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Abstract:Objective To study the mechanical properties of PVP/PVA composite hydrogel and its interaction principle based on the molecular dynamics (MD) theory. Methods MD simulation was applied to investigate mechanical properties and radial distribution functions of PVP, PVA and their mixed system PVP/PVA. Results Compared with the pure PVP, mechanical properties of PVP/PVA were significantly improved, and not affected by temperature. The interaction between PVA and PVP was expected to occur through the interchain hydrogen bonding between the oxygen atom of PVP and the hydroxyl group of PVA. Conclusions MD method revealed the interaction mechanism of PVP/PVA hydrogel at molecular microscopic level and proved it better than pure PVP. Meanwhile, its mechanical properties were stable at different environment temperatures. These results provide a reliable theoretical research method for studying the fabrication process of hydrogel prosthesis in clinic and its mechanical properties.

Abstract:Objective To analyze biomechanical properties of cervical spine after anterior cervical discectomy and fusion (ACDF) and total disc replacement (TDR) surgery. Methods Twelve cadaveric cervical spines (C2-T1) were adopted, and the motion and load distributions of the cervical segments under intact state and after ACDF and TDR surgery were tested using a three-dimensional (3D) optoelectronics measurement system. All the tests were carried out with displacement control in directions of flexion (Flex), extension (Ext), left bending (LB), right bending (RB), left rotation (LR) and right rotation (RR). Motion characteristics of the normal cervical spine and the implant were also discussed. Results In TDR-treated specimens, range of motion (ROM) was well preserved and could restore to the normal ROM distributions, especially in Flex/Ext and LR/RR direction. While in ACDF-treated specimens, ROM presented a large decrease as much as to 73.41% under the same condition compared with TDR, and ROM distributions were also changed obviously in other motions for the segments. Significant changes of ROM in LB/RB direction occurred in both TDR and ACDF group, which were up to 45.92% and 108.06%, respectively. The experimental data indicated that the normal motion of cervical spines was a 3D coupled motion, especially in LB/RB direction, where a 35% rotation around X-axis existed. The cervical spine could recover close to normal coupled motion after TDR surgery. Conclusions TDR surgery can restore the physiological motion of cervical spines more close to the normal state, especially in Flex/Ext and LR/RR direction. The study provides a theoretical basis and quantitative reference for TDR and ACDF surgery in clinic.

Abstract:Objective To explore influences of pre-tension and testing site on the radial compliance of vascular prostheses. Methods Three different textile vascular prosthesis samples (PET woven, PET/PPT woven and PET corrugate warp knitted) with the diameter of 8 mm were selected. Under various pre-tensions and testing sites, the pulsatile pressure and its corresponding radial diameter of these samples were measured by dynamic compliance testing instrument. The radial compliance was calculated according to the compliance formula in ISO 7198-1998. Results The radial compliance of all the three vascular prostheses increased along with the pre-tension increasing. When the pre-tension was 500~550 mN, all the radial compliance reached the maximum (1.03~1.72)%/100 mmHg (1 mmHg=0.133 kPa), and then began to decrease with pre-tension increasing. Furthermore, the radial compliance of corrugate vascular prostheses was slightly bigger at valleys than that at peaks, while the radial compliance at peaks was more stable than that at valleys. Conclusions The pre-tension and test site had vital effects on testing the radial compliance of vascular prostheses. Pre-tension of 500~550 mN should be right to apply to all the three kinds of textile vascular prosthesis at test. As for corrugate vascular prostheses, the radial compliance at peaks should be chosen as the test site to get more stable result and reliable data.

