Abstract
Objectives
Chipping is the most frequent clinical failure of zirconia crowns. Causes of chipping have not been completely understood and different possible reasons have been considered. The study was aimed at evaluating the fracture resistance of 3 different CAD/CAM zirconia frame designs veneered with porcelain.
Methods
Thirty extracted sound premolars were divided into 3 groups ( n = 10). Chamfer preparations were performed, impressions were taken. Three zirconia frame designs (Aadva, GC) were realized: reproduction of the abutment contour (flat design, FD); wax-up as for porcelain-fused-to-metal crowns (PFM); anatomically guided, designed to keep constant the thickness of the overlying porcelain veneering (AG). Porcelain veneering was made with pressure layering technique (Initial Zr, GC). Crowns were cemented utilizing a self-adhesive resin cement (G-Cem, GC). After a 24-h water storage at 37 °C, using a universal testing machine (1 mm crosshead speed), crowned teeth were loaded in the central fossa in a direction parallel to the longitudinal axis of the tooth. Load at fracture was recorded in Newtons (N). Digital photographs of the specimens were taken in order to assess failure patterns. Between-group differences in fracture strength were statistically analyzed (One-Way Analysis of Variance, Tukey test, p < 0.05).
Result
Load at fractures differed significantly among the groups ( p = 0.004). AG exhibited significantly higher fracture resistance 1721.6 (488.1) N than PFM 1004.6 (321.3) N and FD 1179.5 (536.2) N, that were comparable. Repairable failures occurred in 80% of AG, 70% of PFM, and 50% of FD specimens.
Significance
Anatomically guided zirconia frames resisted significantly higher loads than flat and PFM-like frame designs.
1
Introduction
Porcelain layered zirconia crowns were recently introduced and quickly become very popular in dentistry . Success may be defined as the achievement of treatment planning goals and expectations. Failure represents the inability of a restoration to perform as expected under typical clinical and patient conditions. A complication represents an unfavorable and unexpected outcome of dental treatment . In this view, chipping of the veneering porcelain layered on zirconia frameworks represent a failure that could affect the prognosis of the restoration to a various degree, depending on its extension . The studies available in literature focused on the analysis of failure for all-ceramic restorations investigating several parameters involved on the tooth structure-cement-core-veneer complex, in order to improve clinical performances. Some of the parameters, like the framework design, are technique-sensitive and during the manufacturing of the restorations can easily influence the failure rates and fracture modes of final restorations, similarly to metal ceramic crowns .
Chipping of porcelain layered crowns was reported as frequent cause of clinical failure . Among the factors that can determine “chipping fractures” , zirconia sintering process and structural defects , grinding damages produced during laboratory procedures , relationship between cooling rates, Coefficient of Thermal Expansion (CTE) and thickness of zirconia crown have been advocated as possible reasons for such a failure . Other reported factors are possible decrease of zirconia crowns strength during sandblasting procedures , framework design , type of finishing margins , luting procedure and aging of zirconia .
In order to predict the clinical behavior of porcelain layered zirconia crowns, some studies evaluating fracture resistance have been performed . For the present study the focus was on framework design and how three different coping designs may influence possible clinical failures.
The null hypothesis formulated was that different coping designs do not affect the fracture resistance of porcelain layered zirconia crowns.
2
Materials and methods
Thirty extracted sound human maxillary premolars teeth stored in 0.5% chloramine-T (Chloramine-T; Fisher Scientific Co., Pittsburg, PA) were randomly divided into 3 groups ( n = 10).
