Fracture strength, failure type and Weibull characteristics of lithium disilicate and multiphase resin composite endocrowns under axial and lateral forces

Abstract

Objective

Multiphase resin composite materials have been advocated as an alternative to reinforced ceramics but limited information is available to date on their stability. This in vitro study evaluated the effect of axial and lateral forces on the strength of endocrowns made of Li 2 Si 2 O 5 and multiphase resin composite.

Methods

Sound human molars ( N = 60, n = 10 per group) were randomly divided into 6 groups: Group C: Control, no preparation or restoration; Group LI: Endocrown made of Li 2 Si 2 O 5 (IPS e.max CAD) and Group LA: Endocrown made of multiphase resin composite material (Lava Ultimate). After decapitation and endodontic preparation, immediate dentin sealing was performed. Following CAD/CAM fabrication, their cementation surfaces were silica coated (CoJet System) and silanized (ESPE-Sil). Endocrowns were then adhesively cemented (Variolink II). All specimens were thermocycled (×10,000 cycles). While half of the specimens in each group were subjected to axial (C A , LI A , LA A ), the other half was subjected to lateral static (C L , LI L , LA L ) loading (1 mm/min). Failure type and location after debonding/fracture were classified. Data were analyzed using ANOVA and Tukey’s post hoc test ( α = 0.05). Two-parameter Weibull distribution values including the Weibull modulus, scale (m) and shape ( 0 ), values were calculated.

Results

Under axial loading, mean fracture strength ( N ) did not show significant difference between groups: LA A (2675 ± 588) a , LI A (2428 ± 566) a , C A (2151 ± 672) a ( p > 0.05) and under lateral loading, LA L (838 ± 169) A presented significantly lower mean values than those of other groups: C L (1499 ± 418) B , LI L (1118 ± 173) B ( p < 0.05). Both endocrown materials and the control group were more vulnerable to lateral loading than axial loading. Under axial loading, Weibull distribution presented higher shape ( 0 ) for Groups LI A (5.35) and LA A (5.08) than that of the control (3.97) and under lateral loading LI L (7.5) showed higher shape ( 0 ) than those of other groups (4.69–6.46). After axial loading, failure types were mainly cohesive in the material and after lateral loading primarily adhesive between the material and dentin for both LI and LA, most of which were repairable.

Significance

Under axial loading, molars restored with endocrowns performed similar with both Li 2 Si 2 O 5 and multiphase resin composite but the latter was less durable under lateral loading.

Introduction

Severe coronal tooth structure loss due to extensive caries or root canal therapy has been typically restored with a post and core retained full coverage crown in reconstructive dentistry. Due to the advances in adhesive technologies and materials almost two decades ago endocrown type of restorations were suggested for posterior teeth as an alternative to post and core retained ones . An endocrown is a monoblock restoration that is cemented to the internal portion of the pulp chamber and to the remaining tooth margins using adhesive luting cement. Hence, their retention to the tooth is achieved through both macro- and micro-mechanical means. Endocrowns restore the anatomy, seal the root canal opening, preventing bacterial recolonization all of which eventually affect the long-term prognosis of a tooth following endodontic treatment .

Finite element analysis, mathematical modeling and static loading tests from in vitro studies suggest that molar teeth restored by endocrowns could withstand physiological chewing forces without fracture or debonding . They seem to be potentially more resistant to failure than molars restored with glass fiber reinforced composite posts . Several authors described the clinical procedure for the fabrication of endocrowns made of modern ceramics in case reports . Short-term clinical evaluations present promising results with respect to esthetics and functional longevity of endocrowns made of glass ceramic with annual failures rate of 0–0.2% up to 12–35.5 months of follow up .

Recently, multiphase resin composite materials have been advocated as an alternative to reinforced ceramics since they have more biomimetic properties with similar elasticity modulus closer to tooth structure. Limited information is available to date on their durability but they presented promising results for occlusal onlays . The present study aims to expand the current knowledge on structural durability of endocrowns.

