| Abstract|| |
Context: Materials used for the fabrication of interim restorations must satisfy biological, esthetic, and functional needs. Strength and wear resistance are two important physical properties contributing to clinical efficiency.
Aim: The objective of this in vitro study was to evaluate and compare the flexural strength and hardness of five resins used for the fabrication of interim fixed partial dentures.
Materials and Methods: Five groups containing ten specimens of each material were fabricated in customized brass split molds with dimensions 65×10×2.5 mm. The materials subjected to this study were Revotek LC™ (group RLC), Protemp II™ (group PSC), Acry-lux V™ with regular monomer (group AHC), Acry-lux V™ with self-cure monomer (group ASC), DPI™ self-cure tooth molding powder (group DSC). The specimens were polymerized according to the manufacturers' instructions and were evaluated for flexural strength using a universal testing machine and for hardness using a microhardness tester.
Statistical Analysis: The mean of the five groups was compared using one way analysis of variance (ANOVA) and pair-wise comparison was done using Tukeys honesty significance difference (HSD) test. P≤.05 was considered to be statistically significant.
Results: Flexural test results showed that group AHC (79.8950 MPa) had the highest flexural strength followed, in descending order, by group PSC (77.9700 MPa), group ASC (63.7150 MPa), group RLC (58.8110 MPa), and group DSC (51.9840 MPa). Statistically, the difference was found to be highly significant among all the groups. The hardness tests showed that group AHC (17.6900 KHN) had the highest hardness value followed, in descending order, by group PSC (15.9400 KHN), group RLC (12.6000 KHN), group ASC (11.2500 KHN), and group DSC (8.7700 KHN). Statistically, the difference was found to be highly significant among all the groups.
Conclusion: Group AHC, representing a heat-polymerizing resin, showed the highest flexural strength and hardness values as compared to auto-polymerizing resins and light-polymerizing resin.
Keywords: Flexural strength, interim fixed partial dentures, microhardness
|How to cite this article:|
Jo LJ, Shenoy KK, Shetty S. Flexural strength and hardness of resins for interim fixed partial dentures. Indian J Dent Res 2011;22:71-6
Interim restorations are an integral part of fixed prosthodontic treatment. The reasons for providing a fixed interim restoration include: replacement of missing teeth, protection of pulp and preservation of periodontal health, stabilization of tooth position and occlusion, and maintenance of masticatory efficiency.  Interim restorations are also a diagnostic tool used to evaluate patients' response to altered occlusion and appearance as well as to assess the success of periodontal, endodontic, and implant treatment. 
|How to cite this URL:|
Jo LJ, Shenoy KK, Shetty S. Flexural strength and hardness of resins for interim fixed partial dentures. Indian J Dent Res [serial online] 2011 [cited 2014 Apr 20];22:71-6. Available from: http://www.ijdr.in/text.asp?2011/22/1/71/79992
Interim restorations must satisfy biological and esthetic needs as well as mechanical requirements such as resistance to functional loads and wear, especially when it is used for long-term provisionalization, long-span restorations, in areas of heavy occlusal loads, and in cases of parafunction.  During treatment, interim restorations might have to be removed and re-cemented repeatedly without distortion and fracture. 
Currently available interim fixed partial denture materials can be divided into four resin groups, namely, poly (methyl methacrylates), poly (R′ methacrylates), bis-acryl composite resins, and visible light-cured urethane dimethacrylates.  Clinicians should be familiar with the range of commercially available interim restorative materials and the mechanical properties of each so that they can select the best material for a specific treatment plan. Although there are more than one commercially available interim restorative material with the same base resin group, the properties vary depending on the type, amount, geometry, and size of the filler particles as well as the properties of the polymer matrix. 
In clinical situations fixed partial dentures are subjected to various functional loads. In order to assess if an interim restorative material is strong enough to withstand such forces, flexural strength should be determined. These tests evaluate stresses as compressive at the point of application of load and tensile and shear at the point of resistance to the load applied  (making them similar to the stresses produced by multi-unit fixed partial dentures).  Harder materials should be used since they will have good wear resistance.  This reduces the incidence of perforation and plays an important role in maintaining the structural integrity of these restorations for a longer period of time. Hence, hardness of interim restorative materials, which is an indicator of wear resistance, should be evaluated.
