Analysis of the Adherent Cell Response to the Substrate Stiffness Using Tensegrity.

Cell's shape is dependent on the cytoskeleton mechanical properties. Hybrid models were developed that combine the discrete structure for the cytoskeleton and continuum parts for other cell organelles. Tensegrity-based structures that consist of tensile and compression elements are useful models to understand the cytoskeleton mechanical behavior. In this study, we are looking to examine the reaction of the cell to a variety of substrate stiffnesses and explain the relationship between cell behavior and substrate mechanical properties. However, which tensegrity structure is appropriate for modeling a living cell? Is the structure's complexity play a major role? We used two spherical tensegrities with different complexities to assess the impact of the structure on the cell's mechanical response versus substrate's stiffness. Six- and twelve-strut tensegrities together with membrane, cytoplasm, nucleoskeleton, and nucleus envelope were assembled in Abaqus package to create a hybrid cell model. A compressive load was applied to the cell model and the reaction forces versus deflection curves were analyzed for number of substrate stiffness values. By analyzing the difference due to two different tensegrities it became clear that the lower density structure is a better choice for modeling stiffer cells. It was also found that the six-strut tensegrity is sensitive to higher range of substrate stiffness.

In vivo strains at the middle and distal thirds of the tibia during exertional activities.

There is a known variance in the incidence and anatomical site of tibial stress fractures among infantry recruits and athletes who train according to established uniform training programs. To better understand the biomechanical basis for this variance, we conducted in vivo axial strain measurements using instrumented bone staples affixed in the medial cortex, aligned along the long axis of the tibia at the level of the mid and distal third of the bone in four male subjects. Strain measurements were made during treadmill walking, treadmill running, drop jumps from a 45 cm height onto a force plate and serial vertical jumps on a force plate. Significance levels for the main effects of location, type of activity and their interaction were determined by quasi-parametric methodologies. Compared to walking, running and vertical jumping peak axial tensile strain (µÎµ) was 1.94 (p = 0.009) and 3.92 times (p < 0.001) higher, respectively. Peak axial compression strain (µÎµ) values were found to be greater at the distal third than at the mid tibia for walking, running and vertical jumping (PR = 1.95, p-value<0.001). Peak axial compression and tension strains varied significantly between the subjects (all with p < 0.001), after controlling for strain gauge location and activity type. The study findings help explain the variance in the anatomical location of tibial stress fractures among participants doing the same uniform training and offers evidence of individual biomechanical susceptibility to tibial stress fracture. The study data can provide guidance when developing a generalized finite element model for mechanical tibial loading. For subject specific decisions, individualized musculoskeletal finite element models may be necessary.

Biomechanical characterization and modeling of human mesenchymal stem cells under compression.

The application of microelectromechanical systems (MEMS) in biomedical devices has expanded vastly over the last few decades, with MEMS devices being developed to measure different characteristics of cells. The study of cell mechanics offers valuable understanding of cell viability and functionality. Cell biomechanics approaches also facilitate the characterization of important cell and tissue behaviors. In particular, understanding of the biological response of cells to their biomechanical environment would enhance the knowledge of how cellular responses correlate to tissue level characteristics and how some diseases, such as cancer, grow in the body. This study focuses on viscoelastic modeling of the behavior of a single suspended human mesenchymal stem cell (hMSC). Mechanical properties of hMSC cells are particularly important in tissue engineering and research for the treatment of cardiovascular diseases. We evaluated the elastic and viscoelastic properties of hMSC cells using a miniaturized custom-made BioMEMS device. Our results were compared to the elastic and viscoelastic properties measured by other methods such as atomic force microscopy (AFM) and micropipette aspiration. Different approaches were applied to model the experimentally obtained force data, including elastic and Standard Linear Solid (SLS) constitutive models, and the corresponding constants were derived. These values were compared to the ones in literature that were based on micropipette aspiration and AFM methods. We then utilized a tensegrity approach to model major parts of the internal structure of the cell and treat the cell as a network of viscoelastic microtubules and microfilaments, as opposed to a simple spherical blob. The results predicted from the tensegrity model were similar to the recorded experimental data.

