Pros and Cons of IPS e.max® Lithium Disilicate Glass-Ceramics for Dental Restorations

For more than a decade, clinical trials have recorded high chipping rates in porcelain fused to zirconia (PFZ) dental restorations because of the relatively weak bond between the porcelain veneer and strong zirconia core. Porcelain fused to metal (PFM) dental restorations experience veneer chipping as well, however, clinical evidence has shown they perform better than PFZ restorations [1]. The major problem with PFM restorations is the metal substructure does not provide the most appealing esthetics. Therefore, researchers and clinicians developed monolithic all-ceramic dental restorations in an attempt to resolve these issues. One such monolithic all-ceramic dental restoration was developed by Ivoclar-Vivadent called IPS e.max^® lithium disilicate.

When considering U.S. dental patient’s high demand for esthetically driven restorations, a dental professional must make informed decisions about the quality, longevity and esthetics of a dental restoration in order to have a high acceptance rate among their patients. Thus, the dental practice will be more successful by means of new and retained patients. This article will briefly analyze the pros and cons of lithium disilicate glass-ceramics.

Microstructure of Lithium Disilicate (LS₂)

Dental ceramics can be classified by their microstructure (i.e., composition of glass-to-crystalline ratio) [2]. Lithium Disilicate (LS₂) has a glass-based system (mainly silica) with crystalline fillers, formed by the controlled crystallisation of the glass material [2]. The crystalline filler particles are added to increase the strength of the glass-ceramic; cracks caused by stress are “arrested by the lithium disilicate crystals, providing a substantial increase in the flexural strength” [2].

Pros

One pro to lithium Disilicate glass-ceramic restorations is an observed low chipping and fracture rate [1]. Lithium disilicate has an unusual microstructure that contributes to its relatively high flexural strength of 360-400MPa. The spread of cracks is deflected, branched and blunted by lithium disilicate needle-like crystal structure – providing a substantial increase in the flexural strength [2]. In a 2015 study, Valenti evaluated the clinical performance of 110 lithium disilicate crowns over a 9-year period and found 2 crowns had core fractures and 1 crown chipped. They found the “overall survival probability was 96.1% up to 9 years, with a failure rate of 1.8%” [3].

Six other clinical studies (Richter et al. 2009; Nathanson 2008; Reich et al. 2010; Fasbinder et al. 2010; Bind 2011; Sorensen et al. 2009b) showed 97.9% of 237 IPS e.max ^® lithium d­­isilicate (CAD crowns) restorations survived after a mean observation period of 4 years. The 2.1% failure rate included 0.4% irreparable chipping, 0.4% secondary caries and 1.3% fractures. Glass-ceramics exhibit lower strength relative to Porcelain fused to zirconia but higher chip fracture resistance. High chip fracture resistance is due to an elongated grain structure that inhibits crack propagation and contributes to its toughness, while it limits the strength by “promoting crack initiation within a relatively coarse microstructure” [1].

Another pro is lithium disilicate has superior esthetics compared to other ceramic materials. The monolithic structure and glass-ceramic system of lithium disilicate allows the material to take on translucent and light diffusing properties. Natural enamel has translucent qualities that transmits light rather than reflect or scatter it. The optical compatibility between the glassy matrix and the crystalline phase minimizes internal scattering of the light as it passes through the material—high translucency depends on the transmission of light through the core material [2]. Sravanthi et al. study argues that the “core translucency therefore becomes one of the primary factors in controlling aesthetics and is a critical consideration in selection of materials.” The study used a reflectance spectrophotometer (records the amount of light reflected) to compare the translucency of crowns fabricated with three different commercially available all-ceramic materials including alumina - CAD-CAM Procera, Lithium disilicate - Pressable IPS e.max Press, Zirconia - CAD-CAM Lava [4]. Out of all three ceramic materials, lithium disilicate was found to have the maximum translucency even when the coping was at a 0.8mm thickness.

Lastly, glass-ceramics can be “readily etched and silanized at the intaglio surfaces, with attendant superior bonding to a support substrate” [1]. In other words, lithium disilicate has the ability to be easily etched or milled out of a homogenous material by computer-aided-design/computer-aided-manufacturing (CAD/CAM) digital systems. This technology assures acute accuracy of the fabrication of the designed restoration. The silanization of the restoration’s surface helps increase the hydrophobicity of the glass-ceramic.  

