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Friday, September 11, 2020

Venous Biomechanics of Angioplasty and Stent Placement: Implications of the Poisson Effect


Clinical question
Characterize the poisson effect following venous angioplasty and stents placement in veins.

Take-away point
There is a proportional longitudinal foreshortening and narrowing of adjacent venous segments with radial expansion of venous angioplasty and stents, which is more dramatic in veins than arteries.

Reference
Li N., Mendoza F., Rugonyi S., Farsad K., Kaufman J.A., Jahangiri Y., et al. Venous Biomechanics of Angioplasty and Stent Placement: Implications of the Poisson Effect. J Vasc Interv Radiol. 2020 Aug; 31 (8): 1348-1356.

Click here for abstract

Study design
Prospective in Vivo swine model Testing with Real-Time Venous Wall Motion Monitoring and finite-element computational vascular model analysis

Funding source
No reported funding

Setting
Academic setting. Oregon Health and Science University, Portland, Oregon, USA







Figure
A visual synopsis of the 2 study parts and summary of results.

Summary


Venous angioplasty and stent placement for treating venous outflow obstruction has demonstrated improved flow with clinical symptom relief. However, adjacent segment narrowing after stent placement, based on the poisson effect, may carry a negative impact on the quality of venous flow. Research on venous biomechanics for venous-specific stent designs is lacking. The author performed a prospective 3 adult swine in vivo testing with real-time venous wall motion monitoring and finite-element computational vascular model analysis to characterize the poisson effect during angioplasty and stent placement. Statistical analysis including Welch t test, analysis of variance, Pearson correlation, linear regression, and polynomial curve fit were performed using Prism software.

1- Real-Time Venous Wall Motion Monitoring:

Study veins were accessed and baseline venography was obtained followed by RF ablation induced stenosis and repeat venography. Balloon angioplasty, with diameter matching the naive vein(8mm), was performed on the in the stenotic regions of one side. Bare metal stents oversized by 50% (12mm) were deployed in the contralateral stenotic region. Iron particles were injected on the venous endothelium proximal to the intended vessel segment and categorized into near group (mid-point to balloon) and far group (peripheral to angioplasty area). Neodymium magnet was placed externally for iron attraction allowing real-time visualization of the vessel wall under fluoroscopy.

· During balloon angioplasty, the mean venous stenosis decreased by 43.7% ± 2.1. There was significant longitudinal displacement of the iron particles toward the balloon midpoint(1.5% ± 0.2 of the balloon length P <.001) affecting the far group more (2.2%) compared to near group (0.9%)(t test, P< .001). The adjacent luminal collapse correlated with diameter increase of the angioplasty-treated segment (relative collapse % . 0.772 * [relative dilatation %] . 0.255; R2 . 0.8920.

· During bare metal stent deployment the mean venous stenosis reduced to 20.8% ± 0.8, with further reduction to 1.6% ± 1.2 with balloon angioplasty of the stent. There was significant longitudinal displacement toward the midpoint of the balloon(2.0% ± 0.5 of the balloon length P< .01). The longitudinal displacement increased with distance to the midpoint of the balloon in a direct linear relationship (R2 . 0.87).

2- Finite-Element Computational Vascular Model (using ADINA software)

Several parameters were obtained from the literature to simulate a healthy common iliac vein and artery. During angioplasty simulation, the reduction of the adjacent-segment diameter had a significant correlation to the degree of dilation of the treated segment. Angioplasty simulation using arterial wall biomechanical properties showed significantly less adjacent segment narrowing compared with the model using venous properties. Variations of Young moduli in the longitudinal direction had a positive relationship with the amount of adjacent segment collapse, while with circumferential direction had the opposite effect and with the radial direction had no significant impact.

The findings explain the decrease of external iliac vein diameter after stent placement in the common iliac vein. Such phenomenon is due to the nonlinear elastic property of veins secondary to the nonaligned collagen fibers in the predominant adventitia. In contrast, arteries have coaligned collagen fibers in the dominant media layer. Previous and current study showed that veins are more extensible in the circumferential direction than in the longitudinal direction since they have a much higher longitudinal young modulus and a higher longitudinal vs circumferential Poisson ratio (opposite to arteries). With radial outward force applied to venous segments, there is a significant redistribution of forces and venous tissue, resulting in a pronounced Poisson effect.

Commentary


The authors in this study captured an important consequence of venous angioplasty and stent placement. They characterized its dramatic Poisson effect in veins and showed the resulting longitudinal foreshortening and adjacent segment collapse. Although the study has limitations including the small sample size, the possible collagen contraction after RF ablation and the lack of survival period to examine chronic changes. The study can still provide innovative solutions on vein-specific flow optimization. Current stent designs place heavy emphases on maximizing radial resistive force and chronic outward force, which may be counterproductive in venous interventions. Therefore applying and examining this study phenomenon on novel innovative venous stents, with emphasis on overcoming force and tissue redistribution related to the Poisson effect, might maximize venous flow leading to better patient outcomes.

Post Author
Tarig Elhakim, MD
PGY-2 Internal Medicine Residency,
Kendall Regional Medical Center


@TarigElhakim

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