Constitutive properties of adult mammalian cardiac muscle cells.

BACKGROUND: The purpose of this study was to determine whether changes in the constitutive properties of the cardiac muscle cell play a causative role in the development of diastolic dysfunction. METHODS AND RESULTS: Cardiocytes from normal and pressure-hypertrophied cats were embedded in an agarose gel, placed on a stretching device, and subjected to a change in stress (sigma), and resultant changes in cell strain (epsilon) were measured. These measurements were used to examine the passive elastic spring, viscous damping, and myofilament activation. The passive elastic spring was assessed in protocol A by increasing the sigma on the agarose gel at a constant rate to define the cardiocyte sigma-versus-epsilon relationship. Viscous damping was assessed in protocol B from the loop area between the cardiocyte sigma-versus-epsilon relationship during an increase and then a decrease in sigma. In both protocols, myofilament activation was minimized by a reduction in [Ca2+]i. Myofilament activation effects were assessed in protocol C by defining cardiocyte sigma versus epsilon during an increase in sigma with physiological [Ca2+]i. In protocol A, the cardiocyte sigma-versus-epsilon relationship was similar in normal and hypertrophied cells. In protocol B, the loop area was greater in hypertrophied than normal cardiocytes. In protocol C, the sigma-versus-epsilon relation in hypertrophied cardiocytes was shifted to the left compared with normal cells. CONCLUSIONS: Changes in viscous damping and myofilament activation in combination may cause pressure-hypertrophied cardiocytes to resist changes in shape during diastole and contribute to diastolic dysfunction.

The effect of fluid loss on the viscoelastic behavior of the lumbar intervertebral disc in compression.

A viscoelastic finite element model of a L2-L3 motion segment was constructed and used to study: (1) the behavior of the intervertebral disc with different amounts of nucleus fluid loss; and (2) the effect of different rates of fluid loss on the viscoelastic behavior of the disc. The results indicate that: (1) The viscoelastic behavior of the intervertebral disc depends to a large extent on the rate of fluid loss from the disc; the intrinsic properties of disc tissue play a role only at the early stage of compressive loading; (2) the axial strain increases, whereas the intradiscal pressure and the posterior radial disc bulge decrease with increasing fluid loss; (3) a decreasing fluid loss rate with a total fluid loss of 10 to 20 percent (from the nucleus) during the first hour of compressive loading best predicts the overall viscoelastic behavior of a disc.

Gel stretch method: a new method to measure constitutive properties of cardiac muscle cells.

Diastolic dysfunction is an important cause of congestive heart failure; however, the basic mechanisms causing diastolic congestive heart failure are not fully understood, especially the role of the cardiac muscle cell, or cardiocyte, in this process. Before the role of the cardiocyte in this pathophysiology can be defined, methods for measuring cardiocyte constitutive properties must be developed and validated. Thus this study was designed to evaluate a new method to characterize cardiocyte constitutive properties, the gel stretch method. Cardiocytes were isolated enzymatically from normal feline hearts and embedded in a 2% agarose gel containing HEPES-Krebs buffer and laminin. This gel was cast in a shape that allowed it to be placed in a stretching device. The ends of the gel were held between a movable roller and fixed plates that acted as mandibles. Distance between the right and left mandibles was increased using a stepper motor system. The force applied to the gel was measured by a force transducer. The resultant cardiocyte strain was determined by imaging the cells with a microscope, capturing the images with a CCD camera, and measuring cardiocyte and sarcomere length changes. Cardiocyte stress was characterized with a finite-element method. These measurements of cardiocyte stress and strain were used to determine cardiocyte stiffness. Two variables affecting cardiocyte stiffness were measured, the passive elastic spring and viscous damping. The passive spring was assessed by increasing the force on the gel at 1 g/min, modeling the resultant stress vs. strain relationship as an exponential [sigma = A/k(ekepsilon - 1)]. In normal cardiocytes, A = 23.0 kN/m2 and k = 16. Viscous damping was assessed by examining the loop area between the stress vs. strain relationship during 1 g/min increases and decreases in force. Normal cardiocytes had a finite loop area = 1.39 kN/m2, indicating the presence of viscous damping. Thus the gel stretch method provided accurate measurements of cardiocyte constitutive properties. These measurements have allowed the first quantitative assessment of passive elastic spring properties and viscous damping in normal mammalian cardiocytes.

