Lithium Disilicate: Masking Discolored Teeth

INTRODUCTION
Recent developments in ceramic material sciences have led to improvements in modern ceramics and restorative procedures.¹ These improvements have prompted a substantial increase in the clinical use of all-ceramic restorations.¹ Simultaneously, advances in chairside computer-aided design (CAD) and computer-aided manufacturing (CAM) techniques have enabled clinicians to offer restorations that are created faster and easier than ever before.¹ Whether in the office or in the dental laboratory, these new technologies and materials give clinicians in-office treatment options for even the most challenging cases.¹

A Brief Overview of Lithium Disilicate
Lithium disilicate is a ceramic material that contains approximately 70% by volume needle-like crystals in a glassy matrix.^2,3 The controlled size, shape, and density of this structure results in all-ceramic restorations that demonstrate greater strength and durability, with a relatively low refractive index.2-4 Such characteristics allow the material to display outstanding optical properties and optimal aesthetics.^2-4
Presently, lithium disilicate can be fabricated using 2 different processing techniques; it can be either pressed or milled.^3,5 Pressable lithium disilicate restorations are fabricated with a wax hot-press technique, while the milled CAD/CAM version is fabricated with either laboratory or chairside CAD/CAM technology.^3,5 Although both products are composed of the same material, the flexural strength of pressable lithium disilicate is slightly higher at 400 MPa, while the CAD/CAM milled monolithic material (once crystallized in the furnace) posseses a flexural strength of 360 MPa.^3,6 These strengths result from the difference in crystal size found in the 2 different forms of lithium disilicate; however, all other properties are similar.^3,6 Due to the presence of a glassy matrix and this all-ceramic’s excellent strength characteristics, dentists can choose to either conventionally cement or adhesively bond restorations, based on the clinical parameters in each individual case.^3,6

Figure 1. Preoperative view of a discolored tooth with an existing perforated gold onlay.	Figure 2. The appropriately shaded IPS Empress CAD block (Ivoclar Vivadent) was chosen, which closely corresponds to the final IPS e.max shade (Ivoclar Vivadent) after firing the blue block in the furnace.
Figure 3. The old restoration with any recurrent decay was removed.	Figure 4. Initial excavation showing depth cuts. Note the stained amalgam that presented in the canals of the tooth.
Figure 5. After the amalgam was excavated, the tooth was etched with phosphoric acid gel (Ultra-Etch [Ultradent Products]).	Figure 6. The bonding agent (ExciTE F VivaPen [Ivoclar Vivadent]) was used in preparation for a composite resin crown buildup.
Figure 7. The dual-cured resin buildup (LuxaCore [DMG America]) was placed under rubber dam isolation (Flexi Dam [Coltène/Whaledent]).	Figure 8. The buildup material was then light-cured.
Figure 9. View of the initial crown preparation.	Figure 10. View of the completed crown preparation.

CAD Lithium Disilicate
Certain techniques have been developed to maximize the ease of use and aesthetics of lithium disilicate (such as IPS e.max CAD [Ivoclar Vivadent]). CAD/CAM processing techniques allow the clinician and dental laboratory team to efficiently produce restorations stronger than those composed of conventional materials. IPS e.max CAD is indicated for the fabrication of veneers, inlays, and partial or full-crown restorations for individual teeth.⁷ Exhibiting an unusual blue-coloring in the block form, e.max CAD is machined in a soft, intermediate state.⁷ To ensure a proper fit, the material may also be manually cut back or adjusted in just a few quick and easy steps.⁷
It comes in 3 different levels of translucency and 2 sizes.7 High translucency blocks are ideal for minimally invasive, full-contour restorations, such as inlays, onlays, and veneers, and can be characterized with staining materials.7 Low translucency blocks were created for the anterior region, when partial and full crowns are required, allowing for a cut-back and layering technique (using IPS e.max Ceram [Ivoclar Vivadent]) for enhanced aesthetic results.⁷ Because of the material’s opalescence, high opacity blocks can be used to produce frameworks for vital and slightly discolored prepared teeth that are then veneered with e.max Ceram.⁷ The CAD blocks for fabricating full-contour restorations are available in 16 A to D shades and 4 BL (bleach) shades.⁷ When creating the tooth’s opacity, the e.max CAD medium opacity blocks are available in group shades and work exceptionally well when utilizing a layering technique.⁷

