Wettability of three denture base materials to human saliva, saliva substitute, and distilled water: A comparative in vitro study.

AIM: The purpose of this study was to compare the wettability of saliva, a saliva substitute, and distilled water to three denture base materials.
MATERIALS AND METHODS: Thirty specimens of each denture base material: Heat cure polymethylmethacrylate (DPI heat cure), high-impact polymethylmethacrylate (Trevalon HI), and nylon (Valplast) were fabricated. The specimens of each denture base material were divided into three groups of ten specimens. The advancing and receding contact angles of three media: a commercially available carboxymethylcellulose-based saliva substitute (WET MOUTH), human whole saliva, and distilled water, with each denture base material were determined using a goniometer. The contact angle hysteresis was calculated as the difference between the advancing and receding contact angles. The data were statistically analyzed using univariate analysis of variance and Duncan post hoc test.
RESULTS: Low-advancing and receding contact angles were demonstrated on high-impact heat-polymerized polymethylmethacrylate denture base material. Highest hysteresis values were calculated for nylon denture base material.
CONCLUSION: Best wettability was demonstrated on high-impact heat-polymerized polymethylmethacrylate denture base material. Based on the high hysteresis values calculated with nylon denture base material, it would possibly provide better denture retention.

INTRODUCTION

The successful complete denture must provide a desired degree of retention and stability to the prosthesis.[1] Saliva is critical for retention of dentures and provides comfort while wearing removable prostheses.[2]

Complete dentures are retained by a combination of muscular forces exerted by the cheeks, tongue, and lips, and by physical forces acting between the supporting tissues, the denture base, and the interposed film of saliva. Adhesion, defined as the attraction of unlike molecules, is one of the fundamental forces involved in denture retention.[3] The wettability of a liquid to a solid surface plays an important role in determining adhesion.

The wetting power of a liquid is represented by its tendency to spread on the surface of a solid. The wettability of a liquid to a solid surface can be studied by measuring the contact angles formed between them. The lower the contact angle, the better the tendency to wet the surface. Complete wetting occurs when the contact angle is zero.[4]

However, the fundamental requirement suggested for denture retention has been contact angle hysteresis, that is, the difference between the advancing liquid–solid contact angle and the receding contact angle. Advancing contact angle has been defined as the angle that a liquid drop forms on a dry solid surface. Receding contact angle is formed when the liquid recedes on the previously wet surface.[5] Higher the contact angle hysteresis, greater is the retention.

The wetting properties of denture base materials to saliva, therefore, play a vital role in the retention of dentures. Xerostomia, characterized by significantly decreased salivary flow, can make the wearing of dentures very uncomfortable for affected individuals and also affect denture retention. Causes for xerostomia include radiation therapy for oral cancer and systemic conditions such as Sjogren's syndrome, Parkinson's disease, salivary gland hypofunction, or side effects of drug therapies which use diuretics, antihistaminics, antihypertensives, etc.[6789]

Water can be used as a saliva replacement, but it is known that water does not moisten and lubricate the oral mucosa adequately. Besides, salivary mucins present in the saliva possess rheological properties that include elasticity and adhesiveness which aid in retention of dentures.[4] Hence, saliva substitutes which are either mucin or carboxymethylcellulose (CMC based), with sorbitol or xylitol, and salts at concentrations equivalent to those in human saliva, are used. It is important that the wetting properties of these substitutes be comparable to that of human saliva when used with dentures.[10]

Polymethylmethacrylate has been the most commonly used denture base material in dentistry. Acrylic resin wets with water, and this effect is better with saliva, as it adsorbs mucopolysaccharides and proteins from saliva over time.[11] High-impact-resistant acrylic resins are also frequently used. These are polymethylmethacrylates reinforced with butadiene styrene rubber to improve fracture resistance.

Few patients have been found allergic to methylmethacrylates. In such cases, the use of alternative denture base materials such as nylon and polycarbonates have been advocated.[12]

The study of wetting properties of denture base materials is essential to aid the clinician in his choice material.

MATERIALS AND METHODS

In this study, the wettability of three media to three denture base materials was tested.

The three media used were distilled water, a commercially available saliva substitute (WET MOUTH, ICPA Health Products Ltd.) and unstimulated human saliva [Figure 1].

Figure 1

Media used – distilled water, WET MOUTH saliva substitute, and saliva

Human saliva was collected from a healthy individual with normal salivary secretion, atleast 1 h after breakfast, by drooling from the lower lip into a container. The saliva sample was used without further treatment.