The objectives of this in vitro study therefore were to (a) compare the fracture strength of endocrowns made of Li 2 Si 2 O 5 or multiphase resin composite and compare the results with natural teeth under axial and lateral forces, (b) evaluate the failure types after testing. The null hypothesis tested was that material type and loading direction would not affect the fracture strength of endocrowns and the results would not differ from those of unrestored natural teeth.

Material and methods

Specimen preparation

The brands, types, main chemical compositions, manufacturers and batch numbers of the materials used for the experiments are listed in Table 1 . Schematic description of the experimental design is presented in Fig. 1 .

Table 1
The brands, types, chemical compositions, manufacturers and batch numbers of the materials used for the experiments. bis-GMA: Bisphenol A glycol dimethacrylate; TEGDMA: Triethylene glycol dimethacrylate; bis-EMA: Ethoxylated bisphenol A glycol dimethacrylate; UDMA: Urethane dimethacrylate; HEMA: Hydroxyethyl methacrylate; MMA: Methylmethacrylate; PMMA: Polymethylmethacrylate; GPDM: Glycerolphophate dimethacrylate; PAMM: Phathalic acid monoethyl methacrylate.
Brand Type Chemical composition Manufacturer Batch number
Ultraetch Etching agent 38% H 3 PO 4 Ultradent, St Louis, USA 130320
OptiBond FL Adhesive resin Primer: HEMA, GPDM, PAMM, ethanol, water, photo-initiator
Adhesive: TEGDMA, UDMA, GPDM, HEMA, bis-GMA, filler, photo initiator
Kerr, Orange, CA, USA 4706853
4704999
ESPE-Sil Silane coupling agent Ethyl alcohol, methacryloxypropyl, trimethoxysilane 3M ESPE, St. Paul, MN, USA 498021
IPS Empress etching gel Ceramic etching gel <5% hydrofluoric acid Ivoclar Vivadent S20324
CoJet-Sand Blasting particles Aluminum trioxide particles coated with silica, particle size: 30 μm 3M ESPE 506649
Monobond Plus One component primer Ethanol, 3-trimethoxysilsylpropylmetha-crylate, methacrylated phosphoric acid ester Ivoclar Vivadent S14727
Syntac Primer Primer Water, acetone, maleic acid, dimethacrylate Ivoclar Vivadent S12027
Syntac Adhesive Adhesive resin Water, gluteraldehyde, maleic acid, poly-ethyleneglycodi-methacrylate Ivoclar Vivadent S15815
Heliobond Adhesive resin bis-GMA, dimethacrylate, initiators and stabilizers Ivoclar Vivadent S09854
Tetric Flow Photo-polymerized flowable resin bis-GMA, UDMA, Ethoxylated bis-EMA,16.8%
Barium glass filler, Ytterbiumtrifluoride, Mixed oxide 48.5%, Prepolymers 34%, Additives 0.4%
Catalysts and Stabilizers 0.3%, Pigments <0.1%
Ivoclar Vivadent S08370
IPS e.max CAD Lithium disilicate Glass Ceramic 97% SiO 2 , Al 2 O 3 , P 2 O 5 , K 2 O, Na 2 O, CaO, F, 3% TiO 2 , and pigments, water, alcohol, chloride Ivoclar Vivadent S04180
Lava Ultimate Mutiphase resin CAD/CAM material Polymerized dental restorative, consisting of silica nanomers (20 nm), zirconia nanomers (4–11 nm), nanocluster particles derived from the nanomers (0.6–10 μm), silane coupling agent, resin matrix 3M ESPE N357991
N333039
Variolink II Dual polymerized resin cement UDMA, inorganic fillers, ytterbium trifluoride, initiators, stabilizers, pigments Ivoclar Vivadent S09019
S02602

Fig. 1
Flow-chart showing experimental sequence and allocation of groups.

