Highlights
- •
Monolithic Lava Ultimate crowns tolerate fatigue loads 3–4 times higher than IPS Empress CAD.
- •
Lava Ultimate crowns may offer an esthetic alternative for ceramic posterior crowns.
- •
This nanohybrid indirect resin composite has a potential to be used in high stress-bearing applications.
Abstract
Objective
To demonstrate the fatigue behavior of CAD/CAM resin composite molar crowns using a mouth-motion step-stress fatigue test. Monolithic leucite-reinforced glass-ceramic crowns were used as a reference.
Methods
Fully anatomically shaped monolithic resin composite molar crowns (Lava Ultimate, n = 24) and leucite reinforced glass-ceramic crowns (IPS Empress CAD, n = 24) were fabricated using CAD/CAM systems. Crowns were cemented on aged dentin-like resin composite tooth replicas (Filtek Z100) with resin-based cements (RelyX Ultimate for Lava Ultimate or Multilink Automix for IPS Empress). Three step-stress profiles (aggressive, moderate and mild) were employed for the accelerated sliding-contact mouth-motion fatigue test. Twenty one crowns from each group were randomly distributed among these three profiles (1:2:4). Failure was designated as chip-off or bulk fracture. Optical and electron microscopes were used to examine the occlusal surface and subsurface damages, as well as the material microstructures.
Results
The resin composite crowns showed only minor occlusal damage during mouth-motion step-stress fatigue loading up to 1700 N. Cross-sectional views revealed contact-induced cone cracks in all specimens, and flexural radial cracks in 2 crowns. Both cone and radial cracks were relatively small compared to the crown thickness. Extending these cracks to the threshold for catastrophic failure would require much higher indentation loads or more loading cycles. In contrast, all of the glass-ceramic crowns fractured, starting at loads of approximately 450 N.
Significance
Monolithic CAD/CAM resin composite crowns endure, with only superficial damage, fatigue loads 3–4 times higher than those causing catastrophic failure in glass-ceramic CAD crowns.
1
Introduction
In clinical dentistry, there is a shift toward placing metal-free restorations. For direct restorations, resin composite has become the standard material that, depending on risk factors of tooth and patient, provides restorations with good longevity . Indirect restorations allow the dentist to have greater control of the form and function of a restoration, especially for teeth with considerable amount of tooth substance loss. For indirect restorations, several types of ceramic restorative materials have shown good survival rates when applied as full coverage crowns or inlays, although chipping of the ceramic remains a common problem . Indirect resin composite restorations have shown acceptable survivability , but their performance is not better than direct restorations . The increased use of resin composite materials for indirect restorations is a result of several recent trends: significant improvements in their mechanical properties, increased demands for highly esthetic, metal-free, biocompatible restorations , and rapid advances in computer-aided design/computer-assisted manufacturing (CAD/CAM) technology. Recently, resin composite blocks have been introduced for use with CAD/CAM systems as an alternative for machinable ceramics . CAD/CAM resin composite restorations have several advantages over their ceramic counterparts: (1) Resin composite blocks are milling damage tolerant, which allows for a faster milling speed and provides better marginal quality ; a full contour crown takes only 6 min to mill ! (2) No post-milling firing is needed. (3) Indirect resin composite restorations can be easily polished and adjusted for proper occlusion. These properties permit the fabrication and placement of a complete restoration within a single dental office visit , benefiting both the patient and practitioner.
Much effort has been made to improve the mechanical properties of resin composite restorative materials, such as increased filler content, changes in filler particle size and shape, changes in matrix composition, and improvements of polymerization methods . Recently, nanotechnology has been introduced to the dental resin composite manufacturing field. One recently developed nanohybrid resin composite material is Lava Ultimate CAD/CAM Restorative (3 M ESPE, St. Paul, MI, USA); it is heat-cured through a proprietary manufacturing process, which eliminates the need for any further polymerization after milling.
