IPS e.max Lithium Disilicate Glass Ceramic
For more than thirty years, metal ceramic restorations have been considered the restorative materials of choice in dentistry. Their good mechanical properties met the requirements of dental clinicians, while their reasonable esthetics satisfied the tastes of patients.
However, with patients’ increasing interest in life-looking restorations instead of artificial ones, and with the growing concerns regarding the biocompatibility of the metal, researchers have been on the run to find novel restorative materials.
The quest to find a material that transmits and refracts light like a natural tooth has inspired research into all-ceramic restorations. In the last two decades, a number of all-ceramic materials have been available for dental restorations. Among these materials, Lithium Disilicate Glass Ceramic is considered one of the most popular due to its high strength, good color stability, high resistance to wear, and high biocompatibility.
Lithium Disilicate excellent esthetics and good biomechanical properties make it suitable for the fabrication of monolithic restorations and veneered restorations in the anterior and posterior region.
Composition of Lithium Disilicate Glass Ceramics:
Lithium disilicate glass ceramic, particularly Li2O-SiO2 system, is the first material classified as glass ceramic. Discovered by Stookey in the fifties, glass ceramics have proved to have better mechanical properties over base glass.– Since then, the binary Li2O-SiO2 system have been extensively studied and applied in dental restorations.
However, studies have shown that the binary Li2O-SiO2 system has poor chemical durability, inadequate translucency and uncontrolled microcrack formation. Therefore, researchers started to opt for multicomponent glass ceramic, notably adding components such as Al2O3 and K2O to lithium silicate glasses which have been proved to promote the thermo-mechanical properties of the final material.
Several other constituents like ZnO, ZrO2 and P2O5 were also introduced to further improve the properties of glass ceramic. For example, the addition P2O5 can promote the nucleation and crystallization processes in lithium disilicate glasses. For that reason, the adjunction of phosphorus oxide produces fine-interlocking microstructure and therefore resulting in high mechanical strength of the final material.
Manufacturing & processing of IPS e.max lithium disilicate:
In 2005, Ivoclar Vivadent introduced the IPS e.max lithium disilicate glass ceramic. 11 years after its release, this all-ceramic material has been used in more than 100 million restorations with more than 96 percent success rate.
The IPS e.max lithium disilicate is composed of quartz (SiO2), lithium dioxide (Li2O), phosphorus oxide (P2O5), alumina (Al2O3), potassium oxide (K2O), and other components. These powders are combined to produce a glass melt which is then poured in molds to cool down to room temperature. The partially crystallized glass ingots, containing lithium metasilicate (Li2SiO3) and lithium disilicate (Li2Si2O5) crystal nuclei, are then processed differently depending on each technique.
The IPS e.max CAD (Computer aided design) “blue block” is milled and shaped by a computer. The milling procedure doesn’t engender excessive bur wear or chipping of the material, and that is due to the presence of the intermediate lithium meta-silicate crystal structure (Li2SiO3).
A post-milling heat treatment of the partially crystalized fabricated ceramic restoration at approximately 840-850ºC dissolves completely the metasilicate phase and crystallizes the lithium disilicate phase. This heat treatment allows the fabricated restoration to achieve its full density and strength.
For lithium disilicate ceramics, the final crystallization is achieved after only 25 minutes of heat treatment, allowing faster delivery of restorations. In contrast, a zirconia core machined by CAD/CAM can require up to 8 hours of post-milling processing time.
Throughout the heating process, the initially bluish color of the glass ingot changes to a tooth-like shade. In contrary to alumina- or zirconia-based ceramic cores, lithium disilicate ceramic doesn’t require additional porcelain layering for esthetic improvement. A staining procedure is sufficient to give the lithium silicate-based restoration a realistic tooth-like appearance, allowing the initial block to be milled to the final contour.
The glass ingots can also be processed using the lost-wax hot pressing technique (IPS e.max Press). The ingots are processed similarly to the IPS e.max CAD, as they are composed of different powders that are melted and cooled to room temperatures. Following the glass formation, ingots are nucleated and crystallized in one heat treatment. These ingots are then pressed at approximately 920ºC for 5-15 minutes to form a 70% crystalline lithium disilicate restoration.
The crystals of both the IPS e.max Press and IPS e.max CAD are the same in composition (70% of crystalline lithium disilicate Li2Si2O5), but the size and length of these crystals are different. Hence why the material properties such as the coefficient of thermal expansion, modulus of elasticity, and chemical solubility are the same, while fracture strength and fracture toughness are slightly higher for the IPS e.max Press.
The flexural strength of dental restorative materials represents the capacity to tolerate chewing force. As shown in Figure 1, the flexural strength of IPS e.max CAD can reach 360 MPa, while it can go up to 400 MPa for IPS e.max Press. These values are twice greater than those of other ceramic materials that do not require any layering material.
Lithium disilicate glasses offer the same value of flexural strength through the entire restoration. As a result, restorations showcase a monolithic strength that can resist masticatory stress especially in the posterior region. The even distribution of stress without concentration sites is crucial in clinical outcomes. Stress concentration sites in ceramic restorations can result in surface flaws, porosities, and internal disintegration.
Figure 1: Comparison of the flexural strength of pressed ceramics.
