Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 155

Fig. 7.15 Screw models and the bone volumes engaged between the threads: ( a ) Sr1 bone screw,
( b ) Sr2 bone screw, ( c ) Sr3 bone screw
Fig. 7.16 Exploded view of
the implanted cervical unit
represents a compromise between the pullout resistance and the micro-crack
prevention. The design concept of Sr3 bone screw requires an additional locking
screw. This has the purpose of locking the fixation screw into the elliptical hole of
the cervical plate, by expanding the fl ower head of the fixation screw. The locking
screws were designed to be used only with the bone screw Sr3 , and their purpose is
to hold in place the bone screws, and prevent unscrewing, by expanding their heads
against the cervical bone plate. The locking screw is screwing into the bone screw
and, due to its tapered head, it creates a wedge effect. The Sr1 and Sr2 screws have
no locking system into the bone plate and therefore can experience unscrewing dur-
ing the movement of the spine.
Having the models of all anatomical and mechanical parts of the cervical
implanted unit, these were assembled into a SolidWorks assembly file. In order to
create a valid assembly, constraints as coincidence, concentricity, parallelism, dis-
tance, and so on were used. These constraints allow to manipulate the model as it
behaves in reality, excepting the elastic properties of the soft tissue (all the parts are
treated as rigid bodies in the assembly environment). The exploded view of the
assembly is presented in Fig. 7.16 , having all the components visible: 1—spinal
cord, 2—C2 vertebra, 3—interspinous ligament, 4—transversal facet joint, 5—C3