Molar cusp deformation evaluated by micro-CT and enamel crack formation to compare incremental and bulk-filling techniques

Abstract

Objectives

To describe a method of measuring the molar cusp deformation using micro-computed tomography (micro-CT), the propagation of enamel cracks using transillumination, and the effects of hygroscopic expansion after incremental and bulk-filling resin composite restorations.

Methods

Twenty human molars received standardized Class II mesio-occlusal-distal cavity preparations. They were restored with either a bulk-fill resin composite, X-tra fil (XTRA), or a conventional resin composite, Filtek Z100 (Z100). The resin composites were tested for post-gel shrinkage using a strain gauge method. Cusp deformation (CD) was evaluated using the images obtained using a micro-CT protocol and using a strain-gauge method. Enamel cracks were detected using transillumination.

Results

The post-gel shrinkage of Z100 was higher than XTRA (P < 0.001). The amount of cusp deformation produced using Z100 was higher compared to XTRA, irrespective of the measurement method used (P < 0.001). The thinner lingual cusp always had a higher CD than the buccal cusp, irrespective of the measurement method (P < 0.001). A positive correlation (r = 0.78) was found between cusp deformation measured by micro-CT or by the strain-gauge method. After hygroscopic expansion of the resin composite, the cusp displacement recovered around 85% (P < 0.001). After restoration, Z100 produced more cracks than XTRA (P = 0.012).

Conclusions

Micro-CT was an effective method for evaluating the cusp deformation. Transillumination was effective for detecting enamel cracks. There were fewer negative effects of polymerization shrinkage in bulk-fill resin restorations using XTRA than for the conventional incremental filling technique using conventional composite resin Z100.

Clinical significance

Shrinkage and cusp deformation are directly related to the formation of enamel cracks. Cusp deformation and crack propagation may increase the risk of tooth fracture.

Introduction

Resin composites are extensively used to restore teeth . Despite advances in techniques and materials, even bulk-fill resin composites , still exhibit polymerization shrinkage . Conventional resin composites must be inserted and light cured in at most 2 mm increments to reduce shrinkage stress and to achieve acceptable resin properties . On the other hand, light cured bulk-fill resin composites can be used in increments of up to 4–5 mm , and same materials may produce lower shrinkage stress than conventional resin composites depending to the composition, viscosity and elastic modulus .

Polymerization shrinkage can be affected by several factors, such as the composition of the material , cavity configuration or C-factor , and the restorative technique . The stresses generated during the polymerization may cause cusp deformation and enamel cracks . Clinically this may cause increased postoperative sensitivity, more microleakage, marginal discoloration, recurrent caries, and pulpal complications . These have been the most common reasons to replace resin composite restorations . Water absorption causes hygroscopic expansion of the resin composite and can counteract some of the negative effects of polymerization shrinkage , however this takes time to occur. Although this factor is positive for the tooth-restoration complex, if the hygroscopic expansion is greater than the polymerization shrinkage, the expansion will generate stresses within the tooth .

Several devices and methods have been described for analyzing the polymerization shrinkage , whether it be volumetric or linear shrinkage . Measuring the cusp deformation has been reported to be a good technique to analyze and predict the effects of polymerization shrinkage on the restored tooth because it reflects the effects of the internal stresses on the tooth . The cusp deformation under occlusal loading, measured either by the linear displacement or by a strain gauge method have also been used . The use of non-destructive methodologies favors the combination of several methods to achieve different perspectives on the effects of the shrinkage . Determining crack formation in the tooth and its propagation can be directly related to cusp deflection and since the same samples can be used, this facilitates the interpretation and allows direct correlation of the results .

