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 127

Table 6.4 Mechanical properties for bone, interference screw, and tendon
Material Young's Modulus (MPa) Poisson's ratio Density (g/cm 3 )

Interference screw 53,000 0.3 3

Cortical bone 13,400 0.24 2.21

Cancellous bone 283 0.29 283

Tendon 1,604 0.27 1.95

Table 6.5 FEM analysis results for interference screw

Equivalent (von-Mises)
stress (MPa) Runs Angles distal/proximal (°) Total deformations (mm)

1 25°/30° 0.022737 13.213

2 25°/40° 0.212561 12.112

3 30°/30° 0.022221 12.289

4 30°/40° 0.023055 16.857

cortical bone from the extremity of the bone tunnel and fills almost the whole inlet,
thus providing possible initial guidance and ease of manipulation.
This self-tapping type of screw requires the distal zone of the implant to have a
tapered hexagonal cannulation with a different step size so that the tip thickness,
associated with the composite biomaterial properties, prevents breakage during
insertion. The channel continues along the entire length inside screw, enabling the
use of a nitinol guide wire for additional guidance to insert the screw into the bone.
Screw is provided externally with constant pitch double thread, which is continued
up to 1 mm of proximal end so that it can be fully inserted into the bone.
The proximal zone of the screw is rounded to avoid graft damaging and at the
same time to reduce conflict with periarticular tissues where the integration is
incomplete due to implant position.
Thread geometry is different on the two parts of the screw in order to improve
contact with the bone and, consequently, resistance to tearing. Hence, the thread
depth is greater for the cylinder compared to the truncated cone; it ensures better
insertion in cortical bone screw. Also, the conical thread is more aggressive to pen-
etrate more easily into cortical bone and the cylindrical part has a low thread angle
in order not to damage the graft. The transition of the thread profile from one form
to another is progressive, without introducing additional sharp, aggressive edges.
This allows the screw to be inserted without additional tapping and results in opti-
mum compression of the neighboring bone material.
The hexagonal tip of the screwdriver distributes torque along the entire length of
the implant, so the torsional forces created do not lead to screw breakage. The favor-
able load transmission between screwdriver and screw permits the implant to be
manufactured of the mechanically less robust composite biomaterials with poly-
meric matrix.