REINFORCEMENT OF POLYMETHYLMETHACRYLATE (PMMA) RESIN WITH PERFORATED
Jean-François Roulet1a*, Nader Abdulhameed1b, Chiayi Shen1c
1 Department of Restorative Dental Sciences, College of Dentistry, University of Florida, Gainesville FL 32610-0415, USA
aProf. Dr. med. dent., Director of Center for Dental Biomaterials
bBDS, MS, Visiting scientist, Candidate PhD Material Sciences
Prof. Dr. med. dent. Jean-François Roulet, Director of Center for Dental Biomaterials
Department of Restorative Dental Sciences, College of Dentistry, University of Florida,
1395 Center Drive, Room D9-6, PO Box 100415, Gainesville FL 32610-0415, USA
Background: Full dentures show a high rate of fractures under clinical use. Therefore strengthening of the denture base is expected to be beneficial with respect to reducing the fracture risk. Preformed perforated thin metal plates seem to be a simple tool for reinforcement.
Objective: To test the strengthening effect of perforated metal plates incorporated into a PMMA based resin as a function of the bonding strategy.
Materials and Methods: Preformed gold plated stainless steel grid strengtheners were purchased, pressed flat and embedded in wax and processed as dentures using the Ivobase hybrid injection moldable material (Ivoclar Vivadent, Schaan, Liechtenstein). Prior to processing, the surfaces were either not treated or treated with a primer (Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein) and grit blasted and primed. They were then sectioned into beams for three point bending test. Resin beams of identical dimensions without reinforcement were used as control. Fracture strength and flexural Modulus were calculated based on the load to fracture determined by an Instron universal testing machine. The data were analyzed with ANOVA and Tukey’s test.
Results: Fracture strength: control beams fractured at 78.8 MPa ± 5.9 MPa, while, with the exception of the grit blasted samples, the experimental groups showed significantly higher strength (97.2 respect. 95.4 MPa. Flexural Modulus: The control group yielded 2261.7 MPa ± 261.4 MPa, while all experimental groups had a significantly higher modulus (3239.1 – 3952.4 MPa)
The surface treatments did not show significant differences.
Conclusions: The grid strengtheners tested had a significant effect on strengthening. Under the conditions of this study, surface priming did not increase the mechanical properties of the reinforced bars.
Keywords: denture, reinforcement, injection molding process, three point bending test
The shift from vulcanized rubber to polymethacrylate resin (PMMA) has dramatically improved the esthetics of removable dental prostheses. After its invention1 it took several years until it became usable for manufacturing individually formed pieces by reducing its polymerization shrinkage of 21 % to a few percentages by using spherical pre polymerized particles2. These particles which come as a powder could be mixed with liquid MMA and were processed into a dough, that could be used to process dentures of high esthetic appearances. However, being a rather brittle material with modest mechanical properties3 to make it a material, it was still far away from fulfilling the mechanical requirements for an ideal material for dentures4. The result is that full dentures are prone to fractures, which occur in stress concentration areas such as a large frennel notch5, mainly due to fatigue6. Very thin areas, poor fitting dentures and a lack of balanced occlusion are additional factors increasing the fracture risk. In the late 90’s the British Dental Practice Board reported that the cost of repairing annually 1.2 million dentures is 18 million £7. Typically, the ratio of upper to lower denture fractures is about 2:1 with the most common causes of fracture appearing to be poor fit and lack of balanced occlusion8. In another survey on the prevalence of types of fractures, published in 1994, it was reported that 29% were repairs to mid-line fractures most commonly seen in the upper denture9.
It was also reported that 63% of dentures had broken within 3 years of their insertion10. Complete dentures often fracture during normal masticatory function, despite the fact, that an edentulous patient can only exert occlusal forces of 15 to 25% that of dentate patients11.
Maxillary dentures are subject to bending deformation, with tensile stresses occurring at the labial and lingual aspect of the incisors4. A midline fracture of single maxillary complete denture base, especially in patients who have retained their natural mandibular teeth, is at times an inevitable problem9.
