Flaps and grafts

Chapter 19

Flaps and grafts

Ongoing advances in oncological and trauma surgery ensure that the indications for advanced reconstructive surgery in the cat continue to increase. Large skin wounds present the feline clinician with reconstructive challenges and a range of further options can be considered where local skin tension or lack of adjacent skin preclude primary closure. These include promoting second intention healing and the use of local skin flaps, distant skin flaps, axial pattern flaps, and free skin grafts. Although second intention healing may be suitable for many smaller wounds, its use in larger skin deficits is associated with prolonged healing times, a fragile long-term result, and potentially undesirable wound contraction that may impair function. Feline skin, however, lends itself particularly well to the use of random and axial pattern flaps and both can be used early in wound reconstruction even with suboptimal wound beds. Free skin grafts have more fastidious needs; they require an optimal recipient bed and meticulous postoperative management to achieve a favourable outcome.

The indications for muscle flaps are far fewer than the indications for skin flaps but after large oncological resection there may be the requirement for both a muscle flap and a skin flap. For completeness, another flap that is used for reconstructive surgery, amongst other indications, is the omental pedicle and a description of this will be included.

Surgical anatomy

Skin is composed of two layers, an outer epidermis consisting of stratified squamous epithelium and an inner dermis of dense connective tissue. The skin is loosely attached to the hypodermis composed of loose connective tissue, which in turn connects to underlying muscles.1 When considering using skin for flaps or grafts it is essential to have an understanding of the blood supply.

Cutaneous vascular supply (Fig. 19-1)

Direct cutaneous arteries travel through intermuscular fascial planes and then run parallel with the overlying skin before connecting to the cutaneous circulation. Direct cutaneous arteries supply angiosomes composed of cutaneous musculature, subcutaneous tissue and skin. The cutaneous vascular system divides further into three interconnecting levels:1

The subdermal plexus is the major route of arterial supply to the skin.

Skin flaps

A skin flap is a piece of skin that retains at least one source of vascular support from the donor site during its transfer. The terms ‘skin flap’ and ‘pedicle graft’ are synonymous. They are termed ‘subdermal plexus’ or ‘random’ where the integrity of the vascular supply is maintained through an intact attachment of the deep or subdermal plexus. Where a layer of cutaneous muscle is present, the subdermal plexus lies both superficial and deep to this and it should be incorporated when creating a flap. Flaps that derive their primary vascular support from direct cutaneous arteries alone are termed ‘axial’, or ‘island’ where no cutaneous attachment is maintained. Flaps are most commonly moved immediately from donor to recipient site. They provide tissue to achieve coverage of open wounds while redistributing tension away from wound margins. Ideal flap donor sites have ample skin available without excessive tension and motion so as to allow the secondary defect created by the transfer of the flap to be closed readily. Careful planning and meticulous, atraumatic surgical technique to ensure that excessive tension, kinking and circulatory compromise do not develop are necessary for survival of the flap.

Random pattern or subdermal plexus flaps

Random flaps are dependent for their vascular support on the terminal branches of the cutaneous circulation and may be developed anywhere in the skin. Although the vascular integrity of random flaps is somewhat less consistent than axial pattern flaps, they remain a very reliable method of reconstruction. It may be beneficial to align the base of random flaps when possible in the direction of any known direct cutaneous vessels to take advantage of this additional vascular support. Although it is sensible to avoid narrow or excessively long flaps, there is no viability limit to the length of random flaps and predicted base width/flap length ratios are not relevant to flaps in the cat.2 Random flaps may be classified in a number of ways (Box 19-1).

Advancement flaps

Single pedicle advancement flaps (Fig. 19-2) are probably the most commonly used local flap employed in veterinary medicine because of their simplicity and the lack of a significant secondary defect requiring closure. Advancement (or sliding) flaps are rectangular in shape and are created parallel to lines of tension with an attachment at the base opposite the edge covering the wound. Paired advancement flaps resulting in an H-pattern of closure can also be employed although they are more complex. They may have better viability than one large flap, otherwise there is little significant advantage (Fig. 19-3). The single relaxing or release incision is a bipedicle advancement flap by design (Fig. 19-4).

