- Email: [email protected]
Two-year clinical evaluation of lithia-disilicate-based all-ceramic crowns and fixed partial dentures
d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Two-year clinical evaluation of lithia-disilicate-based all-ceramic crowns and fixed partial dentures ¨ b Burak Taskonak a,∗ , Atilla Sertgoz a
Department of Restorative Dentistry, Division of Dental Biomaterials, Indiana University, School of Dentistry, 1121 W. Michigan Street, Indianapolis, IN 46202, United States b Department of Prosthodontics, Marmara University College of Dentistry, Istanbul, Turkey
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aim of this study was to evaluate the clinical performance of crowns and
Received 14 July 2005
fixed partial dentures (FPDs) made with the Empress 2 system over a 2-year period.
Received in revised form
Methods. Twenty anterior or posterior all-ceramic (Empress 2) crowns and 20 anterior or
4 October 2005
posterior, three-unit fixed partial dentures were fabricated for 15 patients. Evaluations of
Accepted 4 October 2005
the restorations were performed at baseline and once a year during the 2-year follow-up period. U.S. Public Health Service criteria were used to examine the marginal adaptation, color match, secondary caries and visible fractures in the restorations. Survival rate of the
Keywords:
restorations were determined using Kaplan–Meier statistical analysis.
Dental ceramic
Results. U.S. Public Health Service criteria showed 100% Alpha scores concerning recurrent
Clinical evaluation
caries for both crowns and FPDs. No crown fractures were observed during the 2-year follow-
Failure of fixed partial dentures
up, however, 10 (50%) catastrophic failures of FPDs occurred. Five (25%) failures occurred within the 1-year clinical period and the others (25%) within the second year. Significance. Single unit Empress 2 all-ceramic crowns exhibited a satisfactory clinical performance over 2-year period. Furthermore, the high fracture rate of Empress 2 FPDs limits the usage of Empress 2 for the fabrication of all-ceramic FPD. © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The improved toughness and strength of dental ceramics have increased their clinical use in multi-unit fixed partial dentures [1–6]. However, toughness and initial strength of dental ceramics are not the only factors that determine long-term survival. Factors like stress corrosion and subcritical crack growth due to repetitive occlusal contact have significant effect on survival of dental ceramics [7]. Metal–ceramic prostheses are commonly used for fixed partial dentures (FPDs) [4]. Previous studies revealed that metal–ceramic FPDs have a high survival rate of 98, 90 and 85% at 5, 10 and 15 years, respectively [6,8,9]. However, there
∗
are very few clinical studies that evaluate the long-term success of all-ceramic restorations [4]. Recent clinical studies reported high failure rates for all-ceramic FPDs compared to metal–ceramic FPDs especially when they are used in the posterior region [10–12]. Fabrication of all-ceramic dental crowns is challenging because exceptional skills of a technician are required to provide minimal stress concentration areas using proper occlusal design and accurate marginal fit [10]. In addition, ceramic crowns must be translucent and resistant to fracture even in clinical situations where inadequate thickness precludes optimal design. Natural translucency is needed to achieve an appearance similar to that of human teeth. The ceramic
Corresponding author. Tel.: +1 317 274 3725; fax: +1 317 278 7462. E-mail address: [email protected] (B. Taskonak).
0109-5641/$ – see front matter © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.11.028
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d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
core of ceramic restorations may be fabricated from feldspathic porcelain, aluminous porcelain, lithia-disilicate-based ceramic, glass infiltrated magnesia aluminate spinel, glassinfiltrated alumina, glass-infiltrated zirconia and mica-based glass–ceramics [13]. However, poor resistance to fracture has been a limiting factor in their use, especially for multi-unit fixed partial dentures [10]. Lithia-disilicate-based (Li2 O·2SiO2 ) IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) is an all-ceramic system developed in response to a high demand for all-ceramic FPDs. However, long-term clinical studies with Empress 2 system are required to determine whether they can serve as a feasible replacement for metal–ceramic systems. The aim of this study was to determine the clinical survival of IPS Empress 2 crowns and FPDs over a 2-year period using modified U.S. Public Health Service evaluation criteria [14,15].
2.
