Shaken Baby Syndrome: Retinal Hemorrhaging. A Biomechanical Approach to Understanding the Mechanism of Causation

Shaken Baby Syndrome (SBS) is a form of abuse where typically an infant, age six months or less, is held and shaken. There may or may not be direct impact associated with this action. Further, there is very little agreement on the actual mechanism of SBS. Clinical studies are limited in showing the exact mechanism of injury and only offer postulations and qualitative descriptions. SBS has received much attention in the media, has resulted in a great deal of litigation and can be the source of unfounded accusations. Therefore, it is necessary to try to quantify the forces that may cause injury due to SBS. The physiology of infants makes injury due to SBS more likely. Infants have relatively large heads supported by weak necks that simply act as tethers (Prange et al., 2003). Therefore, there is minimal resistance to shaking. In addition, the cerebrospinal fluid (CSF) layer surrounding the infant\u27s brain is up to 10 mm thick as opposed to 1–2 mm in older children and adults (Morison, 2002). This thick layer reduces the resistance in rotation of the brain and can cause shearing injuries to the brain tissue. In addition, retinal hemorrhaging has been reported in SBS. The infant\u27s eyes have a vitreous that is typically more gelatinous and with a higher viscosity than in adult eyes. In addition, this vitreous is firmly attached to the retina and is difficult to remove (Levin, 2000). A preliminary parametric model of an infant eye will be presented so that resultant nodal retinal force of the posterior retina can be investigated and compared with a documented shaking frequency and a documented impact pulse. Retinal forces are then compared with various studies that investigate retinal detachment or adhesive strength. This eye model is built using a variety of material properties that have been reported for the sclero-cornea shell, choroids, retina, vitreous, aqueous, lens, ciliary, optic nerve, tendons, extra ocular muscles, optic nerve, and orbital fatty tissue. The geometry of the eye has been carefully optimized for this parametric model based on scaling to an infant from an adult using idealized eye globe geometry and transverse slice tracings of The Visible Human Project. This model shows promise in investigating the forces and kinematics of the infant eye exposed to harmonic shaking and further bolsters some of the few biomechanical studies investigating SBS. However, improvements are necessary to complete the eye model presented. Specifically, improvements on the mechanical properties for the components of the eye and especially the infant eye are needed. There is currently a deficit of biomechanical studies of the materials needed for the infant eye that is specifically geared for use in an explicit finite element code package. Conversions and adaptations of available materials are used in this first version of the infant eye model presented here and are in fair agreement with some of the clinical studies concerning SBS

Simulating Extraocular Muscle Dynamics. A Comparison between Dynamic Implicit and Explicit Finite Element Methods

The finite element method has been widely used to investigate the mechanical behavior of biological tissues. When analyzing these particular materials subjected to dynamic requests, time integration algorithms should be considered to incorporate the inertial effects. These algorithms can be classified as implicit or explicit. Although both algorithms have been used in different scenarios, a comparative study of the outcomes of both methods is important to determine the performance of a model used to simulate the active contraction of the skeletal muscle tissue. In this work, dynamic implicit and dynamic explicit solutions are presented for the movement of the eye ball induced by the extraocular muscles. Aspects such as stability, computational time and the influence of mass-scaling for the explicit formulation were assessed using ABAQUS software. Both strategies produced similar results regarding range of movement of the eye ball, total deformation and kinetic energy. Using the implicit dynamic formulation, an important amount of computational time reduction is achieved. Although mass-scaling can reduce the simulation time, the dynamic contraction of the muscle is drastically altered

