An Analysis Of The Post-Crown Restoration Of Upper Central Incisor With Surgical Crown Lengthening And Forced Eruption By Three Dimensional Finite Element Method

Objective : In order to evaluate the effect of surgical crown lengthening and forced eruption on root retention after the post-crown restoration of upper central incisor, we constructed an ideal three-dimensional finite element model of the maxillary central incisor (including tooth, root, post crown, and alveolar bone) by the combination of Spiral CT (SCT) scanning technology, 3D digital image reconstruction technique, and three dimensional finite element method (3-DFEM). Using this model, we analyzed the variation and distribution of the stress induced in the alveolar bone, and the initial movement of the maxillary central incisor, under different alveolar loss conditions after surgical crown lengthening, foreed eruption and post-crown restoration. This study provides theoretical insights into the treatment of patients with crown fractures under the gingiva.Materials and Methods: The original 3D data of the maxillary central incisor is obtained by spiral CT scanning and then editing the digital image with software Mimics. Based on this data, the 3D FE model of the maxillary central incisor is constructed using 3D finite element method (3D FEM) and software such as Unigraphies and MscMarC/Mentat. This 3D FE model provides a good morphology of the incisor including root, post-crown, alveolar bone, and is a good model for its biomechanics research. There are 42533 elements and 8214 nodes in this model and the material behavior involved in it was simplified as homogeneous, continuous and isotropic. Based on this model, three models of the root of different degrees of fractures under the gingiva were obtained by gradually increasing the pitch of the fractured plane. Six models of post crown and alveolar bone are constructed according to the operating criterion of surgical crown lengthening and the foreed eruption. In each of the six models, a labiolingal 100N static load was applied on incisal 1/3 of the lingual surface at 45 degree to the long axis of the tooth. In the post-process using MscMarC/Mentat, calculation and analysis were done on the stress distribution and initial movement of the central incisor under different alveolar loss conditions.Result:The analysis revealed that with the reduction of the periodontal support, the largest tension of the alveolar bone and the initial displacement of the incisor edge increased. The increment of the stress and displacement were bigger in group B (foreed eruption) than in group A (surgical crown lengthening). The stress of the alveolar bone and the displacement of the incisor edge in model Al were slightly larger than those in model B1. The stress of the alveolar bone and the displacement of the incisor edge in model A2 and A3 were significantly smaller than those in model B2 and B3.Conclusions:1. The increase of the largest tension of the alveolar bone and the initial displacement of the incisor edge is not in proportion to the reduction of the periodontal support or the loss of the root. The more the alveolar bone is lost, the more the largest stress of periodontal support and the displacement of the incisor edge are increased.2. For patients with a relatively short crown fracture under the gingiva (for example less than 1mm), both surgical crown lengthening and foreed eruption are good choices. The latter is slightly better than the former.3. For patients with a relatively deep crown fracture under the gingiva (for example more than 2mm), surgical crown lengthening has a significantly better retention than foreed eruption.4. For patients with a very deep crown fracture under the gingiva, tooth removal or other operations are needed.