Assessment of defects at tooth/self-adhering flowable composite interface using swept-source optical coherence tomography (SS-OCT)

Highlights

  • Adhesive defects were detected and assessed by optical coherence tomography.

  • At enamel, EF showed significantly fewer adhesive defects than PLP.

  • At dentin, EF showed more adhesive defects than PLP.

  • We would recommend clinical use of EF on very small cavities bounded by enamel.

  • The use of EF as a definitive restorative material must be put under scrutiny.

Abstract

Objectives

Assessment of adhesive defects of a self-etch adhesive and a self-adhering flowable composite at the tooth/composite interface before and after water storage by optical coherence tomography (OCT).

Methods

16 extracted human molars ( n = 8 each) with box-shaped, class-V cavities were restored either with an experimental self-adhering flowable composite (EF, DMG) or with the filling system Adper™ Prompt™ L-Pop™/Filtek™ Supreme XT Flowable composite (PLP, 3M ESPE). Restorations of both groups were non-invasively imaged using swept-source OCT before and after storage in water. The OCT signal for adhesive defects at the tooth/composite interface was quantified.

Results

At enamel, significantly fewer adhesive defects were detected at EF restorations than at PLP restorations, before water storage (4%/48%, p < 0.001) and thereafter (8%/49%, p < 0.001); in contrast, at dentin more interfacial defects were observed with EF (before water storage: 75%/11%; p < 0.001, after water storage: 77%/52%; p i = 0.001). In the case of slight initial adhesive defects, water storage caused a statistically verifiable increase in adhesive defects at the enamel interface with EF (before/after storage: 4%/8%; p = 0.023) and at dentin with PLP (before/after storage: 11%/52%; p = 0.008).

Significance

Given the high proportion of adhesive defects with the experimental self-adhering flowable composite, its use as the definitive restorative material in class-V cavities must be critically scrutinized and clinical indications must be investigated further with in vitro and in vivo trials.

Introduction

At the beginning of the 2000s, self-adhesive resin cements were introduced which could be used without any conditioning of the hard tooth tissues, thereby simplifying working procedures and reducing technique sensitivity . Chemical bonding to the tooth substrate is achieved by a reaction between phosphoric acid monomers and hydroxyapatite . As well as resulting in better esthetics, mechanical properties and dimensional stability are improved in comparison with conventional phosphate cements. These self-adhesive cements were expected to have higher moisture tolerance and be less sensitive postoperatively than conventional resin cements using etch and rinse adhesives .

This idea was transferred to the field of filling materials, and recently new flowable composite systems, so-called “self-adhering” flowable composites, have been brought to market. These systems combine a flowable composite with an all-in-one adhesive containing acidic adhesive monomers, thus eliminating the need for separate etching and conditioning of the tooth substrate. The manufacturers suggest that the combination offers good marginal sealing, reduces the risk of overetching and overwetting, and that it may avoid overdrying leading to a collapse of the collagen fiber network. The simplified procedures and lower probability of post-operative sensitivity make the technique popular with clinicians. However, there is little information available about the bonding performance of self-adhering flowable composites. Recent studies investigating this type of material have showed inconsistent results. Bektas et al. suggest that self-adhering flowable composites have an acceptable bond strength and marginal sealing and can therefore be used for clinical applications in general. More commonly, it is suggested that self-adhering flowable composites should be used only as a liner and for repairing small cavities or pits and sealing fissures . The material has shown a lower shear bond strength value than conventional flowable composites using an etch and rinse adhesive system . It has been reported that the self-adhering flowable composite has poor bond strength but better marginal sealing than conventional flowable composites using self-etch adhesive systems , and it has been suggested that routine clinical application of this material should be considered very carefully .

As interfacial adhesive defects seem to have a relevant impact on restoration longevity, additional research is needed to evaluate the adhesive characteristics of self-adhering flowable composites. This study aims to give additional insight into the bonding performance of a new experimental self-adhering flowable composite system. Optical coherence tomography (OCT) was selected as the best method to evaluate interfacial defects before and after water storage. OCT is an established diagnostic tool in retina assessment, and was chosen for this study because it provides non-invasive three-dimensional imaging with high spatial resolution by comparing a reference with the interference caused by light backscattered by microstructures. This tomographic method allows contact-free non-destructive real time 1D–3D images to be generated in 10–20 s. Various studies have shown the suitability of OCT for detecting adhesive defects or evaluating marginal adaptation .