The three denture base materials tested were:

A conventional polymethylmethacrylate acrylic denture base resin (DPI heat cure material) [Figure 2]

Figure 2

Denture base materials tested – Trevalon HI and DPI heat cure

A high-impact polymethylmethacrylate acrylic denture base resin (Trevlon – HI) [Figure 2]

A nylon-based denture base material (Valplast).

Specimen fabrication

Conventional acrylic denture base resin – thirty wax patterns of 21 mm × 16 mm × 2 mm (length × width × thickness) were fabricated. These were invested in flasks and dewaxed. Conventional acrylic denture base resin (DPI heat cure material) was then packed into these moulds and acrylized according to the manufacturer's instructions. The specimens obtained were trimmed and sandpapered to obtain specimens of dimensions of 20 mm × 15 mm × 2 mm with a uniform surface. The specimens were not polished to simulate the tissue surface of dentures [Figures 3–5]

Figure 3

Wax patterns for specimens

Figure 5

Conventional heat-polymerized polymethylmethacrylate denture base resin specimens

High-impact acrylic denture base resin – thirty wax patterns were prepared as with conventional acrylic denture base resin, invested, and dewaxed. The molds were packed with high-impact acrylic denture base resin (Trevalon HI) and acrylized according to the manufacturer's instructions. The specimens were trimmed and sandpapered to obtain specimens of 20 mm × 15 mm × 2 mm dimensions with a uniform surface. The specimens were not polished as with the conventional acrylic resin [Figure 6]

Figure 6

High-impact heat-polymerized polymethylmethacrylate denture base resin specimens

Nylon-based denture base material – thirty specimens of 20 mm × 15 mm × 2 mm with a uniform surface were fabricated using injection molding technique [Figure 7].

Figure 7

Nylon denture base resin specimens

Invested wax patterns

Contact angle measurements

Dynamic contact angle analysis was used to measure the advancing and receding contact angles using a goniometer (Dataphysics, SCA 20) [Figure 8].

Figure 8

Contact angle goniometer

The fluid/media to be tested is dispensed by a syringe onto the specimen [Figure 9]. The system allows for a standardized volume of fluid to be used on the specimen surface while measuring the advancing and receding contact angles. The system uses a high-speed camera to record changes of the drop contour which has been dispensed on to the specimen surface.

Figure 9

Specimen placed for testing

The system's program determines the advancing and receding contact angles. The contact angle is the angle formed by the baseline of the drop and a tangent at the three-phase line (solid/liquid/vapor). The advancing contact angle is measured as the contact angle that the liquid drop forms when dispensed on the dry specimen surface, while the receding contact angle as the contact angle formed after the liquid has receded from the surface [Figures 10 and 11].

Figure 10

Advancing contact angle – θA

Figure 11

Receding contact angle – θR

Before dispensing a different fluid onto the specimen, care was taken to thoroughly rinse the dispensing syringe with water, followed by the fluid to be tested.

Advancing and receding contact angles of each of the three media to ten specimens of each denture base material were measured, that is, a total of 9 groups were tested. The groups were (A, B, and C):

Group A:

Water and conventional heat-polymerized polymethylmethacrylate denture base resin (acrylic)

Water and high-impact heat-polymerized polymethylmethacrylate denture base resin (high impact)

Water and nylon-based denture base material (nylon).

Group B:

Saliva substitute and acrylic

Saliva substitute and high impact

Saliva substitute and nylon.

Group C:

Saliva and acrylic

Saliva and high impact

Saliva and nylon.

Calculation of hysteresis

The hysteresis was calculated as the difference between the advancing and receding contact angles for each of the specimens tested.

RESULTS

The data obtained were statistically analyzed using Univariate analysis of variance and Duncan post hoc tests. The results of the analysis are presented in Tables 1–9 and graphically depicted in Figures 12–14.

Table 1

Univariate analysis of variance for advancing contact angles – descriptive statistics

Table 9

Duncan post hoc comparison of hysteresis values of denture base materials

Figure 12

Mean advancing contact angle values of denture base materials in various media

Figure 14

Mean hysteresis values of denture base materials in various media

Mean receding contact angle values of denture base materials in various media

Univariate analysis of variance showed no statistically significant difference in the advancing contact angle, receding contact angle, and hysteresis values between the three media, that is, distilled water, saliva substitute, and saliva. However, there was a statistically significant difference in the advancing contact angle, receding contact angle, and hysteresis values between the three denture base materials.

