Search PubMed⌕ Search

Biomedical subjects

L E Thibault

Publications and source records attributed to L E Thibault.

36 records · Page 2Linked to original sources
PubMed1989

A system to reproduce and quantify the biomechanical environment of the cell.

An in vitro system that permits application of a uniform biomechanical stimulus to a population of cells with great precision has been developed. The device is designed to subject living cells to reproducible and quantifiable biaxial strains from 0 to 10% at rates from quasi-static to 1 s-1 and frequencies from 0 to 5 Hz. Equations for determining the strain in the substrate upon which the cells are grown, based on easily measured parameters, are derived and validated experimentally. The mechanical properties of the substrate are determined, and it is demonstrated that cells can easily be cultured in the apparatus. By use of the system, cloned bovine pulmonary artery endothelial cell clones are subjected to 5% biaxial strains applied at a peak strain rate of 0.5 s-1 and a frequency of 1 Hz for 7 h with cell viability greater than 84% and cell detachment less than 8%. We demonstrate that cells must be attached to the substrate for them to be stretched and that cell strain and substrate strain are not equal. With the use of fluorescently labeled beads as cell surface markers to measure the actual strain produced in the cells as a result of the deformation of the substrate, cell elongation was found to be approximately 60% of the strain in the substrate. This constant appeared to be affected by both in vitro cell age and morphology.

Cell Physiological Phenomena↗
PubMed1989

Axonal injury in the optic nerve: a model simulating diffuse axonal injury in the brain.

A new model of traumatic axonal injury has been developed by causing a single, rapid, controlled elongation (tensile strain) in the optic nerve of the albino guinea pig. Electron microscopy demonstrates axonal swelling, axolemmal blebs, and accumulation of organelles identical to those seen in human and experimental brain injury. Quantitative morphometric studies confirm that 17% of the optic nerve axons are injured without vascular disruption, and horseradish peroxidase (HRP) studies confirm alterations in rapid axoplasmic transport at the sites of injury. Since 95% to 98% of the optic nerve fibers are crossed, studies of the cell bodies and terminal fields of injured axons can be performed in this model. Glucose utilization was increased in the retina following injury, confirming electron microscopic changes of central chromatolysis in the ganglion cells and increased metabolic activity in reaction to axonal injury. Decreased activity at the superior colliculus was demonstrated by delayed HRP arrival after injury. The model is unique because it produces axonal damage that is morphologically identical to that seen in human brain injury and does so by delivering tissue strains of the same type and magnitude that cause axonal damage in the human. The model offers the possibility of improving the understanding of traumatic damage of central nervous system (CNS) axons because it creates reproducible axonal injury in a well-defined anatomical system that obviates many of the difficulties associated with studying the complex morphology of the brain.

Animals↗
PubMed1987

A silicone gel phantom suitable for multimodality imaging.

A silicone gel phantom material is described. It can be molded into any shape and it can contain internal cavities. The material is selfsealing allowing the cavities to be filled with liquids. A simple geometric phantom consisting of a rectangular solid with a single internal spherical cavity was fabricated and imaged using MRI, CT, and PET. Properties important for imaging are described.

Gels↗
PubMed1987

The shaken baby syndrome. A clinical, pathological, and biomechanical study.

Because a history of shaking is often lacking in the so-called "shaken baby syndrome," diagnosis is usually based on a constellation of clinical and radiographic findings. Forty-eight cases of infants and young children with this diagnosis seen between 1978 and 1985 at the Children's Hospital of Philadelphia were reviewed. All patients had a presenting history thought to be suspicious for child abuse, and either retinal hemorrhages with subdural or subarachnoid hemorrhages or a computerized tomography scan showing subdural or subarachnoid hemorrhages with interhemispheric blood. The physical examination and presence of associated trauma were analyzed; autopsy findings for the 13 fatalities were reviewed. All fatal cases had signs of blunt impact to the head, although in more than half of them these findings were noted only at autopsy. All deaths were associated with uncontrollably increased intracranial pressure. Models of 1-month-old infants with various neck and skull parameters were instrumented with accelerometers and shaken and impacted against padded or unpadded surfaces. Angular accelerations for shakes were smaller than those for impacts by a factor of 50. All shakes fell below injury thresholds established for subhuman primates scaled for the same brain mass, while impacts spanned concussion, subdural hematoma, and diffuse axonal injury ranges. It was concluded that severe head injuries commonly diagnosed as shaking injuries require impact to occur and that shaking alone in an otherwise normal baby is unlikely to cause the shaken baby syndrome.

Battered Child Syndrome↗
PubMed1983

Mechanical characterization of membranelike biological tissue.

Experimental and analytical methods are presented which enable one to examine the local rheological properties of biological tissues which can be captured as flat sheets between matching pressure manifolds and deformed under experimentally prescribed hydrostatic loading conditions. In spite of the fact that most biological tissues, including arteries, are nonlinearly elastic when considered over wide ranges of strain, it was found that the deformation of swine and canine arterial wall specimens in the physiological range of wall strain can be approximated by an isotropic, linearily elastic membrane model. In view of this, the elastic behavior was characterized approximately by an incremental modulus over the range of 0.45 to 0.65 strain. The incremental modulus in both species was shown to increase by a factor of three along the descending thoracic aorta from the ductus scar to the celiac orifice.

