyter EI, Ekstrand K: Effect of water storage on the flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthodont 1998; 11: Vallittu PK: Effect of 180-week water storage on flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthodont 2000; 13: Schneider W, Powers JM, Pierpont HP: Bond strength of composites to etched and silica-coated porcelain fusing alloys. Dent Mater 1992; 8: Saygili G, Sahmali SM, Demirel F: The effect of placement of glass fibers and aramid fibers on the fracture resistance of provisional restorative materials. Oper Dent 2003; 28: MILITARY MEDICINE, Vol. 173, October

8 42. Oshida Y, Hashem A, Elsalawy R: Some mechanistic observation on water-deteriorated dental composite resin. Biomed Mater Eng 1995; 5: Drummond JL, Bapna MS: Static and cyclic loading of fiber-reinforced dental resin. Dent Mater 2003; 19: Nogueira SS, Ogle RE, Davis EL: Comparison of accuracy between compression- and injection-molded complete dentures. J Prosthet Dent 1999; 82: Huggett R, Zissis A, Harrison A, Dennis A: Dimensional accuracy and stability of acrylic resin denture bases. J Prosthet Dent 1992; 68: Archadian N, Kawano F, Ohguri T, Ichikawa T, Matsumoto N: Flexural strength of rebased denture polymers. J Oral Rehabil 2000; 27: Karacaer Ö, Polat TN, Tezvergil A, Lassila LVJ, Vallittu PK: The effect of length and concentration of glass fibers on the mechanical properties of an injection- and compression-molded denture base polymer. J Prosthet Dent 2003; 90: MILITARY MEDICINE, Vol. 173, October 2008