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

INVESTIGATION OF CORTICAL SHELL STRAIN CHANGES DURING
AXIAL LOADING DUE TO SPINAL DEGENERATION
1. ABSTRACT
A. Beckmann 1 , Y.T. Lu 2 , M.M. Morlock 3 and G. Huber 4
The probability of cortical shell fracture increases with age. Aging is also associated
with a degeneration of the spinal components. The effect of spinal degeneration on the
risk of cortical shell fracture remains poorly understood. Studies show that cortical shell
strain is predictive of fracture. Therefore, this study investigates the resulting cortical
shell strain changes during axial loading due to spinal degeneration.
Three finite element (FE) spinal models (healthy, mildly and fully degenerated) were
created. The degenerated FE models were generated by modifying both the
intervertebral disc geometry and material properties of spinal components. The healthy
model was based on CT images of human lumbar spine vertebrae L3-L5 from a 37year-old
male donor. The healthy FE model was validated against experimental data.
Material properties were taken from literature. Minimum and maximum strains at the
cortical anterior middle area and the cortical lateral upper area were compared (Figure
2). A sensitivity study was performed to determine the influence of each of the two
applied modifications, disc geometry and material degeneration.
Full spinal degeneration increased the strain by 67% and 79% at the cortical anterior
middle and lateral upper areas, respectively. Material degeneration was shown to have a
higher impact on cortical strains than disc geometry degeneration. This finding supports
the use of augmentation of degenerated vertebrae as treatment.
2. INTRODUCTION
The material properties and geometry of a functional spine unit (FSU) change due to the
aging process. One of the first geometry changes of the FSU involves the intervertebral
disc [1]. When the nucleus pulposus and annulus fibrosus are subject to dehydration,
they are getting stiffer and their heights decrease [2]. Osteoporosis is also age-related,
and describes an advanced bone mineral density loss. As cortical bone has a higher
mineral density than cancellous bone, the osteoporotic progress increases the loadbearing
role of the cortical shell significantly [3]. It was found that strain is a predictive
variable for fracture risk, indicating that regions with higher strains have a higher risk of
secondary fracture [4]. The purpose of this study was to investigate the influence of agerelated
geometry and material modifications on cortical shell strains.
3. MATERIAL AND METHODS
A FE model of human spinal segments L3-L5 created in another project was adopted for
this study [5]. The annulus fibers were replaced with rebar layer technique. Seven
1 Graduate student, Institute of Biomechanics, TUHH Hamburg University of Technology, Germany
2 Post-doc, Institute of Biomechanics, TUHH Hamburg University of Technology, Germany
3 Professor, Institute of Biomechanics, TUHH Hamburg University of Technology, Germany
4 Chief Engineer, Institute of Biomechanics, TUHH Hamburg University of Technology, Germany