Advancement flaps provide wound coverage by stretching the flap skin along its long axis over the defect, and as a result there is an opposing elastic retraction in the surrounding skin.

Rotating/transposition flaps

Rotation flaps are created by releasing the skin along an arc-shaped incision before moving it along the same axis. They are of limited use in feline reconstruction since relatively large areas of skin need to be raised to achieve wound coverage. The transposition flap is a rectangular pedicle graft, which instead of being moved along its long axis is rotated up to 90 degrees of the wound axis (Fig. 19-5). This is the most common type of rotating flap and is by far the most useful technique to close a variety of small sized problematic wounds. A modification of this flap is the front or hind limb fold flap.

Distant flaps

Direct distant flaps are raised at a location remote from the skin defect and ultimately transferred to the recipient bed by approximating the wound with the donor site. This technique is used almost exclusively for reconstruction of wounds involving the middle or lower extremities and provides durable full thickness skin coverage without the need for skin grafts.3 Flaps are normally raised on the flank and the affected limb brought alongside to allow the flap to be sutured to the recipient bed. Flaps may be single pedicle (hinge; Fig. 19-6) or bipedicle (pouch; Fig. 19-7) grafts. The limb must then be secured against the flank over a ten to 14 day period while the flap attaches to the wound. The pedicle(s) are divided, often in two stages, to complete the transfer.4 Cats are generally well suited to this technique and tolerate the period of limb immobilization better than dogs because of their size, flexible limbs, and the availability of ample loose elastic skin. Distant flaps are multi-staged procedures and consequently are time consuming and expensive. The transplanted skin assumes the same hair growth as its donor site despite its new location. Some patients will not tolerate their limb being immobilized against their flank and this is particularly evident if the limb is positioned in an elevated position. Occasionally, moist dermatitis caused by skin-to-skin contact and moisture accumulation is encountered; transient muscle atrophy of the limb may be noticed in some cases.

Axial pattern flaps (Fig. 19-8)

An axial pattern flap is a pedicle graft based primarily or solely on the vascular supply of a direct cutaneous artery and vein.5 The vessels, of which terminal branches supply blood to the subdermal plexus, extend the length of the flap. Therefore axial pattern flaps have better perfusion compared to subdermal plexus flaps. Large axial pattern flaps can be safely elevated and transferred in a single-stage procedure for closure of major cutaneous defects. These flaps have excellent survival rates and they can be used to reconstruct wounds associated with less optimal conditions such as contamination, uneven surfaces, exposed bone, tendon, and cartilage. Axial or island pattern flaps are classified according to the vascular territory supplied by the direct cutaneous artery (angiosome). These vessels and territories including the thoracodorsal artery, lateral thoracic artery, omocervical artery, caudal superficial epigastric artery have been researched and mapped for clinical use as axial pattern flaps in the cat. Although the location and relative size of each angiosome are similar for both dogs and cats, the extreme elasticity of feline skin and the relative increase in body size to limb length often facilitates more distal coverage of limb wounds in cats.6 Axial pattern flaps sometimes require extensive dissection of the donor site for flap elevation; because of their relatively large size, axial pattern donor sites may require undermining and walking sutures to appose the defect created by the raised flap. The cosmetic appearance of the recipient bed differs from the donor site with regard to hair direction, color, length, glandular tissues, and amount of subcutaneous tissues.

Free skin grafts

Free grafts are areas of skin that are completely detached from the donor site and hence lose their original vascular supply before being transferred to resurface a recipient site. In feline surgery the use of free skin grafts is confined almost exclusively to reconstruction of large skin defects of the extremities where few alternate options are available.7 Secondary healing of limb wounds can lead to unsightly scar tissue development and may also lead to impaired limb function if formed in the vicinity of joints. The use of random flaps is often restricted by the limited availability of adjacent skin. Direct flaps offer an alternative solution although the temperament of some cats may not always be conducive to this option.

