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Soft tissue coverage of the hand presents an
ongoing challenge to the reconstructive surgeon. Skin grafts are not always
appropriate, local flaps are not always available, and distant pedicle flaps are
often too morbid. When conservative approaches are not available or have failed
microvascular transplant of an arterialized venous "flow-through" flap
(VFTF) provides a unique and creative option for difficult reconstructions of
the hand.
Musculocutaneous flaps are appropriate for covering
larger soft tissue defects but can be bulky and restrictive when applied to the
wrist, hand, and digits. Fasciocutaneous flaps are thin and pliable. However,
their transplant not only requires sacrifice of a peripheral artery, but can
lead to significant vessel mismatch when anastomosed to distal vessels. In
general, selection of the best flap for a particular reconstruction balances
optimizing form and function at the recipient site with minimizing the morbidity
at the donor site.
VFTFs are thin and pliable with vessels similar in
size to those of the hand. Their harvest results in minimal donor site
morbidity. These flaps are composed of skin, subcutaneous tissue, and a plexus
of veins. Unlike conventional flaps where the nutrient capillary beds are
supplied by an inflow artery and drained by an outflow vein, the VFTF has no
arterial inflow circuit. All flow proceeds into and out of the flap via the
venous plexus.
The exact physiology of VFTF survival has not been
determined. Three theories have been prosed:
§ Reverse Shunting
§ Reverse Flow
§ Capillary Bypass
Reverse shunting depends on primitive arteriovenous
channels or connections which are able to contract or dilate in response to
nervous and chemical stimuli. With denervation, these channels allow retrograde
flow from the veins to the arterioles and then antegrade flow through the
nutrient capillary beds and out through the veins.
Reverse flow depends on retrograde flow from the
veins through the nutrient capillary bed into the arterioles and then antegrade
flow back out the veins via A-V shunts or other capillary beds.
Both of these theories depend upon some antegrade
flow through the nutrient capillary beds. The capillary bypass theory discounts
the need for the capillary bed. It relies studies, which confirm that normal
tissue can extract up to 50% of the oxygen content of blood prior to it reaching
the capillary beds. It contends that flaps like the VFTF survive on the lower
oxygen content supplied by antegrade flow through the venous plexus. Regardless
of which physiologic mechanism is responsible, sufficient nutrition is available
to keep the flap alive until peripheral neovascularization occurs restoring
conventional antegrade physiology. With the correct flap design, in selected
patients, VFTFs are reliable.
The physiology of the VFTF places unique
restrictions on its design. The tissue farthest away from the venous plexus is
prone to congestion and necrosis. Larger flaps require a more extensive venous
plexus for complete survival. Studies have shown that VFTFs designed with a
central venous plexus with two or more efferent veins have a survival pattern
similar to that of a conventional flaps which have an inflow artery and outflow
vein.
A fine network of veins extending throughout the
flap is not the only factor in the VFTF's survival. Characteristics of the donor
site also play a role. There are several potential VFTF donor sites:
§ Distal Volar Forearm
§ Proximal Volar Forearm
§ Dorsum Digit/Hand
§ Dorsal Foot
§ Medial Thigh/Leg
§ Upper Arm
The superficial venous system located distally on
the extremity is less likely to have valves, has more extensive networking, and
is more intimately associated with and supportive of its overlying skin. This
improves the success rate making the hand, foot, and distal volar forearm
preferred donor sites for VFTFs. When a larger flap is required the proximal
forearm is the next best option. Direct visualization of the venous plexus
through the thin skin of the distal extremities allows precise design of the
VFTFs. The flap can not only be centered over the most appropriate plexus, but
creative inflow and outflow circuits can also be designed in the branching
venous system. The donor sites of small and moderate sized flaps can usually be
closed primarily.
VFTFs harvested from the leg and upper arm are
nourished by the saphenous and basilic vein respectively. These flaps are useful
when long vascular conduits or a larger soft tissue paddle is required. These
flaps are associated with increased subcutaneous tissue between the nourishing
vein and overlying skin. The smaller venous systems cannot be visualized and
their extent cannot be determined at the time of flap design. These flaps are
usually designed over the main vein. Their maximum width is resticted to insure
optimal survival.