Abstract:Objective To construct the three-dimensional (3D) fluid model at the physiological level of shear stresses and study the effects of fluid shear stress (FSS) on adhesion, differentiation and mechanical sensitivity of osteoblasts. Methods The MC3T3-E1 osteoblasts cultured on β-tricalcium phosphate (β-TCP) scaffolds were subjected to various FSSs in the perfusion flow chamber for 6 hours to compare cell adhesion in FSS-loading groups and control group. Nitric oxide (NO) and alkaline phosphatase (ALP) were detected to compare mechanical sensitivity and cell differentiation. The FSS magnitude and distributions corresponding to various fluid rates were calculated with nonlinear fluid-structure coupling analysis. Results Cell adhesion rate was up to 74%~81% when the average FSS magnitude was lower than 0.4 Pa, but reduced to 60.22% when the average FSS was 0.41 Pa. The NO production rate reached the maximal concentration after loading for 5 min, then significantly reduced at 15 min, and gradually diminished to none at 30 min. ALP level significantly increased (P<0.01) at the shear stress range of 0.232 ~ 0.304 Pa, but maintained at the range of 0.304 ~ 0.412 Pa (P>0.05) with the increase of shear stress. Conclusions Majority of the cells kept a normal adherence to the scaffold at the physiological level of shear stresses. The mechanical sensitivity of the cells under 3D condition was dependent on the FSS rate, which was consistent with two-dimensional (2D) condition. When the average FSS was lower than 0.304 Pa in the scaffold, FSS could significantly promote cell differentiation, but no significant change in cell differentiation could be found when FSS was higher than 0.304 Pa. The present study is expected to accelerate the realization of bone tissue engineering.

Abstract:Objective To establish the monosegmental transpedicular fixation model and short segmental fixation model by three-dimensional finite element technique, and evaluate the biomechanical properties of monosegmental transpedicular fixation for thoracolumbar fractures and verify its feasibility for application. Methods T10-L2 motion segment of a young healthy subject was used to establish the normal finite element model. The superior 1/2 cortical bone of the T12 segment was removed and superior 1/2 cancellous bone of the same vertebrae was assigned material property of the injured bone to simulate the thoracolumbar fracture. Transpedicular screw fixation of the T11 and T12 segment was performed in monosegmental fixation model. T11 and L1 segment were instrumented in the short segmental fixation model. All the four finite element models were applied with loading of axial compression, anteflexion, extension, lateroflexion and axial rotation, respectively. Motion difference of each functional unit and the stress of implants were measured to evaluate biomechanical behaviors of monosegmental fixation. Results The motion difference of all the functional units (T10-11, T11-12, T12-L1) in the fractured model was obviously increased under all loading conditions as compared to the normal model, but the motion difference in the fractured models was decreased after monosegmental fixation and short segmental fixation, and no significant differences were found between monosegmental fixation and short segmental fixation. The stress on screws in monosegmental fixation model was significantly lower than that in short segmental fixation under axial compression and anteflexion, but the stress on screws of two fixation models had no significant difference under extension, lateroflexion and axial rotation. The stress on the rods of monosegmental fixation model was apparently higher than that of short segmental fixation under extension and lateroflexion, and lower under axial rotation, but no significant difference was found for two fixation models under axial compression and anteflexion. Conclusions Monosegmental transpedicular screw fixation would give the similar stabilization as short segmental fixtion and could be an effective alternative to treat incomplete fractures in thoracolumbar spine.

Abstract:Objective To study the relationship between comfort and sitting pressure of ejection seat cushion. Methods By measuring sitting pressure distributions on seat cushions with different thickness, indices of sitting pressure such as the contact area, maximum sitting pressure and mean sitting pressure were discussed along with the subjective comfort scores to find the relationship between seat cushion comfort and indices of sitting pressure. Results The contact area, maximum sitting pressure and mean sitting pressure were significantly correlated with the subjective comfort scores. In addition, the comfort score did not increase when thickness of the seat cushion exceeded 4 cm. Conclusions The comfort of seat cushion could be well reflected and evaluated by the contact area, maximum sitting pressure and mean sitting pressure of seat cushion, and thickness of 4 cm is recommended for designing the seat cushion with material used in this paper.

Abstract:Objective To investigate the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts and provide a theoretical guidance for reducing restenosis in artery bypass graft surgery. Methods Five models were employed to numerically investigate the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts. The distributions of hemodynamic parameters such as velocity, second flow, wall shear stress (WSS) and wall shear stress gradient (WSSG) and their change in line with the increase of diameter ratio were analyzed. In addition, comparison of hemodynamic differences in a fully-occluded complete model (Model A), a fully-occluded partial model (Model B) and a 75% stenosis complete model (Model C) with graft-host diameter ratio of 1.0 was conducted to validate the fully-occluded complete model established in this study. Results The hemodynamic performance of Model A was totally different from that of Model C, and the velocity distribution at the graft top had an obvious influence on the WSS distribution at host artery bed in the downstream anastomosis, with maximum WSS differences reaching 79%. A large graft-host diameter ratio resulted in a large size of low WSS region at the host artery bed, but with uniformly distributed WSS and small WSSG. A small graft-host diameter ratio resulted in a small size of low WSS region at the host artery bed, but with large WSSG. Conclusions It is necessary to adopt a complete model to study the influence of graft-host diameter ratio on the flow field of fully-occluded artery bypass grafts. The diameter ratio had a significant impact on the flow field of fully-occluded artery bypass graft, thus a large ratio could be helpful to reduce the occlusion resulted from the restenosis at the downstream anastomosis in artery bypass graft.

