Comparative Analysis of Strength of Differently Activated Denture Base Materials Including Recent Acetal Resin-Based Biodentaplast

Aim and objectives: The aim of this study is to comparatively analyse the compressive and tensile strength of different types of record base materials made of different materials and processing techniques. Materials and methodology: The compressive and tensile strength of 4 types of injection moulded materials were compared with a control of conventional compression moulded material. Twenty test specimens (10 tensile and 10 compressive) were fabricated from each material. A test was done using the Instron 3382 (Norwood, MA, USA) universal testing machine. Results: Compressive and tensile test values showed significant differences between the record base resin groups tested for both compressive and tensile strength tests (p=0.00). The mean tensile strength value was greatest for Group V (66.0 MPa) and lowest for Group III (41.9MPa) and the mean compressive strength value was greatest for Group I (74.5 MPa) followed by Group V (70.2 MPa) and lowest for Group III (10.8 MPa). Conclusion: Injection moulded acetal resin showed the highest tensile strength value; it was comparable to that of conventional compression moulded polymethyl methacrylate (PMMA). Compression moulding is reported to have the highest compressive strength values followed by injection moulded acetal resin material. Injection moulded acetal resin material attributed to its advantages and superior strength value, can be used as a material of choice in various clinical scenarios.


Introduction
Since time immemorial, dentures have been a common modality of restoration in prosthodontics.Prosthetic resins are used widely for fabricating prostheses, including complete or removable partial dentures, transitional prostheses, and implant-supported prostheses [1].Conventional heat polymerized polymethyl methacrylate (PMMA) denture base resin has been in use since the 1940s due to its good biocompatibility, colour stability and minimal water sorption.However, it involves a tedious and time-consuming laboratory procedure and undergoes polymerization shrinkage.In order to overcome these shortcomings as well as to improve its applications, various modifications to its composition and techniques have been tried; additionally, various newer materials have also been introduced.The injection moulding technique of heat processing minimizes the requirements of a conventional flask; it is advantageous as it compensates for dimensional change as well as polymerization shrinkage since it uses fluid resin [2].The evolution of thermoplastic/polyamide resin led to a more commercially attractive denture base material due to its better flexibility and improved patient comfort.Flexible resins in removable partial dentures eliminate the need for a metal framework reviving the success of removable partial dentures [3].The development of polymer chemistry produced alternative materials such as acetal resins (polyoxymethylene-based materials) and ether-based PEEK (polyetheretherketone) to replace PMMA-based resins which possess properties such as good strength and biocompatibility as well as metal alloys with high strength and thermal conductivity used in frameworks to enhance esthetics, physical properties as well as patient acceptance [4].This study comparatively evaluates the compressive and tensile strength of conventional compression moulded PMMA and more recent materials processed by an injection moulding technique.

Materials And Methods
A total of 100 samples were to be fabricated, 20 specimens of each material (n=20) out of which 10 samples were to be tested for compressive load (n=10) and 10 samples for tensile load (n=10).Materials used and their description are available in Table 1.Wax patterns were fabricated by pouring molten wax into the hollow cavity in the milled-metal mould with dimensions in accordance with the use of the universal testing machine.For tensile strength testing, a dumb-bell-shaped specimen measuring 65 mm in length, 3 mm in thickness and 20 mm in breadth at both holding ends and 6.5 mm at the centre is to be made.For compressive strength testing, a rectangular block specimen measuring 30 mm in length, 15 mm in breadth and 3 mm in thickness is to be made.Samples were then fabricated using two techniques, compression moulding and injection moulding (Figure 1-3).

Results
The values obtained for compressive strength (Table 2) and tensile strength (Table 3) of the test groups were tabulated.

Group I (Control)
Group II (Injection moulded

TABLE 4: Descriptive analysis of the data obtained
Following the above an ANOVA test was performed to analyse the compressive and tensile strength data and it was concluded that the p value was significant (<0.001) (Table 5).Tukey HSD (Honest Significance Difference) test was applied for comparison of different groups.

Discussion
To investigate the effectiveness of the Injection moulding system or technique of denture processing as an alternative to conventional compression moulding processing system, the mechanical properties of different denture base materials processed using the Injection moulding processing method were tested.Though transverse strength represents the type of loading in dentures more closely than tensile strength and compressive strength, lack of knowledge and lesser number of previous literature on tensile and compressive testing of denture base materials is the reason for us to test these properties in our study.

