Dynamic viscoelastic behavior of bovine periodontal ligament in compression.

BACKGROUND AND OBJECTIVE: As the main connector of teeth to the surrounding bone, the periodontal ligament (PDL) plays an important role in absorption and distribution of physiological and para-physiological loading. Despite the difference in the mechanical response of the PDL in tension and compression, little information is available on its viscoelastic behavior in compression. To explore the contribution of the fluid phase of the PDL, the aim of the present study was to measure its nonlinear time-dependent behavior in compression. MATERIAL AND METHODS: The in vitro dynamic compressive tests were carried out over a wide range of frequencies at three different preloads. Also, a generalized Maxwell model was proposed based on the experimental data to develop a viscoelastic model for subsequent computational analysis of the PDL. RESULTS: The higher values of loss factor in compression found in the range of 0.03-0.4 than those of 0.04-0.08 reported in tension, implies the focal role of the fluid phase in compressive dynamic response. Furthermore, the model parameters predicted that with an increase in preload, the role of the viscous components decrease, whereas the role of the elastic component increases. CONCLUSION: The viscous effect of the PDL in compression is greater than that in tension. Also, the dependence of the relaxation modulus of the bovine PDL on the applied load indicates its nonlinear viscoelastic behavior in compression.

Effect of geometric dimensions and material models of the periodontal ligament in orthodontic tooth movement.

OBJECTIVES: The aim of the present work was set to explore the role of root diameter, root length, and thickness of periodontal ligament (PDL) and its material properties (linear and non-linear elastic material models) on the stress and strain distribution, tooth displacement and centre of resistance (CR) location. SETTING AND SAMPLE POPULATION: Both the bone and the tooth were considered as rigid bodies, and the PDL was modelled as a paraboloid with different geometric dimensions and material properties. MATERIALS AND METHODS: To achieve this goal, a horizontal force of 1 N was applied in the CR location and the stress and strain distribution and tooth displacement were quantified. Locations of CR were estimated through iterative finite element procedure. RESULTS: It was predicted that the position of CR is in the range of 34%-39% of the root length, slightly higher than one-third of the root length reported in the literature. The geometrical dimensions of the PDL had no significant effect on the position of CR, especially in the non-linear material model of the PDL, while the initial displacement of the tooth was found to be highly dependent on the geometrical and mechanical properties of the PDL. CONCLUSION: The simplified PDL modelling approach with non-linear material behaviour can be suggested for the estimation of initial tooth movement for individual clinical applications without the use of advanced 3D scans.

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.

Effect of temperature on dynamic compressive behavior of periodontal ligament.

Periodontal ligament (PDL) attaches tooth root to the surrounding bone. Its existence between tooth and jaw bone is of utmost importance due to its significant role in absorbing and distributing physiological and para-physiological loading. According to the previous studies, various mechanical tests have been performed to characterize the mechanical properties of the PDL; however, all of them have been done at room temperature. To the best of our knowledge, this is the first study in which the testing was performed at body temperature. The present research was planned to measure the dependency of PDL's viscoelastic behavior on temperature and frequency. Three different temperatures, including body and room temperature, were opted to perform the dynamic compressive tests of the bovine PDL. In addition, a Generalized Maxwell model (GMM) was presented based on empirical outcomes. At 37 °C, amounts of loss factor were found to be greater than those in 25 °C, which demonstrates that the viscous phase of the PDL in higher temperatures plays a critical role. Likewise, by raising the temperature from 25 °C to 37 °C, the model parameters show an enlargement in the viscous part and lessening in the elastic part. It was concluded that the PDL's viscosity in body temperature is much higher than that in room temperature. This model would be functional for a more accurate computational analysis of the PDL at the body temperature (37 °C) in various loading conditions such as orthodontic simulations, mastication, and impact.

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.

Mechanical, material, and biological study of a PCL/bioactive glass bone scaffold: Importance of viscoelasticity.