Another limiting factor in the survival and success
of VFTFs is the recipient bed. Areas with ongoing infection can prolong healing
time and delay neovascularization of the VFTF. This can prolong the time the
flap has to rely on venous physiology and potentially decrease flap survival. In
addition, factors such as infection can result in activation of platelets and
increase thrombosis potential.
Classification of the VFTF is based on the vascular
"hook-up.
§ Arterialized Venous Flap
o A-V-A
o A-V-V
§ Total Venous Perfusion
o V-V-V
The VFTF placed between two arteries in an A-V-A fashion functionally
reconstructs that artery. The VFTF placed between an artery and vein (A-V-V)
functionally creates an A-V fistula.
The A-V-A orientation is useful in difficult
replantations where there is a soft tissue deficit and vessel injury resulting
in devascularization of the distal tissue. The blood, which flows through the
flap not only nourishes the flap but also revascularizes the replanted tissues.
Ring avulsion amputations and devascularizations are good examples where VFTFs
are particularly indicated. The crush component necessitates soft tissue
replacement, and the avulsion component necessitates vessel replacement. A VFTF
can resurface the tissue defect while revascularizing the digit at the sametime.
The A-V-V orientation is particularly useful in
fingertip resurfacing. Digital arteries taper and dorsal vein are just emerging
at the fingertip making them poor targets for anastomoses. In situations such as
soft tissue loss at the fingertip, additional vein length proximal and distal to
the cutaneous portion of the flap can be harvested. This allows the arterial and
venous anastomoses to be performed proximally where vessel size match is better.
This strategy also allows the anastomoses to be performed out of the zone of
injury.
The VFTF placed between two veins (V-V-V) can be
employed to fill soft tissue defects and cover exposed tendon on the dorsum of
the finger. Since veins supply the flap's inflow less perfusion pressure and
less oxygen content are delivered to the flap. This further restricts its
maximum size to less than that of an arterialized VFTF.
VFTFs are pale after transplantation. This usually
lasts for several hours. Viability and flow through the system can be monitored
by palpation of its pulse or Doppler evaluation. After several hours the flap
regains capillary refill. This pattern of delay prior to capillary refill is
related to opening of the arterial-venous shunts triggered by ischemia in the
overlying skin. Over the next days to weeks the flap will appear congested and
ecchymotic. Evaluation by capillary refill is obscured. Palpation of the pulse
or Doppler evaluation is used to confirm viability. After about 2 weeks
congestion resolves and superficial epidermalysis is removed uncovering pink
healthy tissue.
The VFTF can be harvested with additional tissues
creating a composite VFTF. This composite flap is more difficult because success
is dependent on spatial distribution of multiple components rather than just
inflow and outflow circuits. Sensory nerves such as the brachial cutaneous and
saphenous nerves have been included to create a sensate flap or to graft nerve
defects at the recipient site. Tendon has been included to reconstruct tendons,
ligaments, and joints capsules. One author has reported inclusion of tibial bone
with a saphenous venous flap for reconstruction of soft tissue and bony defects
in the hand. Theoretically, these are vascular grafts. Composite VFTFs have been
reported sporatically but no series has evaluated the efficacy of their
components as vascularized grafts.
The VFTF does not replace conventional flaps, which
utilize local tissue. It offers an excellent option when adequate adjacent
tissue sources like cross-finger flaps are not available:
§ Injury to multiple digits
§ Unlar hand injuries
§ Radial hand injuries
§ Defects greater in size than the cross-finger flap
§ Thumb injuries
§ Patient will not accept additional scars on adjacent digits
Diffuse injuries to the hand can limit the use of
pedicled flaps. Reduced mobility secondary to joint injuries or swelling can
make thenar and hypothenar flaps technically impossible. A pedicled groin flap
can cause unnecessary swelling secondary to dependency, and unnecessary
disability secondary to prolonged time to rehabilitation.
When compared to other flaps the VFTF has several
advantages:
§ Revascularized and resurface
§ Single stage procedure
§ Thin/pliable tissue
§ Low donor morbidity
§ Spares donor artery
§ Good cosmetic result
§ Can include composite tissue
VFTFs also have disadvantages:
§ Requires microvascular techniques
§ Requires a two person team
§ Longer hospitalization
Although size constraints limit its use in general reconstructions throughout
the body, it has unique clinical efficacy in small and moderate sized defects.
In selected cases, it represents an excellent option available to the creative
microsurgeon when planning complex reconstructions of the hand.
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