Differences in the principal strain angles during activities performed on natural hilly terrain versus engineered surfaces.

BACKGROUND: Tibial stress fractures in military recruits occur beginning with the fourth week of training. In and ex vivo tibial strain experiments indicate that the repetitive mechanical loading during this time may not alone be sufficient to cause stress fracture. This has led to the hypothesis that the development of tibial stress fracture is mediated by the bone remodeling response to high repetitive strains. This study assesses the differences in the strain and angle of the principal strain during military field activities versus common civilian activities. METHODS: In vivo strain measurements were made from a rosette strain gauge bonded to the midshaft of the medial tibia. Measurements of principal strains and their angles were made while performing level and inclined walking and running on an asphalt surface, while fast walking up and down stairs, while performing a standing vertical jump and while zig-zag running up and down a 30° inclined dirt hill. FINDINGS: The angle of the principal strain varied little (5.40° to +2.74°) during activities performed on engineered surfaces. During zig-zag running on a dirt hill the strain levels were higher (maximum shear = 4290 µÎµ). At the pivot points of zig-zag running the angle of the principal strain varied between -115° to -123° downhill and between -32.8° to -51° uphill. INTERPRETATION: Activities that mimic those performed by infantry recruits on irregular hilly surfaces result in higher tibial strains and have more variation in principal strain angles than activities of ordinary civilian life performed on engineered surfaces.

Understanding the etiology of the posteromedial tibial stress fracture.

Previous human in vivo tibial strain measurements from surface strain gauges during vigorous activities were found to be below the threshold value of repetitive cyclical loading at 2500 microstrain in tension necessary to reduce the fatigue life of bone, based on ex vivo studies. Therefore it has been hypothesized that an intermediate bone remodeling response might play a role in the development of tibial stress fractures. In young adults tibial stress fractures are usually oblique, suggesting that they are the result of failure under shear strain. Strains were measured using surface mounted unstacked 45° rosette strain gauges on the posterior aspect of the flat medial cortex just below the tibial midshaft, in a 48year old male subject while performing vertical jumps, staircase jumps and running up and down stadium stairs. Shear strains approaching 5000 microstrain were recorded during stair jumping and vertical standing jumps. Shear strains above 1250 microstrain were recorded during runs up and down stadium steps. Based on predictions from ex vivo studies, stair and vertical jumping tibial shear strain in the test subject was high enough to potentially produce tibial stress fracture subsequent to repetitive cyclic loading without necessarily requiring an intermediate remodeling response to microdamage.

Enhanced cell adhesion and alignment on micro-wavy patterned surfaces.

Various micropatterns have been fabricated and used to regulate cell adhesion, morphology and function. Micropatterns created by standard photolithography process are usually rectangular channels with sharp corners (microgrooves) which provide limited control over cells and are not favorable for cell-cell interaction and communication. This paper proposes a new micropattern with smooth wavy surfaces (micro-waves) to control the position and orientation of cells. To characterize cell growth and responses on the micro-patterned substrates, bovine aortic endothelial cells were seeded onto surfaces with micro-grooves and micro-waves for 24 h. As a result, the cells on the micro-wavy pattern appeared to have a lower death rate and better alignment compared to those on the micro-grooved pattern. In addition, flow-induced shear stress was applied to examine the adhesion strength of cells on the micro-wavy pattern. Results showed that cells adhered to the wavy surface displayed both improved alignment and adhesion strength compared to those on the flat surface. The combination of increased alignment, lower death rate and enhanced adhesion strength of cells on the micro-wavy patterns will offer advantages in potential applications for cell phenotype, proliferation and tissue engineering.

Transmission of vertical vibration to the human foot and ankle.