The apical third of the root were sectioned in order to obtain an equal specimens length. The specimens were then fixed with sticky wax to a Zeiser plate with pin ledges. A small flask was used to embed all the specimens’ roots in acrylic resin until 1 mm below the enamel-cement junction. After resin setting, each sample/tooth block was sectioned in order to be removable. In order to control further steps during teeth preparations and wax-up, silicon masks were realized. They were prepared leaving the occlusal surface free approximately until the equator of the crown. Zaiser plates were mounted in a verticulator and the rods were set at level 0. Silicon mask were taken for all the specimens. The crowns were subjected to chamfer preparations with 2.0 mm of occlusal reduction and 1.5 mm axial reduction. Impressions of the abutments were taken using 40S Shore Adisil Blau 9:1 (Siladent Dr Bohme & Schops GmbH D, Goslar, Germany) supported by the flask. Dies were prepared with type IV stone (Fuji Rock EP, GC Co., Japan). In the same plate both natural abutments and dies samples can be accommodated. For the framework design, the pin of the verticulator was positioned at minus 1 mm in order to control the wax-up thickness on the occlusal surface of the shell according to the intended 1 mm thickness. The axial wax-up of three different groups based on zirconia frame designs was then manufactured (produced). In Group 1 (flat design, FD) the frame was designed referencing the reproduction of the abutment contour with a controlled wall thickness of 0.5 mm). In Group 2 (PFM) the wax-up followed the indications for porcelain-fused-to-metal crowns design, with an anatomical reduction of 1 mm for the occlusal and wall surfaces, a 1.5 mm collar in the vestibular and palatal region and a interproximal collar up to the 50% of the wall height. In Group 3 (anatomically guided, AG) a wax collar was designed following the PFM group design but 0.5 mm high elevations every 1.5 mm on the axial wall were added in order to support porcelain on axial wall during loading. The minimal thickness of the frame between the elevations was at least 0.5 mm. The designs of the three Groups are showed in Fig. 1 . The wax-up was then sent to the GC milling center where they were scanned, software processed. The Y-ZTP frameworks (Aadva, GC) were the fabricated for the three groups. Porcelain veneering was made with heat pressure layering technique (Initial Zr, GC). A thin layer of Power Frame Modifiers was applied on the untouched zirconia surface in a furnace (P5000, Ivoclar Vivadent Manufacturing Srl, Naturno, Italy) and then the wax was applied using the previously obtained silicon impressions/masks with the verticulator rods set at 0 the in order to reproduce the original anatomy of the sample teeth. The casting procedure was then performed (4 samples each cast, with 200 g GC MultiPressVest and with injection thermo-pressed technique at 970 °C using GC Initial IQ Press Pellets and following manufacturer’s instructions). Luster Paste was applied for all the specimens following the manufacturer instructions. The firing program for Power Frame Modifier and Luster Paste firing were reported in Table 1 . Crowns were checked on the dies and abutments and then luted with self-adhesive resin cement (G-Cem, GC) to the natural abutments. After 24-h water storage at 37 °C, crowned teeth were processed for loading. After placing the specimen on a support, the crowned teeth were loaded in the center with a round-tip stainless steel rod of 2 mm in diameter in a direction parallel to the longitudinal axis of the tooth, using a universal testing machine (Triax 50, Controls, Milano, Italy) at 1 mm crosshead speed. Loading was applied until fracture occurred and load at fracture was recorded in Newtons (N). Digital and SEM photographs of the failed specimens were taken in order to assess failure patterns. According with the classification described by Heintze and Rousson , types of failures were divided in Repairable Fracture (Grades 1 and 2) and Non Repairable Failure (Grade 3). Between-group differences in fracture strength were statistically analyzed. As the data distribution was normal according to the Kolmogorov–Smirnov test and group variances were homogenous according to the Levene test, One-Way Analysis of Variance (ANOVA) was applied, followed by the Tukey test for post hoc comparisons. In all the analyses the level of significance was set at p = 0.05.
| Fired material | Pre dry temp. (°C) | Pre dry time (min.) | Incremental temp. (°C) | Vacuum | Final temp. (°C) | Final temp. time (min) |
|---|---|---|---|---|---|---|
| Power frame modifier | 480 | 4 | 55 | Yes | 900 | 1 |
| Luster paste | 480 | 4 | 45 | No | 810 | 1 |
2
Materials and methods
Thirty extracted sound human maxillary premolars teeth stored in 0.5% chloramine-T (Chloramine-T; Fisher Scientific Co., Pittsburg, PA) were randomly divided into 3 groups ( n = 10).
