Recoil, Foreshortening

The goal of the test is to assess the elastic recoil and foreshortening of balloon expandable stents.

Test Procedure

Empty test tubes are prepared and identified. If required, samples are put in a thermostatic bath at required temperature.

A contactless optical gauging machine (Smartscope, OGP) is used (highest magnification: x145) to measure the undeployed stent length in two perpendicular planes. It is then used to measure diameter at different sections (see below) and two perpendicular planes, both when the balloon is inflated at a specified pressure (measured with a manometer) and per IFU (see below) and when the balloon is deflated. Finally, the length of the expanded stent is measured in both perpendicular planes.

Balloon Expandable Stents Recoil:

The purpose of this test method is to quantify the percentage by which the diameter of a stent decreases from its expanded diameter while still on the delivery balloon to its relaxed diameter after deflating the balloon. Unless specified otherwise this test is performed in air at room temperature.  The test is conducted according to ASTM F2079 and its equivalent.

The recoil is evaluated for different stent lengths at various points along the length of the stent, including the ends. The number of locations along the length of the stent at which recoil is measured should be determined by initial assessment of the stent geometry.  In general, a minimum of ten specimens is recommended. If a single stent geometry is intended to be used for more than one labeled diameter, recoil shall be evaluated for test specimens expanded to the smallest and largest diameters intended for that geometry.

The outer diameter of the stent is measured in two approximately orthogonal rotational positions while the stent is still on the inflated delivery balloon. The balloon is deflated and the outer diameter of the stent is re-measured in the same positions at approximately the same location. The recoil is determined using a calibrated optical system with 5 micrometers accuracy that does not require contact nor deforms the stent. The stent diameter D1 is measured with inflated balloon in five sections. After the balloon is deflated, the stent diameter D2 is measured in five sections. The percentages of elastic recoil of the stent are calculated as percentage of ((D1 – D2) x 100 / D1.

 Stent Foreshortening:

The purpose of this test method is to quantify the percentage by which the length of a stent decreases from its expanded diameter while still on the delivery balloon to its relaxed diameter after deflating the balloon. Unless specified otherwise this test is performed in air at room temperature.  The test is conducted according to ASTM F2081 and its equivalent.

The length of stent is measured using a calibrated optical system with 5 micrometers accuracy that does not require contact nor deforms the stent.  Measurements are taken on each stent in the mounted state and at the labeled diameter. The labeled expanded length is that used to identify the typical size of a particular device.   The mounted (undeployed) length of the stent, L1, provides a baseline for the determination of shortening/lengthening. The stent is deployed using the balloon catheter at nominal pressure recommended by the manufacturer. The balloon is deflated and the stent length L2 is measured.  The percentages of foreshortening of the stent are calculated as a percentage of ((L1 – L2) x 100 / L1).

The recoil is then calculated for each location as:
Recoil (%) = [ diameter(balloon inflated) – diameter(balloon deflated) ] / diameter(balloon inflated) x 100

The foreshortening is also calculated for each stent as:
Foreshortening (%) = [ length(undeployed) – length(expanded) ] / length(undeployed) x 100

The foreshortening is also calculated for each stent as:
Foreshortening (%) = [ length(undeployed) – length(expanded) ] / length(undeployed) x 100

25 to 30 seconds (measured with the timer) were waiting before any measurements in order to allow the balloon and/or stent to stabilize. The stent is free from external stress. Water is used within the dilatation syringe.

The stents are finally retrieved from the balloon, placed within its corresponding test tube and closed with its cap. Samples are tested one at a time and placed back in their container after testing.

Equipment

  • Smartscope Micrometric gauging machine, measurement uncertainty ± 0.01mm
  • A computer with MeasureMind software
  • Calibrated manometer 40bar, measurement uncertainty 0.5 bars between 10 and 40 bars by increasing pressure value
  • Fixture for stents
  • Calibrated chronometer, measurement uncertainty ± 0.03%
  • Calibrated thermometer, measurement uncertainty ±0.5°C
  • Thermostatic bath
  • Dilatation syringe
  • Baseline pins: measurement uncertainty -0.1/0 mm in diameter and ± 0.3 mm in length

Measured Parameters

  • Diameter and length measurement
  • Inflation pressure
  • Time
  • Temperature

Scope

  • Balloon Expandable Stents
  • Stainless Steel (SS), Cobalt Chromium (CrCo), Bioresorbable Stent

When a stent is deployed in a diseased artery to provide mechanical support, it needs to preserve the flow through the stent as any reduction in the lumen diameter would increase the risk of restenosis.
Balloon expandable stent will not keep the diameter imposed by the inflated balloon, and will reach a new equilibrium diameter after balloon deflation. The inflation diameter and the diameter at rest after balloon deflation must be measured to provide insight on proper sizing compatibility of the stent with the artery.

Service Levels

Applicable Standards

  • ISO 25539-2:2020
    Cardiovascular implants — Endovascular devices — Part 2: Vascular stents

  • ASTM F2081 – 06(2017) 
    Standard Guide for Characterization and Presentation of the Dimensional Attributes of Vascular Stents

  • ASTM F2079
    Standard test method for measuring intrinsic elastic recoil of balloon-expandable stents.

  • FDA-1545
    FDA Guidance: Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems – Guidance for Industry and FDA Staff

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OLE0801


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