Split Thickness Skin Grafts

A skin graft is the transfer of skin from a donor site to a recipient site where there is no immediate blood supply. The transplanted skin initially survives by absorbing transudate from the recipient site, a process called plasmatic imbibition.  Neovascularization from the graft bed capillaries then provides a blood supply to the graft over the next 48 to 72 hours. Full circulation is restored within four to seven days.  Skin grafts are indicated for coverage of a skin or soft tissue defect due to trauma, burns, or excision of a tumor if closure cannot be accomplished by approximating the edges of the wound.  There are two major types of skin grafts, split-thickness skin grafts and full-thickness skin grafts.  Split-thickness skin grafts are further discussed below.

Split thickness skin grafts (STSG), also called partial thickness grafts, transfer a portion of the donor site skin including the epidermis and some of the underlying dermis. This allows the donor site to heal from the epidermal elements left behind. STSG can be utilized as an intact sheet or expanded via a mechanical mesher device.  Meshing of a graft increases the surface area that can be covered by a graft.  Wound contraction plays a role in choosing STSG vs. FTSG.  In general, the degree of contraction is less when a graft is thicker, and minimal wound contraction occurs with full thickness skin grafts. However, with a thicker graft, more tissue revascularization is required.  On the face or over joints FTSG are preferable, but in patients with large defects of the trunk or extremities STSG are the standard of care.

The donor site is selected to minimize discomfort and cosmetic issues at the donor site, while maximizing the outcome at the recipient site. The choice of donor sites depends on the desired color, texture, thickness of the skin at the recipient site, prior incisions or skin damage and the possibility of concealing the site in inconspicuous areas.  On the face, it is desirable to replace "like with like" and utilize skin as close to the recipient site as possible.  An open wound of the trunk can be grafted from almost any donor site as the skin color and texture are not important. The upper lateral thigh is a good choice for an area on the trunk.  The recipient site must be well vascularized, as evidenced by the presence of granulation tissue. Surgically created wounds are designed to leave a well vascularized bed so no special preparation is required. Acute traumatic wounds often have a healthy bed, once injured and devascularized tissue is removed.  More chronic wounds may take a period of dressing changes to promote the growth of granulation tissue and neovascularization.  The wound bed must be free of all necrotic or ischemic tissue, cellulitis, purulent drainage, and significant edema. Granulation tissue can be heavily colonized and should be gently debrided back to the wound base. The recipient site should be thoroughly irrigated prior to graft placement. If the base is bony or tendinous a graft may not take and a flap may be needed instead.

STSG are harvested using a mechanical dermatome, which is a device with a large scalpel-like blade and an adjustable depth gauge. Split thickness grafts are harvested 16 to 18/1000 of an inch thick. To harvest the skin, the donor site is put on tension by the assistant after the surface of both the dermatome and the skin has been lubricated with either mineral oil or surgical water based lubricant. The dermatome is engaged on the skin surface at an angle of approximately 30 degrees and gently pushed forward while exerting a modest amount of downward pressure on the dermatome.  The meshing process can be done with varied expansion ratios ranging from 1.5:1 up to 9:1. The larger mesh ratios are reserved for patients with limited donor sites such as major burn patients. The smaller mesh ratios allow for limiting donor site scarring and offer improved drainage through the graft, thereby limiting subgraft fluid accumulation.  Meshed grafts are associated with more scarring.  STSG are transferred dermis-side-down to the recipient site and sutured into place with fine absorbable sutures. If the wound has an unusual surface contour, multiple tacking sutures can also be applied over the course of the graft to the underlying wound bed.  Immobilization of the graft to the recipient bed is imperative to prevent shearing of the graft and/or accumulation of fluid under the graft, which would prevent the process of neovascularization from occurring, thus causing skin graft failure. Meshed grafts drain underlying wound fluid through the interstices, but still need to be immobilized to facilitate healing.  Bolster dressings have long been utilized to stabilize grafts and this can be sewn in place to cover the graft and hold it in place. This is done by placing stitches around the periphery of the graft and leaving them long enough to tie together over the bolster, which is usually a ball of sterile cotton.  Negative pressure wound therapy (eg, vacuum-assisted closure device or VAC™ therapy) provides another method of graft immobilization.  The device is usually left in place for 5 to 10 days after graft placement.  Xeroform gauze is used to cover the donor site which allows for easier removal of the overlying dry gauze dressing. As the wound re-epithelializes underneath, the gauze will separate, usually falling off the wound by 7 to 14 days. Once the dressing is off, the patient should moisturize the donor site daily as the skin will be dry for a period of time.

Graft failure can result from insufficient vascularity of the recipient site, hematoma, seroma, infection, excessive tension, or mechanical shearing forces.  Comorbidities including diabetes, smoking, protein or vitamin deficiencies can affect vascularity and wound healing. In addition, medications such as steroids, immunosuppressive medications, and anticoagulants can interfere with wound healing.  Partial graft loss can be treated with wet to moist saline soaked gauze or other local dressings and usually will heal secondarily. Complete graft loss requires regrafting if the wound bed is found to be suitable.

from Dr. Keith Hood