Modal analysis of the diaphragm of the semicircular canal.

The aim of this paper is to determine the domain of validity of calculated quasi-static deformations of the cupula and of ciliar deflections on the crista ampullaris. Several three-dimensional models of the isolated ampullar diaphragm of the human semicircular canal and of that of the frog are studied theoretically by modal analysis. The four first modes of vibration are determined for each structure. Numerical simulations prove that for the first mode of vibration, the cupular deformation has the same shape as that obtained by applying a static pressure difference across the ampullar diaphragm. We studied also the effect of the mechanical properties (Young's modulus and Poisson's coefficient) of the components of the ampullar diaphragm on the vibration modes and their frequencies. The condition, which must be satisfied by the cupular internal viscosity, to have resonance near the natural frequency of the ampullar diaphragm is determined.

[Crista ampullaris mechanics and possibilities of regulation of the semi-circular canal]. / Mécanique ampullaire et possibilités de régulation du canal semi-circulaire.

A software using a finite element method is used to study the deformation of a bidimensional model of the human cupular diaphragm and the distribution of angular shearing of the hairs of the sensory cells located on the crista ampularis. This study shows that the hairs on the top of the crista are more sensitive to the pressure difference across the cupula due to angular acceleration of the head and those located on the sides of the crista are sensitive to the variation of the inflation pressure of the ampulla. This spatial discrimination of sensibility of the hair cells to the two types of mechanical solicitations leads us to consider the SCC as regulated sensor.

Modal and temporal analysis of head mathematical models.

The basic hypotheses used during these investigations were based on the vibration analysis of the head, which demonstrated that the head is not a solid nondeformable body, but a complex structure including deformable elements. Laboratoire des Systemes Biomecanique (LSBM) has recently proposed three mathematical models: a lumped model, a finite element model of the head in its sagittal plane, and a three-dimensional finite element model. These models were validated by their modal behavior and enabled the lesion mechanisms to be distinguished as a function of the spectral characteristics of the shock. The objective of this study is to complete these modal results by temporal analysis of the models by calculating the evolution of the intracranian mechanical parameters under shock conditions. To describe the head's dynamic behavior in the temporal domain, constant energy shocks of variable duration were simulated to evaluate their influence on different quantities as the intracerebral stresses in terms of compression, tensile, and shearing stresses, the relative brain-skull displacement, and the skull deformation. The importance of modal behavior of the head is illustrated by analyzing its temporal response to variable duration impacts, thus exciting very different frequencies. For a triangular shock, the critical duration times are between 10 and 15 x 10(-3) s, which correspond to impacts that excite the first resonance frequency of the head. Taking modal behavior into consideration in developing the finite element model leads to a harmonization of the calculated intracerebral stresses, even for short duration shocks. So, when the head is considered as a complex structure made up of several deformable elements, risk limitation is conditioned by an impact energy reduction for frequencies close to the natural frequencies of the structure. In the time field, the objective will be to avoid a number of impact shapes and durations. Therefore, the aim will not be to dampen the impact at any cost, but to damper it in an "intelligent" manner. In the future, this will allow the reduction of an injury mechanism-related risk, without increasing the risk of an injury generated by another mechanism.

Mechanisms of brain injury related to mathematical modelling and epidemiological data.

Measurements of the frequency response of head impact points on the exterior and the interior of a car were used to characterize the dynamic behavior of the object that was struck. These points were then arranged in a hierarchy of increasing stiffness. Thirty-two cases in which the distribution of injury to the brain had been recorded were grouped according to the stiffness of the object struck and by the location of the impact on the head. The distribution of the brain lesions were determined for each class of stiffness and location of impact. Three probable mechanisms of brain injury were distinguished: relative motion between the brain and the skull, local bone deformation, and intracerebral stresses. Each mechanism was related to a range of stiffness and natural frequency of the structure impacted. These theories of brain injury mechanisms are consistent with observed epidemiological data and with conclusions drawn from mathematical modelling.

Placental infiltration in congenital neuroblastoma: a case study with ultrastructure.

Examination of a large, oedematous placenta of a still-born infant showed an extensive infiltration of chorionic villi by malignant cells; the nature of these was not apparent by light microscopy. Ultrastructural examination of these cells showed the presence of cytoplasmic granules whose size and appearance were consistent with those characteristically present in neuroblastomas. Whereas no necropsy was carried out on the still-born infant, it may be assumed, in keeping with the available data, that the placental metastases were derived from a congenital neuroblastoma. Infiltration of the villous stroma by metastatic neuroblastoma observed in the present case was not reported previously as in all other known instances the metastatic neuroblastomatous cells were confined to the lumina of the chorionic vessels.