Skin Grafts and Skin Graft Substitutes in the Distal Upper Extremity

James N. Long

Luis O. Vásconez

Jorge I. de la Torre

DEFINITION

Upper extremity wounds that are candidates for skin grafting very closely parallel wounds suitable for skin grafting in other areas of the body. Certain wound conditions must be adhered to, and the principles of grafting remain constant, no matter the location of a wound.

Terminology

Autograft refers to skin that is harvested from the same individual to whom it will be applied at a different location.
Isograft refers to skin harvested from an identical twin of the recipient individual. Isograft behaves like autograft.
Allograft refers to skin harvested from an individual of the same species as the recipient individual. Due to histocompatibility mismatch, these grafts eventually separate from the wound, except in immunosuppressed patients, and so provide only temporary coverage.
Xenograft refers to the use of skin grafts from a species different from the recipient individual. Due to histocompatibility mismatch, these eventually separate from the wound, except in the immunosuppressed patient, and so provide only temporary coverage. Xenograft use is associated with an elevated rate of wound bed infection.
Split-thickness skin grafts contain epidermis, along with a varying thickness of dermis that represents less than the full thickness of the dermis.
Full-thickness skin grafts incorporate the full thickness of dermis and epidermis.
Donor site refers to an area from which either a split- or full-thickness skin graft is harvested. Depending on the thickness of the graft, donor site treatment varies, from topical dressings, which typically are used for split-thickness skin graft donor sites, to direct closure, which is the usual method for addressing full-thickness skin donor defects.
Skin substitutes are semisynthetic or purely synthetic constructs designed to act as replacements for lost skin structures. Ideally, they will be incorporated into the host to act as durable long-term replacements for lost tissue. In 1984, Pruitt and Levine² described the characteristics of ideal biologic dressings and skin substitutes. Their list of qualities considered to be ideal for skin substitutes still holds true more than 20 years later:
- Little or no antigenicity
- Tissue compatibility
- Lack of toxicity
- Permeability to water vapor, as would be seen in normal skin
- Impenetrability to microorganisms
- Rapid and long-term adherence to the wound bed
- Capacity for ingrowth of fibrovascular tissue from the wound bed
- Malleability, which would allow the construct to conform to the wound bed
- Inherent elasticity that would not impede motion
- Structural stability against linear and shear forces
- Smooth surface to hinder bacterial proliferation
- Good to tensile strength that would allow it resist fragmentation
- Biodegradability
- Low cost
- Ease of storage
- An indefinite shelf life

Wound Bed

Before making a decision about using skin grafts or a substitutes, it is important to be familiar with the characteristics of a wound bed that make it suitable for grafting.
- Graft beds should be properly débrided so that they are free of dead tissue and made as clean as possible to help minimize the risk of graft loss from infection.
- Beds that are being considered for grafting must have an appropriate substrate from which the graft can derive its blood supply. In the context of upper extremity wounds, the bed specifically should contain no areas of denuded tendon or bone, as these denuded areas will not support inosculation (ie, neovascularization of the graft).
- A further requirement, once débridement is complete, is the reduction of bacteria in the wound, which usually is effected through the use of a pulse lavage system. Enhanced skin graft survival by means of reducing bacterial counts is supported by studies published by Perry et al¹ in 1989.
A useful tool in maturing a wound bed for grafting is the vacuum-assisted closure (VAC; KCI, Inc., San Antonio, TX) device. This device provides microdébridement of the wound bed and can help to promote the development of healthy granulation tissue, an ideal substrate for the support of skin graft adherence. Moreover, the VAC device can be used over the top of a skin graft applied to a wound and, through its negative pressure effect, limit fluid collection beneath the graft, also helping to ensure contact between graft and bed through an even distribution of pressure across the interface.
Elements key to the development of an adequate graft bed are as follows:
- Débridement of all nonviable tissue
- Minimization of bacterial colonization within the wound bed
- Ensuring that there exists an appropriate substrate for adherence of graft
- Microdébridement and maturation of the graft bed using appropriate dressings, which may include myriad measures ranging from the use of wet-to-moist saline gauze dressings to use of the VAC device.

ANATOMY

The decision-making process in choosing split- versus full-thickness graft in the distal upper extremity involves both gross and microanatomic considerations.
The lack of secondary contraction seen in full-thickness skin grafts supports their use on surfaces that overlie or are juxtaposed to joints. This lack of secondary contraction helps minimize the risk of unwanted joint contracture as the grafts mature.
Over broad flat surfaces, such as the dorsal or volar aspect of the forearm, split-thickness skin grafts perform well.
Wounds that involve the glabrous surface of the hand ideally are replaced with skin that possesses the same characteristics as the adjacent skin.
- Harvest of glabrous skin from the sole of the foot or from the contralateral uninjured hand should be considered for such use.
- In some cases, the wound may be so large that it is not possible to harvest sufficient donor skin while still permitting primary closure of the donor site. When this is the case, the arch within the sole of the foot may yield a full-thickness glabrous skin graft sufficient to cover the area of the original wound; however, the donor site then may require a skin graft itself. The donor site from the arch of the foot can be grafted with nonglabrous, meshed split-thickness graft with minimal morbidity due to its minimal weight-bearing requirement.

