Study Design. A finite element parametric analysis to investigate the
relative load carrying roles of the shell and centrum in the lumbar ve
rtebral body. Objective. To address the issue of the structural role o
f the vertebral shell and clarify some of the contradictions raised by
previous studies. Summary of Background Data. A number of experimenta
l and finite element studies have attempted to quantify the relative s
tructural roles of the shell and centrum, but these studies support no
consensus on the relative contribution of the shell to vertebral body
strength. Methods. The authors developed finite element models to pre
dict the fraction of the total compressive force acting on the lumbar
vertebral body that is carried by the shell. Parametric variations wer
e investigated to determine how the fraction of shell force was affect
ed by changes in shell thickness, shell and centrum modulus, centrum a
nisotropy, and loading conditions. Results. The fraction of compressiv
e force carried by the shell increased from approximately 0 at the end
plate to approximately 0.2 at the mid-transverse plane for a typical c
ase. The shelf force was highly sensitive to the degree of anisotropy
of the trabecular centrum but was relatively insensitive to changes in
shell thickness and the ratio of shell-to-centrum elastic modulus. Co
nclusions. The conflicting conclusions of previous studies about the s
tructural roles of the vertebral shell and centrum can be explained by
differences in their methods. Our findings support the claims that th
e shell accounts for only approximately 10% of vertebral strength in v
ivo and that the trabecular centrum is the dominant structural compone
nt of the vertebral body.