Animals↗
PubMed1982

Diffuse axonal injury and traumatic coma in the primate.

Traumatic coma was produced in 45 monkeys by accelerating the head without impact in one of three directions. The duration of coma, degree of neurological impairment, and amount of diffuse axonal injury (DAI) in the brain were directly related to the amount of coronal head motion used. Coma of less than 15 minutes (concussion) occurred in 11 of 13 animals subjected to sagittal head motion, in 2 of 6 animals with oblique head motion, and in 2 of 26 animals with full lateral head motion. All 15 concussioned animals had good recovery, and none had DAI. Conversely, coma lasting more than 6 hours occurred in one of the sagittal or oblique injury groups but was present in 20 of the laterally injured animals, all of which were severely disabled afterward. All laterally injured animals had a degree of DAI similar to that found in severe human head injury. Coma lasting 16 minutes to 6 hours occurred in 2 of 13 of the sagittal group, 4 of 6 in the oblique group, and 4 of 26 in the lateral group, these animals had less neurological disability and less DAI than when coma lasted longer than 6 hours. These experimental findings duplicate the spectrum of traumatic coma seen in human beings and include axonal damage identical to that seen in sever head injury in humans. Since the amount of DAI was directly proportional to the severity of injury (duration of coma and quality of outcome), we conclude that axonal damage produced by coronal head acceleration is a major cause of prolonged traumatic coma and its sequelae.

Acceleration↗
PubMed1982

Biomechanics of acute subdural hematoma.

Acute subdural hematoma (ASDH) due to ruptured bridging veins occurs under acceleration conditions associated with rates of acceleration onset. That this is due to the strain-rate sensitivity of these veins was confirmed in an experimental model of ASDH. The results of this model were consistent with the clinical causes of ASDH, where 72% are due to high-strain falls and assaults and 24% are due to lower strain-rate vehicular injuries. A mathematical model embodying the known mechanical properties of subdural veins was used to develop tolerance criteria for the occurrence of ASDH. This tolerance curve was consistent with the clinical and experimental data but differed from tolerances previously proposed for head injury.

Acceleration↗
PubMed1981

The effect of ultrasonics and thermal treatment on wounds.

A method of local treatment of wounds to accelerate healing would be a major benefit in those patients in whom abnormal healing is expected. Earlier studies has suggested that local ultrasonic treatment of wounds would stimulate the healing process. We have evaluated the effect of local ultrasound (5 MHz) and thermal treatments on healing in a dermal wound model. Various intensities of ultrasound and heat were employed for 5 min/day (0.05 W/cm2) and 10 min/day (0.05W/cm2), with healing assessed by wound breaking strength measurements obtained 14 days after injury. Subcutaneous temperature measurements demonstrated that equal intensities of ultrasound and heat produced equivalent temperature changes in the tissues. None of the treatments employed resulted in greater wound breaking strengths than the controls, and the higher intensities of ultrasound (0.1 and 0.15 W/cm2) and heat (0.15 W/cm2) resulted in decreased wound breaking strength. Our results failed to support brief daily treatments of local ultrasound or heat as stimulants of wound healing.

Animals↗
PubMed1980

A study of adriamycin-reduced wound breaking strenght in rats. An evaluation by light and electron microscopy, induction of collagen maturation, and hydroxyproline content.

The cause of the reduced wound breaking strength (WBS) in adriamycin-treated animals is unknown. Differential staining of histologic sections, collagen fiber diameter measurements by electron microscopy (EM), induction of maturation, and hydroxyproline content analysis were used to examine this defect. Differential staining and formaldehyde induction of maturation revealed no differences between mature and immature collagen content in scars of adriamycin-treated and control animals. Mean collagen fiber diameters in control rats were 76 +/- 27 nm and, in adriamycin treated rats, 62 +/- 10 nm (mean +/- SD) at 21 days. There was no difference in total hydroxyproline content between ADR and C animals. However, significant differences existed between the specific activity of new or 3H-hydroxyproline (scar collagen) at 14 days (controls, 372 +/- 9 cpm; adriamycin-treated, 129 +/- 5 cpm) and at 21 days (controls, 365 +/- 11 cpm; adriamycin treated, 172 +/- 14 cpm; mean +/- SEM; P less than 0.001). It appears that the defect contributing to reduced WBS in adriamycin treated animals is not due to a collagen maturation defect but rather to a reduction in scar collagen accumulation as measured by new hydroxyproline content and reduced fiber diameter as determined by EM measurements.

Animals↗
PubMed1977

[K+]o clearance in cortex: a new analytical model.

