Abstract
Objectives
To explore fatigue limits of ceramic endocrowns for premolars.
Methods
Forty-eight devitalized premolars were cut at the CEJ. They were restored with standardized CAD–CAM lithium disilicate reinforced ceramic restorations (IPS e.max CAD, Ivoclar-Vivadent) and divided into four Groups (n = 12): overlays (Group A, no endo-core, negative control), endocrowns with an endo-core of 2 mm (Group B), 4 mm (Group C) and crowns with post and core (Group D, positive control). All specimens were first submitted to thermo-mechanical cyclic loading (TCML)(1.7 Hz, 49 N, 600000 cycles, 1500 thermo-cycles). Margins were analysed before and after the loading. Survived specimens were then submitted to cyclic isometric stepwise loading (5 Hz, 200N to 1200N) until completion of 105000 cycles or failure. In case of fracture, fragments were analysed using SEM and failure mode was determined. Results of stepwise loading were statistically analysed by Kaplan–Meier life survival analysis and log rank test (p = 0.05).
Results
All the specimens survived the TCML test except four specimens of Group A (early restorations’ debonding). No difference in percentages of closed margins was found between endocrowns (Groups B, C) and crowns (Group D). After the stepwise test, differences in survival within the groups were not statistically significant. Most of restorations experienced non-reparable fracture.
Conclusions
Endocrowns with both 2-mm and 4-mm long endo-cores displayed outcomes after fatigue equivalent to classical crowns. Results of this study discourage the use of flat overlays with only adhesive retention.
Clinical significance
When restoring extremely destroyed devitalized premolars, adhesive strategies should be coupled to a macro-mechanical retention in the root.
1
Introduction
Endodontically treated teeth are more fragile than vital ones . Among different reasons for this increased weakness, it is nowadays widely accepted that the privation of tooth substance due to the previous pathology and to the endodontic treatment is the most important biomechanical alteration, influencing the long-term prognostic of the tooth . When considering the restoration of devitalized teeth, dental materials should be able to replace this loss of substance in order to guarantee mechanical and functional properties, esthetics and coronal seal . Traditionally, this function is fulfilled by Porcelain Fused to Metal (PFM) or “full-ceramic” crowns which are usually constructed on a core fixed to the root through an endodontic post. The resistance to fracture of a devitalized tooth is related to many factors including the root canal treatment (risk of vertical root fractures), the post-core system (post material and size, core material, ferrule effect) and coronal issues (quality and quantity of remaining tissues, type of restoration, loading context) . Among these factors, post-retained restorations have been widely investigated but still no consensus exists on ideal materials and techniques . While in the past long metal-based cast dowel and prefabricated posts were recognized as gold standards, today bonded prefabricated glass-fiber-reinforced posts (GFRP) have gained more popularity both in clinics and in research. Main reasons for this paradigm shift are a more aesthetic approach, an elastic modulus like dentinal tissues and the possibility to bond these posts inside the root via a resin-dentin interface. Due to these features, bonded GFRP are potentially able to strengthen the root by creating an endodontic “monoblock” . Though this concept is good in theory, ideal bonding inside the root canal is still a challenge in practice. Besides the degradation of the resin-dentin interface with the time which is a generic issue met by the dentin adhesion, the peculiar anatomy of the root canal exacerbates some other concerns during adhesive application such as the tissues moisture control, the smear layer management and the adhesive volatile components removal . Moreover, the anatomy of the canal offers an extremely unfavourable surface geometry for the relief of the shrinkage stresses developed during resin cement polymerization. The C-factor (bonded/unbonded surfaces ratio) in a long and narrow root canal can exceed 1000 indeed , hindering any resin flow during hardening. Plus, all common light- and dual-polymerizable resin cements need a deep light penetration inside the canal and through the fiber post to achieve a proper degree of polymerization , which has an influence over their final bonding effectiveness . As a matter of fact, the need of a deep anchoring with a long GFRP has been criticized, basing on the logical assumption that all the aforementioned issues should increase with the increase of the post length . Also, when the invasiveness of posts over sound tissues is evaluated, the risk of root perforation and root fracture associated to long post placement should not be underestimated .