Most of the studies on flexural strength and hardness of interim restorative materials have been done to assess the effect of different types of reinforcements, such as polyethylene, glass, aramid, nylon, and carbon fibers. Few studies have been done to evaluate the flexural strengthand hardnessof the different commercially available interim restorative materials although these are routinely used. Hence, this study makes an attempt to compare and evaluate the flexural strength and hardness of five resins used for the fabrication of interim fixed partial dentures.
| Materials and Methods|| |
Five groups of ten specimens each were prepared from the following materials:
a) group RLC - Revotek LC™ , (GC Corp., Tokyo) - light-polymerizing/urethane dimethacrylate resin, shade B2; b) group PSC - Protemp II™ (3M-ESPE, Germany) - auto-polymerizing/bis-acryl composite resin, shade A3; c) group AHC - Acry-lux V™ with regular monomer (Ruthinum group, Italy) - heat-polymerizing/polymethylmethacrylate (PMMA), shade A1; d) group ASC - Acry-lux V™ with self-cure monomer (Ruthinum group, Italy) - auto-polymerizing/ PMMA, shade A1; e) group DSC - DPI® self-cure tooth molding powder, (DPI, India) - auto-polymerizing/PMMA, shade E.
The dimensions of the specimens were standardized to 65×10×2.5 mm by using a customized 4-piece brass flask [Figure 1]. Four screws were present at the corners of the flask, which helped in assembling the four pieces. This customized flask was used for the fabrication of the auto-polymerizing and heat-polymerizing resin specimens. For the light-polymerized specimens, a glass lid, identical to the lid portion of the brass flask, was machined to allow the passage of light during polymerization of the specimens. All the materials were manipulated according to the manufacturer's instructions.
For group RLC, the flask was filled with the material using the spatula provided. The flask was closed using pressure in a bench press (Siria, India). Trial closure was done until all excess material was removed. An LED-powered visible light-curing unit (Spectrum 800™ curing unit; Dentsply Caulk, USA) was used for 40s in fast-cure mode (440-480 nm)  for initial polymerization of the material.This was carried out for every 4 mm along the entire length and width of the specimen. The specimen was then retrieved from the flask and was placed in a polymerizing unit (GC Labolight LV-II™ , GC Corp, Tokyo, Japan) for 3 min for the final light polymerization.
For group PSC, the material was dispensed into the mold lubricated with petroleum jelly and the flask was closed using intermittent pressure in a bench press (Siria, India). Trial closure was done. The specimen was retrieved from the flask after 8 min.
The material for group AHC is supplied in powder and liquid form as polymer and monomer, respectively. In a glass mixing jar,  regular monomer was taken and powder was added to it in the ratio of 1:3 by volume, respectively.  This was mixed vigorously with an agate spatula until all the polymer particles were thoroughly wetted with the monomer. In this dough-like stage, it was packed into the flask. The flask was then closed using intermittent pressure in a bench press (Siria, India). Trial closure was done until all the excess material was removed. The flask with resin was allowed to bench cure for 1 hr,  after which it was polymerized (according to the manufacturer's instructions) in luke warm water in an acrylizer which was slowly brought to boiling in about 45 min and then kept at that temperature for 30 min. The flask was allowed to bench cool for 45 min  and the specimen was retrieved.
The material for group ASC and group DSC was manipulated in a similar manner. It is supplied in powder and liquid form as polymer and monomer, respectively. In a glass bowl,  self-curing liquid was taken and powder was added to it in the ratio 1:2 by volume, respectively.  This was mixed for 30 s until all the polymer particles were thoroughly wetted with the monomer and a homogenous mix was obtained. When the material reached the dough-like stage, it was packed into the mold and the flask was closed under intermittent pressure using a bench press  (Siria, India). The brass flask was kept under pressure in bench press for half an hour to ensure complete polymerization.
During the fabrication of the 50 specimens, the completed ones were stored in distilled water. Thereafter, all specimens were finished with 240-grit silicone carbide paperand were stored in artificial saliva for 10 days. They were then washed under running water and air-dried. The specimens were then tested for flexural strength and hardness.
Each specimen was subjected to a 3-point bending test, at a crosshead speed of 2 mm/min, with a support span of 40 mm, using a universal testing apparatus (model 4206, Instron Corp., Canton, Mass.). The load was applied to the center of the specimen [Figure 2]. The loading was continued till fracture occurred. The load at fracture was recorded (in kilograms).