Abstract:Objective To investigate the loading rate-dependent property of different layers for articular cartilage by unconfined compression testing on articular cartilage at different loading rates. Methods The non-contact digital image correlation (DIC) technique was applied to investigate the mechanical properties of different layers for fresh pig articular cartilage at different loading rates. Results At constant loading rate, the compressive strain of superficial layer and deep layer was the largest, while that of middle layer was in between under the same compressive stress. The Poisson’s ratio increased from superficial layer to deep layer along with cartilage depth increasing. The stress-strain curves of cartilage were different at different loading rates, indicating that the mechanical properties of cartilage were dependent on the loading rate. The elastic modulus of cartilage increased with loading rates increasing, and the compressive strains of different layers decreased under the same compressive stress with loading rates increasing. Conclusions The compressive strain decreased while the Poisson’s ratio increased from superficial layer to deep layer along the cartilage depth. The mechanical properties of different layers for cartilage were dependent on the loading rate. This study can provide the basis for clinical cartilage disease prevention and treatment, and is important for mechanical function evaluation of artificial cartilage as well.

Abstract:Objective To analyze and compare hemodynamic features of two different options for modified B-T shunt (MBTS) surgery, namely end-to-side(ETS) and side-to-side (STS), so as to provide references for clinical treatment of single ventricle heart defect syndrome. Methods The real geometric model was reconstructed by medical images obtained from a patient with hypoplastic left heart syndrome (HLHS); MBTS surgery was simulated through virtual operations; a lumped parameter model (LPM) was constructed based on physiological data of the patient; the post-operational boundary conditions of computational fluid dynamics (CFD) models (namely STS model and ETS model) were predicted based on the LPM; numerical simulation was conducted on two CFD models by using finite volume method. Results Flow details and wall shear stress distributions were all obtained for two models. The mean oscillatory shear index (OSI) of ETS model and STS model in part of pulmonary arteries was 3.058×10-3 and 13.624×10-3, respectively, while the energy loss was 116.5 and 94.8 mW, respectively, and blood flow rate ratios of left pulmonary artery to right pulmonary artery (RRPA/LPA) were 0.8 and 1.72, respectively. Conclusions There were nearly no differences between two CFD models in energy loss, which led to a relatively small impact on the surgery. The STS model had a more balanced pulmonary artery blood perfusion and a smaller mean OSI in part of pulmonary arteries, therefore, the STS model was superior to the ETS model. This study provides an important theoretical support and reference for treating patients with HLHS.

Abstract:Objective To study dynamic characteristics of human lumbar spine using three-dimensional finite element method. Methods Finite element model of lumbar spine (L1~5) was developed and validated based on CT images, and the modal analysis was also conducted. Results A total of top 30-order modal parameters were extracted to obtain dynamic characteristics of the lumbar spine under free boundary conditions. Resonance frequencies of the model were concentrately distributed, but the amplitude of each order varied greatly. Amplitude near L5 segment was much larger, indicating L5 was easily to be injured. This lumbar modal analysis could provide a basis for its further dynamic analysis. Parameters such as natural frequency, modal shape and vibration amplitude of the lumbar spine would be helpful for both lumbar dynamic analysis and optimal design of man-machine interface mechanical equipment.

