Biomedical subjects

J P Laible

Publications and source records attributed to J P Laible.

6 recordsLinked to original sources

PubMed2007

Direct measurement of intervertebral disc maximum shear strain in six degrees of freedom: motions that place disc tissue at risk of injury.

Human intervertebral disc specimens were tested to determine the regions of largest maximum shear strain (MSS) experienced by disc tissues in each of three principal displacements and three rotations, and to identify the physiological rotations and displacements that may place the disc at greatest risk for large tissue strains and injury. Tearing of disc annulus may be initiated by large interlamellar shear strains. Nine human lumbar discs were tagged with radiographic markers on the endplates, disc periphery and with a grid of wires in the mid-transverse plane and subjected to each of the six principal displacements and rotations. Stereo-radiographs were taken in each position and digitized for reconstruction of the three-dimensional position of each marker. Maximum tissue shear strains were calculated from relative marker displacements and normalized by the input displacement or rotation. Lateral shear, compression, and lateral bending were the motions that produced the mean (95% confidence interval) largest mean MSS of 9.6 (0.7)%/mm, 9.0 (0.5)%/mm, and 5.8 (1.6)%/ degrees , respectively, and which occurred in the posterior, posterolateral and lateral peripheral regions of the disc. After taking into account the reported maximum physiological range of motion for each degree of freedom, motions producing the highest physiological MSS were lateral bending (57.8 (16.2)%) and flexion (38.3 (3.3)%), followed by lateral shear (14.4 (1.1)%) and compression (12.6 (0.7)%).

Biomechanical Phenomena↗

PubMed1995

Role of muscles in lumbar spine stability in maximum extension efforts.

Many problems of the lumbar spine that cause pain are attributed to instability. The ligamentous spine (without muscles) is unstable at very low compressive loads. This study examined the hypothesis that instability of the lumbar spine is prevented under normal circumstances by the stiffness of spinal musculature, without active responses from the neuromuscular control system. The effect of muscle activity (force and stiffness) on the stability of the lumbar spine was analyzed for maximum voluntary extension efforts with different spinal postures in the sagittal plane. The analysis included realistic three-dimensional representation of the muscular anatomy with muscles crossing several motion segments. The stiffness of motion segments was represented using in vitro measured properties. Under a range of conditions with maximum extension effort, active muscle stiffness was required to prevent the lumbar spine from buckling. The dimensionless value of the muscle stiffness parameter q as a function of activation and length had to be greater than a critical value in the range of 3.7-4.7 in order to stabilize the spine. Experimentally determined values of q ranged from 0.5 to 42. These analyses demonstrate how changes in motion segment stiffness, muscle activation strategy, or muscle stiffness (due to degenerative changes, injuries, fatigue, and so on) might lead to spinal instability and "self-injury."

Elasticity↗

PubMed1994

A dynamic material parameter estimation procedure for soft tissue using a poroelastic finite element model.

A three-dimensional finite element model for a poroelastic medium has been coupled with a least squares parameter estimation method for the purpose of assessing material properties based on intradiscal displacement and reactive forces. Parameter optimization may be based on either load or displacement control experiments. In this paper we present the basis of the finite element model and the parameter estimation process. The method is then applied to a test problem and the computational behavior is discussed. Sequential optimization on different parameter groups was found to have superior convergence properties. Some guidelines for choosing the starting parameter values for optimization were deduced by considering the form of the objective function. For load control experiments, in which displacement data is used for the optimization, the starting values for the elastic modulus should be lower in magnitude than an "anticipated" modulus. The permeability starting values should be higher than an anticipated permeability. For displacement control experiments, the reverse is true. The optimization scheme was also tested on data with random variations.

Anisotropy↗

PubMed1993

A poroelastic-swelling finite element model with application to the intervertebral disc.

The swelling process that occurs in soft tissue is incorporated into a poroelastic finite element model. The model is applied to a spinal segment consisting of two vertebrae and a single intervertebral disc. The theory is an extension of the poroelastic theory developed by Biot and the model is an adaptation of an axisymmetric poroelastic finite element model of the intervertebral disc by Simon. The model is completely three-dimensional although the results presented here assume symmetry about the sagittal plane. The theory is presented in two stages. First the development of the poroelastic theory. Following this, the effects of swelling caused by osmotic pressure are developed and expressed as a modification of the constitutive law and initial stresses. In the case of the disc, this pressure is produced mainly by the fixed negative charges on the proteoglycans within the disc. In this development we assume that the number of fixed charges remains constant over time and that the distribution of mobile ions has reached equilibrium. The variations over time in osmotic pressure, and thus in swelling effects are therefore only dependent on the initial state and the change in water content. Variations of the swelling effects caused by changes in mobile ion concentrations will be the subject of a future paper. The results reported in this article illustrate the dramatic effect of swelling on the load carrying mechanisms in the disc. The authors believe it is likely that this will have important useful implications for our understanding not only of normal disc function, but also of abnormal function, such as disc degeneration, herniation, and others.

Elasticity↗

PubMed1990

Three-dimensional osseo-ligamentous model of the thorax representing initiation of scoliosis by asymmetric growth.

A biomechanical model of the human thorax was constructed to investigate how asymmetric growth of the thorax might initiate spinal lateral curvature and axial rotation as seen in scoliosis deformities. Geometric data specifying nodal points of the model were taken from stereo-radiographs of an adolescent subject. An initially symmetrical geometry was created by 'mirroring' measurements of a hemi-thorax and spine. Published data provided cross-sectional measurements of the ribs, material properties of tissues and global flexibilities of the intervertebral motion segments. The ribs, sternum, intervertebral motion segments and intercostal ligaments were represented by elastic elements. Model deformations were calculated by the direct stiffness finite element method, with growth represented by an initial strain term in the constitutive law. Non-linear behavior was accommodated by running the model recursively, with updated node locations at each step. Both stress relaxation and stress modulation of growth in the component tissues were simulated. Thoracic growth of 20% with asymmetric growth of the ribs was simulated to give rib length asymmetries of 11%. similar to that observed in a previous study of patients with idiopathic scoliosis. This resulted in the model having a small thoracic scoliosis curvature convex toward the side of the longer ribs. Variations of the model which permitted free motion at the costo-vertebral joints or produced changes in the curvature of the posterior parts of the ribs resulted in axial rotation of the vertebrae similar to that observed clinically. The model supports the idea that growth asymmetry could initiate a small scoliosis during adolescence.

Adolescent↗

PubMed1990

Incorporation of spinal flexibility measurements into finite element analysis.

This technical note demonstrates two methods of incorporating the experimental stiffness of spinal motion segments into a finite element analysis of the spine. The first method is to incorporate the experimental data directly as a stiffness matrix. The second method approximates the experimental data as a beam element.

Elasticity↗