Materials and methods
A total of 20 crowns and 20 FPDs were placed on an experimental population of 15 patients (3 men and 12 women, aged 21–59). An informed consent received from each patient mentioning that the study is in compliance with the use of human subjects according to the Turkish Ministry of Health. Crowns were indicated for 20 teeth because existing crown restorations had secondary caries, apical lesions, fracture or lack of esthetics. Indications for FPDs included replacement of an incisor or a first premolar tooth, or an inadequate existing anterior FPD. Three-unit FPDs were fabricated following a design that required a minimum 3.5 mm core thickness buccolingually, occlusogingivally or mesiodistally in the connector areas. Connector thicknesses of each FPD were measured before cementation using a micrometer (Mitutoyo Corp., Kanagawa, Japan) with accuracy to 0.01 mm to ensure that the FPDs met the connector design criteria. The locations, survival time and evaluation time (in months) of the FPDs are shown in Table 1. In addition Table 2 shows the distribution of the Empress 2 crowns according to their evaluation time (in months). Six of the prepared teeth received endodontic treatment. For those that were endodontically treated, three were abutments for FPDs and restored with a cast dowel-and-core (Cosmopost, Ivoclar AG, Liechtenstein). The remaining three endodontically treated teeth did not receive a dowel-and-core restoration. The tooth preparations consisted of a shoulder finish line with rounded, smooth contours to obtain maximum fit of the finished prostheses (Fig. 1). To optimize the load carrying capacity of the ceramic prostheses and to maximize esthetics, a shoulder width of approximately 1.5 mm was prepared. A flat cylinder-shaped dental bur with the diameter of 1.4 mm was used to obtain the shoulder width of approximately 1.5 mm (Shape 109; Dentsply, York, PA). Occlusal reductions of prepared teeth were approximately 2 mm. Fine diamond rotary cutting instruments were used for definitive tooth contouring and finish line enhancement. The smoothness of the finish line and the ability to transfer the details to the refractory die is essential for the precision and the fit of the coping. To ensure high definition of the margins in impressions, retrac-
Table 1 – Location, survival time (in months) and failure type of Empress 2 FPDs Location 31–33 23–25 21–23 33–35 21–23 42–31 11–22 23–25 21–23 21–23 12–11 11–22 13–15 41–43 23–25 13–15 12–21 13–15 33–35 41–32
Survival time (months)
Failure type
9 10 11 11 13 19 20 20 20 21 22 22 22 22 23 23 24 24 25 27
Veneer fracture (chipping) Connector fracture Connector fracture Connector fracture Veneer fracture (chipping) Connector fracture Intact Intact Intact Intact Connector fracture Connector fracture Intact Intact Connector fracture Connector fracture Intact Intact Intact Intact
Table 2 – Number of Empress 2 crowns according to their evaluation time Survival time (months) 19 20 21 22 23 24
Number of crowns Incisors
Premolars
Molars
2 1 2
1
1
1 1 2
2
1 6
Total 2 3 2 3 2 8
tion cord (Stay-put; Roeko, Langenau, Germany) was used. Polyvinylsiloxane impression material [combination of putty and a light body wash] (Extrude; Kerr, Romulus, Mich) was used for complete arch impressions. Provisional crowns and FPDs
Fig. 1 – Facial view of prepared maxillary left central incisor, maxillary left canine, mandibular right central incisor and mandibular left lateral incisor for Empress 2 fixed partial denture.
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d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
Fig. 2 – Definitive prostheses for teeth shown in Fig. 1. Labial view of two Empress 2 fixed partial dentures for maxillary left central incisor to maxillary left canine and mandibular right central incisor to mandibular left lateral incisor region at baseline.
were prepared to maintain gingival health and to maintain tooth position. All prostheses were prepared by the same certified dental technician using a layering technique. Recordings of margin integrity and periodontal health were made after the cementation appointment (Fig. 2). After the trial insertion, the intaglio surface of the ceramic prosthesis was etched (5% HF, IPS ceramic etching gel; Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s, then rinsed, dried and silanated for 60 s (Monobond-S; Ivoclar Vivadent, Schaan, Liechtenstein). Prepared tooth surfaces were conditioned with 37% H3 PO4 (Email Preparator GS; Ivoclar Vivadent, Schaan, Liechtenstein) for 30 s. Dentin primer (Syntac; Vivadent, Schaan, Liechtenstein) and dentin adhesive (Syntac; Vivadent, Schaan, Liechtenstein) were applied to rinsed and dried dentin surfaces. Subsequently, bonding agent (Heliobond; Vivadent,
Schaan, Liechtenstein) was brushed on to dentin surface and intaglio surface of the prosthesis. A low viscosity luting composite catalyst and base (Variolink II; Vivadent, Schaan, Liechtenstein) were mixed and cementation was performed immediately after coating the intaglio surface of the prosthesis with luting agent. Excess cement was removed using a thin brush, explorer and dental floss, respectively. Luting agent was polymerized from each surface using visible light with an irradiance of 480 mW/cm2 (Optilux; Demetron Inc., Danbury, CT) for 60 s. The occlusion and articulation of the IPS Empress crowns were controlled carefully using a 80 m thick artic` ulating paper (Hanel; Coltene/Whaledent GmbH, Langenau, Germany) during the try-in procedure and after the crowns were luted [16]. Occlusal adjustments made after cementation is determined as an exclusion criterion and FPDs that needed occlusal adjustments were not included in the study. Clinical procedures were performed by the same clinician for all the restorations. The United States Public Dental Health (USPHS) criteria were used to evaluate the quality of the restorations (Table 3) [14,15]. Alpha, Bravo and Charlie rankings were recorded and percent distributions were analyzed for each year. Each restoration was evaluated 2 days after cementation (baseline), and after 1 and 2 years. Evaluations were performed by two clinicians using a mirror, explorer and intraoral photographs. Agreement between the two clinicians was 95%. Disagreements were resolved through discussion. Kaplan–Meier statistics were used to analyze the survival rates of the restorations [17].
3.
Results
Of the 20 Empress 2 FPDs evaluated, 50% were rated satisfactory and 100% of the 20 Empress 2 crowns were rated satisfactory after a 2-year evaluation period. Distributions of the scores of the evaluated variables, color and surface, anatomic
Table 3 – Criteria for the direct evaluation of the restorations Category
Score
Anatomic form
Alpha Bravo
Charlie
Marginal adaptation
Alpha Bravo Charlie
Criteria Restoration is continuous with tooth anatomy Slightly under- or overcontoured restoration; marginal ridges slightly undercontoured; contact slightly open (may be self-correcting); occlusal height reduced locally Restoration is undercontoured, dentin or base exposed; contact is faulty, not self-correcting; occlusal height reduced; occlusion affected Restoration is continuous with existing anatomic form, explorer does not catch Explorer catches, no crevice is visible into which explorer will penetrate Crevice at margin, enamel exposed
Color match and surface texture
Alpha Bravo Charlie
Excellent color match and smooth surface Good color match and slightly rough or pitted surface Slight mismatch in color, shade or translucency and rough surface, cannot be refinished
Caries
Alpha Bravo
No evidence of caries contiguous with the margin of the restoration Caries is evident contiguous with the margin of the restoration
Post-operative sensitivity
Alpha Bravo
No sensitivity Slight sensitivity
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d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
Table 4 – FPD restorations Empress 2 FPDs (n = 20)
Baseline
Surface texture and color Anatomic form Marginal adaptation Caries Post-operative sensitivity
Year 1 recall
Alpha
Bravo
Alpha
Bravo
85 75 85 100 90
15 25 15
80 67 60 100 90
20 33 15
10
Year 2 recall Charlie
25
10
Alpha
Bravo
60 70 54 100 100
40 30 13
Charlie
33
Scores of clinical evaluation (%) at baseline, year 1 and year 2 (surface texture and color, anatomic form values were calculated for n = 15 for first year and n = 10 for second year).
