Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 167

interface is very much dependent on the bone screw heads, because their geometries
come in direct contact with the plate, while the bone-screw interface is directly
dependent on the thread's profile.
Based on the simulation results, we may give the answers to the following
questions:

• Are the bone interfaces able to support the applied loads?

• Which screw type provides better pullout strength?

For the first question, the strains in the bone structures are generated by stresses
ranging from 1.2 to 3.5 MPa. Comparing the stress values with the ultimate strength of
the spongy bone (115 MPa for tension and 165 MPa for compression [ 29 ] ), the conclu-
sion is that no interface exceeds the strength limit. Therefore, all bone screws provide
safety fixations for the loading and restricting conditions of the functional unit.
Regarding the second question, better pullout strength is provided by the screw
Sr2 , because the strains in bone volume are well distributed. But, taking into account
that the vertebral body is not entirely a spongy bone but a composite structure of a
spongy core inside a cortical shell, and also the fact that the Sr2 depth of threading
generates micro-cracks in the cortical bone interface, we can affirm that the hybrid
screw Sr3 is more suitable for the fixation purposes. It has a medium thread depth and
a rounded profile, so it cannot cause cracks when using properly. In the same time, the
strains induced in the bone interface have comparable values with the Sr2 screw.
This simulation approach makes a small image about the influence of the thread
type, depth, pitch, and crossing size of the screws, on the bone tissue fixations.
Another important aspect of the simulation must be focused on the stresses and
strains acting at the screw-plate interface. This study can be performed using the
same model of the ICU under the same assumptions as in case of the study of
stresses and strains acting at the bone-screw interface.
The improved models will be subjected to a numerical analysis in order to high-
light the same mechanical aspects related to stresses and strains acting at the bone-
screw interface and screw-plate interface, in a more realistic case.

References

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