Cons

Although lithium disilicate crown applications have a high survival and success rate, they may be “less suitable for applications where stress concentrations can be high, e.g. FDP [fixed dental prosthesis] connectors” [1]. The 360-400MPa flexural strength of lithium disilicate is relatively weak compared to the 1,000MPa flexural strength of zirconia-based restorations. For this reason, FDPs with frameworks made of glass-ceramics require “larger connector areas to meet load-bearing requirements relative to their stronger zirconia counterparts” [1]. Whereas, lithium disilicate single crowns are supported by underlying dentin that exhibits greater strength and stability then lithium disilicate multiple-unit restorations. However, a 10-year study evaluated the long-term outcome of three-unit FDPs made from monolithic lithium disilicate ceramic (IPS e.max Press, Ivoclar Vivadent) and found that the survival and success rates were similar to those of conventional metal-ceramic FDPs [7].

Next, one study analyzed the friction and wear behavior of human enamel that opposed lithium disilicate restorations and found evidence of abrasive wear behavior of the opposing enamel [5]. Lee et al. found the “enamel that opposed lithium disilicate glass ceramic exhibited cracks, plow furrows, and surface loss, which indicated abrasive wear as the prominent wear mechanism.” It’s worth noting that adequate surface finishing of dental ceramics influences wear on opposing enamel and requires competent skills of the dental laboratory to achieve an acceptable polished surface. A rough surface will abrade the opposing enamel of a restoration and “it is highly recommended that the surface is finished and polished appropriately” [6].  

To conclude, the flexural strength of previous glass-ceramic materials was relatively weak until the development of highly esthetic and strong lithium disilicate glass-ceramics. The highly specialized monolithic structure of IPS e.max ^® lithium d­­isilicate by Ivoclar Vivadent can achieve great structural integrity with superior translucent esthetics compared to other all-ceramic materials. For this reason, Lithium disilicate is a viable option for dental patients who are seeking longevity and beautiful esthetics for their dental restoration.

About the Company:

Since 1991, Iverson Dental laboratory has provided dentists with high quality dental lab products to ensure their patients are completely satisfied with the restoration outcome. Iverson’s highly knowledgeable team of certified dental technicians specialize in all-ceramic restorations, dental implants, cosmetic and digital dentistry. Iverson dental labs strongly believe in using high quality certified materials and authentic manufacturing components to fabricate their dental restorations.

To inquire more about all-ceramic dental lab products, please visit http://www.iversondental-labs.com.

Reference:

1. Zhang Y, Chai H, Lee JJ, Lawn BR. Chipping resistance of graded zirconia ceramics for dental crowns. J Dent Res. 2012;91:311–5. [PMC free article] [PubMed]

2. Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent. 2010;13:195–203. [PMC free article] [PubMed]

3. Valenti M, Valenti A. Retrospective survival analysis of 110 lithium disilicate crowns with feather-edge marginal preparation. Int J Esthet Dent. 2015 Summer;10(2):246-57.

4. Sravanthi, Y., Ramani, Y. V., Rathod, A. M., Ram, S. M., & Turakhia, H. (2015). The Comparative Evaluation of the Translucency of Crowns Fabricated with Three Different All-Ceramic Materials: An in Vitro Study. Journal of Clinical and Diagnostic Research : JCDR, 9(2), ZC30–ZC34. doi:10.7860/JCDR/2015/12069.5559

5. Lee, A., Swain, M., He, L., & Lyons, K. (n.d.). Wear behavior of human enamel against lithium disilicate glass ceramic and type III gold. The Journal of Prosthetic Dentistry, 1399-1405.

6. Rashid, H. (n.d.). The effect of surface roughness on ceramics used in dentistry: A review of literature. European Journal of Dentistry Eur J Dent, 571-571.

 

7. Kern, M., Sasse, M., & Wolfart, S. (n.d.). Ten-year outcome of three-unit fixed dental prostheses made from monolithic lithium disilicate ceramic. The Journal of the American Dental Association, 234-240. 

Pros and Cons of Porcelain Fused to Zirconia (PFZ) Dental Restorations

In the last few decades, there has been an increasingly huge demand for highly esthetic dental restorations among U.S. dental patients. New developments and advances in all-ceramic dental materials have replaced metal-ceramic systems, such as porcelain-fused-to-metal (PFM) prostheses, with all-ceramic systems. Advancements in bonding techniques resulted in the development of porcelain fused to zirconia (PFZ) restorations, finding superior esthetics compared with PFM restorations.

Metal-free all-ceramic restorations offer better esthetics and biocompatibility than porcelain fused to metal (PFM) prostheses [1]. Although PFZ prostheses offer better esthetics than metal ceramic systems, dentists must evaluate the pros and cons of PFZ prostheses in order to make an informed decision on providing the best dental lab products to their patients. This article will briefly analyze the cons and pros of porcelain fused to zirconia restorations.