Several in vitro studies have evaluated indirect resin composites. Magne and colleagues conducted a number of studies on Paradigm MZ100 (3 M ESPE) . They compared the fatigue resistance of resin composite and ceramic occlusal veneers and onlays with various thicknesses on posterior teeth (some with endodontic treatments). They reported that posterior occlusal veneers and onlays made of MZ100 had significantly higher fatigue resistance compared to IPS Empress CAD (Ivoclar Vivadent, Schaan, Liechtenstein), IPS e.max CAD (Ivoclar Vivadent), and Vitablocks Mark II (VITA Zahnfabrik, Bad Säckingen, Germany). Kassem et al. examined the effect of cyclic loading using a hardened steel ball ( r = 3 mm) on fatigue resistance and microleakage of monolithic CAD/CAM molar ceramic and resin composite crowns. Their results revealed that MZ100 resin composite molar crowns were more fatigue resistant than Vitablocks Mark II ceramic crowns, after 1,000,000 cycles of cyclic loading at 600 N. However, ceramic crowns exhibited significantly less microleakage relative to resin composite crowns, irrespective of the type of the cement used. Belli et al. compared the fatigue resistance of modern dental ceramic bar specimens versus resin composites using the 4-point bending method. Their results revealed that while resin composite and dental ceramics exhibit similar fatigue degradation, resin composite materials used for direct restorations are more resistant to cyclic flexural loading than glass-rich ceramics used for indirect restorations. Johnson et al. showed superior fracture strength of posterior occlusal veneers made from Lava Ultimate resin composites than Paradigm MZ100 and concluded that Lava Ultimate were able to survive higher occlusal loads than MZ100.
By introducing the new and improved indirect nanohybrid resin composite material, the clinical indications of CAD/CAM resin composite restoration have been extended to full coverage posterior crowns, which require greater mechanical integrity than inlays and onlays. However, to our knowledge, no in vitro fatigue study has been conducted on full-coverage posterior nanohybrid resin composite crowns. The purpose of this in vitro study is, therefore, to investigate the sliding contact mouth-motion fatigue behavior and reliability of a newly developed indirect nanohybrid resin composite (Lava Ultimate) for posterior crown applications.
2
Materials and methods
2.1
Crown fabrication
Anatomically correct mandibular first molar crowns ( n = 24, resin composite Lava Ultimate crowns; n = 24, glass-ceramic IPS Empress CAD crowns) were designed and milled from CAD/CAM blocks using the following systems: Lava Milling System (3M ESPE) in a 3M certified dental lab (Jensen Dental, North Haven, CT) and CEREC System (Sirona, Charlotte, NC, USA). A standard die of a mandibular first molar preparation was scanned into the system adjusted to compensate for the cement layer thickness (50 μm). Tooth preparation was modeled by reducing the proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. After milling the crowns were processed according to the manufacturer recommendation: (1) Lava Ultimate – polished with Meisinger Luster and diamond paste (Meisinger, Centennial, CO, USA); (2) IPS Empress CAD – glazed with IPS Empress Universal Glaze (Ivoclar Vivadent).
2.2
Cementation procedure
All crowns were cemented to aged (stored in distilled water at 37 °C for at least 21 days) resin composite dies (Filtek Z100, 3M ESPE). All cementation procedures followed the manufacturers’ instructions. Z100 was selected as abutment material because it has an elastic modulus of 18 GPa , which is similar to the elastic modulus of 16–18 GPa for human dentin . The effective elastic modulus of the supporting abutment and the luting cement has been found to play a governing role in the fracture resistance of the crown .
- (i)
Lava Ultimate cementation: Prior to cementation all Z100 abutments were thoroughly cleaned with 70% ethyl alcohol and air dried, followed by application of Scotchbond Universal Adhesive (3M ESPE), rubbing it for 20 s and air dried for 5 s. The crowns were cleaned in ultrasonic bath and air dried. The bonding surfaces of the resin composite crowns were sandblasted using aluminum oxide particle (50 μm at 2 bars), avoiding crown margins until the entire bonding surface appears matte. Crowns were then thoroughly cleaned with 70% ethyl alcohol and air dried. Scotchbond Universal Adhesive was applied to the bonding surface of the crown, scrubbed in for 20 s, and air dried for 5 s. The luting agent used was RelyX Ultimate (3M ESPE) dispensed by the mixing tip inside the crown. The crown was then firmly seated and stabilized onto the abutment. The excess cement was chipped away after brief light curing. Polymerization of the luting agent was carried out by using a dental curing light (Ultra Lume LED 5), exposing each surface of the crown for 20 s.