Therefore, over the years, various approaches to strengthening acrylic resin have been suggested. Basically these efforts can be divided into two ways. The first one is to modify the resin composition in order to become tougher (high impact resins). The other way is to incorporate strengthening scaffold made out of metals (mesh, wires, cast plates ore frameworks) or fibers (glass-, carbon-, polyamide, or aramid fibers)12.
Such incorporated scaffolds were not really increasing the strength of the dentures. A study by Smith13 reported that the addition of glass fibers did not provide substantial improvement to the tensile strength. Untreated fibers act as inclusion bodies in the acrylic resin mixture and instead of strengthening, actually weaken the resin4. Furthermore, fibers are difficult to place, create polishing problems, if they surface and may be an aesthetic problem due to their color. Metal frameworks, meshes, wires or cast plates have the same effect as untreated fibers. Studies investigating reinforcement with mesh and a braided wire plate did not report a significant improvement in the transverse strength of acrylic. However, incorporating silanated glass fibers into acrylic resin improved the fracture strength. Vallittu et al 199414 found a linear relationship for the increase. The more fibers were incorporated the greater the increase in strength.
Metal reinforcement could fail at the resin/strengthener interface since areas of stress concentration occur around embedded materials. Various approaches have been used to improve the adhesion between the metal surface and acrylic resin such as sandblasting, silanization and metal adhesive resins. The effect of the metal strengthener’s surface roughness on the fracture resistance of the acrylic denture base material was investigated by Vallittu (1992)15. The investigation showed that the surface roughening of the metal wires used to reinforce the acrylic resin denture base material increased the fracture resistance of the test specimens. The best results were achieved by sandblasting15.
In Europe, preformed perforated gold plated steel plates (Grid strengthener, Dentaurum GmbH &Co, KG, Ispringen, Germany) (Fig1) are sold as strengthener for full dentures, which simplify the incorporation into dentures. However the manufacturer does not give any instructions how to treat the surface before being incorporated into the PMMA resin.
Based on literature data, one would expect a strengthening effect, if such plates were bonded to the resin. Therefore the purpose of the study was to test the strengthening effect of the perforated metal plates as a function of the bonding strategy.
The null hypotheses were as follows:
- The metal grid strengthener does not strengthen acrylic resin.
- The different surface treatments do not affect strengthening.
Materials and Methods
Nine Dentaurum Grid Strengthener (0.4mm stainless steel, gold plated with perforations (Ø 2.5mm, Article No. 318-104-00, Dentaurum GmbH &Co, KG, Ispringen, Germany) were purchased and pressed in a hydraulic laboratory Press (Carver Lab press, Wabash, IN, USA) at 9800 N for 2 days until flat. Using wax plates of different thicknesses (Truwax, USA) (0.5 mm for bottom, 1.6 mm for top) “sandwiches” of 2.5 mm thickness were produced, positioning the metal plate at 0.5 mm from the bottom side. At 3 peripheral sites the wax was removed, so the samples could be repositioned into the flask after boiling out of the wax. Using flasks for the injection technique (IvoBase, Ivoclar Vivadent, Schaan, Liechtenstein) and the appropriate spruing (Fig 2) and yellow microstone, type III (Whip Mix, Louisville, KY, USA) a two part form was created, which allowed a defined 3D reposition of the grid before injecting the resin. For control purposes wax plates 2.5 mm thick were embedded as described above.
Three plates per group were conditioned the following way before being repositioned into the flask:
Group 1: No grid strengthener (control).
Group 2: No surface treatment of grid strengthener
Group 3: Monobond Plus (Ivoclar Vivadent) was applied to grid strengthener with paint brush, let react for 30 s and then the solvent was evaporated for 10 s by blowing with an air syringe.
Group 4: Grid strengthener was grit blasted with Al2O3 100 µm at 0.25 MPa for 10 s, then Monobond Plus was applied as described above.
The next step was to injection mold the Ivobase hybrid material (Ivoclar Vivadent) using the Ivomat Polymerization unit with program #1 for 45 minutes.