Microstructure of PFZ Restorations

Dental ceramics can be classified by their microstructure (i.e., amount and type of crystalline phase and glass composition)[1]. Zirconia oxide (ZrO₂) has a polycrystalline solid-based composition, formed by “directly sintering crystals together without any intervening matrix to from a dense, air-free, glass-free, polycrystalline structure”[1]. Solid-sintered zirconia-oxide ceramic is widely used as a framework material for dental implants, dental crowns, fixed-partial dentures, and other dental lab products.

Conventional dental porcelain is a partially glassy material that contains high amounts of leucite crystals added to aluminum oxide. This pressed-glass ceramic undergoes “dispersion strengthening through the guided crystallization of leucite”[1]. Porcelain is widely used for veneer layering onto a zirconia core (PFZ) or a metal core (PFM).   

Cons of PFZ Restorations

To begin with, clinical research and practices have reported high occurrences of veneer chipping and fractures in all major brands of PFZ systems, especially in posterior prostheses [2]. The porcelain overlay can chip during mastication, most notably at the coping level were thickness is a factor that influences the survival and success of the restoration [3]. According to Agustin et al., porcelain veneer chipping most often occurs as an esthetic defect without affecting the survival of the restoration and is easily repaired by polishing or intraoral repair; it often goes unnoticed by the patient. For this reason, Agustin et al. five-year study reported the survival rates of zirconia-based fixed dental prostheses to be 97-99%.

Although the survival rate is favorable, clinical studies have revealed a high rate of fractures for porcelain-veneered zirconia-based restorations that affect the success of the restoration—varying between 6% and 15% over a 3- to 5-year period. The success of the restoration is dependent on whether the restoration’s esthetics is compromised or not. Agustin et al. believes the reason for the high rate of fractures is uncertain but suspects the bond between the porcelain and the zirconia substructure failed. One likely reason could be the porcelain veneer has a weak flexural strength of 90MPa, contrasted to zirconia’s high flexural strength of 900–1200 MPa [2]. Another likely reason was found in a Baldassarri et al. 2012 clinical study, “the presence of a radial tensile stress in the overlay porcelain of zirconia-ceramic prostheses… may lead to the large clinical chips and fractures of these prostheses.”

Another con to porcelain-veneered zirconia-based restorations is a reported high wear loss of antagonist enamel because porcelain consists of leucite crystal grains that act as an abrasive surface during mastication [4]. Porcelain also consists of feldspathic glass that disappears after wear, leaving large leucite grains to be exposed and act as abrasive materials on the opposing enamel [5]. In fact, Porcelain-veneered zirconia shows higher wear loss on opposing enamel than that of polished zirconia. Rashid reviewed an Odatsu et al. study that used carborundum points, silicone points and diamond polishing paste on zirconia and traditional feldpsathic porcelain to examine the materials surface roughness. They found the “feldspathic porcelain showed highest surface roughness values after finishing and polishing procedures” [7]. They concluded, “a rough surface will abrade the opposing dentition or restoration, and it is highly recommended that the adjusted surface is finished and polished appropriately” [7].

Pros of PFZ Restorations

Despite the low flexural strength of the layered porcelain for PFZ restorations, superior fracture resistance of the zirconia ceramic core is a major pro. According to Miyazaki et al., in a study assessing zirconia-based restorations found the zirconia frameworks rarely got damaged and the complications that did occur were at the veneering ceramic level. In a 10-year clinical study, Sax et al. assessed the long-term survival rate of zirconia-based posterior fixed dental prostheses with zirconia frameworks and found the “zirconia frameworks exhibited very good long-term stability” [6].

In 2012, Agustin et al. analyzed porcelain veneer behavior on zirconia ceramic cores using a scanning electron microscopy (SEM) and found 71.66% of the facture type for zirconia-core restorations were cohesive – meaning the facture only occurred at the surface level without affecting the ceramic-core interface. In another study with the same objective, Saito reported 88.8% of fracture types were cohesive. Since most of the fractures are cohesive in nature, the veneered porcelain can be easily repaired without much concern the ceramic core is fractured [7].

Finally, PFZ restorations have the ability to mask discolored underlying tooth structures because zirconia has opaque characteristics that allow the prepared tooth to be concealed. Since the zirconia core is highly opaque, a porcelain overlay is needed to improve the esthetics of the zirconia core to appear more translucent, more similar to natural teeth. The dual layers of porcelain and zirconia from PFZ restorations allow the prosthetic to hide any discoloration and/or defects from the prepared tooth, making it a top option for patients who may have these issues.