Sound human mandibular molars ( N = 60, n = 10 per group) of similar size and morphology, free of restorations and root canal treatment were selected from a pool of recently extracted teeth that were stored in distilled water. All teeth were screened on the presence of fractures by blue light and those with cracks were eliminated and replaced with new teeth. They were then embedded up to 1 mm below the cement-enamel junction (CEJ) in polyvinylchloride (PVC) tubes (height: 10 mm; diameter: 12 mm) using autopolymerizing acrylic resin (Autoplast, Condular, Wager, Switzerland) and stored in distilled water at 37 °C until preparation . The teeth were randomly divided into 3 groups: Group C: Control, no preparation or restoration; Group LI: Endocrown made of Li 2 Si 2 O 5 (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and Group LA: Endocrown made of multiphase resin composite material (Lava Ultimate, 3M ESPE, St. Paul, MN, USA).

Tooth preparation

Specimens in Groups LI and LA were scanned (Cerec Omnicam, Sirona, Bensheim, Germany) and the data were stored in the Cerec database (version 3.85, Sirona Dental Systems, Bensheim, Germany) in order to be able to restore the teeth to their original anatomy after preparation. An impression (Express 3M ESPE, Seefeld, Germany) was made from each tooth to facilitate the fabrication of a provisional restoration after preparation. Subsequently, the teeth were decapitated to a level 1 mm above the CEJ. Access to the root canal was opened with respect to the anatomy of the pulp chamber. Root canals were prepared using manual instrumentation to a depth of 10 mm relative to the margin of the tooth up to size no. 30 file with an average diameter of 0.9 mm (K-flexofile, Dentsply, Milford, USA). Then, the prepared dentin surfaces were sealed with the so-called Immediate Dentin Sealing (IDS) . This procedure involved etching dentin with 38% H 3 PO 4 (Ultraetch, Ultradent, St Louis, USA) for 15 s, rinsing and subsequent drying for 3–4 s. A primer (OptiBond FL, Kerr, Orange, USA) was applied for 15 s followed by 3–5 s of suction drying. After that adhesive resin (OptiBond FL, Kerr) was carefully applied onto the surface for 20 s, followed by 20 s of polymerization using an LED polymerization device (Bluephase, Ivoclar Vivadent) from a distance of 2 mm. The output of the polymerization device was 1000 mW/cm 2 throughout the experiment (Bluephasemeter, Ivoclar Vivadent). The entrance of the root-canals and undercuts in the pulp chamber were covered with a flowable composite resin (Tetric Flow, Ivoclar Vivadent) followed by 20 s of photo-polymerization. After application of glycerin gel (Panavia Oxyguard, Kuraray, Osaka, Japan), the surface was again photo-polymerized for 40 s and finally, the gel was rinsed away. The IDS layer was checked for the presence of voids and excess adhesive resin was removed under the microscope (Opmipico, Zeiss, Oberkochen, Germany).

The decapitated specimens were scanned again using the Cerec scanner (Cerec Omnicam, Sirona, Bensheim, Germany). Endocrowns were designed and milled (Cerec MC XL, Sirona) according to the original anatomy that was previously stored in the database ( Fig. 2 a and b). Afterwards, a provisional restoration was made (Protemp 4, 3M ESPE, Seefeld, Germany) and cemented (TempBond, Kerr). The specimens were stored in water for another 2 weeks to simulate the typical clinical situation for the provisional phase of indirect restorations.

Fig. 2
(a) Design of endocrown using the Cerec database (version 3.85, Sirona Dental Systems) to be able to restore the teeth to their original anatomy (Mean mesio-distal length: 10.2 mm, Bucco-palatinal length: 10. 2 mm) after preparation, (b) endocrown after milling (Crown height from fissure to wall preparation outline: 2.5 mm; Endocrown depth from preparation outline to the immediate dentin sealing: 2.3 mm).

Adhesive cementation

After 2 weeks, the provisional restorations were carefully removed and the fit of the restorations checked with a probe. The cementation surface of the lithium LI restorations were etched for 20 s with 4.9% hydrofluoric acid (IPS ceramic etch, Ivoclar Vivadent), followed by 30 s of rinsing with water. The restorations were ultrasonically cleaned (Emag, Valkenswaard, The Netherlands) in distilled water for 3 min, dried and silane coupling agent was applied (Monobond Plus, Ivoclar Vivadent) that was further activated at 100 °C for 60 s. Finally, adhesive resin was applied to the surface (Syntac Adhesive, Ivoclar Vivadent) and air thinned.