The breaking load values were converted to flexural strength using the formula  : σ=3 FL/2 bd 2 , where σ=flexural strength; F=load at fracture; L=length of the support span, b=width of specimen; and d=thickness of the specimen.
The flexural strength values obtained were in kg/mm 2 , which were converted into megapascals (MPa) by multiplying it with 9.8. 
The Knoop hardness number (KHN) is the ratio of the load applied to the area of the indentation calculated using the following formula  : KHN=L/l 2 C p , where L=force used to load the instrument in grams; l=length of the longest diagonal in micrometers; [Figure 3] and C p =constant relating l to the projected area of the indentation.
|Figure 3: Computer screen image of the diamond indentation (vertical lines aid in measuring the length of the diagonal)|
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The mean flexural strength values and the hardness values of the five groups were compared using one-way analysis of variance (ANOVA) and pair-wise comparison was done using Tukeys honesty significance difference (HSD) test. The level of significance was set as 95% for this study.
| Results|| |
The flexural strength and hardness values are shown in [Table 1] and [Table 2]. Box-and-whisker plots, which graphically represent the distribution of values of flexural strength and hardness, are shown in [Figure 4] and [Figure 5].
|Figure 4: Box-and-whisker plots showing the distribution of flexural strength values|
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|Figure 5: Box-and-whisker plots showing the distribution of hardness values|
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|Table 1: Descriptive statistics for flexural strength in MPa with mean and standard deviation of each group|
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|Table 2: Descriptive statistics for hardness values in KHN with mean and standard deviation of each group|
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Group AHC (79.8950 MPa) showed the highest flexural strength followed, in descending order, by group PSC (77.9700 MPa), group ASC (63.7150 MPa), group RLC (58.8110 MPa), and group DSC (51.9840 MPa). Pair-wise comparisons showed significant difference among all groups, with P-values less than .001, except between groups PSC and AHC (P=.003). The P-value obtained was less than .001 for each pair-wise comparison among all groups.
Group AHC (17.6900 KHN) showed the highest hardness value followed, in descending order, by group PSC (15.9400 KHN), group RLC (12.6000 KHN), group ASC (11.2500 KHN), and group DSC (8.7700 KHN). Pair-wise comparisons showed significant differences among all groups, with P-values less than .001 among all the groups.
| Discussion|| |
In this in vitro study, rectangular specimens were subjected to 3-point flexure and hardness tests.
The null hypothesis is a statistical hypothesis testing that might be rejected or accepted on the observation of data. The null hypotheses is rejected here, since the P-values were all either significant or highly significant. Thus, the research question - whether there is a comparable difference between the flexural strength and hardness of the materials tested or not - is answered in the affirmative.
The flexural strength test used in this study determined the maximum load at fracture of rectangular specimens supported at each end (an attempt to simulate multiple unit fixed partial dentures supported by two abutment teeth).
For hardness testing, several types of tests like Barcol, Brinell, Rockwell, Shore, Vicker, and Knoop hardness tests can be done. In this study, the Knoop hardness number (microhardness test) was determined since it was considered more reliable for the analysis of the relatively thin specimens of soft acrylic resins and microfilled composites. Surface hardness is used as an indicator of density, and it can be hypothesized that a denser material will be more resistant to wearand surface deterioration. When an interim restoration is fabricated with a material having good wear resistance, the risk of perforation is decreased and it maintains its structural integrity for a longer period of time.
The data indicated that group AHC has the highest flexural strength and hardness. It is a high-molecular-weight PMMA resin combined with special molecular linkers. The possible factors responsible for the high flexural strength and hardness of this material can be its high molecular weight and the cross-linked polymer structure, which makes it less polar, leading to a decrease in the rate of water absorption.  Heat polymerization also eliminates excess residual monomer (0.2-0.5%), leading to a higher degree of polymerization and therefore makes the material stronger.  However, there are also some disadvantages: the longer time taken for fabrication, additional laboratory processes for the preparation of wax patterns, and the need for the use of a split mold.
