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Introduction
Computer aided design/computer aided manufacturing (CAD/CAM) systems have rapidly gained popularity during the last few decades. CAD/CAM systems predominantly utilize ceramic materials for their esthetic qualities, surface finish, and long-term durability. However, recently, composite resin CAD/CAM blocks have become available, opening up a wider range of potential materials.
Composite resin CAD/CAM blocks can be fabricated under controlled conditions offering the highest attainable quality. Composite CAD/CAM blocks are polymerized under standardized parameters at high temperatures and pressures to form hybrid, nano-filled and nano-hybrid composite resins.1 Therefore, the physical and optical color stability are higher compared to conventionally fabricated indirect composite resin restorations.2 Additionally, restorations produced from composite resin CAD/CAM blocks are easier to repair than restorations made from ceramic CAD/CAM blocks.134
Several composite resin CAD/CAM block materials are available for dental restorations. Lava ultimate (LU, 3M ESPE, St. Paul, MN, USA), a resin nano-ceramic (RNC) material, was introduced as a nano-composite CAD/CAM block consisting of a highly cured resin matrix with embedded nano-ceramic particles. It contains filler mixtures of silica particles (20 nm), zirconia nanomers (4 - 11 nm) and aggregated clusters (0.6 - 1.0 µm) with a total filler loading of approximately 80 wt%.5 An experimental nano-hybrid composite CAD/CAM block (EB, Vericom Co., Anyang, Korea), which contains nano-filler (10 - 20 nm) which agglomerate as micro-sized ceramic cluster filler (1 - 5 µm), micro-hybrid particle (0.5 - 1.0 µm), and nano-silica filler (10 - 20 nm).
Marginal adaptation, together with adequate preparation design or cementation, is one of the main factors that determine the long-term success of indirect esthetic restorations. Marginal discrepancies in indirect restorations can increase the risk of cement dissolution and microleakage.6789 In vitro studies reported mean marginal gaps of 50 to 60 µm in full veneer crowns and 150 to 168 µm in mesio-occluso-distal (MOD) inlays fabricated using ceramic CAD/CAM materials.10 However, few data are available concerning the marginal fit of composite resin CAD/CAM restorations. Therefore, this study sought to investigate the marginal and internal fit of nano-composite CAD/CAM restorations by using LU and EB nanocomposite CAD/CAM blocks with a full veneer crown and a mesio-occluso-distal (MOD) inlay with a one-cusp-capping design. The null hypothesis was that there would be no differences in marginal and internal gaps among the CAD/CAM blocks regardless of the measuring point.
Materials and Methods
Specimen preparation
The protocol of this study was approved by the Institutional Review Board of Seoul National University Dental Hospital, Seoul, Korea (CRI14020). Two caries-free extracted human molars were cleaned and stored under moist conditions at room temperature. The teeth were embedded in yellow stone (Snow rock dental stone ND, DK MunGyo Corp., Gimhae, Korea). One was prepared as a full veneer crown with the following dimensions: axial reduction of 1 mm and occlusal reduction of 1.5 mm with a rounded shoulder margin. The other tooth was prepared with an MOD-geometry inlay cavity 2 mm in depth and isthmus width. The finishing lines on the mesial and distal sides of the rounded boxes were 1 mm above the cementoenamel junction. Then, the mesiobuccal cusp was reduced by 2 mm for cusp capping following the anatomical form of the occlusal surface. All marginal preparations were finished with a rounded shoulder design. All surfaces were smoothed and all internal line angles were rounded with fine diamond burs (Mani dia bur TR-26F, Mani Inc., Tochigi, Japan) under a dental surgical microscope (OPMI Pico, Carl Zeiss Surgical GmbH, Oberkochen, Germany) (Figures 1a and 1b). After the preparation, the epoxy resin replicas (Struers, Copenhagen, Denmark) of each restoration design were fabricated.
