Despite the increased use of monolithic crowns, their performance has yet to be determined when the minimal tooth preparation (MTP) principle is considered. The goal of this study was to evaluate the effect of MTP on the mechanical behavior, reliability and translucency of posterior monolithic ceramic crowns.
Dentin analogues were machined using two preparation designs (0.5 or 1 mm reduction) to receive first molar crowns in order to evaluate the monolithic crown performance. Next, 126 crowns were divided (21/g) according to the material (High translucent zirconia – YZHT, Zirconia reinforced lithium silicate – ZLS and Hybrid ceramic – HC) and thickness (0.5 or 1 mm). Tensile stress concentration was calculated using the finite element method. The crowns were adhesivelly cemented and step stress fatigued to calculate reliability for missions at 600 and 1000 N. Translucency was analyzed in 10 discs of each material and thickness.
Higher stress concentration was found in thinner crowns and those with higher elastic modulus. YZHT crowns were suspended when fatigue reached 1500 N load, thus 1-parameter Weibull was used to analyze the data. Reliability was only affected by thickness at 1000 N. ZLS.5 showed lower survival than HC.5, which was similar to the groups that presented 100% survival. YZHT showed the highest strength and data scattering. ZLS1 (22.3 ± 1.4) presented higher translucency than HC1 (19.2 ± 0.6) and YZHT1 (12.0 ± 2.9), whereas ZLS.5 and HC.5 were similar to each other (26.5 ± 2.3, 26.7 ± 2.2) and superior to YZHT.5 (12.7 ± 1.2).
HC.5 combined high reliability and translucency with low stress concentration, yielding better crown performance and tooth preservation.
Advances in dental materials enable different ceramics to be used for the same indication; e.g., high translucent zirconia, reinforced glass ceramics and hybrid ceramics may be indicated [ ]; when a minimal tooth preparation is required to make a posterior crown. According to the manufacturer, 4–6 % yttria stabilized tetragonal zirconia polycrystal with high translucency (YZHT, Vita Zahnfabrick, Bad Sackingen, Germany) has similar chemical composition and mechanical properties to conventional translucent zirconia [ , ], thus enabling its use in monolithic form. Ceramics with different microstructures were developed with optimized mechanical properties, appropriate wear behavior and acceptable aesthetics to overcome disadvantages of the antagonist dentition wear and to match natural tooth aesthetics [ ]. Zirconia reinforced lithium silicate (ZLS) is an example of reinforced glass-ceramic [ ] which can be used for laminate veneers, inlays, onlays, anterior and posterior crowns. Hybrid ceramic (HC) consists of an inorganic ceramic matrix embedded in a polymeric matrix [ , ], with lower elastic modulus than traditional ceramics [ , , ] low hardness [ ], fast machining, and easy polishing [ ]. Laminate veneers, inlays, onlays and crowns are among its indications. In facing different materials for the same indication, clinical questions arise about which material to select and which properties should be considered for the correct material choice.
Despite the increase in posterior monolithic crowns use, their performance has yet to be determined when minimal tooth preparation is considered [ ], as well as which material should be used in these conservative preparations. The occlusal minimal thickness for posterior monolithic crowns suggested by the manufacturer is 0.5 mm for YZHT and 1 mm for ZLS and HC. The restoration thickness influences the mechanical strength [ ], stress distribution [ , ] and translucency [ , ]. As aesthetics are an important factor in selecting a material, the search for a restorative material with high mechanical and desired optical properties has been the focus of several studies [ , , , ].
Posterior monolithic crown failure rate data is very limited with no clinical trial available for ZLS. An up to 5-year laboratory survey of 29,808 zirconia monolithic posterior crowns showed a 0.69% failure rate [ ] and a high success rate of 100% after 23.8 months for 56 posterior crowns without cracking or debonding [ ], and a 98.5% success rate in another study due to marginal discoloration of one crown after 1 year [ ]. The authors suggested that the minimal occlusal thickness of zirconia should be limited to 0.5 mm to withstand occlusal loads [ ]. For the hybrid ceramic, a 2-year follow-up of 35 monolithic posterior crowns with minimal occlusal thickness of 1–1.5 mm reported a 96.8% survival rate. The authors reported the loss of one crown which was fabricated for a previously cracked tooth [ ]. Information on the influence of the preparation type following the principle of minimum tooth preparation on the survival of thin monolithic crowns is still scarce [ ], as well as which material should be used in these conservative preparations.
