An Evaluation on the Flexural Strength of Heat Cure Polymethyl methacrylate Denture Base Resin with and without Reinforcement of Polyethylene Fiber

Introduction: Polymethyl methacrylate as denture base material is not ideal in many aspects. The primary problem is its poor strength characteristics, including low impact and flexural strength. The fatigue failure occurs when the denture base deforms repeatedly through occlusal forces, hence dentures tend to break during usage in due course of time. The aim of this study was to measure and compare the flexural strength of heat cure Polymethyl methacrylate denture base resin with and without reinforcement of polyethylene fiber. Method: A total of 52 specimens (maxillary complete denture) were divided into 2 groups(n=26);Group A(Control): Dentures without reinforcement, Group B: Dentures reinforced with Polyethylene fiber. All specimens were subjected to three-point bending test in UTM. A flexural load was applied to each maxillary complete denture at a speed of 5mm/min until fracture and the flexural load required to break specimen was recorded. Results: The mean values and standard deviation of each test specimen group was calculated and statistical analysis of obtained values was done using ANOVA. Mean flexural strength for Group A was 142.66±29.95 MPa and Group B was 218.88±29.96 MPa. Group comparison showed a statistically significant difference (p<0.001) between group A and B. This suggested that flexural strength for specimens reinforced with Polyethylene fibers was significantly higher than specimens without reinforcement. Conclusion: Reinforcement of heat cure polymethyl methacylate denture base resin with polyethylene fiber resulted in significant increase in flexural strength.


Introduction
P olymethyl methacrylate (PMMA) as denture base material dates back to 1937 1 but is still the most predominantly used denture base material because of its availability, dimensional stability, and excellent esthetics, ease of processing and repair, compatibility with oral tissues. However, it is still far from ideal materials because of its low mechanical strength, brittleness and low thermal conductivity. 2 Denture bases are subjected to diff erent stresses, mainly fl exural and impact 3 during i ts processing stages and after being placed in service. Studies have shown that 68% of acrylic resin dentures break within a few years of fabrication; maxillary complete denture being fractured by a combination of fatigue and impact failure, whereas for mandibular dentures, 80% of fractures by impact. Three approaches to improve the strength are: use of an alternative material (polyamides and polycarbonates) the chemical modifi cation of a denture base material by such as copolymerization with a rubber graft copolymer 4 or the addition of cross-linking agents 5 and the reinforcement of PMMA with other materials such as carbon fi bers 6 , glass fi bers 7 and ultrahigh modulus polyethylene fi bers 8 or metal inserts 9 into denture bases.

Materials and Methods
This in-vitro study was done in Department of Prosthodontics and Maxillofacial Prosthetics, People's Dental College and Hospital, Nepal.
Total of 52 test specimens (maxillary complete dentures) were fabricated for the study, which were divided into 2 Groups of 26 specimens each (n=26) depending upon diff erent reinforcements, Group A (Control) comprised of unreinforced denture and Group B, denture was reinforced with Polyethylene fi ber.

Preparation of experimental specimens
For fabrication of standard specimen, a thermoplastic sheet was used to form a temporary denture base. Wax occlusion rim of ideal dimensions was fabricated with modeling wax followed by ideal teeth arrangement and wax up and carving done. After curing, the standard specimen was defl asked, grossly fi nished and polished. This standard specimen was duplicated into Polyvinyl siloxane (putty consistency) elastomeric material mix to obtain a putty mould [ Figure 1]. Teeth set of same size and mould as used in fabrication of the Standard specimen were used. Each tooth was placed into tooth space in putty mould and the molten wax was poured to fi ll the mould, then stone cast was placed into mould. After cooling duplicated waxed trial denture was retrieved [ Figure 1]. The trial denture retrieved was carved, adjusted again to 2.5mm thickness. 52 of trial dentures were fabricated in similar manner.
The waxed trial dentures were then invested, dewaxing was done and thin layer of sodium alginate separating media (Cold Mould seal, DPI) was applied except on teeth surfaces. Equal amount of dough was placed in both halves of fl ask. After trial packing, reinforcing agents were placed according to groups divided. For Group A, packing was done without any reinforcement, 26 dentures fabricated in similar way. For Group B, Polyethylene fi ber (Ribbond 30mm Χ 2mm) were cut into 3mm in length and wetted in 10 drops of monomer before trial closure and at dough stage, trial packing was done. After that previously wetted Polyethylene fi ber 1% by weight of polymer were placed in between halves of fl ask [ Figure 2].
For both groups, fi nal closure was done by applying pressure in hydraulic press. Bench curing was done for 30 minutes and acrylization was done using short curing cycle. The fl asks were left for bench cooling overnight. Next day the dentures were retrieved, grossly fi nished and polished. After fabrication of all 52 test specimens, they were numbered according to groups (A1-A26, B1-B26). Then they were stored in room temperature distilled water for 1 week before testing to simulate the intra-oral environment.

