Peri-implant cell differentiation in delayed and immediately-loaded dental implant: A mechanobiological simulation.

OBJECTIVE: This study aimed to investigate the effect of immediate versus delayed dental implant placement strategies on cell differentiation in a dental callus. DESIGN: The implant was placed in the mandible with two nearby teeth using an idealized two-dimensional finite element model. Eight weeks after surgery, the mechanobiological modeling of healing was used to estimate cell differentiation. It was assumed that the callus was initially filled by mesenchymal cells. The model then transformed mechanical stimuli received by the callus from loadings in terms of distortional and dilatational strains into predictions of the cellular phenotypes, including fibroblasts, chondrocytes, and osteoblasts, or whether they would remain unchanged or die. RESULTS: The results demonstrated that delayed loading led to greater bone formation than immediate loading. Osteoblast colonies were observed in the base of threads in the immediately-loaded implant, whereas the delayed loading caused distant bone formation from the surrounding bone side towards the implant. The osteoblasts were differentiated from both intramembranous and endochondral mechanisms of ossification. After eight weeks, approximately 61 % of the callus was ossified in the delayed placement model compared to 35 % in the immediate placement model, resulting in a greater amount of fibrocartilaginous tissue on the bone side of the callus. CONCLUSIONS: Immediate and delayed loading models generated different results. In the delayed strategy, bone cells were supplied appropriately during the first few weeks following surgery, whereas the immediate loading caused fibrocartilaginous tissue differentiation. In the form of distant osseointegration, the secondary stability of the dental implant was higher and faster due to the delayed placement.

General muscle fatigue changed joint regulations in static and dynamic balance.

This study was aimed at investigating the joint regulations and body sway after general muscle fatigue during tasks that involve both static and dynamic balance. This cross-sectional study used motion analysis to ascertain the kinematic changes in twelve healthy young individuals before and after running-induced fatigue. Six linear and nonlinear stability metrics were calculated to assess the whole body and joint-related variations. Significant instabilities were observed in the hip and specifically in the knee mechanisms and the whole body during the static condition. Velocity path length and approximate entropy for knee ( p = 0.019, p = 0.027) and hip ( p = 0.016, p = 0.042) were significantly greater after fatigue. These parameters for the whole body center of mass were also higher after fatigue ( p = 0.013, p = 0.013). General muscle fatigue did not significantly affect the ankle during static and dynamic standing ( p > 0.05). Dynamic standing did not reveal the effects of fatigue either on local joint regulations or on the whole body except for the nonlinear metrics of the proximal joints. The knee and hip were adversely affected by fatigue while the ankle strove to compensate for the fatigue-induced instability.

Immediate effects of footwear with vibration applied to the swing phase of the gait cycle on dynamic balance in patients with diabetic peripheral neuropathy.

Sensory and sub-sensory foot vibration have beneficial effects on the static and dynamic balance of patients with diabetic peripheral neuropathy (DPN). Previous foot vibration during walking was applied at sub-sensory threshold in both stance and swing phases of the gait cycle in DPN. This study aimed to investigate the effects of footwear with vibration above the sensory threshold applied to the swing phase of the gait cycle on the dynamic balance of DPN. Fifteen patients with DPN and 10 matched healthy control participants were enrolled in this study. Dynamic balance assessment was investigated by the sit to stand and turn tests while electromyography activity (root mean square, RMS) of the vastus lateralis (VL), tibialis anterior (TA), and peroneus longus (PL) muscles were recorded at baseline, after 10 min of wearing the vibratory sandal without vibration, and after 10 min of wearing the sandal with vibration. Repeated measure analysis of variance was conducted for data analysis. Wearing the vibratory sandal slightly increased the speed of weight transfer in both groups and COP sway in DPN (P > 0.05), and close to significant decrease of turn sway (P = 0.07) in both groups. There were significant differences of the PL RMS in turn test (P = 0.03) in DPN and peak RMS of the PL after 10 min of wearing the sandal with vibration in both groups (P < 0.05). The results of the present study demonstrated that PL activity was significantly influenced by the vibratory sandal as its tendon passing across the sole and in direct contact with the source of the vibration.

Effects of human stature and muscle strength on the standing strategies: A computational biomechanical study.

It has been hypothesized that the muscular efforts exerted during standing may be altered by changes in personal factors, such as the body stature and muscular strength. The goal of this work was to assess the contribution of leg muscles using a biomechanical model in different physical conditions and various initial postures. An optimized inverse dynamics model was employed to find the maximum muscular effort in 23,040 postures. The simulation results showed that mid-range knee flexion could help the healthy and strong individuals maintain balance, but those with weaker muscle strength required more knee flexion. Individuals of weak muscular constitution as well as those with tall stature are at the highest risk of imbalance/falling. The number of imbalanced postures due to deficits in the calf and hamstring muscles was reduced by 7.5 times by strengthening the whole body musculature. The calf and the hamstring muscles play a key role in balance regardless of stature.