Free grafts lack vascular support in the period after transfer and must survive for at least the following 48 hours by absorbing protein-containing fluid from the recipient bed by capillary action, a process known as plasmatic imbibition. During this period the graft becomes attached to the underlying tissue by the development of an interposed fibrin seal that acts as a biological ‘glue’ and provides a scaffold across which capillaries from the recipient bed begin to anastomose with those of the exposed graft plexus to re-establish circulation (inosculation). New capillaries later grow into the graft and the vascular channels remodel (revascularization). Free skin grafts therefore require a vascularized recipient bed and preparation of the recipient bed is a major determinant of successful skin grafting. Fresh active pink granulation tissue is the ideal recipient bed and provides excellent vascular support and the growth factors necessary for these processes to occur. Granulation tissue is not, however, essential for graft survival and any well-vascularized tissue is theoretically capable of accepting and supporting a skin graft. Grafts can therefore be applied over surgically clean and fresh surgical wounds since periosteum, peritenon, and muscle normally have sufficient vascular supply to support skin grafts (Fig. 19-9).

The accumulation of any material such as blood, serum, pus, or foreign material between the graft and the recipient bed will interfere with the early nutrition processes and may ultimately prevent revascularization. It is essential, therefore, that all tissue debris, contamination and superficial infection should be controlled before the graft is applied since the early fibrin seal that fixes the graft to the recipient bed can be easily destroyed by the fibrinolytic activity of proliferating bacteria in the wound. Immobilization of the graft is similarly important, since this attachment can also be mechanically damaged by excessive motion in the graft site during the first week. Methods to immobilize the surgical site include: Robert Jones bandages, splints, or external skeletal fixation. Bandaging and splinting methods vary with the type and location of the graft. Any areas of early epithelialization should be removed from the surface of the granulation tissue because this can prove remarkably resistant to the establishment of the fibrin seal. Grafts are applied with sufficient tension to ensure good contact with the recipient surface; excessive tension may limit capillary circulation while inadequate surface-to-surface contact may prevent interdigitation and delay revascularization.

Meshed or pie-crusted sheets have perforations through the entire thickness of the skin. Perforated grafts have greater flexibility and conform better to an uneven surface than unmeshed skin while permitting the ongoing drainage of any sub-graft exudate. Unmeshed sheets have less flexibility and hence need to be pre-cut more accurately to the outline of the defect. Additionally, they provide no route for drainage, necessitating temporary placement of active suction drains under larger grafts.

Healthy grafts may appear swollen and dark during the first 48–72 hours due to the process of imbibition of hemoglobin degradation products but become pinker and less edematous as a rudimentary circulation becomes established. Non-viable grafts will, however, progress to become white or black and necrotic over a period of three to five days. Occasionally, grafts lose their epidermal layer during the first week, giving rise to the appearance of an apparently complete graft failure. This can be disconcerting but usually an outline of the dermal graft will appear within the next week and the graft will progress satisfactorily to complete coverage of the defect.

In feline surgery, full thickness grafts are preferred for virtually all reconstructive procedures because they are more robust, durable and withstand considerable handling during graft elevation and positioning. Full thickness grafts contain all dermal structures and therefore can provide a better cosmetic appearance. Although they have more demanding nutritional requirements than split-thickness grafts, well-prepared full-thickness grafts can achieve survival rates comparable to thinner grafts. Split-thickness grafts contain the epidermis and only a variable amount of dermal structures. They have a number of disadvantages and are rarely used since they are rather fragile, require more care during collection and transfer and grafted sites tend to have poor durability, sparse hair growth and a greater tendency to contracture due to the absence of dermal structures.

Likewise, almost all grafts are applied as a sheet providing complete wound coverage since this provides the most functional and cosmetically acceptable result. Partial coverage techniques (e.g., stamp, pinch, strip, and mesh) are used only very infrequently and provide a poorer long-term result with significant contribution from epithelialization.

Ideal donor sites for skin grafts have an abundance of easily mobilized durable skin of suitable thickness with hair of a similar color, length and thickness to that it replaces at the recipient site. In practice, the flank usually satisfies most of these criteria and offers the best opportunity to remove sufficient skin for the reconstruction while permitting simple closure of the donor site.

Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Flaps and grafts
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