Finite element analysis of stress in oral mucosa and titanium mesh interface.

BACKGROUND: The stiffness of titanium mesh is a double-blade sword to repair larger alveolar ridges defect with excellent space maintenance ability, while invade the surrounding soft tissue and lead to higher mesh exposure rates. Understanding the mechanical of oral mucosa/titanium mesh/bone interface is clinically meaningful. In this study, the above relationship was analyzed by finite elements and verified by setting different keratinized tissue width in oral mucosa. METHODS: Two three-dimensional finite element models were constructed with 5 mm keratinized tissue in labial mucosa (KM cases) and 0 mm keratinized tissue in labial mucosa (LM cases). Each model was composed of titanium mesh, titanium screws, graft materials, bone, teeth and oral mucosa. After that, a vertical (30 N) loadings were applied from both alveolar ridges direction and labial mucosa direction to stimulate the force from masticatory system. The displacements and von Mises stress of each element at the interfaces were analyzed. RESULTS: Little displacements were found for titanium mesh, titanium screws, graft materials, bone and teeth in both LM and KM cases under different loading conditions. The maximum von Mises stress was found around the lingual titanium screw insertion place for those elements in all cases. The keratinized tissue decreased the displacement of oral mucosa, decreased the maximum von Mises stress generated by an alveolar ridges direction load, while increased those stress from labial mucosa direction load. Only the von Mises stress of the KM cases was all lower than the tensile strength of the oral mucosa. CONCLUSION: The mucosa was vulnerable under the increasing stress generated by the force from masticatory system. The adequate buccal keratinized mucosa width are critical factors in reducing the stress beyond the titanium mesh, which might reduce the titanium exposure rate.

[Influence of attachment type on stress distribution of implant-supported removable partial dentures].

OBJECTIVE: To compare influences of different retention attachments on stress among supporting structures. METHODS: By 3-dimensional laser scanner and reverse engineering computer aided design (CAD) software, a basic partially edentulous digital model with mandibular premolar and molar missing was established. Implant attachment and removable partial dentures (RPD) were added into the basic model to build three kinds of models: RPD only, RPD + implant + Locator attachment, and RPD + implant + Magfit attachment. Vertical and inclined loads were put on artificial teeth unilaterally. By means of 3-dimensional finite element analysis, the stress distribution and displacement of the main supportive structures were compared. RESULTS: A complete 3-dimensional finite element model was established, which contained tooth structure, and periodontal structures. The displacement of the denture was smaller in Locator (9.38 µm vertically, 45.48 µm obliquely) and Magfit models (9.54 µm vertically, 39.45 µm obliquely) compared with non-implant RPD model (95.27 µm vertically, 155.70 µm obliquely). Compared with the two different attachments, cortical bone stress value was higher in Locator model (Locator model 10.850 MPa vertically, 43.760 MPa obliquely; Magfit model 7.100 MPa vertically, 19.260 MPa obliquely).The stress value of abutment periodontal ligamentin Magfit model (0.420 MPa vertically) was lower than that in Locator model (0.520 MPa vertically). CONCLUSION: The existence of implant could reduce maximum von Mises value of each supportive structure when Kennedy I partially edentulous mandible was restored. Comparing the structure of Magfit and Locator attachment, the contact of Magfit attachment was rigid, while Locator was resilient. Locator attachment could improve stability of the denture dramatically. Locator had stronger effect on defending horizontal movement of the denture.

[Finite element analysis of the maxillary central incisor with crown lengthening surgery and post-core restoration in management of crown-root fracture].

