Biomedical Engineering Reference
In-Depth Information
TABLE 9.4
Classification of Vascular Prostheses
Prosthesis
Comments
Surgically implanted biological grafts
Autograt
Graft transplanted from part of a patient's body to another
Example: saphenous vein graft for peripheral bypass
Allograft
Homograft. Transplanted vascular graft tissues derived
from the same species as recipient. Example:
glutaraldehyde-treated umbilical cord vein graft
Xenograt
Heterograft. Surgical graft of vascular tissues derived from
one species to a recipient of another species. Example:
modified bovine heterograft
Surgically implanted synthetic grafts
Dacron (polyethylene terephthalate)
Woven, knitted
PTFE (polytetrafluoroethylene)
Expanded, knitted
Other
Nylon, polyurethane
saphenous vein graft from the same patient. Vein grafts have a failure rate of about 20% in 1 year and up
to 30% in 5 years after implantation. Vein grafts from the same patients are also unavailable or unsuit-
able in about 10-30% of the patients (Abbott and Bouchier-Hayes, 1978). Modified
bovine heterograft
and glutaraldehyde-treated
umbilical cord vein grafts
have also been employed as vascular prostheses
with less success compared to autologous vein grafts.
9.1.4.2 Surgically Implanted Synthetic Grafts
Prostheses made of synthetic material for vascular replacement have been used for over 40 years.
Polymeric materials currently used as implants include
nylon
, polyester,
PTFE
, polypropylene, polyacry-
lonitrile, and silicone rubber (Park and Lakes, 1992). However, Dacron (polyethylene terephthalate) and
PTFE are the more common vascular prosthesis materials currently available. These materials exhibit
the essential qualities for implants—they are biocompatible, resilient, flexible, durable, and resistant to
sterilization and biodegradation. Detailed discussion on the properties, manufacturing techniques, and
testing of Dacron prostheses is included in Guidoin and Couture (1992). Figure 9.9a depicts a Dacron
vascular graft having a bifurcated configuration. Figure 9.9b shows expanded PTFE vascular grafts hav-
ing a variety of configurations and sizes: straight, straight with external reinforcement rings (to resist
external compression), and bifurcated.
Synthetic vascular grafts implanted as large-vessel replacements have resulted in reasonable degrees
of success. However, in medium- and small-diameter prostheses (less than 6 mm in diameter), loss
of
patency
within several months after implantation is more acute. Graft failure due to thrombosis or
intimal hyperplasia with thrombosis is primarily responsible in failures within 30 days after implanta-
tion, and intimal hyperplasia formation is the reason for failure within 6 months after surgery. Soon
after implantation, a layer of
fibrin
and fibrous tissue covers the intimal and outer surface of the pros-
thesis, respectively. A layer of
fibroblasts
replaces the fibrin and is referred to as
neointimal
. In the later
stages,
neointimal hyperplasia
formation occurs and ultimately results in the occlusion of the vessels in
small-diameter vascular grafts. Attempts are being made currently in suitably modifying the surface
characteristics of the prostheses in order to reduce the problems with loss of patency. Studies are also
being performed in order to understand the mechanical stresses induced at the anastomotic region,
which may result in deposits on the intimal surface and occlusion of the vessels (Chandran and Kim,
1994). The alterations in mechanical stresses with the implantation of vascular prostheses in the arterial
circulation may include changes in the deformation and stress concentrations at the anastomotic site.
Altered fluid shear stresses at the intimal surface in the vicinity of the anastomosis has also been sug-
gested as important particularly since the loss of patency is present more often at the distal anastomosis.