After removing from the flasks the resin/metal plates and the control plates were sectioned with a diamond saw (IsoMet 1000 Precision Cutter, Buehler, Lake Bludd, IL USA) under water cooling into approximately 10mm x 75mm x 2.5 mm beams. For testing purposes the thickness and width of every beam was measured individually using a caliper (Model 06-664-16, Fisher Scientific, Pittsburgh, PA, USA). The yield was 4 beams /plate which produced 12 beams per group. The beams were stored in water for 7 days prior to perform the mechanical testing.
The beams were subjected to a 3-point bending test according to ISO standard 1567 at 5 mm/minute. The fracture strength was calculated using the following formula:
The Flexural Modulus was calculated as well according to:
σ is the flexural strength
F is the load at fracture or peak load in strength (specimen embedded with mesh did not break)
L is the span between the two supports
w is the width of the specimen
h is the height (or thickness ) of the specimen
d is the deflection of the specimen due to the load F
Data were analyzed with an ANOVA (SAS, Cary, NC, USA) and multiple pairwise comparisons were performed with the Tukey test.
The results are shown in Figs 3, Fig 4 and Table 1. Note that the reinforcement yielded statistically higher strength with exception to sandblasting. For Flexural Modulus the result was clearer. All reinforced groups were statistically the same, but showed significantly higher modulus than the control. Thus the first null hypothesis can be rejected, which is not the case for the second null hypothesis, which must be accepted.
The objective of the study was to determine if the grid strengthener has an effect on strength and modulus of PMMA and if surface treatment (roughening and bonding) increases this effect. The literature data available were not obtained with the strengthener, which has a specific geometry and were conflicting12,13,15.
A three point bending test was chosen, because its test geometry best represents the load situation in a denture in a patient’s mouth. The grids were positioned close to the bottom of the bars, where tensile stresses, which induce fractures in a brittle material, were expected. Thus it could be expected to have the best possible strengthening of a metal plate to a PMMA structure.
The injection molding technique was chosen, because it allowed better precision in positioning the metal grids within the PMMA plate than a traditional pressing technique, since no vertical forces are subjected to the resin while injecting, as compared to the dough technique, where a trial press is required.
The results show basically that the metal grid had a strengthening effect, since, with one exception (flexural strength of sandblasted and bonded grid), the samples with the incorporated grid were statistically significantly stronger. For Flexural strength all surface treatments yielded the same increase, however grit blasting had removed the gold plated layer, leaving behind a rough steel surface, which was treated with Monobond Plus. This is not a problem, since Monobond Plus contains 3 active compounds: Silane that targets Silica (not used here), a sulfonic acid methacrylate which targets oxides (in this case iron oxides) and a disulfide acrylate which targets gold16. Group 4 missed the significant difference from the control. This may be explained by the low sample number, and the high standard deviation, which varied among all fracture strength groups between and 7.5% and 20.2%. The difficulty to obtain perfectly shaped (same thickness) samples due to using wax plates may be an explanation for this result.
Looking at the results for Flexural Modulus group 2, 3 and 4 did not show any differences, but were significantly different from the control. One may argue that the surface priming with Monobond Plus did not have any effect. However one must consider that the samples were not stored in water or aged, which may have shown different results. This is the weakness of the experiment. Further studies should look at aged samples. Another weakness is that only injection moldable hybrid material was used. It would be interesting to look at an injection moldable high impact material as well as at PMMA resins for classical processing techniques with and without reinforcements.
The results of this study are in contradiction to the results of Vallitu15. This may be explained by the difference in geometry and material of the metal reinforcement used in the two studies.
This study was limited to one type of metal grid. Since significant reinforcements with the injection moldable hybrid materials were found, it would be interesting to look at different other metal reinforcement designs as well as at silanated glass fibers. A finite element analysis of the behavior of the originally shaped metal strengthen grid may help to explain the findings of this experiment and allow a better prognosis of its behavior in clinical use.
Provided the strengthening effect is maintained after ageing, the grid strengthener tested in this experiment could be recommended for reinforcing dentures made out of Ivobase hybrid material.
Under the conditions of this experiment it was not possible to identify the expected positive effect of bonding the grid strengthener to the resin base material.
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