Ata Hashemi

Dynamic tensile properties of bovine periodontal ligament: A nonlinear viscoelastic model.

As a support to the tooth, the mechanical response of the periodontal ligament (PDL) is complex. Like other connective tissues, the PDL exhibits non-linear and time-dependent behavior. The viscoelasticity of the PDL plays a significant role in low and high loading rates. Little information, however, is available on the short-term viscoelastic behavior of the PDL. Also, due to the highly non-linear stress-strain response, it was hypothesized that the dynamic viscoelastic properties of the PDL would be greatly dependent on the preload. Therefore, the present study was designed to explore the dynamic tensile properties of the bovine PDL as a function of loading frequency and preload. The in vitro dynamic tensile tests were performed over a wide range of frequencies (0.01-100Hz) with dynamic force amplitude of 1N and different preloads of 3, 5 and 10N. The generalized Maxwell model was utilized to describe the non-linear viscoelastic behavior of the PDL. The low loss factor of the bovine PDL, measured between 0.04 and 0.08, indicates low energy dissipation due to the high content of collagen fibers. Moreover, the influence of viscous components in the linear region of the stress-strain curve (10N preload) was lower than those of the toe region (3N preload). The data reported in this study could be used in developing accurate computational models of the PDL.

Effect of Thermal Stresses on the Mechanism of Tooth Pain

INTRODUCTION Daily hot and cold thermal loadings on teeth may result in structural deformation, mechanical stress, and pain signaling. The aim of this study was to compare the adverse effects of hot and cold beverages on an intact tooth and, then, to provide physical evidence to support the hydrodynamic theory of tooth pain sensation mechanism. METHODS Three-dimensional finite element analysis was performed on a premolar model subjected to hot and cold thermal loadings. Elapsed times for heat diffusion and stress detection at the pulp-dentin junction were calculated as measures of the pain sensation. RESULTS Extreme tensile stress within the enamel resulted in damage in cold loadings. Also, extreme values of stress at the pulpal wall occurred 21.6 seconds earlier than extreme temperatures in hot and cold loadings. CONCLUSIONS The intact tooth was remarkably vulnerable to cold loading. Earlier changes in mechanical stress rather than temperature at the pulp-dentin junction indicate that the dental pain caused by hot or cold beverages may be based on the hydrodynamic theory.

Biomechanical behavior of bovine periodontal ligament: Experimental tests and constitutive model.

A viscohyperelastic constitutive model with the use of the internal variables approach was formulated to evaluate the nonlinear elastic and time dependent anisotropic mechanical behavior of the periodontal ligament (PDL). Since the relaxation response was found to depend on the applied stretch, the adoption of the nonlinear viscous behavior in the present model was necessary. In this paper, Helmholtz free energy function was assigned to the material as the sum of hyperelastic and viscous terms which is based on the physical concept of internal variables. The constitutive model parameters were evaluated from the comparison of the proposed model and experimental data. For this purpose, tensile response of the bovine PDL samples under different stretch rates was obtained. The good correspondence between the proposed model and the experimental results confirmed the capability of the model to interpret the stretch rate behavior of the PDL. Moreover, the validity of structural model parameters was checked according to the results of the stress relaxation tests.

Do mechanical properties of human fetal membrane depend on strain rate

The objective of this study was to examine the effect of strain rate on the mechanical properties of human fetal membranes.

Thermal Analysis of Dental Implants in Mandibular Premolar Region: 3D FEM Study.

PURPOSE The distribution of temperature in a dental implant following hot food and beverage consumption is essential for evaluating the hazard this process may have on bone health. The purpose of this study was to predict the temperature distribution in the dental implant with/without a crown and the bone crest in contact with it using the finite element method. MATERIALS AND METHODS A 3D model of the implant and the mandible was prepared by using computer-aided design software. Implants were investigated in three cases: without crown (BHI), with ceramic crown (MHIc), and with zirconia crown (MHIz). Subsequently, temperature distribution was numerically determined along the implant system for two heat loadings. RESULTS In loading type I, the maximum temperature of the surrounding bone at the cervical implant/bone interface was obtained in the BHI model (39.1°C), and the lowest was obtained in the MHIc model (37.6°C). The maximum temperature rise in loading type II also took place in the BHI model (41.7°C). Moreover, the BHI model showed a rapid rise to the maximum temperature followed by a fast recovery compared to its two counterparts (MHIc, MHIz). In both loading types, the maximum temperature at the first point of contact between the implant and bone, and apical implant/bone interface was slightly higher in the MHIz than that in the MHIc. The maximum temperature in all the models was higher when subjected to cyclic loading. The maximum temperatures reached in all the models were lower than threshold temperatures, so thermal loading alone does not harm the jawbone. Moreover, the BHI was more vulnerable than the MHIc and the MHIz. CONCLUSIONS The results of this study suggest that dental implants should be covered with crowns as soon as possible, and patients with dental implants should avoid consumption of hot food and beverages without allowing time for the heat to dissipate.

