Vascular Prostheses and Grafts

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Vascular Prostheses and Grafts

• Paul Binghay

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Background

• Hemostasis(normal blood clotting)
• Thrombosis(irregular blood clotting)
• Atherosclerosis(hardening of the arteries)

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Hemostasis

• Dependent factors
• Vascular Wall
• Platelets
• Coagulation System

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Vascular Wall

• Endothelium
• Subendothelial connective tissue

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Platelets

• 3 important reactions when exposed to
  extracellular matrix
• Adhesion and shape change
• Secretion
• Aggregation

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Adhesion

• Attachment of platelets to sites of endothelial
  injury

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Secretion

• Release of platelet granules(Alpha and dense
  granules)
• Alpha granules contain fibrinogen, fibronectin,
  factor V, factor VIII, platelet favtor 4,
  platelet derived growth factor(PDGF) and
  transforming growth factor-beta(TGI-b)
• Electron dense granules store ADP and ATP,
  ionized calcium, histamine, serotonin and
  epinephrine

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Platelet Aggregation

• The combination of thrombin, ADP, and
  thromboxane(TxA2) induces platelet-to-platelet
  adherence
• Platelet aggregation (ADP and TxA2 react)
  primary (temporary)hemostatic plug
• Platelet contraction (thrombin reacts) secondary
  (definitive)hemostatic plug

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Coagulation System

• Transformation of proenzymes to activated enzymes
  that form thrombin,which coverts
  fibrinogen(soluble) to the protein
  fibrin(insoluble)
• Blood Clotting Cascade

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Thromobosis

• Three major influences
• Injury to endothelium
• Alterations in normal blood flow
• Alterations in the blood(hypercoagulability)

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Atherosclerosis

• Arteries narrowed by deposits of material
• Reduced blood into the tissue and organs
• Factors which can result in artherosclerosis
• Smoking
• High blood pressure
• Diabetes

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Applications

• Vascular stent/ angioplasty
• Vascular prostheses/bypass

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Vascular Stents

• Small metallic devices used to keep blood vessels
  open following balloon angioplasty
• 2 types available for use
• balloon expandable stent over an angioplasty
  balloon
• self expanding stent over a delivery system
• Permanent placement in blood vessel
• Material used
• Medical grade stainless steel
• Nitinol (NiTi) shaped memory alloy

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Balloon Expandable Stent

• Over an angioplasty balloon
• Palmaz Stent Balloon expandable stainless steel
  stent most commonly used for tandem short segment
  stenotic lesions. This is the first and oldest
  stent available. 
• Perflex Stent Long Balloon expandable stent
• AVE stentBalloon expandable
• Intrastent Balloon expandable
• Herculink Balloon expandable stent on a very low
  profile delivery

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Self Expanding Stents

• Over a delivery system
• Wallstent Self expanding stainless stent used
  for long lesion and available up to 24 mm
  diameter.. The stent could be use in variety of
  situation due to its flexible nature.
• Mammotherm stent self expanding Nitinol stent
• Symphony stent self expanding Nitinol stent
• Smart stent self expanding Nitinol stent
• Instent coil like stent

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Nitinol

• Special alloy of nickel and titanium (55.6 Ni)
  chemically bonded
• Excellent biocompatibility
• High corrosion resistance
• Properties of thermal memory
• Shaped memory effect (when heated it contracts
  and reforms back to original shape)
• More expansion and contraction than standard
  metals (100x)
• Super-elastic properties
• spring/rubber like behavior

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Vascular bypass
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Vascular prostheses and grafts

• Natural grafts
• Autologous veins and arteries
• Chosen for bypasses from the groin down to lower
  leg, and in the arms
• Synthetic grafts
• Dacron (PET), Gore-Tex (PTFE)
• Chosen for bypasses in the shoulder and neck,
  abdomen and to repair aneurysms

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Natural grafts

• Venous autografts (1949)
• Patients own calf vein (ASV autologous saphenous
  vein)
• Arterial xenografts (1950s)
• Rarely used now because of high incidence of
  thrombosis and rupture

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Synthetic grafts

• Knitted
• Filament running both longitudinally and
  circumferentially
• Strongest but low porosity
• Woven
• Looping of yarns around a thread
• Not as strong but porous and easier to suture

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Dacron (PET)

• Polyethylene Terephthalate
• Used in the 1960s,mainly used for drapes at time
• Properties
• Hard,stiff, stable and hydrophobic
• Still preferred for larger diameter (gt4mm) grafts

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Gore-Tex (PTFE)

• Polytetrafluoroethylene (expanded Teflon)
• Discovered in 1938,by DuPont (Teflon)
• Stretched Teflon to create a strong porous
  material in 1969, (Gore-Tex)
• Properties
• Good biocompatibility and durable
• Hydrophobic and air permeable
• Low friction coefficient

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Risks

• Natural grafts
• Difficult to preserve
• Atherosclerosis
• Synthetic grafts
• Thrombosis
• Turbulence(clots form from static blood flow)

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Future

• Hybrid grafts
• Layer of Endothelial cells to prevent thrombosis
  and clotting