over time. However, if the force remains constant, the tissue will continue to distend, and this property is termed creep.20

Grafts

Transfer of tissue can be performed via the use of grafts or flaps. A graft is tissue that has been excised and transferred to a graft host bed, where a new blood supply develops by a process termed take. This process is dependent on the bulk of the graft and the number of exposed

vessels on the undersurface of the graft as well as the surface of the bed. The initial phase of graft take, imbibition, lasts approximately 48 h. During that phase, the graft survives by “drinking” nutrients from the adjacent graft host bed. The temperature of the graft remains lower than core body temperature. The second phase, inosculation, also requires about 48 h. It is during this phase that true microcirculation is reestablished in the graft and the temperature of the graft rises to core body temperature. The process of take is influenced by both the nature of the grafted tissue and the conditions of the graft

Figure 19.4. demonstration of the viscoelastic properties: (Top) stress relaxation. a force is applied to the tissue, the distension remains constant,the force to maintain that distension decreases; (Bottom) creep. in this model, the force of distension remains constant, over time under that force, the tissue elongates.

host bed. Processes that disrupt or impair the vascularity of the graft host bed thus interfere with graft take.18

The epidermal or epithelial layer is a covering, acting as a barrier to the external environment. It lies adjacent to the superficial dermis or superficial lamina. At approximately that interface is the superficial plexus. This plexus is the intradermal plexus in skin. On the undersurface of the deep dermal layer or deep lamina lies the deep plexus. In skin, this is the subdermal plexus. The deep dermis contains most of the lymphatics and greater collagen content than in the superficial dermal layer. The deep or reticular dermis is generally thought to account for the physical characteristics of the tissue.19

A split thickness graft carries the epidermis, or the covering, and the superficial dermis. This graft exposes the superficial dermal (intradermal or intralaminar) plexus. In most grafts, the superficial plexus is composed of small but numerous vessels allowing for many possible graft-bed connections allowing favorable vascular characteristics. This unit has few lymphatics, and the reticular dermis (and its

Figure 19.5. demonstration of the cross-section of anatomy of the skin, with the layers of a split-thickness graft demonstrated (reprinted with permission from Jordan and schlossberg17. contemporary Urology is a copyrighted publication of advanstar communications inc. all rights reserved).

physical characteristics) are not carried, which accounts for the tendency of split-thickness units to be brittle and, in some cases, less durable (Fig. 19.5).

A full-thickness graft carries the covering, the superficial dermis or lamina and the deep dermis or lamina with all the characteristics attributable to that layer (Fig. 19.6). In skin, the subdermal plexus is exposed. In most cases, that subdermal plexus is composed of larger vessels that are more sparsely distributed (Fig. 19.7). The graft is thus fastidious in its vascular characteristics. A fullthickness unit carries most of the lymphatics, and these physical characteristics are carried with the transferred tissue. If we examine the grafts that are most commonly used in genitourinary reconstructive surgery, the split-thickness skin graft has favorable vascular characteristics but tends to contract. The full-thickness skin graft tends to have more fastidious vascular characteristics, but there is less contraction. There is a difference between genital full-thickness skin (penile and preputial skin grafts) and most areas of extragenital full-thickness skin. This is probably a reflection of the increased mass of the graft

Figure 19.9. axial flap, there is a defined artery in the base of the flap. demonstrated are (a) axial peninsula flap, (b) axial island flap, (c) axial microvascular free transfer flap (reprinted with permission from Jordan and Mccraw15. copyright © american Urological association Education and research, inc).

Figure 19.10. (a) demonstration of the musculocutaneous system flap vascularity (reprinted with permission from Jordan and Mccraw15. copyright © american Urological association Education and research, inc) (b) demonstration of the fasciocutaneous system of vascularity (reprinted with permission from Jordan and schlossberg17. contemporary Urology is a copyrighted publication of advanstar communications inc. all rights reserved).

a

b

Figure 19.11. demonstration of the difference between an axial island flap and a skin island or paddle carried on a flap: (a) true island flap, (b) demonstration of a skin island carried on a fascial flap.

carried on the dermal or laminar plexuses; the dimensions of random flaps can vary widely depending on the individual and the flap donor site. The term axial flap refers to the presence of a defined vessel in the base of the flap (Fig. 19.9). There are three types of axial flaps. The direct cuticular axial flap is a flap based on a vessel superficial to the superficial layer of the deep body wall fascia. The classic example of a direct cuticular flap is the groin flap. A musculocutaneous flap (Fig. 19.10), on the other hand, is based on the vascular supply to the muscle. The overlying skin paddle is carried on perforators. If the muscle alone is carried as a flap, the overlying skin survives as a random unit. The fasciocutaneous flap has a similar vascular pattern to the musculocutaneous system. However, the deep blood supply is carried on the fascia (both deep and superficial layers), and the overlying skin paddle is based again on perforators. Familiar to most urologists are the local genital flaps used for hypospadias or other urethral reconstruction. Thus, one can transfer a fascial flap based on the deep blood supply associated with the flap. It has been argued that fascia is relatively avascular and hence cannot serve as “the blood supply” to the fasciocutaneous unit. In reality, the fascia acts as a trellis and the vessels are carried much like the limbs of a vine.15

Flaps can also be classified by elevation technique. A peninsular flap is a flap in which the vascular supply and the skin of the flap base are left intact. An island flap is a flap in which the skin is divided but the vessels are left intact