Chairside CAD/CAM Fabrication
One of the chairside CAD/CAM systems currently available (used to fabricate the clinical restoration demonstrated in this article) is the CEREC System (Sirona Dental Systems). It provides efficient and high-performance milling machines to save dentists and patients both time and expenses.⁸ The software is easy to use and intuitive and automatically adjusts the restorations into occlusion, and color coding ensures proximal contacts meet the dentist’s specifications before milling.⁸ By using CEREC software (the new 4.0 version was just released in September 2011) and 3-dimensional models, dental professionals can realize predictable and aesthetic outcomes.⁸
The CEREC MC XL, the larger of the 2 milling machines available to dentists, delivers high speed and simplicity, with low noise levels.8 The machine has the ability to produce one-visit single-tooth and quadrant restorations at the chair.⁸ It has a precision in the range of +/- 25 µm, with a 6-minute milling time for full-contour crowns and 3 to 4 minutes for partial coverage crowns.
The second system, the CEREC MC L Compact Milling Unit, delivers economical, highly aesthetic restorations in one visit.8 This unit can efficiently mill restorations for minimally invasive procedures and conservative preparations.⁸
Let’s look at a restorative case that was done in our office using lithium disilicate and chairside CAD/CAM technology to efficiently provide a functional and aesthetic result for our patient.

CASE REPORT
Diagnosis and Treatment Planning
A 53-year-old female presented with a previously treated root canal on tooth No. 19 (Figure 1). The core buildup had been completed with an amalgam and had since leaked corrosive byproduct into the dentin and cementum. This created a severely stained underlying tooth structure that had subsequently been restored with a gold onlay. Since the gold restoration did not cover the buccal aspect of the dark tooth, the patient had been self-conscious when smiling, talking, and laughing.
To correct these issues, the treatment plan consisted of removing the previously placed and leaking amalgam core. Remaining tooth structure that had been affected by recurrent decay would also be excavated, and the core of the tooth would be rebuilt with a dual-cured resin material. To cover the dark tooth, the most opaque CAD block of lithium disilicate would be used.

Figure 11. The preparation was checked to ensure adequate occlusal clearance.	Figure 12. The surrounding gingival tissue was troughed with a diode laser (AMD Lasers) in preparation for final optical impression.
Figure 13. After taking an antagonist bite impression, the rubber dam was replaced, and images of the teeth were taken.	Figure 14. The preparation was powdered, and the final preparation images were taken (CEREC 3D (Sirona Dental Systems).
Figure 15. Final design using the CEREC 3D software before milling shows where to expect some high spots (these were kept for illustration in final milling).	Figure 16. View of the crown in the “blue block” stage after the occlusion was initially checked. (Compare to figure 15).
Figure 17. Glaze and characterizations were applied to the crown before final baking.	Figure 18. View of the final try-in after cooling from oven.Figure 18. View of the final try-in after cooling from oven.
Figure 19. The crown was resin cemented (Multilink Automix [Ivoclar Vivadent]) under rubber dam isolation with easy cleanup.	Figure 20. View of the completed in-office CAD/CAM lithium disilicate (IPS e.max CAD (Ivoclar Vivadent]) full-coverage restoration.