Adaptive Neural Models of Queuing and Timing in Fluent Action

Temporal structure in skilled, fluent action exists at several nested levels. At the largest scale considered here, short sequences of actions that are planned collectively in prefrontal cortex appear to be queued for performance by a cyclic competitive process that operates in concert with a parallel analog representation that implicitly specifies the relative priority of elements of the sequence. At an intermediate scale, single acts, like reaching to grasp, depend on coordinated scaling of the rates at which many muscles shorten or lengthen in parallel. To ensure success of acts such as catching an approaching ball, such parallel rate scaling, which appears to be one function of the basal ganglia, must be coupled to perceptual variables, such as time-to-contact. At a fine scale, within each act, desired rate scaling can be realized only if precisely timed muscle activations first accelerate and then decelerate the limbs, to ensure that muscle length changes do not under- or over-shoot the amounts needed for the precise acts. Each context of action may require a much different timed muscle activation pattern than similar contexts. Because context differences that require different treatment cannot be known in advance, a formidable adaptive engine-the cerebellum-is needed to amplify differences within, and continuosly search, a vast parallel signal flow, in order to discover contextual "leading indicators" of when to generate distinctive parallel patterns of analog signals. From some parts of the cerebellum, such signals controls muscles. But a recent model shows how the lateral cerebellum, such signals control muscles. But a recent model shows how the lateral cerebellum may serve the competitive queuing system (in frontal cortex) as a repository of quickly accessed long-term sequence memories. Thus different parts of the cerebellum may use the same adaptive engine system design to serve the lowest and the highest of the three levels of temporal structure treated. If so, no one-to-one mapping exists between levels of temporal structure and major parts of the brain. Finally, recent data cast doubt on network-delay models of cerebellar adaptive timing.National Institute of Mental Health (R01 DC02852

Isoperimetric Partitioning: A New Algorithm for Graph Partitioning

Temporal structure is skilled, fluent action exists at several nested levels. At the largest scale considered here, short sequences of actions that are planned collectively in prefronatal cortex appear to be queued for performance by a cyclic competitive process that operates in concert with a parallel analog representation that implicitly specifies the relative priority of elements of the sequence. At an intermediate scale, single acts, like reaching to grasp, depend on coordinated scaling of the rates at which many muscles shorten or lengthen in parallel. To ensure success of acts such as catching an approaching ball, such parallel rate scaling, which appears to be one function of the basal ganglia, must be coupled to perceptual variables such as time-to-contact. At a finer scale, within each act, desired rate scaling can be realized only if precisely timed muscle activations first accelerate and then decelerate the limbs, to ensure that muscle length changes do not under- or over- shoot the amounts needed for precise acts. Each context of action may require a different timed muscle activation pattern than similar contexts. Because context differences that require different treatment cannot be known in advance, a formidable adaptive engine-the cerebellum-is needed to amplify differences within, and continuosly search, a vast parallel signal flow, in order to discover contextual "leading indicators" of when to generate distinctive patterns of analog signals. From some parts of the cerebellum, such signals control muscles. But a recent model shows how the lateral cerebellum may serve the competitive queuing system (frontal cortex) as a repository of quickly accessed long-term sequence memories. Thus different parts of the cerebellum may use the same adaptive engine design to serve the lowest and highest of the three levels of temporal structure treated. If so, no one-to-one mapping exists between leveels of temporal structure and major parts of the brain. Finally, recent data cast doubt on network-delay models of cerebellar adaptive timing.National Institute of Mental Health (R01 DC02582

From Parallel Sequence Representations to Calligraphic Control: A Conspiracy of Neural Circuits

Calligraphic writing presents a rich set of challenges to the human movement control system. These challenges include: initial learning, and recall from memory, of prescribed stroke sequences; critical timing of stroke onsets and durations; fine control of grip and contact forces; and letter-form invariance under voluntary size scaling, which entails fine control of stroke direction and amplitude during recruitment and derecruitment of musculoskeletal degrees of freedom. Experimental and computational studies in behavioral neuroscience have made rapid progress toward explaining the learning, planning and contTOl exercised in tasks that share features with calligraphic writing and drawing. This article summarizes computational neuroscience models and related neurobiological data that reveal critical operations spanning from parallel sequence representations to fine force control. Part one addresses stroke sequencing. It treats competitive queuing (CQ) models of sequence representation, performance, learning, and recall. Part two addresses letter size scaling and motor equivalence. It treats cursive handwriting models together with models in which sensory-motor tmnsformations are performed by circuits that learn inverse differential kinematic mappings. Part three addresses fine-grained control of timing and transient forces, by treating circuit models that learn to solve inverse dynamics problems.National Institutes of Health (R01 DC02852

