Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 153

The bone screws have three different designs. Having the same diameter and
length, the differences between the bone screw types consist in the thread's charac-
teristics and screw heads. The threads of the bone screws differ in pitch, helix's
angle, and profile's shape. The three types of developed profiles were coded Pr1 ,
Pr2 , and Pr3 and materialize threads designed to assemble with cortical bone ( Pr1 ),
spongy bone ( Pr2 ), and a combination of cortical-spongy ( Pr3 ) called hybrid. None
of these profiles is standardized, they being designed to perform numerical and
practical simulations to analyze the functionality of the backbone. The threads of
the bone screws were constructed by sweeping the adequate profiles along a helix
curve. The mechanical characteristics of the bone screws are presented in
Table 7.1 .
The correspondences between bone screw types and the type of the bone tis-
sues are the following: the Sr1 screw fits the requirements of self-tapping and
assembling with the cortical bone tissue; the second type Sr2 has design adapted
to interfacing the spongy bone; and Sr3 is a hybrid solution developed as a com-
bination of the first two.
Since the vertebral body is a spongy core within a cortical shell, the hybrid screw
solves the deficiencies of the very specific first two screws. These deficiencies
exhibit when a spongy screw for example is penetrating the thin cortical layer, pro-
ducing micro-cracks due to the large depth of the thread. On the other hand, when a
cortical screw progresses into the spongy bone, the fine thread of the screw gener-
ates a very poor assembling.
Sr1 bone screw (see Fig. 7.15a ) has a cylindrical head with a rounded bottom
edge to materialize a spherical cap. This spherical cap is an essential element in the
screw ability to position and tighten at various angular values. The screw tapping is
performed with a screwdriver, introduced into the hexagonal socket with an aperture
of 1.5 mm.
Sr2 bone screw (see Fig. 7.15b ) has a “ball” head made by combining two differ-
ent spherical caps differentiated by different radii. The lower spherical cap, with a
radius greater than Sr1 , allows screw placement within a larger angle range, making
the screw more flexible in terms of implantation possibilities.
Sr3 bone screw (see Fig. 7.15c ) is completely different from constructive point
of view related to the others. In addition to hybrid type of adopted profile, it was
provided with a particular head. It consists of a fl ower type element which is
deformed by a second screw when these two are assembling. The expanding of the
fl ower has the purpose to achieve a contact between the screw head and the hole of
the plate. The bone screw Sr3 was developed to achieve fixation by one cortical
breakthrough, so it is enrolling in those of unicortical type.
In Fig. 7.15 the models of the bone screws are presented together with the bone
volumes. The bone volumes represent the tissue volumes engaged between threads,
along the entire length of the screw. These are active volumes which hold the screws
in place when a pullout force is applied on the screw. So, higher volumes mean
higher pullout resistance for the screw.
However, in our case, the bone volume corresponding to the hybrid bone screw
will prove to be high enough for a good mechanical strength. This bone screw