- (ii)
IPS Empress CAD cementation: Z100 abutments were also cleaned with 70% ethyl alcohol, followed by application of Monobond Plus (Ivoclar Vivadent), 60 s waiting time, and air dried. The bonding surfaces of the crown were thoroughly cleaned with 70% ethyl alcohol, followed by application of 5% hydrofluoric acid gel (Vita Ceramics Etch – VITA) for 60 s, washed in tap water, and air dried for 20 s. Monobond Plus was applied, followed by 60 s waiting time, and air drying. The luting agent used was Multilink Automix system (Ivoclar Vivadent). The following steps were identical to those described for the resin composite crowns.
After cementation specimens were stored in distilled water at 37 ° C for a minimum of 5 days for polymerization and hydration of the cement layer prior to mechanical testing.
2.3
Mechanical testing
2.3.1
Single cycle load to fracture test
To determine the accelerated sliding contact mouth-motion step-stress profiles, three crowns per group were subjected to the single cycle load to failure test . The crown/tooth replica assembly was mounted in a universal testing machine (Model 5566, Instron); load was applied axially through a tungsten carbide indenter ( r = 3.18 mm) on the central fossa of the occlusal surface using a 10 kN load cell and 1 mm/min load rate.
2.3.2
Accelerated sliding contact mouth-motion step-stress fatigue test
Twenty one crowns from each group were subjected to mouth-motion step-stress fatigue. Mechanical testing was performed by sliding a spherical tungsten carbide (WC) indenter ( r = 3.18 mm) 0.7 mm down the distobuccal cusp toward the central fossa, using an electrodynamic fatigue testing machine (Elf-3300, Enduratec Division of Bose, Minnetonka, MN, USA). Crowns were immersed in distilled water during fatigue testing .
Three step-stress profiles (aggressive, moderate, and mild) were designed for fatigue testing . The 21 crowns per group were distributed across the three profiles in the ratio of 1:2:4, aggressive to mild, respectively, based upon the load to fracture experiments. For the resin composite crowns the mild profile started at 400 N indentation load and went to 1200 N at 170,000 cycles; the moderate profile started at 500 N and went to 1400 N at 120,000 cycles; the aggressive profile at 600 N and went to 1700 N at 90,000 cycles. Whereas for the glass-ceramic crowns, the mild profile started with a 50 N indentation load based on the load-to-fracture experiments and load was increased by predetermined steps until fracture. Similarly, for the moderate and aggressive profiles, indentation load started at 100 N and 150 N, respectively, and was increased successively until fracture.
It is well-established that ceramic and composite materials are sensitive to load/stress rate. Therefore, in the current fatigue test, a clinically relevant load rate (1000 N/s) was utilized . As a result, the loading frequency, including load, slide and lift-off phases, varied from 0.3 Hz at 1700 N to 3 Hz at 100 N.
At the end of each load cycle, all specimens were inspected under polarized light stereomicroscopy for cracks and damage. Bulk fracture and chip-off fracture of the crowns were considered as failures. A Weibull curve and reliability for completion of a mission of 100,000 cycles were calculated (ALTA 7 PRO, Reliasoft) for the glass-ceramic group only (resin composite group did not present failures during step-stress tests).
2.4
Microscopic imaging analysis
After mouth-motion step-stress fatigue tests, all crowns that did not present fracture (from the resin composite group only) were firstly examined for occlusal surface damage associated with the fatigue scar and its surrounding areas using polarized light stereomicroscopy (Leica MZ-APO, Wetzlar, Germany). Fifteen specimens were then embedded in the clear epoxy resin, sectioned, and polished (all 12 samples from the mild profile & 3 samples from the aggressive profile) to evaluate the extent of subsurface damage. Sectioning took place along the direction of sliding contact and slightly away from the center of the fatigue scar, using a water cooled low speed diamond saw (Isomet, Buehler, Lake Buff, IL). The cross-sections were polished up to the center of the fatigue scar with a 1 μm diamond suspension finish and analyzed using optical and scanning electron microscopes for the presence of cracks and damage. Measurements of the actual length and depth of contact-induced partial cone cracks, median cracks, inner cone cracks, and flexural-induced cementation surface radial cracks were performed using Leica QWin software in a polarized light stereomicroscope (Leica MZ-APO). In addition, scanning electron microscopy (SEM, S-3500N, Hitachi Instruments, Tokyo, Japan) was utilized to observe the microstructure of the two materials tested and the interaction between the cracks and the microstructural features.
