Research Evaluation of a Lithium Disilicate Restorative System
The introduction of an all-ceramic material that is indicated for full-coverage crown restorations and 3-unit fixed bridges (IPS Empress® 2) offers exciting possibilities. A clinical study was initiated shortly after the final development of the lithium disilicate formulation to determine the clinical and laboratory efficacy of the system. The inherent characteristics of the IPS Empress®2 ceramic and adhesive cementation facilitate tooth conservation for conventional full-coverage crown preparations.
The original IPS Empress material has achieved tremendous success in the dental profession, making it an advanced alternative to metal supported restorations. As a result of its enhanced aesthetics, achieved through resin cementation of a ceramic that demonstrates a 97.4% success rate has been documented. This enhanced performance has been achieved through the simple and familiar technology of waxing the framework and hot-pressing the ceramic into the mold.
The IPS Empress® system presents the same advantages as the original IPS Empress system, but with a completely new material formulation and expanded clinical indications. Through an extensive clinical study and in vitro examinations at the Oregon Health Sciences University, (OHSU) several additional clinical benefits of the material have been identified.
New material Composition
The original IPS Empress system introduced leucite-reinforced glass ceramics that improved fracture resistance and strength compared to previous ceramics. IPS Empress® 2 is a completely new formulation with a lithium disilicate glass-ceramic structure for fabricating bridge frameworks. Höland and Schweiger developed this highly crystalline (more than 60% volume) microstructure, which is composed of densely arranged and homogenously shaped, elongated lithium disilicate crystals uniformly bonded into a glassy matrix.2 The interlocking structure of the crystals hinders crack propagation, elevating the fracture toughness and flexural strength of the material to approximately 340+20MPa. The IPS Empress® 2 veneering ceramic provided in a powder form in a large variety of dentine, incisal and impulse shades contains fluorapatite crystals that are similar in structure and optical properties to natural dentition.
The microstructural properties of this system present several clinical advantages over conventional metal ceramics. In addition to elevating the strength of the ceramic framework, the high crystallinity promotes the machinability of the ceramic, facilitating a high polish. This will benefit the patient by reducing the tendency toward over contouring at the margin, thereby reducing iatrogenic periodontal disease, and diminishing plaque accumulation as a result of the inherently smoother properties of ceramic compared to metal, or at the metal-opaque-porcelain junction.
Strength and fracture toughness
The strength of the IPS Empress® 2 material is nearly three times greater than the strength of the original IPS Empress material. As a result of the high crystalline content, the fracture toughness has also been increased nearly threefold. Considering that the original IPS Empress system established an approximately 97% success rate for full-coverage crown restorations and a high success rate for inlay and Onlay restorations in long-term clinical studies,4 the enhanced characteristics of the IPS Empress® 2 material are promising for clinical utilization. Since the ingots are sintered at the manufacturing facility under controlled conditions, the pressable ceramic is of high quality with few flaws. The minimization of those flaws in the ceramic substructure is critical to the clinical longevity of the bridge. Flaws contained in the sub-structure act as the origin of crack propagation and ultimate failure.
The fine grain structure and high crystallinity of the glass-ceramic reduce the potential for wear of opposing natural tooth structure. Sintering the fluorapatite veneering ceramic onto the substructure creates apatite crystals similar to the tooth structure. An in vitro study utilizing the OHSU Oral Wear Simulator tested wear of opposing enamel cusps against various ceramics in three-body wear evaluations and recorded less wear with the IPS Empress® 2 material.6 The lithium disilicate framework material and the fluorapatite veneering ceramic also demonstrated less wear of opposing cusps than the bovine enamel control. The composition of the fluorapatite veneering ceramic was found to be closer to natural tooth structure than conventional metal-ceramic, feldspathic porcelains.
Of the teeth measured at six months, half of the natural antagonist dentition exhibited no measurable wear. Approximately 40% of the ceramic occluding surfaces exhibited wear facets. The mean volume (mm3) of wear of tooth structure was 0.0701 0.370. These were early results and represented part of the study group.
