The Comparison of Fracture Resistance between Low Translucent and Ultra-High Translucent Monolithic Zirconia Crown

Abstract Objective  From the beginning of its discovery, the monolithic zirconia crown was highlighted for its remarkable strength; therefore, only available in opaque color. During the past decade, the translucent monolithic zirconia crown was manufactured to meet the aesthetic and restoration demand but was thought to be the cause of decreasing its strength. This study aimed to compare the fracture resistance between two types of translucent monolithic zirconia crowns, that is, low translucent monolithic zirconia (LT) and ultra-high translucent monolithic zirconia (UHT). Material and Methods  The premolar crown model was prepared using a computer-aided design/computer-aided manufacturing system, producing 1 mm of thickness. Ten crown samples were divided into LT and UHT. Then, each sample was measured for its fracture resistance using Universal Testing Machine until a fracture occurred. The differences in fracture resistance were analyzed using an independent t -test with p  < 0.05. Result  The LT showed a higher fracture resistance than UHT ( p  < 0.05). Conclusion  The LT monolithic zirconia crown has strength and can be used for posterior crown restoration.


Introduction
In recent years, public awareness has increased for nonmetal restorations and have been highly in demand. 1 Ceramic crowns have become very popular because they have excellent aesthetics, biocompatibility, and strength. 2][5] Yttrium cation-doped tetragonal zirconia polycrystal (Y-TZP) crown is the most widely used crown.Crowns made of Y-TZP material have been used for anterior and posterior restorations. 6Although the strength of zirconia as a framework is excellent and has high resistance, some studies report fractures in the veneer material that covers it; thus, a monolithic crown was developed.Back then, zirconia restorations were less translucent and only available in low translucent (opaque) colors.][9][10] LT contains 3 mol% of Y-TZP, while HT contains 8 mol% yttria, which results in materials with completely different structures, optical and mechanical properties. 11Increased translucency of the zirconia crown raises doubts about fracture resistance; therefore, research was performed to confirm it. 3onolithic zirconia was the first choice for a single posterior crown, and lithium disilicate was the first choice for a single anterior crown. 12However, a zirconia crown was developed with much better transparency, namely, ultra-high translucent monolithic zirconia (UHT).It aims to obtain the mechanical properties of zirconia and the aesthetic properties of lithium disilicate.
The difference between LT and UHT lies in the composition of the transformation phase. 13The UHT contains significantly more cubic phases than other zirconia, so it has excellent translucency. 14The high content of cubic phase grains will reduce the mechanical properties of HTZ zirconia, especially fracture resistance. 15Based on those problems, it is needed to compare the mechanical properties, especially the fracture resistance between LT and UHT crowns.

Monolithic Zirconia
The monolithic zirconia used in this study was LT (Katana Zirconia, Kuraray Noritake Dental Inc, Aichi, Japan) and HT (Katana Zirconia, Kuraray Noritake Dental Inc).

Crown Sample Model
This study used 10 crown models divided into two groups: five crowns of LT monolithic zirconia and five crowns of UHT monolithic zirconia.The research model was made similar to the premolar tooth using a computer-aided design (CAD)/computer-aided manufacturing system.The premolar tooth was prepared in a crown preparation with a thickness of 1 mm.Chamfer shape was chosen for cervical margin preparation.Prepared premolar teeth are duplicated with wax.The tooth model is then cast in metal form (►Fig. 1A and B).
The model cast was then scanned and transferred to the CAD system for making the crown design with a thickness of 1 mm (►Fig.1C and D).Milling was performed on the zirconia block.Then proceed with the process of sintering and glazing.After completing the crown, we planted the research models into acrylic resin, followed by cementing the monolithic zirconia crown on the tooth model.

Fracture Resistance
The fracture resistance test was performed with UTM test equipment.Two groups of samples were placed below the pressing equipment with the tip of the load test instrument positioned on the fissure of the crown.All the samples were pressed using a starting load test at a speed of 0.225 mm/minute until a fracture occurred; after that, data from the experiment were collected for statistical calculation.

Statistics Analysis
An independent t-test was conducted to determine the fracture resistance between the LT and UHT monolithic crown.The significance level used in this test was 0.05 or 95%; the significance was determined based on a p-value of < 0.05.