Table 5 – Crown restorations Empress 2 crowns (n = 20)
Surface texture and color Anatomic form Marginal adaptation Caries Post-operative sensitivity
Baseline Alpha
Bravo
80 90 70 100 95
20 10 20
Year 1 recall Charlie
10
5
Alpha
Bravo
80 90 40 100 95
20 10 40
Year 2 recall Charlie
Alpha
Bravo
65 80 25 100 100
35 20 55
20
5
Charlie
20
Scores of clinical evaluation (%) at baseline, year 1 and year 2.
Table 6 – Scores of clinical evaluation for plaque and gingival index (%) at baseline, year 1 and year 2 Empress 2 FPDs Baseline Plaque index 0 1
100
Gingival index 0 90 1 10
Empress 2 crowns
Year 1 recall
Year 2 recall
Baseline
Year 1 recall
67 33
80 20
100
70 30
60 40
85 15
85 15
95 5
80 10
75 25
form, marginal integrity and post-operative sensitivity are presented in Tables 4–6. Fractures in the connector area of three FPDs (15%) and two local chipping failures (10%) were recorded at the 1-year recall exam (Fig. 3) (Table 1). In addition, five more connector
Fig. 3 – Fractured connector in anterior Empress 2 fixed partial denture after 11 months of clinical service.
Year 2 recall
fractures (25%) were observed in the remaining FPDs in the second year. All eight (40%) fractures occurred in the connector areas. Even though the score for surface texture parameter was 85% Alpha at the baseline; it decreased to 80% Alpha at the end of the first year and to 60% at the end of the second year (Table 4) that the differences were clinically significant according to USPHS criteria [14,15]. A clinically significant difference between baseline, 1st year and 2nd year was not observed in the scores of anatomic form, caries and sensitivity for the FPDs. Caries was given all Alpha rating. However, the score for marginal integrity decreased from an 85% Alpha score to a 54% Alpha score (Table 4) that was clinically significant. Fractures were not observed in any of the Empress 2 crowns. Crown restorations were rated an 80% Alpha score for color and surface parameter at the baseline exam. This value decreased to 65% at the second year recall examinations (Table 5). The marginal adaptation category received a 70% Alpha score at baseline that decreased to a 40% Alpha score by the end of first year and to a 25% Alpha score by the end of second year (Table 5). The crowns were not associated with secondary caries during the 2-year evaluation period. Even though tooth sensitivity was rated a 95% Alpha score at the baseline exam, it increased to a 100% Alpha score by the end of second year (Table 5). There was no clinically signif-
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d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
Fig. 4 – Kaplan–Meier survival statistics of Empress 2 fixed partial dentures (n = 20).
icant difference between the baseline, first year and second year scores of plaque index and gingival index of FPD and crown restorations (Table 6). Kaplan–Meier statistics revealed that the survival rate for Empress 2 FPDs at 2 years was 50% (Fig. 4).
4.
Discussion
The Empress 2 core ceramic is composed of crystalline and glass phases. The crystalline Empress 2 core consists of elongated lithia-disilicate crystals (Li2 Si2 O5 ) [18,19]. The lithiadisilicate crystalline content in the hot-pressed core ceramic is 70 ± 5 vol.% [18,19]. Empress 2 veneer also consists of a glass and one crystalline phase [20]. The latter is reported to be fluorapatite and the crystal volume fraction is less than that of the Empress 2 core [20]. X-ray diffraction analysis showed that the Empress 2 veneer is an amorphous glass. Empress 2 core ceramic has a flexural strength ( f ) of 215 (±40) MPa, elastic modulus (E) of 96 GPa, characteristic strength ( 0 ) of 231 MPa, Weibull modulus (m) of 5 and fracture toughness of 3.4 MPa m1/2 [19]. Fabrication of three-unit anterior FPDs using Empress 2 system is recommended by the manufacturer because of the high fracture toughness of Empress 2 core ceramic [20]. However, in this situation when the core layer is coated with the low-strength glass veneer, the resulting ceramic composite has a significantly lower strength compared to the core ceramic [3,18,21–23]. Empress 2 glass veneer has a flexural strength of 64 (±6) MPa, elastic modulus (E) of 65 GPa and fracture toughness of 0.7 MPa m1/2 [3,19]. Distribution of stresses is affected by the elastic modulus differences between the glass veneer and ceramic core. These stresses in bilayer ceramic composite are different than those in the monolithic core material. If a material subjected to bending is made of two or more materials with different elastic moduli, the determination of stresses must be modified [3,18,21–23]. For instance, when a bar consisting of two portions of different materials bonded together is tested in flexure, the strain in the bar varies linearly with the distance from the neutral axis of the section. However, the neutral axis of the bilayer bar does not pass through the centroid of the composite section unlike mono-
