e.max® is a trade name for lithium disilicate (Li2Si2O5), which is a glass ceramic that is both strong and esthetic. e.max® is so esthetic, in fact, that it doesn’t require veneering ceramic. Instead, it can be built as a full-contour monolithic restoration.
It’s for these reasons that e.max® restorations have become extremely popular in the last several years.
Monolithic restoration cross-section
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The pressing method involves waxing up the restoration, investing and burning out the wax, and then pressing the superheated lithium disilicate into the negative space left from the burned-out wax. The original patent for lithium disilicate stated that pressing was the desirable production method in order to get the full benefits of the material.
Pressing superheated Lithium disilicate
When we talk about milling, it’s important to point out that there are actually two different methods of milling: cutting and grinding. Cutting is the more precise method and uses burrs that are designed to slice away at the material in a very predictable fashion.
Grinding, on the other hand, uses diamond burrs to wear down the material into the desired shape. The biggest issues with grinding are the uneven milling pattern from the diamond particles. Plus, as the diamond burr wears down, the accuracy of the milling is reduced.
A study of dental CAD/CAM systems which was published in the Journal of Dentistry reported findings of microcracks with depths of 40-60 microns when using the grinding method of milling.
Grinding burs can lead to micro-fractures on the restoration
Cutting burs are preferred for milling
One of the benefits of lithium disilicate is its fracture toughness. Fracture toughness is the ability of a material to resist surface fractures. Since a surface fracture will continue to propagate through the material, the goal is to avoid them in the first place. That’s why a higher fracture toughness is important for the occlusal surface.
According to Ivoclar Vivadent® in a document published in 2010, pressed lithium disilicate has a fracture toughness of 2.75 MPa and a flexural strength of 400 MPa. On the other hand, milled disilicate which has a fracture toughness of 2.25 MPa and a flexural strength of 360 MPa.
Pressed e-max crystals (simulated)
Milled e-max crystals (simulated)
Fracture toughness is based on the size and formation of the crystals. The reason that milled e.max® has a lower fracture toughness is that the crystals are smaller and, therefore, less resistant to fracture.
The reason for the difference in crystal size has to do with how and when the material is crystallized. Lithium disilicate that is created for the purpose of pressing is fully crystallized at the factory, resulting in long crystals. These long crystals are what give lithium disilicate its high flexural strength and fracture toughness.
They are also the reason why lithium disilicate can’t be milled in its fully crystallized state – it’s too hard and would cause grinding burrs to wear down too quickly. To make e.max® millable, the lithium disilicate is only partially crystallized at the factory.
Lithium metasilicate is a lot softer than lithium disilicate and is rated at about 130MPA of flexural strength. This allows the product to be milled with diamond grinding burrs.
After the milling is complete, the lithium metasilicate can go through its final crystallization cycle. The resulting lithium disilicate is very different than its pressed counterpart. This is due to the shorter crystals formed when using the two-step crystallization method.
These short crystals are the reason for the drop in both flexural strength and fracture toughness when using the milling methods of cutting and grinding.
At O’Brien, it’s our goal to always give our customers the very best product we can. This is why we only use the pressing method for fabricating our e.max® restorations.