Marginal and internal adaptation of ceramic crown restorations fabricated with CAD/CAM technology and the heat-press technique Hisham A. Mously, BDS, MS,a Matthew Finkelman, PhD,b Roya Zandparsa, DDS, DMD, MSc,c and Hiroshi Hirayama, DDS, DMD, MSd Tufts University School of Dental Medicine, Boston, Mass; Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia Statement of problem. The accuracy of chairside computer-aided design and computer-aided manufacturing (CAD/CAM) restorations is questionable, and the effect of the die spacer settings is not well stated in the literature. Purpose. The purpose of the study was to evaluate the marginal and internal adaptation of E4D crowns fabricated with different spacer thicknesses and to compare these crowns with those fabricated with the heat-press technique. Material and methods. The E4D system was used to fabricate 30 crowns for the ﬁrst 3 groups, with different spacer thickness settings: 30 mm, 60 mm, and 100 mm. In the fourth group, 10 lithium disilicate crowns were fabricated with the heat-press technique. The occlusal gap, axial gap, vertical marginal gap, and absolute marginal discrepancy were evaluated by x-ray microtomography. Statistical signiﬁcance was assessed with the Kruskal-Wallis test (a¼.05). For post hoc analyses, the Mann-Whitney U test was used alongside the Bonferroni correction for multiple comparisons (a¼.008). Results. Within the CAD/CAM groups, the 30-mm spacer thickness resulted in the lowest median axial gap (90.04 mm), whereas the 60-mm spacer thickness resulted in the lowest median occlusal gap (152.39 mm). The median marginal gap values of the CAD/CAM-60 group (49.35 mm) and CAD/CAM-100 group (46.65 mm) were lower than those of the CAD/CAM-30 group (55.18 mm). No signiﬁcant differences among the CAD/CAM groups were observed for absolute marginal discrepancy. The heat-press group had signiﬁcantly different values than those of the CAD/CAM groups. Conclusion. The spacer thickness and fabrication technique affected the adaptation of ceramic crowns. The heat-press group yielded the best marginal and internal crown adaptation results. The 30- or 60-mm spacer settings are recommended for the E4D CAD/CAM system. (J Prosthet Dent 2014;-:---)
Clinical Implications The results of this study may aid in the clinical determination of the most accurate spacer thickness settings for the optimal adaptation of CAD/CAM crown restorations, thereby improving clinical success and longevity. Crown adaptation along with esthetic value and fracture resistance are important to the clinical success a
and longevity of crown restorations.1-7 Crown adaptation is deﬁned by the measurements of the marginal and
internal gaps of crown restorations. Holmes et al8 stated that the internal gap is the perpendicular distance from
Assistant Professor, Division of Prosthodontics, Department of Oral and Maxillofacial Rehabilitation, Faculty of Dentistry, King Abdulaziz University; Former Resident, Division of Postgraduate Prosthodontics, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine. b Assistant Professor, Department of Public Health and Community Service, Tufts University School of Dental Medicine. c Clinical Professor, Division of Postgraduate Prosthodontics, Department of Prosthodontics and Operative Dentistry, Tufts University School of Dental Medicine. d Professor, Division Head of Postgraduate Prosthodontics; Director, Graduate and Postgraduate Prosthodontics; Director, Advanced Education in Esthetic Dentistry; and Director, Advanced Dental Technology and Research Program, Tufts University School of Dental Medicine.
