Dr. Prachi Agrawal1, Dr. Jyoti Karani2,Dr. Saloni Mistry3, Dr. Anshul Khanna4,Dr. Shruti Gill4, Dr. Praveen Badwaik5, Dr. Nikita Gharat6
1Private Practitioner
2Ex Head of Department and Professor, Department of Prosthodontics, Crown & Bridge.,
T.P.C. T’s Terna Dental College, Navi Mumbai
3Head of Department and Professor, Department of Prosthodontics,
Crown & Bridge., Yerala Medical Trust and Dental college, Navi Mumbai
4Associate Professor
5Professor
6Lecturer
Department of Prosthodontics, Crown & Bridge.,
T.P.C. T’s Terna Dental College, Navi Mumbai
ABSTRACT
Heat polymerized acrylic resins are choice materials for complete dentures. However, the fracture of complete dentures constitutes a challenge. Also, discoloration of denture base may be a cause of concern for denture wearer. An alternative to acrylic resins are nylon-based denture base materials.
Thus, the purpose of this study was to evaluate flexural strength and color stability of commonly used denture base materials.
Materials and methodology: Materials tested were nylon based (Lucitone FRS, Valplast) and conventional heat polymerized acrylic resin (DPI). For flexural strength, ten specimens from each test material were prepared. Five specimens from each material were stored in dry condition for 24 hours and five others were stored in water at room temperature for 7 days. Flexural strength (Mpa) was measured with universal testing machine. For colour stability, twenty specimens from each type of material were prepared. All specimens were stored in distilled water at 37°C for 24 hours. While in water, the color (T0) of all specimens was spectrophotometrically measured. Then five specimens of each material were immersed in coffee, Coca-Cola, curry solution and tea at 37°C. Colorimetric measurements were done after 1 (T1), 3 (T2), and 6 (T3) month and converted into NBS units. Data was statistically analyzed using two-way ANOVA.
Results: The difference in flexural strength (heat polymerized resin - 75.05 Mpa and Valplast - 42.70 Mpa) of all the three groups was statistically significant in both conditions (p < 0.05). Significant color change occurred in heat-polymerized acrylic resins (least) and nylon-based flexible denture base materials (greatest) in all solutions after 6 months (p < 0.05).
Conclusion: The flexural strength of DPI heat polymerized acrylic resin was greatest followed by Lucitone FRS and Valplast. The alternate hypothesis was rejected. The effect of staining solutions on the color of each test material in each session was perceivable by the human eye (∆E > 1). The color shifts of all test materials evaluated with NBS system were in range of 5 to 12+ NBS units.
Keywords: Denture base materials, heat polymerized acrylic resin denture base, nylon polymer denture base.
Citations: Agrawal P, Karani J, Mistry S, Khanna A, Gill S, Badwaik P et al. Comparative evaluation of flexural strength and color stability of different nylon polymer denture base materials with conventional heat polymerized acrylic resin denture base material: an in vitro study. J Prosthodont Dent Mater 2022;3(1):24-38.
Introduction
Different varieties of resins have been used for denture base fabrication. Polymethyl methacrylate (PMMA) resin is currently the most widely used denture base material.1,2 PMMA was introduced as denture base material in 1937.3 Dr. Walter Wright clinically evaluated PMMA and found that it fulfilled almost all the requirements of an ideal denture base material. It became instantly successful due to its desirable properties of excellent aesthetics, low water sorption and solubility, relative lack of toxicity, ease of repair and reline and simple processing technique.4 However, it is not an ideal material in every respect.5 It has certain poor mechanical properties like unsatisfactory transverse strength, due to which the denture fractures easily.2,6 Other common clinical problem encountered with PMMA is the inability to choose a suitable path of insertion while maintaining close adaptation to the tissues in the presence of soft and hard tissue undercuts.7,8 It may be acceptable in case of complete dentures but in case of partially edentulous situations the need for flexible dentures arises.
