 Stent recoil

Stent recoil is a reduction in the diameter of an expanded vascular stent after the expanding load is removed. For a balloon-expandable device, the relevant acute comparison is commonly the diameter during balloon expansion against the diameter after balloon deflation. It is distinct from later biological tissue growth inside the treated segment.^[ref]

Using matched diameter measurements $d_e$ during expansion and $d_r$ after release, defineThe denominator is the expanded diameter. An original bench example with $d_e=3.20\mathrm{mm}$ and $d_r=3.04\mathrm{mm}$ gives $R=5\%$. If the sections are circular, the corresponding area loss isDiameter and area percentages should not be interchanged, and neither alone specifies the resulting change in blood flow.

The mechanical distinction is between permanent expansion and recoverable deformation. A loading–unloading test relates stent recoil to the device’s material response and geometry. Imaging should compare the same location along the device; averages can hide differences near its ends. The primary FDA engineering guidance consequently asks for diameter-specific measurements and attention to variation with length and position.^[ref]

The ratio also has a useful measurement property. Multiplying both diameters by the same scale factor leaves $R$ unchanged, while an equal additive edge-location offset generally does not. For independent diameter standard uncertainties $u_e,u_r$, first-order uncertainty propagation givesWith $u_e=u_r=0.01\mathrm{mm}$ in the example, $u_R\approx 0.431$ percentage points. Shared image-scale or fitting errors require a covariance term instead of the independent-error formula.^[ref]