Group PSC had the second highest flexural strength and hardness values. The material used to represent this group was Protemp™ II. It is a bis-acryl resin containing bifunctional methacrylate (70%), silicone dioxide as filler (25%), vinyl copolymers (4%), inorganic fillers (56%), and bifunctional esters (40%).It attains a flexible cross-linked polymer structure, imparting strength and hardness to the material. , It is hydrophobic,  ensuring minimal water uptake and thus reducing the plasticizer action. In addition, vinyl copolymers are included to increase the flexural strength. The high proportion of silicone dioxide filler improves the wear resistance of the material.  Bis-acryls have a rigid central structure that reduces the dissolution of the resin-filler particles during its immersion in saliva.  These features impart to it higher flexural strength and hardness as compared to the other autopolymerizing and light-cure resins used in this study. The results of this study are similar to those obtained in a study done by Haselton et al. comparing the transverse strength of five auto-polymerizing methacrylate-based resins and eight bis-acryl resins.  The authors reported that most bis-acryl resins demonstrated significantly superior flexural strength over traditional auto-polymerizing methacrylate resins. The hardness values obtained in this study are also similar to those obtained in a study by Diaz et al,  who evaluated the microhardness of three bis-acryl resin composites and two poly (methyl methacrylate) acrylic resins. All of the bis-acryl resin composite materials exhibited superior microhardness over traditional methyl methacrylate resins.
The material used in group RLC contains urethane dimethacrylate (45-50%), and crystalline silica powder (10-15%) as filler. The flexural strength was low compared to group AHC, group PSC, and group ASC. The reasons for this result can be that less filler particles are found in interim composites (15-35% by weight) as compared to normal composites (85% by weight).  These glass fillers are slowly leached out in the presence of saliva, thus reducing the mechanical properties of the interim composites. 
Of all the groups in this study, group DSC had the lowest flexural strength and hardness. It is a low-molecular-weight PMMA resin. The low molecular weight and absence of fillers may be the major factors  compromising the mechanical properties of this material. Water absorption, facilitated by the polarity of PMMA resin, interferes with the entanglement of polymer chains, thereby acting as a plasticizer.  The degree of polymerization is also low, leading to a higher residual monomer (3-5%) content, which acts as a plasticizer.  The residual monomer is soluble and leaches out during the storage period,further decreasing its strength and hardness. 
The standard deviation of flexural strength of AHC, PSC, ASC, RLC, and DSC were found to be 1.77%, 1.53%, 1.14%, 2.03%, and 1.16% of their mean values, respectively. The standard deviation of hardness values of AHC, PSC, ASC, RLC, and DSC were found to be 2.17%, 1.67%, 1.14%, 2.62%, and 4.59% of their mean values, respectively. This clearly indicates that the standard protocol of specimen preparation was well maintained.
PMMA resins are relatively inexpensive, with good color stability, excellent polishability, and good marginal adaptation. The major drawbacks of this group of resins include exothermic polymerization, high polymerization shrinkage, low strength and wear resistance, and pulpal irritation associated with excess free monomer. Poly (R′ methacrylates) have low polymerization shrinkage and low exothermic reaction when compared to PMMA resins; but low strength, low wear resistance, and low color stability limits its use.Bis-acryl composite resins have low polymerization shrinkage, low exothermic reaction, good wear resistance, and good strength; but, these materials are expensive, brittle, have less polishability, and their repair is difficult. Visible light-cured urethane dimethacrylates have controllable working time, good wear resistance, low temperature changes, and good color stability. Their disadvantages include poor marginal fit, brittle nature, and high cost. Selection of a material should take into consideration all the properties of the material in addition to its flexural strength and hardness.
Limitations of the study
- In spite of following a standard protocol for preparing, curing, and finishing of the test specimens, the homogeneity of mix, presence of internal porosity, pressure, and the release of stresses during finishing and polishing procedures was not controlled.
- In the oral cavity, the provisional restoration is exposed to forces of varying magnitudes acting in different directions, and there are also temperature variations. The same situation was not simulated in this in vitro study.
- No correlation between effects of varying span length and different types of food solvents on the mechanical properties of the provisional restorations was done. Therefore further investigations are required under more closely simulated clinical conditions.
- The shape of the specimens did not reflect the shape of a multiple unit fixed partial denture.
- In this study, only one commercial product was chosen to represent a group. Extrapolation of these results to similar products of the same group should be done with care.
| Conclusion|| |
Within the limitations of the study, group AHC showed the highest flexural strength and hardness and may be considered as the material of choice for long-term provisionalization, when there is a history of frequent breakage, when long-span restorations are needed, in areas of heavy occlusal loads, and in cases of parafunction.
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Liju Jacob Jo
Department of Prosthodontics, KMCT Dental College, Kozhikode, Kerala
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]