Results
The fracture resistance from pressure in LT was 3527 AE 20.59 mm/minute and UHT was 2972.2AE 19.95 mm/minute.The LT showed a higher fracture resistance than UHT (p ¼ 0.046) ►Fig.2.

Discussion
This study compared the LT and UHT zirconia crown seen from the mechanical properties of fracture resistance.It is similar to the research conducted by Johansson et al 16 and Nordahl et al, 3 which compared LT, UHT, and monolithic lithium disilicate crowns.The results of this study indicate that the LT monolithic zirconia crown has higher fracture resistance than the UHT monolithic zirconia crown.This was caused by manipulation of grain size and dopant changes can affect the mechanical properties of zirconia crowns. 3LT monolithic zirconia crowns have a polymorphic structure.With the change of phase from the tetragonal phase to the monolithic phase, there will be an increase in volume, which will cause cracks at room temperature. 11Adding a stabilizer will prevent cracks due to the addition of volume.8][19][20] It was also proved in the study by Nordahl et al, who stated that monolithic zirconia crowns had higher fracture resistance than monolithic lithium disilicate crowns. 3he composition of the phase changes in the UHT monolithic zirconia crown differs from the LT monolithic zirconia crown.The yttria (Y 2 O 3 ) stabilizer content in UHT monolithic zirconia is around 9.42 wt%, while in the LT it is around 5.15 wt%, which makes this zirconia contain cubic phase (c-ZrO 2 ) resulting in lower alumina content. 21,22The higher content of the cubic phase makes the UHT monolithic zirconia crown more stable hydrothermally because the zirconia grains in the cubic phase (c-ZrO 2 ) do not change to the monoclinic zirconia (m-ZrO 2 ) phase at room temperature.The reduced transformation of the zirconia phase from the tetragonal phase to the monoclinic phase will undoubtedly reduce the mechanical properties of UHT monolithic zirconia because this transformation increases the strength of the zirconia.Thus, the higher cubic phase content in UHT monolithic zirconia will affect the decreased mechanical properties, especially in strength and fracture resistance. 23ne method for increasing the mechanical strength of ceramics is to add alumina.Alumina is a rigid and opaquecolored material that is less susceptible to cracking when compared with ceramics.The mechanism of increasing strength with alumina is that alumina will act as a "crack stopper" that can prevent the spread of cracks to all parts of the restoration.The high modulus of elasticity of alumina is 350 GPA which will ensure that the interface between the ceramic surface and alumina particles is free from stress or tension, so it does not trigger the spread of cracks around alumina particles. 24Lower alumina content in the UHT monolithic zirconia makes the mechanical properties of fracture resistance lower than the crown of LT in this study.
The microstructural analysis also revealed that the crown grain size of the UL and UHT monolithic zirconia were distributed differently.The UHT monolithic zirconia crown contains larger grain sizes because the grain size in the cubic phase was greater than the grain size in the tetragonal phase.In addition, the temperature in the sintering process will also affect the size of the zirconia grains. 25Higher temperatures in the sintering process will result in larger grain sizes.In manufacturing, the UHT monolithic zirconia was sintered at 1550°C, while the LT crown was sintered at 1450°C.However, zirconia grain sizes produced by different zirconia brands sintered at the same temperature will make different grain sizes. 21In principle, the increase of the crown's translucence will decrease its strength.The limitation of this study was that the test was not conditioned in the oral environment.
Future research needs to be performed to evaluate its strength or fracture resistance in different types of anterior and posterior teeth.

Conclusion
The LT monolithic zirconia has higher strength than UHT monolithic zirconia and can be used for posterior crown restoration.

Ethical Approval
Not requiredthe study did not involve human subjects.

Fig. 1 (
Fig.1(A) The model of toot was created from metal, (B) and the model of preparation with a thickness of 1 mm and Chamfer shape was chosen for cervical margin preparation.(C,D) Then, a scanning model was prepared for zirconia crown casting.

Fig. 2 (
Fig. 2 (A) Low-translucent monolithic zirconia (LT) and ultra-high translucent monolithic zirconia (UHT) zirconia crown model of fracture resistance value (B), and the fracture resistance difference between the two types of crown.