Compressive strength
Conventional compression moulded PMMA (Meliodent) showed the highest compressive strength values (74.5MPa).Acetal resin-based BioDentaplast (70.2 MPa) showed comparative values of compressive strength followed by injection moulded PMMA (SR-Ivocap HI -51.6 MPa).Polyamide-based injection moulded resins showed lesser compressive strength values (Breflex -24.5 MPa) of which Valplast (10.8 MPa) was the least (Figure 5).The compressive strength of conventional compression moulded PMMA denture base material (Meliodent -74.5 MPa) was comparable to that in previous literature by Hashem et al. of 71.9 Mpa [9].From testing for tensile and compressive strength, it was found that conventional compression moulded showed better values in terms of both mean compressive (74.5 MPa) and tensile strength (57.8MPa) than injection moulded PMMA (mean compressive = 51.6Mpa and mean tensile = 52.1MPa).
Polyamide-based thermoplastic denture base materials, unlike acrylic dentures, are made from a thermoplastic nylon resin that is ultrathin, very flexible and transparent and thus the esthetics are far superior to conventional acrylic/metal partial dentures.It allows for the fabrication of metal-free clasps or removable partial dentures.They are difficult to finish and polish.Though flexible, these polyamide-based materials require a certain level of strength to fulfil the specifications of denture-based materials.In our study, the polyamide-based Valplast and Breflex materials showed lower strength values compared to other materials.The mean compressive strength (45.7MPa) and tensile strength (24.5MPa) of Breflex polyamide material were better than that of Valplast (compressive = 10.8MPa and tensile = 41.9Mpa).
Biodentaplast is a techno-polymer made from acetal (polyoxymethylene) resin.The polymerization of formaldehyde produces Acetal resins.An oxygen molecule connects a chain of alternating methyl groups.It is recommended for use as a clasp in removable partial dentures because of its various advantages, including wear resistance [10], superior abrasion resistance, and higher stiffness.It has a low water absorption rate, making it an ideal material for making interim surgical prostheses for larger oral defects.In view of better flexibility in comparison with cobalt-chromium clasps, the retentive clasp arm of acetyl resin engages deeper undercuts on the abutments [11].In our study, BioDentaplast (Group V) showed the highest tensile strength (66.0 MPa) among all the groups tested and compressive strength value (70.2 MPa) only next to conventional compression moulded Meliodent (74.5 MPa).Wettability with saliva is an important factor that aids in denture base retention.Farcasiu and Pauma noted BioDentaplast to have superior wettability in comparison to other heat-cured acrylic resins tested [12].This serves as an indication for the use of BioDentaplast as a material of choice for patients with decreased salivary flow or Xerostomia [12].Acetal resin (BioDentaplast) frameworks may be as thin as 0.3-0.5 mm without any macroscopic defects so that the material is suitable for removable partial dentures with flexible, esthetic clasps [4].Acetal resin can be used as an alternative material to conventional heat-cured acrylic resin [10].

Limitations
Determination of the flexural strength of the denture base materials would have contributed to better clinical relevance.Understanding the application of acetal resin-based clasps for removable partial dental prostheses and its efficiency requires for further clinical studies.

Conclusions
Injection moulded acetal resin showed the highest tensile strength values; it was comparable to that of conventional compression moulded polymethyl methacrylate (PMMA).Compression moulded is reported to have the highest compressive strength values followed by the injection moulded acetal resin material.
Between the two PMMA-based resins tested, compression moulded showed better properties than injection moulded denture material.Polyamide (nylon-based) materials recorded inferior compressive and tensile strength values in comparison with other types of resins (PMMA and acetal resin).Based on our study, it can be suggested that conventional compression moulded and injection moulded PMMA-based denture base resins still remain the material of choice for the fabrication of dentures.Acetal resin-based denture material BioDentaplast also shows comparable strength properties; additionally, its application as a removable partial denture framework material and as an esthetic tooth-coloured clasp enables its selection as a material for the fabrication of removable partial dentures.

FIGURE 4 :
FIGURE 4: Testing for compressive strength

FIGURE 5 :
FIGURE 5: Testing for tensile strength

TABLE 1 : Description of various groups of materials used
PMMA: Polymethyl methacrylate

TABLE 3 : Tensile strength values obtained of various material samples (in MPa)
The obtained data (Table2, 3) was statistically analysed using the statistical software Statistical Package for Social Sciences (SPSS), version 21.0 (IBM Corp., Armonk, NY) (Table4).The Shapiro-Wilk normality test was performed.