A potential alternate to PMMA is polyamides. 1, 9 Nylon is a generic name for certain types of thermoplastic polymers belonging to the class of polyamides. The use of nylon as a flexible denture base material has been described in the literature since 1950s. It has several advantages similar to PMMA such as optimum aesthetics, accuracy, and biocompatibility. However, its main advantage over PMMA is its flexibility and subsequently management of severe undercuts and comfort to patients. Some of the disadvantages reported in the early forms of nylon included a tendency to colour deterioration, staining, high water sorption, and the development of a rough surface after a short period of time.
Two very important desirable properties of a denture base material are flexural strength and colour stability. Flexural strength indicates the resistance to deformation or fracture under a flexural load. It closely represents the type of loads a denture is subjected in the mouth.10 Second important property of a denture base material is colour stability. Staining of denture base material can lead to poor aesthetics. A denture base material should not get stained and not change colour in clinical use.5
One of the factors affecting the strength of a denture base is the effect of water storage. There is no sufficient data concerning the comparison of flexural strength of nylon denture materials with PMMA stored in wet conditions, hence this study was carried out to evaluate and compare dry and wet flexural strength of different denture base materials.
Although there have been studies on the colour stability of acrylic denture bases, the comparison of these resins with polyamides after immersion in staining solutions for a long term remains unknown. The second purpose of this study was to determine colour stability of denture base materials by immersing them in four staining solutions (coffee, Coca-Cola, curry, tea) for 6 months. These solutions were selected as immersion solutions, because of their stainability and regular consumption by Indian population.
Null hypotheses of the present study were that the flexural strength and colour of different denture base materials is not affected by different storing conditions and that all the denture base materials is not dependent upon the type of solution or duration of time for which it is immersed.
MATERIALS AND METHODS:
Lucitone FRS and Valplast, two commercially available nylon denture base materials (injection moulded, heat activation) and DPI (compression moulded) heat polymerized acrylic resin material (PMMA) (for control group) were used. The material details are given in Table 1.
*PA – Polyamide, PMMA – Poly (methyl methacrylate)
All the materials used in this study were Type 3 denture base resins: thermoplastic blank or powder as defined in ISO 20795-1: 2008, Dentistry-based Polymers-Part 1: Denture base polymer. 11
Fabrication of specimens :
The test specimens were fabricated from stainless steel blanks. For flexural strength, two stainless steel blanks in the shape of bar [64mm length, 10mm width, 3.3mm thickness (±0.2 mm)] as per ISO specification 1567 were prepared.12 For colour stability, two stainless steel blanks in the shape of round discs (20mm diameter, 3 mm thickness) were prepared (Fig. 1).9
Figure 1: Stainless steel blanks (bar and disc shaped) for fabrication of specimens
Specimens for Lucitone FRS and Valplast were processed using an injection system provided by the manufacturer. These materials were supplied as a single component in a cartridge form. Table 2 shows heating temperature, heating time and injection pressure used in injection moulded technique.
The specimens were polished as per manufacturer’s instructions. The DPI heat polymerized acrylic resin specimens were fabricated as recommended by manufacturer. The acrylic resin specimens were cured in a water bath at 74°C for 2 hours and 100°C for 1 hour.13 Waterproof abrasive papers and pumice were used to finish and polish the specimens.
Flexural strength test :
Ten specimens were made from each group (Total 30 specimens). The samples were wet polished using 600-grit waterproof (silicon carbide) abrasive paper on felt wheel (ISO standard 21948:2001) to simulate the polished denture base.11,14 The specimens were divided into 2 subgroups (dry condition and wet condition) to provide a sample size of 5 of each material. Five specimens were stored in dry condition for 24 hours. For wet conditions, the remaining specimens were stored under distilled water for 7 days at room temperature. A flexural 3-point bending test of bar specimens (PMMA and nylon polymer resin denture base materials) was performed in accordance with ISO standard (1567). Each specimen was mounted on an Instron universal testing machine (Fig. 2).