Clinical Protocol
The patient’s soft tissues were initially anesthetized with one carpule of 4% prilocaine HCL (Citanest 4% [DENTSPLY Pharmaceutical]). One and a half carpules of 2% lidocaine with 1:100,000 epinephrine (Lidocaine ANES 50 [Patterson Dental]) were then used to anesthetize the mandible. A latex-free rubber dam (Flexi Dam [Coltène/Whaledent]) was then placed to protect the patient from the mercury vapors produced during the removal of the old amalgam. Then, by using an IPS Empress CAD block (Ivoclar Vivadent) as a comparison for shade taking against the natural tooth, the proper shade of IPS e.max CAD block was selected (Figure 2). It is important to note that in this author’s opinion, the e.max block shades are extremely similar to Empress shades, which made it possible to use these blocks as a guide. As an alternative and more traditional method, a shade guide (such as the Chromoscope Shade Guide [Ivoclar Vivadent]) can also be used.
The old restoration was removed (Figure 3), and the tooth was then prepared with a 1.5 mm shoulder and 1.5 mm of occlusal clearance (Figure 4). A total etch of 35% phosphoric acid (Ultra-Etch [Ultradent Products]) was then used on the surface of the preparation (Figure 5), after which the preparation was cleansed and dried. A fifth-generation bonding agent (ExciTE F VivaPen [Ivoclar Vivadent]) was then placed on the tooth (Figure 6), air-dried, and then light-cured. A dual-cure resin (LuxaCore [DMG America]) then placed for the core buildup and cured (Figures 7 to 10).
The preparation was checked to ensure adequate occlusal clearance (Figure 11). The surrounding gingival tissue was troughed with a diode laser (AMD Lasers) in preparation for final optical impression (Figure 12). An antagonist bite impression was then taken (Figure 13). To facilitate digital imaging, the tooth was powder coated (PowderPro [Advanced Dental Instruments]), and the digital impression (CEREC 3D [Sirona Dental Systems]) was taken (Figure 14).
The crown was digitally designed using the CEREC 3D software (Figure 15) and then milled (Figure 16). Upon completion of the milling process, the crown was tried into the mouth in the “blue block” stage to verify the fit and occlusion. Any necessary adjustments to the crown were completed during this time prior to crystalization in a chairside furnace (V.I.P. Universal X-Press [Jelrus]). The crown was then glazed (IPS e.max Universal Glaze [Ivoclar Vivadent]) and customized using subtle orange, cream, and blue stains to create natural characteristics that would make it indistinguishable from the surrounding natural dentition (Figure 17). The restoration was then ready for the final firing.
After the crown had cooled, it was tried in the patient’s mouth once again to verify fit (Figure 18). A rubber dam was placed once again. The internal surface of the crown was treated with silane (Monobond [Ivoclar Vivadent]) in preparation for its cementation with a resin cement, per manufacturer instructions. The restoration was subsequently bonded using a self-adhesive composite (Multilink Automix [Ivoclar Vivadent]) (Figure 19). Once seated with the uncured cement in place, the crown was tacked for 7 seconds with a light-curing unit (FLASHlite [Discus Dental]). Then, any excess cement was carefully trimmed away from the margins and the tooth was gently flossed to clean interproximally. Immediately following, to complete the polymerization of the resin cement, the restoration was light-cured again for a full 60 seconds.
Next, the rubber dam was removed, and the occlusion was checked in centric occlusion and all excursive movements. The patient was dismissed from the office after satisfactory function and aesthetics were verified in the completed restoration. The patient was very pleased with the aesthetics and function of the chairside fabricated lithium disilicate restoration and the final results of treatment (Figure 20).
The total treatment time from seating the patient to dismissal was approximately 70 minutes. This included the 40-minute cycle in the oven, during which time other treatments or hygiene checks could have been performed.

CONCLUSION
Through the use of the latest developments in material sciences and CAD/CAM fabrication technologies, proper training and teamwork, and the wise use of chair time, dental professionals are able to complete many cases in a highly efficient manner. By utilizing appropriate modern dental materials, indications such as discolored teeth and amalgam leakage can be addressed quickly and easily, thus saving time and reducing expenses for both the doctor and the patient. Patients can also expect to receive highly aesthetic restorations since materials like lithium disilicate (even when milled chairside) can be customized artistically in the dental office to provide lifelike results.

References

1. Fasbinder DJ, Dennison JB, Heys D, et al. A clinical evaluation of chairside lithium disilicate CAD/CAM crowns: a two-year report. J Am Dent Assoc. 2010;141(suppl 2):10S-14S.
2. McLaren EA, Phong TC. Ceramics in dentistry—Part I: classes of materials. Inside Dentistry. 2009;5:94-103.
3. Tysowsky GW. The science behind lithium disilicate: a metal-free alternative. Dent Today. 2009;28:112-113.
4. Reynolds JA, Roberts M. Lithium-disilicate pressed veneers for diastema closure. Inside Dentistry. 2010;6:46-52.
5. Helvey GA. Chairside CAD-CAM: Lithium disilicate restoration for anterior teeth made simple. Inside Dentistry. 2009;5:58-67.
6. Fabianelli A, Goracci C, Bertelli E, et al. A clinical trial of Empress II porcelain inlays luted to vital teeth with a dual-curing adhesive system and a self-curing resin cement. J Adhes Dent. 2006;8:427-431.
7. Baltzer A. All-ceramic single-tooth restorations: choosing the material to match the preparation—preparing the tooth to match the material [in English, German]. Int J Comput Dent. 2008;11(3-4):241-256.
8. Poticny DJ, Klim J. CAD/CAM in-office technology: innovations after 25 years for predictable, esthetic outcomes. J Am Dent Assoc. 2010;141(suppl 2):5S-9S.

Dr. Horwitz graduated from Ohio State University’s College of Dentistry and he received postgraduate training at the Dawson Center for Advanced Dental Study, completing the curriculum in 2003. He has completed numerous comprehensive functional aesthetic courses mentored by Drs. Bill Strupp, John Kois, and Frank Spear, and has graduated from The Hornbrook Group’s full course series. Dr. Horwitz has been practicing comprehensive restorative, rehabilitative, and cosmetic dentistry for more than 10 years. Additionally, he serves as a regional CEREC trainer for Sirona, and as a lab consultant for Knight Dental Group in Oldsmar, Fla. Dr. Horwitz maintains private practices in Palm Harbor and Trinity, Fla. He can be reached at hordent@yahoo.com, or at teethmakeadifference.com.

Disclosure: Dr. Horwitz reports no disclosures.