Microsphere sintering method was used to fabricate bone tissue engineering scaffolds made of polycaprolactone (PCL)/bioactive glass 58S5Z (58S modified with 5 wt% Zinc). First, the effect of PCL/58S5Z ratio on the mechanical properties (elastic modulus and yield strength) was investigated. It was found that samples with 5 wt% 58S5Z (named 5%BG) had the highest elastic modulus and yield strength among all samples, i.e., with 0, 5, 10, and 20 wt% bioactive glass. Then, considering the importance of viscoelastic properties of bone, the viscoelastic behavior of 0%BG (scaffold with only PCL) and 5%BG samples was determined by performing compressive stress relaxation test and subsequently a Generalized Maxwell model was developed. Findings indicated a similar amount and pattern of predicted storage and loss moduli and loss factor of the composite scaffolds to those of the bone. In the next step, the analysis of biological behavior of the scaffolds using MTT assay, DAPI and Alizarin red staining demonstrated that 5%BG scaffolds had higher in vitro cell viability and bone formation compared to 0%BG ones. Furthermore, in vivo study employing H&E staining of the scaffolds implanted in rats' calvarium for 50 days, confirmed the earlier findings and showed that 5%BG-filled defects had higher and more uniform bone formation compared to both 0%BG-filled and empty defects.

Biaxial flexural modulus of antibiotic-impregnated orthopedic bone cement.

Previously reported antibiotic-impregnated cement strengths have been based on uniaxial and fatigue testing methodologies. These methods may not provide an accurate characterization of bone cement's true load-bearing capacity in total joint replacement (TJR). The present study utilized biaxial testing to report on the properties of antibiotic-impregnated cement. Test groups included: PMMA mixed with Vancomycin, Gentamicin, Tobramycin, or no antibiotic (control). In comparison to the control group, PMMA samples mixed with powdered gentamicin resulted in an increase in the mean elastic modulus by 6.50% versus a drop noted with powdered vancomycin and tobramycin by 2.65 and 1.37% respectively. The mean elastic modulus in samples containing liquid gentamicin dropped by 11.6%. This study supports the continued use of powdered antibiotics when clinically indicated, but suggest caution in the use of liquid gentamicin in TJR.

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 Delfino's 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.

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.

Pullout strength of pedicle screws augmented with particulate calcium phosphate: an experimental study.

BACKGROUND CONTEXT: Pressure-injected and in situ curing bone cements have been studied as alternatives in augmenting lumbar pedicle screw fixation but are frequently found to leak outside the confines of the target vertebra. PURPOSE: The objective is set to determine the mechanical efficacy of a porous granular/particulate calcium phosphate (CP) bone augmentation product (Skelite) applied manually without pressurized injection in this application. STUDY DESIGN/SETTING: The biomechanical analysis compared the axial pullout strength and insertional torque of augmented and nonaugmented pedicle screws in cellular polyurethane foams. METHODS: The insertion torque and pullout strength of 6.5-mm pedicle screws inserted (via 3.5-mm pilot holes) into polyurethane blocks mimicking the porosity of cancellous bone were measured. New pilot holes were then packed with granular particles of Skelite and retested. Last, those blocks initially tested to failure without augmentation were packed with Skelite and retested. Measurements were performed for polyurethane densities of 0.16 and 0.32 g/cc (corresponding to the porosity of osteoporotic and normal bone) and strain rates of 0.5 and 5mm/min. RESULTS: Peak pullout force averaged 2132.5+/-119.3 N and 1840.1+/-216.7 N in high density samples without and with augmentation and 688.2+/-91.4 N and 861.6+/-74.5 N in low density samples without and with augmentation. After failure, approximately 50% and 77% of the peak pullout force of original high and low density samples was regained by augmentation. Statistical analysis revealed significant (p<.0001) correlation between the addition of CP, peak pullout resistance, and insertion torque. CONCLUSION: Granular CP augmentation improved the pullout strength in both failed (pulledout) samples and low density (porosity of osteoporotic cancellous bone) polyurethane blocks.