Grafts, as free segments of tissue, depend on the delivery of nutrients from the vascular bed onto which they are transferred. Flaps carry their own blood supply or have it surgically reestablished after it is transferred.
Blood Supply to the Skin
Blood is supplied either through a longitudinal artery arising dorsally that lies deep to the muscle or fascia, supplying perforators to the subdermal and intradermal plexus in the overlying skin or through longitudinal vessels arising ventrally that lie superficial to the fascia, connecting directly to the plexuses in the skin ( Fig. 3.1 ). These systems are interconnected by a complex network of vessels of varying sizes. They are very delicate and cannot withstand compression in forceps, twisting, or undue stretching. Skin hooks and stay sutures are essential tools to preserve them.
Grafts, bereft of vascular connections, must acquire nutrients by diffusion from the recipient bed for the first 24 to 48 hours (imbibition) and during the next 2 days must establish local vascular connections (inosculation). This requires that the graft remain immobilized and closely applied to the bed, which in turn must be well vascularized. Seromas and hematomas block these steps, as do infection and scar tissue.
Thickness of Skin Grafts
Grafts may be full thickness to include the entire dermis to the adipose layer, or they may be split thickness and include a portion of the dermis ( Fig. 3.2 ).
Full-Thickness Skin Grafts
Full-thickness grafts, made up of all skin layers, contract only 5% to 25%. These provide a durable skin covering and are less likely than split-thickness grafts to become hyperpigmented, but they also carry the skin adnexal structures, making hair growth a potential problem. They are more demanding than split-thickness grafts in regard to the vascularity and quality of the recipient bed, not only because they are thicker (bulkier) and thus require a greater supply of blood but also because they depend almost completely on new vascular connections to the disrupted subdermal plexus, which characteristically has fewer vessels available for the process of inosculation. The requirements for “take” are a well-vascularized bed and absolute immobilization.
Full-thickness grafts must be cleared of underlying fatty areolar tissue to allow the vessels of the subdermal plexus direct contact with the new bed ( Fig. 3.3 ).
A good compromise for grafts from the lower abdomen may be a thick split-thickness graft (>0.19 inch) because it has most of the favorable qualities of the full-thickness graft and little of the tendency to contract as a thinner split-thickness graft would. Full-thickness grafts from the prepuce, the bladder, or the mouth, on the other hand, are thin and pliable and inherently have little subcutaneous fat. They too must have their deep surface meticulously prepared to expose the deep laminar plexus optimally.
Split-Thickness Skin Grafts
Split-thickness skin grafts may be thin (to include a minimal amount of dermis, from 0.010 to 0.015 inch), intermediate (approximately half the thickness of the dermis, from 0.016 to 0.19 inch), or thick (three fourths or more of the dermis, more than 0.19 inch). Composed of only part of the dermis along with the epidermis, split-thickness skin grafts take better than full-thickness grafts but provide a more fragile covering. Split-thickness skin grafts can contract about 50% or even more in unsupported areas.
Dermal grafts , cleared of both epidermis and fat, can be more elastic than full-thickness grafts and become vascularized on both sides. They are useful for replacement of deep structural layers such as penile tunica albuginea and fascia.
Acellular dermal matrix harvested from cadaveric donors may be an alternative to autologous dermal grafts.
Application of Split-Thickness and Meshed Grafts
Meshing the skin graft in a mesher allows for expansion and provides greater coverage if necessary, but more important for the genitourinary surgeon, allows escape of serum and blood. However, such a graft may contract more because the openings in the mesh heal by secondary intent. A meshed graft conforms more easily to curved and irregular recipient surfaces. The graft must be placed with good hemostasis, be relatively free of contamination, and also be immobilized. Mesh grafts are placed with the slits parallel to the existing skin lines. They can be expected to contract 30% to 60%, except on the back of the hand and on genital tissue.
A nonmeshed split-thickness graft is applied to a functioning penis because a meshed graft offers no advantages and can be expected to contract from 30% to 60%, creating a cosmetically unsightly appearance to the reconstructed penis. For reconstruction of the scrotum, however, a meshed graft allows for better contact with the underlying complex contours, avoids collections in the contour interfaces, and creates a cosmetically pleasing appearance because the mesh scars are similar in appearance to the rugae seen in normal scrotal skin ( Fig. 3.4 ).
The deep surface of a cutaneous flap is composed of fat, a fasciocutaneous flap is composed of fascia, and a musculocutaneous flap is composed of muscle. Flaps may be used to cover (skin flaps), to provide structure and function and contribute to revascularization (muscle flaps), to provide sensation (sensible fasciocutaneous flaps), or for a combination of these purposes.
In contrast to grafts, flaps carry their own blood supply. Flaps can also be reattached directly to a new supply by microvascular techniques. Flaps may be random pattern flaps for transposition, rotation and tube flaps, or axial pattern flaps. For a flap to be classified as either random or axial depends on the inherent pattern of vascularity of the flap itself. Random flaps have no defined cuticular vascularity, which varies from individual to individual and is somewhat undependable. In contrast, axial flaps have an organized, self-contained blood supply and defined cuticular vascular territories that vary little from one individual to another and thus are predictable and dependable.
Another approach to classifying flaps is to divide them into peninsular, island, and microvascular free transfer (MVFT) flaps. These classifications address the design and shape of the flap itself. Peninsula flaps , as the name implies, are shaped like a peninsula, and thus both the cuticular and vascular portions of the flap remain attached to the “mainland” (body). A random peninsula flap (all random flaps are peninsula flaps by definition) is thus mobilized so that the skin survives on the random distribution of the skin plexuses. In the past, surgeons attempted to make random flaps more dependable by defining length-o-width ratios for the flap (i.e., if the flap was 3 cm long, its base needed to be 3 cm wide, for a 1 : 1 ratio). However, ratios are more limiting than useful because certain areas in the body with a 1 : 2 ratio or even a 1 : 3 ratio allow reliable survival, but in other areas, a 1 : 1 ratio approaches the limit.
