ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

a) b) c)
Fig. 5 – Contact interface: a) proximal extension with distal extension b) distal extension with
distraction lock c) proximal extension with distraction lock
4. RESULTS
Overall, analyzed Von Mises stress remains under the titanium yield stress (≈930 MPa)
for all designs. Within the distal extension, Von Mises stress increases up to
approximately 400 MPa. Regarding the proximal extension, Von Mises stress remains
under 600 MPa with the exception of the rod and the underside of the rectangular cross
section. Regarding the distraction lock, Von Mises stress distribution does not exceed
400 MPa, excepting a small portion of the pin, where Von Mises stress increases up to
900 MPa. Fig. 6, Fig. 7, Fig. 8 and Fig. 9 show the Von Mises stress distribution of the
standard VEPTR, the decreased diameter configuration (from 6mm to 5mm), the
decreased width configuration (from 6.7mm to 5.7mm) and the decreased height
configuration (from 6mm to 5mm), respectively.
Maximum Von Mises stress is reached around the hole of the distal extension, where
the distraction lock is inserted, in the standard design (diameter 6mm), as well as in the
decreased diameter design (from 6mm to 5mm) and in the decreased width design (from
6.7mm to 5.7mm). This maximum is likely due to the sharp edge in contact between the
distal extension and the distraction lock. This also explains the peak stress within the
distraction lock. In the decreased height design (from 6mm to 5mm), the maximum Von
Mises stress is reached at the end of the rod of the proximal extension.
Fig. 6 – Von Mises stress distribution within the standard
VEPTR