The internal adaptation of the restorative material in the cavity, the restorative material itself and the dental structures can be examined using two dimensional (2D) and three dimensional (3D) micro-CT images with a high spatial resolution (maximum resolution of 1.10 μm) . This method has demonstrated its efficacy to analyze the polymerization shrinkage vectors and to evaluate the presence of any gaps after silver nitrate infiltration without damaging the specimens . This method is also able to quantify the volumetric shrinkage of resin composites , and can characterize both the pattern and volume of polymerization shrinkage . Despite the diverse uses of the micro-CT, to the authors knowledge, there are no studies that have analyzed the cusp deformation and the effect of hygroscopic expansion in molar class II resin composite restorations using micro-CT.

The aim of this study was to describe and validate a method of measuring cuspdeformation using micro-CT, the propagation of enamel cracks using transillumination, and the effects of hygroscopic expansion of both incrementally filled and bulk-filled resin composite restorations. The null hypotheses were:

  • 1.

    There would be no correlation between the cusp deformation evaluated with micro-CT, and the strain gauge method.

  • 2.

    That the bulk-fill resin composite would not exhibit different post-gel shrinkage and would not generate different amounts of cusp deflection and enamel cracking compared to the incrementally placed conventional resin composite.

  • 3.

    The bulk-fill resin composite and the conventional resin composite would not exhibit different amounts of hygroscopic expansion after 7 days in water.

Materials and methods

Study design

Twenty human molars received standardized Class II mesio-occlusal-distal (MOD) cavity preparations. Restorations were made with two restorative protocols according to manufacturer’s instructions: using a bulk-fill resin, XTRA (X-tra fil, VOCO, Cuxhaven, Germany) or an incrementally placed conventional composite resin, Z100 (Z100 Restorative, 3 M ESPE, St. Paul, MN, USA). The number of samples for each methodology was based on the coefficient of variability and the sample calculation. The power of the test was 80% with a minimum detectable difference of 20%, a residual standard deviation of 15% and a significance level of 0.05, which resulted in a number of samples of n = 10 per group. Considering the data variation of micro-CT methodology and the cost of each scan, the number of sample was reduced (n = 5).

The composition of the resin composites as provided by the manufacturers are listed in Table 1 . The two resin composites were tested for post-gel shrinkage (Shr) using the strain gauge method. Teeth were tested for cuspal deformation using strain gauges as they were restored with the resin composite. Enamel cracking was detected and classified using a standardized transillumination technique. Cusp deformation (CD) was evaluated using micro-CT and the novel protocol developed in this study.

Table 1
Resin composite information based on manufacturer’s reported data.
Material Code LOT Shade Composite type Increment size and light activation time Organic matrix Filler Filler% w/vol
Z100 Z100 472109 A3 Microhybrid composite 2.0 mm – 40 s Bis-GMA, TEG-DMA Zirconia and silica 85/66
X-tra Fil XTRA 15300244 Universal Bulk-fill paste composite 4.0 mm – 20 s Bis-GMA, UDMA, TEG-DMA Barium, Boron, Alumino-silicate glass 86/70

Post-gel shrinkage (Shr)

The post-gel linear shrinkage was determined using a previously described strain gauge method . This method allows real time measurement of shrinkage strain. When the composite sample is exposed to the light curing, the strain is recorded and the measurement is continued for ten minutes . Ten specimens were tested for each restorative material (n = 10). The materials were shaped into a hemisphere on top of a biaxial strain gauge (CEA-06-032WT-120f) that measured the shrinkage strains in two perpendicular directions. A strain conditioner (ADS0500IPg) converted electrical resistance changes in the strain gauge to voltage changes through a quarter-bridge circuit with an internal reference resistance of 120 Ω. The strain values measured along the two axes were averaged because the material properties were considered to be homogeneous and isotropic on a macro scale. Both resin composites were inserted in increments that were 1 mm thick, and 2 mm × 2 mm, by the same operator, and light activated using the same Bluephase G2 (Ivoclar Vivadent, AG, Schaan, Liechtenstein) light-curing unit according to manufacturer’s instructions: for 20 s for XTRA and 40 s for Z100, with the light tip held at 1 mm distance from the surface of the resin composite. The strain values were collected for 10 min after light activation to monitor the post-gel shrinkage of the resin composites. Strain recorded from the two axes of the strain gauge were averaged and plotted as a function of time for each sample. Then the strain values obtained from the ten samples was used for created a mean curve of the strain at each material . The final values, which represented the linear shrinkage, was converted to percentage and then multiplied by three to obtain the volumetric shrinkage .

