Introduction

Maintaining the health of oral mucosal tissues is a significant responsibility of the clinician after the insertion of removable prosthesis. Several determinants influence the health of oral tissues after post insertion of the denture. Systemic metabolic disorders like diabetes mellitus, nutritional deficiencies, infections, renal and hepatic disorders, bone disorders, radiation, and hormonal imbalance greatly hamper the tissue response to functional loading after insertion of the prosthesis. As a result multiple changes could be observed in the denture bearing hard and soft tissues ¹. The bone may resorb, altering the topography of the denture bearing foundation and the soft tissue may undergo inflammation and compromised function could be experienced by the patient.

Among the multiple methods of maintaining health in such situations they can be classified broadly into surgical and non-surgical modalities. Among the non-surgical methods employed a potential treatment option is with resilient denture liners. A soft liner absorbs some of the energy, produced by masticatory impact that would otherwise be transmitted through the denture to the soft basal tissue thus minimizes trauma and affords protection to the denture bearing tissues².

The denture resilient liners have a commendable capacity to restore the health of the inflamed and distorted mucosa. The liners are commercially marketed based on their chemical composition as acrylic and silicone liners³. Despite their advantages the liners possess a few disadvantages also. Among them the chief disadvantages is bond failure occurring between the liner and denture base which will create a potential interface for microleakage which sub-sequently results in delamination of the liner from the denture thus negating the effective function of liner⁴.

Owing to this problem several methods to improve adhesion between the liner and denture base have been suggested^3,4. These include various surface pre-treatment of the tissue surface of the denture base by chemical, mechanical and mechano-chemical modalities reported with varying degrees of success⁵.

Hence this study attempts to evaluate the various pre-treatment methods to improve the adhesion between a permanent acrylic denture liner and denture base to prolong the function of the prosthesis.

This aim of this study is

To estimate the tensile bond strength between denture base resin and acrylic resilient liner following various denture base surface pre-treatments.

To compare and evaluate the efficacy of various surface pre-treatments influencing the bond strength of the denture base with liners at varying time intervals in the presence of artificial saliva.

Materials and Method

The materials used for this study are listed in Table 1. The specimens of 280 rectangular acrylic blocks measuring 8 x 8-mm² cross-sectional area and 40-mm length were prepared. For preparation of the blocks, 20 metal dies were prepared of equal length and soldered to the plate which covers the lower flask. Elastomer (putty consistency) material was mixed and packed in to the lower flask and the upper lid was pressed into the lower flask mixed material. After the material set, it was separated from the set elastomeric material. Heat cure acrylic material was mixed and packed in dough stage into the mold created by the metal dies; the flask was compressed and closed using clamps and the excess material was removed. After bench press the flask was kept for curing at 75⁰ c for 9 hours. After polymerization, PMMA blocks were stored in artificial saliva.

Two PMMA blocks with 3 mm thick metal spacer between them were invested in denture flask using silicone rubber to allow easy removal of the specimens from the flask. The PMMA blocks were removed from the flask and the surface to be bonded were pre-treated with different chemical, mechanical and chemico-mechanical methods as stated in Table 2. Next, the PMMA blocks were placed back into the molds, and the acrylic based resilient liner is then packed into the space made by the brass spacer. After 10 minutes, the specimens were removed carefully with a scalpel and were stored in artificial saliva for 24 hours, 1 week and 1 month before the tensile bond strength was determined for each specimen until failure. Universal testing machine (Lloyd instruments / England / Model LR100K) at a crosshead speed of 5mm/min was used for this test. Bond strength (Mpa) was calculated as stress at failure divided by the cross-sectional area of the specimen. The data was analysed using t test and ANOVA.

Results

The results of the afore mentioned tables gave inference as following

Table 3 inferred that the bond strength of acrylic blocks pre-treated with acetone followed by resilient liner application was more than the control group for all categories, 24 hours, 1 week, 1 month immersion in artificial saliva respectively with p < (0.001***) which was statistically significant.

Table 4 inferred that the bond strength of acrylic blocks pre-treated with methyl chloride followed by resilient liner application was more than the control group for all categories, 24 hours, 1 week, 1 month immersion in artificial saliva respectively with p < (0.001***) which was statistically significant.

