Choosing a Durable and Minimally Invasive Alternative: Posterior Partial-Coverage Lithium Disilicate Restorations

INTRODUCTION
Partial coverage all-ceramic restorations are among dentistry’s minimally invasive treatment options for posterior teeth. Contributing to their conservative potential is the fracture resistance of materials selected for posterior partial coverage restorations; those fabricated with lithium disilicate have demonstrated lower failure risks than those fabricated with other all-ceramic materials.¹

Lithium disilicate partial coverage restorations can be fabricated relatively thin (ie, one mm) and still demonstrate enhanced fracture resistance compared to other metal-free materials when bonded to tooth structure.² As a result, conservative preparation depths (eg, 0.5 to 1.0 mm) do not lower the fracture resistance of pressable lithium disilicate partial coverage restorations.³ In fact, the fracture resistance of inlays may be affected by the amount of tooth structure removed and has been shown to decrease as the amount of preparation increases from inlays to onlays.⁴ Although the majority of lithium disilicate partial coverage restorations have survived loads within the range of mastication forces,⁵ the cementation and placement techniques used can affect the quality of these minimally invasive treatments.⁶

This article describes the clinical protocol for the placement of aesthetic posterior lab-fabricated lithium disilicate inlay restorations.

CASE REPORT
Diagnosis and Treatment Planning
A female patient in her mid-40s presented with an old mesial-occlusal amalgam restoration on tooth No. 2 and an occlusal amalgam restoration on tooth No. 3 that were placed 20-plus years earlier (Figure 1). Quadrants 2, 3, and 4 were previously restored with direct composite and lab-fabricated all-ceramic restorations.

Thorough intraoral, photographic, and radiographic examinations were performed, in addition to extraoral and temporomandibular joint examinations that were uneventful, showed no significant findings, and demonstrated sound occlusion. She maintained regular oral hygiene and examination appointments.

Figure 1. Preoperative view of existing amalgam restorations on teeth Nos. 2 and 3.	Figure 2. The shade was verified using the “polar eyes” filter (Emulation North America) attached to the Canon Macro Ring Lite MR-14EX II.

A conservative treatment was planned to remove the amalgam restorations on both teeth, prepare them for the placement of all-ceramic inlay restorations that would ensure lifelike aesthetics and predictable strength. Lithium disilicate inlays would be ideal in this case to reinforce the remaining tooth structure and to withstand posterior mastication forces.

Preparation and Impression-Taking
Treatment began by taking digital photographs of the adjacent teeth. Shade selection was done preoperatively of the hydrated teeth using the Chromoscop shade guide (Ivoclar Vivadent); body shade 140 was selected for the restorations. This shade was verified using the “polar eyes” filter (Emulation North America) attached to the Canon Macro Ring Lite MR-14EX II (Figure 2). A preoperative impression of the existing restorations was then taken using a light-body vinyl polysiloxane (VPS) impression material (Virtual XD Extra Light Body Fast Set [Ivoclar Vivadent]) in a dual-arch tray (Triple Tray [Premier Dental Products]) lined with clear bite registration material (Clear Bite Matrix [DenMat]) (Figure 3). Virtual XD Light Body is injected around the teeth for accuracy as it flows and is unlikely to incorporate voids. Clear Bite is the matrix material and is injected into the Triple Tray and seated on the teeth. It is a stiffer material than the light body for matrix rigidity, and the fact that it is clear makes it easy to seat when making the provisionals.

Rubber dam isolation was achieved, and the amalgam restorations were removed using a 1557 carbide bur (Razor Operative Carbide [Kerr Rotary]). The teeth were prepared for inlay restorations with a tapered, flat-end coarse diamond bur (NTI C845KR-025 [Kerr Rotary]) and a bullet-nosed fine diamond bur (NTI F856-020 [Kerr Rotary]) (Figure 4). Interproximal finishing was completed using inlay diamonds and an ultrasonic scaler (SONICflex [KaVo]) (Figure 5). The finished preparations were then checked for adequate occlusal clearance, smooth tapering walls, and visible margins (Figure 6). The rubber dam was removed and the preparation shade ND 6 (IPS Natural Die Material Guide [Ivoclar Vivadent]) was selected to match the dentin shade at the base of the inlays and photographed (Figure 7).

Figure 3. A preoperative impression of the existing dentition was taken for the fabrication of the provisionals.	Figure 4. The inlay preparations were created using a coarse diamond bur (NTI C845KR-025 [Kerr Rotary]).
Figure 5. The preparations were completed using the SONICflex (KaVo).	Figure 6. The finished preparation walls and margins were visually inspected.
Figure 7. Preparation shade ND 6 (IPS Natural Die Material Guide [Ivoclar Vivadent]) was selected based on the dentin.	Figure 8. A light-body impression material (Virtual XD Light Body Fast Set [Ivoclar Vivadent]) was placed around the preparations.
Figure 9. A layer of extra light-body material was injected into the heavy-body impression material (Virtual XD).	Figure 10. The impressions were inspected to ensure replication of the margins.

