Lecture 3

Question 1

Atherosclerosis

Answer 1

• buildup of fats and cholesterol in/on artery walls
• complications include ischemia, cerebral infarction, and myocardial infarction

Question 2

Populations with Atherosclerosis

Answer 2

• initiates during youth (95% of people 36-39 yrs have some form of lesion)
• mechanical intervention is essential in older population

Question 3

Treatment of atherosclerosis

Answer 3

• utilize intravascular stents, which can expand the blockage and restore normal blood flow
• percutaneous coronary intervention
• over 1 million stents implemented annually in the US, 80% of stents worldwide are produced in Galway

Question 4

Vascular stents

Answer 4

• three generations, formed of struts made of different metallic materials

Question 5

Adverse events for vascular stents

Answer 5

• in-stent thrombosis: formation of a blood clot at implant site (stent under-expansion, asymmetry, persistent slow flow, and other causes)
• in-stent restenosis: tissue growth around stent (neo-intimal growth, or proliferation of epithelial cells, triggered by placement; initiated by similar events as thrombosis; bare metal stents treat SYMPTOMS of atherosclerosis)
• stent fracture: results in more likely restenosis, 3-29% incident rate

Question 6

FDA Guidelines on Intravascular Stents

Answer 6

• Class III, including balloon expandable and self-expanding
• definition:
• synthetic tubular structure intended for permanent implant
• provides mechanical radial support, and is meant to enhance vessel patency (openness)
• is expanded by a balloon or through self-expansion

Question 7

Sample testing

Answer 7

• follows specific design controls and design validation procedures
• devices must conform to defined user needs and be tested under actual or simulated use conditions
• test full range of sizes, each sample should represent finished product

Question 8

Four corners paradigm

Answer 8

• recommended for testing, involves using a 2 x 2 factorial of the largest and smallest diameters and lengths (should test each corner to see the different extremes)

Question 9

Material Characterization

Answer 9

• material composition testing is used as a baseline for evaluation of the effects of future material changes

Question 10

Dimensional Verification

Answer 10

• accurate dimensions help physicians to achieve proper sizing and accurate placement
• also affects functional behavior

Question 11

Percent Surface Area

Answer 11

• area over which stent contacts a vessel, affects biologic response (ex: tissue ingrowth)
• equation:
  Percent Surface Area = 100 x (Area in contact) / Full cylindrical surface area (pi * D * L)

Question 12

Percent Foreshortening

Answer 12

• dimensional changes that may occur when deploying, can impact final length
• knowing this can aid in proper length selection and placement
• equation:
  Percent Foreshortening = 100 x (Change in Length / Loaded Length)

Question 13

Stent Integrity

Answer 13

• stent defects contribute to complications
• certain processes like laser cutting can induce flaws, and plastic deformation during loading or expansion can lead to cracks
• use optical or electron microscopy to look for defects

Question 14

Recoil for balloon expandable stents

Answer 14

• influences appropriate device selection, sizing, acute results, and long-term outcomes
• function of stent design and material selection
• present as a percentage of expanded diameter:
  Stent Recoil (%) = 100 x (1 - (Diameter final or deflated / Diameter inflated))

Question 15

Stress/Strain analysis

Answer 15

• failure of loaded stent can lead to loss of support or perforation of surrounding vessel
• stress/strain analysis can help show device durability
• use computational FE models

Question 16

Radial Outward Force of Self-Expanding Stents

Answer 16

• excessive radial force = injuring surrounding tissue
• too low radial force = incomplete apposition of stent to vessel wall

Question 17

Crush Resistance

Answer 17

• only in peripheral stents, which can experience external distributed loads (non-cardiac, focal)
• can cause stent deformation
• include these varied loads in testing, determining load required to cause buckling or deflection of at least 50% as well as load required to permanently deform
• determine whether stent recovers its original geometry

Question 18

Crush Resistance testing

Answer 18

• local compression test, uses localized compressive force via a probe
• used also for self-expanding devices and transcatheter heart valves
• pinch stiffness: slope of load displacement curve, shows device’s ability to achieve circular deployment (trans. heart valves should have high pinch stiffness for better circularity)

Question 19

Radial stiffness and radial strength

Answer 19

• characterize stent’s ability to resist collapse under external loads
• stiffness: change in stent diameter as function of uniformly applied external radial pressure
• strength: pressure at which the stent deforms irrecoverably

Question 20

Radial testing

Answer 20

• stents experience radial loading through vessel elasticity, cardiac pulse pressure, and musculoskeletal motions
• can use segmented head (iris) radial force tester