Scanning Electron Microscopic Evaluation of Composite Resin-Dentin, liner-Dentin Interfacial Gap with Composite Resin Restora

INTRODUCTION

The ability of a restorative material to seal the interface with tooth structure is perhaps the most significant factor in determining resistance to formation of future caries. Quality and durability of the marginal seal have always been major considerations in selection of a restorative material¹ . Studies have demonstrated that dental materials such as silicate cement, zinc phosphate cement, resin composites and amalgams are biologically compatible, even if directly applied to dentin, provided that the latter is hermetically sealed and protected from any subsequent bacterial leakage². Moreover dental pulp has been shown to have its own reparative capacity, capable not only of healing but also of producing dentinal bridge in the absence of calcium hydroxide.³

The problem of marginal infiltration is still unresolved because resin composites contract during polymerization, thus detaching from the walls of the cavity. It is therefore necessary to isolate dentin with intermediate materials, so that after the passage of fluids in the gap, there is no possibility of bacterial penetration inside the tubules, causing postoperative sensitivity, pulpal complications and secondary caries.

Certain clinical and biological studies have reported that prepared dentin and underlying pulp must be protected with liners and bases that supposedly impart biological and structural integrity to tissues. But recent studies imply that clinicians need to re-evaluate the generous use of calcium hydroxide and glass ionomer liners and bases. Light cure glass ionomer cements have superior features such as immediate curing with visible light irradiation, no water sensitivity after curing, higher translucency than conventional glass ionomer cement and most importantly improved bond strength⁴. As there is residual flexibility of glass ionomer liners after light curing, absorption of the contraction stress of resin composite upon polymerization may result.^{5, 6}

New adhesive systems provide etching of dentin and combined use of monomers with bifunctional groups (primers) as well as hydrophilic resins able to penetrate dentinal tubules and chemically and mechanically bind to peri tubular and intertubular dentin. Further acid etching eliminates smear layer and demineralizes the dentinal surfaces, thus allowing the penetration of resin tags in the tubules and formation of a demineralized resin dentin inter diffusion zone the hybrid layer.

The objective of the present study was to compare marginal sealing capacity of adhesive restorative system and combination of this system with different lining materials. 

MATERIAL AND METHODS

Thirty non carious third molars (maxillary and mandibular) belonging to subjects in age group of 18 to 22 years were used in present study. Teeth were cleaned free of debris and fixed by immersion in 10% neutralized formalin solution.

Preparation of sample

2 mm deep heels were made on the buccal and lingual surfaces at the level of cemento enamel junction of each tooth. The specimens were then ground flat with cooled orthodontic cast trimmer until the heels disappear. Dentin was then ground with sand paper, in decreasing grits. All the preparations were rinsed with water and dried with compressed air. The teeth were randomly divided into 3 groups of ten each.

Group 1

A self hardening calcium hydroxide (Calcimol, VOCO) was mixed and a thin layer was applied over exposed dentinal buccal and lingual surfaces, adjacent to area of pulp chamber.

Group 2

A self-hardening glass ionomer lining cement (GC Corporation) was mixed and a thin layer was applied over exposed dentinal buccal and lingual surfaces, adjacent to area of pulp chamber. The cement was allowed to set at room temperature for 5 minutes.

Group 3

Aqueous solution of 20% polyacrylic acid with 3% aluminium chloride (cavity conditioner,GC international) was applied to exposed dentinal surface for 10 seconds, and washed off for 5 seconds with air water spray. Resin modified glass ionomer linear (GC FUJI BOND LC) was applied over exposed dentinal buccal and lingual surfaces, adjacent to area of pulp chamber and photo cured for 10 seconds.

The remaining exposed dentinal surfaces were etched with scotch bond multipurpose etchant gel for 15 seconds. The etchant was rinsed for 15 seconds with air water spray. Next Adper Single Bond 2 adhesive was applied over calcium hydroxide/glass ionomer lining cement/ resin modified glass ionomer liner and etched dentinal surfaces with the help of applicator tip and gently dried for 5 seconds with air until shiny surface was established and photo cured for ten seconds. Thereafter a layer of hybrid resin composite Filtek Z 100 not more than 2mm was applied to dentinal surfaces and light cured for 40 seconds.