Figure 2: Bar specimen mounted on universal testing machine for evaluation of flexural strength
The distance between the two supporting jigs was 50 mm. The crosshead speed was set 5mm per minute. The width and thickness of specimens were measured with digital vernier caliper and entered into the program for computation. The load was applied at the centre of specimens till it fractured or reached a point of deflection. The acrylic specimens fractured while nylon specimens came off the rollers of the jig (point of deflection). This maximum load of fracture (or deflection) was recorded. The following formula was used to calculate the flexural strength: 1,2
Flexural strength (S) =3PL/2bd²
Where, P: the load at fracture for PMMA or the maximum load recorded from the load deflection curve for nylon denture base materials, L: span between the two jigs (mm), b: width of the specimen (mm) and d: thickness of specimen (mm).
Colour stability test :
Twenty-disc specimens (20mm diameter, 3mm thickness) of each material were used to test the colour stability (total – 60 specimens). This specification was selected to permit the fitting of specimens for diffraction grating of the spectrophotometer (dimensions were modification of ADA specification no. 12).9,15 Specimens were wet-polished with following order of abrasiveness – 600, 800, 1000 and 1200-grit silicon carbide sandpapers on felt wheel, to simulate polished denture.11 After storing the specimens in distilled water at 37±20C for 24 hours initial colour (T0) was evaluated. After the initial colour measurements (l, a, b coordinates of CIElab system), the specimens were divided into four groups (5 specimens of each material) according to the staining solutions - coffee, Coca cola, curry solution, and tea and soaked at 37±20C for 6 months. Fresh solutions were prepared every 10 days for the same concentration.
Preparation of Staining Solutions 16 :
Coffee solution - 20 g of coffee (Nescafe Classic, Nestle India Limited) was poured into 1000 ml of boiling water.
Coca cola (Hindustan Coca-cola Beverages Pvt. Ltd.), 1000 ml at room temperature. Curry solution - 20 g of curry powder (Mangal curry powder, Mangal Motilal Masalawala, Mumbai, India) was mixed with 1000 ml of boiling water. The solution was filtered using sieve, to remove suspended particles of curry powder.
Tea solution - immerse five teabags (Brooke Bond, Taj Mahal, Hindustan Unilever Limited, India) into 1000 ml of boiling water.
Colour measurements :
The colour was evaluated using an ultraviolet-visible reflectance spectrophotometer (Spectrophotometer 5100+, Rayscan equipments and services pvt ltd, Mumbai, India). The spectrophotometer was calibrated with white calibration standard which sets the maximum reflectance to 100%. The measurement was conducted under the standard illuminant D65 corresponding to average daylight with standard observer at 10 degrees.9,11 Colour measurements were recorded in 3 randomly selected areas. The average of the 3 readings was automatically calculated by the spectrophotometer. Once the initial recordings were completed, the colour measurements for all samples were made after 1 month (T1), 3 months (T2) and 6 months (T3) of exposure to staining solutions. When taking the subsequent readings, the specimens were removed from the solutions and rinsed thoroughly with running water. Excess water on the surfaces was removed with tissue paper and the specimens were allowed to dry.
Colour changes were measured using the Commission Internationale d’Eclairage L*a*b* colour space (CIE L*a*b*). Total colour difference was calculated with the help of the following formula: 9
ΔE* = {(ΔL*) 2+ (Δa*) 2+ (Δb*) 2}1/2
where Δ L*, Δ a*, and Δ b* were the differences between L*, a*, and b* values of T0 and T1, T0 and T2, and T0 and T3.
Mean values (MPa and ΔE) and standard error of mean were assessed with a Windows based statistical package Medcalc® version 12.7.5.0 (64 bit) (Belgium). The results of each test were statistically analyzed by a two-way analysis of variance (ANOVA) followed by post-hoc multiple comparisons using Bonferroni's method.
RESULTS :
Flexural Strength :
The mean flexural strength of heat polymerized acrylic resin was highest in both conditions, dry (77.89 MPa) and wet (72.20 MPa) (Table 3 and Fig. 3).
Figure 3: Disc specimen mounted in spectrophotometer for evaluation of colour change
The mean flexural strength of Valplast was lowest in both conditions, dry (43.49 MPa) and wet (42.70 MPa).
There were statistically significant differences in the mean flexural strengths between the three denture base materials (p<0.001) (Table 4).
