Biomedical Engineering – From Theory to Applications

Orthopaedic Modular Implants Based on Shape Memory Alloys
Step 2
Fig. 21. Total displacements in the bone-implant assembly (a), von Mises stress in femur (b),
von Mises in the element (c)
Step 3
Fig. 22. Total displacements in the bone-implant assembly (a), total displacements in the
plates (b), von Mises stress in the element (c) and von Mises stress in the plates (d)
4.2 Orthopaedic modular centro-medullar rods based on shape memory alloys
Centro-medullar rods can be used only for diaphyseal fracture fixation of long bones
(femur, tibia, and humerus) which limit their use. They make a good centring but
compaction is quite poor. When these rods are blocked by passing a proximal screw and a
distal one, transversely, trough the bone and rod, it results in the cancellation of compaction
forces and implicitly the delay of consolidation, with the development of pseudarthrosis.
Disadvantages of classical centro-medullar rods are that their shape and length do not
adapt to the bone channel and that they allow rotation of bone fragments from fractures
(the main cause of pseudarthrosis). The rods also get stuck in the medullar canal of the bone
and they are difficult to extract after the reduction of the fracture centre and bone healing.
If the centro-medullar rod is not well calibrated, it does not prevent rotation of bone
fragments and, therefore, does not always permit a good compaction of the fragments,
causing pseudoarthrosis. Also in the fracture centre, micro-movements can occur leading to
fatigue of the rod’s material and, implicitly, to breaking. Centro-medullar rods that have
mobility can cause important degenerative-dystrophic injuries at the interface with the
fracture centre.
The technical solution consists in designing and execution of a centro-medullar rod whose
dimensional characteristics (length and diameter) can be adapted to the medullar canal of
the bone.
The total length of the centro-medullar rod can be adjusted by simply substituting the two
modules which can be adapted for different bone lengths. Also, two modules may slide
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