Testing of anatomical crowns and tooth preparations tend to simulate clinical fracture modes [ , , , , , ] when evaluated under fatigue [ , ]. The step-stress accelerated-life test (SSALT) has been widely used to evaluate the lifetime of restorations [ , , , , ]. Inferences on restoration critical fracture initiation sites are enabled when SSALT fatigue is associated with the stress distribution analysis. This study evaluated the effect of minimum tooth preparation on the mechanical behavior, reliability and translucency of posterior monolithic crowns of three different CAD/CAM ceramic materials. The hypotheses tested were that the preparation type and ceramic material would not change the crown’s: (1) mechanical behavior; (2) reliability; or (3) translucency.
Materials and methods
Step stress accelerated life test (SSALT)
First, one hundred twenty-six (126) crown preparations were machined in epoxy resin (G10, Protec, São Paulo, Brazil) [ ] using CAD/CAM (Computer Aided Design/Computer Aided Machine) technology. The occlusal and axial walls were uniformly reduced to 1 (conventional preparation) and 0.5 mm (conservative preparation) thickness. Therefore, the difference in the whole thickness of the preparations was uniformly defined as 0.5 mm. Both preparations presented rounded internal angles, chamfer preparation, and total occlusal convergence of 20° [ ]. The abutments were cleaned in an ultrasonic bath with distilled water for 5 min and embedded in acrylic resin (25 mm Ø x20 mm height) up to 1.5 mm bellow the chamfer.
A thin layer of titanium dioxide-based powder was sprayed onto each abutment for scanning (inEos Blue, inLab SW4.2, Sirona, Benshein, Germany) and subsequent crown design. A crown was generated after delimiting the end of the conservative preparation. The crown was then used as a standard for the software to generate another crown with similar external anatomy to the conservative on the conventional preparation. The following ceramic blocks were milled using the Cerec inLab (5884742 D329, Sirona for Dental Systems, Benshelm, Germany): High translucency zirconia HT (YZHT), Vita Suprinity (ZLS) and Vita Enamic (HC) (all from Vita Zahnfabrik, Bad Säckingen, Germany). Next, the restorations were separated from the remaining blocks with the aid of a diamond blade and fine-grained diamond bur under abundant irrigation. After cleaning in an ultrasonic bath with isopropyl alcohol for 5 min, YZHT and ZLS ceramic restorations were cleaned and sintered (1450 °C, Infire HTC speed, Sirona) or crystallized (840 °C, Vacumat 600 M P, Vita Zahnfabrik, Bad Säckingen, Germany) according to manufacturer’s instructions. All crowns were polished using the 2-stage polishing system with pre- (7000–12,000 rpm speed for 15 s) and high-gloss polishers (4000–8000 rpm for 15 s).
The G10 abutments were conditioned with 10% hydrofluoric acid for 60 s, washed with air/water jet for 30 s, and then dried [ ] before receiving the adhesive system (ED primer A + B, Kuraray Medical, Tokyo, Japan). The crowns were sandblasted with 50 μm alumin oxide (10 s, 10 mm and 2.5 Bar), etched with phosphoric acid during 5 s for surface cleaning and then a silane agent (Clearfil Ceramic Primer, Kuraray Medical) was applied for 60 s. Panavia F 2.0 self-etching resin cement (Kuraray Medical) was manipulated in 1:1 ratio and applied onto the intaglio surface of the crowns, which were then placed on the respective abutment and kept in a 750 g weight for 5 min. Excess cement was removed and then the cement was light cured (Valo, Ultradent Products, Utah, USA) for 20 s on each face. The finished specimens were immersed in distilled water and stored at 37 °C for 24 h before the fatigue test. Fig. 1 shows a flowchart of the study design and methods.
Prior to the SSALT, three specimens per group were submitted to single load to fracture test (load cell of 1000 kgf and 0.5 mm/min of cross-head speed) in a universal testing machine (DL-1000, EMIC, São José dos Pinhais, Brazil). The profiles used in the SSALT were determined from the mean load value (in N) of each group. The samples were randomly distributed among 3 profiles: mild, moderate, and aggressive [ , ] in the ratio of 3:2:1, according to the load increase and number of cycles ( Fig. 2 ). The samples were positioned on a fatigue machine (Instron Electropuls E3000 Linear Torsion system, USA) and a fatigue load was delivered (6 mm diameter, stainless steel, water, 10 Hz) on the occlusal fossa [ ]. The fatigue machine has been set to stop at the moment of the failure. The load value and the number of cycles until the specimen failure or suspension were used for statistical analysis. All crowns were inspected after the total number of cycles predetermined for each SSALT profile ( Fig. 2 ) using the transillumination technique. Either bulk or cohesive (cracks) fractures of the crowns were determined as the failure criterion. Representative failed crowns were subjected to fractographic analysis under polarized light (Stereo Discovery.V20, Carl Zeiss, LLC, USA) and scanning electron microscopes (SEM) (XL 20, Philips, Eindhoven, the Netherlands) to identify the failure origin, following the guidance described by Scherrer et al. (2017) [ ].