The preparation area was again isolated using OptraGate (Ivoclar Vivadent) and dry angles (NeoDrys [Microcopy]). (Note: OptraGate acts as self-retracting lip retractors, improving visibility and access.) An extra light-body VPS material (Virtual XD) was injected around the inlay crown preparations (Figure 8) while the assistant loaded a full posterior tray with a heavy-body VPS material (Virtual XD). To reduce the likelihood of the pulling and dragging that can occur when combining light-body and heavy-body impression materials (as also seen when using a putty-wash technique), the assistant injected extra light-body material directly into the middle of the heavy-body material in the tray (Figure 9). Next, the loaded tray was placed over the maxillary teeth, allowed to set, and then removed and checked for accuracy. Care was taken to ensure the absence of any bubbles or voids that could negatively impact the integrity of the final restorations (Figure 10). A bite registration was then taken using a VPS material (Virtual Bite Registration Material [Ivoclar Vivadent]).

Fabricating the Provisional Restorations
Inlay temporary material (Telio CS [Ivoclar Vivadent]) was placed about 1.0 mm short of the cavosurface margins and then light cured (Figure 11). Shade A2 of a self-curing acrylic resin (Telio Crown and Bridge [Ivoclar Vivadent]) was added to the preoperative impression of the existing restorations and placed on top of the Telio CS inlay-covered preparations. After a 2-minute setting time, the impression was gently teased off the teeth, leaving a “crust” of acrylic on top of the inlay material (Figure 12).

Figure 11. Inlay provisional material was added to within 1.0 mm of the cavosurface margins.	Figure 12. Self-curing acrylic resin was added to the preoperative impression of the existing restorations and finished after setting.

This technique was done with these materials because when the soft and pliable inlay material is used alone, it has a risk of falling out. When the more robust acrylic material is used, it can lock onto the preparation, making it difficult to remove. This “Godiva chocolate” technique combines the best of both materials and allows for easy removal of the provisionals at the seating appointment.

The provisionals were finished with appropriate finishing carbides and polishers. Next, the occlusion was adjusted and the patient dismissed. The impressions and bite records, a detailed laboratory prescription, digital photographs, and the shade information were then sent to the dental laboratory team.

Laboratory Fabrication
At the laboratory, the models were poured and mounted on an articulator (Panadent). The restorations were then waxed, sprued, and pressed using IPS e.max Press Multi (Ivoclar Vivadent) lithium disilicate ingots. The Press Multi ingots were selected for this case because they demonstrate multiple levels of translucencies depending on how they are positioned on the ring. After pressing, the restorations were divested, stained, and then glazed.

Seating Appointment
The patient received local anesthetic while the restorations were inspected for marginal fit, color, and deemed to be free from any defects (Figures 13 and 14). Slots were cut in the middle of the provisional restorations and they were then removed using a spoon excavator (Figure 15). The lithium disilicate inlays were tried in dry to verify proper fit, and the margins were checked with an explorer. Next, the inlays were tried again using the “warm” shade of Variolink Esthetic Try-in Paste (Ivoclar Vivadent) to verify color. The try-in paste is designed to closely resemble the cured shade of the final resin cement.

Figure 13. The finished lithium disilicate (IPS e.max Press Multi [Ivoclar Vivadent]) restorations were returned from the laboratory team.	Figure 14. Marginal adaptation to the working die was verified.
Figure 15. A slot was cut in the provisional restorations, after which they were removed.	Figure 16. After the try-in procedures, a universal cleaning paste (Ivoclean [Ivoclar Vivadent]) was applied for 20 seconds to the internal aspects of the previously rinsed and dried restorations, then rinsed and dried again.

Ivoclean (Ivoclar Vivadent) was applied to the internal aspect of the rinsed and dried restorations, left on for 20 seconds, and then rinsed and dried. This process cleansed the porcelain surface of saliva, try-in paste, and any unseen debris (Figure 16). To optimize the strength of the bond between the resin cement and the lithium disilicate restorations, a universal primer (Monobond Plus [Ivoclar Vivadent]) was applied to the internal surfaces of the inlays for one minute and then lightly air-dried (Figure 17).

A fluoride-free polishing paste (Proxyt [Ivoclar Vivadent]) was used to clean the tooth preparations, then they were then etched for 15 seconds using 37% phosphoric acid (Total Etch [Ivoclar Vivadent]) (Figure 18). The etchant was then removed with water, and high volume suction and a microtip applicator were used to remove excess water while ensuring a moist tooth surface.

An adhesive bonding agent (Adhese Universal [Ivoclar Vivadent]) was scrubbed onto the preparation surface for 10 to 15 seconds, and the solvent evaporated with gentle warm air for 5 seconds (Warm Air Tooth Dryer [A-dec]). After confirming a uniform glossy appearance of preparation surfaces (Figure 19), the adhesive was cured for 10 seconds using an LED curing light (Bluephase Style [Ivoclar Vivadent]).

