Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 152

Fig. 7.14 Bone screws positioning ranges with respect to the bone plate

Since the development trends in cervical bone plates are increasingly turning to
more dynamic assembly type, the modeled system presents two major features:

• Capability of angular positioning and mobility within an important interval of

the bone screws
Possibility of longitudinal and transverse migration of bone screws along the •

bone plate axes

These two functional characteristics are coming to eliminate the stress shielding
effect produced by the rigid fixation to the bone graft. The large angular positioning
capability provides an appropriate flexibility for intra-operatory fixation of the
implant. It also allows an appropriate placing of the plate during the osteointegra-
tion. The longitudinal and transverse migration capacity assures adapting of the
relative position of the plate to the bone structure. The allowed migrations must be
within certain limits and developed under the action of loads greater than gravita-
tional force generated by own masses.
The intra-operative bending capacity of the plate is sustained by its geometrical
configuration together with the longitudinal and transversal grooves. This design
facilitates bending around longitudinal and transversal axes, and suites with ana-
tomical shape of the vertebral body. The four mounting holes have large dimensions
and elliptical shape to allow appropriate angular, longitudinal, and transverse posi-
tioning of the bone screws. The nine fine holes having the diameter of 1.5 mm are
necessary to temporarily position the plate in implantation site, before the final
fixing. The connection radius is 3.5 mm facilitating a possible longitudinal bending
of the plate.
The relative positioning between the bone plate and bone screws can be seen in
Fig. 7.14 , where for a better visualization the screws were declared transparent.
The angular positioning of the bone screw in front plane ranges from 0 to a maxi-
mum of ±10°. This means that the angle of convergence of two bone screws on the
same line is 20°, which defines an appropriate triangulation effect. A wider angular
range could be obtained by reducing the screw head or plate's height. Figure 7.14
shows also the allowed relative translation of the screws along the transversal axis
of the plate. This value although very small (0.3 mm) produces a significant mobil-
ity of the bone plate with respect to the vertebrae. A third possibility is the large
longitudinal movement (see Fig. 7.14 ). The value of this displacement is of 2.7 mm
for a screw.