OBJECTIVE: To construct the finite element models of maxillary central incisor and the simulations with crown lengthening surgery and post-core restoration in management of different crown-root fracture types, to investigate the stress intensity and distributions of these models mentioned above, and to analyze the indications of crown lengthening from the point of view of mechanics. METHODS: An extracted maxillary central incisor and alveolar bone plaster model were scanned by Micro-CT and dental impression scanner (3shape D700) respectively. Then the 3D finite element models of the maxillary central incisor and 9 simulations with crown lengthening surgery and post-core restoration were constructed by Mimics 10.0, Geomagic studio 9.0 and ANSYS 14.0 software. The oblique static force (100 N) was applied to the palatal surface (the junctional area of the incisal 1/3 and middle 1/3), at 45 degrees to the longitudinal axis, then the von Mises stress of dentin, periodontal ligament, alveolar bone, post and core, as well as the periodontal ligament area, were calculated. RESULTS: A total of 10 high-precision three-dimensional finite element models of maxillary central incisor were established. The von Mises stress of models: post>dentin>alveolar bone>core>periodontal ligament, and the von Mises stress increased linearly with the augmentation of fracture degree (besides the core). The periodontal ligament area of the crown lengthening was reduced by 12% to 33%. The von Mises stress of periodontal ligament of the B2L2c, B2L3c, B3L1c, B3L2c, B3L3c models exceeded their threshold limit value, respectively. CONCLUSION: The maxillary central incisors with the labial fracture greater than three-quarter crown length and the palatal fracture deeper than 1 mm below the alveolar crest are not the ideal indications of the crown lengthening surgery.

[Analysis of human mandible and temporomandibular joint using three-dimensional finite element method under different mechanical models].

OBJECTIVE: Analysis the result of human mandible and temporomandibular joint using two different three-dimensional finite element method under different mechanical models. METHODS: The 3-dimensional model including cortical and cancellous bone for human mandible was obtained through computed tomography (CT) scan. Then the model was meshed in the software ICEM CFD. The passive and active muscle-force loadings were separately applied on the FE model to simulate the anterior clenching. Stress distributions in two models were compared. RESULTS: The stress distributions of two models were apparently different. In the passive muscle-force model, high stress was mainly distributed in mandibular angle, retromolar trigone, notch and bite point on crown. In the active muscle-force model, high stress was mainly distributed in condylar vertex and neck, mandibular angle, retromolar trigone and bite point on crown. There were some similarities between passive and active muscle loadings. However, large difference existed in condylar region due to the vertices reaction force disparity. CONCLUSION: Closer to actual stressing state of human mandible and temporomandibular joint, the active muscle-force model is a proper biomechanical model for human mandible under anterior clenching.

Patient-specific modeling of facial soft tissue based on radial basis functions transformations of a standard three-dimensional finite element model.

BACKGROUND: An important purpose of orthodontic treatment is to gain the harmonic soft tissue profile. This article describes a novel way to build patient-specific models of facial soft tissues by transforming a standard finite element (FE) model into one that has two stages: a first transformation and a second transformation, so as to evaluate the facial soft tissue changes after orthodontic treatment for individual patients. METHODS: The radial basis functions (RBFs) interpolation method was used to transform the standard FE model into a patient-specific one based on landmark points. A combined strategy for selecting landmark points was developed in this study: manually for the first transformation and automatically for the second transformation. Four typical patients were chosen to validate the effectiveness of this transformation method. RESULTS: The results showed good similarity between the transformed FE models and the computed tomography (CT) models. The absolute values of average deviations were in the range of 0.375 - 0.700 mm at the lip-mouth region after the first transformation, and they decreased to a range of 0.116 - 0.286 mm after the second transformation. CONCLUSIONS: The modeling results show that the second transformation resulted in enhanced accuracy compared to the first transformation. Because of these results, a third transformation is usually not necessary.

[Individualized three-dimensional finite element model of facial soft tissue and preliminary application in orthodontics].

OBJECTIVE: To get individualized facial three-dimensional finite element (FE) model from transformation of a generic one to assist orthodontic analysis and prediction of treatment-related morphological change of facial soft tissue. METHODS: A generic three-dimensional FE model of craniofacial soft and hard tissue was constructed based on a volunteer's spiral CT data. Seven pairs of main peri-oral muscles were constructed based on a combination of CT image and anatomical method. Individualized model could be obtained through transformation of the generic model based on selection of corresponding anatomical landmarks and radial basis functions (RBF) method. Validation was analyzed through superimposition of the transformed model and cone-beam CT (CBCT) reconstruction data. Pre- and post-treatment CBCT data of two patients were collected, which were superimposed to gain the amount of anterior teeth retraction and anterior alveolar surface remodeling that could be used as boundary condition. Different values of Poisson ratio ν and Young's modulus E were tested during simulation. RESULTS: Average deviation was 0.47 mm and 0.75 mm in the soft and hard tissue respectively. It could be decreased to a range of +0.29 mm and -0.21 mm after a second transformation at the lip-mouth region. The best correspondence between simulation and post-treatment result was found with elastic properties of soft tissues defined as follows. Poisson ratio ν for skin, muscle and fat being set as 0.45 while Young's modulus being set as 90.0 kPa, 6.2 kPa and 2.0 kPa respectively. CONCLUSIONS: Individualized three-dimensional facial FE model could be obtained through mathematical model transformation. With boundary condition defined according to treatment plan such FE model could be used to analyze the effect of orthodontic treatment on facial soft tissue.

