The skin comprises the epidermis and dermis, and beneath these layers lies the subcutis or subcutaneous fat layer. Skin thickness ranges throughout the body and is thinnest on areas such as the eyelids, and thickest on areas such as the scalp and back. The variation in skin thickness according to anatomic location is primarily due to differences in dermal depth or thickness. This can be helpful to note when training residents new to surgery who are first developing their surgical skills. For example, suturing on the back is generally easier and the added depth of tissue facilitates practice of deep dermal buried sutures for the resident. The epidermis (not the dermis) has four or five layers or strata (depending on anatomic location). From deep to superficial these are: the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Only the deeper three layers have live cells, with the lucidum and corneum having dead or cornified keratinocytes present. The epidermis provides the protective outer layer for the skin and typically represents around 5% of total skin depth, not 25%. The stratum corneum is only found in glabrous skin which is present on the foot soles and palms. The main skin appendages are found in the dermis although they can extend into the subcutis, and this is relevant when transferring full-thickness grafts as the appendages will be transferred too. For example, hair follicles may be transferred to the recipient site resulting in hair growth, so donor sites are generally selected to avoid this transfer from happening.1
Grafts are generally classified according to their composition or origin into autografts, allografts, xenografts, composite grafts, and alloplasts. The most commonly used grafts are from the patient themselves and these are termed autografts. They represent the benchmark against which all grafts and other biomaterials are compared because of their high tolerance, and host incorporation. Their main disadvantage is the requirement for a donor site. Allografts in contrast are derived from another donor of the same species, so all cadaveric tissues used in clinical practice represent allografts. There are many of these in common use including skin for burn patients, and tendon, bone, or fascia for more complex reconstructions. There is generally no limitation to such tissue availability; however, skin, for example, has only a short-term use as it will subsequently be rejected and an alternative cover will still be required. Xenografts are derived from a different species. Again these sources can be plentiful but integration with host tissues may be less reliable than with autologous grafts. Composite grafts are most commonly autologous in nature and a key example would be a skin and cartilage graft harvested from the ear for nasal alar reconstruction. Alloplasts are synthetic grafts or implants such as silicone breast prostheses or joint replacements. They are associated with a foreign body response and never truly integrate with the donor in the way that biological tissue does.1,2
Split skin grafting is indicated in large wounds where direct closure is not possible providing the wound bed is clean, infection free, and well vascularized. There should be healthy viable tissue at the wound base without avascular structures being present such as exposed tendon, bone, or cartilage. This is because these structures do not have sufficient vascularity to support the graft successfully. If they are covered with granulation tissue, or paratenon, periosteum, or perichondrium, however, then skin grafting is a viable option. For this reason, in cases where there are exposed structures, a period of treatment with negative pressure wound therapy can be beneficial as this can condition the wound to take a graft by creation of new granulation tissue, thereby avoiding more complex reconstructive processes such as flap surgery. One exception to this rule is the cranium where if the outer calvarial layer is burred, it is possible to successfully utilize a skin graft. This is because the process improves the vascularity and adherence of the wound base. Split skin grafting is, of course, best performed in patients where health status is good and significant comorbidities are absent. However, having comorbidities is not a specific contraindication as grafts can be performed under local anesthetic and can avoid more complex intervention. Bear in mind that donor site healing may be poor in patients with multiple medical problems (which can be painful and inconvenient for many weeks) and the risk of graft failure is increased, so a careful discussion about risks and benefits is particularly important in this setting. Grafting in the presence of active infection is contraindicated; however, once infection is treated, grafting is entirely appropriate. In some cases, it is advisable to have evidence of negative wound cultures prior to grafting (e.g., group A streptococcus); however, in most cases clinical assessment is sufficient particularly where a final wound debridement is performed at the time of grafting. Grafts are often used in combination with flap surgery to close donor site defects such as scalp transposition, radial forearm fasciocutaneous, or fibula osseocutaneous flaps.1,2
There are a number of reasons why grafts fail to take and the most common reasons include hematoma which stops the graft from adhering to the recipient site and obtaining a blood supply, infection resulting in the inability for the graft to take, or a physical/mechanical reason, such as shearing of the graft. For this reason, care is taken to ensure adequate hemostasis before graft placement, ensure absence of infection, and secure the graft to the recipient site. When raising a radial forearm flap, it is important to retain paratenon over the long flexor tendons so that a viable graft bed is preserved. Once the graft is secured it is important to minimize movement beneath the graft where it lies on the flexor tendons. For this reason, it is important to splint the hand and wrist to stop this movement in the early phases of graft take. Usually a period of immobilization of 1–2 weeks is sufficient. Compression is not usually required in this scenario although many surgeons will still use a low adherence dressing on the graft followed by gauze or foam to secure and contour the graft. Few surgeons would recommend leaving this site exposed until the graft has taken at 1–2 weeks as it would be likely to sustain inadvertent damage. Daily wound inspection would serve no useful purpose and antibiotics would not normally be indicated.
