To determine, by means of finite element analysis (FEA) and static fracture testing the effect of the tooth preparation design on the stress state and the structural integrity of the cemented, machined ceramic crown-tooth complex.
Method:
Extracted human maxillary premolar teeth were prepared for all-ceramic crowns using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita Mk II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3, Sirona AG). The machined all-ceramic crowns were resin bonded to the tooth structure using a resin-based lute (Panavia F 2.0, Kuraray). FEA Analysis: A specimen of each group (Standard and Flat) was digitized with a micro-CT scanner (Skyscan 1172®, Bruker-Micro CT) to create a 3D model. Professional CAD modelling software (Haptic technology) was used to create the supporting structures; periodontal ligament and alveolar bone. The generated 3D-models were imported into an FEA software program (Hypermesh version 10.0, Altair Engineering, Inc). The maximum principal stress and the von Mises stress values were calculated. Mechanical Testing: Ten specimens of each group were subjected to an axial compressive force through a 4mm diameter steel ball at a crosshead speed of 1 mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the fracture point was recorded for each specimen in Newtons (N).
Result:
The von-Mises and maximum-principal-stress values within the restored crown-tooth complex were lower in the flat occlusal design than the standard design. One-way ANOVA (p=.000) with Tukeys pairwise comparison demonstrated a significant difference in the fracture strength between the flat occlusal design (833.4±147.5N) and the standard design (531.6±133N).
Conclusion:
There is a strong correlation between the stress state and the structural integrity of the crown-tooth complex for different tooth-preparation designs.