Direct and indirect effects of mechanical creep of passive structures on the load sharing in the human spine and on risk of working tasks

Status:

completed 10/2019

Aims:

The load distribution among lumbar spine structures - still an unanswered question - has been in the focus of this combined experimental and simulation study.

Activities/Methods:

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 custommade trunk bending machine. Second, a computer model of the human body that incorporates a detailed lumbar spine was used to simulate the human-machine interaction in accordance with the experiments and validate the modelled 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 ten 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 non-linear flexion characteristics.

Results:

The subsequent extension branch of a flexion-extension torque-angle characteristic could be 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 wellestablished literature sources, had to be distinctly reduced in order to approach our measured overall lumbar stiffness values. Even after such parameter adjustment in comparison to experimental data, the computer model still predicts too stiff lumbar spines in most cases. 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 incompletely documented.

Yet, our model reproduces literature data of measured pressure values within the lumbar disc at level L4/5, well. 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 can not be predicted ad hoc, because, for example, the load distribution itself crucially determines the structures' current lever arms. Simulation of fatigued ligaments due to long term loading revealed load transmission to muscle-tendon-complexes and, thus, a different inner load scenario.

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 worth while, to take more consciously care of trunk flexion during daily activities, for instance, regarding long-term effects like lasting repetitive flexions or sedentary postures.

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