Tooth selection and cavity preparation

Twenty extracted intact caries-free mandibular third human molars were used (Ethics Committee in Human Research approval #1.372.102). The teeth were stored in distilled water in the refrigerator before use. To standardize the samples, the tooth dimensions were measured with digital micrometer (Absolute AOS, Mitutoyo Sul Americana Ltda., Suzano, SP, Brasil). Any deviations in the intercuspal width size were kept to a maximum deviation of 10% from the mean value. Five measurements of the crown were made to calculate the volume: buccal and lingual cusp height, intercuspal distance, buccal/lingual, and mesio/distal width. The teeth were randomly allocated to the two groups (n = 10) and the mean crown volume before preparation (mm 3 ) for XTRA was 762.9 ± 183.4 and for Z100 it was 748.0 ± 107.2. These volumes were statistically similar (P = .827).

To allow some freedom of movement, the roots were covered with a 0.3 mm layer of a polyether impression material (Impregum F; 3 M, St Paul, MN, USA), and then embedded in a polystyrene resin (Cristal, Piracicaba, SP, Brazil) to 2 mm below the cemento–enamel junction. This simulated the periodontal ligament and the alveolar bone . Large Class II MOD cavities were prepared in all specimens that were 4/5 of the intercuspal width and 4 mm deep in occlusal box with a cylindrical with a rounded ended diamond bur (#3099 diamond bur, KG Sorensen, Cotia, SP, Brazil) using copious air–water spray and a mechanical cavity preparation device . In an attempt to standardize the preparations, the thickness of hard tissue of each cusp was measured during the preparation and the location of the pulp horns were evaluated using a radiograph. The cusp deformation was measured with strain gauges and then transillumination was used to determine the presence of enamel cracks in the teeth.

Cusp deformation using the strain gauge method

To validate the results from the micro-CT, the cusp deformation was measured using strain gauges (PA-06-060CC- 350 L, Excel Sensores, Embú, SP, Brazil) that had an internal electrical resistance of 350 ohm, a gauge factor of 2.14, and a grid size of 21.02 mm 2 . Following the cavity preparation, the buccal and lingual tooth enamel was etched with 37% phosphoric acid (Fusion Duralink 37%, Angelus, Londrina, PR, Brazil) for 30 s. After washing with copious water-spray for 30 s, the cavity was dried with absorbent paper and the self-etching adhesive system (Clearfil SE Bond, Kuraray, Japan) was applied according to the manufacturer’s instructions. Two strain gauges were then bonded to the external surface of the buccal and lingual cusps, using cyanoacrylate-based adhesive (Super Bonder; Loctite, Itapeví, SP, Brazil), and connected to a data acquisition device (ADS2000; Lynx, São Paulo, SP, Brazil) as previously reported . The strain gauges were located in the region where previous finite element model analysis had reported the highest polymerization stresses . In addition, two strain gauges were bonded to another tooth with the same cavity preparation (a passive control sample) to compensate for any dimensional deviations due to temperature.

The teeth were restored with either a bulk-fill composite resin − XTRA, using one increment that was 4.0 mm thick, or using the conventional resin composite (Z100) that was placed and cured in eight increments with two increments in each proximal box and four increments in center, starting at one proximal box. For the incremental technique with Z100, a Teflon matrix used to standardize each composite resin increment before the insertion into the cavity . The composites were light cured using the LCU Bluephase G2 (Ivoclar Vivadent) by placing from the occlusal direction closest to the cavity according to manufacturer’s instructions: 20 s (s) for XTRA over the mesial box and 20 s over the distal box, and 40 s for each increment of Z100. The total energy for each filling technique was 50.2 J/cm 2 for the bulk-fill restorations, and 208.4 J/cm 2 for the incremental technique. The samples were restored in a device created to simulate an adjacent premolar and molar to allow interproximal contact during restoration . The cusp deformation data were obtained from the data analysis software (AqDados 7.02 and AqAnalisys data acquisition software; Lynx São Paulo, SP, Brazil). The strain values were recorded at 4 Hz during the restoration procedures and continued for 10 min after curing the last increment.

