To prospectively evaluate the clinical long-term outcome of tooth-supported crowns (SCs), implant-supported crowns (ISCs), and fixed dental prostheses (FDPs) made of a lithium-disilicate glass-ceramic framework material (IPS Empress 2).
Materials and methods
Between 1997 and 1999, a total of 184 restorations (106 SCs, 32 ISCs, 33 FDPs, and 13 diverse restorations) were placed in 73 patients. Kaplan-Meier estimation was applied for survival and chipping-free rates. Inter-group comparison of both rates was realized by a log rank test and a 2 × 2 contingency table. Also, SCs and FDPs were compared regarding adhesive vs. conventional cementation, and anterior vs. posterior positioning, for impact on survival.
Due to 14 dropouts (34 restorations) and reasonable exclusion of 19 other restorations, the final dataset included: i) 87 SCs [37 patients, mean observation time 11.4 (±3.8) years]; ii) 17 ISCs [12 patients, mean observation time 13.3 (±2.3) years; and iii) 27 FDPs [19 patients, mean observation time 8.9 (±5.4) years]. The 10-year survival rate/chipping-free rate for SCs were 86.1%/83.4%, for ISCs 93.8%/94.1%, and for FDPs were 51.9%/90.8%. Both ISCs and SCs had a significantly higher survival than FDPs (ISCs vs. FDPs: both tests p = 0.001; SCs vs. FDPs: p = 0.001 and p = 0.005). Differences in the chipping-free rates did not reach significance. Also, neither the cementation mode nor positioning of the restoration had an impact on survival.
SCs had a slightly lower outcome than can generally be expected from single crowns. In contrast, ICSs had a favorable outcome and the FDPs predominantly failed.
The practitioner’s choice of dental materials is based (at best) on long-term experience. The present 10-year results are based on comprehensive data analyses and show the high potential of lithium-disilicate as a reliable material, especially for single-unit restoration.
The global demand for ideal dental esthetics is based on the assumption that beauty is associated with favorable personality traits . As control of substrate color and translucency can nowadays be accomplished by decisive selection of a specific ceramic system, all-ceramic dental restorations can be considered fail-safe concerning the esthetic matter. Furthermore, in terms of biocompatibility all-ceramic materials provide low solubility , reduced plaque accumulation and sufficient marginal fit . Nevertheless, the ceramic’s brittle nature has led to high fracture rates, especially in cases of glass-ceramic systems inserted in the posterior region . Additionally, in high-strength, bi-layered all-ceramic systems, vulnerability of the veneering material’s adhesion has caused high chipping rates . Lithium-disilicate glass-ceramic is a unique glass-ceramic material, containing an interlocking microstructure from needle-like lithium-disilicate crystals of up to 70 Vol% embedded in a glassy matrix. On the one hand it provides translucency similar to natural teeth and, on the other, a biaxial strength of up to 400 MPa, which is almost three times as high as conventional glass-ceramic. In addition, since the chemical durability of lithium-disilicate fulfills the standards of a veneering material, it can be used for monolithic restorations .
The first version of the lithium-disilicate restoration material IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) has been improved, particularly concerning the strength, the assortment of translucency levels, and the optional use as monolithic restoration. This second generation of lithium-disilicate was named IPS e.max Press (Ivoclar Vivadent) and became commercially available in 2006. In retrospective evaluations, IPS Empress 2 single crowns achieved a 9- and 10-year survival rate of 96.1% and 95.5% , respectively, whereas a prospective study yielded an up to 9-year survival rate of only 87.1% . Prospectively, IPS e.max Press single crowns showed a survival rate of 94.8% after 8 years . Another retrospective evaluation after up to 11 years of clinical service, that included IPS Empress 2 and IPS e.max Press crowns, reported an overall cumulative survival rate of 98.2% . In terms of single tooth replacement, three-unit fixed dental prostheses (FDPs) made from IPS Empress 2 prospectively demonstrated a 10-year survival of 71.4% and FDPs made from monolithic IPS e.max Press, 87.9% .
However, reliable clinical application needs to be confirmed by data on prospective clinical outcome after at least 10 years of clinical service. Unfortunately, because such studies are scarce, the results of more long-term studies are needed to provide important insight into the evaluation of lithium-disilicate restorations in different fields of clinical application.
The approach of the present research (dating from 2012/2013) was to re-recall patients of a prospective clinical study on IPS Empress 2 restorations that was initiated in 1997 and which systematically evaluated clinical service up to 2005. The aim was to collect data on clinical complications and survival of tooth-supported single crowns, implant-supported single crowns, and FDPs after at least 10 years of clinical service. The impact of the cementation mode and oral positioning on survival was also tested. Moreover, attention was paid to whether (or not) chipping of the veneering material had occurred.
For tooth-supported restorations (i.e. tooth-supported single crowns and FDPs) the following null hypotheses were formulated:
The mode of cementation would have no negative impact on survival.
