Comparative evaluation of residual monomer content and polymerization shrinkage of a packable composite and an ormocer.

AIM: The aim of this study is to evaluate the residual monomer content and polymerization shrinkage of a packable composite (Surefil) and an ormocer (Admira).
MATERIALS AND METHODS: The study was conducted in two parts. In Part I, 10 samples of each material were prepared in a standardized split brass mould, using incremental curing technique. The residual monomer content was measured by observing change in weight before and after Soxhlet Extraction procedure. In part II, the volumetric polymerization shrinkage was calculated by measuring the difference in specific gravities of 10 uncured and 10 cured samples of each material using a modified version of ASTM D-792 method. The data obtained was put to statistical analysis using student's 't' test.
RESULTS: Part I - The percentage change in weight for Surefil was 0.525% while that for Admira was 0.374%, which was found to be statistically significant. Part II - The volumetric percentage shrinkage for Surefil ranged between 1.04-3.42% and that for Admira between 1.01-2.31%, which was not found to be significant statistically.
CONCLUSION: Admira may be considered more biocompatible than Surefil due to the lower residual monomer content in the former; however, both are comparable with regards to their polymerization shrinkage.

INTRODUCTION

The overriding goal of conservative dentistry is to improve the functional life of a carious tooth by judicious use of a restorative material. Hence, any restorative material should be biocompatible, have a good bond with the tooth structure and should possess physical and mechanical properties similar to those of tooth tissues along with chemical inertness. In today's era of esthetic dentistry, composite resins are the material class satisfying many of the above requirements. This was made possible by the acid-etch technique of Buonocore[1] and a silica-reinforced polymer manufactured by Bowen[2] which was the forerunner of present day composite resins. Composites are being heavily researched today in order to overcome many of their deficiencies like polymerization shrinkage, residual monomer content, low wear resistance etc. Manufacturers’ have come up with several new varieties with modifications in the filler technology, filler distribution and loading as well as alterations in their matrices. The introduction of packable composites is one such innovation as they are less technique-sensitive and can be handled like amalgam.[3] These are based on PRIMM technology i.e. polymer reinforced inorganic matrix material and hence exhibit improved physical and mechanical properties and high filler content leading to high viscosity resulting in less polymerization shrinkage.

Another promising development has been the synthesis of organically-modified ceramics or ORMOCERs. While composites are based on a purely organic resin matrix, ormocer consists of an inorganic-organic network matrix formed through polycondensation i.e. inorganic backbone based on silicon dioxide (SiO2) functionalized with polymerizable organic units.[4] These long ormocer molecule chains are very rigid, have extremely high molecular weight with a preformed structure, exhibit low polymerization shrinkage and more bio-compatible than the currently available composites owing to their cross-linked structure leaving least amounts of residual monomer. Despite technological advancements and claims of the manufacturers, any new composite should be tested for certain inherent weaknesses as these may cause failure of restorations.

For instance, polymerization shrinkage may lead to development of shrinkage stresses (13 MPa), causing defects in the composite tooth bond, and leading to bond failure and micro-leakage with attendant sequelae of marginal discoloration, post-operative sensitivity, recurrent caries, hypersensitivity and development of pulpal pathology.[56] Polymerization process of composites is usually incomplete under clinical conditions and almost every component can be detected in the extracts of polymerized materials.[78] A high percentage (25-50%) of methacrylate groups may remain unreacted after setting, of which approximately 1/10th are present as residual monomer.[9-11] The components released are biologically active and their use has occasionally been associated with necrosis and irritation of the pulp.[12] Apart from bio-compatibility issues, monomers trapped in the set composite may reduce the clinical serviceability of the restoration. This study was conducted to comparatively evaluate the residual monomer content and polymerization shrinkage of a packable composite (Surefil-Dentsply) and an ormocer (Admira-VOCO).

MATERIALS AND METHODS

The study was conducted in two parts.