In this context, monolithic CAD-CAM endocrowns, which extend inside the pulp chamber and partially inside the root canal with a short “endo-core”, could represent an alternative to classical treatments to restore endodontically treated teeth. Endocrowns are full-composite or full ceramic overlays which restore partially or totally the coronal part of a devitalized tooth extending inside the previous pulp chamber (multirooted teeth) or the root canal (single rooted) with a dowel, namely an endo-core. The role of this extension is to stabilize the restoration inside the cavity during the cementation process – more often in case of molars – or to improve its adhesive retention inside the root, typically in severely destroyed premolars and single rooted teeth, depending on quantity and quality of remaining tissues available for adhesion. While for post-retained crown restorations the length of the intra-radicular portion has been widely debated with different results and opinions, few studies exist about the importance of this endo-core and its length over the in-vitro effectiveness of posterior endocrowns . Therefore, the aim of this in-vitro research was to test the influence of the endo-core length on the marginal adaptation, fatigue resistance and fracture mode of ceramic endocrowns to restore severely destroyed upper premolars. Different endo-core lengths were tested for endocrowns and compared to flat overlays (no endo-core, negative control) and to classical post-core-crown restorations (positive control). Considering both marginal integrity and fatigue resistance of restorations, it was hypothesized that a) endocrowns perform better than flat overlays b) the length of the endo-core has an influence on endocrowns performances and c) endocrowns perform better than crowns.
2
Materials and methods
Forty-eight freshly extracted human upper first premolars with nearly identical size were used for this study. Bucco-lingual/mesio-distal dimensions and root length of each tooth were measured using digital calipers. The inclusion criteria are absence of carious lesions, visible fracture lines in the root and a complete root formation. The teeth were stored in a sodium azide solution (0.2%) at 4 °C until the experiment onset.
2.1
Endodontic procedure
The pulp chamber of all the specimens was opened with diamond burs following a standardized procedure. A size No. 10 K-file (Dentsplay-Maillefer, Ballaigues, Switzerland) was placed to visually determine the working length. Then the root canals were prepared using manual K-files 10, 15, 20 (Dentsply-Maillefer) and rotary nickel-titanium instruments (Pro Taper, Dentsply-Maillefer), according to manufacturer’s instructions. 1 mL of 4,2% sodium hypochlorite was used to irrigate root canals during the preparation. All canals were obturated with gutta-pherca until the pulp chamber orifice using a warm vertical condensation technique (Calamus, Dentsply Tulsa Dental Specialties, Johnson City, USA). Teeth were then stored in distilled water for 24 h at 4 °C before cavity preparation.
2.2
Teeth preparation
Each specimen was fixed on a metallic holder (Baltec, Balzer, Liechtenstein) – in a vertical position – with light-curing composite; then, the root base was embedded with self-curing acrylic resin until 1 mm below the cemento-enamel junction (CEJ) to complete the tooth stabilization. For the first three Groups (n = 36), the crown of each tooth was completely cut and flattened till the CEJ. Enamel was completely removed. The roots were then randomly assigned to three Groups (n = 12): in the first Group A, roots were left with the pulp chamber filled of gutta-percha while for Groups B and C, gutta-percha was removed from the pulp chamber until 2 mm (Group B) and 4 mm(Group C) below the CEJ. For the control Group D(n = 12), the crown of each tooth was cut 1 mm above the CEJ and gutta-percha was removed from the pulp chamber and inside the palatal root canal until 5 mm below the CEJ. The outer limits of the teeth of Group D were then prepared with a chamfer (1-mm width, 1-mm high) to create a dentin core and a “ferula” effect. Enamel was completely removed. All these procedures were accomplished using coarse diamond coated burs (Cerinlay, Intensiv, Viganello, Switzerland) and finished with fine grained burs of the same shape under profuse water spray cooling.
Then, the cavity dentin surfaces of all specimens were sealed with a layer of an adhesive system (Adhese Universal,Ivoclar-Vivadent, Schaan, Liechtenstein) used in a self-etch procedure according to manufacturer recommendations. The bonding resin was polymerized for 10 s with a second generation LED high-power device (Bluephase, Ivoclar-Vivadent).