lithic bars [21]. The location of neutral axis must be calculated according to the elastic modulus and thickness of each layer. In this specific situation, the outer layer is a glass and will result in lower failure stress in core/veneer composite than in the monolithic core ceramic since the neutral axis moves towards the glass layer due to its low elastic modulus [3,21–23]. The large number of pre-existing ceramic cracks of differing sizes, coupled with low fracture toughness, limits the strength of ceramics and causes a large variability in strength [22,24]. Additionally, in a severe contact event surface cracks can propagate to cause fracture [7,25]. Consequently, it is reported in a previous paper that crack propagation in Empress 2 core/veneer bilayer specimens continued through the Empress 2 veneer–core interface [3,18,22]. Thus, the increased toughness of the core compared with the veneer did not affect crack initiation or propagation. For any bilayer composite, the combination of loading and toughness of interfaces will determine if failure occurs because of delamination of the interface or flexural/tensile failure of the composite [23,26]. In the absence of interface fracture, toughened multiphase and laminated ceramics are susceptible to brittle failure in a manner similar to fine grained, homogenous ceramics [23,26,27]. In the present study, a high fracture rate (50%) was observed for veneered Empress 2 FPDs during the examined period. This outcome might be related to the presence of a low toughness Empress 2 glass veneer and flaws at the surface of the veneer layer (0.7 MPa m1/2 ) [3,18]. Specific emphasis should be placed on the crack initiating flaws and crack origins of the specimens retrieved from clinical studies to understand the certain cause of failures. In a previous study, 94.6% satisfactory rate was reported in 37 IPS Empress crowns after the mean evaluation period of 24.56 months [28]. The results of this study confirmed these results for crown restorations; however, failure rates for FPD restorations were significantly higher. Reasons for this result might be because of longer follow-up period, difference in luting cements, veneer thickness and thickness of the overall restoration. In addition, occlusion types in the patient pool can have an effect on the outcome. Even though special attention was paid during occlusal adjustment sessions to minimize the occlusal loads on FPD connectors, most of the fractures occurred at the connector areas. This outcome suggests that the connector is the likely site of failure in the entire FPD structure. This finding is in agreement with the results reported in a previous in vitro study [29]. In this study, a clinically significant difference between the base line, first year and second year scores of surface and color, plaque index and gingival index parameters was not observed [14,15]. Periodic recall sessions motivate patients to pay special interest to their oral hygiene. This factor has a great effect on preserving the baseline scores for plaque and gingival index as well as surface and color [28]. There was a clinically significant difference between the baseline, first year and second year scores of marginal integrity parameter [14,15]. This might be because of the dissolution of high viscosity luting cement from the marginals [28]. Additionally, the precision of fit of fixed partial dentures is an
d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
important factor effecting the marginal adaptation parameter and dissolution of luting cement. Low scores for marginal adaptation observed in this study (85% Alpha at the baseline for FPDs and 70% at the baseline for crowns) are in accordance with a previous in vitro study that reported low precision of fit for Empress 2 crowns [30].
5.
Conclusions
Satisfactory clinical performance can be achieved with Empress 2 crowns however; Empress 2 FPDs have a high tendency to fracture at the connector regions.
references
[1] Esquivel-Upshaw JF, Anusavice KJ, Young H, Jones J, Gibbs C. Clinical performance of a lithia-disilicate-based core ceramic for three-unit posterior FPDs. Int J Prosthodont 2004;17:469–75. [2] Qualtrough AJE, Piddock V. Ceramics update. J Dent 1997;25:91–5. [3] Taskonak B, Mecholsky Jr JJ, Anusavice KJ. Residual stresses in bilayer dental ceramics. Biomaterials 2005;26:3235–41. ´ [4] Suarez MJ, Salido MP, Mart´ınez F. Three-year clinical evaluation of In-Ceram® zirconia posterior FPDs. Int J Prosthodont 2004;17:35–8. [5] Kelly J. Ceramics in restorative and prosthetic dentistry. Annu Rev Mater Sci 1997;27:443–68. [6] Scurria MS, Bader JD, Shugars DA. Meta-analysis of fixed partial denture survival: prostheses and abutments. J Prosthet Dent 1998;79:459–64. [7] Zhang Y, Lawn B. Competing damage modes in all-ceramic crowns: fatigue and lifetime. Key Eng Mater 2005;17: 697–700. ¨ [8] Creugers NHJ, Kayser AF, van’t Hof MA. A meta-analysis of durability data on conventional fixed bridges. Community Dent Oral Epidemiol 1994;22:448–52. [9] Walton TR. An up to 15-year longitudinal study of 515 metal-ceramic FPDs: Part 1 outcome. Int J Prosthodont 2002;15:439–45. [10] Kelly JR, Nishimura I, Campbell SD. Ceramics in dentistry: historical roots and current perspectives. J Prosthet Dent 1996;75:18–32. [11] Sorensen JA, Kang S-K, Torres TJ, Knode H. In-ceram fixed partial dentures: three year clinical trial results. J Calif Dent Assoc 1998;26:207–14.
