Clin Oral Invest DOI 10.1007/s00784-013-1183-0


Retention and surface changes of zirconia primary crowns with secondary crowns of different materials Işıl Turp & Ergün Bozdağ & Emin Sünbüloğlu & Cem Kahruman & İbrahim Yusufoğlu & Gülsen Bayraktar

Received: 6 April 2012 / Accepted: 30 December 2013 # Springer-Verlag Berlin Heidelberg 2014

Abstract Objectives To evaluate zirconia as a substitute for gold alloy in primary crowns facing secondary crowns manufactured with different materials, in terms of long-term retention force changes, wear, and phase transformation was aimed. Materials and Methods A total of 12 groups, each containing six samples, consisting of gold alloy primary crownelectroformed gold secondary crowns (AA), zirconia primary crown-electroformed gold secondary crowns (ZA) and zirconia primary crown-casted non-precious alloy secondary crowns (ZC) with conus angles of 0°, 2°, 4°, and 6° were evaluated. Samples were subjected to 10,000 insertion–separation cycles in artificial saliva and retention force was

I. Turp (*) Bezmialem Vakıf University, Adnan Menderes Bulvarı (Vatan Cad.) P.K. 34093, Fatih, Istanbul, Turkey e-mail: [email protected] E. Bozdağ : E. Sünbüloğlu Faculty of Mechanical Engineering, Istanbul Technical University, Gümüşsuyu, Istanbul, Turkey E. Bozdağ e-mail: [email protected] E. Sünbüloğlu e-mail: [email protected] C. Kahruman : İ. Yusufoğlu Department of Metallurgical and Materials Engineering, Faculty of Engineering, İstanbul University, Avcılar, Istanbul, Turkey C. Kahruman e-mail: [email protected] İ. Yusufoğlu e-mail: [email protected] G. Bayraktar Department of Prosthodontics, Faculty of Dentistry, İstanbul University, Çapa, Istanbul, Turkey e-mail: [email protected]

measured. X-ray diffraction and scanning electron microscope analysis were performed on the sample surfaces. Results The highest retention forces were obtained from ZC-0° group (72.09–71.26 N) and the lowest were obtained from ZA4° (12.73–19.44 N) and ZA-6° (5.36–19.73 N) groups in the beginning and after 10,000 cycles, respectively. Retention force increased as the conus angle decreased. The monoclinic phase ratio of the zirconia primary crowns decreased after the experiments. No wear was observed in zirconia primary crowns except for the ZC-0° and ZC-2° groups. The use of zirconia primary crowns resulted in a less excursive retention force. Conclusions A more predictable and less excursive retention force can be obtained using a hard and rigid primary crown material like zirconia. Clinical Relevance Despite a lack of knowledge about the aging of zirconia without a veneer layer in the oral environment, zirconia primary crowns are more advantageous in terms of retention force development and wear. Keywords Double crown . Zirconia . Retention force . Wear . X-ray diffraction

Introduction Double crowns are frequently used as retentive elements to connect teeth or implants to dentures. They can be designed as resilient or rigid for splinting, stabilization, and support as well as retention in order to meet patients’ requirements [1]. New technologies have improved manufacturing techniques and new materials and manufacturing processes have been introduced [1–13]. Contemporary manufacturing techniques are intended to overcome the problems faced when a conventional casting method is used with gold alloys for the fabrication of double crowns. In cases where metal alloys are used as the primary crown material, if the patient has a high lip line when smiling, and thin and delicate gingival tissue, then aesthetic problems with

