Local or Subdermal Plexus Flaps

Chapter 78

Local or Subdermal Plexus Flaps

Subdermal plexus flaps constitute a large and readily available group of skin flaps used to close acute and chronic wounds in small animals. They are full-thickness “tongues” of skin that are generally detached from the surrounding skin along three of four quadrants and then stretched or rotated to fill a nearby defect. Their survival relies on collateral circulation from the remaining cutaneous attachment and its vasculature, the subdermal plexus.9 Successful use is dependent on sound decision making, which should be based on an understanding of cutaneous physiology and factors affecting wound healing and on evaluation of local skin elasticity, mobility, and lines of tension. Lines of tension (see Figure 77-5) in dogs and cats have been identified;9 however, the surgeon should spend some time manipulating the local skin in each patient to determine the most appropriate way to harvest and move a skin flap without an excessive amount of stretching.

Anatomy and Physiology

The skin comprises approximately 12% of the total body weight in a 6-month-old dog. Hairy skin ranges in thickness from 0.5 to 5 mm, depending on location. The thickest skin occurs on the nose.1 Although the skin itself consists simply of the epidermis and dermis (corium), skin flaps also usually include subcutaneous tissue and portions of the thin subcutaneous muscles.

Cutaneous Circulation

The skin has a segmental arterial supply from simple and mixed cutaneous arteries that run between or through muscles, respectively, to arborize in three distinct plexuses that run parallel to the skin surface (Figure 78-1). These plexuses include the subcutaneous (deep) plexus, the cutaneous (middle) plexus, and the subpapillary (superficial) plexus. There are extensive connections between these vascular elements, providing excellent collateral flow to adjacent areas of skin.1,16 Traditional subdermal plexus flaps, which include skin and subcutaneous tissue, receive their blood supply from collateral connections to the subcutaneous plexus. Composite flaps may either be based on the subdermal plexus or on a direct cutaneous artery.

Skin Elasticity

The skin is richly supplied with collagen and elastin fibers that impart strength and compliance. Collagen accounts for 90% of the fiber content.1 The skin’s properties of mobility, elasticity, and rich blood supply facilitate its use in formation of flaps to cover adjacent defects. Releasing incisions, undermining, and pre- and intraoperative skin stretching all contribute to successful mobilization. Assuming there is no underlying disease, such as neoplasia or infection, the most critical issues associated with prompt healing of a defect closed with a skin flap are preservation of blood supply, closure with minimal tension, and appropriate systemic nutritional status.

Delay Phenomenon

The “delay phenomenon” encompasses a group of mechanisms by which flap survival is enhanced through staged flap development. Flaps may be physiologically “trained” to rely on vascular support from their pedicle by gradually restricting blood supply from other sources. Practically speaking, this might include incision and suture apposition of proposed flap borders without subcutaneous elevation, partial division of the pedicle of a flap that has developed some vascular supply from underlying tissues, or temporary occlusion of one pedicle of a bipedicle flap. Tissue expansion is another form of delay, providing additional skin surface area that contains an adequate vascular supply.

Extensive research in experimental animals shows that perfusion decreases to 10% and 40% of normal after initial elevation of single pedicle and bipedicle flaps, respectively.5 In delayed flaps, circulation then rises to 120% to 150% of normal after approximately 3 weeks. Re-elevation of a flap after a 3-week delay results in a much smaller decrease in blood flow (to 90 % of normal), resulting in improved survival rate. Staged development of flaps therefore helps offset the negative impacts of flap rotation and stretching.

Major factors contributing to the delay phenomenon are alteration in sympathetic tone, dilation of choke vessels, reorientation of vessels, early and late changes in tissue metabolism, and neovascularization.5,19 Immediately after surgery, the elevated flap is in a hyperadrenergic state, leading to vasoconstriction. After a delay, blood supply is preserved with subsequent flap elevation because of vasodilation secondary to norepinephrine depletion. Additionally, the number and size of blood vessels to the flap increase, and their orientation is changed to favor vessels parallel to the long axis of the flap. In particular, the “choke vessels” that link adjacent vascular territories increase in size and number.

Delay may also act as a form of ischemic preconditioning, whereby resistance of a tissue or organ to ischemia increases after short episodes of vascular occlusion. Ischemic preconditioning has been proven effective in the heart, liver, kidney, and free and pedicled skin flaps. Late effects on tissue metabolism are also seen, including decreased production of prostaglandin F, which normally causes vasoconstriction in acutely elevated flaps. In contrast, levels of prostaglandin E2 increase preferentially after elevation of a delayed flap, facilitating vasodilation.