Oral positioning (anterior vs. posterior) of the restoration would have no negative impact on survival.For inter-group comparisons (implant-supported single crowns vs. tooth-supported single crowns vs. FDPs) the following null hypotheses were formulated:
The survival probability of all lithium-disilicate applications would be the same.
The chipping-free probability of all lithium-disilicate applications would be the same.
Material and methods
Patients of the department of Prosthodontics of the Uniklinik Aachen (RWTH Aachen University), with medical need for prosthetic treatment with crowns, FDPs and/or implants were recruited for the clinical study. Excluded were patients that suffered from bruxism, poor oral hygiene or periodontal diseases, as well as adolescent patients with (presumably) large pulp chambers. The prospective abutment teeth had to be vital or state-of-the-art endodontically treated.
A total of 73 patients were assessed for eligibility. The requirements of the Helsinki Declaration were observed and all patients provided informed consent. The ethical board of the RWTH Aachen University reviewed and approved the study protocol (No. 641).
Between November 1997 and October 1999 a total of 184 restorations were inserted: 106 tooth-supported crowns, 33 FDPs, 32 implant-supported crowns, and 13 diversely designed restorations (i.e. 5 resin-bonded FDPs, 7 inlay-retained FDPs, 1 implant-supported FDP) ( Fig. 1 ).
All dental technicians involved in this study were specially trained by experts employed by the manufacturer. Nine trained and calibrated clinicians performed the treatment. All clinicians were fully informed about the study protocol including the rationale, objectives, and design. All clinical processes were monitored by the principal investigator. Determination of the color of the restoration was performed with the help of a shade guide (Chromascop, Ivoclar Vivadent). The abutment teeth were prepared as follows:
The location of the finishing line was oriented on the clinical conditions either subgingival, equigingival or supragingival.
Margin design, 1 mm wide rounded shoulder/chamfer.
Equatorial reduction, 1.5 mm.
Occlusal/incisal reduction, 1.5–2 mm.
total occlusal/incisal convergence, 6–10°.
Particular attention was paid to rounded line angles.
After preparation the dentin shade of the abutment teeth was identified.
Direct temporary restorations were fabricated (Protemp 2, 3 M Espe, Seefeld, Germany) and placed with eugenol-free temporary cement (Kerr Life, Kerr, Orange, CA, USA). In case of FDPs, the dimensions of the connector area of those temporary restorations were measured to ensure that there was enough space to fulfill the stipulated IPS Empress 2 FDP design: the aim was 16 mm 2 . Although (preferably) a connector area of 4 × 4 mm was targeted, if the linguo-vestibular portion had to be reduced it was permitted to enlarge the incisal-cervical portion. However, if only limited space was available, the treating clinician could decide whether a surgical crown lengthening would be performed, or whether a restoration material other than the IPS Empress 2 would be used. No detailed documentation in terms of connector dimensions was performed.
In case of implant superstructure, an impression was made of the implants with a transfer coping (Nobel Biocare Branemark, Abformpfosten). As abutment material either CerAdapt (monolithic Alumina Ceramic, Nobel Biocare, Gothenburg, Sweden), TiAdapt (Titanium, Nobel Biocare), or abutments based on monolithic zirconia ceramic (Wohlwend Innovative, Zurich, Switzerland) were prepared by the dental technician. After placement onto the implant, the preparation was adjusted to the adequate gingival margin design intraorally by the dentist. Finally, impressions (polyether material, Permadyne, 3 M Espe) were made and implant retained crowns were fabricated in the dental laboratory, as described for tooth-supported crowns.
In case of tooth-supported restorations, the impressions were made with a simultaneous, dual-mix technique using polyether material (Permadyne, 3 M Espe), or with an A-Silicone (President, Coltene/Whaledent AG Altstätten, Switzerland) using the two-step putty wash impression technique.
An interocclusal registration was made with self-curing resin (Pattern Resin, GC Europe, Leuven, Belgium).
In the dental laboratory a master cast was fabricated with type IV gypsum (GC-Fuji Rock EP, Leuven, Belgium) and articulated in a semi-adjustable articulator. Die spacer (Vita In-Ceram die spacer, Vita Zahnfabrik, Bad Säckingen, Germany) was applied twice on the model die with a distance of 1 mm from the preparation line. In case of FDPs, an additional layer of die spacer was applied at the abutment aspect facing the pontic area. This was done to prevent any kind of friction in this area. A full wax-up was established and secured with a silicone key. Then, the prospective area of layering material was systematically reduced (cutback technique), so that the prospective framework thickness measured at least 0.8 mm. A special investment material was used for these wax patterns (IPS Empress 2 Special Investment material, Ivoclar Vivadent). The wax burnout took place in a conventional pre-heated furnace at 850 °C. Then, in a press furnace (EP500 Ivoclar Vivadent), a ceramic ingot (IPS e.max Press) that matched the desired tooth shade was plastified at 920 °C and pressed into the mold of the investment under vacuum. After cooling down, controlled removal of the investment ring was followed by rough [polishing jet medium (Ivoclar Vivadent) at 4 bar] and fine divestment (polishing jet medium at 2 bar), elimination of the reaction layer (30-min bath in Invex liquid: Ivoclar Vivadent), water flush, and blasting (Al 2 O 3 Type 100 μm at 1 bar). Thereafter, fine diamond disks (that were guided through a wet sponge) were used to cut the sprues. Further adjustments were obtained by fine-grained diamond instruments with water spray cooling at a maximum speed of 15,000 rpm.