Teeth were mounted vertically by embedding half of the root portion in self cure acrylic resin. Teeth were then sectioned with a carborundum disc along the longitudinal axis, thus passing through the center of restoration [fig 1]. Sectioned surfaces were cleaned with 10% ortho phosphoric acid for 3 to 5 seconds to remove smear layer. All the specimens were dehydrated through increasing concentration of ethyl alcohol (30, 50, 70, 90 &100%) for one hour in each concentration. Specimens were mounted with silver paste on metallic stubs and gold coated with a sputtering system under vacuum and then examined under scanning electron microscope at an acceleration voltage of 7 to 15KV.

The interfacial gaps between calcium hydroxide / glass ionomer lining cement / resin modified glass ionomer liner and unetched dentinal surface and composite resin- etched and primed dentin surface was observed under scanning electron microscope at a magnification of 200X [Fig 2], 750X [Fig 3,4] & 3000X [Fig 5].The interfacial gaps were measured and noted in a tabular form. The data was analyzed with one-way analysis of variance (ANOVA). Duncan's multiple range test was used to compare the group means.

RESULTS

Interfacial gaps between the liner- dentin and composite - dentin were measured under scanning electron microscope at a magnification of 200X, 750X and 3000X. The measured values wear subjected to statistical analysis.

Means and standard deviations were calculated for each test condition (N=15) and compared for significant difference using ANOVA (=0.05). The P values obtained for testing equality of means among A1, A2 and A3 (Composite resindentin interface) is equal to 0.066. Since this is greater than the level of significance (=0.05), we conclude that means of A1, A2 and A3 do not differ significantly at 5% level of significance. The P value obtained for testing the equality of means among B1, B2 and B3 is very close to zero (= 0.0001). Hence we can conclude that the means of B1, B2 and B3 do differ significantly. [Table 1]

Duncan's multiple range tests was used to test the significance of difference between specific means. If the difference between two means exceeds the corresponding critical value we conclude that two means differ significantly. [Table 2]

Results indicated that calcium hydroxide lining material had highest degree of interfacial gap, according to variance (ANOVA) and Duncan's multiple comparison range test, calcium hydroxide lining material differed significantly from glass ionomer lining cement and resin modified glass ionomer liner. Resin modified glass ionomer lining cement produced values less than auto set glass ionomer lining cement at P =0.05. However no significant difference was found between values for resin modified glass ionomer liner and glass ionomer lining cement.

DISCUSSION

For in vitro evaluation of a restorative material or technique, investigations typically focus on marginal adaptation at the resin-dentin interface and liner-dentin. For interfacial analysis, different methods have been employed: three dimensional analysis⁷, dye penetration test⁸, and SEM observation⁹. Although micro leakage evaluation is one of the most common methods for assessing the sealing efficiency of a restorative material, a gold standard has not been established 8 for this evaluation method yet. It is also noteworthy that even a minor micro leakage might induce recurrent caries, which implies that it may not be appropriate to evaluate resin-cavity interfaces based on micro leakage rankings or scores. In the present study, the crosscut surfaces of restorations were observed using SEM. Direct observation by SEM is difficult due to the presence of the liquid phase in the tooth tissues. The vacuum procedure during SEM causes artifacts like cracks, which can look like true gap formation if the liquid is not removed in a proper way.¹⁰

During sample preparation the buccal and lingual surface was taken as dentin bonding site. This was done to get a consistent superficial dentin and on flat surfaces tensile stresses can be more producible. On flat dentin surface, the bond strength could withstand the contraction forces. This configuration allowed a large, free and unbonded surface, which permitted the flow of the resin across the free surface during its polymerization shrinkage, thereby minimizing stresses at the bonded surface as suggested by Davidson & de Gee 1984. ¹¹

Mechanical properties of calcium hydroxide are less than ideal (Farah et al 1983).¹² High solubility may result in contamination of bonding agents and increased marginal leakage (Krejci and Lutz 1990).¹³ Macroscopic dentinal bridge formed does not constitute a continuous seal, but may allow bacterial leakage through numerous tunnel defects¹⁴ . In the present study it was seen that the bond strength of adhesive resin to calcium hydroxide was higher than the bond strength of calcium hydroxide to dentin. Interfacial gaps of 8 to 26 m wide were observed between calcium hydroxide and dentin in 100% of the cases. This could be interpreted as a sign that calcium hydroxide did not even resist the contraction stress, because of its weak adhesive property. 