At the end of fatigue loading, we evaluated latent damage in survivors using a sectioning technique rather than a residual strength measurement using the monotonic load to failure test . We believe that more information can be gleaned by identifying surface and subsurface structural changes than from very high load fracture patterns or reduction in initial strength.
2
Materials and methods
2.1
Crown fabrication
Anatomically correct mandibular first molar crowns ( n = 24, resin composite Lava Ultimate crowns; n = 24, glass-ceramic IPS Empress CAD crowns) were designed and milled from CAD/CAM blocks using the following systems: Lava Milling System (3M ESPE) in a 3M certified dental lab (Jensen Dental, North Haven, CT) and CEREC System (Sirona, Charlotte, NC, USA). A standard die of a mandibular first molar preparation was scanned into the system adjusted to compensate for the cement layer thickness (50 μm). Tooth preparation was modeled by reducing the proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. After milling the crowns were processed according to the manufacturer recommendation: (1) Lava Ultimate – polished with Meisinger Luster and diamond paste (Meisinger, Centennial, CO, USA); (2) IPS Empress CAD – glazed with IPS Empress Universal Glaze (Ivoclar Vivadent).
2.2
Cementation procedure
All crowns were cemented to aged (stored in distilled water at 37 °C for at least 21 days) resin composite dies (Filtek Z100, 3M ESPE). All cementation procedures followed the manufacturers’ instructions. Z100 was selected as abutment material because it has an elastic modulus of 18 GPa , which is similar to the elastic modulus of 16–18 GPa for human dentin . The effective elastic modulus of the supporting abutment and the luting cement has been found to play a governing role in the fracture resistance of the crown .
- (i)
Lava Ultimate cementation: Prior to cementation all Z100 abutments were thoroughly cleaned with 70% ethyl alcohol and air dried, followed by application of Scotchbond Universal Adhesive (3M ESPE), rubbing it for 20 s and air dried for 5 s. The crowns were cleaned in ultrasonic bath and air dried. The bonding surfaces of the resin composite crowns were sandblasted using aluminum oxide particle (50 μm at 2 bars), avoiding crown margins until the entire bonding surface appears matte. Crowns were then thoroughly cleaned with 70% ethyl alcohol and air dried. Scotchbond Universal Adhesive was applied to the bonding surface of the crown, scrubbed in for 20 s, and air dried for 5 s. The luting agent used was RelyX Ultimate (3M ESPE) dispensed by the mixing tip inside the crown. The crown was then firmly seated and stabilized onto the abutment. The excess cement was chipped away after brief light curing. Polymerization of the luting agent was carried out by using a dental curing light (Ultra Lume LED 5), exposing each surface of the crown for 20 s.
- (ii)
IPS Empress CAD cementation: Z100 abutments were also cleaned with 70% ethyl alcohol, followed by application of Monobond Plus (Ivoclar Vivadent), 60 s waiting time, and air dried. The bonding surfaces of the crown were thoroughly cleaned with 70% ethyl alcohol, followed by application of 5% hydrofluoric acid gel (Vita Ceramics Etch – VITA) for 60 s, washed in tap water, and air dried for 20 s. Monobond Plus was applied, followed by 60 s waiting time, and air drying. The luting agent used was Multilink Automix system (Ivoclar Vivadent). The following steps were identical to those described for the resin composite crowns.
After cementation specimens were stored in distilled water at 37 ° C for a minimum of 5 days for polymerization and hydration of the cement layer prior to mechanical testing.
2.3
Mechanical testing
2.3.1
Single cycle load to fracture test
To determine the accelerated sliding contact mouth-motion step-stress profiles, three crowns per group were subjected to the single cycle load to failure test . The crown/tooth replica assembly was mounted in a universal testing machine (Model 5566, Instron); load was applied axially through a tungsten carbide indenter ( r = 3.18 mm) on the central fossa of the occlusal surface using a 10 kN load cell and 1 mm/min load rate.