Colour Stability :
The graphs for ΔET0T1, ΔET0T2, ΔET0T3 showed that nylon resins discoloured maximum in curry solution while acrylic resins discoloured maximum in tea solution. Amongst nylon resins, Valplast exhibited maximum colour change at all intervals (Fig. 4).
Figure 4: Colour change of disc specimens after 6 months of immersion in different staining solutions
Heat polymerized acrylic resin and Lucitone FRS showed a decrease in color change at third month interval. Table 5 shows color change between three denture base materials in different staining solutions occurring over period of time [T0T1, T0T2, T0T3].
According to two-way ANOVA and Bonferroni pairwise differences (Table 6), in T0T1, T0T2, T0T3 the most severe staining was apparent with curry solution (P<0.001) followed by tea (P<0.001), coffee and Coca-Cola (Graph 1).
To relate colour change of denture base materials to clinical environment, ΔE is converted to NBS units through the formula NBS units = ΔE ×0.92 (Table 7).9
Valplast after 6 months of exposure in curry solution, showed colour changes in the category of 'very much'. Lucitone FRS, after 6 months of exposure in all staining solutions showed colour changes in the category of ‘much’. Heat polymerized acrylic resin after 6 months of immersion in staining solutions, exhibited 'appreciable' colour changes except in tea where it was 'much' (Fig. 7).
DISCUSSION :
The present study compared the flexural strengths of two nylon denture base materials with control group heat polymerized acrylic resin. According to the ISO standard, Type 3 denture base materials require more than 65 MPa of flexural strength. Thus, the study indicates, PMMA correlates with standard values, but PA did not.
The higher flexural strength of PMMA can be attributed to medium to high molecular weights and linear polymer molecules with monofunctional groups. On the other hand, reduced flexural strength of nylon denture base materials may be due to its low molecular weight, linear polyamide chains which exhibit decreased strength and rigidity.10 Such differences may exhibit plastic deformation and necking without fracture even after exceeding the proportional limits.11 It has been suggested that flexural strength and flexural deflection depend upon method of moulding, method of curing, and curing time.10 The present study results are consistent with previous studies.1,2,10,11 There are few studies which have obtained greater flexural strength of nylon polymer denture base materials than PMMA.17-19 The reason could be attributed to the difference in sample size (no. of specimens) and testing conditions.
The two nylon denture base resins used in this study had the same base chemical structure of a polyamide. However, flexural strength of Valplast is significantly lower than Lucitone FRS in both dry and wet conditions which may be due to difference in filler content composition (not specified by respective manufacturers).
Flexural strength of specimens, when stored in water was lower than those stored in dry condition. The findings are in agreement with literature.20-24 The reason for reduction in flexural strength can be attributed to water sorption. PMMA absorb water slowly over a period of time. A diffusion mechanism is primarily responsible for the ingress of water. Water molecules penetrate the PMMA mass and occupy positions between polymer chains interfering with entanglement of polymer chains.13 Nylon denture base materials are hygroscopic in nature. This is explained by the hydrophilicity of the numerous amide bonds forming the main chains of the polyamide resin. Amide groups are polar in nature, making nylon sensitive to water (water being polar solvent).25 When water molecule comes in contact with polyamide, a weak bond is formed between water and polyamide. The water molecule diffuses through the material, forcing polymer chains to apart. The separation of the polymer chains reduces the polar attraction between polyamide chains. This allows for increased chain mobility, resulting in diminished mechanical properties.26 The water absorption occurs among the molecular chains of amide bond. Thus, higher the amide group concentration, the greater the water sorption.11
In the present study, the decrease in flexural strength of denture base materials, stored in water was insignificant. Thus, the null hypothesis stating that the flexural strength of different denture base material is not affected by different storing conditions is not rejected.