In an island flap, the skin is detached from its origin. The term island flap implies that the cuticular continuity is interrupted but the vessels remain attached (the flap dangles on its vessels). If the vessels are detached, then the flap becomes an MVFT flap or free flap.
The musculocutaneous or fasciocutaneous flap has come to be viewed as an island flap, but it is only truly an island flap if the muscle and fascia is totally detached, both origin and insertion, with the flap unit moved on the vessels supplying it. For most clinical uses, the muscle is left attached at the origin and transposed to the adjacent defect. To be accurate in both theory and semantics, the surgeon must view the muscle or fascia as the flap and the attached skin unit as a passenger on the flap. The proper term then is skin island or paddle. To use the gracilis as an example, the flap is properly thought of and termed “a gracilis flap with a skin island or paddle.” Fascial flaps, almost by definition, cannot be elevated as islands. To use an example of a flap that has become common in urology, the preputial or penile skin island flap should correctly be designated a dartos fascial flap with a preputial or penile skin island or paddle.
The musculocutaneous flap , useful in reconstructive urology, is formed by elevating skin and muscle, together with their independent cutaneous vascular territory, on a single pedicle on the superficial inferior epigastric, superior epigastric, or superficial circumflex iliac artery.
Preparation of a Flap
Choose a flap with a size and ability to arc into place, with adequate vasculature, accessibility, proper composition, and an acceptable donor site remaining. Outline the defect to be grafted with a marking pen; then quickly press a piece of glove-wrapper paper against it to obtain a pattern for the graft. Skin grafts and flaps are viscoelastic, so stretch the graft in place to overcome the elastic fibers in the skin. A pull for 10 or 15 minutes enlarges a flap, owing to stress relaxation and creep. However, undue tension may compromise vascularity.
Secondary contraction between the skin graft and its bed occurs with maturation of the scar tissue, beginning after the 10th day and continuing for 6 months. Thin grafts, flexible beds, and complete take all reduce the chances for contraction. Sensation begins to return to a graft in 3 weeks if dense scarring does not intervene. Skin grafts and flaps grow as the patient grows, stimulated by tension from the surrounding skin.
Avoid marks in the skin that result from tension on the sutures. Tie the suture just tight enough to approximate the edges and no tighter. Subcutaneous sutures reduce the tension, as does placing the incision parallel to the skin lines. The length of time that the sutures remain is also a factor: 6 or 7 days is usually adequate, but allow 10 to 14 days for heavy skin on the back. Small bites of tissue close to the edge are associated with less apparent skin marks; infection is accompanied by more prominent ones. Of course, a patient prone to keloid formation is at greatest risk.
Slight eversion of the skin edges results in a flat scar; inverted edges leave a depressed scar. In some areas, a vertical mattress suture is necessary to stabilize the skin edges. If skin clips are used, they should grasp the skin with equal bites and should be angled so that they slightly evert the skin. Microporous skin tape, used in conjunction with buried sutures, may be placed initially as primary skin closure or applied at the time of suture or clip removal. It helps adherence to wipe the skin with alcohol or acetone before application. Skin tapes have the advantages of quick application and avoidance of suture marks, and they do provide added tensile strength. Their disadvantages are that they do not evert the skin edges, and they may come off prematurely.
Use 1% lidocaine with 1 : 200,000 epinephrine; for a child, use 0.5% lidocaine with 1 : 400,000 epinephrine. Hyaluronidase may aid in diffusion of the agents. Inject it slowly while explaining the procedure to the patient. Stop for a minute if the injection is causing pain. Regional block is often better than local infiltration.
Use of Langer Lines
Make incisions parallel to Langer lines ( Fig. 3.5 ). These are oriented at right angles to the line of maximal tissue extensibility. By orienting excisions or incisions with the lines, the wound tension (not to be confused with innate skin tension) is minimized.
The island flap maintains all of the favorable vascular qualities of the peninsular flap but has the advantage of a maneuverable narrow vascular pedicle that contains the essential axial artery and vein. By completely severing attachments to the skin, the little blood supply lost from the random cutaneous vascular plexuses is made up by gain in greater mobility ( Fig. 3.6 ).
Choose a flap for size, ability to arc into place, and presence of adequate vessels. Although an island flap can be transposed much more easily than a peninsular flap, the supplying vessels are fragile and easily injured. Flaps that can be transposed with much more freedom are the muscle flaps or fascial flaps and their respective skin island paddles.
Correction of Dog Ear
Retract the center of the longer edge that remains beyond the last stitch ( Fig. 3.7, A ).
Incise the skin in the line of the incision for a short distance on the opposite side ( Fig. 3.7, B ).
Incise the skin on the redundant side, also in the line of the incision, to excise the flap of excess skin ( Fig. 3.7, C ).
Close the remainder of the wound ( Fig. 3.7, D ).
Elevation of muscle and the overlying skin on a single pedicle produces a musculocutaneous flap. These flaps are useful in the repair of urogenital defects, especially when based on the gracilis and inferior rectus abdominis muscles.
Examples ( Fig. 3.8 ): Muscles with perforators that supply the overlying cutaneous vascular territories suitable for formation of musculocutaneous flaps are shown in Fig. 3.8 . On the left are the deep inferior epigastric vessels supplying the rectus abdominis muscle. On the right is the medial circumflex femoral artery to the gracilis muscle.