2.3.2
Accelerated sliding contact mouth-motion step-stress fatigue test
Twenty one crowns from each group were subjected to mouth-motion step-stress fatigue. Mechanical testing was performed by sliding a spherical tungsten carbide (WC) indenter ( r = 3.18 mm) 0.7 mm down the distobuccal cusp toward the central fossa, using an electrodynamic fatigue testing machine (Elf-3300, Enduratec Division of Bose, Minnetonka, MN, USA). Crowns were immersed in distilled water during fatigue testing .
Three step-stress profiles (aggressive, moderate, and mild) were designed for fatigue testing . The 21 crowns per group were distributed across the three profiles in the ratio of 1:2:4, aggressive to mild, respectively, based upon the load to fracture experiments. For the resin composite crowns the mild profile started at 400 N indentation load and went to 1200 N at 170,000 cycles; the moderate profile started at 500 N and went to 1400 N at 120,000 cycles; the aggressive profile at 600 N and went to 1700 N at 90,000 cycles. Whereas for the glass-ceramic crowns, the mild profile started with a 50 N indentation load based on the load-to-fracture experiments and load was increased by predetermined steps until fracture. Similarly, for the moderate and aggressive profiles, indentation load started at 100 N and 150 N, respectively, and was increased successively until fracture.
It is well-established that ceramic and composite materials are sensitive to load/stress rate. Therefore, in the current fatigue test, a clinically relevant load rate (1000 N/s) was utilized . As a result, the loading frequency, including load, slide and lift-off phases, varied from 0.3 Hz at 1700 N to 3 Hz at 100 N.
At the end of each load cycle, all specimens were inspected under polarized light stereomicroscopy for cracks and damage. Bulk fracture and chip-off fracture of the crowns were considered as failures. A Weibull curve and reliability for completion of a mission of 100,000 cycles were calculated (ALTA 7 PRO, Reliasoft) for the glass-ceramic group only (resin composite group did not present failures during step-stress tests).
2.4
Microscopic imaging analysis
After mouth-motion step-stress fatigue tests, all crowns that did not present fracture (from the resin composite group only) were firstly examined for occlusal surface damage associated with the fatigue scar and its surrounding areas using polarized light stereomicroscopy (Leica MZ-APO, Wetzlar, Germany). Fifteen specimens were then embedded in the clear epoxy resin, sectioned, and polished (all 12 samples from the mild profile & 3 samples from the aggressive profile) to evaluate the extent of subsurface damage. Sectioning took place along the direction of sliding contact and slightly away from the center of the fatigue scar, using a water cooled low speed diamond saw (Isomet, Buehler, Lake Buff, IL). The cross-sections were polished up to the center of the fatigue scar with a 1 μm diamond suspension finish and analyzed using optical and scanning electron microscopes for the presence of cracks and damage. Measurements of the actual length and depth of contact-induced partial cone cracks, median cracks, inner cone cracks, and flexural-induced cementation surface radial cracks were performed using Leica QWin software in a polarized light stereomicroscope (Leica MZ-APO). In addition, scanning electron microscopy (SEM, S-3500N, Hitachi Instruments, Tokyo, Japan) was utilized to observe the microstructure of the two materials tested and the interaction between the cracks and the microstructural features.
At the end of fatigue loading, we evaluated latent damage in survivors using a sectioning technique rather than a residual strength measurement using the monotonic load to failure test . We believe that more information can be gleaned by identifying surface and subsurface structural changes than from very high load fracture patterns or reduction in initial strength.
3
Results
The microstructures of the resin composite Lava Ultimate and the glass-ceramic IPS Empress CAD are shown in Fig. 1 . SEM and EDS analyses revealed that the microstructure of the resin composite material consisted of a polymer matrix with high ceramic filler loading ( Fig. 1 a). However, due to the relatively low magnification of our SEM, only the nanoparticle clusters of silica and zirconia were observed, which were in the range of 0.6–10 μm. It was not possible to identify the dispersed silica and zirconia nanoparticles described by the manufacturer of this material (silica particles ≈ 20 nm, zirconia particles ≈ 4–11 nm). The microstructure of the glass-ceramic consisted of a glassy matrix with evenly distributed leucite crystals ( Fig. 1 b). The diameter of these crystals was about 1–5 μm; the crystal content was 35–45% by volume.
