Highlights
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Capacity of crown restorations to sustain high bite force has been measured on manufactured, anatomically-correct specimens for three classes of monolithic materials.
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Extended finite element modeling has been used to follow the progression of embedded cracks in the crown/dentin structure, revealing a complex evolution to failure that cannot be inferred from more traditional FEM.
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Zirconia is confirmed to be the most durable of available monolithic ceramics, with lithium disilicate intermediate and dental composite weakest. All perform favorably relative to natural tooth enamel.
Abstract
Objective
To quantify the splitting resistance of monolithic zirconia, lithium disilicate and nanoparticle-composite dental crowns.
Methods
Fracture experiments were conducted on anatomically-correct monolithic crown structures cemented to standard dental composite dies, by axial loading of a hard sphere placed between the cusps. The structures were observed in situ during fracture testing, and critical loads to split the structures were measured. Extended finite element modeling (XFEM), with provision for step-by-step extension of embedded cracks, was employed to simulate full failure evolution.
Results
Experimental measurements and XFEM predictions were self-consistent within data scatter. In conjunction with a fracture mechanics equation for critical splitting load, the data were used to predict load-sustaining capacity for crowns on actual dentin substrates and for loading with a sphere of different size. Stages of crack propagation within the crown and support substrate were quantified. Zirconia crowns showed the highest fracture loads, lithium disilicate intermediate, and dental nanocomposite lowest. Dental nanocomposite crowns have comparable fracture resistance to natural enamel.
Significance
The results confirm that monolithic crowns are able to sustain high bite forces. The analysis indicates what material and geometrical properties are important in optimizing crown performance and longevity.
1
Introduction
Monolithic all-ceramic crowns are increasingly favored over traditional prostheses with porcelain veneers, minimizing issues associated with complex fabrication, presence of residual stresses, and delamination from an inner core . Dense zirconia and lithium disilicate glass-ceramics are prime candidates, for their relatively high modulus and fracture resistance along with ever-improving esthetics . A stiff all-ceramic crown shields the inner dentin–pulp region by absorbing the bulk of occlusal stresses. However, brittleness remains a concern—failures for monolithic ceramic systems have been reported . These failures are sometimes described as ‘abrupt’, with complete cracking of the crown and sometimes of the entire tooth. But little attempt has been made to examine how cracks evolve in relation to material properties. One effort has been made to elucidate the mechanics of the less disruptive fracture mode of edge chipping in monolithic ceramics , but while such studies are useful for quantifying crack resistance properties of the crown material, they do not address the more serious issue of total crown failure.
An alternative, more recent approach to crown restoration is to fabricate monolith prostheses from particle-filled resins . While much more compliant and therefore less protective of the underlayer, dental composite crowns are seemingly less prone to fracture, if not to deformation. They appeal to the practicing dentist because of their ease of fabrication and installation. However, their capacity to sustain high bite forces and to avoid debonding is in question. Again, little attempt has been made to determine the mechanics of failure in composite restorations.
Clues to the modes of potential crown failure can be drawn from recent studies of fracture in natural teeth. Apart from incidental edge chipping , extensive experimental testing and theoretical fracture mechanics analysis of tooth breakdown under occlusal loading reveals a common mode of fracture in human teeth to be longitudinal cracking from the bite surface to the tooth base (or vice versa) . Longitudinal cracks start from ‘tuft’-like defects in the enamel close to the junction with dentin and run to the surface before vertical extension along the tooth walls . They can remain fully contained as visible ‘lamellae’ within the enamel, and can even heal to some extent, without causing immediate failure—the fracture to that point is ‘contained’. Longitudinal cracks can also act as precursors to whole tooth splitting through the dentin to the root, at which point the tooth is lost . The question arises as to whether similar fracture processes operate in monolithic crown restorations and, if so, which are the best materials to contain them?