Cusp deformation on micro-CT

The micro-CT virtually divides the object into cuts that are visualized in the form of a two-dimensional image. The slices can be transformed into a three-dimensional image by combination of the coordinates using associated software (Nrecom software version 1.6.10.1; DataViewer software version 1.5.1.2; CTAn, version 1.13; CTVol, version 2.0; SkyScan Bruker Belgium). To obtain the tomographic image two steps are necessary, first through the acquisition of the tomographic image and, later, the reconstruction of this image by specific software using the computer clustering system. From the reconstructed image, it is possible to analyze the internal structure of the sample. To evaluate the cusp deformation produced by the resin restorations, five teeth from each group were scanned using a micro-CT device (SkyScan 1272, Bruker, Belgium) at the following three time points: after cavity preparation; immediately after the photoactivation of the restorative materials; and after the restored teeth had been stored in water at 37 °C for 7 days ( Fig. 1 A–C.). To standardize and allow superimposition of the images, the teeth were placed in the micro-CT in the same position with the buccal face facing the door. On average, it took forty-five minutes to scan each tooth using the following parameters: exposure time of 1000 milliseconds, energy 100 KV–100 μA, 180° rotation at the 0.400 step, Cu filter of 0.11 mm thickness, and a 10 μm voxel size.

Fig. 1
Method developed to measure the cusp deformation using micro-CT; A. image of prepared tooth; B. image immediately after tooth restoration; C. image immediately after storage in water for 7 days – showing the hygroscopic expansion; D. overlapping the prepared and restored teeth demonstrating an inadequate image fit; E. fitting images using root volume of the restored teeth over prepared tooth; F. ROI rectangle in the area of interest of diff image; G. binarization of the difference between prepared/restored (blank area); H. examples of the volume of cusp deformation in 3D using micro-CT: occlusal view of volume of cusp deformation after polymerization shrinkage of resin composite and proximal view of volume of cusp deformation after polymerization shrinkage of resin composite, in sequence.

The scan images acquired by micro-CT was imported to a computer and rebuilt using Nrecom software (version 1.6.10.1, Skyscan, Bruker, Belgium) in approximately 1.000 slices, respecting the anatomical limits of the samples. The reconstructed images were overlaid using DataViewer software (version 1.5.1.2, SkyScan, Bruker, Belgium). To align the different images of the prepared, restored, and after storage in water, a reference point was selected that was distant from the area affected by shrinkage. The volume of root portion of the tooth below cementoenamel junction which included both the pulp chamber and canals were used as a reference ( Fig. 1 D and E.). The prepared tooth image (reference) and the restored tooth image (target) were superimposed, this generated a volume of difference image (Diff). This Diff image represented the volume of the cusp deformation caused by the polymerization shrinkage of the resin composite restoration.

The micro-CT analyzer software (CTAn, version 1.13, SkyScan, Bruker, Belgium) was used to threshold the regions of interest, and to calculate the difference between the overlapping 2D images ( Fig. 1 F and G.). The number of layers was the same for all analyzed Diff images, at total of 600 layers, each with a resolution of 0.4 μm. The regions of interest were positioned in the same area of the cusp where the strain gauge was positioned in the sample. To determine the effects of hygroscopic expansion, identical analyses were made using the same tooth obtained after storage in water for 7 days. The cusp deformation volume values were obtained in mm 3 and the percentage of this deformation was calculated as a function of the total volume of each cusp. The volumes measured by using micro-CT of the prepared and restored teeth with Z100 and XTRA were similar for buccal (P = .582) and lingual cusp (P = .365). Using the CT VOL software (CTVol, version 2.0, SkyScan, Bruker, Belgium), 3D images of the volume of the cusp deformation caused by the resin polymerization shrinkage and after hygroscopic expansion were generated ( Fig. 1 H).