The replicas were coated with anti-reflection powder (VITA CEREC Powder, VITA Zahnfabrik, Bad Säckingen, Germany), and the optical impression was taken using an intraoral camera (CEREC Bluecam, Sirona Dental Systems GmbH, Bensheim, Germany). The images were transferred to CEREC software version 4.00 and used to design the restorations. The parameters for the spacers (luting space) and adhesive gaps were set to 140 µm and 20 µm, respectively, according to the manufacturer recommended settings for LU. Ten crowns and ten inlays were milled with either of LU and EB.
The milled restorations were inspected for any defects, and placed in the replica to assess fitness. All restorations were cleaned and sandblasted with aluminum oxide (grain size ≤ 50 µm at 30 psi ± 2 psi) until the entire bonding surface had a matte finish, according to the manufacturers' instructions. After removing the sand, adhesive (Scotchbond Universal Adhesive, 3M ESPE) was applied to the bonding surface and agitated for 20 seconds. After that, uniform layer of cement material (RelyX Ultimate Clicker Adhesive Resin Cement, 3M ESPE) was applied and the restorations were set completely to the restoration.
Gap measurement
After cementation, the specimens were examined under a dental surgical microscope (×40, OPMI pico, Carl Zeiss) and were photographed with a digital camera to assess the external marginal gap at selected points. Then, each specimen was embedded in an acrylic resin block, and sectioned bucco-lingually and mesio-distally to obtain 10 segments using a water-cooled low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) 0.5 mm in thickness (Figures 1c and 1d). A total of 20 cross-section images from the both sides of sectioned segment were obtained for each specimen, and the internal gap was measured at 10 points along each internal cavity outline, and at 3 points in each transitional area. Gap measurement was performed by determining the shortest distance between the foot of the perpendiculars connecting the tangent of the abutment (replica)-restoration interfaces.
Statistical analyses
The collected gap data were pooled according to the measuring points (occlusal, axial, transitional, or marginal) in both the crown and inlay, while the data at the occlusal, axial and transitional points of the cusp capping area in inlays were additionally organized (Figures 1e and 1f). All gap measurements of each preparation design were separately analyzed with two-way analysis of variance (ANOVA) using the type of CAD/CAM block and the measuring point as the main factors, followed by Tukey's post-hoc comparisons. The significance level was set at α = 0.05. All statistical analyses were performed by using SPSS 21.0 (SPSS Inc., Chicago, IL, USA).
Results
Gap data of both crowns and inlays are presented in Tables 1 and 2, respectively. Two-way ANOVA suggested that the two main factors (the type of CAD/CAM block and the measuring point) and their interaction were significant in both crowns and inlays (p = 0.000). Gap data of CAD/CAM restorations from both blocks showed the occlusal gap to be the largest among the measuring points and the marginal gap to be the smallest in the two preparation designs (p = 0.000). EB restorations showed a significantly larger gap at all measuring points, including the cusp capping area of inlays (p < 0.001), except the axial walls of crowns (p > 0.05). In EB inlays, gap data at the transitional area were significantly larger in the cusp capping area than in other cavity areas (p = 0.000), while LU inlays showed similar values in the transitional areas of the internal cavity outlines (p > 0.05).
Discussion
The aim of this study was to examine and compare the external and internal marginal gap of the two different nano-composite CAD/CAM blocks in different restoration designs. We found that the nano-composite CAD/CAM block type and measuring point had a significant influence on external and internal marginal fit of CAD/CAM restorations in both preparation designs. Between the blocks, the restorations milled from EB had a significantly larger gap at all measuring points compared to those milled from LU. In crowns and inlay restorations made from the two blocks, gap data were significantly larger in the occlusal than in the axial area, while those in the marginal area were the smallest.