At the next clinical appointment, the fit of the framework was evaluated intraorally by fit checker material (Xantopren blau, Heraeus Kulzer, Germany). Particular attention was paid to a passive fit of FDP frameworks. For adjustments, red-coded, ball-shaped diamond burs were used with water spray cooling. Anterior restorations were veneered with wax (Chromowax nach Wohlwend, Benzer Dental, Zurich, Switzerland) to allow a realistic forecast of the future morphologic outcome. The interocclusal relation between the framework and the antagonists was recorded with a bite registration made from self-curing resin (Pattern Resin, GC Europe, Leuven, Belgium). Taking into account the clinical information, occasionally the master dies had to be re-articulated. Frameworks were prepared for the firing process by careful blasting with aluminum oxide (grain size 100 μm, 1 bar), cleaning with steam, and drying with oil-free air. After foundation firing, fluorapatite veneering ceramic (IPS Eris for E2, Ivoclar Vivadent) was applied using a layering technique.
The finalized restorations were examined clinically. If grinding and polishing was necessary to achieve a perfect fit, the temporary restorations were cemented again and the restorations were finished in the laboratory by additional glaze firing.
Selection of the cementation mode was performed based on the following guidelines (precondition-oriented cementation protocol):
Adhesive bonding (IPS Ceramic etchant/Monobond S/Variolink II, Ivoclar Vivadent) was preferred in the following situations:
Abutment height of 4 mm or less
Angle of convergence of more than 10°
Conventional cementation with Glass-ionomer cement (Ketac Cem Maxi Cap, 3 M Espe) was used in the following situations:
Angle of convergence of less than 10°
Abutment height of more than 4 mm
Patients with allergy to any component of the adhesive technique
The working field was located in an area difficult to isolate.
In case of adhesive cementation, if possible absolute isolation was performed using a rubber dam. Otherwise, relative isolation and placement of a retraction cord (size 0, Ultrapak, without impregnation) were used for moisture control. Adhesive and conventional cementation was conducted according to the manufacturer’s instructions.
Clinical examinations for baseline and follow-up data
Baseline data were recorded immediately after cementation.
The first follow-up appointment was at 6 months after insertion. Annual appointments were conducted up to 3 years after cementation, followed by a 5-year follow-up. Finally, in the years 2012 and 2013 (approximately 7 years after the last regular recall) all case report forms and all regular patient files of the Uniklinik Aachen were screened for incidents related to the IPS Empress 2 restorations. Moreover, patients were requested to allow data transfer between the dentist that had performed the dental check-up during the previous years and the authors of the present study. Finally, patients were again invited to participate in another comprehensive clinical examination.
The same experienced clinician performed all the recall examinations. Also, each clinical finding (e.g. complication, or failure) was reviewed by two of the co-authors.
During screening of the case report forms/medical files, as well as during i) patient interviews, ii) data transfer between private practices and the Uniklinik Aachen and iii) clinical examinations, data on the following were collected:
Biological complications such as endodontic or periodontal disease, occurrence of caries, tooth fracture, and trauma.
Technical complications such as marginal discrepancy, loss of retention, minor chipping (according to Heintze/Rousson 2010 : clinical polishing or repair is possible), major chippings (according to Heintze/Rousson 2010 : clinical polishing or repair is not possible, replacement is necessary), and fracture of the framework material.
In-situ presence of the restoration, or if not: the reason why restoration was replaced, such as fracture of the framework or teeth, catastrophic periodontal or endodontic disease, or tooth decay that caused extraction of the abutment teeth, or demand for a new prosthetic treatment which required renewing the study’s restorations (hereafter referred to as ‘Change of the prosthetic treatment plan’).
Moreover, during the clinical appointments, the following variables were assessed:
Periodontal parameters such as plaque index , gingival index , probing depth, and bleeding on probing of the abutment teeth.
California Dental Association (CDA) criteria .
In summary: after this ‘data mining’, five sources were available for updating our state of knowledge concerning the in-situ presence or absence of the IPS Empress 2 restorations: 1) clinical examination, 2) data transfer with private practices, 3) patient files, 4) case report forms of the Uniklinik Aachen, and 5) the patient interview itself ( Table 1 ). For analysis of the clinical quality (i.e. periodontal parameters and the CDA criteria) only the latest clinical data detected by the authors were used.