Initial Displacement and Stress Distribution of Upper Central Incisor Extrusion with Clear Aligners and Various Shapes of Composite Attachments Using the Finite Element Method.

A clear aligner is an esthetic and more comfortable option for patients who need orthodontic treatment. However, some types of tooth movement, such as extrusion, are difficult with this tool. Therefore, composite attachments have been suggested to improve tooth movement. This study aims to evaluate the initial displacement and stress distribution during upper central incisor extrusion using the conventional composite attachments. Maxillary models with the upper teeth, clear aligners, and composite attachments placed on the labial surface of the upper right central incisor were constructed. Four models were created to simulate upper central incisor extrusion: (1) without any composite attachment; (2) rectangular beveled attachment; (3) ellipsoid attachment; and (4) horizontal rectangular attachment. Clear aligners were designed to perform upper central incisor extrusion. The constructed models were analyzed using the finite element method. Initial displacement and stress distribution were analyzed. Output analysis found that the upper right central incisor in the model with a horizontal rectangular attachment had the greatest extrusive movement, followed by the model with ellipsoid attachment and the model with beveled attachment. Maximum compressive stress was seen at the cervical region of the composite attachment. Composite attachments including horizontal rectangular attachment, ellipsoid attachment, and rectangular beveled attachment can be used to perform upper central incisor extrusion.

Determination of the centre of resistance during en masse retraction combined with corticotomy: finite element analysis.

OBJECTIVE: To determine the effect of corticotomy on the change in the centre of resistance of the six maxillary anterior teeth Materials and methods: Three-dimensional finite element models of the maxillary anterior teeth with and without corticotomy were constructed. Brackets (size 0.022 inch × 0.028 inch) were placed passively on all anterior teeth that were set at the centre of the labial surface in the mesio-distal dimension and 3 mm from the incisal edge to the bracket slot in the vertical direction. The power arm was set mesial of the canine bracket. For the model with corticotomy, the bone density was decreased from initial value at 5% to 25%. The point of force application was varied in order to locate the centre of resistance. The centre of resistance was located by measurement of the difference of the displacement between the apical and incisal edges. The position of force was varied by moving apically parallel to the occlusal plane to simulate tooth movement. RESULTS: As the alveolar bone density decreased from initial value to 25%, the location of the centre of resistance moved apically from the bracket slot from 10.8 mm to 11.2 mm, respectively. CONCLUSIONS: The change of alveolar bone density due to corticotomy was associated with the location of the centre of resistance. The location of the centre of resistance moved apically as the alveolar bone density decreased but it was not clinically noticeable.

Finite element analysis of bone around a dental implant supporting a crown with a premature contact.

OBJECTIVE: To investigate the influence of a premature contact caused by an implant-retained crown (IRC) on stress and strain distributions in bone surrounding the implant using the finite element method. MATERIAL AND METHOD: A 3D finite element (FE) model of a section of a mandible with a single tooth dental implant, an IRC, and two adjacent teeth was created. Three rigid plates were used to represent the antagonist teeth. Modeling the antagonist teeth using the rigid plates removed the necessity to create FE models for the antagonist teeth, their periodontal ligament, and the maxilla. Moreover, this new approach also allowed the premature contact height to be easily varied by changing the positions of the rigid plates. In the present study, premature contact heights of 0, 50, 100, 150, 200 and 250 microm were considered. The FE contact analysis was employed. All materials were assumed to be linear elastic and isotropic. RESULTS: The magnitudes of von Mises stresses in the bone change drastically when there was a premature contact. For example, the von Mises stress increased from 9.68 MPa in the case with no premature contact to 49.92 MPa in the case with the premature contact height of 50 microm. In addition, the magnitude of the major principal strain in the marginal bone reached the pathologic overload of 4000 microepsilon when the premature contact height was 100 microm or higher. CONCLUSION: The influence ofpremature contacts is very high and the premature contact height of an IRC over 100 microm should be avoided as much as possible to provide longevity of dental implants.

Fit-and-fill analysis of trochanteric gamma nail for the Thai proximal femur: a virtual simulation study.

The present study present a three-dimensional virtual simulation method to evaluate the fit-and-fill effect of the insertion of a trochanteric gamma nail (TGN) in 98 Thai dadaveric proximal femora. The circular best fit of the 2-dimensional cross-section of the femoral canal and the nail at 4 levels [d100, d120, d140 and d160] which were located at 100, 120, 140 and 160 mm distal to the tip of the greater trochanter were calculated. The evaluation of each level included, 1) the diameters of the medullary canal, 2) the percentage of area filled by the nail in the unreamed medullary canal, 3) the minimal reamer diameter that required enlargement of the canal to accommodate TGN insertion, 4) the minimal inner cortical reaming thickness that needed to be removed, 5) the percentage of cortical bone area that needed to be removed prior to nail insertion and 6) the deviation of the nail center from the center of the medullary canal. The results showed that at 4 studied locations the diameter of unreamed medullary canal averaged 10.3 to 11.8 mm. The nail cross-section that could fill the medullary canal averaged 86.9-95.1%. The minimal reaming diameter for the medullary canal to accommodate the TGN insertion averaged 11.3 to 12.3 mm. The inner cortical thickness that should be removed averaged 0.6 to 0.8 mm. The cortical bone that needed to be removed averaged 13.6 to 19.3% of the total cortical area. The deviation of the nail center from the canal center averaged 0.3 to 0.8 mm. The present study showed some mismatching of the TGN to that of the Thai proximal femur. Appropriate reaming to prepare the medullary canal should be considered prior to TGN insertion to prevent technical problem. Future re-design of the implant may be considered for Thai patients.