Development of Bioabsorbable Interference Screws: How Biomaterials Composition and Clinical and Retrieval Studies Influence the Innovative Screw Design and Manufacturing Processes - Biologically Responsive Biomaterials for Tissue Engineering - page 117

6.2.1 Major and Minor Diameters of the Screw

The thread depth or the ratio between minor and major diameters determines the
contact area between the screw and the bone, thus influencing bone regrowth. Asnis
et al. [ 37 ] demonstrated that the root diameter has an important effect on the holding
power, while Chapman et al. [ 40 ] experimentally studied different factors affecting
the pullout strength, the major diameter, and the thread depth being critical in this
sense. In another experimental study, DeCoster et al. [ 60 ] showed a roughly linear
dependence between major diameter and pullout force. Costi et al. [ 61 ] experimen-
tally confirmed the influence of screw diameter on the torsional strength, testing
screws from different manufacturers subjected to manually applied torque. Tensile
and compressive forces are proportional to the square of the minor diameter and
shear forces are proportional to the cube of the minor diameter.
The insertion of the screw determines not only a compression of the graft, but
also an enlargement of the tunnel [ 62 ]; therefore, ideally, after an initial conical
shape for easy insertion, most of the screw diameter should remain constant, for
intimate contact both with the graft and the tunnel. Interference screws of 9-10 mm
in diameter and around 35 mm in length are used for tibial site fixation, while sizes
for femoral screws are closer to 7 mm diameter by 28 mm length.

6.2.2 Thread

The shape of the thread is considered another critical influence factor for the pullout
strength [ 40 ]. A general design rule states that, in order to avoid lacerations of the
graft, the thread must not contain sharp edges. The thread profile ideally should be
symmetrical, because the screw is subject to diametrically opposed stress on the side
of the bone plug and on the side of the tunnel wall. Analysis of several thread designs
[ 63 ] showed that the pullout strength is not significantly different for the analyzed
types of threads: buttress vs. V-shape. However, Wang et al. [ 64 ] reported that a value
of 30° for proximal half angle ensures an equal distribution of the pullout force on
the recipient bone. Also, Hansson and Werke [ 50 ] used fi nite elements method to
assess the effect of thread shape and size on the stress peaks for the cortical bone.

6.2.3 Screw Length

Black et al. [ 65 ] performed biomechanical experiments, which showed that the
interference screw length does not influence in a significant manner the insertion
torque, load to failure, displacement, or stiffness. However, Lima et al. [ 66 ] showed
that the implant torque for both cylindrical and parallel miniscrews increases with
the screw length. Herrera et al. [ 42 ] experimentally demonstrated that longer and
wider interference screws provide better fixation in ACL graft fixation, especially in