Abstract:Objective To compare the postoperative stability of unstable distal radius fractures fixed by locking screws with different length and analyze the stress distributions of distal screws and callus at different healing periods, so as to provide biomechanical references for choosing appropriate length of screws for treating distal radius fractures. Methods The three-dimensional finite element models of unstable distal radius fractures with fracture section and callus were established, respectively, and fixed by volar locking plates and locking screws with different length. Then different periods of fracture healing were simulated by assigning callus with different material properties. Stress distributions on callus and distal screws at different postoperative periods were analyzed, and compression stiffness of the whole fixation system was calculated according to the maximum axial displacement at fracture section. Results Under the same axial loads, the compression stiffness was basically the same by using unicortical screw with over 75% length or bi-cortical screw. With the screw length increasing, the maximum stress of callus was decreased gradually during the period of early healing; while the maximum stress of distal screws was increasing gradually with the increase of screw length at middle and last period of fracture healing, and the stress of distal bi-cortical screw was the largest. Conclusions Using the unicortical distal locking screws with at least 75% length can not only produce early stability， but also avoid extensor tendon injuries due to dorsal screw prominence.

Abstract:Objective To provide an optimization method and a basic configuration for the configuration design of mandibular defect-repair implant. Methods A topology optimization computation based on density method was adopted to make topology optimization on reconstruction titanium plate for repairing mandibular defect. Results The effects of volume fraction F and penalization factor p on the optimized configuration were compared and analyzed, respectively, and the optimum parameters were chosen to be F=0.65 and p=3.5. Using this set of parameters, optimized computation was performed on the mandibular defect-repair implant, and the threeopening configuration was proved to be the basic optimization configuration. The result also showed that unilateral molar clenching had more significant effect on stress distributions in the implant as compared to incisal clenching. Conclusions A set of parameters suitable for optimizing configuration of mandibular defect-repair implant, as well as the basic configuration of optimized titanium plate were obtained, which could provide references for the design of mandibular defect-repair implant in clinic.

Abstract:Objective To establish an ultrasonic viscoelastic detection system to measure the viscoelasticity of in vitro tissues. Methods Based on the method of shear wave dispersion ultrasound vibration (SDUV), this system applied acoustic radiation force to excite harmonic vibration in soft tissues. The propagation of shear waves induced by the vibration was detected and the tissue viscoelasticity properties were calculated. The standard phantom and rat liver experiment were conducted using this system, and preliminary assessment of the system was completed. Results The measured result of standard phantom was close to the calibration value. The viscous and elastic coefficient of rat liver were (1.12±0.41) Pa?s and (0.81±0.40) kPa, respectively. Conclusions The results about the standard phantom and rat liver experiment approved feasibility of the system for viscoelasticity measurement on in vitro animal experiment, which is a preliminary exploration for the realization of liver fibrosis detection of human body.

Abstract:Hematogenous metastasis is one of the most important ways for metastasis of tumor cells and this is a complex pathophysiological process. Tumor cells enter the bloodstream and move with the blood circulation, meanwhile interact with leukocyte, platelets via integrins, or directly interact with endothelial cells to cause a series of biological behaviors and promote the metastasis of tumor cells. These activated integrins, collaborating with other molecules (e.g. integrins, selectins, cytokines and chemokines), will induce various signal cascades to mediate tumor cell adhesion and migration, and form a new metastatic foci. Hence, better understanding of hematogenous metastasis process is of great significance for treating malignant metastasis of tumor cells and improving life of tumor patients. In this review, the roles of integrins during hematogenous metastasis of tumor cells and their signal transduction were summarized, and new perspectives for future investigation were also discussed. The elucidation about the mechanism of hematogenous metastasis of tumor cells will help to provide a rational basis for anticancer drug development and drug target discovery.

Abstract:Mechanical stimulation affects the onset of cell apoptosis, thereby leads to the structural and functional changes of the tissues and organs. Such force-affected cell apoptosis plays an important role in physiological and pathological processes of organism and is closely related with occurrence and development of many diseases. In recent years, the mechanism of signal transduction affected by force-induced cell apoptosis has aroused more interests of researchers in the field of cellular mechanics, and experiment results at present stage showed that the form of mechanical stimulation, its intensity and the type of cells on which the mechanical stimulation applied would affect cell apoptosis with different process and results. During the process, the signal pathways, genes and protein expression related with cell apoptosis were quite complex and the specific mechanism of signal transduction still remain unclear. In this article, the recent advances on characteristics of cell apoptosis and its related signal pathways, as well as effects of different mechanical stimulations on cell apoptosis and its signal pathways were reviewed and discussed.