1. It has been suggested that passive diffusion is the principal mechanism of the clearance of locally elevated extracellular potassium, K+o, in the cerebral cortex. This concept was based on the assumption that elevated K+ could be modeled as a point source. In the present study, the functional role of passive diffusion was reevaluated in the anesthetized cat cortex following local electrical stimulation. 2. The initial spatial distribution of extracellular potassium activity, [K+]o, elevated by monopolar stimulation, could be modeled by a two-dimensional Gaussian function at and below the 500-micron cortical depth. Azimuthal symmetry around the stimulating electrode was assumed and cylindrical spatial coordinates were used. 3. The observed clearance of transiently elevated [K+]o as a function of space and time was much more rapid than that predicted by an analytical model consisting of the homogeneous diffusion equation whose initial condition was the Gaussian spatial distribution of [K+]o at the onset of the clearance process. 4. It is concluded that passive diffusion does not significantly contribute to the rapid clearance of locally elevated extracellular potassium in the cortex. Active uptake of potassium by cortical cells should be more seriously considered as being primarily responsible for the potassium clearance.

Adenosine Triphosphatases↗
PubMedDate not supplied

Strain measurements in cultured vascular smooth muscle cells subjected to mechanical deformation.

Early work in the field of biomechanics employed rigorous application of the principles of mechanics to the study of the macroscopic structural response of tissues to applied loads. Interest in the functional response of tissues to mechanical stimulation has lead researchers to study the biochemical responses of cells to mechanical loading. Characterization of the experimental system (i.e., specimen geometry and boundary conditions) is no less important on the microscopic scale of the cell than it is for macroscopic tissue testing. We outline a method for appropriate characterization of cell deformation in a cell culture model; describe a system for applying a uniform, isotropic strain field to cells in culture; and demonstrate a dependence of cell deformation on morphology and distribution of adhesion sites. Cultured vascular smooth-muscle cells were mechanically deformed by applying an isotropic strain to the compliant substrate to which they were adhered. The state of strain in the cells was determined by measurement of the displacements of fluorescent microspheres attached to the cell surface. The magnitude and orientation of principal strains were found to vary spatially and temporally and to depend on cell morphology. These results show that cell strain can be highly variable and emphasize the need to characterize both the loading conditions and the actual cellular deformation in this type of experimental model.

Animals↗
PubMedDate not supplied

Model for short-term intracranial pressure changes following traumatic injury.

Results from primate studies show a transient increase in intracranial pressure (ICP) after a nonimpact inertial loading condition. The measured ICP increase varies linearly with the peak tangential load of these experiments. These experiments point to possible alterations in cerebral blood flow. This paper investigates the possible etiology of this particular phenomenon, and presents a simple analytical model that could explain the changes in intracranial pressure. The model combines the effects of cerebral venous constriction, arterial dilatation, and raised mean blood pressure to yield the characteristic immediate rise and exponential decay of ICP. The main contributor to the increase in intracranial pressure is believed to be vasodilation of cerebral arteries following venous constriction. Passive release of cerebrospinal fluid (CSF) is believed to mediate the long-term decay of intracranial pressure and possibly contribute to local hyperemia.

Animals↗
PubMedDate not supplied

An in vitro traumatic injury model to examine the response of neurons to a hydrodynamically-induced deformation.

A novel in vitro system was developed to examine the effects of traumatic mechanical loading on individual cells. The cell shearing injury device (CSID) is a parallel disk viscometer that applies fluid shear stress with variable onset rate. The CSID was used in conjunction with microscopy and biochemical techniques to obtain a quantitative expression of the deformation and functional response of neurons to injury. Analytical and numerical approximations of the shear stress at the bottom disk were compared to determine the contribution of secondary flows. A significant portion of the shear stress was directed in the r-direction during start-up, and therefore the full Navier-Stokes equation was necessary to accurately describe the transient shear stress. When shear stress was applied at a high rate (800 dyne cm-2 sec-1) to cultured neurons, a range of cell membrane strains (0.01 to 0.53) was obtained, suggesting inhomogeneity in cellular response. Functionally, cytosolic calcium and extracellular lactate dehydrogenase levels increased in response to high strain rate (> 1 sec-1) loading, compared with quasistatic (< 1 sec-1) loading. In addition, a subpopulation of the culture subjected to rapid deformation subsequently died. These strain rates are relevant to those shown to occur in traumatic injury, and, as such, the CSID is an appropriate model for studying the biomechanics and pathophysiology of neuronal injury.

Calcium↗
PubMedDate not supplied

The mechanical properties of the human cervical spinal cord in vitro.

The response of spinal cord tissue to mechanical loadings is not well understood. In this study, isolated fresh cervical spinal cord samples were obtained from cadavers at autopsy and tested in uniaxial tension at moderate strain rates. Stress relaxation experiments were performed with an applied strain rate and peak strain in the physiological range, similar to those seen in the spinal cord during voluntary motion. The spinal cord samples exhibited a nonlinear stress-strain response with increasing strain increasing the tangent modulus. In addition, significant relaxation was observed over 1 min. A quasilinear viscoelastic model was developed to describe the behavior of the spinal cord tissue and was found to describe the material behavior adequately. The data also were fitted to both hyperelastic and viscoelastic fluid models for comparison with other data in the literature.

Adult↗