Skin graft survival, or “take,” is based upon the ability of the graft to adhere to a recipient site, receive nutrients from the underlying wound bed, and progress to vascular ingrowth and maturity. Split-thickness skin grafts initially adhere to the wound with formation of a fibrin layer over the first 24 hours. Over the next 24–48 hours, the graft swells and obtains nutrients from the wound bed by diffusion. This process is called serum imbibition. During the following days, recipient and donor site vessels align. This process is called inosculation and occurs in nature when tree branches join together. The final process is called revascularization, which involves full ingrowth of host capillaries into the graft. Once revascularization has occurred, there is continued ingrowth of fibrous tissue and vessels producing a long-term permanent adherence and tissue integration. The process of skin graft take is often discussed in clinical examinations.1,2 Timings for the various processes are as follows: • 0–24 hours: Fibrin adherence • 24–48 hours: Serum imbibition • 48–72 hours: Inosculation • 4–6 days: Revascularization
HARVEST AND APPLICATION OF SPLIT-THICKNESS SKIN GRAFT (STSG) There are two types of skin graft contraction: primary and secondary. Primary contraction occurs intraoperatively when a graft is harvested. Secondary contraction occurs later in the healing phase. The greater the amount of dermis present, the greater will be the primary contraction and conversely there will be less secondary contraction. Primary contraction is thought to be related to elastin content within the dermis, whereas secondary contraction is thought to be related to myofibroblast activity. It is standard practice to mesh split-thickness grafts where there are large areas of defect to be covered such as in large total body surface area (TBSA) burn injuries. Common meshing ratios include 1.5:1, 2:1, and 3:1. This will allow for greater surface coverage while minimizing donor site size. However, meshing ratios have been shown to be inaccurate representations of final graft size where, for example, a 1.5:1 meshing resulted in 1.36× expansion, and a 3:1 mesh resulted in 1.8× expansion. This is important to consider when deciding whether to mesh or not and also to gauge how much donor skin needs to be harvested for any given defect. In addition, when going ahead with meshing of the graft, this process may improve graft take over sheet graft as fluids such as blood and serum can escape from the wound bed, which may otherwise compromise the graft take. The downside to meshing is that the cosmetic outcome is poorer than with sheet graft so it should be avoided in cosmetically sensitive areas, and also the functional outcome can be worse so, for example, sheet should be preferentially used around joints in the hand. When using a dermatome to harvest skin the settings are measured in 1000ths of an inch and not 10,000ths of an inch. A typical graft harvest will be at 10–12/1000ths of an inch. This should harvest all of the epidermis and part of the dermis. Although the most common donor site is the thigh, the scalp should be considered as a potential donor site in head and neck reconstruction, particularly in the setting of burns. The benefits include a better color match, a more rapid donor healing, and better long-term donor site aesthetics due to camouflage and hair regrowth.1,2
Split-thickness and full-thickness skin grafts each have their own advantages and disadvantages and consequent indications for use. The main advantages of split-thickness grafts are that they have a higher chance of graft survival due to the decreased dermal component, greater availability of donor sites, an ability to reharvest from the same site, and faster reinnervation. Furthermore, the surgical time can be reduced compared with full- thickness grafts especially as donor site closure is not required. In contrast, full-thickness grafts provide a better color and texture match with more robust reinnervation, they display less secondary contraction, and the donor site can be closed directly. Downsides are more limited availability, a lower chance of graft survival, and increased surgical time. Given the above, split-thickness grafts are indicated in larger areas of noncosmetically sensitive regions and full-thickness grafts are indicated in smaller defects where aesthetics and function are paramount (Table 19.1).1–3
One of the key differences in split-thickness versus full-thickness skin grafts is that only full-thickness grafts will transfer the skin appendages which include hair follicles. Therefore, in this scenario it is possible to conclude that a full-thickness graft has been used. The absence of any obvious donor site is not a reliable guide to surgery performed. Both split- and full-thickness donor sites can be discretely placed and well concealed, especially if the graft size is small. Wound contracture will be present in both full-thickness or split-thickness grafts and this, too, will be influenced by how well the graft took. If there was partial graft loss, then some of the wound would heal by secondary intention with associated scarring. This could cancel the benefits of reduced secondary contracture by a full-thickness graft. In general, skin color match is better in full-thickness grafts, but only where due consideration has been given to donor site selection to optimize the match, e.g., cheek or preauricular donor sites for central face reconstruction. Evidence of graft failure may make it more likely that a full-thickness graft was used but as both can fail, it would not exclude one or the other.
