Three-dimensional Finite Element Study On The Comparison Of Periodontal Tissue Stress Of Tilted Abutment Attachment Denture And Porcelain-fused-to-metal Fixed Bridge

It was very common to see proximal teeth incline and slope caused by teeth missing of protracted restoration, especially the second mandibular molar because of the missing first one. The resultant force of tilted abutment was not axis force ,and medial stress can not be transmitted and disperse through medial teeth, all of which made periodontal tissue stress distribute unevenly and harmed the periodontal tissue. So reasonable restoration of such teeth gradually become a popular consideration.Theoretically, the best therapy method was to restore after Minor Orthodontics Tooth Movement. Actually, it was too difficult to do that for kinds of reason. Normally, removable partial denture was unstable and had inefficient mastication. Nowadays, some scholars testified that fixed restoration benefited the periodontal health of tilted abutment by biomechanics. But much more dental hard tissue need to be cut in traditional fixed bridge denture, even dental pulp was devitalized sometimes. As a result. patients became more and more distressed and the cost increased. A few scholars had proposed that attachment can be used as retainer to solve the problem of the insertion path of abutments when tilted molar was planned to be fixed. the stress distribution of the tilted abutment teeth can be improved when restored with attachment.This experiment used modified key-way attachment-an extracoronal attachment. Modified key-way attachment placed positive part outside the full crown of medial abutment, while put negative key-way inside the pontic. Due to distal abutment inclined to midst, we had to consider the path insertion. So.it was unnecessary for extracoronal attachment to prepare box preparation in vital dental pulp after negative key-way was placed outside the full crown, which preserved tooth tissue and did no harm to dental pulp. Though this attachment was already used clinically, there was no report concerning its biomechanical nature.This experiment organically integrated CT scan technology with finite element method, which was applied to the establishment of three-dimension finite element model of PFM fixed bridge denture and modified key-way attachment denture in which the second molar were tilted 30 degrees towards midst. Compare stress distribution of alveolar bone and periodontal membrance of two types of restorations abutments when load came from different direction.This research concluded in two parts. The results showed as follows: 1)Establishment of three-dimension finite element model of attachment denture and porcelain-fused-to-metal fixed bridge denture. Two three-dimension finite element models of attachment denture and porcelain-fused-to-metal fixed bridge denture with CT scan technology and software were established. There were 63661 nodes and 312217 tetrahedron elements respectively.2) Stress comparison of different directions of two abutments' periodontal tissue. The stress distribution of periodontal tissue of attachment abutment was similar with PFM fixed bridge, and the stress concentration part had no apparent difference, all of which testified the point that fixed bridge can improve tilted abutment to some extent. The stress concentrated on periodontal tissue of cervix and root apiex as well as alveolar bone between7 roots.Conclusion: Modified key-way attachment denture of tilted abutment not only can obtain favorable retention and stability but also can improve the stress distribution of abutments and their supporting tissue. Attachment denture can eliminate the exposure of mental clasp arm and lead to betteresthetics compared with RPD. Attachment denture obtain favorable insertion path other than cutting much dental hard tissue which might hurt dental pulp . It owned the advantages of rigidly-fixed bridge as for bearing bite force, and the advantages of cantilever fixed bridge convenience as for the denture insertion. But the stress of periodontal tissue of cervix and root apiex kept strong whatever despite the restoration of attachment and PFM fixed bridge, which showed that fixed restoration just reduced stress value but can not thoroughly alter stress concentration. So indication should be seriously chosen when we rehabilitate a tilted molar. Meanwhile, in order to improve the curative effect of restoration ..restore projects should be planned carefully and various methods reducing bite force should be taken for those cases that gradient was too large and periodontal conditions was in bad condition.