This study addresses these questions, first by conducting in vitro laboratory axial-loading fracture tests on manufactured model zirconia, lithium disilicate and dental nanocomposite crown specimens, and then by extending earlier fracture mechanics computations to analyze the fracture modes. The study focuses on anatomically-correct molars, centrally loaded with a hard spherical indenter to wedge open adjacent cusps and thereby split the crown along a fissure plane. However, the conclusions drawn have a broader reach to other tooth forms. Particular attention is given to the role of crown material properties, notably toughness and modulus, as well as to key geometrical variables such as tooth dimensions and sphere radius. The manner in which cracks progress from the occlusal surface to the enamel/cementum junction will be elucidated. It will be confirmed that monolith ceramics are capable of providing long-lasting crown restorations, provided due caution is exercised in fabrication and cementation procedure.
2
Materials and methods
2.1
Crown preparation and testing
Three candidate restorative materials for monolithic dental crowns were selected for study, each procured in the form of commercially available CAD/CAM blocks: (i) high translucency zirconia, 3 mol% Y-TZP (LAVA Plus, 3M ESPE, St Paul, MN); machinable lithium disilicate glass-ceramic (IPS e.max CAD, Ivoclar-Vivadent, Amherst, NY); and resin-based composite with nanoparticle ceramic filler (LAVA Ultimate Restorative, 3M ESPE, St Paul, MN). Six blocks of each material were machined into anatomically-correct mandibular first molar crowns by inputting scanned profiles of standard dental composite dies (Filtek Z100, 3M ESPE, St Paul, MN) into the following machining systems: Lava Milling System (3M ESPE, St Paul, MN) for zirconia and resin-based nanocomposite, and CEREC System (Sirona, Charlotte, NC) for lithium disilicate. The ensuing occlusal crown thickness was 1.0 mm for zirconia and 2.0 mm for lithium disilicate and nanocomposite, with a common proximal wall thickness 1.5 mm. Allowance was made for a cementation thickness 50 μm in all cases. The dies were engineered to provide common external crown dimensions for all materials, specifically tooth radius 5.0 mm (one quarter of sum of maximum buccal/lingual and mesial/distal widths) and tooth height 7.0 mm (average occlusal to enamel/cementum junction distance over all cusps).
Values of representative mechanical properties of the three crown materials in their finished states are given in Table 1 , drawn from manufacturers’ specifications. The Z100 die material used as substrate in the crown fabrication has near-identical filler content to that of the LAVA Ultimate dental nanocomposite, and is therefore taken to have similar properties. For comparison, some values for enamel and dentin are included in the table. Uncertainties in the values presented, from variations in published literature properties, are estimated at a nominal ∼20%.
| Material | Modulus E (GPa) | Poissons ratio | Toughness T (MPa m 1/2 ) | Strength S (MPa) |
|---|---|---|---|---|
| Zirconia (Lava Plus) | 210 | 0.30 | 4.0 | 1200 |
| Lithium disilicate (IPS e.max CAD) | 95 | 0.23 | 1.5 | 376 |
| Resin composite (Lava Ultimate) | 13 | 0.35 | 2.0 | 200 |
| Enamel | 90 | 0.22 | 1.0 | – |
| Dentin | 18 | 0.35 | 3.0 | – |
After milling, the crowns were processed as follows. The surfaces of the zirconia and lithium disilicate crowns were glazed at elevated temperatures and times according to manufacturers’ recommendations. The intaglio surfaces of zirconia were sandblasted (aluminum oxide particles, 50 μm at pressure 2 bars, 100 mm standoff distance) to promote a mechanically adherent bond, while those of lithium disilicate were acid-etched (5% HF for 20 s). As-machined resin-based nanocomposite crowns were polished (1 μm diamond paste) on their outer surfaces and sandblasted at their intaglio surfaces, taking care to avoid damage to the margins.
