| With the outstanding aesthetics advantages of the all-ceramic crown to metal crown and porcelain fused to metal Crown, the all-ceramic restoration has been improved dramatically in the recent decades. More and more patients prefer to choose this new prosthodontics. Although all-ceramic materials is developing rapidly,there are still have failure case reported in the clinical occasionally. The reason of failure concludes bulk fracture of crown ,cohesive fracture in the veneer, etc. So, in addition to increasing the strength of all-ceramic materials, optimum design of all-ceramic crown plays a very important role in guaranteeing its long-term performance. The design of crown includes: margin configurations, crown thickness, adhesive thickness, cusp inclination, etc. Compared with other factors, the design of crown thickness affects the biomechanical properties of the crown most obviously. However, there are no uniform conclusions for crown thickness design; and which thickness should the crown be is a long-term problem confused clinicians.Objective: This study aimed to create a 3D finite element model for continuous variation of occlusal thickness and shoulder width of monolithic all-ceramic crown ,and veneer and core occlusal thickness of bilayered all-ceramic crown , thereby identifying their optimal range under biomechanical consideration.Methods: The solid model of mandibular first molar all-ceramic crown were constructed by means of 3DSS-STDC-II scanning, digital image transfer, reverse engineering software Geomagic, computer aid design program Unigraphics NX6. Then the solid model was imported to Ansys Workbench software by bidirectional parameters transmitting between Unigraphics NX6 and Ansys Workbench to build finite element model.In monolithic all-ceramic crown, occlusal thickness(H) and shoulder width(W) were set as design parameter(DP). H ranged from 0.2mm to 3mm, and W ranged from 0.1mm to 1.5mm. The Maximum Principal stress in crown was set as objective function(OBJ). A simulated chewing force of 225N load was applied vertically at small area (SA load) or large area (LA load) on the monolithic crown. Optimum design was carried by observing how H and W affect the OBJ during two loading scenarios.In bilayered all-ceramic crown, veneer occlusal thickness(V) and core occlusal thickness(C) were set as DP. V ranged from 0.1mm to 2mm,and C ranged from 0.1mm to 1mm . The Maximum Principal stress in veneer and core were set as objective function(OBJ). A simulated chewing force of 225N load was applied at the mesial-distal cusp ridge on the veneer layer vertically(Vertical load) or with angle of 25°to the tooth axis(Angular load). Optimum design was carried by observing how V and C affect the OBJ during two loading scenarios.Results:1.Two parametric three-dimensional finite element model of the first mandibular first molar monolithic all-ceramic crown which includes DP of H and W as well as bilayered all-ceramic crown parametric finite element model with DP of V and C were established.2.For monolithic all-ceramic crown, High MPS levels were observed directly below the load region in contact with the cement layer under both SA load and LA load, and higher stress levels were also observed in areas around the loading point under SA load.3.Under SA load, Max MPS in crown decreased by 35.48% with H and W increasing. While under LA load, those decreased by 54.78%. The Max MPS of all-ceramic crown under SA load is considerably higher than that on LA load, consequently the crown is easier to be broken under SA load which should be avoid in clinical.4. For monolithic all-ceramic crown, W has no influence to Max MPS under SA load. When W ranged from 0.56mm to1.50mm under LA load, H ranged from 1.35mm to3mm under SA load, and when H ranged from 1.25mm to2.61mm under LA load, the tangent slope rate of OBJ response curves ranged from-1 to 1.5.For bilayered all-ceramic crown, under vertical load, the Max MPS of veneer was observed in areas around the loading points, and the Max MPS of core concentrated in the bottom part of core Layer, directly beneath the loading position. While under angular load,the peak MPS of veneer was observed on the central fossa aspect of the loading points, opposing the direction in which the horizontal component load was applied, and the Max MPS of the core was similar to the area the vertical load concentrated in the bottom part of core Layer.6.Under vertical load, Max MPS in veneer and core decreased by 20.9% and 74.6% respectively with V and C increasing. And under angular load, those decreased by 52.2%.and 79% respectively. The Max MPS under angular load is higher than it under vertical load; therefore large cusp inclination should be avoided in making crown. On the condition of vertical loading, Max MPS of veneer in experiment three is smaller than Max MPS of crown in experiment two, so the existence of core can reduce the stress in veneer.7. For bilayered all-ceramic crown, under vertical load ,when V ranged from 0.88mm to 1.57mm, the tangent slope rate of veneer response curves ranged from-1 to 1; when V ranged from 0.75mm to 2mm, the tangent slope rate of core response curves ranged from-1 to 1; C has no influence to veneer Max MPS; when C exceed 0.52mm, the tangent slope rate of core response curves ranged from-1 to 1. Under angular load, with increase of V, Max MPS of veneer is on the rise; when V exceed 0.75mm, the tangent slope rate of core response curves ranged from-1 to 1; core thickness has no influence to Max MPS of veneer; when C exceed 0.39mm, the tangent slope rate of core response curves ranged from-1 to 1.Conclusion: Biomechanically, occlusal thickness ranged from 1.35mm to 2.61mm and shoulder width exceed 0.56mm are the combination with optimal properties for a monolithic all-ceramic crown, and veneer occlusal thickness ranged from 0.88mm to 1.57mm and core occlusal thickness exceeded 0.52mm are the combination with optimal properties for a bilayered all-ceramic crown. |
PDF Full Text Request