Table 5 inferred that the bond strength of acrylic blocks pre-treated with Methyl methacrylate monomer followed by resilient liner application was more than the control group for all categories, 24 hours, 1 week, 1 month immersion in artificial saliva respectively with p < (0.001***) which was statistically significant

Table 6 inferred that the bond strength of acrylic blocks pre-treated mechanically with Sand blasting followed by resilient liner application was more than the control group for all categories, 24 hours, 1 week, 1 month immersion in artificial saliva respectively with p < (0.001***) which was statistically significant

Table 7 inferred that acrylic blocks which were pre-treated mechanically with Sand paper, showed more tensile bond strength than control group for 24 hrs P < (0.001^***).which was significant. 1 week samples showed more bond strength than 24 hrs samples, one month samples showed decreased bond strength which was also significant.

Table 8 inferred that acrylic blocks pre-treated Mechanically by Sand paper and Chemically with Methyl Methacrylate followed by resilient liner application was more than the control group for all categories, 24 hours, 1 week, 1 month immersion in artificial saliva respectively with p < (0.001***) which was statistically significant

Table 9 infers when comparison of tensile bond strength done within the Chemical group, high bond strength was found with the Methyl methacrylate sample group and Methyl chloride sample group, when compared to the Acetone sample group which was found to be least.

Table 10 infers When comparison done within the Mechanical group, Tensile bond strength was more for sand blasting group for first 24 hrs but, 1 week sand paper pre-treated samples showed more Tensile bond strength than the sand blasting samples.1 month samples had no significant difference they were having same bond strength.

Table 11 infers when comparison of Tensile bond strength of Mechano – Chemical group with Mechanical group done highest bond strength was found with the mechano – chemical sample group.

Table 12 infers when comparison of tensile bond strength of Mechano – Chemical group with Chemical group done, highest bond strength was found with the mechano – chemical sample group.

The result of One way ANOVA for tensile bond strength values obtained 24 hours, 1 week and 1 month post bonding are shown in Tables 13,14 and 15. The result shows that the difference in tensile bond strength values obtained in each of the 7 groups are statistically significant.

Figure 1

TABLE 1 LIST OF MATERIALS USED

Figure 2

TABLE 2 SURFACE PRETREATMENTS AND GROUPS

Figure 3

TABLE – 3 COMPARISON OF CONTROL GROUP WITH ACETONE GROUP

Figure 4

TABLE -4 COMPARISON OF CONTROL GROUP WITH METHYL - CHLORIDE GROUP

Figure 5

TABLE -5 COMPARISON OF CONTROL GROUP WITH METHYL METHACRYLATE GROUP

Figure 6

TABLE -6 COMPARISON OF CONTROL GROUP WITH SANDBLASTING GROUP

Figure 7

TABLE -7 COMPARISON OF CONTROL GROUP WITH SANDPAPER GROUP

Figure 8

TABLE -8 COMPARISON OF CONTROL GROUP WITH CHEMICO - MECHANICAL GROUP

Figure 9

TABLE -9 COMPARISON WITH IN THE CHEMICAL GROUP

Figure 10

TABLE -10 OMPARISON WITH IN THE MECHANICAL GROUP

Figure 11

TABLE -11 COMPARISON OF MECHANICAL GROUP WITH MECHANO – CHEMICAL GROUP

Figure 12

TABLE -12 COMPARISON OF CHEMICAL GROUP WITH MECHANO – CHEMICAL GROUP

Figure 13

TABLE -13 Result of One– way ANOVA for tensile bond strength test of 24 hour groups

Figure 14

TABLE -14 Result of One – way ANOVA for tensile bond strength test of 1 week groups

Figure 15

TABLE -15 Result of One – way ANOVA for tensile bond strength test of 1 month groups

Discussion

From the results of this study the null hypothesis stating that is there is no difference in tensile bond strength of heat polymerized permanent acrylic soft liner with various denture base surface pre treatments was rejected with the significant confidence level of (p=0.000). Hence, it could be inferred that denture base surface pre treatment significantly increased tensile bond strength of the heat polymerized permanent acrylic soft liner and the combined mechano chemical method increasing the strength to the largest extent followed by mechanical and chemical methods respectively. The tensile bond strength of all the pretreated specimen decreased with sub sequent time intervals during immersion in artificial saliva by all the three methods of denture base pre treatment.

Denture soft lining materials have been used in dentistry for many years. Denture soft liners have a key role in modern removable prosthodontics because of their capability of restoring health of inflamed and distorted mucosa^6,7,8. The use of resilient lining material is helpful in fabricating removable complete and partial dentures because of their ability to alleviate inflamed mucosa, resulting in a more equal distribution of functional load on the denture-bearing tissues and improving the intaglio denture surface and retention of the prosthesis⁸.