After the dentin sealing procedure, cavities of Groups A, B, and C were optically impressed (Cerec Omnicam, Sirona, Germany) and then coated with a water-based glycerin gel (Airblock, DeTrey-Dentsply, Constance, Germany) before they were provisionally restored with a soft light-curing resin (Telio CS Onlay, Ivoclar-Vivadent) and kept in saline for 7 days at 32 °C. For teeth of Group D, a pre-calibrated glass-fiber post (FRC Postec Plus,Ivoclar-Vivadent) was inserted in the palatal canal and luted with an universal dual-curing resin cement (Multilink Automix, Ivoclar-Vivadent) following manufacturer recommendations. A 4-mm high core was then fabricated with a restorative nano-hybrid resin composite (Tetric EvoCeram, Ivoclar-Vivadent). Digital impressions (Cerec, Omnicam) were taken and root cavities were provisionally restored as Groups A, B and C.
2.3
Endocrowns and crowns fabrication
All root cavities were restored with lithium disilicate reinforced CAD-CAM ceramic material (IPS e.max CAD, Ivoclar-Vivadent) and divided in four Groups (n = 12) as shown in Table 1 :
Group A (negative control): flat CAD-CAM overlays with no endo-core.
Group B: CAD-CAM endocrowns with a 2-mm high endo-core.
Group C: CAD-CAM endocrowns with a 4-mm high endo-core.
Group D (positive control): conventional CAD-CAM crowns.
Before teeth preparation, one of the extracted maxillary first premolar was chosen as a master model. An optical impression of the crown was used to fabricate the crown anatomy of the CAD-CAM restorations using the software Cerec 4.0 in Biogeneric Copy Design mode.
2.4
Luting procedures
The internal surfaces of all ceramic restorations were etched with 5% HF acid (IPS Ceramic Etching Gel, Ivoclar-Vivadent) for 20 s following manufacturer’s instructions, rinsed and dried. Etched surfaces were cleaned in an ultrasonic bath for 2 min. A layer of silane agent (Monobond Plus, Ivoclar-Vivadent) was applied over the surfaces for 60 s and then air-dried.
Root cavities (Groups A–C) and the core preparations (Group D) were submitted to airborne-particle abrasion with 27 μm alluminium-oxide particles at about 0.2 MPa pressure for 5 s (Kavo EWL, Type 5423, Biberach, Germany), rinced with profused water and dried. The silane agent (Monobond Plus, Ivoclar-Vivadent) was then applied on all the sandblasted surfaces ( Fig. 1 ).
A thin layer of dual-curing resin cement (Multilink Automix, Ivoclar-Vivadent) was spread over the preparation of all specimens. CAD-CAM restorations were then put in place first with manual pressure and then with the assistance of a specific ultra-sonic device (Cementation tip, EMS, Nyon, CH). After removal of excesses, each restoration surface was light-cured for 90 s. Restorations were then immediately finished and polished, using fine diamonds burs (first 40 μm, then 25 μm grain size) (Intensiv No 4205L, 4255, 5205L and 5255, Intensiv) and discs of decreasing grain size (Pop On XT, 3 M, St. Paul, MN, USA), from coarse (80 μm) to superfine (20 μm). Materials used in the study are shown in Table 2 .
