In Vitro Study On The Thickness Design Of Resin-matrix Ceramics In Implant Crown Restoration

Objective1、The fracture resistance of resin-matrix ceramics crowns with different thickness supported by titanium abutment was compared by preparing specimens,which could provide guidance for the application of resin-matrix ceramics in implant restoration.2、Finite element method was used to compare the stress distribution of different CAD/CAM all ceramic restoration materials and abutment design in implant crown restoration.Methods1、According to the standard crown shape of the first molar,CAD/CAM data were imported.Through the use of specific CAD design tools to reduce the size respectively to ensure uniform material thickness of the crown.The RKO8 cutting machine was used to cut and make individual titanium abutments（n=8）.The adhesive thickness was set as 100μm,and then the data were imported into CEREC to process corresponding crowns with thickness of 1 mm and 2 mm（n=8）and the crown thickness error was controlled within±0.1 mm.After adjusting the height of prefabricated titanium abutment to be consistent with customized titanium abutment,the model data of customized abutment and prefabricated abutment are imported into computer software.Copy the crown shape of customized abutment group,and 3D print the resin-matrix ceramics crown of prefabricated abutment group（the thickness is about 3mm）.The internal surface of the restoration and the surface of the titanium abutment were sandblasted（Al₂O₃）and coated with Single Bond Universal.Titanium implant abutments were fixed on implant analogs,and the Vita Enamic crowns were bonded with Rely X Ultimate to abutments.After thermocycling for 10,000 cycles（5 to 55°C,60 seconds dwell time）,The specimens were subjected to compressive load in a universal testing machine to record initial load to failure.The data were statistically analyzed.2、The finite element model of implant crown restoration of mandibular first molar was established.Four crown thicknesses（1 mm,2 mm,3 mm and 4 mm）and four different crown restoration materials,namely,resin-matrix ceramics（Lava Ultimate and Vita Enamic）,lithium disilicate ceramics（IPS e.max CAD）and zirconia ceramics were designed.The first molar occlusal surface of the mandibular was loaded with 600 N.The finite element software ANSYS 10.0 was used to analyze the stress distribution and record the principal stress in the tensile direction.Results:1、One-way ANOVA indicated that different thickness had a statistically significant effect on the fracture resistance of resin-matrix ceramics crowns supported by titanium abutment.The 2 mm group showed the highest fracture resistance of 1707.09±289.31 N,and the 3 mm group showed the lowest fracture resistance of 1204.96±130.50 N.In terms of fracture morphology,Most of the 1mm and 2 mm groups were type II failure.The crown prosthesis was partially detached from the abutment.The abutment exposed area was more than 10%and less than or equal to 50%.Most of the 3 mm group were IV failure.The crown prosthesis completely fell off from the abutment or the exposed area of abutment was more than 90%.2、The crown stress analysis showed that 211.30 MPa was the highest in 1 mm Cercon group and 11.56 MPa was the lowest in 4 mm Lava Ultimate group,the abutment stress analysis showed that 38.84 MPa was the highest in 3 mm-Lava Ultimate group and 11.68 MPa was the lowest in 1 mm Cercon group.With the increase of crown thickness in Lava Ultimate group,Vita Enamic group and e.max CAD group,the stress of crown prosthesis increased.With the increase of crown thickness,the stress in cement of each material showed an upward trend,while the stress of implant showed a downward trend.The stress concentration of resin-matrix ceramics crown and cement was higher than that of personalized abutment.The stress in the surrounding alveolar bone changed little.Conclusion:1、In this study,the fracture resistance of resin-matrix ceramics crowns with different thickness supported by titanium abutment were compared.The fracture resistance of 2 mm group was higher than that of 1 mm and 3 mm.In terms of fracture morphology,1 mm and 2 mm crowns were mostly partial fractures which could be repaired with resin,and 3 mm groups were longitudinal fractures with crown separated from abutment.2、The results of finite element analysis showed that the maximum tensile stress concentration of resin-matrix ceramics Lava Ultimate and Vita Enamic increased with the crown thickness;the maximum tensile stress concentration in cement increased with the increase of crown thickness.It is beneficial to reduce stress concentration when resin-matrix ceramics materials are used in dental crown restoration.The effect of dynamic impact on implant and surrounding bone needs further study.