These results contradict the documented destructive nature of conventional ceramic when opposing natural dentition, To the authors’ knowledge, this is the first clinical study to detect wear facets on the ceramic surface that suggest the material is gentler to natural tooth structure. This new ceramic structure could have tremendous impact on clinical dentistry, offering an alternative to the compromises of conventional ceramic occluding surfaces.
Conservation of tooth structure
Previously, dentists have been under the false impression that the virtues of an all-ceramic crown’s superior aesthetics must be achieved by sacrificing greater amounts of tooth structure required for tooth preparation. Sorensen et al. demonstrated in a clinical study on adhesively cemented, leucite-reinforced pressed ceramic crowns (IPS Empress) that less than 1.3 mm of axial reduction was necessary. A potential additional benefit of a higher strength yet translucent ceramic was a decrease in the amount of axial tooth reduction required for full-coverage crown restorations. While 1.4 mm to 1.7 mm of axial reduction is recommended for metal ceramics,678 and 1.3mm reduction for adhesively cemented IPS Empress crowns, it is suggested that only 1.0 mm of reduction was necessary for the lithium disilicate ceramic restorations (IPS Empress® 2)
1. Recommended preparation design for anterior full-coverage crown restorations and fixed bridges
2. Recommended preparation design for a premolar full-coverage crown restoration and a fixed bridge. The IPS Empress® 2 system, therefore, represents a more biocompatible restoration, since less axial tooth reduction is required.
Another potential application of the adhesively cemented all-ceramic technology is fixed bridges utilizing inlay and onlay abutments. Tremendous amounts of tooth structure could be conserved by minimum inlay/onlay abutment preparations, compared to full-coverage crown preparations. Superior aesthetics could theoretically be achieved, since the dentist/ceramic team would only be required to match the pontic to the shade of the natural dentition. The resin cement optically connects the translucent core ceramic to the adjacent tooth structure, enabling the conservative preparations to blend in with the natural dentition. No published clinical studies have evaluated this mode of treatment, and essentially no research has been conducted to define the clinical parameters of adhesive preparation design. This has resulted in a poor definition of the conservative preparation design; most articles refer to anecdotal case reports that assume that for anterior bridges, a Maryland bridge wing design can be utilized. However, this design does not provide sufficient bulk for the strength-conferring core material, whether it is a ceramic or fiber-reinforced composite. From the results of this study – in which none of the inlay and onlay bridges have fractured – it was postulated that the adhesively cemented IPS Empress® 2 ceramic could be utilized for the fabrication of conservative bridge restorations. Based upon the amount of remaining tooth structure, the extent of preexisting restorations, and the anatomic location of the tooth, when deemed appropriate, three conservative restorative designs were utilized. The experimental hybrid designs included onlays, inlays, and what is called a distal slice preparation for maxillary canines. The clinical results showed that the distal slice preparation provided little mechanical retention, resulting in the bridge retainer having to rely almost completely on the adhesive mechanism. The conservative bridge retainers must have opposing vertical axial walls of box forms and grooves to successfully retain the bridge on a long term basis. Note: At this time, the manufacturer (Ivoclar) does not recommend the use of IPS Empress® 2 for inlay or onlay (anterior or posterior) bridges, pending further clinical evaluation.
Although the adhesive cementation technique may be more time-consuming and technique-sensitive than conventional cementation, it is still the preferred method of placement. Adhesive cementation technology enables the clinician to utilize conservative preparation designs and limit axial wall reduction for full-coverage crown preparations to about 1.0 mm; conventional metal-ceramic restorations require approximately 50% more tooth reduction in order to achieve similar aesthetics. However, in situations where the clinical environment does not permit the adhesive isolation required for the adhesive technique (subgingival margins), the further clinical investigation has confirmed the successful utilization of an ultra-low expansion hybrid ionomer (Pro-Tec CEM) for conventional cementation of IPS Empress® 2 full-coverage crown and bridge restorations.