The composite dies were aged in distilled water for at least 21 days, and then air dried for receipt of the crowns. The zirconia and lithium disilicate crowns were first coated at their intaglio surfaces with a bonding agent (Monobond Plus, Ivoclar Vivadent, Amherst, NY), then cemented onto the dies (Multilink Automix, Ivoclar Vivadent, Amherst, NY). The dental nanocomposite crowns were similarly coated (Scotchbond Universal, 3M ESPE, St Paul, MN) and cemented (RelyX Ultimate, 3M ESPE, St Paul, MN). After cementation, the mounted specimen crowns were stored in distilled water at 37 °C for a minimum of 5 days to allow complete polymerization and hydration of the cement layer prior to mechanical testing.
The crowns were then subjected to failure testing by axial loading with a tungsten carbide sphere located in the central fossa . This particular mode of loading is the easiest way to deliver wedge-opening forces on adjacent cusps, and simulates biting on a hard food object or opposing molar tooth or crown cusp. A sphere radius 3.2 mm was chosen simply because it was convenient to handle in the experiments. The load was applied in a mechanical testing machine (Model 5566, Instron Corp, Norwood, MA) at a crosshead speed of 0.1 mm/min. A video camera was placed to observe the crown side walls during the loading, in order that the failure evolution may be followed in situ .
2.2
Fracture mechanics analysis
Analysis of the failure evolution was conducted using an extended finite element model (XFEM) package (Abaqus 6.9-EF1, Simulia, Providence, RI), incorporating provision for step-by-step progression of an embedded crack. The model used is depicted in Fig. 1 , and is the same as in a preceding article describing the failure of natural molar teeth , but with the crown material replacing the enamel shell. It consists of a shell structure with four truncated hemispherical cusps of radius R /2 and uniform thickness d = 1 mm (zirconia) or 2 mm (lithium disilicate and dental nanocomposite) on a cylindrical base of radius R = 5.0 mm and total occlusal-to-margin height H = 7.0 mm, i.e. closely representative of the anatomically-correct crowns. The shell is fully bonded to an interior dentin-like substrate. The presence of an intervening cement layer is considered to have a negligible effect on the fracture mechanics, because of its thinness (∼50 μm) and comparable modulus and toughness relative to the dentin-like interior, and was therefore omitted from most computations. Nevertheless, a simple comparative computation with cement layer included was run for confirmation. A vertical load is applied axially onto a rigid sphere of radius r = 3.2 mm in frictionless contact on all four cusps, so that horizontal wedging forces are delivered across symmetry fissure planes (dashed lines in Fig. 1 b). Note that these fissure planes are distant from the indenter contact points ( Fig. 1 a). Input material properties for the crown materials are taken from Table 1 .
The XFEM algorithm contains provision for progressive stable growth of an embedded precursor starter crack with increasing load, up to and beyond the point of final failure, corresponding to total splitting of the crown/substrate assembly. Emplacement of the starter crack on a fissure plane and immediately adjacent to the contact points circumvents meshing complexities in the crack initiation region , and is tantamount to assuming that the tooth structure contains preexisting cracks. This is justifiable in the case of tooth enamel, in the form of tufts and lamellae , but is an assumption that needs to be examined in the case of crown restoratives. Contact with the cusp walls is made by controlling the downward displacement of the indenting sphere in small increments. The outer shell and inner portions of the structure are meshed sufficiently finely that the computations attain convergence. The emplaced crack is allowed to advance mesh cell by cell in accordance with a cohesive zone criterion outlined in detail elsewhere , with parameters for the sequential rupture of each cell adjusted to match the appropriate material modulus and toughness. In this way entire progression of the cracks through both the outer shell and inner filler material could be followed.
2.3
Statistical analysis
Experimental critical splitting load data were analyzed by one-way ANOVA. The independent factor was material group (zirconia, lithium disilicate, nanocomposite). The mean critical load and its 95% confidence interval were calculated for each crown group. Post Hoc Tukey HSD test for multiple comparisons was performed to determine which specific groups differed significantly from one other. The significance level was set at p ≤ 0.05 for all analyses.