The warm value shade of Variolink Esthetic DC (dual-cure) resin cement was selected and applied directly onto the restorations (Figure 20). The restorations were transferred to the mouth using an OptraStick (Ivoclar Vivadent), held down with a ball burnisher, and then spot tacked (cured) in place for one second using a 2.0-mm curing tip (Figure 21). The excess “gelled” cement was removed using a scaler and the interproximal areas were flossed (Figure 22). Glycerin gel (Liquid Strip [Ivoclar Vivadent]) was placed on the margins (to prevent formation of an uncured oxygen inhibited layer), and then the final light curing was done for 10 seconds from each aspect (Figure 23).

The occlusion was checked (Figure 24), and the inlays were polished after adjusting with fine ceramic polishers (NTI CeraGlaze [Kerr Dental]) (Figure 25). Contact points were checked with floss to ensure assess for hygiene.

Figure 17. A universal primer (Monobond Plus [Ivoclar Vivadent]) was applied to the internal surface of the inlays for 60 seconds and then lightly air-dried.	Figure 18. The inlay preparations were etched with 37% phosphoric acid for 15 seconds.
Figure 19. A bonding adhesive (Adhese Universal [Ivoclar Vivadent]) was applied to the etched inlay preparations.	Figure 20. The warm value shade of a dual-cure resin cement (Variolink Esthetic DC [Ivoclar Vivadent]) was selected and applied directly onto the restorations.
Figure 21. The restorations spot-tacked in place for one second using a 2-mm tacking tip.	Figure 22. Excess “gelled” cement was removed using a scaler, and the interproximal areas were flossed.
Figure 23. With glycerin on the margins, the restorations underwent the final light-curing steps using the LED curing light (Bluephase Style [Ivoclar Vivadent]).	Figure 24. The occlusion was then checked and adjusted as needed.
Figure 25. The adjustments were polished with a fine porcelain polisher (NTI CeraGlaze [Kerr Dental]).	Figure 26. Immediate postoperative occlusal view of the lithium disilicate inlay restorations on teeth Nos. 2 and 3.

CLOSING COMMENTS
The immediate postoperative view of this case reveals a return to aesthetic and functional harmony (Figure 26), helping to confirm that all-ceramic inlays are now a staple in the restorative dentist’s arsenal for posterior restorations. The blessings of so many dental material innovations and advances in clinical and laboratory techniques, as used and described herein, greatly benefit patients and everyone involved in the treatment process.

References

1. Kois DE, Isvilanonda V, Chaiyabutr Y, et al. Evaluation of fracture resistance and failure risks of posterior partial coverage restorations. J Esthet Restor Dent. 2013;25:110-122.
2. Bakeman EM, Rego N, Chaiyabutr Y, et al. Influence of ceramic thickness and ceramic materials on fracture resistance of posterior partial coverage restorations. Oper Dent. 2015;40:211-217.
3. Guess PC, Schultheis S, Wolkewitz M, et al. Influence of preparation design and ceramic thicknesses on fracture resistance and failure modes of premolar partial coverage restorations. J Prosthet Dent. 2013;110:264-273.
4. Saridag S, Sevimay M, Pekkan G. Fracture resistance of teeth restored with all-ceramic inlays and onlays: an in vitro study. Oper Dent. 2013;38:626-634.
5. Stappert CF, Guess PC, Chitmongkolsuk S, et al. All-ceramic partial coverage restorations on natural molars. Masticatory fatigue loading and fracture resistance. Am J Dent. 2007;20:21-26.
6. Ruiz JL. Anterior and posterior partial-coverage indirect restorations using supragingival dentistry techniques. J Mass Dent Soc. 2012;61:16-19.

Dr. Lowe, a graduate of the University of British Columbia’s Faculty of Dentistry, maintains a full-time private practice devoted to comprehensive functional aesthetic and reconstructive dentistry in Vancouver, BC. He is an Accredited Member and Accreditation Examiner in the American Academy of Cosmetic Dentistry (AACD) and holds Fellowships in the International Academy of Dental Facial Esthetics, American College of Dentists, and the International Congress of Oral Implantologists. He sits on the editorial advisory board of the Dental Chronicle, Journal of Implant and Advanced Clinical Dentistry, and Inside Dentistry, and he is the editor-in-chief of the AACD’s Journal of Cosmetic Dentistry. In addition, he is an opinion leader and consultant for THE DENTAL ADVISOR. Dr. Lowe mentors the TEST study club in Vancouver and is currently adjunct faculty at the University of British Columbia. He has published more than 60 articles and given more than 200 lectures internationally on aesthetic, restorative, and implant dentistry. He can be reached at (604) 683-2483 or via email at edlowe@mac.com.

Disclosure: Dr. Lowe discloses that materials were provided by Ivoclar Vivadent for this article; however, he is not a paid consultant.

Mr. Rego owns and operates Smile Designs by Rego, a progressive dental laboratory specializing in ceramics in Santa Fe Springs, Calif. He and Juan Rego, CDT, established the laboratory in 1980. He is a certified dental technician and is an Accredited Member of the American Academy of Cosmetic Dentistry. He has written more than 68 articles pertaining to dental materials and techniques, which have appeared in many of the leading dental journals. He is also an evaluator and speaker for several dental product manufacturers. He can be reached via email at nelson@regosmiles.com.

Disclosure: Mr. Rego reports no disclosures.