Biomedical Engineering – From Theory to Applications

Orthopaedic Modular Implants Based on Shape Memory Alloys
Easy to manufacture thanks to components with simple shapes, most components
having two threaded surfaces which are used to assemble the next components.
Easy to extract by cooling the shape memory material
Provide good compaction of the bone fragments, lowering or eliminating the risk of
non-union (pseudo-arthrosys);
does not allow micro-movements between bone fragments found in fracture centre
Motion stability is ensured by continuous inter-fragmentary compression
Avoid the appearance of important degenerative-dystrophic lesions on the contact
surface of the fracture centre.
4.3 External fixator actuated by shape memory alloys elements
In the open fractures with important coetaneous lesions (type III) using the osteosynthesis
materials (plates, centro-medullar rods) is a real danger for infection. In these cases one can
use the external fixator which comprises threaded rods or Kirschner brooch which are
fixated in the bone fragments at a certain distance above and below the fracture centre,
passing through the healthy tissue. These structures are linked externally with rods or
circles. In the case of an external fixator, the resistance required to stabilize and consolidate
the fracture changes in time, the initial fixation must be rigid enough in order to withstand
the mechanical stress that appear once the patient can walk, without fracture
disequilibrium. In the same time, the fixator rigidity has to be under certain limits in order
to allow the development of pressures at the fracture centre which stimulate the callus
formation. In order to obtain the highest resistance for the fixator, several requests must be
fulfilled: the distance between the rod and bone to be reduced, the pins diameter to be
augmented, the pins located near fracture to be close one to other, the pins thread to be
totally inserted in the bone.
Fig. 25. Sequential frames of the femur external fixator – showing the osteosynthesis
process
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