2
Materials and methods
2.1
Crown preparation and testing
Three candidate restorative materials for monolithic dental crowns were selected for study, each procured in the form of commercially available CAD/CAM blocks: (i) high translucency zirconia, 3 mol% Y-TZP (LAVA Plus, 3M ESPE, St Paul, MN); machinable lithium disilicate glass-ceramic (IPS e.max CAD, Ivoclar-Vivadent, Amherst, NY); and resin-based composite with nanoparticle ceramic filler (LAVA Ultimate Restorative, 3M ESPE, St Paul, MN). Six blocks of each material were machined into anatomically-correct mandibular first molar crowns by inputting scanned profiles of standard dental composite dies (Filtek Z100, 3M ESPE, St Paul, MN) into the following machining systems: Lava Milling System (3M ESPE, St Paul, MN) for zirconia and resin-based nanocomposite, and CEREC System (Sirona, Charlotte, NC) for lithium disilicate. The ensuing occlusal crown thickness was 1.0 mm for zirconia and 2.0 mm for lithium disilicate and nanocomposite, with a common proximal wall thickness 1.5 mm. Allowance was made for a cementation thickness 50 μm in all cases. The dies were engineered to provide common external crown dimensions for all materials, specifically tooth radius 5.0 mm (one quarter of sum of maximum buccal/lingual and mesial/distal widths) and tooth height 7.0 mm (average occlusal to enamel/cementum junction distance over all cusps).
Values of representative mechanical properties of the three crown materials in their finished states are given in Table 1 , drawn from manufacturers’ specifications. The Z100 die material used as substrate in the crown fabrication has near-identical filler content to that of the LAVA Ultimate dental nanocomposite, and is therefore taken to have similar properties. For comparison, some values for enamel and dentin are included in the table. Uncertainties in the values presented, from variations in published literature properties, are estimated at a nominal ∼20%.
| Material | Modulus E (GPa) | Poissons ratio | Toughness T (MPa m 1/2 ) | Strength S (MPa) |
|---|---|---|---|---|
| Zirconia (Lava Plus) | 210 | 0.30 | 4.0 | 1200 |
| Lithium disilicate (IPS e.max CAD) | 95 | 0.23 | 1.5 | 376 |
| Resin composite (Lava Ultimate) | 13 | 0.35 | 2.0 | 200 |
| Enamel | 90 | 0.22 | 1.0 | – |
| Dentin | 18 | 0.35 | 3.0 | – |
After milling, the crowns were processed as follows. The surfaces of the zirconia and lithium disilicate crowns were glazed at elevated temperatures and times according to manufacturers’ recommendations. The intaglio surfaces of zirconia were sandblasted (aluminum oxide particles, 50 μm at pressure 2 bars, 100 mm standoff distance) to promote a mechanically adherent bond, while those of lithium disilicate were acid-etched (5% HF for 20 s). As-machined resin-based nanocomposite crowns were polished (1 μm diamond paste) on their outer surfaces and sandblasted at their intaglio surfaces, taking care to avoid damage to the margins.
The composite dies were aged in distilled water for at least 21 days, and then air dried for receipt of the crowns. The zirconia and lithium disilicate crowns were first coated at their intaglio surfaces with a bonding agent (Monobond Plus, Ivoclar Vivadent, Amherst, NY), then cemented onto the dies (Multilink Automix, Ivoclar Vivadent, Amherst, NY). The dental nanocomposite crowns were similarly coated (Scotchbond Universal, 3M ESPE, St Paul, MN) and cemented (RelyX Ultimate, 3M ESPE, St Paul, MN). After cementation, the mounted specimen crowns were stored in distilled water at 37 °C for a minimum of 5 days to allow complete polymerization and hydration of the cement layer prior to mechanical testing.
The crowns were then subjected to failure testing by axial loading with a tungsten carbide sphere located in the central fossa . This particular mode of loading is the easiest way to deliver wedge-opening forces on adjacent cusps, and simulates biting on a hard food object or opposing molar tooth or crown cusp. A sphere radius 3.2 mm was chosen simply because it was convenient to handle in the experiments. The load was applied in a mechanical testing machine (Model 5566, Instron Corp, Norwood, MA) at a crosshead speed of 0.1 mm/min. A video camera was placed to observe the crown side walls during the loading, in order that the failure evolution may be followed in situ .
