Biomechanical evaluation of syndesmotic screw design via finite element analysis and Taguchi's method.

BACKGROUND: Screw fixation of syndesmotic injuries facilitates ligament healing and restoration of ankle stability, but failure of the screw might threaten the success of the treatment. Screw design parameters, such as outer diameter, inner diameter, thread pitch, leading edge radius, trailing edge radius, leading edge angle, and trailing edge angle, might have effects on the stresses that occur in the screws. This is the first study, to our knowledge, to investigate which geometric screw parameters play key roles in stresses that occur in screws used for syndesmotic fixation. METHODS: A three-dimensional finite element model of an ankle was reconstructed. Four different types of titanium screws­4.5-mm malleolar, 4-mm cancellous, 4-mm machine, and 3.5-mm cortical­were placed on this model. Physiologic load was applied to evaluate the stress in the screw. Then the contribution of each design factor to stress in the screws was analyzed systematically by Taguchi's robust design method. RESULTS: The maximum equivalent ductile failure (von Mises equivalent stress) value was found in the 4-mm cancellous screw (402 MPa). Taguchi's analysis showed that the descending order of contribution of the design factors to stress emerging on the screw is inner diameter, leading edge angle, thread pitch, outer diameter, and trailing edge angle. CONCLUSIONS: Stress that occurs in syndesmotic screws is closely related to their geometry and dimensions. According to the results, a 3.5-mm cortical screw with the ideal screw design regarding optimal parameters to resist against stresses in the syndesmosis seems more reasonable to choose in syndesmotic fixation.

Alterations in pituitary gland volume in polycystic ovary syndrome: a structural magnetic resonance imaging study.

The purpose of this prospectively designed cross-sectional observational study was to evaluate the effect of polycystic ovary syndrome (PCOS) on pituitary gland volume (PGV) under the hypothesis that endocrinologic changes may lead to morphologic changes of the pituitary gland. Twenty-six PCOS patients and 31 control subjects underwent magnetic resonance imaging (MRI) of the pituitary. Informed consent was obtained from all subjects. PGV was significantly larger in PCOS patients than in control subjects. Luteinizing hormone/follicle-stimulating hormone ratio was the only predictor of PGV. The association between pituitary gland enlargement and PCOS should be kept in mind when pituitary hypertrophy is detected on MRI.

Biomechanical evaluation of syndesmotic screw position: a finite-element analysis.

OBJECTIVES: To evaluate the stresses in syndesmotic screws and widening of syndesmosis under loading after placement of the screws at different levels from the ankle joint line and to determine the optimal level. METHODS: From a set of computed tomographic data of an ankle, a 3-dimensional finite-element model was reconstructed. Six fixation configurations of the syndesmosis with placement of 3.5 or 4.5 mm single tricortical screws at 20-45 mm from the tibiotalar joint were performed on this model. Physiological loads approximating those during both midstance and heel-off states of stance phase of normal walking were applied to evaluate the stress in the screw and widening of the syndesmosis. RESULTS: Among the 6 fixation configurations, the lowest von Mises stress was found in the screws placed 30-40 mm above the joint line (373.31-380.17 MPa for 3.5 mm cortical screw and 284.06-327.31 MPa for 4.5 mm cortical screw in midstance phases), whereas the least syndesmosis widening was determined when the screw was placed 30 mm above the tibial plafond (0.005 mm) for 3.5 mm cortical screw and 20, 25, and 30 mm above the tibial plafond (0.004 mm for each, respectively) for 4.5 mm cortical screw during midstance phases. CONCLUSIONS: This study showed that syndesmosis fixation at the level of 30-40 mm above the tibiotalar joint has advantages with regard to stress in screws in comparison with the other evaluated levels.

Three-dimensional finite element analysis used to compare six different methods of syndesmosis fixation with 3.5- or 4.5-mm titanium screws: a biomechanical study.