Recent work on neovascularization of ischemic myocardium and limbs has cast light on other mechanisms that might also be important in the delay phenomenon. Revascularization occurs as a result of angiogenesis (growth of microvessels from the existing capillary bed) and vasculogenesis (development of new vessels in situ from bone marrow–derived endothelial progenitor cells). Investigation in a delay or vascular ischemia mouse model showed an increase in bone marrow–derived progenitor cell numbers and evidence of functional vessels produced by vasculogenesis by day 21.5 This finding supports clinical recommendations that at least 2 weeks is required for optimal vascular delay. The need for, and advantage gained by, delay is dependent on initial vascularity of the flap. Flaps with adequate circulation do not benefit from delay, but ischemic flaps potentially do. Pavletic15 describes a practical approach to delay of tubed flaps in dogs based on the available literature and taking into account that extrapolation between species may not be valid. He recommends a total 3-week delay. At 18 days, half of the pedicle is divided, and the remainder of the pedicle is severed 3 days later.

Patient Preparation

The patient should be carefully evaluated to ensure the greatest chance of flap survival and timely wound healing. Systemic illnesses known to delay wound healing should be ruled out, or steps taken to treat them, before the procedure. A catabolic state secondary to poor nutrition is a common problem in animals with chronic, open wounds and other unresolved diseases and can be a major impediment to successful reconstructive surgery. Thought should be given to ensuring patients are in positive nutritional balance before surgery, and if appetite or ability to eat is anticipated to be a problem after surgery, provisions should be made for enteral or parenteral feeding.

Local preparation of the recipient site should be undertaken to ensure that the exposed tissue is free of infection or contamination with inorganic material. Necrotic tissue should be removed. Granulation tissue should be well vascularized and areas of dense, mature connective tissue debrided to ensure good vascularity. Margins of chronic open wounds may be excised to remove thin layers of fragile epithelium that would otherwise interfere with cosmetic result. A healthy bed onto which to place the flap may be as important as the health of the skin edges itself because prompt healing onto subcutaneous tissue provides additional blood supply and reduces tension on skin edges.

The patient should be clipped and prepped widely and positioned for surgery in a way that allows access to mobile skin around the defect and does not create undue tension on the skin to be manipulated. Similarly, the area should be draped widely to allow for a variety of contingencies. The surgical plan may change when the procedure is underway, and the need for a different type of flap or multiple flaps might become evident. Close communication with staff performing the surgical preparation is essential because the surgical plan may not be intuitively obvious to them.

Guidelines for Flap Development

Subdermal plexus flaps are used when vascularized skin in the local vicinity of the defect is sufficient but strategic division of its attachments is required to enable movement toward the defect from areas with loose skin. Relaxing incisions are included in the discussion of subdermal plexus flaps because they essentially produce a bipedicle subdermal plexus flap. The aim of a flap is to transfer tension from edges of the original wound to fresh wound edges created by relocation of donor skin. Healing of a wound closed with a subdermal flap may be more reliable than primary closure of wound edges because the flap is not affected by local disease, has not been subjected to previous surgical interference, and has a normal blood supply.

Closure of each wound must be planned individually because issues associated with wound healing and flap development are unique to each patient. In some instances, abundant loose skin is present in the immediate vicinity, making the decisions easy. In other cases, creativity must be applied to determine the best way of moving loose skin to the target area. Flaps should not be created in areas of high motion unless the incisions used to create the flap reduce mobility or tension (e.g., skin fold flaps). Skin flaps created near limbs must be carefully planned to ensure that dynamic tension does not occur during ambulation. Removal of skin from the donor site should not expose vital structures.

Before surgery, skin surrounding the wound should be manipulated to determine lines of tension and confirm the directions in which skin will most easily move. The surgeon should seek the area with the most mobile skin and consider different ways of mobilizing that skin, even if it is somewhat distant to the wound to be closed. It can be helpful to first create a template of the proposed flap, using cloth or rubber, to check whether it will move in the appropriate direction and for the required distance.

In general, flaps and relaxing incisions are most effective when developed adjacent to the skin defect. The surgeon must make a commitment to create flaps large enough to provide the required result while still being able to close the donor site without tension. Creation of a smaller flap facilitates donor site closure; however, insufficient flap size may result in inadequate coverage and more tension, negating the original purpose of the flap. To ensure sufficient mobility, the length of an advancement flap should be as long as, or preferably longer than, the length of the wound to be closed. A relaxing incision parallel to the wound should be at least as long as the wound and is more effective if it is approximately 1.5 times the length of the wound.

