Limb flexion-induced axial compression and bending in human femoropopliteal artery segments

BACKGROUND

High failure rates of femoropopliteal artery (FPA) interventions are often attributed in part to severe mechanical deformations that occur with limb movement. Axial compression and bending of the FPA likely play significant roles in FPA disease development and reconstruction failure, but these deformations are poorly characterized. The goal of this study was to quantify axial compression and bending of human FPAs that are placed in positions commonly assumed during the normal course of daily activities.

METHODS

Retrievable nitinol markers were deployed using a custom-made catheter system into 28 in situ FPAs of 14 human cadavers. Contrast-enhanced, thin-section computed tomography images were acquired with each limb in the standing (180 degrees), walking (110 degrees), sitting (90 degrees), and gardening (60 degrees) postures. Image segmentation and analysis allowed relative comparison of spatial locations of each intra-arterial marker to determine axial compression and bending using the arterial centerlines.

RESULTS

Axial compression in the popliteal artery (PA) was greater than in the proximal superficial femoral artery (SFA) or the adductor hiatus (AH) segments in all postures (P = .02). Average compression in the SFA, AH, and PA ranged from 9% to 15%, 11% to 19%, and 13% to 25%, respectively. The FPA experienced significantly more acute bending in the AH and PA segments compared with the proximal SFA (P < .05) in all postures. In the walking, sitting, and gardening postures, average sphere radii in the SFA, AH, and PA ranged from 21 to 27 mm, 10 to 18 mm, and 8 to 19 mm, whereas bending angles ranged from 150 to 157 degrees, 136 to 147 degrees, and 137 to 148 degrees, respectively.

CONCLUSIONS

The FPA experiences significant axial compression and bending during limb flexion that occur at even modest limb angles. Moreover, different segments of the FPA appear to undergo significantly different degrees of deformation. Understanding the effects of limb flexion on axial compression and bending might assist with reconstructive device selection for patients requiring peripheral arterial disease intervention and may also help guide the development of devices with improved characteristics that can better adapt to the dynamic environment of the lower extremity vasculature.

Limb flexion-induced twist and associated intramural stresses in the human femoropopliteal artery

High failure rates of femoropopliteal artery (FPA) interventions are often attributed to severe mechanical deformations that occur with limb movement. Torsion of the FPA likely plays a significant role, but is poorly characterized and the associated intramural stresses are currently unknown. FPA torsion in the walking, sitting and gardening postures was characterized in n = 28 in situ FPAs using intra-arterial markers. Principal mechanical stresses and strains were quantified in the superficial femoral artery (SFA), adductor hiatus segment (AH) and the popliteal artery (PA) using analytical modelling. The FPA experienced significant torsion during limb flexion that was most severe in the gardening posture. The associated mechanical stresses were non-uniformly distributed along the length of the artery, increasing distally and achieving maximum values in the PA. Maximum twist in the SFA ranged 10-13° cm^-1, at the AH 8-16° cm^-1, and in the PA 14-26° cm^-1 in the walking, sitting and gardening postures. Maximum principal stresses were 30-35 kPa in the SFA, 27-37 kPa at the AH and 39-43 kPa in the PA. Understanding torsional deformations and intramural stresses in the FPA can assist with device selection for peripheral arterial disease interventions and may help guide the development of devices with improved characteristics.

The choice of a constitutive formulation for modeling limb flexion-induced deformations and stresses in the human femoropopliteal arteries of different ages

Open and endovascular treatments for peripheral arterial disease are notorious for high failure rates. Severe mechanical deformations experienced by the femoropopliteal artery (FPA) during limb flexion and interactions between the artery and repair materials play important roles and may contribute to poor clinical outcomes. Computational modeling can help optimize FPA repair, but these simulations heavily depend on the choice of constitutive model describing the arterial behavior. In this study finite element model of the FPA in the standing (straight) and gardening (acutely bent) postures was built using computed tomography data, longitudinal pre-stretch and biaxially determined mechanical properties. Springs and dashpots were used to represent surrounding tissue forces associated with limb flexion-induced deformations. These forces were then used with age-specific longitudinal pre-stretch and mechanical properties to obtain deformed FPA configurations for seven age groups. Four commonly used invariant-based constitutive models were compared to determine the accuracy of capturing deformations and stresses in each age group. The four-fiber FPA model most accurately portrayed arterial behavior in all ages, but in subjects younger than 40 years, the performance of all constitutive formulations was similar. In older subjects, Demiray (Delfino) and classic two-fiber Holzapfel-Gasser-Ogden formulations were better than the Neo-Hookean model for predicting deformations due to limb flexion, but both significantly overestimated principal stresses compared to the FPA or Neo-Hookean models.

Abstract 121: Effects of Tethering Branches on Limb Flexion-induced Deformations of the Human Femoropopliteal Artery

Introduction: High failure rates of femoropopliteal artery (FPA) interventions are commonly attributed to severe mechanical deformations that occur with limb movement. Recent data demonstrate large variability in FPA bending and torsion. We hypothesize that FPA branch tethering significantly influences arterial deformations.

Methods: Under fluoroscopic guidance, nitinol markers were endovascularly deployed into 22 limbs of 11 human cadavers. Dilute contrast was injected into the FPA, and thin-section CT images were acquired with the limbs in straight (180°) and acutely bent (60°) postures. Image segmentation and 3D reconstruction were used to measure bending and torsion of the FPA by tracking spatial movement of the artery and markers. FPA bends were measured as radii of inscribed spheres, while torsion was calculated by measuring the angle of twist between each pair of consecutive markers in the straight and bent leg configurations. FPA branches were mapped and Pearson correlation r was used to assess whether bending and torsion values correlated with the distance to the nearest branch.

Results: At the adductor hiatus the FPA demonstrated a bending radius of 3.43±9.14 mm with 15.03±9.95 °/cm twist, while below the knee the most acute bending radius was 2.21±7.03 mm and the maximum twist was 32.03±10.68 °/cm. Bending and torsion below the knee were more severe than at the adductor hiatus (p<0.01). On average, large branches were observed 6.82± 8.26 mm in proximity to the bend and 5.29± 5.16 mm to the twist in the adductor hiatus and 7.96± 6.52 mm and 6.31± 6.06 mm to the bend and twist below the knee. There was no correlation ( r =-0.22, p=0.32) between bending and branch proximity at the adductor hiatus or in the below the knee arterial segments ( r =-0.11, p=0.30). Similarly, no correlation was observed between twist and branch proximity ( r =0.02, p=0.91 at the adductor hiatus; r =-0.02, p=0.87 below the knee).

Conclusions: Limb flexion-induced deformations of the FPA demonstrate severe bending and torsion at the adductor hiatus and below the knee; however, neither bending nor torsion appear to be affected by the proximity of the FPA branches. These data suggest that tethering branches have minimal effect on limb flexion-induced bending and torsion of the FPA.