In Part I, residual monomer content was evaluated by Soxhlet extraction procedure. This part was divided in two groups: Group A (Surefil) and Group B (Admira). 10 samples of each material were prepared in a standardized split brass mould, using incremental curing technique. A total of 3 increments, each of 2 mm thickness cured for 20 seconds each using CU-100 A Light cure unit (Rolence Enterprises, Taiwan) were used. Each sample was weighed using an electronic weighing balance (Afcoset, India) and the readings were noted. The weighed samples were individually put into separate Whatman filter paper cones which were placed in separate Soxhlet extractors. The extractor was fitted with a condenser on the top and a round bottom flask below, containing 200 ml of methanol (Merck, Germany). The entire assembly was heated slowly and refluxing was carried out for total 108 hours. Samples were taken out at regular intervals of 36 hours and weighed after drying in a hot air oven followed by cooling in a desiccator.

Residual monomer content was calculated as follows:

Change in weight=Original weight – Final weight

The change in weight represented the residual monomer content. The data obtained was compiled, tabulated and put to statistical analysis.

In part II, the volumetric polymerization shrinkage was calculated by measuring the difference in specific gravities of uncured and cured samples using a modified version of ASTM D-792 method of “Specific gravity and density of plastics by water displacement”.[13] This part was also divided in two groups A’(Surefil) and B’(Admira) with 10 uncured and 10 cured samples of each material. Sample preparation was done in the same way as in part I except that half samples of each material were left uncured. The samples were weighed in air and readings noted. Next, they were weighed under water.

Specific gravity was calculated as follows:-

The percentage volumetric shrinkage was calculated as follows

RESULTS

Part I – The mean change in weight after 108 hours in Group A was 0.184 mg and that for Group B was 0.117 mg [Figure 1a]. The percentage change in weight for group A was 0.525% while that for group B was 0.374%. Using student ‘t’ test, the above differences were found to be statistically significant at 5% level of significance [Table 1].

Figure 1a

Change in weights of group A v/s group B in the study

Table 1

Comparison of residual monomer content observed by change in weight of group A and group B using student ‘t’ test in the study

Part II- The volumetric percentage shrinkage for Group A’ ranged between 1.04-3.42% with a mean of 2.276 and that for Group B’ between 1.01-2.31% with a mean of 1.812 [Figure 1b]. Using student ‘t’ test, the observed difference between the two groups was not statistically significant [Table 2].

Figure 1b

Volumetric shrinkage of group A v/s group B in the study

Table 2

Comparison of volumetric percentage shrinkage in group A and group B using student ‘t’ test in the study

DISCUSSION

The physical properties and handling characteristics of the currently available composites can be considered clinically sufficient; however, the biologic acceptability is still under consideration. Biocompatibility issue is closely related to residual monomer content and polymerization shrinkage and its sequelae, micro-leakage. Several newer varieties of composite resins have come up in order to overcome these drawbacks. In the present study, we evaluated two such materials i.e. a packable composite (Surefil) and an Ormocer (Admira) for their residual monomer content and polymerization shrinkage. The residual monomer content was evaluated by Soxhlet extraction procedure[14] from the polymerized samples. Various methods suggested to identify the residual monomer, such as, high performance liquid chromatography, Gas chromatography, Fourier transform infra-red spectroscopy, Nuclear magnetic resonance, etc generally require expensive and elaborate equipment. These methods are usually employed for structural characterization of the leachable components. Our objective was to measure the quantity of residual monomer; hence we used the Soxhlet extraction procedure which is the primary procedure before any of the above methods can be used.