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[12] Olsson K, Furst B, Andersson B, Carlsson GE. A long term retrospective study and clinical follow-up study of In-ceram alumina FPDs. Int J Prosthodont 2000;13:131–5. [13] Campbell SD, Sozio RB. Evaluation of the fit and strength of all-ceramic fixed partial denture. J Prosthet Dent 1988;59:301–6. [14] Ryge G, Snyder M. Evaluating the clinical quality of restorations. J Am Dent Assoc 1973;87:369–77. [15] Ryge G. Clinical criteria. Int Dent J 1980;30:347–58. [16] Shillinburg HT, Hobo S, Whitsett LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence; 1997. p. 451. [17] Kaplan EL, Meier P. Non-parametric estimation from incomplete observations. J Am Stat Assoc 1958;53:457–65. [18] Taskonak B, Anusavice KJ, Mecholsky Jr JJ. Role of investment interaction layer on strength and toughness of ceramic laminates. Dent Mater 2004;20:701–8. [19] Della Bona A, Mecholsky JJ, Anusavice KJ. Fracture behavior of lithia-disilicate- and leucite-based ceramics. Dent Mater 2004;20:956–62. ¨ [20] Holand W, Schweiger M, Frank M, Rheinberger V. A comparison of the microstructure and properties of the IPS Empress 2 and the IPS Empress glass–ceramics. Appl Biomater 2000;53:297–303. [21] Beer FP, Johnston ER. Mechanics of materials. 3rd ed. New York: McGraw-Hill; 2002. p. 209–30. [22] Della Bona A, Anusavice KJ, DeHoff PH. Weibull analysis and flexural strength of hot-pressed core and veneer structures. Dent Mater 2003;19:662–9. [23] Thompson GA. Influence of relative layer height and testing method on the failure mode and origin in a bilayered dental ceramic composite. Dent Mater 2000;16:235–43. [24] Mecholsky Jr JJ. Fracture mechanics principles. Dent Mater 1995;11:111–2. [25] Lawn BR. Fracture of brittle solids. 2nd ed. Cambridge: Cambridge University Press; 1993. p. 106–12. [26] Zhang Y, Pajares A, Lawn B. Fatigue and damage tolerance of Y-TZP ceramics in layered biomechanical systems. J Biomed Mater Res 2004;71B:166–71. [27] Prakash O, Sarkar P, Nicholson PS. Crack deflection with ceramic/ceramic laminates with strong interfaces. J Am Ceram Soc 1995;78:1125–7. [28] Gemalmaz D, Ergin S. Clinical evaluation of all-ceramic crowns. J Prosthet Dent 2002;87:189–96. ¨ [29] Oh W, Gotzen N, Anusavice KJ. Influence of connector design on fracture probability of ceramic fixed partial dentures. J Dent Res 2002;81:623–7. [30] Konstantinos P, Griggs JA, Woody RD, Guillen GE, Miller AW. Fracture resistance of three all-ceramic systems for posterior applications. J Prosthet Dent 2004;91:561–9.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Two-year clinical evaluation of lithia-disilicate-based all-ceramic crowns and fixed partial dentures ¨ b Burak Taskonak a,∗ , Atilla Sertgoz a
Department of Restorative Dentistry, Division of Dental Biomaterials, Indiana University, School of Dentistry, 1121 W. Michigan Street, Indianapolis, IN 46202, United States b Department of Prosthodontics, Marmara University College of Dentistry, Istanbul, Turkey
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aim of this study was to evaluate the clinical performance of crowns and
Received 14 July 2005
fixed partial dentures (FPDs) made with the Empress 2 system over a 2-year period.
Received in revised form
Methods. Twenty anterior or posterior all-ceramic (Empress 2) crowns and 20 anterior or
4 October 2005
posterior, three-unit fixed partial dentures were fabricated for 15 patients. Evaluations of
Accepted 4 October 2005
the restorations were performed at baseline and once a year during the 2-year follow-up period. U.S. Public Health Service criteria were used to examine the marginal adaptation, color match, secondary caries and visible fractures in the restorations. Survival rate of the
Keywords:
restorations were determined using Kaplan–Meier statistical analysis.
Dental ceramic
Results. U.S. Public Health Service criteria showed 100% Alpha scores concerning recurrent
Clinical evaluation
caries for both crowns and FPDs. No crown fractures were observed during the 2-year follow-
Failure of fixed partial dentures
up, however, 10 (50%) catastrophic failures of FPDs occurred. Five (25%) failures occurred within the 1-year clinical period and the others (25%) within the second year. Significance. Single unit Empress 2 all-ceramic crowns exhibited a satisfactory clinical performance over 2-year period. Furthermore, the high fracture rate of Empress 2 FPDs limits the usage of Empress 2 for the fabrication of all-ceramic FPD. © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The improved toughness and strength of dental ceramics have increased their clinical use in multi-unit fixed partial dentures [1–6]. However, toughness and initial strength of dental ceramics are not the only factors that determine long-term survival. Factors like stress corrosion and subcritical crack growth due to repetitive occlusal contact have significant effect on survival of dental ceramics [7]. Metal–ceramic prostheses are commonly used for fixed partial dentures (FPDs) [4]. Previous studies revealed that metal–ceramic FPDs have a high survival rate of 98, 90 and 85% at 5, 10 and 15 years, respectively [6,8,9]. However, there
∗
are very few clinical studies that evaluate the long-term success of all-ceramic restorations [4]. Recent clinical studies reported high failure rates for all-ceramic FPDs compared to metal–ceramic FPDs especially when they are used in the posterior region [10–12]. Fabrication of all-ceramic dental crowns is challenging because exceptional skills of a technician are required to provide minimal stress concentration areas using proper occlusal design and accurate marginal fit [10]. In addition, ceramic crowns must be translucent and resistant to fracture even in clinical situations where inadequate thickness precludes optimal design. Natural translucency is needed to achieve an appearance similar to that of human teeth. The ceramic
Corresponding author. Tel.: +1 317 274 3725; fax: +1 317 278 7462. E-mail address: [email protected] (B. Taskonak).