Do bending, twisting, and diurnal fluid changes in the disc affect the propensity to prolapse? A viscoelastic finite element model.

STUDY DESIGN: A finite element model of a lumbar motion segment was constructed. OBJECTIVES: The model was directed toward understanding the effect of compression, bending and twisting, and diurnal fluid changes in the disc on the propensity to disc prolapse. Tensile stresses in the anulus fibers were computed and used to determine the successive steps required to create a fissure in the disc. SUMMARY OF BACKGROUND DATA: Disc prolapse is more likely under combined loading involving compression and bending and twisting. Changes in fluid content in the disc also affect the mechanical behavior of the disc. METHODS: The three-dimensional model accounted for the viscoelastic material properties of the anulus fibers and ligaments. Diurnal fluid exchange was simulated by changing the fluid content in the nucleus of the disc. Combined with bending and twisting, a compressive load was applied at different loading rates. RESULTS: The maximum tensile stress in the anulus fibers always occurred in the fibers at the inner posterior anulus at the junction of the disc and the endplate. Of the three models tested, the "weakest" (or the first to fail) was the saturated disc subjected to compression and bending and twisting. As the loading rate increased, anulus fiber failure was initiated at a lower value of compressive load. An increasing compressive load applied to a flexed, twisted, and saturated disc resulted in progressive failure, or fissure propagation, starting at the posterior inner anulus at the junction of the disc and the endplate. CONCLUSIONS: The results from this study suggest that there are several key factors involved in the initiation and propagation of anulus failure: axial compressive load, bending and twisting, and disc saturation. If one of these is lacking, anulus failure is harder to achieve.

Can variations in intervertebral disc height affect the mechanical function of the disc?

STUDY DESIGN: The finite element method was used to investigate the effect of variations in disc height on the mechanical behavior of the intervertebral disc. OBJECTIVES: The effect of disc height on the mechanical behavior of a human lumbar spine segment in terms of axial displacement, intradiscal pressure, posterolateral disc bulge, tensile stress in the peripheral anulus fibers, and longitudinal stress distribution at the end plate-vertebra interface was evaluated. SUMMARY OF BACKGROUND DATA: Disc height varies with individuals, disc level, abnormal conditions, and clinical management. METHODS: A three-dimensional finite element model of L2-L3 disc body unit was developed. Parametric studies were undertaken by studying discs of three different heights: 8 mm, 10 mm, and 12 mm, whereas disc cross sectional area, finite element mesh density, and all other parameters were kept constant. The model accounted for geometric nonlinearity but assumed that the material properties were linear. RESULTS: Variations in disc height had a significant influence on the axial displacement, the posterolateral disc bulge, and the tensile stress in the peripheral anulus fibers, but the effect on the intradiscal pressure and the longitudinal stress distribution at the endplate vertebra interface was minimal. CONCLUSIONS: Variations in disc height may compromise the general conclusions reached from experimental work and analytic studies in which geometric parameters (especially disc height and disc cross-sectional area) are not taken into consideration.

Cracks emanating from slipping inclusions near a bone-implant interface.

It has been hypothesized that mechanical fracture at the bone-cement-implant interface is the initial cause for loosening of orthopedic implants. Previous investigators have observed cracks to emanate from methacrylate beads, apparently acting as inclusions within the cement. It is believed that the bond between these inclusions and the surrounding matrix breaks prior to emanation of radial cracks from the inclusion. An analytical model is developed for radial cracks emanating from circular inclusions that allow slip along their interface. The solution to the interaction of a single dislocation and a slipping inclusion is used as a Green's Function to model the crack. The Mode I stress intensity factors are calculated for arbitrary orientations of the crack and for varying relative stiffness of the matrix and the inclusion to test feasibility of crack growth.

Cracks emanating from circular voids or elastic inclusions in PMMA near a bone-implant interface.

Mechanical fracture is believed to be a primary reason for loss of fixation at the bone-cement-implant interface. In addition to the expected cracks at the bone-cement interface, cracks are also observed to be formed at voids and inclusions within the cement. An analytical solution is presented for cracks emanating from circular voids or elastic inclusions under uniaxial tension using the solution for a single dislocation as a Green's function. Stress intensity factors are calculated for arbitrary orientations of the cracks, and for varying relative stiffnesses of the inclusion and the matrix, to determine the most favorable combination of parameters for crack growth.