The aim of this study was to test whether the load bearing capacity of anterior zirconia crowns veneered with overpressed or layered, is similar and to evaluate the failure types.
Standardized zirconia frameworks were fabricated and randomly divided into 8 groups ( N = 120, n = 15 per test group). Four groups were veneered with one of the layered veneering porcelains: Zirox, GC Initial ZR, VITA VM9 or IPS e.max Ceram and the other four groups were veneered with overpressed veneering porcelains: PressX Zr, GC Initial LF, VITA PM9 or IPS e.max ZirPress. The crowns were cemented on their corresponding CoCr abutment and the specimens were loaded at an angle of 45° in a Universal Testing Machine to determine the fracture load. Data were analyzed using one-way and two-way ANOVA, followed by a post hoc Scheffé test, t -test and Weibull analysis (alpha = 0.05).
Within three manufacturers of veneering porcelain, fracture load values were not statistically significant between overpressed and layered porcelain systems. Within one manufacturer of veneering porcelain, the overpressed crowns (IPS e.max ZirPress: 1519 ± 334 N) demonstrated significantly higher ( p < 0.05) fracture load than that of the layered one (IPS e.max Ceram: 894 ± 160 N). Except with IPS e.max ZirPress, where exclusively only chipping of the veneering porcelain was observed, all other porcelain systems showed predominantly framework fractures together with fracture of the veneering porcelain.
Overpressed veneering porcelains for zirconia frameworks exhibited similar or better fracture load compared with layered ones.
Zirconia-based CAD/CAM reconstructions are produced by milling techniques. They have the potential to substitute the metal-ceramic fixed-dental prosthesis (FDP) due to their high biocompatibility and similar mechanical properties with those of metal–ceramics . In principle, zirconia framework materials are veneered using layering technique where meticiulous work and expertise of the dental technician is required. In general, this procedure takes minimal three firing cycles, namely dentin, enamel and glaze firings. The consistency of the mixed powder-liquid veneering ceramic plays a major role on the durability of the reconstruction since the dental technician has to consider shrinkage level of the porcelain and therefore build it larger than the final dimensions. In this procedure, operator factor on the possible inhomogenity of the porcelain mass may result in fracture initiation and propagation in clinical applications. In fact, small impurities in the veneering ceramics in the form of inhomogenities, pores, and crazes could initiate catastrophic failures since under oral conditions such cracks cannot be healed. Above critical values, tensile and shear stresses yield to additional crack forms. Furthermore, slow crack growth may also occur with cyclic loading .
For more than a decade zirconia is on the dental market and many in vitro studies exist indicating its use for the anterior and posterior regions of the mouth with high initial flexural strength above 1000 MPa . Despite its favorable mechanical properties, several clinical studies reported often chipping of the veneering porcelain and less frequent framework fractures . The clinical findings indicate that the weak link in the zirconia FDPs is the veneering porcelain-zirconia interface.
Depending on the manufacturer, the flexural strength of the veneering porcelain ranges between 70 and 100 MPa . Studies on the bond strength between the zirconia framework and veneering porcelain have reported that the failure type was mainly cohesive within the veneering porcelain even on polished zirconia surfaces . This could be related to the possible flaws that may occur during porcelain build up. In order to overcome such failures, recently overpressed veneering porcelains have been developed. Such veneering porcelains could be overpressed and thereafter with different colors individualized. This technique also allows for quick and easy production compared to the conventional layering technique . Additionally, the shrinkage related problems as well as consequences of possible sintering procedures are eliminated. Although veneering porcelain demonstrates similar chemical compositions to those of the overpressed ones, the operator factor in the layered technique could be eliminated . Hence, it can be hypothesized that the overpressed veneering porcelain would result in similar or better fracture load results compared to the layering technique.
The objectives of this study therefore were to test whether the ultimate fracture load of zirconia frameworks for single crowns veneered with overpressed or layered are similar and further evaluate the failure types.