There are several methods for evaluating the fit of CAD/CAM restorations. Marginal fit can be assessed using microphotography and light microscopy, silicone replicas of the fit between the abutment and the restoration, silicon weight and density evaluation, virtual 3D analysis with a noncontact scanner and specific software, and micro-computerized tomographic (micro-CT) technology with no impression of cementation.11121314151617 In this study, the tooth-restoration spacing was measured using light microscopy after cementation and sectioning was done. This nonconservative method has some shortcomings, such as the limited number of sections and the risk of specimen loss during sectioning. However, it is advantageous for evaluating both external and internal marginal fit in the same specimen after cementation, which is not easy with the micro-CT technique because external and internal gap data are usually combined to calculate overall mean values. Additionally, it is possible to evaluate the actual gap with various levels of microscopic magnification, which is not possible when using other methods.
Most authors agree that 120 µm of indirect restoration could be a clinically acceptable scale for external marginal spacing, but the criteria for internal fit have not yet been determined.13181920212223 A systematic review of external and internal marginal fit for ceramic indirect restorations was carried out by Boitelle et al.12 They reviewed 230 articles, and 90 articles were selected for data analysis, including various materials such as zirconia, feldspathic ceramic, leucite-reinforced feldspathic ceramics, and CAD/CAM systems. They concluded that the external marginal gap ranged from 39.1 to 201.0 µm and the internal marginal gap ranged between 23 and 230 µm. In terms of the marginal fit of CEREC systems, Nakamura et al. reported that the external marginal fit of all CAD/CAM ceramic crowns varied from 30 to 108 µm according to the pre-set milling value.13
Little is known about the marginal fit of the restorations with nano-composite CAD/CAM blocks. There have been two studies dealing with nano-composite CAD/CAM block Paradigm MZ 100 (3M ESPE).2425 A study examining the marginal fit of crowns with three different margin designs by Tsitrou et al. reported an external marginal fit of 77 to 105 µm in CAD/CAM crowns fabricated with Paradigm MZ100 using a replica and sectioning method.24 Another study by Akbar et al. reported that the mean marginal gap ranged from 46.0 to 65.9 µm in composite CAD/CAM crowns.25
The gap difference between the two blocks was even higher in complicated abutment designs, such as the transitional gap of the cusp capping area in MOD inlays. This result is in accordance with previous researches reporting that a retentive preparation design (including cusp capping) resulted in a larger mean gap.2627 The larger gap in the transitional area indicates a poor fit of the cusp capping area in EB inlays. Improper fit of the restoration in such internal structure could not be detected if the external marginal fit is acceptable. Moreover, most capped cusps are functional cusps. The tendency toward a larger gap in such areas may result in thinner restoration thickness, and may increase the risk of restoration fracture under occlusal forces. In clinical situations, extra care is required during the cementation procedure and resin cements should be used to strengthen and support the restoration.
In this results, the internal gap of the restorations differed according to the measuring point. The occlusal gap of both crowns and inlays was larger than the preset parameter, while the axial gaps were not. This regional discrepancy is in accordance with previous studies regardless of the CAD/CAM block materials, experiment method, or preparation designs, which implies a possible consequence of milling process and restoration-designing software of the CAD/CAM system.10262728 Aside from this regional discrepancy, such milling software might be a main cause of the differences between the two blocks. Both tested blocks were milled using the same settings in an LU-specific program originally pre-built using CEREC software. CEREC software is a closed system, and thus the milling program is not shared with other blocks. Different milling conditions might produce different results. These data could be utilized to determine the milling characteristics of the tested blocks and further to their clinical performance. In-depth investigation of mechanical properties and milling conditions as well as clinical study are required to determine the clinical relevance of nano-composite CAD/CAM restorations.
Conclusions
Within the limitations of this study, the type of CAD/CAM block and the measuring point significantly affected the marginal and internal fit of the restorations. EB restorations had significantly larger gaps than LU restorations. Occlusal gap data from the two composite resin CAD/CAM blocks were the largest, while marginal values were the smallest.
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