The load absorbing capability of the foot and ankle system (FAS) was characterized by measuring the transmissibility and the phase delay at the medial malleolus and the tibial tuberosity. The FAS of twenty subjects were exposed to sinusoidal vertical excitation (10-50 Hz with 5 Hz increments and peak to peak acceleration of 17.9 m/s(2)) while sitting as a function of the external mass (0, 2.3, and 4.5 kg) and the foot postures (midstance, plantarflexion, and dorsiflexion). The results showed that the FAS plays important role in vibration transmission of lower leg. Adding extra mass affected a resonant frequency at the medial malleolus: 15-25, 30-35, and 35 Hz for with no additional mass, 2.3, and 4.5 kg, respectively. However, the changed postures of the FAS did not show significant effect on the resonant frequency. The applied mass affected the stiffness increase of the FAS and consequently resulted in the increase of the resonant frequency. This result supports the assertion that the resonant frequency of overweight or obese persons is similar to the major frequency component (25-35 Hz) of the heel strike.

The natural frequency of the foot-surface cushion during the stance phase of running.

Researchers have reported on the stiffness of running in holistic terms, i.e. for the structures that are undergoing deformation as a whole rather than in terms of specific locations. This study aimed to estimate both the natural frequency and the viscous damping coefficient of the human foot-surface cushion, during the period between the heel strike and the mid-stance phase of running, using a purposely developed one degree-of-freedom inverted pendulum state space model of the leg. The model, which was validated via a comparison of measured and estimated ground reaction forces, incorporated a novel use of linearized and extended Kalman filter estimators. Investigation of the effect of variation of the natural frequency and/or the damping of the cushioning mechanism during running, using the said model, revealed the natural frequency of running on said foot-surface cushion, during the stance phase, to lie between 5 and 11 Hz. The "extended Kalman filter (EKF)" approach, that was used here for the first time to directly apply measured ground forces, may be widely applicable to the identification process of combined estimation of both unknown physiological state and mechanical characteristics of the environment in an inverse dynamic model.

Reliability and validity of goniometric turnout measurements compared with MRI and retro-reflective markers.

There is no consensus on a valid and reliable method of measuring turnout. However, there is a building awareness that such measures need to exist. Total turnout is the sum of hip rotation, tibial torsion, and contributions from the foot. To our knowledge, there has been no research that directly measures and then sums each individual component of turnout to verify a total turnout value. Furthermore, the tibial torsion component has not previously been confirmed by an imaging study. The purpose of this study was to test the validity and reliability of a single total passive turnout (TPT) test taken with a goniometer by comparing it with the sum of the individual components. Fourteen female dancers were recruited as participants. Measurements of the subjects' right and left legs were gathered for the components of turnout. Tibial torsion was measured using Magnetic Resonance Imaging (MRI). Retro-reflective marker assisted measurements were used to calculate the static components of TPT. Hip external rotation, TPT, and total active turnout (TAT) were measured by goniometer. Additional standing turnout values were collected on rotational disks. Tibial torsion and hip rotation were summed and compared with three whole-leg turnout values using Two-Tailed T-Tests and Pearson product-moment correlation coefficients. Tibial torsion measurements in dancers were found to demonstrate substantial variation between subjects and between legs in the same subject. The range on the right leg was 16 degrees to 60 degrees, and the range on the left leg was 16 degrees to 52 degrees. Retro-reflective markers and biomechanical theory demonstrated that when the knee is extended and locked, "screwed home," it will not factor into a whole-leg turnout value. TAT and turnout on the disks were not statistically significant when compared with the summed total. Statistical significance was achieved in four of the eight measurement series comparing TPT with the summed value of tibial torsion and hip rotation. The advantages of a standard, valid, and reliable method of measuring turnout are many, and the risks are few. Some advantages include improved training techniques, mastery of the use of turnout at an earlier age, better dancer and teacher compliance with suggested turnout rates, understanding the use of parallel position, understanding the etiology of many dance-related injuries, and possible development of preventative measures.

A biomechanical analysis of the effect of lateral column lengthening calcaneal osteotomy on the flat foot.