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the metallic collar of the primary crown can arise [6]. Preparation of a supragingival crown margin is also advised for periodontal health but the metal margin of the restoration can again be displayed at the collar [1, 14]. The same unaesthetic situation can also show up when gingival regression occurs, even if the restoration was initially placed subgingivally [13]. Thermal irritation can also occur in the abutment teeth because of the high thermal conductivity of metals [15]. None of these problems is observed when the primary crowns are fabricated with a tooth-colored material like zirconia [4, 10, 15]. Improvements in the manufacturing techniques have made the use of zirconia as primary crown material possible [3, 5–7, 9, 10, 15, 16]. Its high biocompatibility, tooth color, and resistance to wear have promoted its usage in recent years. The use of tooth-colored ceramic materials also is stated to have a positive psychological effect on patients and promotes improvements in oral hygiene [5, 13]. Zirconia has a lower thermal conductivity than metal, and no cold-welding or galvanic current develops between zirconia and electroformed gold, which is another advantage of this application [11, 15]. When compared with the conventional casting method, a better fit of primary and the secondary crowns can be achieved when the secondary crowns are fabricated by electroforming directly on the primary crowns. The gap between the primary crown and the electroformed gold secondary crown is less than 5 μm and its cost is lower than a casted precious alloy [11, 12]. Non-precious metal alloys can also be used for fabricating double crowns with the conventional casting method. Their cost is also lower and these crowns presumably will experience less wear as secondary crown material because their hardness values are closer to zirconia than electroformed gold, which has a purity of 99.9 % [21]. Several case reports have described the use of zirconia as primary crown material [5–7, 9, 10, 15], but only one short-term and two long-term in vitro studies in the current literature [3, 16, 17] have evaluated the retention force development. These latter studies also only evaluated conus angles of 0° and 2°; no conus angles higher than 2° have been investigated. In addition, no controlled clinical trials or surface analysis with long-term results are presently available [10, 15]. The changes occurring on the surface of zirconia, as the primary crown of double crowns, when subjected to the moist mouth environment and friction is still unknown. In vivo and in vitro analysis is needed to determine wear and surface changes of zirconia primary crowns not covered by any veneer layer and in direct contact with saliva [7]. The use of non-precious metal alloys as secondary crown materials facing zirconia primary crowns has also not been evaluated yet. For these reasons, the objectives of this study are to evaluate the following: &

The retention development of double crown assemblies with material couples consisting of gold alloy primary crown-electroformed gold secondary crowns (AA),

& & &

zirconia primary crown-electroformed gold secondary crowns (ZA), and zirconia primary crown-casted nonprecious alloy secondary crowns (ZC) The effect of conus angles, including the uninvestigated 4° and 6°, on retention force of these double crown assemblies The effect of insertion–separation cycles carried out in artificial saliva on the phase integrity of zirconia primary crowns The changes taking place on the contacting surfaces of primary and secondary crowns, for 10,000 insertion–separation cycles in vitro

Our five-part working hypothesis is that (1) primary crown material has no influence on retention force; (2) sufficient retention force can be obtained with non-precious alloy secondary crowns; (3) retention force increases as the conus angle decreases; (4) no wear will be observed on the zirconia primary crowns after 10,000 insertion–separation cycles; and (5) the monoclinic content increases in both the ZA and ZC groups.

Materials and methods A schematic drawing of a cross-section of the setup used to attach the stainless steel dies to the system can be seen in Fig. 1. The scheme of the stainless steel dies imitating the tooth or implant abutment is also magnified to show its dimensions. All dies had the conus angles (α/2) of the test samples tested on them, which were 0°, 2°, 4°, and 6°. A 0.6-mm-deep groove was formed vertically with a bur (S6881, Komet) (1.2 mm diameter) to ensure consistent seating of the primary crowns on the abutment. A total of 12 groups were evaluated; each contained six samples, consisting of AA, ZA, and ZC material couples with conus angles of 0°, 2°, 4°, and 6°. The gold alloy primary crowns of the AA groups were modeled in wax (Light Dip, Yeti Dentalprodukte GmbH) directly on stainless steel dies. Wax models were carefully shaped with a milling device to apply low pressure and unidirectional milling (Exacto 1, HeimerleMeule) with wax burs (H364RA, H356RA; Komet) having conus angles of 0°, 2°, 4°, and 6° until they had a thickness of 0.8 mm. The wax models were invested (Presto Vest II, Siladent Dr. Böhme & Schöps GmbH) and cast in gold alloy (Solaro 3, Metalor Dental AG) according to the directions of the manufacturer. After casting, colored spray (WP Occlusion, W+P Dental) was applied inside the primary crowns and they were placed on the dies without force to check for irregularities that might prevent seating of the gold alloy primary crowns on the dies. The traces inside the crown were removed and a complete fit was achieved. After all crowns were adapted by the same experienced dental technician, they were milled again with adequate burs (H364RE, H356RSE, H364RF, H356RF; Komet) to assure conus angles of 0°, 2°, 4°, and 6°. Rubber polishers for the gold alloy (AuR22, Eve Ernst Vetter GmbH) were applied after their conus angles were arranged with the dressing block