Aerospace Medicine and Biology: A continuing supplement 180, May 1978

This special bibliography lists 201 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1978

Aerospace Medicine and Biology. A continuing bibliography with indexes

This bibliography lists 244 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1981. Aerospace medicine and aerobiology topics are included. Listings for physiological factors, astronaut performance, control theory, artificial intelligence, and cybernetics are included

A subject-specific software solution for the modeling and thevisualization of muscles deformations

Today, to create and to simulate a virtual anatomical version of a subject is useful in the decision process of surgical treatments. The muscular activity is one of the factors which can contribute to abnormal movements such as in spasticity or static contracture. In this paper, we propose a numerical solution, based on the Finite Element (FE) method, able to estimate muscles deformations during contraction. Organized around a finite element solver and a volumetric environment, this solution is made of all the modeling and simulation processes from the discretization of the studied domain to the visualization of the results. The choices of materials and properties of the FE model are also presented such as the hyperelasticity, the contention model based on inter-meshes neighboring nodes pairing, and the estimation of nodal forces based on the subject-specific muscular forces and action line

A Numerical Exploration of the Crystalline Lens: from Presbyopia to Cataracts and Intraocular Lenses

Esta tesis aborda, de forma numérica, la resolución de tres problemas relacionados con el cristalino. En primer lugar, se ha construido un modelo de elementos finitos del cristalino humano para abordar la simulación de la acomodación, gracias a la incorporación de la contracción muscular del músculo ciliar. El modelo se ha validado con resultados experimentales comparando con Ramasubramanian & Glasser, 2015. Con el mismo modelo, se ha estudiado como afecta el cambio de las propiedades mecánicas de los tejidos del cristalino en la pérdida de amplitud de la acomodación con la edad para entender si la rigidización de los tejidos juega un papel importante en la presbicia. La conclusión principal del estudio numérico ha sido que las propiedades mecánicas y tensiones iniciales de la cápsula del cristalino proporciona la fuerza necesaria para acomodar, es decir, cambiar su curvatura para enfocar de cerca. Especificamente, el ratio de rígidez entre el núcleo y el cristalino gobierna cómo el cristalino cambia de forma. Con la edad, se produce una rigidización del núcleo, y el incremento de la relación entre ambas rigideces (núcleo y corteza) podría ser el principal responsable de la pérdida de la amplitud de acomodación con la edad. En segundo lugar, se ha estudiado la estabilidad biomecánica de diferentes diseños de lentes intraoculares (IOL). Las IOLs sustituyen las funciones del cristalino en pacientes con cataratas, es por ello necesario garantizar su estabilidad en el interior del saco para garantizar una visión adecuada. Entre los aspectos estudiados destaca la caracterización mecánica de los materiales acrílicos con los que se fabrican las lentes. Para ello, se han combinado ensayos uniaxiales con ensayos de indentación. Éstos últimos se han utilizado para caracterizar la respuesta visco-elástica del material. El definir la respuesta del material mediante modelos visco hiperelásticos es necesario para posteriormente analizar la estabilidad de la IOL mediante elementos finitos. Este análisis se ha defino a dos niveles, en un primer nivel se analiza la estabilidad de la IOL simulando el ensayo establecido en la norma ISO 11979-3:2012. Esta norma es de obligado cumplimiento para los fabricantes antes de introducir un nuevo diseño en el mercado. Se ha realizado un estudio estadístico para estudiar el efecto de la geometría de los hápticos tipo C-loop en la estabilidad mecánica de la IOL, obteniendo que el entronque, la unión entre el háptico y la lente, es el parámetro más influyente. Para validar la metodología numérica, se fabricaron varios diseños y se analizaron experimentalmente para comparar los resultados correspondientes con biomarcadores mecánicos (desplazamiento axial, rotación y la inclinación de la IOL) que están relacionados con la calidad visual resultante de la IOL. En un segundo nivel, se ha simulado la respuesta de la IOL en el interior del saco capsular, estudiando la influencia de diferentes parámetros del paciente, como geometría y propiedades mecánicas