| Material | Brand name (manufacturer) | Chemical Composition | Application Mode | Batch Number |
|---|---|---|---|---|
| Adhesive System | Adhese Universal (Ivoclar-Vivadent) | Methacrylates, ethanol, water, highly dispersed silicon dioxide, initiators and stabilizers. | The adhesive is scrubbed into the tooth surface for at least 20 s. Light-curing for 10 s. | T02457 |
| Hydrofluoric Acid | IPS Ceramic etching Gel (Ivoclar-Vivadent) | 5% hydrofluoric acid. | Application for 20 s then rinsing and drying. | S45272 |
| Silane | Monobond Plus (Ivoclar-Vivadent) | Alcohol solution of silane methacrylate, phosphoric acid methacrylate and sulphide methacrylate. | Application of a thin coat of Monobond Plus with a brush or a microbrush to the pre-treated surfaces (avoid pooling when treating crowns). Allow the material to react for 60 s. Subsequently, disperse any remaining excess with a strong stream of air. | S49812 |
| Resin Cement | Multilink Automix (Ivoclar-Vivadent) | The monomer matrix is composed of dimethacrylate and HEMA. The inorganic fillers include barium glass, ytterbium trifluoride and spheroid mixed oxide. The particle size is 0.25–3.0 μm. The mean particle size measures 0.9 μm.The total volume of inorganic fillers is approximately 40%. | T03802 | |
| Fiber glass Post | FRC Postec Plus (Ivoclar-Vivadent) | FRC Postec Plus is a light-conducting, radiopaque root canal post made of glass fibers. The polymer matrix is composed of aromatic and aliphatic dimethacrylates. It also contains ytterbium trifluoride. | S13174 | |
| Composite Resin | Tetric EvoCeram (Ivoclar-Vivadent) | The following percentages are in weight: Bis-GMA, Urethane dimethacrylate, Ethoxylated Bis-EMA 16.8% | S46221 | |
| Barium glass filler, Ytterbiumtrifluoride, Mixed oxide 48.5%,Prepolymers 34.0%,Additives 0.4%,Catalysts and stabilizers 0.3%,Pigments < 0.1%. | ||||
| Lithium Disilicate reinforced ceramic Blocks | IPS e.max CAD (Ivoclar-Vivadent) | SiO2, Li2O, K2O, MgO, Al2O3, P2O5 and other oxides. | S54611 | |
| Temporary resin | Telio CS Onlay (Ivoclar-Vivadent) | The monomer matrix consists of monofunctional and difunctional methacrylates (36.3 wt.%). The fillers are highly dispersed silicon dioxide and copolymers (62.6 wt.%). Fluoride (1500 ppm), catalysts, stabilizers and pigments (0.6 wt.%) are additional ingredients. | S34966 |
2.5
Thermo-mechanical fatigue loading
After 24 h the stress test was carried out with an established thermo-mechanical fatigue method, in a chewing simulator for Thermal Cycling and Mechanical Loading (TCML). All specimens were subjected to 600000 cycles with 49N axial occusal loading force applied with a ball (diameter of 4 mm) on the buccal cusp at a 1.7 Hz frequency following a one-half sine wave curve. Dimensions of the indenter were limited by the peculiar occlusal anatomy of the upper premolars, which presents a small intercuspal angle. By having the specimen holder mounted on a hard rubber disc, a sliding movement of the tooth is produced between the first contact on an inclined plane of the buccal cusp (mesial or distal) and the central fossa ( Fig. 1 ). A total of 1500 thermo-cycles (5 °C to 50 °C to 5 °C) were performed simultaneously.
2.6
Marginal analysis
Before the TCML test, as well as after completion of the loading phase, gold sputtered epoxy resin replicas (Epofix, Struers, Rødrove, Denmark) are made from polyvinylsiloxane impressions (President light, Coltène). The following interfaces are observed: the restoration-cement (RC) interface and the cement-dentin (CD) interface. These two interfaces are analyzed semi-quantitatively by scanning electron microscopy (SEM) (Digital SEM XL20, Philips, Eindhoven, Netherlands) by employing an established evaluation method. The margins are observed at a standard 200× magnification or when necessary for assessment accuracy, higher magnifications up to 1000× can be used. The following evaluation criteria are tentatively considered: perfect adaptation (continuity), overfilling, underfilling, marginal opening, restoration or tooth fracture. Results for the marginal adaptation, before and following the loading phase, are expressed as percentages of “perfect adaptation” (defect free) for both CD and RC interfaces. Percentages were calculated as the ratio between the cumulative distance of all segments showing the same morphological quality and the whole interface length.