We hypothesized that the experimental self-adhering flowable composite would not show significantly more interfacial adhesive defects at enamel and dentin before/after water storage when compared to the reference group in which a conventional self-etch adhesive system was used.

Materials and methods

Preparation of specimens

16 intact caries-free human molars were selected after extraction and immediately immersed in 0.5% chloramine solution at 4 °C. The teeth were cleaned mechanically. Standardized, box-shaped class V cavities with dimensions of 4 mm mesio-distally, 3 mm coronal-apically and an average depth of 1.5 mm were prepared with a rounded cylindrical diamond bur (107 μm, 836 KR 314014, Komet ® /Gebr. Brasseler, Lemgo, Germany). The cavities were placed at the vestibular cemento-enamel junction (CEJ) in order to deliberately produce cavity edges of equal length at enamel and dentin. The cavity margins in enamel were beveled (1 mm wide at the sagittal middle line at an angle of 45° relative to the cavity wall), whereas the cavity-surface margin in dentin was left as a butt joint ( Fig. 1 ).

Fig. 1
(a) Preparation of class V cavity and restoration with EF/PLP, (b) OCT imaging, (c) water storage for 100 day at 37 °C, (d) OCT imaging after water storage, (e) selection of 10 B-scans from OCT 3D volume data, scan field (box), 512 B-scans per specimen (dashed lines), composite restoration (Co, dashed circle). (b,d) OCT equipment; E: enamel, D: dentin.

The cavities were prepared by patting with a wet dabber, so that the cavities were neither overdried nor overwetted. The cavities were directly restored with an experimental self-adhering flowable composite (EF; DMG GmbH, Germany), and the other 8 cavities were restored with self-etch adhesive Adper Prompt L-Pop (PLP; 3M ESPE AG, Germany) and flowable composite Filtek™ Supreme XT Flowable (3M ESPE AG). Table 1 indicates the properties/compositions of the materials used, while Table 2 summarizes the application of the restorative materials. The restored teeth were stored in water at 37 °C for 100 days. The composite restorations were imaged initially (24 h after restoration) and after the water storage, using swept-source optical coherence tomography (SS-OCT).

Table 1
Properties and compositions of restorative materials used.
Material
Batch-no.
Company
Composition Polymerization shrinkage
(vol.%)
Filler content
(vol.%)
Water sorption
(μg/mm 3 )
Water solubility at 20 °C
(g/L)
pH Refractive index
Experimental self-adhering flowable composite (shade: A2)
F-142890
DMG GmbH, Hamburg, Germany
Barium glass in a Bis-GMA-based matrix 4.1 43.2 49.4 1 ≈3.0 1.49
Adper Prompt L-Pop
405560
3M ESPE AG, Seefeld, Germany
Liquid 1: Methacrylated phosphoric esters, Bis-GMA
Liquid 2: water, HEMA, polyalkenoic acid, co-polymer
1.5
Filtek™ Supreme XT flowable composite (shade: A2)
N166300
3M ESPE AG, Seefeld, Germany
Bis-EMA, BisGMA,
TEGDMA
nanosilica filler
agglomerated zirconia
silica nanocluster
≈3.8 59.5 17.1 −4.0 1.45

Table 2
Procedure for application of used restorative materials according to manufacturer’s recommendations.
Material Workflow
Experimental self-adhering flowable composite 1. Blowing of the tooth using water- and oil free air in order to avoid overdrying of the dentin
2. Application of the material onto the cavity surface with the aid of the micro-brush by pressing the syringe and massaging a thin layer (≈0.5 mm) into the entire surface of the cavity for 20 s
3. Removal of any excess material
4. Light-cure for 20 s (1000 mW/cm 2 , HS LED 1200, Henry Schein ® , Langen, Germany)
5. Build-up of layers of 2 mm thickness max. and curing of each layer for 20 s
6. Finishing and polishing with a diamond bur (46 μm, 8836KR314014, Komet/Gebr. Brasseler) and polishing bur (Politip F, 533602 and Politip P, 533584, Ivoclar Vivadent)
Filtek Supreme XT flowable with Adper Prompt L-Pop 1. Blowing of the tooth using water- and oil free air in order to avoid overdrying of the dentin
2. Application of Adper Prompt L-Pop onto the cavity surface for 15 s (pressure: ∼3.5 N)
3. Drying of adhesive to a thin film using a gentle stream of air
4. Application of a second coat of adhesive
5. Drying of adhesive to a thin film using a gentle stream of air
6. Light-cure for 10 s (>1000 mW/cm 2 , HS LED 1200, Henry Schein, Langen, Germany)
7. Application of Filtek Supreme XT Flowable onto the cavity surface
8. Build-up of layers of 2 mm thickness max. and curing of each layer for 20 s
9. Finishing and polishing with a diamond bur (46 μm, 8836KR314014, Komet) and polishing bur (Politip F, 533602 and Politip P, 533584, Ivoclar Vivadent)