The apical third of the root were sectioned in order to obtain an equal specimens length. The specimens were then fixed with sticky wax to a Zeiser plate with pin ledges. A small flask was used to embed all the specimens’ roots in acrylic resin until 1 mm below the enamel-cement junction. After resin setting, each sample/tooth block was sectioned in order to be removable. In order to control further steps during teeth preparations and wax-up, silicon masks were realized. They were prepared leaving the occlusal surface free approximately until the equator of the crown. Zaiser plates were mounted in a verticulator and the rods were set at level 0. Silicon mask were taken for all the specimens. The crowns were subjected to chamfer preparations with 2.0 mm of occlusal reduction and 1.5 mm axial reduction. Impressions of the abutments were taken using 40S Shore Adisil Blau 9:1 (Siladent Dr Bohme & Schops GmbH D, Goslar, Germany) supported by the flask. Dies were prepared with type IV stone (Fuji Rock EP, GC Co., Japan). In the same plate both natural abutments and dies samples can be accommodated. For the framework design, the pin of the verticulator was positioned at minus 1 mm in order to control the wax-up thickness on the occlusal surface of the shell according to the intended 1 mm thickness. The axial wax-up of three different groups based on zirconia frame designs was then manufactured (produced). In Group 1 (flat design, FD) the frame was designed referencing the reproduction of the abutment contour with a controlled wall thickness of 0.5 mm). In Group 2 (PFM) the wax-up followed the indications for porcelain-fused-to-metal crowns design, with an anatomical reduction of 1 mm for the occlusal and wall surfaces, a 1.5 mm collar in the vestibular and palatal region and a interproximal collar up to the 50% of the wall height. In Group 3 (anatomically guided, AG) a wax collar was designed following the PFM group design but 0.5 mm high elevations every 1.5 mm on the axial wall were added in order to support porcelain on axial wall during loading. The minimal thickness of the frame between the elevations was at least 0.5 mm. The designs of the three Groups are showed in Fig. 1 . The wax-up was then sent to the GC milling center where they were scanned, software processed. The Y-ZTP frameworks (Aadva, GC) were the fabricated for the three groups. Porcelain veneering was made with heat pressure layering technique (Initial Zr, GC). A thin layer of Power Frame Modifiers was applied on the untouched zirconia surface in a furnace (P5000, Ivoclar Vivadent Manufacturing Srl, Naturno, Italy) and then the wax was applied using the previously obtained silicon impressions/masks with the verticulator rods set at 0 the in order to reproduce the original anatomy of the sample teeth. The casting procedure was then performed (4 samples each cast, with 200 g GC MultiPressVest and with injection thermo-pressed technique at 970 °C using GC Initial IQ Press Pellets and following manufacturer’s instructions). Luster Paste was applied for all the specimens following the manufacturer instructions. The firing program for Power Frame Modifier and Luster Paste firing were reported in Table 1 . Crowns were checked on the dies and abutments and then luted with self-adhesive resin cement (G-Cem, GC) to the natural abutments. After 24-h water storage at 37 °C, crowned teeth were processed for loading. After placing the specimen on a support, the crowned teeth were loaded in the center with a round-tip stainless steel rod of 2 mm in diameter in a direction parallel to the longitudinal axis of the tooth, using a universal testing machine (Triax 50, Controls, Milano, Italy) at 1 mm crosshead speed. Loading was applied until fracture occurred and load at fracture was recorded in Newtons (N). Digital and SEM photographs of the failed specimens were taken in order to assess failure patterns. According with the classification described by Heintze and Rousson , types of failures were divided in Repairable Fracture (Grades 1 and 2) and Non Repairable Failure (Grade 3). Between-group differences in fracture strength were statistically analyzed. As the data distribution was normal according to the Kolmogorov–Smirnov test and group variances were homogenous according to the Levene test, One-Way Analysis of Variance (ANOVA) was applied, followed by the Tukey test for post hoc comparisons. In all the analyses the level of significance was set at p = 0.05.