Finite element analysis (FEA)
A three-dimensional (3D) model of an upper first molar was used to evaluate the tensile stress of the monolithic crown. The model containing root, pulp chamber, periodontal ligament, medullary and cortical bones [ ] was imported to Rhinoceros CAD software (version 5.0 SR8, McNeel North America, Seattle, USA). A cross-section was made at the cementoenamel junction to separate root and crown, and then a conservative preparation following the in vitro specimen dimension was modeled to receive the crown with minimum occlusal thickness of 0.5 mm. The conventional preparation was generated from the conservative with a reduction of 0.5 mm in all directions. A crown with minimal occlusal thickness of 1 mm was also generated. The cement layer was modeled with 80 μm for both conditions.
The volumetric solids were imported to the analysis software (ANSYS 17.2, ANSYS Inc., Houston, USA) in STEP (STandard for the Exchange of Product model data) format. A mesh convergence test (10%) determined 169,672 nodes and 95,263 tetrahedral elements for the conventional preparation; and 173,428 nodes and 101,112 elements for the conservative preparation. A static structural analysis was used and the material properties were reported for each solid component as isotropic and homogeneous ( Table 1 ) [ , ]. The groups were divided according to the crown occlusal minimum thickness and the ceramic material (high translucent zirconia, zirconia reinforced lithium silicate and hybrid ceramic). The contacts were considered perfectly bonded between the bodies and the fixation was defined on the base of the cortical bone. The loading was similarly applied (600 and 900 N) as performed in the in vitro test ( Fig. 3 ). The Maximum Principal Stress criterion was used to assess the distribution of tensile stresses in the monolithic crowns.
|Periodontal ligament||0.069||0.45||[ ]|
|High translucency zirconia||210||0.33||Vita Zhanfabriek|
|Zirconia reinforced lithium silicate||65.6||0.23||[ ]|
|Hybrid ceramic||34.7||0.28||[ ]|
|Resin cement||7.5||0.25||[ ]|
Ten (10) discs from each group (6 mm Ø × 0.5 mm and 1 mm) were polished with sandpaper up to #1200 grit under water cooling [ ] in an automatic precision polishing equipment (EcoMet™/AutoMet™250, Buehler, Illinois, USA). A spectrophotometer (CM-2600d Konica Minolta, Japan) was used with D65 illuminant in reflectance mode, 100% UV, observer angle at 2°, specular reflection, small area view of 3 mm Ø and with the presence of polyethylene glycol [ ]. The translucency was calculated applying the formula [(L B – L W ) 2 + (a B – a W ) 2 + (b B – b W ) 2 ] 1/2 , where the subscripts B and W [ ] correspond to the black (L*: 2.58; a*: −0.15; b*: −0.24) and white (L*: 84.95; a*: −0.38; b*: 2.93) standard backgrounds colors.
Fatigue data of number of cycles and failures were submitted to the Weibull analysis (Minitab 18.1, PA, USA). The estimated survival probability of each group at loads 600 and 1000 N was calculated. Weibull modulus ( m ) and characteristic strength ( σ ) considering 62.3% failure were also determined for samples that were subjected to single load to failure testing after fatigue. Tensile stress results (MPa) in the crown were organized with identical scale for direct comparison in a color graph. Tensile stress peaks were exported from the occlusal surface and are summarized in Table 2 . Translucency data were submitted to one-way ANOVA and Tukey test, all with α = 5%.
|Groups||Stress peak (MPa)||m (CI)||σ (CI)|
|600 N||1000 N|
|YZHT.5||88.1||142.8||5.8 *||3.370 (3.141–3.615) *|
|YZHT1||60.2||96.9||3.8 *||5.323 (4.783–5.925) *|
|ZLS.5||72.1||121.8||7.2||1.499 (1.380–1.518) A|
|HC.5||55.3||102.6||35.8||1.497 (1.486–1.508) A|