The underlying principle of all surgical flaps is the ability to maintain a viable blood supply upon transfer of flap tissue from a donor website to a recipient website. Given this fundamental capacity to retain vascular circulation, surgical flaps might be classified in numerous methods. One approach is by composition, as a flap might be made up of many various kinds of tissue.
Another is by vascularity, and several different schemata have been developed to categorize flaps by the kind of vascular supply. A third manner of categorizing flaps is by method of movement, and it's essential to understand the basic techniques of flap transfer.
Unlike a graft, which is wholly dependent upon the recipient bed to offer blood supply, a flap by definition is able to preserve its own vascular supply for survival. Thus, regardless of whether classifying a flap by composition, vascularity or method of movement, the core principle essential to all flaps is how to maintain blood supply so that the flap tissue will remain robust after transfer to its new site.
The most basic way to think about a flap is to think about what tissues are contained within it. A flap may contain skin, fascia, muscle, bone or various combinations of these tissues. As the underlying principle of any flap is its ability to retain its own blood supply, the amount of tissue that might be carried within it is dictated by the minimum or maximum quantity of tissue that can be transferred with intact vascularity. When more than one kind of tissue is contained within a flap, it is called a "composite flap."
The simplest type of flap is the skin flap. The blood supply of the skin is contained largely in the dermal and subdermal plexus and derives from two main sources: a musculocutaneous vascular system along with a direct cutaneous vascular system. When the blood supply to the skin is via a named artery, the skin flap is known as an "axial flap."
When the blood supply to the skin lacks a substantial pattern in its vascular design, the skin flap is known as a "random flap." Either way, the survival of a cutaneous flap depends on the number and kind of blood vessels at the base of the flap. For an axial flap, the survival pattern of the flap is based on the length of the underlying feeding artery.
For a random pattern flap, the length and width should be created in a 2:1 ratio, as a wider base width increases the chance that a large vessel will be incorporated to provide an adequate blood supply to the enclosed dermal-subdermal plexus. Even in an axial flap, the distal borders of the flap are also random pattern with distal perfusion from the dermal-subdermal plexus.
Skin flaps might also be transferred based on the vascular plexus of the deep fascia, in which case they're termed "fasciocutaneous flaps." The blood supply of the deep fascia is derived from perforating vessels of regional arteries that pass along the fibrous septa of muscle bellies or muscle compartments. Including the deep fascia along with the skin avoids tedious dissection and might also preserve adjacent subfascial arteries.
Among the advantages of fasciocutaneous flaps in reconstructive surgery are ease of elevation and transfer, decreased bulk, good reliability, and decreased functional morbidity at the donor website. Depending on the size of the skin paddle, however, the secondary defect at the donor site may need coverage with a split-thickness skin graft.
Progressing one layer deeper still, an additional typical flap in reconstructive surgery is the "myocutaneous" or "musculocutaneous" flap, which combines muscle, skin, and the intervening fascia and subcutaneous tissue. Supplied by one or more dominant vascular pedicle within the muscle instead of a direct cutaneous arterial source, the essential feature of a myocutaneous flap is that the underlying muscle "carries" the blood supply for the overlying skin. Myocutaneous flaps have two key advantages.
First, the increased bulk better allows it to fill dead space. Secondly, myocutaneous flaps are also more resistant to bacterial infection than fasciocutaneous flaps by a factor of 100. This makes them extremely dependable and useful, particularly when increased bulk is needed having a robust arterial supply to fill a defect that has been subjected to chronic infection. If a skin paddle is not needed, muscle may also be transferred alone, without the overlying fascial and cutaneous tissue.
A final kind of tissue commonly incorporated into a flap is bone. When taken with the overlying skin, this is known as an "osseocutaneous flap." A dominant vascular pedicle with perforating branches supplies the skin and periosteum. Generally taken as a free flap, the bone is harvested with a cuff of muscle and/or skin to reconstruct a skeletal framework with soft tissue. The long bones of the extremities, such as the fibula, are frequently used as they provide more length for shaping according to the needed need.
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