e.max® Production Methods
Senior Team Leader Karli Luksch introduces the two ways e.max® (lithium disilicate) restorations are produced and how they are different.
e.max® restorations have become extremely popular in the last several years and in this video, I’m going to talk about the two production methods for e.max® and how they differ.
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 and instead can be built as a full contour monolithic restoration.
The two methods for creating an e.max® restoration are pressing and milling. The pressing method involves waxing up the restoration, investing and burning out the wax, and then pressing the superheated lithium disilicate in to the negative space left from the burned-out wax
To get the full benefit of lithium disilicate, it needs to be pressed instead of milled. In fact, in the original patent for lithium disilicate it was argued that pressing is the desirable method.
According to a 2010 document published by Ivoclar Vivadent®, pressed lithium disilicate has a fracture toughness of 2.75 MPa and a flexural strength of 400 MPa.
Compare this to milled lithium disilicate which has a fracture toughness of 2.25 MPa and a flexural strength of 360 MPa.
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.
The fracture toughness is based on the size and formation of the crystals. The reason that milled e.max® has a lower fracture toughness is because 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 crystalized. Lithium disilicate that is created for the purpose of pressing is fully crystalized at the factory which results in these 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 crystalized 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 which creates a material called lithium meta silicate (Li2SiO3). Lithium meta silicate is blue in color because the shade characteristics are only gained after crystallization.
Lithium meta silicate 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.
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 uses diamond burrs to wear down the material in to the desired shape. The biggest issues with grinding are the uneven milling pattern from the diamond particles and the fact that 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 micro cracks with depths of 40-60 microns when using the grinding method of milling.
After the milling is complete, the lithium meta silicate can go through its final crystallization cycle. The resulting lithium disilicate is very different than its pressed counterpart which has to do with the shorter crystals that are formed when using the two-step crystallization method. These short crystals are the reason for the drop in both flexural strength and fracture toughness.
At O’Brien, it’s our goal to always give our customers the very best product we can which is why we only use the pressing method for fabricating our e.max® restorations.