Sample preparation was done in a standardized split brass mould because metal mould correlates more closely to the cavities in extracted teeth.[15] The mould had the internal diameter of 6 mm and depth of 5 mm simulating those of a deep class II cavity. Moreover, with this mould diameter, it can be completely covered by a light tip of 8 mm, to cure the sample in one position. An incremental curing technique was used because it compensates for the polymerization shrinkage.[1316] A total of 3 increments (2 mm) were used to fill the mould cavity as it has been suggested that at depths greater than 2 mm, poor polymerization of composite results.[17]

In the Soxhlet extraction procedure, the monomer is leached out from the polymerized samples using only the vapors of methanol, thus avoiding harsh conditions. Hence, only the monomer, since it is soluble in methanol, is extracted whereas the polymer being insoluble is not affected. Methanol was chosen as a solvent because it disintegrates the polymer to a lesser extent during chemical analysis as compared to other strong solvents like acetone and chloroform.[18] The monomer content in group A was found to be 0.525% while that for group B was 0.374%. Earlier studies employing different techniques and solvents have reported somewhat similar and slightly higher amounts of residual monomer in the commercially available composite resins. For instance, Inoue and Hayashi,[19] using tetrahydrofuran as a solvent found a residual quantity of Bis-GMA (0.4-1.2 wt%) in the then available composites. Ferracane[11] reported that approximately 5-10% of the unbound substances elute into an aqueous solution which corresponds to approximately 2 wt% of the resin matrix. Spahl et al.[20] calculated the residual quantity of the basic monomers as 0.2-1.3 wt% in ethanol. These values are in the order of magnitude reported by Tanaka et al.[21] i.e. 0.04-2.3 wt% in methanol and 0-0.4 wt% in water. Similarly, Polydorou O et al.[22] investigated the elution of monomers from two light-cured materials (nanohybrid and ormocer) and from a chemically cured composite, after different curing times (0, 20, 40 and 80 seconds) and different storage periods (24 hours, 7 days, 28 days, and 1 year after curing) in 1 ml of 75% ethanol. The amount of monomers released from the nanohybrid and the chemically cured composite was significantly higher than released from the ormocer. Vishnu S et al.[23] using the Fourier transform infrared spectroscopy also reported that packable composite resin showed significantly lower degree of conversion values when compared to hybrid and flowable composite.

From the above studies, it can be concluded that the materials used in our study, Surefil and Admira are better than the previous composites used by the above investigators, as regards the residual monomer content. This might be because of different techniques employed or due to a high filler loading in both the materials which reduces the resin matrix content. As compared to Surefil, Admira fared better because of a pre-polymerized structure and a high molecular weight. Polymerization shrinkage was calculated using the method suggested by Puckett and Smith.[13] There are 2 methods to determine the change in density i.e. water displacement method and Infra-red spectroscopy. In our study, water displacement method was used because it has been found to be accurate as it uses large sized specimens that can be made more precisely and chances of error are less, as compared to thin films of composites used in Infra-red spectroscopy which are difficult to make precisely.[24] The volumetric percentage shrinkage for both the Groups ranged between 1.01 – 3.42%. When the 2 Groups were compared, the difference was not found to be significant statistically when student ‘t’ test was applied (P=0.075). Therefore, it can be concluded that Surefil and Admira are comparable with respect to their polymerization shrinkage.

The results of our study are in concurrence with those of Puckett and Smith,[13] who found that percentage shrinkage of different composites ranged between 1.35 – 3.22%. Al-Harbi and Farsi[25] assessed the differences in the degree of microleakage according to the cavity wall location for Ormocer (Admira) and a composite (Restorative Z-100) and found no statistically significant differences between the 2 materials or the location of cavity walls. David A G et al.[26] evaluated the polymerization shrinkage by disk deflective method and microleakage (dye penetration method) of packable composite (Filtek P60), a compomer (Compoglass F), an ormocer (Admira) and a resin-modified glass ionomer (Fuji II LC). The ormocer and the packable composite exhibited the best sealing ability and lowest polymerization shrinkage. George J et al.[27] also investigated the polymerization shrinkage of two packable resin composites Surefil and Solitaire in air using a strain gauge and showed that Surefil exhibited significantly lower polymerization shrinkage than Solitaire.

CONCLUSION

From the present study, following conclusions can be drawn:

Admira may be considered more biocompatible than Surefil due to the lower residual monomer content in the former.

Both Surefil and Admira are comparable with regards to their polymerization shrinkage when cured incrementally.

Further, long term studies employing a large sample size are necessary to validate the above findings with regards to their clinical behavior.