0109-5641/$ – see front matter © 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.11.028
1009
d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
core of ceramic restorations may be fabricated from feldspathic porcelain, aluminous porcelain, lithia-disilicate-based ceramic, glass infiltrated magnesia aluminate spinel, glassinfiltrated alumina, glass-infiltrated zirconia and mica-based glass–ceramics [13]. However, poor resistance to fracture has been a limiting factor in their use, especially for multi-unit fixed partial dentures [10]. Lithia-disilicate-based (Li2 O·2SiO2 ) IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) is an all-ceramic system developed in response to a high demand for all-ceramic FPDs. However, long-term clinical studies with Empress 2 system are required to determine whether they can serve as a feasible replacement for metal–ceramic systems. The aim of this study was to determine the clinical survival of IPS Empress 2 crowns and FPDs over a 2-year period using modified U.S. Public Health Service evaluation criteria [14,15].
2.
Materials and methods
A total of 20 crowns and 20 FPDs were placed on an experimental population of 15 patients (3 men and 12 women, aged 21–59). An informed consent received from each patient mentioning that the study is in compliance with the use of human subjects according to the Turkish Ministry of Health. Crowns were indicated for 20 teeth because existing crown restorations had secondary caries, apical lesions, fracture or lack of esthetics. Indications for FPDs included replacement of an incisor or a first premolar tooth, or an inadequate existing anterior FPD. Three-unit FPDs were fabricated following a design that required a minimum 3.5 mm core thickness buccolingually, occlusogingivally or mesiodistally in the connector areas. Connector thicknesses of each FPD were measured before cementation using a micrometer (Mitutoyo Corp., Kanagawa, Japan) with accuracy to 0.01 mm to ensure that the FPDs met the connector design criteria. The locations, survival time and evaluation time (in months) of the FPDs are shown in Table 1. In addition Table 2 shows the distribution of the Empress 2 crowns according to their evaluation time (in months). Six of the prepared teeth received endodontic treatment. For those that were endodontically treated, three were abutments for FPDs and restored with a cast dowel-and-core (Cosmopost, Ivoclar AG, Liechtenstein). The remaining three endodontically treated teeth did not receive a dowel-and-core restoration. The tooth preparations consisted of a shoulder finish line with rounded, smooth contours to obtain maximum fit of the finished prostheses (Fig. 1). To optimize the load carrying capacity of the ceramic prostheses and to maximize esthetics, a shoulder width of approximately 1.5 mm was prepared. A flat cylinder-shaped dental bur with the diameter of 1.4 mm was used to obtain the shoulder width of approximately 1.5 mm (Shape 109; Dentsply, York, PA). Occlusal reductions of prepared teeth were approximately 2 mm. Fine diamond rotary cutting instruments were used for definitive tooth contouring and finish line enhancement. The smoothness of the finish line and the ability to transfer the details to the refractory die is essential for the precision and the fit of the coping. To ensure high definition of the margins in impressions, retrac-
Table 1 – Location, survival time (in months) and failure type of Empress 2 FPDs Location 31–33 23–25 21–23 33–35 21–23 42–31 11–22 23–25 21–23 21–23 12–11 11–22 13–15 41–43 23–25 13–15 12–21 13–15 33–35 41–32
Survival time (months)
Failure type
9 10 11 11 13 19 20 20 20 21 22 22 22 22 23 23 24 24 25 27
Veneer fracture (chipping) Connector fracture Connector fracture Connector fracture Veneer fracture (chipping) Connector fracture Intact Intact Intact Intact Connector fracture Connector fracture Intact Intact Connector fracture Connector fracture Intact Intact Intact Intact
Table 2 – Number of Empress 2 crowns according to their evaluation time Survival time (months) 19 20 21 22 23 24
Number of crowns Incisors
Premolars
Molars
2 1 2
1
1
1 1 2
2
1 6
Total 2 3 2 3 2 8
tion cord (Stay-put; Roeko, Langenau, Germany) was used. Polyvinylsiloxane impression material [combination of putty and a light body wash] (Extrude; Kerr, Romulus, Mich) was used for complete arch impressions. Provisional crowns and FPDs
Fig. 1 – Facial view of prepared maxillary left central incisor, maxillary left canine, mandibular right central incisor and mandibular left lateral incisor for Empress 2 fixed partial denture.
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Fig. 2 – Definitive prostheses for teeth shown in Fig. 1. Labial view of two Empress 2 fixed partial dentures for maxillary left central incisor to maxillary left canine and mandibular right central incisor to mandibular left lateral incisor region at baseline.