The Duncan post hoc comparison of advancing contact angles with the different denture base materials found that the highest advancing contact angle values were observed with Nylon (79.38 ± 3.98 standard deviation [SD]), followed by acrylic (74.94 ± 2.18 SD) and high impact, which had the lowest advancing contact angle value (72.04 ± 4.86 SD).

The Duncan post hoc comparison of receding contact angles with the different denture base materials showed a significantly higher receding contact angle value with acrylic (54.38 ± 2.75 SD) than high impact or nylon. There was no statistically significant difference between the receding contact angle values with nylon and high impact.

The Duncan post hoc comparison of the hysteresis values with the different denture base materials showed nylon had the highest statistically significant hysteresis value (29.06 ± 0.72 S.D). There was no statistically significant difference between the hysteresis values of acrylic and high impact.

DISCUSSION

Denture retention is related to forces necessary to completely remove the denture from its basal seat.[13] The wetting of the denture and the palate through the respective adhesive forces at the two interfaces is a necessary prelude to retention.

According to Stanitz, the retention force is a function of saliva surface tension, liquid film thickness, surface of contact, and liquid-denture contact angle.[14]

Theoretical considerations and experimental results have demonstrated that, with the exception of some specific cases such as perfectly wettable solids, the contact angle of the advancing liquid front on a dry solid surface (advancing contact angle θA) is different than the receding contact angle (θR) which is formed when the liquid recedes on a previously wet surface.

The hysteresis of the contact angle (θA–θR) for pure liquids is mainly caused by surface flaws (geometric flaws and surface roughness) or the heterogeneous chemical composition of the surface. In case of polymers, the presence of liquid in contact with a solid may provoke the reorientation of surface groups, leading to contact angle hysteresis.

When instead of pure liquids, solutions containing different surface-active agents (such as surfactant or proteins) are used in contact angle measurements, adsorption of these molecules at the liquid–solid interface induces an important hysteresis.

Monsenego and Proust's analysis of denture retention showed that retention occurs only when hysteresis of denture-saliva contact angle exists.

According to their study,

Where

Fmax is the maximum retentive force of the denture

θA is the advancing contact angle

θR is the receding contact angle

m is the mass of the denture and g is the gravitational force.

This shows that higher the contact angle hysteresis, greater is the force required to dislodge the denture.

High values of denture mass and advancing contact angle θA, that is, θA close to 90° are factors favorable to denture retention. The most favorable values for θR is θR close to 0°. The results of this study is contrary to previous views that perfect wettability is necessary to obtain good retention and implies poor initial wettability (advancing contact angle) is favorable to good retention.[13]

Wettability of denture materials have been studied by Monsenego et al., Waters et al., and Zissis et al., by measuring the advancing and receding contact angles and hysteresis.

Monsenego et al. concluded from their in vitro study that the most convenient denture base material would be that exhibiting the highest contact angle hysteresis, such as high advancing contact angle (θA) and low receding contact angle (θR), and found that sand-abraded heat-polymerized resin would fulfill this condition better than the other materials studied.[5]

Waters et al. concluded that higher contact angle hysteresis values of soft-lining denture materials in comparison to polymethylmethacrylate denture base material gave an indication that the all soft lining materials would improve denture stability under dislodgement forces.[15]

Zissis et al. also applied contact angle hysteresis as an indicator of retention and found two of the soft liners tested showed greater contact angle hysteresis and concluded that, this indicted better retention properties.[16]

In this study, high-impact heat-polymerized polymethylmethacrylate denture base resin demonstrated the best wettability with the lowest advancing and receding contact angle values. Nylon denture base material, however, exhibited poor initial wettability with the highest advancing contact angle values. However, it also had the lowest receding contact angle values and the highest hysteresis value. This implies that nylon denture base would provide the best retention among the three denture base materials tested.

