ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

essential [5]. The following components are important for a numerical spine model to
investigate the influence of posture and muscle strength on spinal loading: Individual
thoracic vertebral bodies allowing a variable degree of kyphosis, intra-abdominal
pressure (IAP), inclusion of all relevant long and short muscles, the intervertebral disc
(IVD) stiffness and paraspinal ligaments. Recently developed models from de Zee M. et
al. [6], Han et al. [5] or Spreiter G. [7] have some, but not all of these components
included.
This work combines the properties of these previous models and includes additionally a
translational stiffness between the VBs. The model allows also scaling the erectus
spinae muscle according to the physiological cross section area (PCSA) derived from
MRI data to mimic different stages of sarcopenia. Our first hypothesis is that a higher
degree of kyphosis will produce a higher spinal load. Our second hypothesis is that
sarcopenia also contributes to a higher spinal load through sub-optimal stabilisation.
3. MATERIALS AND METHODS
The model was constructed with the AnyBody Modeling System (AnyBody
Technology, Aalborg, Denmark). The software is a three-dimensional multi-body
dynamic simulation package which defines the model in a text-based, declarative,
object-oriented language called AnyScript. The Anybody Modeling System uses inverse
dynamics to calculate forces and moments. The developed model replicates a box lifting
task. The simulated motion is a flexion of the lumbar spine of 30 degrees over a period
of 4 seconds. The upper extremities, the cervical spine, the skull and the box are
represented by boundary conditions of applied moments and forces simplify the model
and to avoid problems with modelling the shoulder complex in combination with the
newly defined thoracic VBs. The lower extremities are also excluded and the pelvis is
fixed to the ground. Intervertebral joints in the previous spinal models were spherical
joints. In this work, the connection between all VBs was represented by a 6 degree-offreedom
(DOF) joint with linear stiffnesses in all DOF. This allows motion in both
translation and rotation.
The lumbar paraspinal ligaments were represented by the supraspinous, interspinous,
anterior and posterior longitudinal, intertransverse ligaments and ligamentum flavum.
All ligaments have a linear stress-strain relationship, with the properties of the
ligaments taken from the AnyBody repository and based on Pintar F.A. et al. [8] and
Chazal J. et al. [9].
The curvature of the kyphosis is defined with the Cobb angle between T12 and T4. The
three representative degrees of kyphosis are 37°, 50° and 70° for the normal, moderate
and severe case [10]. To interpolate the coordinates of each joint between the vertebral
bodies, a third-degree polynomial function was used (Fig. 1). The individual muscle
fascicles have a constant maximal strength as their only input parameter (Fig. 2). To
represent varying degrees of sarcopenia, we scaled the maximum strength of the erector
spinae muscle with two components; the cross-section area and the fat percentage. Both
can be read out from MRI data and were represented in the AnyScript by a factor (ES-
Factor). These erector spinae scaling factors combined with the three thoracic
curvatures to result in 12 situations.