Results of clinical study
60 fixed bridges were placed in 57 patients. Of the patients, 24 were female and 33 were male. The mean age of the patients was 45.7 years (±12.8 years). The subjects’ ages ranged from 21 to 75 years. Two groups of bridge restorations were made as defined by the type of bridge retainer. Conventional bridge restorations were prepared with full-coverage crown retainers with at least 1.0 mm of axial reduction and 1.5 mm to 2.0 mm of occlusal reduction. All restorations were adhesively bonded (Syntac Dentin Bonding System, Variolink II Ceromer Cement). The mean service time for the bridges was 11.2 months.
Sorensen et al. in a study of 75 IPS Empress crown restorations that were resin cemented observed only one failure in a full-coverage molar crown (occurring at 27 months of function) and a small chip in the incisal porcelain of an incisor. This calculated to a failure rate of 2.7%. The shortest duration of function for the 75 crown restorations is now 4 years. The dentition was prepared with 1.3 mm of axial reduction and a shoulder margin design. For the leucite-reinforced core material (IPS Empress), a flexural strength of 126 MPa was recorded.
In the present study of IPS Empress® 2 bridge restorations with only 1.0 mm of axial reduction for the full coverage crown preparations, no ceramic fractures occurred through the retainers. Four bridges failed by catastrophic fracture through the connector. Of these failures, three exhibited a deficient occlusal gingival connector height, according to the guidelines presented below. The success rate of the IPS Empress® 2 bridges fabricated according to the critical connector-size guidelines was 98.3%.
While the inherent strength of the lithium disilicate glass-ceramic (IPS Empress® 2) has been significantly improved, the overall strength of the fixed bridge is dependent on several factors. The occlusal-gingival dimension or vertical height of the connector is critical. Consequently, this vertical dimension should be maximized in the hot-pressed core material during fabrication; every effort should be made by the ceramist to minimize flaw content. These flaws may initiate the propagation of cracks that can increase to critical size in the oral environment with cyclic loading and stress corrosion fatigue.
Therefore, the biomechanical engineering principles and the Law of Beams are fundamental considerations when treatment planning for all types of bridge restorations. The deflection of a beam varies directly with the cube of the length of the span, and inversely with the cube of the height.14,15 Therefore, of the two dimensions of the bridge connector, the vertical height of the connector has a significantly greater effect on the flexure or strength of the bridge than the buccal-lingual width.16 A connector with a given occlusal-gingival dimension will bend eight times as much if the height is reduced to half, while a similar reduction in the buccal-lingual dimension results in only a two times greater increase in flexure. Since the occlusal-gingival connector height is the critical dimension, the clinical evaluation of the ability to achieve this dimension is the primary determinant of whether to utilize the IPS Empress® 2 system for 3-unit bridges.
The occlusal contact and the gingival tissues define the limits of the connector height. A gingival embrasure must be maintained for oral hygiene access and to avoid iatrogenic periodontal disease.
If the minimal vertical height dimension is not available, the clinician may consider a minor gingival procedure with electrosurgery to remove an increment of the soft tissue in order to gain adequate space for the connector height. There are limits to the degree of tissue that can be removed, and the biologic width must be respected. If this critical minimum vertical dimension cannot be achieved, the utilization of the IPS Empress® 2 glass-ceramic for the fabrication of a bridge prosthesis is contraindicated.
The more posterior the location of the pontic, the greater is the occlusal forces and functional requirements of the occlusal-gingival connector dimension.
3. Minimum occlusal-gingival and buccal-lingual framework connector dimensions as a function of the position of bridge connectors and occlusal forces.
4. Determination of the occlusal contact point to establish the clinically acceptable occlusal limit of the connector height or the distance from the opposing cusp contact or incisal embrasure to the original crest measured with a periodontal probe
5. Measurement of potential occlusal-gingival connector height from the crest of the gingiva to the occlusal contact point For a first premolar pontic, the connector dimension between the second premolar retainer and the pontic should be 5.0 mm occlusal-gingivally, and 4.5 mm buccal-lingually. The connector dimension between the canine and lateral incisor pontic should be 4.0 mm occlusal-gingivally, and 4.0 mm buccal-lingually.