The cementation surface of LA endocrowns were silica coated (CoJet, 3M, ESPE) using a chairside air-abrasion device (Dento-Prep™, RØNVIG A/S, Daugaard, Denmark) from a distance of 10 mm, angle of 45° and 2 bar pressure until the surface became matt for 5 s. Silane coupling agent was applied (ESPE Sil, 3M ESPE) and further activated at 100 °C for 60 s. Finally, adhesive resin was applied to the surface (Syntac Adhesive, Ivoclar Vivadent) and air thinned.

On the tooth surface the IDS layer was silica coated as described above (CoJet, 3M ESPE). Enamel was etched with 38% H 3 PO 4 (Ultraetch, Ultradent) for 30 s, rinsed and dried for 30 s. Silane coupling agent was applied on the IDS layer (ESPE Sil, 3M ESPE), followed by primer (Syntac Primer, Syntac Adhesive, Ivoclar Vivadent) and adhesive resin (Heliobond, Ivoclar Vivadent) application on both the tooth and the restoration surfaces. The dual polymerizing cement (Variolink II, Ivoclar Vivadent) was mixed and distributed on the cementation surface of the restoration. The endocrown was placed on the tooth under standardized occlusal pressure (50 N) using a custom-made device. Excess cement was removed from the margins, an oxygen inhibition gel (Liquid Strip, Ivoclar Vivadent) was applied at the margins and the specimens were photo-polymerized from occlusal, buccal, lingual, mesial and distal directions for 40 s each. Excess cement was removed and margins were finished and polished.

Aging and fracture test

All specimens were thermocycled (Willytec, Munich, Germany) for 10,000 times between 5 °C and 55 °C with a dwell time of 30 s in each bath. After aging, digital photos of the specimens were made.

The fracture test was performed in a Universal Testing Machine (MTS 810, Eden Prairie, USA). While half of the specimens were mounted in a metal base and the stainless steel round load cell was applied perpendicular to the occlusal plane, at the central fissure (axial loading), the other half was loaded by means of a v-shaped stainless steel load cell that was placed on the interface between the tooth-endocrown margin interface (lateral loading) ( Fig. 3 a and b). The maximum force to produce fracture was recorded.

Fig. 3
The position of the load cell in relation to the occlusal surface and to the endocrown-tooth interface during (a) axial loading and (b) lateral loading, respectively in the universal testing machine where loading was applied until fracture.

Failure analysis

Failure sites were initially observed using a dental microscope (OPMIpico, Zeiss, Oberkochen, Germany), and digital photos were made from the specimens. Failure types were classified as follows: Type I: Cohesive failure in the endocrown material; Type II: Adhesive failure between the endocrown material and dentin; Type III: Cohesive failure in enamel/dentin; Type VI: Fracture extending to root. Failures above CEJ were considered as “Repairable” and those below Cemento Enamel Junction (CEJ) extending the root were classified as “irrepairable”.

Statistical analysis

Kolmogorov–Smirnov and Shapiro–Wilk tests were used to test normal distribution of the data. As the data ( N ) were normally distributed, 2-way analysis of variance (ANOVA) were applied to analyze possible differences between the groups using a statistical software program (SPSS, PASW statistics 18.0.3, Chicago, USA). Due to significant difference ( p = 0.000), Tukey’s post hoc test was applied to compare the significant differences between groups where the fracture strength ( N ) was the dependent variable and endocrown materials (2 levels: LI and LA) and force direction (2 levels; axial and lateral) were independent variables. Maximum likelihood estimation without a correction factor was used for 2-parameter Weibull distribution, including the Weibull modulus, scale ( m ) and shape ( 0 ), to interpret predictability and reliability of endocrown materials (Minitab Software V.16, State College, PA, USA). p < 0.05 was considered to be statistically significant in all tests.

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Fracture strength, failure type and Weibull characteristics of lithium disilicate and multiphase resin composite endocrowns under axial and lateral forces
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