Resilient denture liners are advantageous for patients who are capable of delivering a relatively heavy occlusal load to unfavourable denture-bearing tissues. Duration of softness of the liners varies from a few months to as long as 5 years. Without a resilient liner, surface hardness of polymethyl methacrylate may lead to chronic soreness, due to pressure on the mental foramen, sharp bony spicules, thin, atrophic mucosa, bony undercuts, particularly in the mylohyoid region, irregular bony resorption, poor fit of the denture base, incorrect occlusal relationship, bruxism, and/or debilitating disease.

The resilient denture lining materials can also be divided into 2 types: plasticized acrylic resins and silicone elastomers. Resilient reline materials are available in auto polymerizing and heat-polymerizing forms ⁹. Acrylic resin-based resilient denture liners often contain plasticizers that may leach out of the material, resulting in the hardening of the liner with time. It has been suggested that initial softness of the plasticized acrylics is due to the large quantity of plasticizers in the liquid, and the plasticizers are responsible for maintaining material softness. The purpose of the plasticizer is to lower the glass transition temperature of the polymer to a value below mouth temperature so that the amount of permanent deformation of the resilient material is reduced to a satisfactory level. In silicone-based resilient lining materials, the polymer is an elastomers, which does not require an external plasticizer and is, therefore, more stable over time ¹⁰.

One problem with resilient liners is the failure of adhesion between a resilient liner and a denture base. Bond failure between the liner and the denture base also creates a potential interface for microleakage. Therefore effective bonding is important for longevity, and long term bonding cannot be accomplished without also preventing leakage of fluids between the liner and denture base. The patient with chronic soreness from dentures presents an extremely difficult problem for prosthodontic treatment. This condition is caused mainly by irritation from faulty dentures, by bruxism, or by denture irritation secondary to a systemic condition ^11,12. Abused soft tissues supporting dentures often distort and destroy underlying bone resulting in continued escalation of the deformation. The soft lining materials are used for patient comfort, for the treatment of the atrophic ridge, bone undercuts, bruxism, xerostomia, and denture opposing natural teeth ¹³. They are also used to secure dynamic impressions, as tissue conditioners to restore the traumatized oral mucosa to a healthy state, as temporary reliners to maintain the fit of a denture and prevent trauma, and for trial evaluation of border extension. It is necessary to apply the soft lining material to the fitting surface of a denture in order to act as a ‘cushion’ which will enable traumatized soft tissues to recover before recording an impression for a new denture¹⁴.

During clinical use, soft liners are in saliva and during storage of the denture; they may be soaked in an aqueous cleansing solution or in water. When immersed, soft lining materials undergo two processes: plasticizers and other soluble materials are leached out & water or saliva is absorbed ¹⁵.

Both processes are important as they are going to have an impact on the physical properties of the material and its dimensional stability. To predict clinical behaviour, both the amount of water absorbed and the amount of soluble material lost must be measured over a period which is comparable with the proposed period of use in the oral environment ¹⁶.

Resilient denture liners have several problems associated with their use, as loss of softness, change of permanent deformation characteristics, water absorption, colonization by Candida albicans, and bond failure between liner and denture base^17,18 . Bond failures also create potential surface for bacterial growth, plaque, and calculus formation. Therefore, frequent clinical evaluation and periodic replacement of resilient denture liners is required. Ideally, denture liners should bond well enough to PMMA resin denture base to avoid interface failure during the service life of the prosthesis ^19,20.

Several factors may affect the bond strength between the resilient lining material and the denture base, such as aging in water, use of adhesive, and the nature of the denture base material. When the resilient lining material absorbs water, stress occurs between the bonding surfaces and viscoelastic properties of the resilient liners change. The material becomes brittle and transfers the external loads to the bond area. To simulate these natural conditions in the present study, the specimens of the tensile bond strength test are thermal cycled²¹.

Bonding between the denture base resin and liner material relies completely on the adhesive, a solvent that dissolves the denture base resin surface, such as acrylic resin monomer. The specimens were placed in artificial saliva to simulate oral environmental conditions and tensile bond strength was evaluated over varying intervals of time, 24 Hrs, 1 Week and 1 Month respectively.

The group 1 control specimens showed least bond strength than other pre-treated groups.