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Volume the internal surface of the restoration to the axial wall of the preparation, whereas the marginal gap is the perpendicular distance from the internal surface of the restoration to the ﬁnish line of the preparation.8 An increase in the marginal or internal gap could increase cement dissolution, thereby increasing the potential for microleakage, recurrent caries, and periodontal disease.9-15 Furthermore, an increase in the internal gap could decrease the fracture strength of ceramic restorations because these areas with a higher internal gap induce different load concentrations16-21 Paint-on die spacers have been used successfully for the conventional lostwax and heat-press techniques to provide appropriate marginal and internal gaps, thereby facilitating the complete seating of crown restorations.22-25 In contrast, the spacer thicknesses of crown restorations fabricated with chairside computer-aided design and computer-aided manufacturing (CAD/ CAM) technology usually are determined during the software’s design phase. However, the spacer thickness setting most conducive to appropriate crown adaptation remains unknown. Although chairside CAD/CAM technology has improved considerably over the past few years, the accuracy of dental restorations fabricated with these systems remains questionable. Many factors can affect the accuracy of these restorations, including the type of dental restoration, material properties,26 preparation design,7,18 scanning device accuracy, software design, spacer settings, and milling machine accuracy.27 Few studies, however, have examined these factors and their effects on the marginal and internal gaps of different dental restorations.7,18,26,27 Most of these studies used either subjective or inaccurate methods to evaluate the marginal and internal gaps of crown restorations,6 including the use of the explorer, visual examination,28 and radiographic techniques.29 The impression technique is a popular method, which uses a putty-wash technique with low-viscosity impression
material.6,30-32 This technique is considered difﬁcult and inaccurate when the discrepancies are small because the impression material may be distorted or damaged.1,5,6 The proﬁlometry method is considered to be an accurate method, except in restorations with overextended margins.6,33 Scanning electron microscopy for the study of presectioned specimens also is a popular technique.19,34-36 Computerized x-ray microtomography (micro-XCT), a nondestructive method, produces high-resolution images for both quantitative and qualitative analyses of the tooth, bone, and implants, which makes it a powerful tool in dental research.6,37,38 Micro-XCT has been used in dental research to measure enamel thickness39 and dental restorations,40,41 and to assess the marginal and internal gaps of crown restorations.6,38 No consensus has been reached regarding the biologically acceptable values of marginal and internal gaps. The clinically acceptable value for the marginal gap has been discussed in the literature, with proposed values that range from 39 to 120 mm.9,42 Theoretically, the acceptable marginal discrepancy for cemented crown restorations ranges between 25 and 40 mm30,43,44; however, several in vitro studies have reported mean marginal gaps of 64 to 83 mm in CAD/ CAM-generated ceramic single-tooth restorations.31,45-47 The internal gap of conventional ceramic crowns has been reported to be within the range of 123 to 154 mm.48,49 May et al5 reported that the marginal ﬁt of the Procera CAD/CAM system ranged between 54 and 64 mm, with an internal gap of 49 to 63 mm. Thus, the accuracy of the chairside CAD/CAM restorations remains questionable, and the effect of the spacer settings has not been studied in detail. The main objective of this in vitro study was to evaluate the marginal and internal adaptation of lithium disilicate restorations fabricated with a CAD/ CAM system and to compare them with the conventional heat-press technique by using micro-XCT. In particular, this
The Journal of Prosthetic Dentistry
study aimed to evaluate the inﬂuence of different spacer thicknesses on the accuracy of lithium disilicate restorations fabricated with the E4D CAD/CAM system. In addition, the accuracy of lithium disilicate restorations fabricated with the E4D CAD/CAM technology was compared with the accuracy of restorations fabricated with the conventional heat-press technique. The null hypotheses for this study were that different spacer thicknesses will not inﬂuence the marginal and internal adaptation of the lithium disilicate restorations and that different fabrication techniques will not inﬂuence the marginal and internal adaptation of the lithium disilicate restorations.
MATERIAL AND METHODS A complete coverage preparation on the mandibular right ﬁrst molar was made for a typodont model (Model D95SDP-200; Kilgore Intl Inc) with a coarse, tapered diamond rotary instrument with a rounded end (no. 6856; Brasseler USA). The following parameters were used: total occlusal convergence, 12 degrees; occlusal reduction, 2 mm; and chamfer ﬁnish line, 1 mm, with rounded internal angle margins. Then, 40 acrylic resin master dies were fabricated by duplicating the prepared tooth with an In-lab CAD/CAM system (Tizian Cut 5; Schütz Dental Gmb) (Fig. 1). The master dies were divided into 4 groups (10 dies per group). Each master die was assigned to each of these 4 groups. The ﬁrst CAD/CAM system was applied to the ﬁrst 3 groups, whereas the heat-press technique was applied to the fourth group. Optical impressions of each master die were obtained with an E4D camera (E4D Dentist system; D4D Technologies), which was calibrated before each master die was scanned. The E4D software was then used to evaluate the clarity of the scanning process. The crown margins were marked manually on the ﬁnish lines of the preparations. Crown shape A was selected from the available designs within the E4D library.
Mously et al
The autogenesis feature was not selected. Scanning and design was performed by the same clinician (H.M.).
Spacer settings for lithium disilicate complete coverage restorations To fabricate the restorations, in the CAD/CAM-30 group, the internal and marginal gaps were set at 30 mm with a 1-mm marginal ramp; in the CAD/ CAM-60 group, the internal and marginal gaps were set at 60 mm with a 1-mm marginal ramp; in the CAD/ CAM-100 group, the internal and marginal gaps were set at 100 mm with a 1-mm marginal ramp.