BACKGROUND: Use of thicker and longer (four cortices) screws or of multiple screws seems to be more stable and efficient for syndesmosis fixation. METHODS: A three-dimensional finite element model of an ankle was constructed from serial axial sections from an existing two-dimensional computed tomographic image. Constructions of syndesmosis fixation with 3.5-mm single tricortical, 3.5-mm single quadricortical, 3.5-mm double tricortical, 3.5-mm double quadricortical, 4.5-mm single tricortical, and 4.5-mm single quadricortical screws were performed on this model. Physiologic loads approximating those during stance phase normal walking were applied to this ankle system. Stress values on the screws using the six fixation methods were compared. RESULTS: The highest maximum stress was determined over 3.5-mm cortical screws applied as single quadricortical, and the lowest maximum stress was determined over the 4.5-mm cortical screw applied as single quadricortical. Stress on the 3.5-mm single screw with quadricortical application was found to be higher than that with tricortical application and also compared with the 4.5-mm quadricortical screw application. Differences between the 4.5-mm single tricortical and quadricortical screws and between the 3.5-mm single tricortical and 3.5-mm double tricortical screw applications were not significant. CONCLUSIONS: Quadricortical application of 3.5-mm single screws and tricortical application of 3.5-mm double cortical screws are not good choices for syndesmosis fixation. If the plan is tricortical application, a 3.5-mm single cortical screw is adequate. If quadricortical application of syndesmosis fixation is planned, a 4.5-mm cortical screw should be used.

Anatomical comparison and evaluation of human proximal femurs modeling via different devices and FEM analysis.

BACKGROUND: Breuckmann optical scanning, Metris laser scanning and CT are general devices used for modeling hard or soft tissues in the biomedical field. Whereas the CT device is able to model internal and external structures of hard tissues, Breuckmann and Metris devices can only model the exterior portions of tissues. In this study, a human proximal femur was modeled using these devices, and the matching accuracy thereof is presented. METHODS: The human proximal femur was modeled by scanning with Breuckmann optical scanning, Metris laser scanning and CT devices. The 3D/3D registration method was performed in two ways: coordinate to coordinate based, and 2D contour based matching. Matching accuracies of the three models were developed with statistical deviation and local deviation. To determine the significance value between the deviations obtained, one way ANOVA, and for intragroup comparisons Tukey and Thamhane tests were used. After statistical analysis, stresses on the models were evaluated using ANSYS software taking boundary conditions on human standing position into consideration. RESULTS: In this study, the value of the 2D contour based accuracy deviation of the femur head zone between CT and Metris models was obtained as 0.4 ± 0.2 mm while it was 0.3 ± 0.1 mm between CT and Breuckmann. The highest matching deviation obtained as a result of the ANOVA test among these three models was found in the femur trochanter region (0.0142 ± 0.0164 mm), the lowest value was found in the femur head region (0.0070 ± 0.0132 mm). The stress of the CT-Breuckmann pair was found close to each other in stress analysis. CONCLUSION: The deviation values obtained by matching models created by three different methods showed statistically significant results (P < 0.05). Values obtained from the CT-Breuckmann model were lower than those obtained from CT-Metris. In order to lower deviation values, applications such as increasing the resolution of images, using stronger algorithms, meshing methods and enhancing surface form should be implemented.

Anatomical evaluation and stress distribution of intact canine femur.

BACKGROUND: In the biomedical field, three-dimensional (3D) modeling and analysis of bones and tissues has steadily gained in importance. The aim of this study was to produce more accurate 3D models of the canine femur derived from computed tomography (CT) data by using several modeling software programs and two different methods. METHOD: The accuracy of the analysis depends on the modeling process and the right boundary conditions. Solidworks, Rapidform, Inventor, and 3DsMax software programs were used to create 3D models. Data derived from CT were converted into 3D models using two different methods: in the first, 3D models were generated using boundary lines, while in the second, 3D models were generated using point clouds. RESULTS: Stress analyses in the models were made by ANSYS v12, also considering any muscle forces acting on the canine femur. When stress values and statistical values were taken into consideration, more accurate models were obtained with the point cloud method. CONCLUSION: It was found that the maximum von Mises stress on the canine femur shaft was 34.8 MPa. Stress and accuracy values were obtained from the model formed using the Rapidform software. The values obtained were similar to those in other studies in the literature.