TEVAR

Thoracic Endografts and Future GoalsVahe VarzhapetyanResearch InternDr. Khoynezhad LabHeart Institute Cedars Sinai Medical Center

Content

The Thoracic Aorta

Endograft Design

Frontiers

Thoracic Endografts TEVAR approved in 2005 for

descending aortic aneurysm repair.

It is important to understand endograft design and possible advancements as strides are being made for use in aortic dissection, in supraaortic pathologies and thoracoabdominal disease.

Lower perioperative mortality, neurological injury, hospital stay compared to open repair

Midterm and long term outcomes have yet to be determined

Challenges in the Thoracic Aorta

Pulsatile turbulent blood flow

Respiratory movement

Curved anatomy

Diverse pathologies (retrograde and antegrade dissections)

Larger lumen

Distance from access point



High tension in the thoracic wall aorta. Tension=(Pressure) x (radius).

In the disease state the scattered loss of elasticity throughout the length of the aorta produces a complex change in the vessel stiffness.

The Pressure-Strain Modulus, a measure of stiffness

is utilized in studies of computational analysis and fluid dynamics. These are areas of active research to improve endograft design.

(Ps-Pd)Dd P=Pressure(Ds-Dd) D=Diameter s=systolic d=diastolic

Endograft Design

Ease of Deployment

Exclude the lesion with a good seal and fix

Durable

Conformatible

Biocompatible

Endograft Design

Nitinol stent frame

Bare metal stent

Polyester fabric graft

Endograft Design (Considerations for good seal and fixation)

Sufficient landing zone with consideration for thrombus and calcifications.

Optimal radial pressure of the stent frame is essential to provide good apposition to the aortic wall with minimal trauma.

Radial pressure determined by stent frame geometry, length, spacing, wire material and oversizing.

Endograft Design (Considerations for good seal and fixation)

Proximal bare metal stent provides good anchorage and fixation with allowance to branching vessels. (extends the landing zone)

Staples, screws, clips, balloon angioplasty, Bowden cable and fibrin glue may be utilized

The use of bars in the stent frame design and stiffer fabric in the graft increases longitudinal stiffness decreasing the risk of migration

Endograft Design (Considerations for conformability and durability)

Stent frame spacing, wire and graft material are major determinates of conformability.

Nitinol is a nickel and titanium alloy. It is super elastic (allows for crimping) and has shape memory. Shape memory allow the stent to expand in the artery during deployment when the sheath is removed and the stent is exposed to the temperature in the vessel.

Biocompatible and resistant to fatigue

Endograft Design (Considerations for conformability and durability)

Graft covering composed of either polyester or expanded polytetrafluoroethylene ePTFE (Gor-tex). They are flexible, conformable, lightweight, biocompatible with low porosity.

Manufacturing techniques affect the filament and weave pattern which in turn determine conformability and durability.

Endograft Design (Considerations for deployment)

High torque adjustable catheters with hydrophilic coverings improve accessibility.

Different deployment methods are utilized to maintain accuracy.

Gore TAG addresses this issue by starting the deployment in the middle of the graft followed by inflation of a balloon that results in apposition to the aortic wall.

Valiant Captiva system uses a two step process. First the graft is deployed with the bare metal stents closed. Then after adjustment the bare metals are deployed to make the seal

Frontiers

Hybrid procedures, chimney technique, fenestrated devices allow the landing zone to extend and to treat the ascending aorta.

Developing tapered endografts utilizing the clinically measured aortic taper ratio will allow better fixation especially in dissection.

New materials are under investigation to decrease the profile of the graft while maintaining the same durability.

Frontiers The Cardiatis Multilayer utilizes a bare metal stent

composed of braided cobalt alloy and currently is in clinical trails.

It reduces flow velocity and increases laminar flow in the aneurysm.

Allows laminar flow in branched vessels

Frontiers

Computation analysis of flow velocity in thoracic aorta

Same analysis after Cardiatis stent placement

Frontiers

Flow preserved in branched vessels

References

1.Greenberg RK, Lu Q, Roselli EE et al. Contemporary analysis ofdescending thoracic and thoracoabdominal aneurysm repair: a comparison of endovascular and open techniques.Circula-tion 2008;19;118(8):808 – 817

2. Jenna M. Weidman, Malhar Desai, Arif Iftekhar, Kevin Boyle, Judith S. Greengard,bLois M. Fisher, Richard L.S. Thomas, Simona Zannett. Engineering Goals for Future Thoracic Endografts — How CanWe Make Them More Effective? Progresses in Cardiovascular Diseases 56 (2013) 92-102

3. Desai ND, Burtch K, Moser W, et al. Long-term comparison ofthoracic endovascular aortic repair (TEVAR) to open surgery

for the treatment of thoracic aortic aneurysms. J Thorac Cardiovasc Surg. 2012;144(3):604-609.

4. Canaud L, Alric P, Laurent M, et al. Proximal fixation ofthoracic stent-grafts as a function of oversizing and increas-ing aortic arch angulation in human cadaveric aortas.J Endovasc Ther.2008;15(3):326-334.

5. Humphrey JD, Holzapfel GA. Mechanics, mechanobiology, andmodeling of human abdominal aorta and aneurysms.J Biomech.2012;45(5):805-814.

6. Cheng D, Martin J, Shennib H, et al. Endovascular aorticrepair versus open surgical repair for descending thoracic aorticdisease a systematic review and meta-analysis of comparativestudies.

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