MODELING MECHANICAL PROPERTIES

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MODELING MECHANICAL PROPERTIES OF
ENDOVASCULAR STENTS IN THEIR EXPANDED
STATE

• determine radial force that a stent can sustain
  depending on the
• mechanical and geometrical stent parameters
• Youngs modulus and shear modulus of strut
  material
• Number of stent struts in the circumferential and
  axial direction
• Reference radius and length of stent in expanded
  state
• Width and thickness of stent struts
• determine pressure-displacement relationship
  (radial and axial) for
• a stent with given mechanical and geometrical
  characteristics
• stents of arbitrary geometry, for example

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METHODS

• MATHEMATICAL THEORY OF SLENDER
• CURVED RODS TO APPROXIMATE STENT STRUTS
• DEVELOPMENT OF IN-HOUSE CUSTOM MADE
• FINITE ELEMENT METHOD-BASED SOFTWARE
• (1000 times faster than commercial software
  using 3D approx)
• EXPERIMENTAL VALIDATION

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RESULTS

• NON-UNIFORM GEOMETRY INFLUENCES STENTS
• MECHANICAL RESPONSE SMALLER DIAMONDS
• IMPLY HIGHER STIFFNESS

pressure0.5 atm
pressure0.5 atm
pressure5 atm
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MAXIMALLY EXPANDED STENTS ARE STIFFER
Colored by radial displacement
Colored by longitudinal displacement
R 1cm
Deformation (15 )
R 0.8cm
Larger deformation (23.5 )
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STENTS PLACED IN A SMOOTH, UNIFORM LUMEN CAN
SUPPORT LARGER UNIFORM PRESSURE

• if radial force is not uniformly applied to a
  stent due to,
• for example, non-axially symmetric lumen
  cross-section,
• or a plaque deposit that has not been uniformly
  pushed
• against the wall during balloon angioplasty, a
  stent is
• less stiff than when it is applied to a
  uniform lumen
• surface.

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MOVIE GALLERY
Non-uniform Stent, Pres. 0.5atm
Uniform Stent, Pres.0.5 atm
Uniform Stent, Pres. 5 atm
Non-uniform Stent, Pres. 5atm
Non-uniform Stent, Pres. 5atm Sideways
Uniform Stent, Pres. 5 atm Sideways
Non-uniform Stent, Pres. 5atm Pres. 0.5atm Middle
Uniform Stent, Pres. 5atm Pres. 0.5atm Middle
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CONCLUSIONS

• non-uniform stent geometry provides more support
  in
• the regions where diamonds are smaller (e.g.,
  convenient
• for use in tortuous geometry or, as a base for
  a
• bio-artificial aortic valve).
• if radial force is not uniformly applied to a
  stent due to,
• for example, non-axially symmetric lumen
  cross-section,
• or a plaque deposit that has not been uniformly
  pushed
• against the wall during balloon angioplasty, a
  stent is
• less stiff than when it is applied to a
  uniform lumen
• surface.

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A STUDY OF CORONARY STENT RESPONSE TO
COMPRESSION AND TO BENDING FORCES
STENTS CONSIDERED

• Express-like stent
• (Express stent by Boston Scientific)
• Cypher-like stent
• (Cypher stent by Cordis)
• Xience-like stent
• (Xience stent by Abbott)

COMPRESSION (MOVIES)
BENDING (MOVIES)
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CONCLUSIONS

• open-cell design provides more flexibility to
  bending
• Express-like stent has high flexibility
  (bending) while keeping
• high radial strength (radial displacement
  0.24)
• Xience-like stent is very flexible but it is
  also soft to
• radial force (compression) (radial
  displacement 2)
• Xience-like stent has the smallest longitudinal
  extension
• under cyclic loading (struts in phase)
• New design more flexibility than Express with
  higher
• radial strength..Cypher-like stent with
  open-cell design

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DOGBONING AT INFLATION
CAN BE AVOIDED BY SHORTER END STRUTS
STIFFER ENDS
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OBTAINED LAPLACE LAW FOR UNIFORM
STENTS (SIMPLIFIED PRESSURE-DISPLACEMENT FORMULA)
LAPLACE LAW (lin.elastic membrane)
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THIS ANALYSIS CAN HELP

• DECIDE WHICH STENT IS BEST FOR
• A GIVEN ENDOVASCULAR PROCEDURE
• OPTIMAL STENT DESIGN DEPENDENT ON A
• PARTICULAR ENDOVASCULAR PROCEDURE