Flaps

Unlike a graft a flap is transferred with its own essential blood supply or is connected to the recipient site vessels.

Although the skin graft is often simpler, there are cases in which a flap is required or may be more desirable. Flaps are usually needed for covering recipient beds that have poor vascularity so blood supply is the critical factor to select the proper type of flap.

Classification of skin flaps is related to three distinct groups2:

• Method of movement

Local or distant flap, according to the distance between the donor and the recipient site. Local flaps include advancement, pivot (often a graft is required to close the donor site) and interpolation (more complex). Distant flaps are divided into direct, tube or free. A free flap permits the immediate transfer of composite tissue supported by its own blood supply in a basis of detached blood vessels which are attached at the existing vessels at the recipient area of the wound.

Flaps are also divided according to the existing vessels, which are the musculocutaneous (major branches supplying muscle and dermal plexi) and septocutaneous arteries (supplying fascia and skin).

In the musculocutaneous flaps belong the random cutaneous (composed of skin, fat and the synonymous arteries) subdivided in: advancement, pivot, and interpolation flaps, and the myocytaneous ( plus the presence of muscle). The last ones have a vascular bed of greater length and credibility

In the septocutaneous belong the fasciocutaneous flaps (incorporation of deep fascia into a skin flap) and the axial or arterial flaps (nourishing by a direct cutaneous artery and vein) • Composition

A simple skin flap is usually efficient for the majority of defects but in complicated cases more tissues need to participate in the composition of the flap.

According to the involved tissues flaps are divided in: cutaneous, fasciocutaneous, myocutaneous, skin graft and muscle, and specialized flaps subdivided in sensory neurovascular, osseocutaneous, and composite flaps.

4. SKIN GRAFT SURVIVAL

Skin grafts, being avascular, are particularly susceptible to hypoxic insult. Graft survival depends on the recipient site blood supply.

When the skin graft is detached from the donor site its biological behavior is dramatically changed as its circulation, lymphatic drainage and nerve continuity are acutely interrupted.

Consequently the survival of the threatened graft is dependent on restoration of blood supply and the time till the reestablishment of circulation plays a critical role.

In the first 24-48 hrs post-transplantation the graft is nourished from the host bed via "plasmatic circulation" by serum imbibition.

Imbibition is supported by animal studies in which skin graft gained weight in the first 20 hours through fluid absorption1.

Despite histologic, biochemical, tissue chamber, microangiograhic and vital microscopy studies, revascularisation of skin grafts is still an issue of argument.

A combination of three processes is related to revascularisation of the graft3:

a) Direct anastomosis of the graft and host vessels known as "inoculation"

b) In-growth of host vessels into the endothelial channels of the graft and c) Random penetration of the host vessels into the graft dermis creating new endothelial channels.

Enzyme studies of markers for viable endothelium indicate degeneration within the first 4-5 days and then a restart of activity suggesting in-growth of vasculature stemmed from the host bed.

Updated interpretation is that early filling of the graft endothelial spaces with serum-like fluid and erythrocytes follows the anastomosis of graft vessels with host vessels, coupled with early in-growth and penetration of host endothelium.

During the imbibition period there is edema of the graft due to entrance of nourishing fluid. Progressively and in parallel with the reestablishment of circulation the edema is reduced.

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