Clin Oral Invest Fig. 1 The cross-sectional scheme of the specially developed setup for attaching the die holding the primary crown to the lower part of the device. 1 stainless steel die, 2 brass holder. 3 screw holding the die in place, 4 O-rings holding the artificial saliva chamber in place without leakage, 5 artificial saliva chamber, 6 iron base holding the magnet, 7 screw attaching the iron base to the brass holder, 8 magnet connecting the setup to the Z-type load cell

(Abrichtblock, Komet). The polishing procedure was completed with the application of polishing paste (Abraso-Starglanz asg, Bredent) and a final thickness of 0.6 mm was achieved. The zirconia primary crowns were produced by a CAD/CAM system (D30, Yenamak) using the software for double crowns with desired conus angles and with a thickness of 0.8 mm. The primary crowns were milled from zirconia blocks (Zirkon Transluzent 98H14, Zirkonzahn GmbH) 20 % larger than the original dimensions. After sintering to full density at 1,500 °C, the original dimensions were reached. The crowns were checked for fit with the same procedure applied for gold alloy primary crowns. Irregularities that prevented the seating of primary crowns were removed under water cooling with a diamond bur developed for zirconia surfaces (ZR8801L, Komet). The primary crowns were seated by the same dental technician and milled with burs developed for zirconia primary crowns with colour codes of blue, red, yellow and white (W882.014, W882.025, DiT-Dental-Instrumente GmbH; 356.031, 356.040, Edenta AG) and having the desired conus angles. The milling was carried out under water cooling with an air turbine (342 Art, Silfradent) mounted in a milling device (Exacto C, Heimerle-Meule). The conus angles of polishers specifically designed for zirconia (H7DCmf, H7DC, Eve Ernst Vetter GmbH) were arranged with the dressing block on the milling device, so the surface of the shaped polisher was perfectly planar and applied to the zirconia primary crowns on the milling device. The polishing procedure was finished with the application of zirconia polishing paste The secondary crowns of the AA and ZA groups were fabricated with electroforming according to the manufacturer’s instructions. The primary crowns were filled with pattern resin (Pattern Resin LS, GC Corporation). Two layers of silver lacquer

(Silberleitlack, Gramm Technik GmbH) were applied with the specific brush of the system and the conductivity was checked after drying. The amount of time required, liquid (Ecolyt SG 200, Gramm Technik GmbH), and activator (Activator SG200-T, Gramm Technik GmbH) were determined automatically by the system (Gammat optimo 2, Gramm Technik GmbH) to obtain a thickness of 250 μm and electroforming was conducted. The secondary crowns of the ZC group were modelled directly on the primary crowns with light-curing wax (Light Dip, Yeti Dentalprodukte GmbH). Wax models were invested (Presto Vest II, Siladent Dr. Böhme & Schöps GmbH) and cast with a non-precious alloy (Wiroloy, BEGO Bremer Goldschlaegerei Wilh. Herbst GmbH & Co) according to the manufacturer’s instructions. All cast secondary crowns were adapted to the primary crowns by the same experienced dental technician and a thickness of 0.7 mm was achieved. All primary crowns were cemented to the stainless steel dies with resin cement (C&B Cement, Bisco Inc.) according to the manufacturer’s instructions. The insertion–separation cycles were conducted with a Mini Bionix II device (Mini Bionix II, MTS Systems Corporation). The stainless steel dies with primary crowns cemented on them were attached to a Z-type load cell (STCS50kgC3, Esit) with a measuring capability of 500 N at the lower part of the device with a specially developed setup (Fig. 1). The brass setup held the die with the aid of a screw, and connection with the load cell was achieved with a magnet embedded in a socket underneath the iron base, which was connected to the brass portion with another screw. The secondary crowns were attached to the upper part of the device with the aid of a cylindrical brass connector 8 cm long and 1 cm in diameter.

