Axial stiffness of human lumbar motion segments, force dependence.

This paper addresses the axial stiffness of human lumbar motion segments while subjected to moderate loads. Impacts in axial direction were applied to Functional Spinal Units while they were subjected to weights acting as static pre-load. Accelerations were recorded proximal and distal of the FSU. The transfer function and the resonant frequency were calculated from this data. The stiffness was calculated from the resonant frequency and the load. A simple non-linear model was fitted to the data and a linear relationship was found between stiffness squared and force. The non-linear component in the model strongly affected the stiffness within the chosen load range. The present model may allow in vivo dynamic force determination with improved accuracy, e.g. in experiments where accelerometers have been fixated to pins inserted into the spinous processes of lumbar vertebrae if the static force is known.

In vivo dynamic stiffness of the porcine lumbar spine exposed to cyclic loading: influence of load and degeneration.

The dynamic axial stiffness of the L2-3 motion segment subjected to vibratory loading under intact and injured states of the intervertebral disc was studied using an in vivo porcine model. Three groups of animals with the following states of the intervertebral discs were studied: intact disc, acutely injured disc, and degenerated disc. A miniaturized servo-hydraulic exciter was used to sinusoidally vibrate the motion segment from 0.05 to 25 Hz under a compressive load with a peak value of either 100 or 200 N. The dynamic axial stiffness of the intervertebral disc was calculated at 1-Hz intervals over the frequency range. The results showed that the dynamic axial stiffness was frequency dependent. A positive relationship was found between an increase in mean dynamic stiffness and load magnitude. An increase in mean stiffness with successive exposures at the same load magnitude was observed, despite the allowance of a recovery period between loading. The greatest difference was noted between the first and second load sets. No significant change in stiffness was found due to an acute disc injury, whereas a significant increase in mean stiffness was found for the degenerated disc group as compared with the intact group. The form of the frequency response curve, however, remained relatively unaltered regardless of the degenerated state of the disc. With heavier loads, repeated loading, and/or disc degeneration, the stiffness of the intervertebral disc increases. An increase in stiffness can mean a reduction in the amount of allowable motion within the motion segment or a potentially harmful increase in force to obtain the desired motion. This may locally result in greater stresses due to an altered ability of the disc to distribute loads.

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.

Resonant frequency of a pin-accelerometer system mounted in bone.

Invasive measurements of spinal motion using intraosseous metal pins have become common. For this reason, the resonant frequency of intraosseous pins attached with accelerometers was determined using two different methods. It was concluded that plucking the pin is a reliable method for determining the resonant frequency and, in order to accurately measure bone movement at frequencies up to 32 Hz, the pin diameter should be 2.0 mm or more. With a mass of the accelerometer assembly equal to 27 g, the total pin length should not exceed 80 mm with a bone-accelerometer distance of 25 mm and a pin diameter of 2.4 mm.

Effect of backrest inclination on the transmission of vertical vibrations through the lumbar spine.

UNLABELLED: Various studies have demonstrated a positive relationship between low back pain and driving of vehicles. Little work has been done to establish how posture or seat design can attenuate vibrations. By means of skeletally mounted accelerometers it was demonstrated that the inclination of the backrest has only a minor role in vibration attenuation in the 4-6 Hz range. RELEVANCE: The increasing evidence of a positive relationship between driving and low back pain emphasizes the need for ways to attenuate vibrations on the spine. In the present study, the inclination of a backrest was studied.