Enamel crack analysis

The samples were evaluated at these four time points during the experiment to detect presence or propagation of enamel cracks in the buccal and lingual cusps: (A) intact tooth before preparation; (B) after cavity preparation; (C) after restoration; and (D) after hygroscopic expansion (samples immersed in water for seven days). The images of the sample were captured under standardized conditions, all were taken with the same camera (Canon EOS REBEL T5i, Canon Inc., Tokyo, Japan; Tokina AT-X 100 mm f/2.8D macro lens, Kenko Co. Ltd., Tokyo, Japan), at the same settings (ISO 200, f/18, 1/200 s). A transillumination light (Photonita, P1050, Florianópolis, SC, Brazil) was used with the optic fiber illuminator positioned on the occlusal surface of the tooth . The samples were positioned in a custom device specifically developed for this study. The size (mm) of the cracks were evaluated, the location (base of preparation or top close to the cusp; mesial, distal or at the center) and direction (vertical, horizontal and oblique). The images were uploaded onto a computer and the crack location was determined using public domain software (ImageJ, National Institutes of Health, Bethesda, MD, USA) as shown in Fig. 2 , by three calibrated operators in a blinded process.

Fig. 2
Examples of crack detection: A. intact tooth before preparation; B. after tooth cavity preparation; C. immediately after resin composite restoration; D. cervical-occlusal and mesial-distal location of the crack. Red arrows indicate crack limits. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Statistical analysis

The tooth crown volume (mm 3 ), post-gel shrinkage, cusp deformation (μS), the volume of the cusp deformation after restoration, and the hygroscopic expansion (mm 3 ) data were tested using the Shapiro-Wilk test for normality and equality of variances (Levene’s test), followed by parametric statistical tests. The Student’s t -test was used to compare the crown volume (mm 3 ) of the teeth randomly allocated on XTRA or Z100 and the post-gel shrinkage of the two composites (α = 0.05). ANOVA 2-way tests were used to compare effect of the composite resin, cusp type and their interactions for cusp deformation as measured using strain gauge and micro-CT. Pearson correlation was used to correlate the cusp deformation measured using the micro-CT and strain gauge methods. All tests used an α = 0.05 significance level and all analyses were carried out with the statistical package Sigma Plot version 13.1.

Materials and methods

Study design

Twenty human molars received standardized Class II mesio-occlusal-distal (MOD) cavity preparations. Restorations were made with two restorative protocols according to manufacturer’s instructions: using a bulk-fill resin, XTRA (X-tra fil, VOCO, Cuxhaven, Germany) or an incrementally placed conventional composite resin, Z100 (Z100 Restorative, 3 M ESPE, St. Paul, MN, USA). The number of samples for each methodology was based on the coefficient of variability and the sample calculation. The power of the test was 80% with a minimum detectable difference of 20%, a residual standard deviation of 15% and a significance level of 0.05, which resulted in a number of samples of n = 10 per group. Considering the data variation of micro-CT methodology and the cost of each scan, the number of sample was reduced (n = 5).

The composition of the resin composites as provided by the manufacturers are listed in Table 1 . The two resin composites were tested for post-gel shrinkage (Shr) using the strain gauge method. Teeth were tested for cuspal deformation using strain gauges as they were restored with the resin composite. Enamel cracking was detected and classified using a standardized transillumination technique. Cusp deformation (CD) was evaluated using micro-CT and the novel protocol developed in this study.

Jun 17, 2018 | Posted by in General Dentistry | Comments Off on Molar cusp deformation evaluated by micro-CT and enamel crack formation to compare incremental and bulk-filling techniques
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