Glass ionomer has been utilized as a liner/base in an attempt to take advantage of two highly desirable properties; chemical bond to tooth structure and fluoride release. Abdulla Ali, Davidson et al 1993 reported that use of glass ionomer liners has been demonstrated to improve marginal integrity and decreased marginal leakage¹⁵ . Less bulk of composite resin is required to fill the preparation, reducing the amount of polymerization shrinkage, and improve marginal adaptation.¹⁶ Glass ionomer liners also reduce the rise in temperature associated with application of the curing light during incremental insertion procedure. Data presented in this study also indicates that glass ionomer may not have sufficient adhesion to the tooth structure to resist the polymerization shrinkage forces. A gap up to 16 m was seen between glass ionomer liner and dentin.

Resin modified glass ionomer has recently been introduced as a light cured liner bonding system for dentin and enamel. It is claimed to develop a dynamic bond and be flexible enough to cushion against strong forces. (S. K Sidhu and T. F Coatson). Marginal leakage can be minimized because light cured ionomers show much better marginal seal to dentin than other restorative material, thereby reducing the chance of post operative sensitivity. (Tjan and Dunn 1990, Wibouro, Stockon and Suzuki 1999)^{18, 19}. Tolidis and others 1998 found that the use of resin modified glass ionomer liner reduced volumetric polymerization contraction for the light cured composite material to approximately 61%. A possible explanation of the relief of stress that results from the use of an intermediate layer of resin modified glass ionomer cement with partial dimethacrylate cross linking is the movability of the clusters of molecules in the early set materials. This gives the total restoration the flexibility required to compensate for the part of the polymerization contraction stress that exceeds the bond strength. As a result, the adhesive bond will remain intact and marginal integrity will be preserved. 

Ciucchi B, Bouillaguet.B and Holz.J 1997 reported that a strong chemical and mechanical bond was achieved between the resin modified glass ionomer and restorative material. They concluded that the bond to dentin on the one hand, could not resist the polymerization stress generated during bulk insertion but, on the other hand appeared to be sufficiently adhesive to resist the stress produced during the 20 polymerization of multiplayer or indirect technique .Resin modified glass ionomer consistently presented the smallest interfacial gap (mean 4.26m), followed by the glass ionomer lining cement (mean 7.73m), and calcium hydroxide lined specimens (mean13.53m).

Composite resin- dentin interface single bond multipurpose dental adhesive showed a gap free attachment, hybrid layer and resin tag penetration at X 3000 magnification. This result is consistent with other studies by Goracci et al and Ciucchi et al.^{20, 21}

A definitive and precise location of the gaps was not possible. Gaps could have formed at different levels of the interface, inside the materials or even in the dentinal tissue. However, any space formed can be considered deleterious mechanically, as it permit's the material to deform under occlusal load.²²  Under masticatory forces, the fluid which accumulated in these gaps could be forced inside the dentinal tubules towards the pulp, thus triggering pain.²³ The gap and the fluid it contains could become a biologically deleterious reservoir of substances leaching from the materials or denatured proteins, a perfect environment for the growth of bacteria possibly on the dental surface or inside the smear layer. ^23,24,25

Therefore, to improve the longevity of restoration, more effort should be directed at increasing adhesive and sealing properties of restorative materials placed on dentin.

SUMMARY AND CONCLUSION

On analyzing this result, following conclusions could be drawn. All liners tested exhibited a gap between liner and dentinal walls, amount of interfacial gap formed between calcium hydroxide- dentin interfaces differed significantly from glass ionomer lining cement and resin modified glass ionomer liner. Results indicated significantly better performance of light cured glass ionomer liners compared with auto set glass ionomer liners. Amount of interfacial gap formed between composite resin - dentin interface is not statistically significant. Interposition of calcium hydroxide between composite resin possesses some clinical disadvantage. Hence it should be recommended only in selective clinical situations.

Multilayer and indirect restoration techniques reduce the formation of gaps. More efforts should be directed towards increasing the adhesive and sealing properties to be used, so that a gap free attachment can be developed and maintained.