The Science Behind Lithium Disilicate: A Metal-Free Alternative

Dentists and laboratory technicians today require materials that offer outstanding aesthetics, high strength, and efficient productivity. Historically, dentistry also has always been faced with the challenge of finding ways to combine 2 incompatible materials in a synergistic way, whether that combination is metal with metal-ceramic or zirconia with zirconia layering ceramic. Simultaneously, dental practices and dental laboratories are increasingly seeking ways to realize the wonderful op-portunities presented by CAD/CAM and digital fabrication techniques that offer the benefits of consistency in the production of a restoration and expanded material options. To satisfy these requirements, lithium disilicate glass ceramic represents a unique material choice available to both doctors and laboratories. What’s more, lithium disilicate has the present and future potential to provide new options for improving patient care.

For the dentist, lithium disilicate is a highly aesthetic, high-strength material that can be conventionally cemented or adhesively bonded.1 In terms of dealing with dissimilar material combinations, lithium disilicate offers a unique solution with its ability to offer a full contour restoration fabricated from one high-strength ceramic, thereby eliminating the challenge of managing 2 dissimilar materials. Lithium disilicate is a material that can be used in all areas of the mouth, when specific considerations are taken into account. For laboratory ceramists, the versatility and performance of lithium disilicate enables them to optimize their productivity when fabricating restorations using this material, since either lost-wax pressing or CAD/CAM milling fabrication techniques can be used.
Glass ceramics are categorized according to their major crystalline structure and/or application.2 Lithium disilicate is among the best known and most widely used types of glass ceramics. IPS e.max (Ivoclar Vivadent) lithium disilicate, for example, is composed of quartz, lithium dioxide, phosphor oxide, alumina, potassium oxide, and other components (Figure 1). Overall, this composition yields a highly thermal shock resistant glass ceramic due to the low thermal expansion that results when it is processed. This type of resistant glass ceramic can be processed using either well-known lost-wax hot pressing techniques or state-of-the-art CAD/CAM milling procedures.

The pressable lithium disilicate (IPS e.max Press [Ivoclar Vivadent])) is produced according to a unique bulk casting production process in order to create the ingots. This involves a continuous manufacturing process based on glass technology (melting, cooling, simultaneous nucleation of 2 different crystals, and growth of crystals) that is constantly optimized in order to prevent the formation of defects (eg, pores, pigments). The microstructure of the pressable lithium disilicate material consists of approximately 70% needle-like lithium disilicate crystals that are embedded in a glassy matrix. These crystals measure approximately 3 to 6 µm in length.

Polyvalent ions that are dissolved in the glass are utilized to provide the desired color to the lithium disilicate material. These color-controlling ions are homogeneously distributed in the single-phase material, thereby eliminating color-pigment imperfections in the microstructure.

Machineable lithium disilicate blocks are manufactured according to a similar process, but only an “intermediate” crystallization is achieved in order to ensure that the blocks can be milled efficiently in a crystalline intermediate phase (blue, translucent state). The intermediate crystallization process leads to the formation of lithium metasilicate crystals, which are responsible for the material’s processing properties, machineability, and good edge stability. It is after the milling procedure and the restorations are fired that they reach their final crystallized state and their high strength. The microstructure of intermediate crystallized IPS e.max CAD lithium disilicate consists of 40% platelet-shaped lithium metasilicate crystals embedded in a glassy phase (Figure 2). These crystals range in length from 0.2 to 1.0 µm. Postcrystallization microstructure of IPS e.max CAD lithium disilicate material consists of 70% fine-grain lithium disilicate crystals embedded in a glassy matrix.

Similar to the pressable lithium disilicate, the millable IPS e.max CAD blocks are colored using coloring ions. However, the coloring elements are in a different oxidation state during the intermediate phase than in the fully crystallized state. As a result, the lithium disilicate exhibits a blue color. The material achieves its desired tooth color and opacity when the lithium metasilicate is transformed into lithium disilicate during the postmilling firing process.