Fracture toughness – and why it’s important for monolithic zirconium oxide.
To stay on top of things in the “jungle” of zirconium oxide types
and to select the right indication, it’s important to gain an
understanding of the mechanical parameters. In particular it’s worth taking a closer look at the highly translucent zirconium oxides for veneer-free monolithic restorations.
MPa – Flexural strength
The flexural strength, expressed in megapascals (MPa), is an important criterion in the data sheet and it’s often the focus of the dental technician when selecting a resilient material. Flexural strength describes how much force is needed to deform and ultimately break a specimen with a defined diameter. High values in this discipline are essential for multi-unit restorations. For bridges larger than four elements, a value of 800 MPa must be achieved in a test according to the essential standard for dental ceramics (DIN EN ISO 6872).
In addition to initial strength, fracture toughness (crack toughness) should always be taken into account.
KIC – Fracture toughness
Fracture toughness describes the resistance of a material to crack propagation. The higher the value of the critical stress intensity factor (KIC [MPa√m]), the better the prognosis for the long-term clinical behavior of a material. The ideal combination of flexural strength and fracture toughness is based on a material’s ability to transform its crystalline phase at stresses to stop the progression of a crack and close the crack itself.
DD cube ONE® is the only cubic, monolithic zirconia with fracture protection factor
The higher the KIC, the more robust the ceramic
So the KIC value provides information on how robustly an all-ceramic behaves, e.g. during processing in the laboratory or during insertion by the dentist.
The classic, high-strength 3Y TZP zirconium oxide grades generally possess a high fracture toughness (>5 MPa√m)—an essential factor for their clinical success. The super high translucents (SHT) 5Y TZP have a lower initial KIC, which is recommended for 3-unit molar bridges with at least >3 MPa√m . In this way, the crystalline structure of the 5Y TZPs offers little to no potential for phase transformation and therefore for strengthening.
DD cube ONE® is the ideal choice for long-span bridges with high translucency and reliable robustness!
Since 2010, more than 16 million tooth elements have been made from our proven, high-strength 3Y-TZP zirconium oxide
types DD Bio Z and DD Bio ZX2. Thanks to the documented clinical success with these types, our developers have defined a fracture toughness of KIC > 9 MPa√m for the fracture protection factor.
And don’t forget: The aging behavior
The property of a material for phase transformation strengthening is also relevant for the aging behavior of a ceramic. The long-term behavior of the material is analyzed in simulated chewing load tests, which are more practical than standardized material tests. The chewing simulation is a test procedure that allows reliable conclusions to be drawn about the behavior of the material in its clinical application. This simulates the typical aging process of a dental restoration in an artificial oral environment. The influence of moisture and thermal (hydrothermal) alternating stress under mechanical chewing load subjects the crowns to realistic demands.
This test setup simulates the mechanical and thermal stresses to which a dental restoration in the patient's mouth is exposed for a period of 5 years.
In this simulation, eight crowns of the same type were destroyed respectively before and after chewing. The required force to failure is measured in N-Newton. With a high-strength 3Y TZP (DD Bio ZX2) and the new 4Y TZP (DD cubeONE® ML) with increased translucency, a significant Newton increase is evident. The mechanical stress of chewing therefore leads to a phase transformation, which stops the cracks and closes any residual porosity in the ceramic. This increase in stability cannot be observed with the 5Y TZP (DD cubeX2® ML). Although the DD cubeX2® zirconium oxide with a specified flexural strength of 750–800 MPa already comes within the range of the required 800 MPa for the ISO 6872 indication “large bridges”, it is limited to use for 3-unit bridges due to its lower fracture toughness (KIC >4 MPa√m SEVNB). In this context, the high initial values offer sufficient stability (>1000 N) even after the aging process, for example to withstand the maximum force of someone with bruxism.
(KIC*) Conclusion: Anyone who wants to use an aesthetic and safe bridge material should consider both the bending strength and the fracture toughness.
DD cube ONE® ML (4Y TZP) has a hybrid crystal structure of about 30% cubic and 70% tetragonal crystals. The KIC is ≈10 MPa√m SEVNB. Aluminum oxide (Al2O3) was added in a balanced concentration to positively influence the aging behavior. The cubic, large crystals are responsible for increasing translucency. The KIC value of fracture toughness is just as high in the incisal area of the multilayer variant as it is in the rest of the body. This should provide the best possible security even when corrections are made to the occlusal surfaces.