This finding is in accordance with previous studies where in PMMA resin specimens showed insignificant difference in flexural strength after 90 and 180 days of water storage.20,23,24
Colour Stability:
Colour stability is important for all dental restorative materials in terms of acceptable aesthetic appearance.6 It can be measured by Munsell or CIE l*a*b system
Present study has reiterated the fact that, PMMA specimens have better colour stability than nylon denture base materials. The reason could be attributed to hydrophobicity of PMMA and hydrophilicity of nylon.9 The water sorption of PMMA is moderate compared to nylon polymer denture base materials because of PMMA’S greater contact angle with water.11
Lucitone FRS has better colour stability than Valplast. The reason could be attributed to difference in filler content composition.29
Valplast and Lucitone FRS specimens showed colour changes ranging from ‘appreciable’ to ‘very much’ with curry solution throughout the study. Turmeric powder (from curry solution) contains conjugated diarylhepnoids like curcumin which are responsible for orange colour and staining capacity of curry solution.30 Staining occurs due to the physical penetration of pigments between the molecular lattices or the adsorption of pigments on the surface of specimens. Nylon polymer resin denture base materials tested contain chromophores (>C=O), and auxochromes (>N-). Polarized chromophores and auxochromes are responsible for staining.11
Coca cola stains because of caramel present in it, but it does not produce as much staining as others because it contains phosphoric acid. Due to phosphoric acid its pH is in the range of 2.5 - 4.2. Low pH damages the integrity of the materials; thus, continuous surface erosion of the object takes place. Hence, it results in reduced staining.9
Heat polymerized acrylic resin (DPI) showed maximum colour change in tea as compared to any other staining solution.31-33 Yellowish brown stains caused by tea are due to presence of high amounts of flavanoid and methylxanthine in tea leaves. 34
Staining property of the coffee changes with storage time, temperature and humidity. The caffeine and caffeic acid present in coffee are responsible for discoloration of polymeric materials.6,34 Lai et al., demonstrated that coffee produced greater colour changes than tea on denture base resins which may be partly attributable to different polar properties of the tested materials.33
Comparison amongst the staining solutions at all three intervals showed statistically significant difference in colour change (p<0.001). Thus, the null hypothesis that the colour stability of denture base material is not dependent upon the type of solution or time for which it is immersed was rejected.
In this study, there was initial increase in colour change for 1 month reading, and then a decrease in the colour change values of the denture base materials at 3rd month. This was seen for Lucitone FRS in Coca cola and curry solution, Valplast in curry solution and heat polymerized acrylic resin in all solutions except coffee. This result was attributed to removal of the accumulated layer. As tea and curry layers reach a certain thickness (on the specimen), they break away from the surface of the specimens and return to the solution.6,34
Colour change of polymer could also be intrinsic discoloration due to degradation of various polymers, and oxidation of the amine accelerator.
According to the present study, PMMA still remains material of choice for removable and complete denture prosthesis. But in compromised situations, where nylon will be choice of material, Lucitone FRS is preferred over Valplast.
LIMITATIONS OF THE STUDY:
The actual oral environment is comparatively different because of the pellicles formed by the proteins and glucoproteins in saliva that led to the formation of plaque adhering to the dentures. So, in future studies, the staining solutions can be combined with artificial saliva.Also, the specimens can be cleaned with brush every day in order to simulate cleaning protocol of denture. The effect of thermocycling on colour change of denture bases can also be studied. In-vivo studies can also be carried out as they give more precise results.
CONCLUSION:
Within the limitations of the study, the following conclusions were drawn:
1. Flexural Strength:a. The flexural strength of nylon polymer denture base materials was lower than heat polymerized acrylic resin in both dry and wet storage conditions (p<0.001).
b. The flexural strength of Lucitone FRS was greater than Valplast in both dry and wet storage conditions (p<0.0001).
c. The dry and wet flexural strength of each tested material were comparable (p>0.05).
2. Colour Stability:
a. The greatest chromatic changes were found with immersion in the curry solution, followed by tea, coffee and Coca cola.
b. Lucitone FRS exhibited colour stability similar to heat polymerized acrylic resin at 1 month and 3 month readings (p>0.05), but showed lower colour stability in 6th month reading (p<0.0001).
c. Valplast specimens exhibited least colour stability at all 3 intervals (∆ET0T1, ∆ET0T2, ∆ET0T3) (p<0.0001).
CONFLICT OF INTEREST - None
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