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Sockets for the secondary crowns were prepared at the ends of the connectors. The inner surface of the sockets and outer surface of the secondary crowns were sandblasted with 50 μm Al2O3 particles to increase the surface area. After attaching the brass connector to the upper part of the device, the secondary crowns were cemented to the sockets in the connectors with a pressure of 20 N and with the same resin cement used for luting the primary crowns. The brass connectors in this case also acted as the tertiary structure of the electroformed secondary crowns (Fig. 2). After the primary and secondary crowns were attached to the device, ten insertion–separation cycles were conducted slowly to allow the system to align itself, with the magnet providing retention vertically but freedom laterally (Fig. 2). The experiments then began and the initial retention force measurements were made. Retention force measurements were repeated after every 500 insertion–separation cycles until 10,000 cycles were completed. The measurements were made for ten times at every period. Two different programs were assembled for conical and cylindrical double crowns to conduct the insertion–separation cycles and retention force measurements. For conical double crowns, the speed of insertion and separation of the secondary crown was 300 mm/min with a frequency of 2.2 Hz during insertion–separation cycles. During the retention force measurements, the speed of insertion was set at 10 mm/min and separation was set at 20 mm/min. The vertical displacement of the secondary crown was 1.5 mm and the secondary crowns were inserted on the primary crowns with a load of 50 N during the insertion–separation cycles and retention force measurements. For cylindrical double crowns, the experimental procedure had to be altered because of their continuing contact all along the lateral surface of the primary crown. During the insertion and separation cycles, the speed of insertion of the secondary crowns was changed to 15 mm/min and the separation to 12 mm/min with a frequency of 2.1 Hz. During the retention force measurements, the speeds of insertion and Fig. 2 a Cylindrical brass connector of the secondary crowns with an electroformed secondary crown cemented inside. b The complete setup in place. The system could align itself benefitting from the property of the magnet providing lateral freedom

separation were 5 and 20 mm/min, respectively. In theory, the retention force of a cylindrical double crown is equal to the load of insertion, so the load needed to seat the secondary crown on the primary crown was increased to 120 N because the retention force of cylindrical double crowns could exceed 50 N during the experimental procedure, and the vertical displacement of the secondary crown was increased to 4 mm. All insertion–separation cycles and retention force measurements were conducted in artificial saliva, which was produced according to Shannon [18]. A plastic chamber was attached to the brass setup connecting the primary crown and the stainless steel die to the lower part of the device by the aid of an o-ring to achieve a seal between the detachable chamber and serve as a reservoir for the artificial saliva (Figs. 1 and 2). The primary and the secondary crowns were totally immersed in artificial saliva throughout the experimental procedure. One sample from each group, which had the retention force value nearest to the group mean, was chosen for SEM (scanning electron microscopy) (JSM-6500, Jeol) analysis for comparison with samples which were not subjected to experiments. The secondary crowns of all samples were vertically cut into two pieces, applying low pressure. The samples that had been subjected to the experimental procedure were cut with their brass housing surrounding them and the control samples, which were not subjected to the experimental procedure, were cut alone so that the surfaces of both primary and secondary crowns could be examined. The SEM analyses were conducted especially at the parts near the collar of the double crown but away from cut edges. The samples were placed with their collars parallel to the lower margin of the image so that the inclinations of wear traces could also be evaluated. The surfaces of the zirconia primary crowns were coated with gold in order to achieve conductivity. After the experiments, X-ray diffraction (XRD; D-Max2200/PC, Rigaku) was used to quantify the crystalline phases in primary crowns of the samples from each zirconia primary crown containing ZA and ZC groups and in a control sample.

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The time period between manufacturing the samples and XRD analysis was approximately 4 months. The samples were immersed in cork dust (which showed no peaks in XRD) and their lateral surfaces were placed parallel to the holder so that the surfaces with the highest applied friction and highest expected phase transformation could be evaluated. A fast scan was first made on the control sample from 5° to 65° 2θ at 4°/min to determine the nature and number of the phases. Slower scans were then performed on all samples with Cu/Kalpha1 radiation (1.54056 Å) at 20 mA and 40 kV. Diffraction data were collected from 27° to 33° 2θ, where monoclinic and tetragonal phase peaks are observed, with a step size of 0.1° and a scanning time of 6 s/step. The relative amount of monoclinic zirconia (Xm) was determined according to the method recommended by Garvie and Nicholson [19] from the integrated intensities of monoclinic and tetragonal peaks: Xm ¼ ½I m ð−111Þ þ I m ð111ފ=½I m ð−111Þ þ I m ð111Þ þ I t ð111ފ;