Some studies, particularly Agustin et al., have concluded the survival rate is not dependent on esthetic defects but on the functional survival of the restoration. But with an ever-growing society that demands perfection in esthetics for dental restorations, functionality is not the only characteristic desired for dental restorations by the majority. Although the 1,000 MPa zirconia substructure exhibits good long-term stability, the failure of the layering porcelain can be seen as ultimately a failure of the restoration. After much clinical research, it is safe to say the porcelain layer on PFZ restorations is not reliable for the long-term success of esthetics. Consequently, PFZ restorations are at the risk of failing as an acceptable restoration among U.S. dental patients.

Resource Box:

Iverson Dental Laboratories is a cutting edge dental lab that utilizes advances in dental technology and procedures to fabricate high quality dental lab products. Their highly knowledgeable team of certified dental technicians specialize in all-ceramic restorations, dental implants, cosmetic and digital dentistry. Iverson strongly believes in using high quality certified materials and authentic manufacturing components to fabricate all their dental restorations. They proudly make 100% of their dental lab products at their Southern California dental labs, allowing them to have one of the fastest turnaround rates in the industry.

To learn more about quality options for dental lab products, please visit http://www.iversondental-labs.com.

References:

1. Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent. 2010;13:195–203. [PMC free article] [PubMed]

2. Baldassarri M, Stappert CF, Wolff MS, Thompson VP, Zhang Y. Residual stresses in porcelain-veneered zirconia prostheses. Dent Mater. 2012;28:873–9. [PMC free article] [PubMed]

3. Triwatana P, Nagaviroj N, Tulapornchai C. Clinical performance and failures of  zirconia-based fixed partial dentures: a review literature. J Adv Prosthodont. 2012;4:76–83. [PMC free article] [PubMed]

4. Agustin-Panadero R, Román-Rodríguez JL, Ferreiroa A, Solá-Ruíz MF, Fons-Font A. Zirconia in fixed prosthesis. A literature review. J Clin Exp Dent. 2014;6:e66–73. [PMC free article] [PubMed]

5. Miyazaki, T., Nakamura, T., Matsumura, H., Ban, S., & Kobayashi, T. (n.d.). Current status of zirconia restoration. Journal of Prosthodontic Research, 236-261.

 

6. Sax C, Hammerle CH, Sailer I. 10-year clinical outcomes of fixed dental prostheses with zirconia frameworks. Int J Comput Dent. 2011;14:183–202. [PubMed]

 

7. Saito A, Komine F, Blatz M, Matsumura H. A comparison of bond strength of layered veneering porcelains to zirconia and metal. J Prosthet Dent. 2010;104:247–57. [PubMed]

7. Rashid, H. (n.d.). The effect of surface roughness on ceramics used in dentistry: A review of literature. European Journal of Dentistry Eur J Dent, 571-571.

How Do Dental Implants Work?

Dental implants are considered to be the best long-term solution for missing teeth. Though dental bridges are a cheaper alternative, over time a dental implant is more cost-effective, as it can last decades. A patient can treat an implant as they would their real teeth, remembering that brushing and flossing is just as important! Dental implants look, feel and function like real teeth.

A dental implant, usually made of titanium, takes the place of a missing tooth’s original root, connecting to the bone. They help to maintain the jaw bone’s shape and density. It can take two to six months for the bone to fuse around the implant. Once healing is complete, an abutment is attached to it. This is the device that joins the implant to a crown, which replaces the actual missing tooth.

Crowns are custom-made at a dental laboratory, like Iverson, to match your existing teeth. The crown is cemented or screwed onto the abutment to permanently keep it in place. Once the dental implant procedure is complete, the crown should be indistinguishable from the rest of your teeth.

For fixed multiple tooth replacement, custom-made crowns or bridgework can be attached to the abutments to replace missing teeth without disturbing the healthy teeth next to them, and while stopping bone loss in the jaw and maintaining facial features.

If you are missing all of either your lower or upper teeth (or just all of your teeth), there’s also the option of getting a removable implant-supported tooth replacement. Two to six implants may be used to support a lower denture, depending on the design of the removable replacement. On the other hand, a minimum of four implants are required to support an upper denture. Removable dentures are usually used to replace extensive tooth, bone, and gum-tissue loss, providing support for the facial skeleton, lips, and cheeks. An advantage of a removable denture is making the cleaning of the dental implants significantly easier.

Over time, dental implants have become much more efficient and affordable. They’re very widely used, as roughly 70% of adults in America are missing at least one tooth. If you think you may be a good candidate for dental implants, speak to an experienced dentist to review your options. If you don’t have enough existing bone in your jaw or you’ve had radiation to your jaw, possibly for treating cancer or something similar, dental implants may not be the best option for you. Also, if you’re a smoker, your dentist will probably ask you to quit, as smoking can increase infection and rejection risks for dental implants.