OCT imaging

The restorations were 3-dimensionally imaged by SS-OCT (OCT1300SS, Thorlabs Inc., New Jersey, USA, Thorlabs GmbH Dachau, Germany). SS-OCT ( Fig. 1 ) is a variant of Fourier domain OCT (FD-OCT), which uses a tunable light source with a narrow wavelength spectrum. Table 3 summarizes the specifications of the equipment used.

Table 3
Technical specification of the used SS-OCT.
Center wavelength 1325 nm
Bandwidth (3 dB) 100 nm
A-scan/line rate 16 kHz
B-scan frame rate (512 lines/frame) 25 fps
Axial (air)/lateral resolution, μm 12/25
Sensitivity 100 dB
Maximum field of view ( L × W × D ) 10 mm × 10 mm × 3 mm
Pixel count 512 × 512
Power on sample 1.5 mW

The images obtained by OCT were analyzed using Image J v1.45 (Wayne Rasband, National Institutes of Health, Bethesda, MD, USA). 512 B-scans were extracted from the 3D volume dataset and 10 of these 512 B-scans were selected with equal mesio-distal distribution.

An OCT signal indicating an interfacial adhesive defect was defined as a signal peak resulting in a white line extending along the interface. This was in accordance with the findings of Park et al. , who confirmed that the presence of gaps caused increased signal intensities due to light at the phase boundaries of media ( i.e. , air and composite/adhesive or air and cavity floor) being reflected with highly different refractive indices. The analysis of interfacial zones for the presence of defects was performed visually by expert judgment comparable with common practice in in vivo imaging-based diagnostic techniques ( e.g. , interpretation of X-ray or ultrasound images). The operator has several years old practical experience in OCT dental applications and has verified the OCT method to detect gap formations at the composite-tooth interface. Inter-observer measurement errors of weighted means were 6.21% and 4.25% at enamel and dentin, respectively . Visual definition of threshold was chosen over computerized thresholding for better exclusion of false positive signals ( e.g. cracks, internal tooth structures, pores) and inclusion of false negative signals which would otherwise often not be detected by computerized thresholding method ( e.g. shadowing by pores or filler).

“Defect lengths” indicated by OCT signal at the enamel/composite and dentin/composite interfaces on the cavity edges and floor were measured by means of ten mesio-distally oriented B-scans per tooth. Measurements were made between both line angles at the cavity floor (line angle: junction of two surfaces of a tooth or of two walls of a tooth cavity). Length was measured in pixels using ImageJ. The length of defect-induced signals at the enamel margins and at the cavity floor was related to the total length of these interfaces and expressed as a percentage. Weighted average values of defect-induced signals (length, %) at enamel and dentin, respectively, were determined from ten OCT B-scans per tooth. Mean value per group as well as standard deviation was determined and the groups were compared statistically using Mann–Whitney U-/Wilcoxon-test (two-sided, α adj. = 0.0125/0.025, Bonferroni correction).

Materials and methods

Preparation of specimens

16 intact caries-free human molars were selected after extraction and immediately immersed in 0.5% chloramine solution at 4 °C. The teeth were cleaned mechanically. Standardized, box-shaped class V cavities with dimensions of 4 mm mesio-distally, 3 mm coronal-apically and an average depth of 1.5 mm were prepared with a rounded cylindrical diamond bur (107 μm, 836 KR 314014, Komet ® /Gebr. Brasseler, Lemgo, Germany). The cavities were placed at the vestibular cemento-enamel junction (CEJ) in order to deliberately produce cavity edges of equal length at enamel and dentin. The cavity margins in enamel were beveled (1 mm wide at the sagittal middle line at an angle of 45° relative to the cavity wall), whereas the cavity-surface margin in dentin was left as a butt joint ( Fig. 1 ).

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Assessment of defects at tooth/self-adhering flowable composite interface using swept-source optical coherence tomography (SS-OCT)
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