Milling process and crystallization After each crown was designed with the E4D software, the information was electronically sent to the milling unit. IPS e.max CAD lithium disilicate blocks (Ivoclar Vivadent) were used to fabricate the crowns. The standard mode was selected for the milling process of all the crowns. After the completion of the milling process, all the crowns were crystallized in a porcelain furnace. In the heat-press group, each master die was copied by using a polyether impression material (Impregum Penta Soft; 3M ESPE) to produce identical stone dies with a Type IV dental stone (Resin Rock; Whip Mix Corp). Two layers of die spacer (Euro Quick Set; Kerr Dental Laboratory Products) were applied to the stone dies. The layers were applied uniformly with a brush, starting 1 mm short of the ﬁnish lines of the preparations. Next, an anatomic contour waxing was created for each master die. All of the anatomic contour waxings were invested with ﬁne-grained phosphate-bonded investment material (Multi Press Vest; GC America Inc). Finally, the pressing process was completed according to the manufacturer’s instructions. IPS e.max Press lithium disilicate ingots were used to fabricate the crowns. Four silicone indices were attached with stone to the metal base of a
Mously et al
3 manual milling device (Straumann gonyX; Straumann USA LLC). Each index was used to stabilize and position each surface (buccal, lingual, mesial, and distal) of the master die. By using a carbide round bur (no. H71; Brasseler USA) attached to the milling device, 4 reference indentations were prepared on the buccal, lingual, mesial, and distal surfaces of each master die. Because these indentations were identical in position and size for each master die, they could be used to standardize the computed tomography images for each specimen. The marginal and internal gaps were evaluated by means of micro-XCT (mCT 40; Scanco Medical). Each specimen was stabilized in the scanning tube and positioned perpendicularly to the x-ray beam for scanning. Scanning
was performed at 70 KVp and 114 mA, with an integration time of 300 ms. Two-dimensional images, with an image size of 2048 2048 pixels and a resolution of 10 mm, were reconstructed from the x-ray shadow images. After computerized reconstruction of the images, Image J software (National Institutes of Health) was used to evaluate and analyze the scans. The reference indentations were used to standardize the images of each specimen. By using the Image J software, 5 equidistant vertical cuts were made in the buccolingual direction, with 50 slices between these cuts; in addition, 5 equidistant vertical cuts were made in the mesiodistal direction, with 25 slices between these cuts, depending on the slice numbers of each specimen (Fig. 2).
1 Master die fabrication with acrylic resin block.
2 Micro-XCT scan image (horizontal cut): 10 vertical cuts obtained with 50 slices between buccolingual cuts and 25 slices between mesiodistal cuts.
Volume Twelve points of measurement were selected for each vertical cut (2 measurements for vertical marginal gap [MG], 2 for absolute marginal discrepancy [AMD], 4 for occlusal gap [OG], and 4 for axial gap [AG]). Thus, 20 measurements for MG and AMD, and 40 measurements for OG and AG were obtained for each specimen (Figs. 3, 4). All of the measurements were averaged for each outcome. The magniﬁcations and measurements of the marginal and internal gaps were made with the Image J software.
The MG, AMD, OG, and AG were evaluated at 120 different points for each crown. The MG and AMD were measured by using the criteria for the evaluation of the marginal adaptation provided by Holmes et al.8 The overextended and underextended margins for the AMD were both assembled as positive values. The OG was measured separately from the AG for the evaluation of internal adaptation. All standardizations and measurements of the specimens were performed by the same clinician (H.M.).
3 Micro-XCT scan image (zoomed at margin area): measurement of marginal gap (for overextended crown, perpendicular distance between internal surface of crown and master die margin was measured, whereas, for underextended crown, perpendicular distance between internal surface of master die and crown margin was measured) and absolute marginal discrepancy (distance between crown and master die margins).
4 Micro-XCT scan image (vertical cut): Measurement of axial gap and occlusal gap (measured distances were perpendicular to ceramic crown and master die internal surfaces at selected points).
The Journal of Prosthetic Dentistry
The statistical power was calculated with statistical software (nQuery Advisor, v7.0; Statistical Solutions Ltd). The variance of means and common standard deviation were 257.3 and 15.8 (results from a pilot study), respectively; accordingly, a sample size of n¼10 per group was considered adequate to obtain a type I error rate of 5% and a power greater than 99%. All the outcomes (AG, OG, MG, and AMD) were analyzed separately. Descriptive statistics were computed for each group with SPSS v19.0. One-way ANOVA was initially performed with the Tukey honestly signiﬁcant difference for post hoc tests; however, the analysis could not be reported because of the violation of the assumption of equal variances among groups (P values of the Levene test were less than .05). Therefore, nonparametric testing was undertaken: the Kruskal-Wallis test was performed for each outcome, with P