The group 2 specimens pre-treated with acetone showed there was significant increase in the bond strength after 24 Hrs and 1 week of evaluation. But subsequently due to constant immersion in artificial saliva for a period of 1 month there was decrease in bond strength. It was postulated that Acetone being an organic solvent penetrated into the surface of the poly Methyl methacrylate resin creating microvoids in the resin surface thus increasing the surface area for the liner to flow inside.

The group 3 specimens pre-treated with methyl chloride showed there was significant increase in the bond strength after 24 Hrs and 1 week of evaluation. But subsequently bond strength decreased. Methyl chloride being organic solvent had similar action as Acetone.

The group 4 specimens pre-treated with methyl methacrylate monomer showed there was significant increase in the bond strength after 24 Hrs and 1 week of evaluation. But subsequently bond strength decreased. Monomer provided additional etching and increased the surface area by creating microvoids.

Among the chemical groups, bond strength was more in the specimens pre-treated with methyl methacrylate followed by methyl chloride and acetone.

The group 5 specimens pre-treated with sandblasting with 50 microns alumina particles showed there was significant increase in the bond strength after 24 Hrs and 1 week of evaluation. But subsequently bond strength decreased due to continuous immersion in the artificial saliva. Alumina particles created microvoids, which increased the surface area available for bonding to the denture liner.

The group 6 specimens pre-treated with sandpaper showed there was significant increase in the bond strength. Bond strength was more in 1 week specimens than 24 Hrs specimens. But 1 month specimen showed gradual decrease in bond strength as other groups. Sand papering was done with very fine, 1000 grit paper. This created micro roughness of the surface resin block which helped to bond stronger.

The group 7 specimens pre-treated with sandpaper and monomer showed that bond strength was more in 24 hour specimens than 1 week specimens. But 1 month specimen showed gradual decrease in bond strength as other groups. Sand papering was done first then followed by monomer pre-treatment. Sand papering caused microvoids and monomer acted as superficial solvent of the poly methyl methacrylate resin which enhanced bond strength.

Among the chemical methods, pre-treatment with monomer exhibited highest bond strength of (2-39 MPa) followed by methyl chloride (2.03 MPa) and acetone (0.83) group respectively when evaluated at 24 Hrs. 1 Week results showed (2.17 MPa), (1.67 MPa) and (0.57 MPa) and 1 Month results were (1.29 MPa), (0.76 MPa) and (0.42 MPa) after immersion in the artificial saliva.

Among the mechanical methods, sandblasting exhibited highest bond strength of (2.76 MPa) and sand paper exhibited (2.46MPa) with in 24 Hrs. But when 1 week samples were tested bond strength was more in sand paper group with bond strength of (2.68 MPa) and Sand blasting group with bond strength of (2.47 MPa). 1 month samples exhibited bond strength of (1.55 MPa) and (1.34 MPa).

The results of mechanochemical method showed highest bond strength of (3.91MPa) when tested with in 24 Hrs. 1 week results exhibited (3.50 MPa) and 1 month exhibited (2.47 MPa).

The result of One way ANOVA for tensile bond strength values obtained 24 hours, 1 week and 1 month post bonding shows that the difference in tensile bond strength values obtained in each of the 7 groups are statistically significant (P< .05). Therefore it was concluded that each method of surface pretreatment of the denture base resin increased the tensile bond strength between the denture base resin and the resilient liner differently.

There are few limitations pertaining to this study. The behaviour of bonding between denture liner and pre-treated denture base resin should be assessed over a longer period of time to assess the efficacy in a broader spectrum. This is an In-vitro study and hence some variations might be experienced when tested under in vivo conditions owing to fluctuations in the pH of saliva, varying concentrations of ions in the saliva, presence of immunoglobulin’s and serum markers in the saliva, different occlusal schemes, use of disinfectants and stain removers during denture maintenance. These factors could modify bond strength during function.

This study opens new scope for further research such as evaluation of the effect of denture base surface pretreatment on the degree of water sorption and co-efficient of thermal expansions of the liner and microscopic evaluation of bonding behavior after pretreatment.

Conclusion

Within the limitations of this study it could be concluded that

The surface pre-treatment of denture base significantly increased the tensile bond strength and adhesive capacity with resilient liners.

Among the various methods of pre-treatment of denture base Viz chemical, mechanical, and mechanochemical, it was inferred that the mechanochemical surface pre-treatment with sand paper abrasion followed by monomer application exhibited superior bond strength compared to the other methods.