2.2
Fracture mechanics analysis
Analysis of the failure evolution was conducted using an extended finite element model (XFEM) package (Abaqus 6.9-EF1, Simulia, Providence, RI), incorporating provision for step-by-step progression of an embedded crack. The model used is depicted in Fig. 1 , and is the same as in a preceding article describing the failure of natural molar teeth , but with the crown material replacing the enamel shell. It consists of a shell structure with four truncated hemispherical cusps of radius R /2 and uniform thickness d = 1 mm (zirconia) or 2 mm (lithium disilicate and dental nanocomposite) on a cylindrical base of radius R = 5.0 mm and total occlusal-to-margin height H = 7.0 mm, i.e. closely representative of the anatomically-correct crowns. The shell is fully bonded to an interior dentin-like substrate. The presence of an intervening cement layer is considered to have a negligible effect on the fracture mechanics, because of its thinness (∼50 μm) and comparable modulus and toughness relative to the dentin-like interior, and was therefore omitted from most computations. Nevertheless, a simple comparative computation with cement layer included was run for confirmation. A vertical load is applied axially onto a rigid sphere of radius r = 3.2 mm in frictionless contact on all four cusps, so that horizontal wedging forces are delivered across symmetry fissure planes (dashed lines in Fig. 1 b). Note that these fissure planes are distant from the indenter contact points ( Fig. 1 a). Input material properties for the crown materials are taken from Table 1 .
The XFEM algorithm contains provision for progressive stable growth of an embedded precursor starter crack with increasing load, up to and beyond the point of final failure, corresponding to total splitting of the crown/substrate assembly. Emplacement of the starter crack on a fissure plane and immediately adjacent to the contact points circumvents meshing complexities in the crack initiation region , and is tantamount to assuming that the tooth structure contains preexisting cracks. This is justifiable in the case of tooth enamel, in the form of tufts and lamellae , but is an assumption that needs to be examined in the case of crown restoratives. Contact with the cusp walls is made by controlling the downward displacement of the indenting sphere in small increments. The outer shell and inner portions of the structure are meshed sufficiently finely that the computations attain convergence. The emplaced crack is allowed to advance mesh cell by cell in accordance with a cohesive zone criterion outlined in detail elsewhere , with parameters for the sequential rupture of each cell adjusted to match the appropriate material modulus and toughness. In this way entire progression of the cracks through both the outer shell and inner filler material could be followed.
2.3
Statistical analysis
Experimental critical splitting load data were analyzed by one-way ANOVA. The independent factor was material group (zirconia, lithium disilicate, nanocomposite). The mean critical load and its 95% confidence interval were calculated for each crown group. Post Hoc Tukey HSD test for multiple comparisons was performed to determine which specific groups differed significantly from one other. The significance level was set at p ≤ 0.05 for all analyses.
3
Results
3.1
Experimental data
Representative top and side views of failures in each monolithic crown system are shown in Fig. 2 . In all cases final fracture occurred by splitting through the dentin to the crown base into two and sometimes more parts. The splits deviated slightly from the symmetry fissure plane between cusps, suggesting that fracture may equally well find a path through part of a cusp, at least for the particular sphere on four-cusp loading configuration investigated here. In all cases the crown failure appeared to be abrupt at the critical load, with no load drop or visible sign of precursor longitudinal or other stable cracking prior to splitting during in situ side wall viewing. This is suggestive of relatively defect-free crown materials where, unlike in enamel, starter cracks must first be initiated before any propagation may ensue. Whereas the section views of the splits in the zirconia and lithium disilicate crowns are ‘clean’ in Fig. 2 , indicative of a brittle fracture mode, those in the nanocomposite crown show substantial plastic deformation in the substrate below the occlusal contacts, indicative of a diminished capacity of the composite material to provide adequate support under high loads. Closer fractographic inspection of the broken crown segments confirmed sites of crack initiation to lie in the occlusal region within the crown shell, between the cuspal load points.
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