BACKGROUND: Biomechanical models have been used to study the plantar aponeurosis, medial arch height, subtalar motion, medial displacement calcaneal osteotomy, subtalar arthroereisis and the distribution of forces in the normal and flat foot. The objective was to examine the hypothesis that increased load on the medial arch in the adult flat foot can be reduced through a 10mm lateral column lengthening calcaneal osteotomy 10 mm proximal from the calcaneal cuboid joint. METHODS: A three dimensional multisegment biomechanical model was used with anatomical data from a normal foot, a flat foot and a foot corrected with a 10mm lateral column lengthening calcaneal osteotomy. The response of a normal foot, a flat foot and a flat foot with a 10mm lateral column lengthening calcaneal osteotomy to an applied load of 683 N was analyzed using the biomechanical model. Data for the biomechanical model was obtained from a cadaver foot using the direct linear transformation method. Direct linear transformation uses multiple cameras to determine the spatial location of anatomical landmarks. FINDINGS: Load on the first metatarsal increases to 37% body weight in the flat foot compared to 12% for the normal foot and the moment about the talo-navicular joint increases from 5.6 N m to 21.6 N m. Lateral column lengthening shifts the load toward the lateral column, decreasing load on the first metatarsal to 10% and decreasing the moment about the talo-navicular joint to 8.1 N m. INTERPRETATION: The analysis shows that a 10mm lateral column lengthening calcaneal osteotomy reduces the excess force on the medial arch in an adult flat foot and adds biomechanical rationale to this clinical procedure.

Dynamic loading on the human musculoskeletal system -- effect of fatigue.

OBJECTIVE: A study was conducted to investigate the effects of fatigue on the ability of human musculoskeletal system to deal with the onslaught of the heel strike initiated shock waves. DESIGN: Running on a treadmill at the anaerobic threshold level for 30 min was used to acquire the experimental data on the foot strike initiated shock waves. BACKGROUND: Muscles act to lower the bending stress on bone and to attenuate the dynamic load on human musculoskeletal system. Fatigue may diminish their ability to dissipate and attenuate loading on the system. Knowledge of the effects of fatigue on the ability of the human musculoskeletal system to attenuate the shock waves may help in design of the training procedures and exercises. METHODS: Twenty-two young healthy males participated in this study. Each one was running on the treadmill at the speed corresponding to his anaerobic threshold for 30 min. The heel strike induced shock waves were recorded every 5 min on the tibial tuberosity and sacrum. The data obtained were analyzed in both temporal and frequency domains. RESULTS: The results reveal significant increase in the dynamic loading experienced by the human musculoskeletal system with fatigue. This may be attributed to the inability of the fatigued system to provide an efficient way to attenuate shock waves. CONCLUSIONS: The analysis of the recorded signals suggests that fatigue contributes to the reduction of the human musculoskeletal system's capacity to attenuate and dissipate those shock waves. This capacity appears to be a function not only of the fatigue level, but also of the vertical location along the skeleton. RELEVANCE: Fatigue during running may affect the ability of the human musculoskeletal system to attenuate and dissipate the heel strike induced shock waves. The study of the fatigue effect on shock wave attenuation provides information that may benefit the runner.

Shock Transmission and Fatigue in Human Running.

The goal of this research was to analyze the effects of fatigue on the shock waves generated by foot strike. Twenty-two subjects were instrumented with an externally attached, lightweight accelerometer placed over the tibial tuberosity. The subjects ran on a treadmill for 30 min at a speed near their anaerobic threshold. Fatigue was established when the end-tidal CO2 pressure decreased. The results indicated that approximately half of the subjects reached the fatigue state toward the end of the test. Whenever fatigue occurred, the peak acceleration was found to increase. It was thus concluded that there is a clear association between fatigue and increased heel strike-induced shock waves. These results have a significant implication for the etiology of running injuries, since shock wave attenuation has been previously reported to play an important role in preventing such injuries.