Effects of geometric individualisation of a human spine model on load sharing: neuro-musculoskeletal simulation reveals significant differences in ligament and muscle contribution.

In spine research, two possibilities to generate models exist: generic (population-based) models representing the average human and subject-specific representations of individuals. Despite the increasing interest in subject specificity, individualisation of spine models remains challenging. Neuro-musculoskeletal (NMS) models enable the analysis and prediction of dynamic motions by incorporating active muscles attaching to bones that are connected using articulating joints under the assumption of rigid body dynamics. In this study, we used forward-dynamic simulations to compare a generic NMS multibody model of the thoracolumbar spine including fully articulated vertebrae, detailed musculature, passive ligaments and linear intervertebral disc (IVD) models with an individualised model to assess the contribution of individual biological structures. Individualisation was achieved by integrating skeletal geometry from computed tomography and custom-selected muscle and ligament paths. Both models underwent a gravitational settling process and a forward flexion-to-extension movement. The model-specific load distribution in an equilibrated upright position and local stiffness in the L4/5 functional spinal unit (FSU) is compared. Load sharing between occurring internal forces generated by individual biological structures and their contribution to the FSU stiffness was computed. The main finding of our simulations is an apparent shift in load sharing with individualisation from an equally distributed element contribution of IVD, ligaments and muscles in the generic spine model to a predominant muscle contribution in the individualised model depending on the analysed spine level.

The control effort to steer self-propelled microswimmers depends on their morphology: comparing symmetric spherical versus asymmetric L-shaped particles.

Active goal-directed motion requires real-time adjustment of control signals depending on the system's status, also known as control. The amount of information that needs to be processed depends on the desired motion and control, and on the system's morphology. The morphology of the system may directly effectuate or support the desired motion. This morphology-based reduction to the neuronal 'control effort' can be quantified by a novel information-entropy-based approach. Here, we apply this novel measure of 'control effort' to active microswimmers of different morphology. Their motion is a combination of directed deterministic and stochastic motion. In spherical microswimmers, the active propulsion leads to linear velocities. Active propulsion of asymmetric L-shaped particles leads to circular or-on tilted substrates-directed motion. Thus, the difference in shape, i.e. the morphology of the particles, directly influence the motion. Here, we quantify how this morphology can be exploited by control schemes for the purpose of steering the particles towards targets. Using computer simulations, we found in both cases a significantly lower control effort for L-shaped particles. However, certain movements can only be achieved by spherical particles. This demonstrates that a suitably designed microswimmer's morphology might be exploited to perform specific tasks.

Intuitive assessment of modeled lumbar spinal motion by clustering and visualization of finite helical axes.

A variety of medical imaging procedures, cadaver experiments, and computer models have been utilized to capture, depict, and understand the motion of the human lumbar spine. Particular interest lies in assessing the relative movement between two adjacent vertebrae, which can be represented by a temporal evolution of finite helical axes (FHA). Mathematically, this FHA evolution constitutes a seven-dimensional quantity: one dimension for the time, two for the (normalized) direction vector, another two for the (unique) position vector, as well as one for each the angle of rotation around and the amount of translation along the axis. Predominantly in the literature, however, movements are assumed to take place in certain physiological planes on which FHA are projected. The resulting three-dimensional quantity - the so-called centrode - is easily presentable but leaves out substantial pieces of available data. Here, we investigate and assess several possibilities to visualize subsets of FHA data of increasing dimensionality. Finally, we utilize an agglomerative hierarchical clustering algorithm and propose a novel visualization technique, namely the quiver principal axis plot (QPAP), to depict the entirety of information inherent to hundreds or thousands of FHA. The QPAP method is applied to flexion-extension, lateral bending, and axial rotation movements of a lumbar spine within both a reduced model as well as a complex upper body system.

Loads distributed in vivo among vertebrae, muscles, spinal ligaments, and intervertebral discs in a passively flexed lumbar spine.

The load distribution among lumbar spinal structures-still an unanswered question-has been in the focus of this hybrid experimental and simulation study. First, the overall passive resistive torque-angle characteristics of healthy subjects' lumbar spines during flexion-extension cycles in the sagittal plane were determined experimentally by use of a custom-made trunk-bending machine. Second, a forward dynamic computer model of the human body that incorporates a detailed lumbar spine was used to (1) simulate the human-machine interaction in accordance with the experiments and (2) validate the modeled properties of the load-bearing structures. Third, the computer model was used to predict the load distribution in the experimental situation among the implemented lumbar spine structures: muscle-tendon units, ligaments, intervertebral discs, and facet joints. Nine female and 10 male volunteers were investigated. Lumbar kinematics were measured with a marker-based infrared device. The lumbar flexion resistance was measured by the trunk-bending machine through strain gauges on the axes of the machine's torque motors. Any lumbar muscle activity was excluded by simultaneous sEMG monitoring. A mathematical model was used to describe the nonlinear flexion characteristics. The subsequent extension branch of a flexion-extension torque-angle characteristic could be significantly distinguished from its flexion branch by the zero-torque lordosis angle shifted to lower values. A side finding was that the model values of ligament and passive muscle stiffnesses, extracted from well-established literature sources, had to be distinctly reduced in order to approach our measured overall lumbar stiffness values. Even after such parameter adjustment, the computer model still predicts too stiff lumbar spines in most cases in comparison with experimental data. A review of literature data reveals a deficient documentation of anatomical and mechanical parameters of spinal ligaments. For instance, rest lengths of ligaments-a very sensitive parameter for simulations-and cross-sectional areas turned out to be documented at best incompletely. Yet by now, our model well reproduces the literature data of measured pressure values within the lumbar disc at level L4/5. Stretch of the lumbar dorsal (passive) muscle and ligament structures as an inescapable response to flexion can fully explain the pressure values in the lumbar disc. Any further external forces like gravity, or any muscle activities, further increase the compressive load on a vertebral disc. The impact of daily or sportive movements on the loads of the spinal structures other than the disc cannot be predicted ad hoc, because, for example, the load distribution itself crucially determines the structures' current lever arms. In summary, compressive loads on the vertebral discs are not the major determinants, and very likely also not the key indicators, of the load scenario in the lumbar spine. All other structures should be considered at least equally relevant in the future. Likewise, load indicators other than disc compression are advisable to turn attention to. Further, lumbar flexion is a self-contained factor of lumbar load. It may be worthwhile, to take more consciously care of trunk flexion during daily activities, for instance, regarding long-term effects like lasting repetitive flexions or sedentary postures.