Nylon, which is the generic name of a thermoplastic polymer belonging to the class of polyamides, was first considered for dentures in the 1950s. High-impact acrylic denture base materials are methylmethacrylates reinforced with butadiene–styrene rubber to improve the impact strength of conventional polymethylmehacrylates. In general, polymethylmethacrylate is highly biocompatible and patients suffer few problems. Nevertheless, some patients will show an allergic reaction. This is most probably associated with various leachable components in the denture such as any residual monomer or benzoic acid. When a patient has a confirmed delayed hypersensitivity to methacrylate resins, then an alternative denture base material, such as polycarbonate or nylon may be considered.[12]

The chief advantage in nylon lies in its exceptional mechanical properties of resistance to shock and repeated stressing. However, coupled with a high flexibility, it is doubtful whether this is what is required in a denture base. For a given masticatory stress, the deformation of nylon will be higher than acrylic resin, and increased mechanical irritation of the tissues may well follow.[17]

Nylon also has the disadvantage of staining badly in the mouth and encouraging bacterial growth.[18]

A nylon denture base material, suitably stiffened, could be extremely useful in the treatment of patients for whom acrylic prostheses are not suitable. This would include patients who demonstrate repeated fracture of dentures and those that show tissue reactions of a proven allergic nature.[19]

In the current study, no statistically significant difference was found in the advancing and receding contact angle and hysteresis values of the three media: Distilled water, a saliva substitute, and saliva, which were tested.

In this aspect, it is similar to an in vitro study by Sharma and Chitre who studied the wettability of four commercially available saliva substitutes and distilled water on heat-polymerized acrylic resin. The study revealed no statistically significant difference in the advancing and receding contact angle, and hysteresis values between distilled water and WET MOUTH, a commercially available CMC-based saliva substitute.[6]

A study by Vissink et al. on the wetting properties of human saliva and saliva substitutes found that contact angles of CMC preparations and human whole saliva were comparable on the human mucosa. However, the contact angle of water on human mucosa was significantly higher than that of whole human saliva. Furthermore, on ground polished enamel, the contact angles of water, CMC, or mucin-containing saliva substitutes were significantly lower than whole human saliva.[10]

As mentioned earlier, when solutions contain surface-active substances, the surface and interfacial tensions, and the contact angles, depend on adsorption kinetics of these substances at the interfaces and are therefore time dependent.[13]

Craig et al. stated that the contact angle showed better wetting of the dentures if the dentures were previously soaked in saliva before use. The length of the soaking period was not given.[14]

It has been seen that the contact angle for saliva freshly applied to the acrylic plastic surface is 75°, which is the same as that of water. When saliva is allowed to stand overnight in contact with the plastic material, the contact angle of saliva was reduced to approximately 68°, which indicates that the surface wetting is somewhat improved after remaining in contact with saliva.[4]

The effect on the contact angle values due to prolonged contact of the media with the denture base materials was not considered in this study. Further studies incorporating this factor would be useful.

Saliva aids in the preservation and maintenance of oral health. It plays a significant role in prosthodontic rehabilitation with complete dentures by aiding in retention and providing comfort.

Niedermeier and Kramer in their study emphasized that the secretion of the palatal salivary glands is primarily responsible for the physical retention of maxillary complete dentures.[20]

Loss of salivary flow or xerostomia is both unpleasant and harmful to the patient. In addition to tissue irritation, it predisposes to candidal infections and periodontal disease. It affects denture retention and causes discomfort.

Studies by Nakamoto and Duxbury et al. have found commercially available saliva substitutes such as VA-Oralube (CMC based) and Saliva Orthana (mucin-based) as effective substitutes.[78]

Mucin-based saliva substitutes have been proved to show better wettability than carboxymethyl cellulose-based saliva substitute,[6] but they are of bovine or porcine origin and may not be accepted by the Indian population.

The commercially available saliva substitute (WET MOUTH) tested in this study was carboxymethyl cellulose-based and was found to have wetting properties not significantly different from human saliva.

CONCLUSION

Within the limitations of this study, it could be concluded that:

The wettability of the commercially available saliva substitute tested (WET MOUTH) was comparable to that of human saliva as there was no statistically significant difference between the wettability of the three media tested

High-impact heat-polymerized polymethylmethacrylate denture base material (Trevalon HI) was the most easily wetted as it demonstrated low advancing and receding contact angles

Nylon denture base material (Valplast) could possibly provide the best retention of the three denture base materials tested as it had the highest hysteresis value.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Table 2

Advancing contact angle: Tests for between-denture base material and between-media effects

Table 3

Duncan post hoc comparison of advancing contact angles of denture base materials

Table 4

Univariate analysis of variance for receding contact angles – descriptive statistics

Table 5

Receding contact angle: Tests for between-denture base material and between-media effects

Table 6

Duncan post hoc comparison of receding contact angles of denture base materials

Table 7

Univariate analysis of variance for hysteresis – descriptive statistics

Table 8

Hysteresis: Tests for between-denture base material and between-media effects