Density Functional for van der Waals Forces at Surfaces.
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Journal Article↗
Biomedical subjects
E Hult
Publications and source records attributed to E Hult.
Intervertebral disc response to cyclic loading--an animal model.
The viscoelastic response of a lumbar motion segment loaded in cyclic compression was studied in an in vivo porcine model (N = 7). Using surgical techniques, a miniaturized servohydraulic exciter was attached to the L2-L3 motion segment via pedicle fixation. A dynamic loading scheme was implemented, which consisted of one hour of sinusoidal vibration at 5 Hz, 50 N peak load, followed by one hour of restitution at zero load and one hour of sinusoidal vibration at 5 Hz, 100 N peak load. The force and displacement responses of the motion segment were sampled at 25 Hz. The experimental data were used for evaluating the parameters of two viscoelastic models: a standard linear solid model (three-parameter) and a linear Burger's fluid model (four-parameter). In this study, the creep behaviour under sinusoidal vibration at 5 Hz closely resembled the creep behaviour under static loading observed in previous studies. Expanding the three-parameter solid model into a four-parameter fluid model made it possible to separate out a progressive linear displacement term. This deformation was not fully recovered during restitution and is therefore an indication of a specific effect caused by the cyclic loading. High variability was observed in the parameters determined from the 50 N experimental data, particularly for the elastic modulus E1. However, at the 100 N load level, significant differences between the models were found. Both models accurately predicted the creep response under the first 800 s of 100 N loading, as displayed by mean absolute errors for the calculated deformation data from the experimental data of 1.26 and 0.97 percent for the solid and fluid models respectively. The linear Burger's fluid model, however, yielded superior predictions particularly for the initial elastic response.
Animals↗
In vivo measurement of spinal column viscoelasticity--an animal model.
The goal of this study was to measure the in vivo viscoelastic response of spinal motion segments loaded in compression using a porcine model. Nine pigs were used in the study. The animals were anaesthetized and, using surgical techniques, four intrapedicular screws were inserted into the vertebrae of the L2-L3 motion segment. A miniaturized servohydraulic exciter capable of compressing the motion segment was mounted on to the screws. In six animals, a loading scheme consisting of 50 N and 100 N of compression, each applied for 10 min, was used. Each loading period was followed by 10 min restitution with zero load. The loading scheme was repeated four times. Three animals were examined for stiffening effects by consecutively repeating eight times 50 N loading for 5 min followed by 5 min restitution with zero load. This loading scheme was repeated using a 100 N load level. The creep-recovery behavior of the motion segment was recorded continuously. Using non-linear regression techniques, the experimental data were used for evaluating the parameters of a three-parameter standard linear solid model. Correlation coefficients of the order of 0.85 or higher were obtained for the three independent parameters of the model. A survey of the data shows that the viscous deformation rate was a function of the load level. Also, repeated loading at 100 N seemed to induce long-lasting changes in the viscoelastic properties of the porcine lumbar motion segment.
Animals↗
Bone mineral content and fixation strength of femoral neck fractures. A cadaver study.
We measured the bone mineral content (BMC) of 21 human femoral autopsy specimens using Radiographic Attenuation Technique (RAT). The specimens were then tested for stiffness in a material-testing machine. Osteosynthesis was performed after medial femoral neck osteotomy, using 1 of 3 different pinning methods: 2 hook pins (LIH), 2 cannulated screws (Uppsala), and 3 cannulated screws (CHP). The specimens were allocated to each method by grouping them in triplets with similar BMC. Cyclic loading at 500-1000 N and 1 Hz was performed during 50 minutes. The amount of axial compression during loading was measured. Finally, loading to failure was done and post-fixation stiffness and ultimate compression strength recorded. With one exception, all cases failed where the BMC was less than 0.4 g/cm2. No differences between the methods of osteosynthesis were found. The fixation strength was mostly dependent on the strength of the bone, i.e., the degree of osteoporosis.
Aged↗
Structural properties of the anterior longitudinal ligament. Correlation with lumbar bone mineral content.
The relationship between the amount of bone in the lumbar spine and ligamentous properties has not been studied. This article reports the tensile structural properties of bone-ligament-bone preparations of the anterior longitudinal ligament from 15 human lumbar spine segments. Significant correlations were found between the vertebral bone mineral content expressed as BMC (g/cm) and BMA (g/cm2) and BMD (g/cm3) and the structural properties of the vertebral bone-anterior longitudinal ligament-bone complex determined at yield and failure. These findings suggest that the amount of bone tissue in the spine may be functionally related to structural properties of the spinal ligaments.
Adult↗
In vivo measurements of spinal column vibrations.
Recent epidemiological studies have indicated high risk factors for persons exposed to vibrations. We measured the in vivo responses of the lumbar vertebrae and sacrum of five volunteers who were subjected to pure sinusoidal vertical vibrations in the seated position. Two acceleration amplitudes were used, one and three meters per second squared, with frequencies ranging from two to fifteen hertz. Spinal vibration was measured for two lumbar vertebrae using a transducer that was attached directly to the spinous processes. Axial, horizontal, and rotatory accelerations in the sagittal plane were determined for each vertebra. Vertical acceleration at the sacrum was also measured. The amplitude ratios of the accelerations at the vertebrae and sacrum with respect to the acceleration of the seat were calculated. Our results show that the pure vertical sinusoidal input vibration at the seat produced vibrations of the lumbar vertebrae not only vertically but also horizontally, as well as a rotational vibration. The resonance frequency of the lumbar vertebrae in the vertical direction was an average of 4.4 hertz. Horizontal and rotatory resonance frequencies could not be determined.
Acceleration↗
HYOMEX: a miniature universal testing machine for in vivo biomechanical studies.
A miniaturized universal testing machine, HYOMEX, designed for biomechanically testing spinal structures of an anaesthetized research animal, such as a pig, is presented. A variety of mechanical applications such as static and dynamic tests, creep and relaxation tests, and fatigue tests with frequencies up to 20 Hz are possible. The system permits force control as well as displacement control. Continuous surveillance is made possible through interfacing the system to a computer. An application is presented where the viscoelastic behaviour in a porcine lumbar spine was measured.
Animals↗