A tensile machine with a novel optical load cell for soft biological tissues application

Abstract The uniaxial tensile testing machine is the most common device used to measure the mechanical properties of industrial and biological materials. The need for a low-cost uniaxial tension testing device for small research centers has always been the subject of research. To address this need, a novel uniaxial tensile testing machine was designed and fabricated to measure the mechanical properties of soft biological tissues. The device is equipped with a new low-cost load cell which works based on the linear displacement/force relationship of beams. The deflection of the beam load cell is measured optically by a digital microscope with an accuracy of 1 µm. The stiffness of the designed load cell was experimentally and theoretically determined at 100 N mm−1. The stiffness of the load cell can be easily adjusted according to the tissue’s strength. The force-time behaviour of soft tissue specimens was obtained by an in-house image processing program. To demonstrate the efficiency of the fabricated device, the mechanical properties of amnion tissue was measured and compared with available data. The obtained results indicate a strong agreement with that of previous studies.

A novel haemostatic powder delivery device applicable in minimally invasive surgery.

Abstract Haemostatic powder is an effective solution commonly used in various open surgeries. However, there is no specific intra-abdominal delivery device for application of haemostatic powder at the bleeding site during minimally invasive surgery (MIS). In this study, design, construction and test of a novel powder delivery device were carried out. The device uses pressurized gas to deliver the haemostatic powder to the bleeding point. The effect of the gas pressure and the spraying distance on the geometry of the powder dispersion surface area was investigated and found to be significant. The findings indicate that the driving gas pressure range of 60–80 mmHg and the spraying distance range of 2–5 cm achieve the most concentrated powder dispersion surface area. Additionally, in vivo experiments confirmed the effectiveness of the device in live tissue.

Impact Resistance of Antibiotic-Impregnated Orthopedic Bone Cement

The usess of antibiotic--impregnated bone cement in total joint replacement (TJR) is a common practice but remains extremely variable. The benefit of adding antibiotics to bone cement as a method of infection prophylaxis in TJR needs to be weighed against a number of drawbacks primarily a drop in the mechanical properties of the bone cement. The present study utilized the pendulum impact strength testing to report on the properties of antibiotic-impregnated cement. All testing was conducted in accordance with ASTM D256-06. Test groups included: PMMA mixed with vancomycin, gentamicin, erythromycin or no antibiotic (control). Also, the effect of aging samples in air versus saline on the impact strength was studied. The effect of antibiotic addition to cement on the impact strength was found statistically significant (p 0.05).

Finite element investigation of human maxillary incisor under traumatic loading: Static vs dynamic analysis

BACKGROUND AND OBJECTIVE Traumatic loading is the main form of injury sustained in dental injuries. In spite of the prevalence of dental trauma, little information is available on traumatic dental damage and the evaluation of tooth behavior under traumatic loading. Due to the short period of traumatic loading, at first sight, a dynamic analysis needs to be performed to investigate the dental trauma. However, it was hypothesized that dental traumatic loading could be regarded as quasi-static loading. Thus, the aim of the present study was to examine this hypothesis. METHODS Static and dynamic analyses of the human maxillary incisor were carried out under traumatic loading using a 3D finite element method. Also, modal analysis of the tooth model was performed in order to evaluate the assumption of the dental traumatic loading as a quasi-static one. RESULTS It was revealed that the static analysis of dental trauma is preferred to the dynamic analysis when investigating dental trauma, mainly due to its lower computational cost. In fact, it was shown that including the inertia of the tooth structure does not influence the results of the dental trauma simulation. Furthermore, according to the modal analysis of the tooth structure, it was found that the mechanical properties and geometry of the periodontal ligament play significant roles in the classification of dental traumatic loading as a quasi-static one, in addition to the time duration of the applied load. CONCLUSIONS This paper provides important biomechanical insights into the classification of dental loading as quasi-static, transient or impact loading in future dental studies.

A comparison of the material properties of natural and synthetic vascular walls

Characterization of the mechanical properties of native and synthetic vascular grafts is an essential task in the process of designing novel vascular constructs. The aim in this study was to compare the mechanical behavior of ovine left Subclavian artery with that of POSS-PCU (a commercial biomaterial which is currently under clinical investigation. ClinicalTrials.gov Identifier: NCT02301312). We used Delfinos strain energy potential within the framework of quasilinear viscoelasticity theory to capture the viscoelastic response of the considered materials. The material parameters of the quasilinear viscoelastic constitutive equation were determined through a combination of experimental and computational method. First, a uniaxial tensile testing device was used to perform a series of stress relaxation tests on ring samples. Then, the derived quasilinear viscoelastic models were implemented into finite element system. With the aid of mechanical experimentation and finite element simulation, the material parameters were obtained, modified and used for comparison of the mechanical properties of vascular walls. The results showed that the stiffness and the long term viscoelastic parameters of POSS-PCU may lead to different stress responses of the vascular walls. These two factors can be improved by modifications in manufacturing parameters of the synthetic vessel.