Negative pressure wound therapy (NPWT) can be a very effective dressing for a split skin graft recipient site because it ensures uniform pressure across graft, minimizes shear, splints the wound, and can accelerate the process of inosculation. A bolster is commonly used following skin grafting in order to apply external pressure to the graft and maintain a clean moist environment. This can be achieved with a nonadherent base layer and either cotton wool balls or foam. The pressure effect is, however, very short-lived. However, neither of these approaches should normally be used on free tissue muscle flaps with split-thickness skin grafts as this would result in undue pressure on the flap and an inability to assess flap viability. The ideal dressing is therefore one which is nonadherent and allows for fluid egress from the wound and yet has some ability to absorb fluid, while allowing for regular flap monitoring. For this reason, a simple nonadherent dressing such as Mepitel™ in combination with gauze is ideal for this purpose and a viewing window in the flap is readily created. Alginate or Duoderm™ dressings are more commonly used for donor site dressings and can be left in situ for 1 week. Aquacel AG™ is a silver-impregnated antimicrobial absorbent nonocclusive dressing that again may be used for split-thickness donor site wounds. Duoderm™ is a hydrocolloid dressing that absorbs exudate to form a soft gel. It promotes a moist environment, is occlusive, is waterproof, and can improve epithelialization. It is used for some donor site dressings and may also be useful as a sealant at the interface of normal skin and wound where a negative pressure dressing is used (Table 19.2).1–3 Table 19.1 STSG versus FTSG Advantages Disadvantages STSG All epidermis and part of dermis: Thin: 5–12/1000 inch Medium: 12–16/1000 inch Thick: >16/1000 inch - Higher chance of graft survival due to decreased dermis - Ability to reharvest from prior donor site - Greater availability of donor sites - Faster reinnervation - Greater secondary contracture - Poor color/texture match - Hair growth not possible FTSG Includes all of dermis and epidermis - Less secondary contracture - Ability to close donor site - Better color/texture match - Dermal appendages more likely to regenerate - More robust reinnervation (pain, touch, then temperature) - Lower chance of graft survival - Need to close donor site limits size of harvested graft FTSG, full-thickness skin grafts; STSG, split-thickness skin grafts.
There are a number of pitfalls to avoid when taking full-thickness skin grafts. Of these one of the most important is to ensure that the graft is sufficiently thinned and contains no residual subcutaneous fat on the undersurface, as this can lead to an increased graft failure rate. Skin grafts are avascular once harvested and need to obtain nutrition and a new blood supply from the recipient bed. If the graft is too thick, it is more difficult for it to successfully achieve this in the early phases of healing. The other key elements of graft failure should also be borne in mind and these include infection, inadequate hemostasis, inadequate debridement or tumor excision, and inadequate graft fixation. Full-thickness grafts are usually harvested in an elliptical design with a scalpel ideally just beneath the dermis with no fat. If there is residual fat on the undersurface, this should be debulked or removed to promote survival. Where the dermis is thick, this should be thinned. The use of a bolster dressing is the standard of many surgeons although others choose to leave the graft exposed with little more than petroleum jelly covering it. The use of a bolster is unlikely to be the cause of graft failure in this scenario and even the absence of one may well not be the primary cause. Where grafts are left exposed, it is important to remember that they display a cyanotic discoloration during revascularization, which can be disconcerting for both patient, nurse, and surgeon if not anticipated. Full-thickness grafts are not meshed but may be fenestrated to allow fluid egress. In this scenario, there would have to be a significant volume of residual tumor for incomplete tumor excision to be the cause of graft failure. Table 19.2 Common Donor Site Dressings Xeroform (Covidien, Dublin, Ireland) Mepitel (Molnlycke, Norcross, Georgia) Aquacel Burn (ConvaTec, Bridgewater, NJ) DuoDERM (ConvaTec, Bridgewater, NJ) Bismuth tribromophenate- impregnated petroleum gauze Transparent breathable silicone film Silver-impregnated hydrofiber Hydrocolloid - Common dressing - Widely available - Inexpensive - Antimicrobial - Becomes dry dressing, forming eschar - May demonstrate increased donor site pain - Nonadherent - Moist wound environment - Nonocclusive, allowing fluid egress - Persistent drainage until epithelialization - Can remain in place for 14 days - Antimicrobial - Absorbent - Nonocclusive - Can remain in place for up to 21 days - Forms dry eschar similar to Xeroform that is peeled off the donor site - Hydrocolloid absorbs exudate to form soft gel -Promotes moist environment - Occlusive, waterproof - Improved degree of epithelialization - Dressing can remain in place for 7 days