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

INFLUENCE OF KYPHOSIS ON SPINAL LOADING
G. Spreiter 1 , P.F. Heini 2 , P. Galibarov 3 , S. Dendorfer 4 and S.J. Ferguson 1
1. ABSTRACT
Osteoporosis affects an estimated 75 million people in Europe, USA and Japan. The
biggest risk of fracture is in the spine. Literature shows that an excessive thoracic
curvature is an important risk factor for future fractures. The objective of the present
study was to evaluate the influence of kyphosis on local spinal segment loading, using a
numerical musculoskeletal model. This study extends a previous musculoskeletal model
of the spine to include the multi-axial stiffness of all lumbar and thoracic intervertebral
discs (6 DOF). The model was constructed with the AnyBody Modeling System. The
improved model incorporates intra-abdominal pressure and lumbar ligaments inherited
from the base model. The simplified thorax model has been refined, adding individual
thoracic vertebral bodies to allow the degree of kyphosis to be altered. The multifidus
and semispinalis muscles for the thoracic spine have been added. Segmental
compression and shear forces were compared for three different kyphosis severities
(37°, 50°, 70°) during simulated box lifting. Simulation results for these different
kyphosis severities showed good agreement with literature for compression forces
between T6 - T9. The most severe kyphosis curvature lead to an increase in segmental
compression forces at these levels of up to 20%, compared to the model with normal
curvature. For a moderate kyphosis curvature, the increase in compression was
approximately 10%, compared to the normal curvature. This study has demonstrated
that increasing kyphosis is associated with higher compression forces between T6 - T9,
and therefore may increase the risk of vertebral fractures.
2. INTRODUCTION
Osteoporosis affects an estimated 75 million people in Europe, USA and Japan. The
biggest risk of fracture is in the spine. A 50 year old woman has a 16% lifetime risk of
experiencing a vertebral fracture [1]. Literature shows that an excessive thoracic
curvature is an important risk factor for future fractures, even independent of low bone
mineral density or prior fracture [2,3]. Recently, the influence of sarcopenia, the agerelated
decrease of muscle volume and quality, has been implied to be also a potential
risk factor. This is based the work of Sinaki M. et al. [4], who has shown that stronger
back muscles reduce the incidence of vertebral fractures in estrogen-deficient women.
However, the relative importance of these factors is unknown and, therefore, the goal of
this study was to evaluate the influence of kyphosis and sarcopenia on local spinal
segment loading, using a numerical musculoskeletal model. For a comprehensive spinal
musculoskeletal model, the inclusion of passive soft tissue structures seem to be
1 Institute for Biomechanics, ETH Zürich, Zürich, Switzerland
2 University of Bern, Bern, Switzerland
3 AnyBody Technology, Aalborg, Denmark
4 Regensburg University of Applied Sciences, Regensburg, Germany