buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 247
Graphical representation of the equivalent stress von Mises for the
oesteosynthesis tibia-malleolar screw, obtained with the software ANSYS, is
represented in Fig. 3. Analyzing the stresses, graphically represented with
different colours, there can be observed that they are varying between 156 MPa
and 941 MPa. The unit measure Pa, called Pascal, is equivalent with N/m 2 , and
MPa represents 10 6 · Pa = 1 N/mm 2 .
From the analysis of stresses in longitudinal section through 3D model,
there can be seen the fact that the biggest values of stress occur at the limit
between the screw thread and porous bone tissue, that corresponding to
observed reality. Comparing the value of stress with the admissible one, there
can be observed that if the fractured ankle is not immobilized, but subjected to
normal gait, the maximum stresses could exceed the admissible values, the bone
tissue could break and the osteosynthesis assembly could be destroyed.
3. CAD Model of the System Bone-Kirschner Broaches–link with Metalic
Wire and Finite Element Analysis of the Osteosynthesis Assembly
The CAD model of the two Kirschner broaches was obtained very easy
with the software ProEngineering, the broach being modeled as a steel rod, as
can be seen in Fig. 4.
Fig. 4 – Kirschner broach.
The CAD model of the assembly tibia-Kirschner broaches–metalic wire
(wire of thickness 3 [mm]) is presented in Fig. 5, in overall view, and in Fig. 6,
in section view.
The metalic wire was taken as a closed loop, because the spliced
representation could not be possible.
Analysis of the state of stress and strain done with the software ANSYS,
emphasyzed the following aspects for this technique of osteosynthesis of the
tibia malleolus fracture:
i) when the separation plane of the fracture is traction loaded, the
mechanical stress varies between 121 MPa and 364 MPa, as can be seen in
Fig.7;
ii) the stresses in the lower malleolar fragment have big values at the
interface between broaches and bone, so that, for the given load, the bone tissue
could be destroyed;
iii) compared with the osteosynthesis technique with malleolar screw, the
mechanical stresses are smaller in this case, the maximum value being much
smaller, this meaning that this techinque is more suitable than the first one;