Stress area of the mandibular alveolar mucosa under complete denture with linear occlusion at lateral excursion.

BACKGROUND: The rocking and instability of a loaded complete denture (CD) during lateral excursion reduce the bearing area under the denture base, causing localized high stress concentrations. This can lead to mucosal tenderness, ulceration, and alveolar bone resorption, and the linear occlusion design was to decrease the lateral force exerted on the denture and to ensure denture stability. But it is not known how the bearing areas of linear occlusal CDs (LOCDs) and anatomic occlusal CDs (AOCDs) differ. The purpose of this study was to analyze and compare the distributions of the high and low vertical stress-bearing areas in the mandibular alveolar mucosa under LOCDs and AOCDs at lateral excursion. METHODS: Computerized tomography (CT) and finite element analysis were used to establish three-dimensional models of an edentulous maxilla and mandible with severe residual ridge resorption. These models were composed of maxillary and mandibular bone structure, mucosa, and the LOCD or AOCD. Lateral excursion movements of the mandible were simulated and the vertical stress-bearing areas in the mucosa under both mandibular CDs were analyzed using ANSYS 7.0. RESULTS: On the working side, the high stress-bearing (-0.07 to -0.1 MPa) area under the LOCD during lateral excursion was smaller than that under the AOCD, while the medium stress-bearing (-0.03 to -0.07 MPa) area under the LOCD was 1.33-fold that under the AOCD. The medium stress-bearing area on the non-working side under the LOCD was 2.4-fold that under the AOCD. Therefore, the overall medium vertical stress-bearing area under the LOCD was 20% larger than that under the AOCD. CONCLUSIONS: During lateral excursion, the medium vertical stress-bearing area under a mandibular LOCD was larger and the high vertical stress-bearing area was smaller than that under an AOCD. Thus, the vertical stress under the LOCD was distributed more evenly and over a wider area than that under the AOCD, thereby improving denture stability.

[Stress distribution in alveolar bone around implants under implant supported overdenture with linear occlusion at lateral occlusion].

OBJECTIVE: To analyze stress distribution in alveolar bone around implants of implant supported overdentures (ISO) with linear occlusion and with anatomic occlusion at lateral mandibular position, and to justify the possibility of decreased injurious force around implants in ISO with linear occlusion. METHODS: Computerized tomography scan and finite element analysis (FEA) were used to set up two 3-D FEA models of maxillae and mandible with severe residual ridge resorption. The mucosa, linear and anatomic occlusal ISO with bar attachments, and two implants inserted between mandibular foramina were also established in the models. With the condition of imitating the loading of masseter muscles, these models were loaded to simulate the stress distributions in alveolar bone around implants under ISO at lateral occlusion position. RESULTS: At lateral occlusion, the stress distributions in alveolar bone around implants under ISO with anatomic occlusion were mainly on the lingual and distal sides of the working side implants. However, stress distributions under ISO with linear occlusion were on the distal sides of bilateral implants. Both the stress peaks of ISOs with linear occlusion and with the anatomic one appeared in the working side. In anatomic occlusion model, sigma(z): -6.47 MPa and 6.81 MPa, sigma(1): -4.20 MPa and 7.20 MPa (negative value: compressive stress, positive value: tensile stress); in linear occlusion model, sigma(z): -4.86 MPa and 3.04 MPa, sigma(1): -3.48 MPa and 5.33 MPa. CONCLUSIONS: At lateral occlusion, when comparing the ISO with two different occlusion schemes, stress peak in alveolar bone around implants in the linear occlusion model was lower than that in the anatomic occlusion model at equal loading situation. Stress in the alveolar bone under ISO with linear occlusion distributed more evenly than that under ISO with anatomic occlusion.