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 118

the tibial tunnel. In a comparative study, Walton [ 12 ] demonstrate that absorbable
interference screws of 28 mm or 35 mm provide similar or even better fixation than
metallic screws. Selby et al. [ 67 ] demonstrated that it is easier to place the insertion
torque force near the articular line with long screws, making the graft less prone to
failure mechanisms as the “windshield” effect.

6.2.4 Cutting Flutes

The effect of cutting flutes (as number and length) was investigated by several
authors [ 38, 47, 63, 68 ] . Rubel et al. [ 69 ], comparing the pullout strength for regular
and self-tapping screws, showed that the latter provide less pullout strength despite
the designs and lengths of the cutting flutes, while Schatzker et al. [ 68 ] and Koranyi
et al. [ 63 ] reported no significant difference between the self-tapped and pretapped
screws in terms of pullout strength. A comparison between self-tapping screws with
three short cutting flutes, and with two long cutting, showed that the pullout strength
is superior for the former. Yerby et al. [ 47 ] proved that the insertion torques decrease
if the length of the cutting flutes increases, for the same number of flutes, perform-
ing experiments of six different designs of self-tapping screw inserted in human
cadaveric femurs.

6.2.5 Screw Core/Screwdriver Shape

The screwdriver shape has to be designed to ensure that the insertion torque will be
uniformly distributed over a larger area, in order to avoid the screw breakage during
the insertion.
Cannulated screws in comparison with non-cannulated screws improve the inser-
tion torque and enhance the surface area for tissue regrowth, but at similar sizes,
they have worst performances in terms of pullout strength due to a smaller thread
depth [ 70 ]. However, Kissel et al. [ 43 ] showed that there is no significant difference
on pullout strength between cannulated and non-cannulated screws with 2.5 mm
diameter or less.
Weiler et al. [ 49 ] showed that turbine-like shape of the driver determines less
stress concentration at the interface, increasing the torque at failure.

6.2.6 Pitch

A higher number of screw threads in contact with the bone and a higher thread depth
increase the screw-bone interface, thus the axial shear area [ 37, 60, 71, 72 ] . Ricci
et al. [ 46 ] investigated the effect of the pitch on the insertion torque and on pullout