Study On The Fracture Mechanisms Of All-Ceramic Crown Restorations

All-ceramic crowns have been widely used in prosthodontics because oftheir stable biocompatibility and aesthetic characteristics. However, asthey are brittle materials and vulnerable to tensile failure, the surviverate is not satisfied. It is important to study the failure mechanisms ofall-ceramic crowns to improve the service life. Due to limitations oflaboratory test methods, the initiation and propagation of cracksoccurring in opaque stressed all-ceramic crowns was unable to beexamined. This doctorial dissertation proposed a unique experimanentaland numerical method and then studied the influences of all-ceramicmaterials, cement aging, cement types and resin substrate on theload-bearing capacity of all-ceramic crowns with intention to provideguidance for clinical applications. The main contributions of the studyare listed as follows：1. The experimental and numerical methods for studying failuremechanism of all-ceramic crown restorations have been proposed.Sectioned all-ceramic crowns with2mm thickness have been designedand fabricated to study the failure mechanisms of all-ceramic crowns. DIC was carried out to detect the crack initiation and propagation inlayer structures during loading. The3D numerical model is developedbased on the geometry of the first mandible molar using reverseengineering technology. The convergence of the model was verified.2. The fracture modes and critical fracture loads of all-ceramiccrowns for IPS e.max and Zirconia have been studied. Sub-surface yielddamge occurring in veneer was detected by DIC analysis during thequasistatic contact loading. Cracks initiated from subsurface penetratedinto the IPS e.max Press core and led to bulk fracture in IPS e.maxspecimens,while deflected in Zirconia core and caused chipping in theveneer layer. It is found that cracks are unlikely to propagate fromveneer with low-modulus and low-toughness ceramic (Vita VM9) to theceramics with high-modulus and high toughness ceramic (Zirconia).Results of the three-dimensional axisymmetric contact model showedthat cracks were caused by shear stress and were driven by tensile stress.The three-dimensional numerical simulation of all-ceramic crownsshowed that the core ceramics with high elastic modulus can bear largebite forces and protect veneer and dental substrate. 3. The effect of fatigue on fracture toughness of ceramics has beenstudied. The laboratory studies indicated that the fracture toughness ofIPS e.max Press reduced significantly with fatigue loads, while noappreciable variation was found in Zirconia. The fatigue resistance ofZirconia is much stronger than that of IPS e.max Press. The quasistaticcontact tests also suggested that the load-bearing capacity of IPS e.maxspecimens can be significantly affected by fatigue loads. Bothexperimental results were consistent.4. The influence of cement aging on load-bearing capacities andfracture modes of all-ceramics has been studied. Quasi-static contactload is applied on buccal cusp bridge of dry and water aging specimens.Chipping was the dominant failure mode for dry specimens regardless ofthe ceramic types. An apparent debonding between the core andsubstrate was detected at a low loading level in water aging specimens.The load-bearing capacity for IPS e.max crowns were significantlyreduced with bulk failure due to cement aging, while zirconia specimens still underwent chipping with a slight drop of the load-bearing capacity.Numerical results indicated that cement aging could lead to tensile stressconcentration on the lower surface of the core and increase the risk ofradial cracks, which was detrimental to low-strength dental ceramics.The results of this study rationally explained the differences in thelaboratory tests and clinical examinations.5. The effect of the elastic modulus of cements (Pavavia F E=18.3GPaand Variolink II E=8.3GPa) on load-bearing capacities and fracturemodes of all-ceramic has been studied. Quasi-static contact tests showedthat there was a significant difference in load-bearing capacities in thetwo types of adhesive specimens. The cement with high elastic moduluscould improve load-bearing capacity of all-ceramic crowns. Numericalresults demonstrated that the high elastic modulus of cement coulddecrease the tensile stress level in core ceramics. The optimal cementthickness of90μm was suggested from the four enumerated thicknesses(60,90,120and150μm). 6. The effect of elastic modulus of the resin substrate (Z100E=16.6GPa和Surefil E=9.3GPa) on load-bearing capacity and fracture mode ofall-ceramic crowns has been studied. Quasi-static contact tests showedthat there was a significant difference in load-bearing capacities in thetwo types of resin substrates. Numerical results demonstrated that thehigh elastic modulus of the resin substrate could decrease tensile stresslevels in core ceramics.