| 1、BACKGROUNDFull crown has come up to be the mainstream method for the repair of tooth defects.All-ceramic crown has become one of the first choice to repair the defected anterior teeth due to its outstanding aesthetic and biocompatible advantages.Tooth preparation is the first and most important step in full crown restoration.About 1/3 of the tooth structure is removed to eliminate the undercut and provide the necessary repair space for the prosthesis.However,improper tooth preparation will impair the fracture resistance of the abutment,and directly affect the survival rate of the all-ceramic crown and the affected teeth.Anterior tooth preparation of all-ceramic crowns would be affected by aesthetic and mechanical factors.However,researches on the aesthetic and mechanical properties of all-ceramic crowns mostly focused on the effects of crown thickness or crown structure.There are less reports on tooth preparation modes of anterior crown considering different ceramic types or ceramic structures.Moreover,with the continuous progress of ceramic materials and manufacturing technology,the monolithic all-ceramic crowns designed and manufactured digitally,are widely used in clinic.Many porcelain manufacturers or denture laboratories declared that monolithic all-ceramic crowns can achieve ideal restoration results with less tooth preparation and thinner prosthesis thickness.However,there is still a lack of scientific and standard guidance for the anterior tooth preparation method of monolithic all-ceramic crowns at present,which need to be evaluated by experimental research.2、OBJECTIVESIn this study,an experimental model of monolithic all-ceramic crowns based on different "tooth preparation mode + crown thickness + porcelain type" was constructed to study their effects on the shade and mechanical properties of the full-crown prosthesis.The results of the present study would provide a theoretical guidance for the selection,design and clinical operation of monolithic all-ceramic crowns.It is of great significance to promote the standardization and popularization of anterior all-ceramic crown restoration and improve the long-term success rate of all-ceramic crown restoration.3、METHODS3.1 The colorimetric research model of the monolithic all-ceramic crown of the anterior teeth was constructed using IPS Nature Die abutment colorimetric system.This model included resin abutments with different discoloration degree(ND2,ND5,ND8)and prepared according to different degree,the corresponding anterior monolithic all-ceramic crowns of with different thickness and ceramic types,and the resin cement with different shades as well.The all-ceramic crown was designed and manufactured through an all-digital process.The shade of different regions of the crown restoration was measured by spectrophotometer under the environment of standard D60 light source using a silicon rubber testing guide.The values of L,a,and b were recorded and the color differences of△E were calculated to investigate the effects of tooth preparation mode,abutment color,porcelain type and shade of resin cement on the color of anterior porcelain crown prosthesis.3.2 With the help of Micro-CT scanning,the scanned tissue data of the upper central incisor were imported into the reverse engineering software(Mimics,Geomagic,Hypermesh)to establish a three-dimensional finite element model containing elements such as monolithic all-ceramic crown,cement layer,abutment and periodontal membrane.The accuracy of the model was tested for standby.Then the three-dimensional finite element analysis software(Abaqus)was used to energize and design the contact of each part of the model.Finally,static loading was carried out to simulate anterior teeth occlusion and boundary limits were set.The influence of different tooth preparation mode and ceramic type on stress distribution of anterior monolithic all-ceramic crown was analyzed.3.3 The elastic modulus and mechanical properties of the two experimental resins for abutments were tested by three-point bending test to screen their mechanical properties.With the help of digital tools,we designed and made the experimental central incisor abutment with G10 resin.The monolithic all-ceramic crowns were bonded to the abutments simulating the clinical bonding process and the occlusal loads were then conducted by the universal mechanical tester.The fracture load of the all-ceramic crown in different experimental groups were tested and the fracture mode of the crown and the abutments were analyzed by scanning electron microscope.4、RESULTS & CONCLUSIONS4.1 For both glass ceramic and high translucent zirconia crowns,the two-way analysis showed that tooth preparation mode,abutment color and the interaction between the two factors had significant effects on the shades of the monolithic all-ceramic crowns(p<0.01).When the same color of the abutment teeth,the △E value of the crown restorations decreased with the increase of the amount of the tooth preparation,possibly due to the increasing crown thickness with the increase of the tooth preparation amount.With the same amount of tooth preparation amount,the higher degree of abutment discoloration is,the larger △E value of the monolithic all-ceramic crown is: ND8>ND5>ND2(p<0.05).For abutments with light and moderate discoloration(ND2 and ND5),when the tooth cut in 1/3of middle and cervical region reached 1mm(preparation 2 and preparation 3)in glass ceramic groups,the △E value was less than 3.46 and satisfactory aesthetic restoration can be achieved;when the tooth cut in 1/3 of middle and cervical region were 0.5mm(preparation 1)in zirconia groups,the △E values were less than 3.46 with satisfactory aesthetic performance.For the abutment teeth with severe discoloration(ND8),the △E values of each group were all higher than 3.46 in glass