Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 164

Fig. 7.26 Strain effects on bone, induced by Sr1, Sr2, and Sr3 superior and inferior screws
type. The stresses were sampled along the screw sections, in the thread's adjacency.
This area usually represents a stress concentration due to geometrical leaps.
In the case of Sr1 bone screw, the stress values decrease from a maximum of
22 MPa. The nonsymmetrical distribution toward the geometrical axis of symmetry
can be observed in Fig. 7.27a , b . This behavior is generated by the complex loading
of the structure and is leading to bone strain distribution.
The Sr2 bone screw stress conditions are similar to the Sr1 bone screw. The
maximum stress recorded was 30 MPa, with a descendent trend from head to tip.
The high stresses recorded (see Fig. 7.27c , d ) are caused by the threading depth of
the screw. In the stress distribution, an important role is played by the contact sur-
face between the screw head and the plate, this surface being responsible to load
sharing. So, the uniform distribution aspect of the stresses for the superior and infe-
rior Sr2 bone screws can be caused by the spherical screw head.
The maximum stress value for Sr3 bone screw was 35 MPa, recorded in the
proximity of the screw head (see Fig. 7.27e , f ). The stress decreases from the
head to the tip of the screw. For the inferior screw, a uniform stress distribution
can be observed along the screw and also toward the symmetry axis. This effect
can be assigned to the thread profile and the rounded contact between the screw
head and bone plate.
Better stress distribution in bone screws leads to good strain distribution in bone
structure.
The stress tendencies for all the bone screws in superior and inferior vertebral
assembling are presented in Fig. 7.28 . For each screw in superior assembling, a
descendent tendency from the head to the tip can be observed. This stress is gener-
ated by the pullout loadings, applied directly on the screws. For the inferior screw,
the effect is attenuated due to the complex load sharing. The load sharing effect in
inferior screws is related to the contact and relative positioning between the bone
screw heads and bone plate.