were prepared to maintain gingival health and to maintain tooth position. All prostheses were prepared by the same certified dental technician using a layering technique. Recordings of margin integrity and periodontal health were made after the cementation appointment (Fig. 2). After the trial insertion, the intaglio surface of the ceramic prosthesis was etched (5% HF, IPS ceramic etching gel; Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s, then rinsed, dried and silanated for 60 s (Monobond-S; Ivoclar Vivadent, Schaan, Liechtenstein). Prepared tooth surfaces were conditioned with 37% H3 PO4 (Email Preparator GS; Ivoclar Vivadent, Schaan, Liechtenstein) for 30 s. Dentin primer (Syntac; Vivadent, Schaan, Liechtenstein) and dentin adhesive (Syntac; Vivadent, Schaan, Liechtenstein) were applied to rinsed and dried dentin surfaces. Subsequently, bonding agent (Heliobond; Vivadent,
Schaan, Liechtenstein) was brushed on to dentin surface and intaglio surface of the prosthesis. A low viscosity luting composite catalyst and base (Variolink II; Vivadent, Schaan, Liechtenstein) were mixed and cementation was performed immediately after coating the intaglio surface of the prosthesis with luting agent. Excess cement was removed using a thin brush, explorer and dental floss, respectively. Luting agent was polymerized from each surface using visible light with an irradiance of 480 mW/cm2 (Optilux; Demetron Inc., Danbury, CT) for 60 s. The occlusion and articulation of the IPS Empress crowns were controlled carefully using a 80 m thick artic` ulating paper (Hanel; Coltene/Whaledent GmbH, Langenau, Germany) during the try-in procedure and after the crowns were luted [16]. Occlusal adjustments made after cementation is determined as an exclusion criterion and FPDs that needed occlusal adjustments were not included in the study. Clinical procedures were performed by the same clinician for all the restorations. The United States Public Dental Health (USPHS) criteria were used to evaluate the quality of the restorations (Table 3) [14,15]. Alpha, Bravo and Charlie rankings were recorded and percent distributions were analyzed for each year. Each restoration was evaluated 2 days after cementation (baseline), and after 1 and 2 years. Evaluations were performed by two clinicians using a mirror, explorer and intraoral photographs. Agreement between the two clinicians was 95%. Disagreements were resolved through discussion. Kaplan–Meier statistics were used to analyze the survival rates of the restorations [17].
3.
Results
Of the 20 Empress 2 FPDs evaluated, 50% were rated satisfactory and 100% of the 20 Empress 2 crowns were rated satisfactory after a 2-year evaluation period. Distributions of the scores of the evaluated variables, color and surface, anatomic
Table 3 – Criteria for the direct evaluation of the restorations Category
Score
Anatomic form
Alpha Bravo
Charlie
Marginal adaptation
Alpha Bravo Charlie
Criteria Restoration is continuous with tooth anatomy Slightly under- or overcontoured restoration; marginal ridges slightly undercontoured; contact slightly open (may be self-correcting); occlusal height reduced locally Restoration is undercontoured, dentin or base exposed; contact is faulty, not self-correcting; occlusal height reduced; occlusion affected Restoration is continuous with existing anatomic form, explorer does not catch Explorer catches, no crevice is visible into which explorer will penetrate Crevice at margin, enamel exposed
Color match and surface texture
Alpha Bravo Charlie
Excellent color match and smooth surface Good color match and slightly rough or pitted surface Slight mismatch in color, shade or translucency and rough surface, cannot be refinished
Caries
Alpha Bravo
No evidence of caries contiguous with the margin of the restoration Caries is evident contiguous with the margin of the restoration
Post-operative sensitivity
Alpha Bravo
No sensitivity Slight sensitivity
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Table 4 – FPD restorations Empress 2 FPDs (n = 20)
Baseline
Surface texture and color Anatomic form Marginal adaptation Caries Post-operative sensitivity
Year 1 recall
Alpha
Bravo
Alpha
Bravo
85 75 85 100 90
15 25 15
80 67 60 100 90
20 33 15
10
Year 2 recall Charlie
25
10
Alpha
Bravo
60 70 54 100 100
40 30 13
Charlie
33
Scores of clinical evaluation (%) at baseline, year 1 and year 2 (surface texture and color, anatomic form values were calculated for n = 15 for first year and n = 10 for second year).
Table 5 – Crown restorations Empress 2 crowns (n = 20)
Surface texture and color Anatomic form Marginal adaptation Caries Post-operative sensitivity
Baseline Alpha
Bravo
80 90 70 100 95
20 10 20
Year 1 recall Charlie
10
5
Alpha
Bravo
80 90 40 100 95
20 10 40
Year 2 recall Charlie
Alpha
Bravo
65 80 25 100 100
35 20 55
20
5
Charlie
20
Scores of clinical evaluation (%) at baseline, year 1 and year 2.
Table 6 – Scores of clinical evaluation for plaque and gingival index (%) at baseline, year 1 and year 2 Empress 2 FPDs Baseline Plaque index 0 1
100
Gingival index 0 90 1 10
Empress 2 crowns
Year 1 recall
Year 2 recall
Baseline
Year 1 recall
67 33
80 20
100
70 30
60 40
85 15
85 15
95 5
80 10
75 25
form, marginal integrity and post-operative sensitivity are presented in Tables 4–6. Fractures in the connector area of three FPDs (15%) and two local chipping failures (10%) were recorded at the 1-year recall exam (Fig. 3) (Table 1). In addition, five more connector
Fig. 3 – Fractured connector in anterior Empress 2 fixed partial denture after 11 months of clinical service.
Year 2 recall
fractures (25%) were observed in the remaining FPDs in the second year. All eight (40%) fractures occurred in the connector areas. Even though the score for surface texture parameter was 85% Alpha at the baseline; it decreased to 80% Alpha at the end of the first year and to 60% at the end of the second year (Table 4) that the differences were clinically significant according to USPHS criteria [14,15]. A clinically significant difference between baseline, 1st year and 2nd year was not observed in the scores of anatomic form, caries and sensitivity for the FPDs. Caries was given all Alpha rating. However, the score for marginal integrity decreased from an 85% Alpha score to a 54% Alpha score (Table 4) that was clinically significant. Fractures were not observed in any of the Empress 2 crowns. Crown restorations were rated an 80% Alpha score for color and surface parameter at the baseline exam. This value decreased to 65% at the second year recall examinations (Table 5). The marginal adaptation category received a 70% Alpha score at baseline that decreased to a 40% Alpha score by the end of first year and to a 25% Alpha score by the end of second year (Table 5). The crowns were not associated with secondary caries during the 2-year evaluation period. Even though tooth sensitivity was rated a 95% Alpha score at the baseline exam, it increased to a 100% Alpha score by the end of second year (Table 5). There was no clinically signif-
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Fig. 4 – Kaplan–Meier survival statistics of Empress 2 fixed partial dentures (n = 20).
icant difference between the baseline, first year and second year scores of plaque index and gingival index of FPD and crown restorations (Table 6). Kaplan–Meier statistics revealed that the survival rate for Empress 2 FPDs at 2 years was 50% (Fig. 4).