ceramic groups,which exceeded the discernible range of naked eyes and significantly affected the aesthetic performance;when the tooth cut was 1.5mm in the incisal region and 1mm in the middle region(preparation 3)in zirconia groups,the △E values was less than 3.46 with the clinical acceptable aesthetic performance.However,in all groups even the tooth cut in cervical region was 1.0mm,and the zirconia groups value was higher than 3.46,which showed the abutment color would be difficultly covered.Therefore,when selecting monolithic porcelain crown for anterior teeth restoration in clinic,crown thickness and the corresponding tooth preparation should be reasonably designed according to different abutment color and porcelain type to obtain the satisfactory aesthetic performance.4.2 The ceramic type,the tooth preparation mode and the color of resin cement,as well as the interaction between two and three factors all had significant effects on the color difference values of anterior monolithic all-ceramic crown(p<0.05).When ceramic type and the tooth preparation mode were the same,the △E values of the crown restorations decreased with shielding ability of cement color: WO < B0.5 < TR(p<0.05).When the color of cement and the ceramic type were the same,the △E values of the crown restorations decreased with the increase of tooth preparation amount.When the tooth preparation amount and cement color were the same,the △E values of the crown restorations in zirconia groups were significantly lower than those in glass ceramic groups(p < 0.05).4.3 In this study,a three-dimensional finite element restoration model of anterior teeth with a monolithic porcelain crown was successfully constructed,including the monolithic all-ceramic crown model,the adhesive layer model,the abutment preparation model,the periodontal membrane model,etc.The suitability of each part of the model and the mesh division quality were confirmed.The stress distribution was studied using this model.The results showed that the amount of tooth preparation had an effect on the stress peak value of monolithic all-ceramic crown and the abutment.When the ceramic type is the same,the stress of crown decreased with the increase of tooth preparation amount.The stress peak mainly concentrated on the inner side of the crown of the palatal part corresponding to the loading point of static force,which may be because the stress between the crown and the abutment is mainly tensile stress.When the tooth preparation amount is the same,the stress of crown in zirconia groups were larger than those in glass ceramic groups,which may be because high translucent zirconia has larger elastic modulus and less mechanical denaturation for less buffering effect.The stress on abutments increased with the increase of the amount of preparation,and the stress peak mainly concentrated in the 1/3 of the proximal neck of the root and the corresponding alveolar ridge in the alveolar bone.With the increase of tooth preparation amount,the stress distribution gradually approach the incisal end of the remaining dental tissue,and the stress concentration distribution range decreased and gradually tend to be uniform.It is suggested that the increase of crown thickness and abutment preparation should not be done simply in pursuit of good aesthetic effect,which may lead to increased stress of abutment and increased risk of crown fracture.4.4 The results of three-point bending experiment show that G10 resin has more similar elastic modulus and bending strength to the dentin than P60 composite resin.Moreover,digital processing can be used to prepare specimens with P60 resin,which would be helpful to ensure the consistency of abutment specimens.Therefore,P60 resin was selected as the abutment replacement materials for fracture load tests in this study.When the tooth preparation is the same,the stress load were significantly greater in zirconia groups than those in glass ceramic groups(p<0.05).The fracture mode of the specimens were observed by scanning electron microscope.When the amount of abutment preparation increased in preparation 3 and preparation 4 groups,different number of abutment fractures existed in both zirconia and glass ceramic groups,which was consistent with the results of threedimensional finite element analysis.It further confirmed that the increase of abutment preparation would reduce the fracture resistance of teeth.The fracture load of monolithic porcelain crown in all experimental groups was greater than 700 N,which exceeded the range of 75-270 N of the anterior teeth reported in the literature.This indicated that the preparation volume of palatal teeth of 0.5mm of the two porcelain materials could meet the clinical requirements.5、conclusionWhen glass ceramic crowns and translucent zirconia crowns are used for anterior monolithic porcelain crown in clinic,the tooth preparation amount of abutment teeth and crown thickness should be reasonably designed according to the color of abutment teeth,and cement with appropriate color should be selected for bonding.The amount of abutment preparation had an effect on the stress distribution and fracture strength of monolithic allceramic crown restorations.Reducing the amount of abutment preparation could increase the stress of porcelain full crown,but decreased the stress value of corresponding abutment preparation pairs.On the contrary,increasing the amount of tooth preparation could also increase the stress to the abutment and increase the risk of tooth fracture In clinical practice,the color and the fracture resistance of the abutment should be considered comprehensively in the design of anterior monolithic all-ceramic crown restorations,so as to obtain a minimally invasive,aesthetic,and long-lasting restorations... |
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