Experimental results for the validation of dynamic finite element models

Little is known about the loads and stresses of the musculoskeletal system, this holds especially for the scope of the lumbar spine, which is in particular interest for occupational medicine and social economics. Reliable data on stresses derive from measurements of intradiscal pressure and from implants equipped with sensors. Those measurements provide data for investigated structures. Yet it is indistinct how implants influence the native force flow. Finite element (FE) models of the anatomy of the lumbar spine, which are embedded into whole body models, deliver powerful tools for evaluate vibration stresses of the lumbar spine e.g. for a sitting person in a construction machine. The quality of this assessment depends mainly on the knowledge of the structural material parameters. A multitude of material parameters can be determined post mortem with in vitro experiments. Extensive studies exist for the stress-strain behaviour of functional spine units. Predominantly these studies were executed without axial preload and concentrate mostly on extensional and flexional movement, as it is present when bending backward and forward. Dynamical loads in the frequency range around 10 Hz in axial and frontal or lateral shear direction are much more relevant with regards to whole body vibrations. Based on relaxation measurements, a dynamic stiffening of spinal segments was calculated in this range of frequencies. Aim of this study was to determine the dynamic stiffening of non-degenerated functional spine units within a frequency and load range relevant to occupational medicine, such to provide data to validate dynamic FE models. Anthropometrical data of 30 young donors (33±6 years) were collected post mortem. Functional spinal units L4/L5 were harvested and deep frozen afterwards. CT scans of the deep frozen functional spine units were made to enable numerical reconstruction of the vertebrae and to identify characteristics such as the height of intervertebral discs. After thawing functional spine units were prepared and tested dynamically (1-12 Hz) with a biaxial hydraulic material testing machine in axial compression and anterior-posterior as well as lateral shear. Besides deformations and forces, the intradiscal pressure was measured. All measurements were performed in a physiological saline solution at 37°C. Valuable data were acquired due to the homogeneous young collective of donors and the high amount of specimens. Standard errors of all measurements were at 3-5%. The results show a distinct dependency of the total stiffness and the frequency. At a frequency of 12 Hz the stiffness was 8-20% higher than at 1 Hz. As a result of the nonlinear material behaviour, the axial preload as well as the load amplitude have a substantial contribution to the total stiffness. In order to investigate the influence of whole body vibrations with numerical models, dynamic measurements should be used in addition to quasistatic measurements to validate the structures.

DDC Class

610: Medicine, Health

620: Engineering

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Experimental results for the validation of dynamic finite element models