Testing of specimens
The fl exural load was evaluated by the threepoint bending test at a cross head speed of 5 mm/min using Universal Testing Machine (Shimadzu). The dentures were placed on platform of machine with customized jig and fl exural load was applied to center of tissue surface of each denture with a 25mm diameter ball attachment [ Figure 3]. The specimen was loaded until fracture [ Figure 3] and the fl exural load required to break specimen was recorded from digital monitor of testing machine for all specimens.
Then Flexural strength was calculated in N/mm 2 (MPa) by means of the standard relation. S = 3 LP/2 WT 2 , where S is the Flexural strength, P is the fracture load (N), L the distance between supports(mm), W the specimen width(mm), and T the specimen thickness(mm).
After calculation, data was entered on SPSS ver20 and statistical analysis was done. The mean values and standard deviation of each test specimen group was calculated and statistical analysis of obtained values was done using ANOVA. All results were considered statistically signifi cant if P<0.05.    Table 2]. This suggested the greater fl exural strength for specimens reinforced with Polyethylene fi bers as compared to unreinforced specimens.

Discussion
Although individual denture bases may be made from metals or metal alloys, majority of denture bases are fabricated using common polymers chosen based on availability, excellent appearance, ease in processing, reparability, dimensional stability, handling characteristics. Despite the widespread use of PMMA, low impact and fatigue strength 10 of material itself may result in failure of dentures and expense in terms of fi nancial cost and inconvenience to the patient when repairs are carried out. Fracture of a denture is an important problem not only for patients but also for dentists and dental laboratory technicians. 11 In most situations, fractures occur in the midline of the denture base. 12 This location of fracture occurs more often in maxillary dentures than in mandibular dentures (ratio of maxillary to mandibular denture base fractures is 2:1). 13 Although various materials and methods have been proposed for repairing fractured denture bases, repairing with auto polymerized resin is much weaker (approximately 60% to 65% of the original material 14 than the originally used heat polymerized denture resin. 15 Doubling the thickness of the denture was one of the early attempts to increase the strength of acrylic dentures. 16 Recently, much attention has been directed towards the addition of various types of fi bers like carbon fi ber 17,18 polyethylene fi ber 19,20 and glass fi ber 21,22 to acrylic resins in an attempt to improve its mechanical properties. Reinforcement with fi bers enhances the mechanical strength characteristics of denture bases, such as the transverse strength, ultimate tensile strength and impact strength. 23 Gutteridge DL 24 in 1993 suggested that the use of short lengths of 3mm or 6mm Ultra-high modulus polyethylene (UHMPE) fi ber is an appropriate, eff ective and technically acceptable means of reinforcing PMMA. Some investigators have found that the fl exural properties of UHMWPE-reinforced denture base polymer showed no signifi cant increase in fl exural strength. 19,25 Flexural strength was used in this study because this test more closely represents the type of loading applied to a denture in the mouth. 26 The test specimens were maxillary complete denture because typical fatigue failure occurs in maxillary complete denture as evidenced by a midline fracture. 13,27 Yoshida et al. 9 in 2011, Yadav et al. 28 in 2012 also have used maxillary complete denture as test specimens and threepoint bending test as measure for fl exural strength in their study.

Limitation
1. In-vitro static load tests diff er from intraoral dynamic conditions.
2. Since test specimen in this study involves complex design, uniform thickness of specimen cannot be ascertained which might aff ect fl exural strength.
3. Some variability might be incorporated during selection of material, fi nishing and polishing and storage of specimens.

Conclusion
Within the limitations of this in vitro study, the following conclusion can be drawn: Reinforcement of heat cure polymethyl methacylate denture base resin with polyethylene fi bre resulted in signifi cant increase in fl exural strength.