DOS BULLETIN - Dansk Ortopædisk Selskab

2010-378_DOS nr. 3 2010 29/09/10 10:08 Side 103
The influence of muscle forces on the stress distribution
in lumbar spine
Christian Wong, John Rasmussen, Erik B. Simonsen, Lone Hansen,
Mark de Zee, Sebastian Dendorfer
Department of Orthopaedic Surgery, University Hospital of Hvidovre;
Department of Mechanical Engineering, Aalborg University; Department
of Neuroscience and Pharmacology, University of Copenhagen;
Team Denmark, Idraettens hus; AnyBody Technolog A/S, Aalborg
Background: Previous studies of bone stresses in the human lumbar
spine have relied on simplified models when modelling the spinal musculature,
even though muscle forces are likely major contributors to the
stresses in the vertebral bones. Detailed musculoskeletal spine models
have recently become available and show good correlation with experimental
findings.
Purpose: A combined inverse dynamics and finite element analysis
study was conducted in the lumbar spine to investigate the effects of
muscle forces on a detailed musculoskeletal finite element model of the
4th lumbar vertebral body.
Methods: The muscle forces were computed with a detailed and validated
inverse dynamics musculoskeletal spine model in a lifting situation,
and were then applied to an orthotropic finite element model of the
4th lumbar vertebra. The muscle model consisted of roughly 1000 individually
activated muscle fascicles of which about 188 are located in the
lumbar spine area. The FE model consisted of 63974 volumetric elements
for trabecular bone and 8886 shell elements with a thickness of
0.6 mm for cortical bone. Static analysis of the FE model was performed
and the results were compared with those from a simplified load case FE
model without muscles.
Findings: In general inclusion of muscle forces in the FE model
increased the Von Mises stress by 30 % in the superio-anterior and central
part of the vertebral body and in the pedicles.
Conclusion: The application of spine muscles to a finite element model
shows quite clearly that muscle forces play a large and non- negligible
role for the stress distribution in the vertebrae. This is especially important,
when considering compression fractures in the elderly population
with decreased trabecular strength, where small changes in the load axis
combined with muscle forces would lead to such fractures.
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