2.7
Stepwise fatigue loading
After the TCML test all specimens were subjected to a cyclic loading test with a MTS Mini Bionix 858.02 servohydraulic testing system (Mini Bionix II, MTS, Eden Prairie, MN, USA) according to a stepwise loading method. The system was equipped with a load cell with a range of 0–2500 N. The chewing cycle was simulated by an isometric contraction. The loading member was a stainless steel ball (diameter of 4 mm). Dimensions of the indenter were limited by the peculiar occlusal anatomy of the upper premolars, which presents a small intercuspal angle. Fatigue testing was carried out with unidirectional axial force and under water. Because of the standardized anatomy, all restorations were adjusted in the same position with the loading sphere contacting both buccal and palatal cusps, halfway of the slope ( Fig. 1 ). The load varied sinusoidally between a nominal peak value F and 10% of this value (R = 0.1). The loading frequency was 5 Hz. The first 5000 cycles was a warm-up load at 200 N, followed by stages at 400, 600, 800, 1000 and 1200 N of a maximum of 20000 cycles each. Specimens were loaded until fracture or to a maximum of 105000 cycles and the number of endured cycles was registered. The integrity of the specimens was monitored throughout the test with a peak detector (Peak/Valley detector, MTS) which recognizes the difference between current loading and prescribed loading curve. The deviation was usually connected with excessive wear, accidental movements and first micro fractures inside the restoration.
2.8
Fractography
After fracture all the specimens were visually examined in order to establish which fragments were suitable for fractographic analysis. The first examination of the broken specimens was performed using a stereomicroscope (SZX9, Olympus optical Co. LTD, Tokyo, Japan). Characteristic features like compression curl, hackle and arrest lines were identified. Different magnifications (ranging from 6.3× to 50×) were used depending on the size of the characteristic marks detected. Angled illumination was used to better view the fracture surface. All recognizable features were photographed and documented. Scanning Electron Microscopy (SEM) (Digital SEM XL20, Philips, Amsterdam, Netherlands) was then used for a more detailed analysis of the fractured surfaces. In order to remove all of the impurities, all fragments were cleaned in an ultrasonic 10% sodium hypochlorite bath for 3 min, rinsed with water, dried and then fixed on the support for the microscope. The specimens were gold coated prior to the analysis with the SEM. Magnifications up to 2000× were used to obtain higher definition of identified crack features in selected areas of interest. The overall direction of crack propagation and failure origin(s) were systematically mapped for all specimens.
The modes of fracture were analyzed by optical stereo microscopy and classified as (1) catastrophic fracture, propagating to the root, under the CEJ or (2) non-catastrophic fracture, over the CEJ. Classification was based on an agreement between three examiners.
2
Materials and methods
Forty-eight freshly extracted human upper first premolars with nearly identical size were used for this study. Bucco-lingual/mesio-distal dimensions and root length of each tooth were measured using digital calipers. The inclusion criteria are absence of carious lesions, visible fracture lines in the root and a complete root formation. The teeth were stored in a sodium azide solution (0.2%) at 4 °C until the experiment onset.
2.1
Endodontic procedure
The pulp chamber of all the specimens was opened with diamond burs following a standardized procedure. A size No. 10 K-file (Dentsplay-Maillefer, Ballaigues, Switzerland) was placed to visually determine the working length. Then the root canals were prepared using manual K-files 10, 15, 20 (Dentsply-Maillefer) and rotary nickel-titanium instruments (Pro Taper, Dentsply-Maillefer), according to manufacturer’s instructions. 1 mL of 4,2% sodium hypochlorite was used to irrigate root canals during the preparation. All canals were obturated with gutta-pherca until the pulp chamber orifice using a warm vertical condensation technique (Calamus, Dentsply Tulsa Dental Specialties, Johnson City, USA). Teeth were then stored in distilled water for 24 h at 4 °C before cavity preparation.
2.2
Teeth preparation
Each specimen was fixed on a metallic holder (Baltec, Balzer, Liechtenstein) – in a vertical position – with light-curing composite; then, the root base was embedded with self-curing acrylic resin until 1 mm below the cemento-enamel junction (CEJ) to complete the tooth stabilization. For the first three Groups (n = 36), the crown of each tooth was completely cut and flattened till the CEJ. Enamel was completely removed. The roots were then randomly assigned to three Groups (n = 12): in the first Group A, roots were left with the pulp chamber filled of gutta-percha while for Groups B and C, gutta-percha was removed from the pulp chamber until 2 mm (Group B) and 4 mm(Group C) below the CEJ. For the control Group D(n = 12), the crown of each tooth was cut 1 mm above the CEJ and gutta-percha was removed from the pulp chamber and inside the palatal root canal until 5 mm below the CEJ. The outer limits of the teeth of Group D were then prepared with a chamfer (1-mm width, 1-mm high) to create a dentin core and a “ferula” effect. Enamel was completely removed. All these procedures were accomplished using coarse diamond coated burs (Cerinlay, Intensiv, Viganello, Switzerland) and finished with fine grained burs of the same shape under profuse water spray cooling.