Additionally, the maximum length of the pontic span in the posterior is the average mesiodistal width of a premolar, or about 9.0 mm. In the anterior, it is the average mesiodistal dimension of the central, or about 11.0 mm
6. Maximum length of the pontic span is equal to the average width of the premolar in the posterior segment (9.0 mm), and of the central in the anterior (11.0 mm). To maximize the strength of bridge restorations, and since the core ceramic is significantly stronger than the veneer ceramic, it may be suggested to apply little or no veneer ceramic at the gingival embrasure of the connectors. This will maximize the strength-conferring core material. Both the laboratory technician and clinician must be careful if modifying the connectors with rotary instruments to avoid inducing micro-cracks and critical flaws in these most important connector areas. In fact, it is highly recommended that no rotary instruments be used on the connector areas once the bridge has been fabricated.
Unique advantages to pressed ceramics
The IPS Empress® 2 system incorporates a relatively simple lost-wax hot-pressed ceramic fabrication technology that is familiar to dental technicians. The fabrication processes for other all ceramic systems are more complicated and involve many additional steps to create a special model or die for producing the framework for IPS Empress® 2, the substructure pattern is merely waxed on the master die, invested, and then pressed.
Frequently, in clinical practice there are situations in which the existing preparation retention and resistance form are deficient and require augmentation. An advantage of the hot-pressed, lithium disilicate glass-ceramic system is that it can reproduce auxiliary retention and resistance form, such as boxes and grooves, which expand the clinical applications of this system.
The patient was presented with a missing central incisor that had been lost more than ten years earlier
7. Preoperative facial view of the patient presenting with a large edentulous space. The mesiodistal width of the edentulous space was significantly larger than the remaining central incisor
8. Close-up preoperative view of the missing central incisor. This created a very challenging treatment situation in terms of trying to fill the edentulous space, maintain good portioning of the central and lateral incisors, and insure the ascetic appearance of the definitive fixed-bridge. An IPS Empress® 2 substructure shade was selected from custom-made shade tabs
9. The appropriate shade was selected utilizing customized shade tabs. The substructure was hot-pressed in the ceramic and the fluorapatite veneer ceramic artistically applied
10. The lithium disilicate glass-ceramic restoration. The majority of the lingual surface was made in core material to maximize the dimensions of the high-strength lithium disilicate glass-ceramic substructure material and yet still have a wear friendly surface in contact with the natural dentition
11. Postoperative palatal view of the IPS Empress® 2 3-unit fixed bridge, demonstrating framework material lingually The bridge aesthetically replaced the missing tooth and filed the large edentulous space with good proportioning
12. Immediate postoperative view of the IPS Empress® 2 restorations. Note natural aesthetics and gingival harmony The facial aesthetics were restored with a pleasing smile
13. Postoperative full-smile view of the definitive IPS Empress® 2 full-coverage 3-unit fixed bridge.
The recently introduced lithium disilicate restorative system (IPS Empress® 2) is a major advancement in restoring form, function, and aesthetics to compromised dentition. Its unique material formulation brings dental materials closer to replicating the function and composition of natural tooth structure. Its clinical success, when prepared and fabricated according to proper protocol, suggests that clinicians may confidently restore missing dentition. IPS Empress® 2 provides a more biocompatible restoration since less axial tooth reduction is required, the iatrogenic periodontal disease potential is diminished, and less wear of opposing natural tooth structure is experienced when compared to conventional metal ceramics and other metal-free alternatives.
The authors acknowledge the technical assistance of Oscar Raffeiner, MDT, and Hans Peter Foser, MDT.
IPS Empress and IPS Empress® 2 are products of Ivoclar, Schaan, Liechtenstein.
Pro-Tec CEM, Syntac Dentin Bonding System, and Variolink II Ceromer Cement are products of Vivadent, Schaan, Liechtenstein.
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John A Sorensen, DMD, PhD is the ODA Centennial
Professor of Restorative Dentistry, Director, Dental research Center,
Oregon Health Sciences University, Portland, Oregon.
Mark Cruz DDS, is an Assistant Adjunct Professor, Oregon Health Sciences University, and maintains a private practice in Laguna Niguel, California.
Wayne T. Mito, CDT, is a Senior Research Associate, Oregon Health Sciences University, Portland, Oregon.