| It has been shown that root canal treatment leads to loss of tooth structure and affects the fracture resistance of the tooth.The occurrence of irreparable root fractures is a major cause of tooth failure in pile crown restorations.The survival rate of a tooth restored with a pile crown is closely related to the position of the tooth in the dental arch,the material of the pile core,and the length and diameter of the pile.For teeth with moderate to severe defects or combined periodontal-pulpal lesions,cast piles have a more obvious advantage.Cobalt-chromium alloy is a commonly used material for cast piles in clinical restorations because of its corrosion resistance and low price.The emergence of the new discipline of oral biomechanics has provided us with more diversified research methods.Among them,the finite element method can transform a complex oral system into a computer data model to simulate the forces on teeth in the oral cavity and to optimize the design of pile cores.Objective:The three-dimensional finite element model of the mandibular first premolar was established by the finite element method,and the length and diameter of the cobalt-chromium metal cast pile were grouped in more detail and comprehensively,and the magnitude of the Von Mises stress in the root combined with the distribution of the stress clouds were observed under specific loads,so as to obtain the optimal length and diameter scheme of the cobalt-chromium metal cast pile core restoration of the mandibular premolar,with a view to providing a basis for clinical restoration of to reduce the occurrence of root fracture.Method:A mandibular first premolar that met the criteria for orthodontic reduction extraction was selected,and the root canal treatment was completed in vitro.2 mm of crown-side tooth tissue was retained for pile preparation.Image data were obtained by CBCT scanning and exported in DICOM format.Mimics Research 20.0,Geomagic Studio 12,and Abaqus 6.14 were used to construct 3D finite element models of the mandibular first premolar and cobalt-chromium cast piles of different lengths and diameters for the pile core crown restoration.The model was loaded by simulating the stresses on the mandibular first premolar in the mouth.Analysis was performed to compare the changes in the Von Mises maximum stress peak and stress clouds to derive the optimized range for the cobalt-chromium metal cast pile restoration of mandibular first premolar.Result:1、The finite element model of mandibular first premolar restored by pile core crown was successfully constructed.2、 When the diameter of the cobalt-chromium metal casting pile is constant,the maximum stress peak of the root Von Mises shows a decreasing trend with the increase of the pile length;when the length of the cobalt-chromium metal casting pile is constant,the maximum stress peak of the root Von Mises shows a decreasing trend with the increase of the pile diameter and then increases.Among them,when the diameter of the cobalt-chromium metal casting pile was 3.0 mm and the length was 14.0 mm,the maximum stress peak of Von Mises of the root was the smallest,73.70 MPa;when the diameter of the cobalt-chromium metal casting pile was 2.0 mm and the length was 8.0mm,the maximum stress peak of Von Mises of the root was 84.34 MPa,with a variation of 14.44%.When the pile length was 12mm-14mm(about 69.4%-80.9% of the root length)and the pile diameter was 2.6mm-3.0mm(about 32.5%-37.5% of the buccolingual diameter of the root),the maximum stress peak of Von Mises in the root fluctuated between 73.7.Mpa-78.82 Mpa,which was relatively small.3、The stress distribution clouds on the external surface of the root showed that the stress distribution in the buccolingual direction of the root was more concentrated,mainly located in the lingual side and the root 1/3 of the buccal side,followed by the cervical 1/3 of the buccal side;the stress distribution in the proximal and distal mesial directions was relatively uniform.When the diameter of the cobalt-chromium cast pile remained unchanged,the distribution of the strongest stress level(corresponding to the surface area marked with red,orange,and yellow)in the cross-sectional stress distribution cloud gradually decreased as the length of the pile increased,while the distribution of the lowest stress level(corresponding to the surface area marked with green to dark blue The regional distribution of the lowest strength stress level(corresponding to the surface area marked in green to dark blue)gradually increases and even becomes dominant;the stress distribution pattern does not change when the length of the pile changes with constant diameter.4、Under the condition of constant diameter of the cobalt-chromium metal casting pile,the Von Mises maximum stress peak of the pile increases with the increase of the pile length.Under the condition of constant pile length,there is no obvious pattern in the change of Von Mises maximum stress peak of the pile as the diameter of the pile changes.The correlation between the Von Mises maximum stress peak and the length of the pile is more significant.Among them,when the diameter of the cobalt-chromium metal casting pile was 2.0 mm length 8.0 mm,the Von Mises maximum stress peak of the tooth root was the smallest,172.9 MPa;when the diameter of the cobalt-chromium metal casting pile was 2.4 mm length 14.0 mm,the Von Mises maximum stress peak of the pile was the largest,366.9 MPa,with a variation of 112.2%.Conclusion:1、The Von Mises stresses on the roots of mandibular first premolar teeth restored with cobalt-chromium cast piles are mainly concentrated on the lingual and buccal cervical portions of the root and the outer surface of the apical 1/3 of the tooth.Therefore,as much lingual and buccal cervical and apical 1/3 tooth tissue should be preserved during root canal treatment and subsequent pile crown restoration,thus reducing the risk of root fracture.2 、 When using cobalt-chromium metal casting pile to restore the mandibular first premolar,the pile length is about 69.4% of the root length or more,on the basis of ensuring 4mm apical closure,the pile length can be increased appropriately to increase the retention of the pile;the diameter of the pile is about 32.5%-37.5% of the buccolingual diameter of the root,the root bearing stress is minimal;it is conducive to protecting the root and reducing the occurrence of root fracture.3、The trend of Von Mises stress of the root is opposite to the trend of Von Mises stress of the cobalt-chromium metal casting pile,suggesting that: as the length of the pile increases,the bearing capacity of the cobalt-chromium metal casting pile increases and the bearing capacity of the root decreases significantly;the cobalt-chromium metal casting pile bears the main stress load about 2-3 times of the stress load borne by the root.Therefore,casting pile restorations are clinically recommended for teeth with weak roots. |
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