Initial balance in human standing postures: Roles of the joint mechanisms.

The static initial postures of standing before applying perturbations may affect the maintenance of postural balance. The goal of this article was to find the stable set of postures and then determine the roles of joint mechanisms. The set of posture was defined in a biomechanical model based on three joint angles of the lower limbs. Optimized inverse dynamics method was used to solve for muscle forces in a precise model of the human musculoskeletal system posed in 4096 static sets of posture using AnyBody software. Results showed that the overall body muscular activity in standing is reduced by knee flexion. Moderate knee angles between 20° and 60° provided safer postures against possible perturbations because of higher collaboration levels of the joint mechanisms. About 36% of the overall postural infeasibilities were attributed to the inability of the ankle muscles to more sustain the exerted loads. Although the roles of the joint mechanisms were closely dependent on the postures, there was no direct relation between the joint kinematics and activation levels of their supporting muscles. Lower extremity muscle groups collaborate to maintain the balance in a considerable number of static postures.

Effects of postural and cognitive difficulty levels on the standing of healthy young males on an unstable platform.

Standing on an unstable platform requires continuous effort of the neuro­musculoskeletal system. The aim of the present study is to evaluate the ability to remain standing on an unstable platform at different levels of postural and cognitive difficulty. Healthy young males stood in the sagittal plane on an unstable platform supported by a pair of springs with modifiable stiffness. The balance test also assessed different levels of vision and cognitive function. Linear and nonlinear metrics of standing, based on motion captured kinematic data, were assessed to analyze the stability of standing. Results showed that vision plays a significant role in maintaining balance in terms of linear metrics. Elimination of visual feedback changed the direction of body sway and increased standing instability. Placement of low stiffness springs led to unstable standing. The cognitive dual task, however, had no effect on the stability metrics and merely could be revealed in the simplest test condition. Standing on an unstable platform was closely related to visual feedback and decreasing the spring stiffness significantly reduced stability. The roles of cognitive involvement were subdued by increasing the postural difficulty in standing on an unstable platform.

Effects of visual and cognitive interference on joint contributions in perturbed standing: a temporal and spectral analysis.

Postural balance requires using joint strategies which may be changed from normal conditions by interfering with the sensory information. The goal of the present study was to quantitatively evaluate the role of the joint mechanisms during perturbed stance. Visual and cognitive interference was imposed to sixteen healthy young males under rotational toes-down or up perturbations. Power spectral analysis was employed to distinguish the joint contribution and their in- or out-phase co-works. Results showed that addition of cognitive loads reduce the stability by increasing the center of mass (CoM) power to three times greater. Besides the CoM, the knee and hip powers were also significantly enhanced by the cognitive loads (p < .004), but the ankle was not influenced by cognition involvement (p > .05). Elimination of the vision had lower effect on the time and spectral functions of the knee and hip while the ankle rotations were increased due to the lack of visual feedback (p = .001). The toes-down perturbations resulted in more prominent contribution of the knee while the toes-up evoked the hip joint to keep the balance more than the other joints. Addition of the cognitive loads hindered the reactions of the joint mechanisms and vision caused more conservative responses of the joints.

Nonlinear dynamics analysis of the human balance control subjected to physical and sensory perturbations.

Postural control after applying perturbation involves neural and muscular efforts to limit the center of mass (CoM) motion. Linear dynamical approaches may not unveil all complexities of body efforts. This study was aimed at determining two nonlinear dynamics parameters (fractal dimension (FD) and largest Lyapunov exponent (LLE)) in addition to the linear standing metrics of balance in perturbed stance. Sixteen healthy young males were subjected to sudden rotations of the standing platform. The vision and cognition during the standing were also interfered. Motion capturing was used to measure the lower limb joints and the CoM displacements. The CoM path length as a linear parameter was increased by elimination of vision (p<0.01) and adding a cognitive load (p<0.01). The CoM nonlinear metric FD was decreased due to the cognitive loads (p<0.001). The visual interference increased the FD of all joints when the task included the cognitive loads (p<0.01). The slightly positive LLE values showed weakly-chaotic behavior of the whole body. The local joint rotations indicated higher LLEs. Results indicated weakly chaotic response of the whole body. Increase in the task difficulty by adding sensory interference had difference effects on parameters. Linear and nonlinear metrics of the perturbed stance showed that a combination of them may properly represent the body behavior.

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.