del saco. También se ha analizado la influencia de parámetros de la cirugía de la catarata, como es el diámetro y posición de la capsulorexis. En este último nivel, se ha estudiado tanto la respuesta instantánea, es decir, tras la cirugía, como a largo plazo, cuando sucede la huella de fusión (fusion footprint) entre la cápsula y la IOL. Para que los modelos computacionales sean de ayuda a los cirujanos o puedan servir en tiempo real, se ha planteado una metodología basada en inteligencia artificial. En este caso la base de datos de partida corresponde a modelos numéricos altamente fiables y con ellos, se genera datos con los que se entrena la red neuronal. En esta tesis, se estudia la estabilidad de la IOL en función del diámetro de compresión del paciente y la edad, que a su vez influye en las propiedades mecánicas del saco. Por último, se ha evaluado experimentalmente la influencia del material de la IOL (hidrófobo o hidrofílico) y su geometría durante la inyección de la IOL en el saco, registrando la fuerza de inyección que debe realizar el cirujano. De cara a evitar complicaciones (se dañe la IOL o el tejido corneal) durante la cirugía, es conveniente que la fuerza a ejercer sea baja. Se ha comprobado que su valor está fuertemente influenciado por el material de la lente.¿Por qué el cristalino es de vital importancia?El cristalino es el responsable tanto del cambio dinámico de la potencia refractiva del ojo a través del mecanismo de acomodación como de la corrección de las aberraciones de la córnea. El cambio óptico dinámico es consecuencia de un cambio geométrico del cristalino. Sin embargo, a medida que el cristalino envejece, disminuye este cambio óptico dinámico y se opacifica, lo que da lugar a las dos patologías comúnmente asociadas al envejecimiento como es, la presbicia y las cataratas. Por este motivo, en esta tesis doctoral se ha profundizado en el estudio mecánico del cristalino y tras su sustitución mediante una lente intraocular artificial durante la cirugía de catarata. La metodología establecida pueden ayudar en un futuro tanto al diseño de nuevos implantes como a los oftalmólogos a seleccionar la IOL adecuada a cada paciente para mejora su calidad visual.This thesis addresses three different case studies related to the crystalline lens. Firstly, the mechanical causes of the loss of accommodation amplitude with age, called presbyopia, were analysed through the finite element method. A high-fidelity simulation of the mechanism of accommodation including the contraction of the ciliary muscle was developed. This allowed us to analyse accommodation in depth, showing that although the lens capsule provides the force to accommodate, the stiffness ratio between the lens cortex and lens nucleus could have a higher effect on how the lens changes its shape. Secondly, the biomechanical stability of intraocular lenses (IOLs) was analysed. IOLs are essential for post-cataract patients as they substitute the functions of the crystalline lens. In this thesis, a wide variety of solutions were addressed: from the visco- and hyper-elasticity characterisation of IOL acrylic materials from depth sensing indentation and uniaxial tests to the simulation of the IOL biomechanical stability inside the capsular bag. We also performed a high-fidelity simulation of the IOL compression standards tests required by the IOLs to be commercialised and the results obtained were compared with clinical data. Lastly, we developed a patient-specific methodology to customise the IOL haptic design. Most of the numerical methology developed is intended to be used in the IOL pre-design phase to avoid costs and time. Thirdly, the IOL delivery during cataract surgery according to haptic and material design and injector characteristics was experimentally studied to avoid any possibility of IOL and eye damage. Apart from the injector size, the IOL material was the most influential parameter in the force exerted in IOL delivery. Why is the crystalline lens of vital importance? The crystalline lens is the responsible for both the dynamic change of the refractive power of the eye through the mechanism of accommodation and the correction of cornea aberrations. The dynamic optical change is consequence of change of the lens shape. However, as the lens ages over time, it decreases this dynamic optical change and becomes cloudy, what leads to the two most common lens-related pathologies, presbyopia and cataracts. Therefore, it is of utmost importance to study the lens mechanics and all issues related to the artificial intraocular lens that substitutes the lens during cataract surgery.<br /