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		Finite-element study of Existent medical stent

Company Business:
Stent technologies.

Analysis Goals:
To calculate the stress-strain fields that develops during the various stages of stent deployment into a blood vessel. To calculate the fatigue behavior and safety factor of the stent under in vivo conditions.


Simulation Details: The FE model of the stent contained the smallest repeating pattern of the struts under appropriate symmetry conditions. Figure 1 shows the geometry of the model. In order to simulate the physical conditions as closely as possible, FEA was carried out in 6 stages: Stage 1: Crimping. The stent was radially compressed to reduce its diameter until all struts were in tight contact and gaps were eliminated. The solution was obtained by solving a contact problem between a rigid cylinder surface (initially the outer diameter of the stent) and the outer surface of the stent. Stage 2: Spring-back. The crimping surface was gradually removed by increasing its diameter and letting the stent assume its equilibrium, load-free configuration. The unloaded (recoiled) configuration was then obtained. Stage 3: Inflation. The stent balloon was inflated to its final shape (inner diameter at 110% of the nominal diameter). Solving a contact problem between the balloon and the inner surface of the stent simulated inflation in the FE model. Stage 4: Recoil. The inflation surface was gradually removed, allowing the stent to recoil to its equilibrium state. The unloaded configuration was obtained. Stage 5: The stent was recoiled to a 3% reduction of its maximum expanded diameter. Stage 6: Alternating load. The solution of Stage 5 was loaded by an inner pressure level that matches the maximum level of pressure existing in a blood vessel.		Figure 1
Notes: Strain-stress levels were monitored at all stages to see if the values exceeded critical material limits. The fatigue and safety factors were calculate, based on the alternating stress value (Goodman-Soderberg criterion). Calculation of the fatigue-safety envelope for the stent included the highest stress level in its recoil configuration, along with the change in this stress level due to the alternating pressure loads in the blood vessel. The change in stress levels due to alternating in vivo pressure was obtained by applying an equivalent pressure load to the stent inner surface in its recoil configuration. The elements used in this model to mesh the stent were first-order brick elements using enhanced strain formulation (Figure 2), which allow for large shear and bulk deformation without mesh locking. The struts' cross sections were meshed with 3x3 element divisions to accurately capture any high stress/strain gradients that might develop.		Figure 2
Results: Figures 3 and 4 show the strain field obtained at the end of the crimping and inflation stages respectively. Figure 5 shows the modified Goodman-Soderberg fatigue diagram that was used to determine the fatigue/safety factors and life expectancy of the stent.	Figure 3	Figure 4

Benefits:
Using FEA early in the design stage is a valuable tool for evaluating the entire manufacturing and deployment process of the stent. The strain/stress field, as well as the geometric configuration, are monitored throughout the process, which gives a reliable 3D view of the device performance. Moreover, the ability to numerically calculate the fatigue behavior of the stent substantially reduces the need for long and expensive physical experiments.

Figure 5