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Besides strength testing, a chewing simulation testing performed on various restorative material for crowns (e.g leucite glass ceramic, metal ceramic, zirconia) to examine the nature of fatigue on these materials, showed that lithium disilicate demonstrates superior results.
From an esthetic standpoint, ceramics are the best when it comes to mimicking the natural tooth appearance. The optical behavior of ceramic materials differ from system to system and this should be taken into consideration during the selection of which system to be used. Lithium disilicate material is very versatile, as its availability in four translucencies allows its usage in different types of restorations (Veneers, Inlays/Onlays, anterior crowns, posterior crowns, etc.).
Wear resistance and compatibility are vital properties of all restorative materials. In some cases, the wear can concern the restorative material itself (e.g composites), while in case of lithium disilicate and other ceramic materials, the wear concerns more the enamel of the antagonist tooth. The wear of the opposing enamel by lithium disilicate is considered to be the lowest when compared to other ceramics and even enamel.
The biocompatibility of ceramic materials have been extensively studied. In 2008, Brackett, Wataha, and others examined the cytotoxicity response of lithium disilicate materials and concluded that: “In spite of the mitochondrial suppression caused by the lithium disilicate materials in the current study, these materials do not appear to be any more cytotoxic than other materials that are successfully used for dental restorations. The lithium disilicate materials were less cytotoxic than several commonly used composite materials and were comparable to cytotoxicity reported for several alloys and glass ionomers.”
Dental Applications of IPS e.max lithium disilicate glass ceramic:
The lithium disilicate (LS2) IPS e.max effectively merges esthetics and efficiency. The high-strength glass-ceramic can be applicated in a variety of clinical situations. The indication spectrum ranges from thin veneers (0.3 mm) and minimally invasive inlays and onlays to partial crowns, full crowns and implant superstructures. The material is also suitable for fabricating crowns, splinted crowns or 3 unit bridges up to the second premolar on top of implant abutments. In addition, three-unit bridges in both the anterior region and the premolar region can be produced. Lithium disilicate is also used to fabricate posterior bridges as long as it is supported by zirconium oxide.
The concept of modern dentistry is preserving as much saine tooth structure as possible. This modern vision has instructed clinicians to opt for minimally invasive restorations. IPS e.max lithium disilicate has enamel-like properties, hence offering durable solutions for restoring the function, esthetics and biomechanics of teeth when using conservative techniques. The combination of the high flexural strength, ideal fracture toughness and clinical longevity of IPS e.max lithium disilicate allow the fabrication of full-contour crowns of only 1 mm thickness, which then can be placed using the adhesive cementation method.
Posterior restorations withstand high stresses resulting from masticatory forces. The masticatory loading in posterior regions questions the durability of minimally invasive restorations in said regions. Several studies showed that the superior mechanical properties of Lithium disilicate make it suitable for the fabrication of posterior restorations.
Although the flexural strength of zirconia is 2.5 times higher than lithium disilicate glass ceramic, the relative load-bearing capacity changes when both of these materials are bonded to and backed by tooth structures.
When supported by dentin, the fracture load of zirconia restorations is about 1.8 times higher than lithium disilicate glass-ceramics. This ratio drops dramatically to only 1.3 times when the restorations are supported by enamel backed by dentin. This is noticeable when ceramic restorations thickness varies between 0.6 mm and 1.4 mm, implying that lithium disilicate can be convenient for use in conservative restorations in which the preparation can be confined to enamel.
IPS e.max lithium silicate glass ceramics offer a wide variety of translucency levels. Therefore, practitioners can camouflage dark tooth structure that results from stained teeth or titanium abutments. You can inform the laboratory about the shade of the tooth structure, and the dental technician will pick the IPS e.max lithium disilicate ingot with the suitable opacity level for maximum esthetic results.
Contraindications of IPS e.max lithium disilicate:
All-ceramic materials have a major drawback represented in their susceptibility to fatigue mechanisms, which can remarkably diminish their strength resulting in higher risks of fracture. The mastication forces can reach 250 N, while the forces due to clenching/grinding can reach up to 800 N.
The rehabilitation of heavily abraded occlusion on patients with parafunctional habits is a major challenge in restorative dentistry. Therefore, patients with increased chewing forces due to bruxism or other, can not benefit from all-ceramic crowns, including lithium disilicate ones. Special design features, such as cantilevers, maryland bridges, inlay retained
bridges are also considered to be a contraindication for all-ceramic restorations.
On their turn, wide spans (4 and more unit bridges) and posterior bridges reaching into the molar region are not recommended to be fabricated using all-ceramic systems.
Other contraindications of lithium disilicate IPS e.max can be deep subgingival preparations and failure to observe the necessary minimum connector dimensions and layer thicknesses.
Since its release, IPS e.max lithium disilicate glass ceramics showcased outstanding clinical performance. The Oceanic Dental Laboratory offers dental clinicians across Australia the chance to meet the expectations of their patients with e.max crowns, bridges and inlays. The perennity of IPS e.max lithium disilicate monolithic restorations can give credibility to your practice and, on the long run, help you generate organic leads. Until the release of a new all-ceramic system with a wider spectrum of applications, the lithium disilicate IPS e.max will offer a great chance for dental clinicians to match intraoral and esthetic requirements.
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