Because it is dependent for survival on the subdermal plexus at its base, the flap must not be too narrow. Converging incisions that result in a flap with a wider tip than base should also be avoided. If anything, creation of a flap with a wide base that converges toward its far extremity is safer. Although restriction of pedicle width may negatively impact flap survival, an increase in flap width beyond an optimum diameter does not increase survival rates; rather, it simply increases the chance that a direct cutaneous artery might be incorporated in the base.12 Flaps should therefore be designed with a base slightly wider than the overall width of the flap and a length sufficient to cover the defect while still enabling closure of the donor site and maintenance of flap blood supply.16 Specific recommendations for flap length to width are not possible because blood supply varies among individuals and regions of the body.

Care must be taken when undermining the flap not to damage the subdermal plexus. A decision should be made as to whether subcutaneous musculature is included with the flap. Elevation of the flap away from underlying musculature reduces the metabolic requirements of the flap and the demands on the subdermal plexus. In many instances, however, muscle is too closely associated with the skin to permit elevation without damage to vasculature. In some patients, subdermal plexus flaps may be purposefully developed to include structures underlying the skin. Most commonly, these “composite” flaps include underlying muscle, oral mucosa, or both.

Closure of flaps is usually performed in two or three layers with absorbable buried sutures in the subcutaneous tissues and nonabsorbable sutures or staples in the skin. In one study, the cosmetic impact of subdermal plexus flaps secured with cyanoacrylate was evaluated. The authors concluded that the resultant scars were thinner and more esthetic than those of sutured flaps.21 Cyanoacrylate closure may therefore be worthy of consideration, especially for wounds of the head and those in show animals.

Types of Subdermal Plexus Flaps

Flaps are often classified as local or distant based on their location in relation to the recipient bed. Local flaps are developed adjacent to the recipient bed and can be used in most areas of the body. Distant flaps are constructed at a distance from the recipient bed and are most commonly used for closure of wounds on extremities. Distant flaps may be transferred directly to the defect (direct flaps) or indirectly by delayed transfer of a tubed flap.

Flaps are also classified according to the way the skin is moved or stretched. Advancement flaps shift skin without rotation. They rely on the inherent elasticity of local skin to stretch over the defect. Rotating flaps, which include rotation, transposition, and interpolation flaps, pivot around a point central to their base into a defect. Rotation flaps are semicircular rotating flaps that cover a triangular defect along one border of the flap. Transposition flaps also share a common border with the defect; however, the flap is rotated across intact skin to reach an adjacent defect. Transposition flaps bring in new skin to the defect and are thus more versatile than advancement or rotation flaps. Interpolation flaps lack a common border with the recipient bed and therefore must be tubed or incorporated into a “bridging incision” between the recipient and donor beds.

Advancement Flap

Advancement flaps can be developed with two incisions made roughly perpendicular to the wound to leave a single base (single pedicle) or by making a single (“relief,” “relaxing,” or “releasing”) incision parallel to the length of the wound to leave two vascular pedicles (bipedicle). Ideally, the base of a single pedicle flap or the releasing incision of a bipedicle flap should be situated in a region of loose skin, increasing the amount of stretching that can be achieved for skin advancement and donor site closure.

A single pedicle flap is usually formed by two skin incisions that are equal in length to that of the defect. These incisions are started at each end of the wound bed, the margin of which comprises the third side of the flap. Incisions diverge toward the flap base to prevent restriction of blood supply (Figure 78-2). The skin of the flap and area surrounding the recipient bed are undermined to release subcutaneous attachments and enable better skin stretching. Wounds can be closed with two single pedicle flaps (an H-plasty) to reduce flap length and maintain blood supply. Bipedicle advancement flaps are created with an incision made parallel to the long axis of the wound. The width of the flap should approximate the width of the defect to be closed (Figure 78-3), and the total length of the flap should be no more than twice the width of the flap base. The flap is undermined and slid into the defect, where it is sutured in place. When tension is minimal at the donor site, the surrounding skin is undermined, and the site is closed primarily. If tension is too great, donor sites are allowed to heal by second intention.

Advancement flaps can be used on many areas of the body, particularly the trunk, face, and forehead.14 The main disadvantage of an advancement flap is that elastic retraction and innate tension will be transmitted to the wound edge. If minimal skin tension is desired or if advancement results in dysfunction (e.g., loss of the ability to close or open the eyelids), a rotation or transposition flap is a better choice.

Rotation Flap

Rotation flaps are usually developed in a stepwise fashion in dogs and cats. A curved incision (Figure 78-4) is begun at a point adjacent to the shortest side of the triangular wound (away from the pivot point). The incision is gradually lengthened and the flap undermined until it is of sufficient length to cover the wound without tension. For rectangular wounds, bilateral rotation flaps can be developed. Excessive tension may develop along the end of the incision opposite the wound. Tension can usually be relieved with a small stab wound to backcut the flap at that site.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Local or Subdermal Plexus Flaps

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