where I is the intensities at angular position 2θ degrees from the diffraction, t and m are the tetragonal and monoclinic peaks. The monoclinic volume content (Vm) was calculated using the method of Toraya et al. [20]: V m ¼ 1:311X m =ð1 þ 0:311X m Þ Statistical analyses were performed using the SPSS statistical software package for Microsoft Windows (version 16.0, SPSS Inc.). Descriptive analysis was carried out with the Chisquare test. Normalities of distributions were explored by means of the Kolmogorov–Smirnov test. The influence of conus angle and material pair was compared with the Kruskal–Wallis test; further comparisons were made with the Mann–Whitney U test. The retention force values of groups were compared with a one-way ANOVA test and the Tamhane multiple comparison test if significant differences were noted after each period. The retention force values of a group measured after each period was compared with the Wilcoxon signed-rank test. The significance level was set at 5 %.

Results Statistically significant increases were seen between initial the retention force values and values after 10,000 insertion–separation cycles in all groups except AA-0°, AA-2°, ZC-0°, and ZC-2°, according to the Tamhane multiple comparison test. The ZC-0° group had the highest retention force values, with a mean of 72.09 N initially and 71.26 N finally. The ZA-4° and ZA-6° groups had the lowest values, with initial means of 12.73 and 5.36 N and final means of 19.44 and 19.73 N, respectively (Table 1). The ZC material displayed the highest

retention force values compared to the other material couples for all conus angles both initially and finally, although not all differences were statistically significant. The retention force values decreased as the conus angle increased for all material couples, with statistical significance occurring between groups that had more than a 2° difference in conus angles. No significant decrease in retention force values occurred after 10,000 insertion–separation cycles in any group. The retention force values of a group measured after each period was also evaluated with the Wilcoxon signed rank test. The changes in mean retention force values of groups throughout the entire 10,000 insertion–separation cycles can be seen in Fig. 3. The retention force development in the conical double crowns of the AA groups was more stable when compared with AA-0° group. The ZA-0° and ZC-0° groups also had a more stable retention force development than did the AA-0° group, with more predictable increases and decreases. The final mean retention force value for the ZA-0° group was significantly higher than the initial mean retention force value (p≤0.0001), but there was no significant difference between the initial and final retention force values for the ZC-0° group. An increase in the retention force was observed in conical crown groups with electroformed secondary crowns (AA-2°, AA-4°, AA-6°, ZA-2°, ZA-4°, and ZA-6°) after the first 500 insertion–separation cycles. The SEM images of one sample from the AA, ZA, and ZC groups and one control sample can be seen in Fig. 4. Wear traces were evident in all primary and secondary crowns in the AA-0°, AA-2°, AA-4° and AA-6° groups. In the ZA-0°, ZA2°, ZA-4°, ZA-6°, ZC-4° and ZC-6° groups, wear traces were observed only in the secondary crowns. The traces seen on the primary crowns of these groups were horizontal to the collar of the sample and not in the direction of insertion-separation and resulted from the grinding and polishing processes. Wear traces could be observed on zirconia primary crowns only in the ZC-0° and ZC-2° samples. The peaks of the fast scan performed in the range of 5–65° 2θ on the control zirconia primary crown matched the XRD standards file for tetragonal zirconium oxide (PDF#50-1089). The XRD patterns of all samples acquired from the scans performed in the range 27–33° 2θ and the calculated monoclinic volume content of zirconia primary crowns are shown in Fig. 5 and Table 2. The monoclinic volume content of the control zirconia primary crown was 19.7 %, whereas the rest were under 5 %, indicating that the experimental procedure reduced the monoclinic volume content in all groups.

Discussion No standard protocols have been established for in vitro evaluation of the double crowns, so the methods used in this study were chosen very carefully to imitate the clinical situation

Clin Oral Invest Table 1 The initial and final mean retention force values (N) of groups with different material couples and conus angles and their comparisons

Group Material AA

ZA Letters indicate comparisons carried out with the Tamhane multiple comparison test (vertical) and Wilcoxon signed rank test (horizontal) (p

Retention and surface changes of zirconia primary crowns with secondary crowns of different materials.

To evaluate zirconia as a substitute for gold alloy in primary crowns facing secondary crowns manufactured with different materials, in terms of long-...
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