Materials and methods
In order to produce standardized frameworks, a metal tooth analog with the shape of an anatomically prepared maxillary canine with a chamfer preparation of 1 mm was cast from a CoCr alloy (ZENOTEC NP, Wieland Dental, Pforzheim, Germany). After scanning (3Shape D 250, Wieland Dental), an anatomically supported zirconia framework was constructed (ZENO TEC Wieland Dental). Zirconia frameworks were milled (ZENO 4030 M1, Wieland Dental) in the green state (ZENO TEC ZR Bridge, Wieland Dental) and subsequently sintered using a predefined firing schedule (ZENO TEC Fire, Wieland Dental).
The zirconia frameworks were then divided into eight groups ( N = 120, n = 15 per group). Groups 1–4 were veneered according to manufacturers instructions with the following layered veneering porcelains for zirconia: Zirox (Wieland Dental; Group 1), GC Initial ZR (GC Europe, Leuven, Belgium; Group 2), Vita VM9 (Vita Zahnfabrik, Bad Säckingen, Germany; Group 3) and IPS e.max Ceram (Ivoclar Vivadent, Schaan, Liechtenstein; Group 4). The remaining four groups (Groups 5–8) were veneered according manufacturers ‘instructions with one of the following overpressed veneering porcelains for zirconia: PressX Zr (Wieland Dental; Group 5), GC Initial IQ LF (GC Europe; Group 6), VITA PM9 (Vita Zahnfabrik; Group 7) and IPS e.max ZirPress (Ivoclar Vivadent; Group 8). The list of materials used in this study is presented in Table 1 and the compositions of the veneering ceramics are listed in Table 2 .
|Framework||Veneering technique||Veneering porcelain||CTEs according manufacturer’s (10 −6 K −1 )||3-Point flexural strength according manufacturer’s (MPa)||Manufacturers||Lot-number||Groups|
|ZENO Zr (Wieland Dental, Pforzheim, Germany)||Layered||Zirox
|9.3||120||Wieland Dental, Pforzheim, Germany||1/05
|GC Initial ZR
|9.4||90||GC Europe, Leuven, Belgium||4651
|9.0||100||Vita Zahnfabrik, Bad Säckingen, Germany||13340
|IPS e.max Ceram
|9.5||90||Ivoclar Vivadent, Schaan, Liechtensten||L37100
Investment: PressX Zr
Glaze: P: PressX Zr Glaze/L: Stain Liquid Special
|9.3||125||Wieland Dental, Pforzheim, Germany||2/07
|GC Initial IQ LF
Investment: GC Multi PressVest
Glaze: P: GC Initial IQ/POZr L-N/L: GC Initial Diluting Liquid
|9.4||90||GC Europe, Leuven, Belgium||200710101
P: 200806051A/L: 0805151
P: 200805201/L: 0941
Investment: VITA PM Investment
Glaze: Vita SHEDING PASTE Glaze SP15
|9.0||100||Vita Zahnfabrik, Bad Säckingen, Germany||25870
P: 2851198/L: 14540
|IPS e.max ZirPress
Investment: IPS PressVest
Glaze: P: IPS e.max Ceram Glaze Powder/L: IPS e.max Ceram Glaze and Stain Liquid
|9.8||110||Ivoclar Vivadent, Schaan, Liechtensten||L28227
P: LL2004/L: LL2053
P: L36521/L: 32899
|Veneering ceramic||Zirox||GC Initial ZR||Vita VM9||IPS e.max Ceram||PressX Zr||GC Initial IQ||Vita PM9||IPS e.max Zirpress|
|Al 2 O 3||7–23||8–10||7–10||8–12||17–24||14–17||16–19||12–16|
|K 2 O||3–10||5–6||7–10||6–8||7–11||9–13||6–8||2–4|
|Na 2 O||3–13||10–12||4–6||6–9||5–9||5–7||5–8||6–8|
|B 2 O 3||0–4||3–5||0–2||1–2||1–3|
|P 2 O 5||<1||<6||1–2|