Page 43 - Advances in Biomechanics and Tissue Regeneration

3.2 IDEAL MECHANICAL PROPERTIES FOR COLONIC STENTS 37

• Radial compression resistance (Fig. 3.2C): Stiffness of the stent when compressed by uniformly distributed radial
pressure. A higher value is better. In the case of use materials that present hysteresis loops in their strain-stress curve
(e.g., Nitinol), two parameters related to compression and expansion are defined. In such a case, the radial
compression resistance is controlled by two terms. The first is the radial compression resistance (RCR): stiffness of
the stent when compressed by uniformly distributed radial pressure (crimping process). A higher value is better.
The second is the chronic radial expansion force (CEF): radial force exerted by the stent against the duct wall in
deployment (release process). A higher and progressive (low slope) value evolution is better.
• Buckling resistance (Fig. 3.2D): Stent resistance to inelastic collapse when compressed by pressure above a critical
value. A higher value is better.
• Longitudinal flexibility (Fig. 3.2E): Flexibility of the stent when globally bent as a tubular beam. A higher value
(lower stiffness) is better.
• Perimetral adaptability (Fig. 3.2F): Ability to tightly adapt to the shape of the conduit cross- section for whose
patency the device is maintained. A higher value is better.
• Longitudinal adaptability (Fig. 3.2G): Ability to tightly adapt to the shape of the conduit longitudinal section for
whose patency the device is maintained. A higher value is better.
• Mesh opening: Total amount of conduit tissue area not covered by metal on each of the deformed cells of the
expanded stent. A lower value is better.
• Migration tendency: Probability of stent displacement after correct implantation due to peristaltic movement in
combination with poor mechanical performance. A lower value is better.

3.2.2 Commercial Stents

3.2.2.1 Self-Expanding Stainless Steel Stents
3.2.2.1.1 WALLSTENT (FIG. 3.3)
This stent is based on braided wires in a tubular configuration with two families of helicoidal springs rolled up in
inverse directions. There is no connection between the wires in the helicoidal shape, which results in the formation of

FIG. 3.3 Wallstent: (A) geometrical scheme; (B) real view. (Source: Boston Scientific.)

I. BIOMECHANICS