Then, the cavity dentin surfaces of all specimens were sealed with a layer of an adhesive system (Adhese Universal,Ivoclar-Vivadent, Schaan, Liechtenstein) used in a self-etch procedure according to manufacturer recommendations. The bonding resin was polymerized for 10 s with a second generation LED high-power device (Bluephase, Ivoclar-Vivadent).
After the dentin sealing procedure, cavities of Groups A, B, and C were optically impressed (Cerec Omnicam, Sirona, Germany) and then coated with a water-based glycerin gel (Airblock, DeTrey-Dentsply, Constance, Germany) before they were provisionally restored with a soft light-curing resin (Telio CS Onlay, Ivoclar-Vivadent) and kept in saline for 7 days at 32 °C. For teeth of Group D, a pre-calibrated glass-fiber post (FRC Postec Plus,Ivoclar-Vivadent) was inserted in the palatal canal and luted with an universal dual-curing resin cement (Multilink Automix, Ivoclar-Vivadent) following manufacturer recommendations. A 4-mm high core was then fabricated with a restorative nano-hybrid resin composite (Tetric EvoCeram, Ivoclar-Vivadent). Digital impressions (Cerec, Omnicam) were taken and root cavities were provisionally restored as Groups A, B and C.
2.3
Endocrowns and crowns fabrication
All root cavities were restored with lithium disilicate reinforced CAD-CAM ceramic material (IPS e.max CAD, Ivoclar-Vivadent) and divided in four Groups (n = 12) as shown in Table 1 :
Group A (negative control): flat CAD-CAM overlays with no endo-core.
Group B: CAD-CAM endocrowns with a 2-mm high endo-core.
Group C: CAD-CAM endocrowns with a 4-mm high endo-core.
Group D (positive control): conventional CAD-CAM crowns.
Before teeth preparation, one of the extracted maxillary first premolar was chosen as a master model. An optical impression of the crown was used to fabricate the crown anatomy of the CAD-CAM restorations using the software Cerec 4.0 in Biogeneric Copy Design mode.
2.4
Luting procedures
The internal surfaces of all ceramic restorations were etched with 5% HF acid (IPS Ceramic Etching Gel, Ivoclar-Vivadent) for 20 s following manufacturer’s instructions, rinsed and dried. Etched surfaces were cleaned in an ultrasonic bath for 2 min. A layer of silane agent (Monobond Plus, Ivoclar-Vivadent) was applied over the surfaces for 60 s and then air-dried.
Root cavities (Groups A–C) and the core preparations (Group D) were submitted to airborne-particle abrasion with 27 μm alluminium-oxide particles at about 0.2 MPa pressure for 5 s (Kavo EWL, Type 5423, Biberach, Germany), rinced with profused water and dried. The silane agent (Monobond Plus, Ivoclar-Vivadent) was then applied on all the sandblasted surfaces ( Fig. 1 ).
A thin layer of dual-curing resin cement (Multilink Automix, Ivoclar-Vivadent) was spread over the preparation of all specimens. CAD-CAM restorations were then put in place first with manual pressure and then with the assistance of a specific ultra-sonic device (Cementation tip, EMS, Nyon, CH). After removal of excesses, each restoration surface was light-cured for 90 s. Restorations were then immediately finished and polished, using fine diamonds burs (first 40 μm, then 25 μm grain size) (Intensiv No 4205L, 4255, 5205L and 5255, Intensiv) and discs of decreasing grain size (Pop On XT, 3 M, St. Paul, MN, USA), from coarse (80 μm) to superfine (20 μm). Materials used in the study are shown in Table 2 .
