Study of microleakage prevention in recently developed aesthetic dental restorative materials

IntroductionDental caries occurs as the result of metabolic activity of bacteria in dental plaque, leading to acid production and tooth demineralisation. The conventional restorative material, amalgam, is increasingly being replaced by, more aesthetic, glass-ionomer cements (GICs). GICs were first described by Wilson and Kent in the early 1970s 1. This material consists of a glass powder and a polyacid solution, which when mixed set in minutes to produce a solid. The main features of GICs are their adhesive property and ability to release fluoride. The major benefit of using adhesive materials is the bonding between the filling material and the tooth can prevent micro-leakage. Microleakage is the passage of bacteria between a restoration and dentine. It can cause recurrent caries and pulpal infections. The fluoride released from GICs can also increase caries resistance of the tooth adjacent to the restoration, and inhibit the growth of bacteria. GICs, however, cannot be used in load bearing regions due to their low flexural strength. This limitation has been partially overcome by incorporating resin, i.e. methacrylate monomers, into the GIC formulation. The resultant cements are known as Resin Modified Glass Ionomer Cements (RMGIC). The Constant Depth Film Fermentor (CDFF) 2 provides a new in vitro method to model the oral condition allowing studies of oral biofilms. In this work, CDFF was used to investigate the microleakage of bacteria into the microspace between 3 different filling materials and dentine in restored dentine cylinders.

D. Leung, K. Gulabivala, J. Pratten, D. Spratt, A.M. Young Department of Oral Microbiology, Eastman Dental Institute 256 Grays Inn Road, London,

WC1X 8LD

University College London

SummaryFig. 1a –During acid / glass reaction, the proton from the acid group is lost and a COO- group formed. Consequently, this reaction causes the peaks marked on the left and on the right to decrease, and gives rise to the peak marked in the centre. The rate of acid / glass reaction can be determined by monitoring any absorbance changes as function of time.

Fig. 1b - During polymerisation, the methacrylate C=C and C-O peaks decrease. The levels of change observed are consistent with full polymerisation within 2 min after the start of exposure to a dental light. The changes in spectra at latter time indicates the occurrence of acid / glass reaction (compare Figure 1a and 1b). This process is required to generate fluoride ions.

Fig. 2- This graph shows fluoride release from the GIC and RMGIC are comparable. The initial linear relationship with square root of time suggests a diffusion controlled process.

Fig. 3a,b – The reduced number of bacteria observable on the GICs and RMGICs in comparision with the amalgam specimens are consistent with reduced bacterial microleakage. Much of the roughness on the GIC and RMGIC surfaces may be due to a combination of a smear layer and nanoleakage of artificial saliva.

Fig. 3c,d - Without the smear layer, the structure of dentine tubules can be seen clearly, and the imprinted structure of dentine tubules were observed on the cement surface. This result suggests better interactions between the cements and the dentine in the absence of smear layer.

Materials and MethodsBoth GICs and RMGICs (Fuji IX GP and Fuji II LC, GC Corporation, respectively) were prepared according to the manufacturer's directions and powder / liquid ratios.

FTIR – Materials of 1 mm thickness were placed onto the single reflection diamond ATR attachment in the sample chamber of a Perkin Elmer 2000 series FTIR spectrometer. Spectra were then taken at vairous times.

Fluoride release – The cements were made into discs and the amount of F- ions released into distilled water measured, using a fluoride meter, as a function of time.

Microleakage studies – Dentine cylinders were prepared from bovine teeth. Each was then centrally drilled and restored with either Amalgam (Tytin FC), a GIC (Fuji IX) or an RMGIC (Fuji II LC from GC Corporation) according to manufacturer’s instructions. Human saliva was used as an inoculum to provide a multi-species biofilm consisting of organisms found in the oral cavity. The biofilms were formed on the dentine cylinders and grown in a Constant Depth Film Fermentor (CDFF) with a continuous supply of artificial saliva. At selected time periods, samples were removed and examined by Scanning Electron Microscopy (SEM).

Smear layer – The effect of conditioning on smear layer (cutting debris) was also investigated. The drilled cavities were washed with GC dentine conditioner thoroughly before being restored. After submerged in sterile artificial saliva for 10 weeks, the samples were examined as described above.

ConclusionsDue to the toxicity of HEMA, the methacrylate in Fuji II LC, it is important to ensure completion of polymerisation. From this study, FTIR showed practically 100 % polymerisation of this monomer. The fluoride release level from the RMGIC was found to be comparable to that of GIC. This suggests the anti-bacterial properties of RMGIC and GIC should be equally effective.

The use of the CDFF model simulated the oral condition and accelerated the microleakage at the dentine restoration interface. A larger number of bacteria were observed from the amalgam restorations. RMGICs were shown to be equally effective at preventing microleakage as GICs. This technique, however, could not show us whether this result was due to the sealing abilities of GIC and RMGIC or the anti-bacterial effect from fluoride release. Longer experimental time and larger sample numbers are needed to distinguish differences between GICs and RMGICs.

The presence of the smear layer prevents interactions of the adhesive material with dentine. The removal of the smear layer can therefore improve the bonding between the cements and dentine, which can potentially improve the sealing properties and to prevent microleakage to a greater extent.

References1. A D Wilson and B E Kent. The Glass Ionomer Cement, a New Translucent Dental Filling Material. J. Appl. Chem. Biotechnol., 1971, vol. 21, pp 313

2. J Pratten and M Wilson. Antimicrobial Susceptibility and Composition of Mircosom Dental Plaque Supplemented with Sucrose. Antimicrobial Agents and Chemotherapy, July 1999, pp 1595-1599

AimsThe aims of this study were to assess

1) GIC and RMGIC setting reactions by Fourier Transform Infra-red Spectroscopy (FTIR);

2) whether recently developed resin modified glass ionomer cements are as effective as conventional GICs in preventing bacterial microleakage using a Constant Depth Film Fermentor (CDFF).

For Oral Health Care Scienceshttp://www.eastman.ucl.ac.uk

Eastman

Dental

Institute

http://www.eastman.ucl.ac.uk

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Fig. 2 – Fluorie ion released from Fuji IX and Fuji II LC

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Fig. 3a – SEM picture of the amalgam sample at 10 weeks

Fig. 3b – SEM picture of the GIC sample at 10 weeks

Fig. 3c – SEM picture of the RMGIC surface without conditioning on the smear layer prior to restoration

Fig. 3d – SEM picture of the RMGIC surface with conditioning on the smear layer prior to restoration

Fig. 1b – IR spectra of Fuji II LC as function of time

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