4.
Discussion
The Empress 2 core ceramic is composed of crystalline and glass phases. The crystalline Empress 2 core consists of elongated lithia-disilicate crystals (Li2 Si2 O5 ) [18,19]. The lithiadisilicate crystalline content in the hot-pressed core ceramic is 70 ± 5 vol.% [18,19]. Empress 2 veneer also consists of a glass and one crystalline phase [20]. The latter is reported to be fluorapatite and the crystal volume fraction is less than that of the Empress 2 core [20]. X-ray diffraction analysis showed that the Empress 2 veneer is an amorphous glass. Empress 2 core ceramic has a flexural strength ( f ) of 215 (±40) MPa, elastic modulus (E) of 96 GPa, characteristic strength ( 0 ) of 231 MPa, Weibull modulus (m) of 5 and fracture toughness of 3.4 MPa m1/2 [19]. Fabrication of three-unit anterior FPDs using Empress 2 system is recommended by the manufacturer because of the high fracture toughness of Empress 2 core ceramic [20]. However, in this situation when the core layer is coated with the low-strength glass veneer, the resulting ceramic composite has a significantly lower strength compared to the core ceramic [3,18,21–23]. Empress 2 glass veneer has a flexural strength of 64 (±6) MPa, elastic modulus (E) of 65 GPa and fracture toughness of 0.7 MPa m1/2 [3,19]. Distribution of stresses is affected by the elastic modulus differences between the glass veneer and ceramic core. These stresses in bilayer ceramic composite are different than those in the monolithic core material. If a material subjected to bending is made of two or more materials with different elastic moduli, the determination of stresses must be modified [3,18,21–23]. For instance, when a bar consisting of two portions of different materials bonded together is tested in flexure, the strain in the bar varies linearly with the distance from the neutral axis of the section. However, the neutral axis of the bilayer bar does not pass through the centroid of the composite section unlike mono-
lithic bars [21]. The location of neutral axis must be calculated according to the elastic modulus and thickness of each layer. In this specific situation, the outer layer is a glass and will result in lower failure stress in core/veneer composite than in the monolithic core ceramic since the neutral axis moves towards the glass layer due to its low elastic modulus [3,21–23]. The large number of pre-existing ceramic cracks of differing sizes, coupled with low fracture toughness, limits the strength of ceramics and causes a large variability in strength [22,24]. Additionally, in a severe contact event surface cracks can propagate to cause fracture [7,25]. Consequently, it is reported in a previous paper that crack propagation in Empress 2 core/veneer bilayer specimens continued through the Empress 2 veneer–core interface [3,18,22]. Thus, the increased toughness of the core compared with the veneer did not affect crack initiation or propagation. For any bilayer composite, the combination of loading and toughness of interfaces will determine if failure occurs because of delamination of the interface or flexural/tensile failure of the composite [23,26]. In the absence of interface fracture, toughened multiphase and laminated ceramics are susceptible to brittle failure in a manner similar to fine grained, homogenous ceramics [23,26,27]. In the present study, a high fracture rate (50%) was observed for veneered Empress 2 FPDs during the examined period. This outcome might be related to the presence of a low toughness Empress 2 glass veneer and flaws at the surface of the veneer layer (0.7 MPa m1/2 ) [3,18]. Specific emphasis should be placed on the crack initiating flaws and crack origins of the specimens retrieved from clinical studies to understand the certain cause of failures. In a previous study, 94.6% satisfactory rate was reported in 37 IPS Empress crowns after the mean evaluation period of 24.56 months [28]. The results of this study confirmed these results for crown restorations; however, failure rates for FPD restorations were significantly higher. Reasons for this result might be because of longer follow-up period, difference in luting cements, veneer thickness and thickness of the overall restoration. In addition, occlusion types in the patient pool can have an effect on the outcome. Even though special attention was paid during occlusal adjustment sessions to minimize the occlusal loads on FPD connectors, most of the fractures occurred at the connector areas. This outcome suggests that the connector is the likely site of failure in the entire FPD structure. This finding is in agreement with the results reported in a previous in vitro study [29]. In this study, a clinically significant difference between the base line, first year and second year scores of surface and color, plaque index and gingival index parameters was not observed [14,15]. Periodic recall sessions motivate patients to pay special interest to their oral hygiene. This factor has a great effect on preserving the baseline scores for plaque and gingival index as well as surface and color [28]. There was a clinically significant difference between the baseline, first year and second year scores of marginal integrity parameter [14,15]. This might be because of the dissolution of high viscosity luting cement from the marginals [28]. Additionally, the precision of fit of fixed partial dentures is an
d e n t a l m a t e r i a l s 2 2 ( 2 0 0 6 ) 1008–1013
important factor effecting the marginal adaptation parameter and dissolution of luting cement. Low scores for marginal adaptation observed in this study (85% Alpha at the baseline for FPDs and 70% at the baseline for crowns) are in accordance with a previous in vitro study that reported low precision of fit for Empress 2 crowns [30].
5.
Conclusions
Satisfactory clinical performance can be achieved with Empress 2 crowns however; Empress 2 FPDs have a high tendency to fracture at the connector regions.
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