Microanatomy

As suggested earlier, the surgeon must be concerned with the microanatomic conditions of the wound bed.
- An appropriately vascular substrate is required to ensure proper graft take. Healthy fat, muscle, paratenon, or periosteum must be present within the base of the wound to ensure success.
- Additional considerations include proper débridement of nonviable tissues from the wound bed as well as the minimization of bacterial contamination.

Donor Sites

Glabrous skin
- The sole of foot within the arch, beginning at the junction of glabrous and nonglabrous skin along the medial aspect of the arch
- The ulnar aspect of the hand, beginning at the junction of the glabrous and the nonglabrous skin along the ulnar aspect of the palm
Full-thickness skin
Redundant areas of full-thickness skin available for harvest that maintain ease of primary closure of the donor defect include the lower abdomen, running from the anterior superior iliac spine in a gentle arc around the lower portion of the abdomen to the contralateral anterior superior iliac spine
- Skin harvested from this area may be hair bearing. Depending on requirements of the recipient site, selection of full-thickness skin graft can range from the relatively hairless portions found laterally to the hirsute areas found centrally.
Smaller areas of satisfactory full-thickness skin can be harvested from the upper inner arm. This skin, located at the junction of the medial biceps and triceps muscle groups, is thin and usually hairless.
Split-thickness skin graft
- Traditionally preferred sites have included the anterior thighs due to the ease of harvest and postoperative care of these areas.
- Another site that has favorable characteristics in terms of quality of graft donor, as well as healing of donor site, includes the scalp.
- Harvest of split-thickness skin graft from the scalp requires shaving of the head and the injection of epinephrine-containing wetting solution, for example, Pitkin solution or Klein solution, which is directed via puncture into a subgaleal plane to help minimize blood loss from the harvest.
- The very rich vascular supply to the scalp makes split-thickness skin grafts from this site quite robust.
- If the harvest is kept within the hair-bearing portions of the scalp, little to no donor defect can be detected once hair has grown back. Moreover, because of the high density of epidermal appendages in the scalp, reepithelialization of this area is more rapid than at other sites on the body. This rapid reepithelialization helps to minimize the potential for donor deformity (ie, scarring and dyspigmentation).

Harvest

Skin harvest is greatly facilitated by proper preparation of the chosen site.
First, a template of the bed to be grafted should be transferred to the donor site to ensure an adequate harvest. This is easily done with gentian violet and a sterile glove wrapper.
Limiting blood loss from the harvest site is desirable and is easily achieved by preinjecting the hypodermis of the planned harvest area with an epinephrine-containing local anesthetic.
If a long-acting local anesthetic such as Marcaine with epinephrine is used, the patient will have the additional benefit of prolonged donor site anesthesia postoperatively.
- As split-thickness donor sites are typically quite painful, use of long-acting local anesthesia is a real benefit and is appreciated by the patient.
When a large area is planned for harvest, attention must be paid to the appropriate maximum dosage for the local anesthetic selected. Dilute solutions in these cases can provide the benefits sought for these larger surface areas while still respecting the maximum allowed dosages.

PATHOGENESIS

Wounds in the distal upper extremity requiring coverage arise from a host of different mechanisms. Among the most
common are traumatic injuries, which commonly result in avulsive loss of skin. Other causes include burn injury to the upper extremity as well as defects created by tumor removal.
Any one of these mechanisms may result in a wide range of injuries, from simple skin loss to injuries of deeper structures, including loss of paratenon or periosteum.

NATURAL HISTORY

Skin graft healing varies from site to site on the body, and each location will vary from person to person.
Skin in young adults is thick and healthy; however, in about the fourth decade, the skin begins to thin.
Despite differences in skin thickness at differing anatomic locations, the overall dermal-to-epidermal ratio remains relatively constant: about 95% dermis to 5% epidermis.
Blood vessels form arborizations into the dermis of the skin through access portals in the dermal papillae.

How Do Grafts Work?

After application to an appropriately prepared wound bed, both split- and full-thickness grafts undergo a process that has been commonly termed take.
The process involved in adherence of skin graft to wound bed is complex and involves an initial hypermetabolic condition within the graft, supported by plasmatic imbibition. Plasmatic imbibition is the process whereby nutrients and oxygen are drawn into the graft by absorption and capillary action. During this time, the graft remains adherent by a thin and friable film of fibrin between wound bed and graft.
This early phase of graft support is followed by inosculation and capillary ingrowth. Before inosculation, there is a period during which ischemia and, therefore, hypoxia within the graft, with attendant histologic findings, are present.
Once capillary ingrowth occurs and makes contact with the vascular network inherently present within the graft, blood flow is reestablished, and the skin graft takes on a pinkish hue. This process likely involves both the use of the inherent network of vessels within the graft and new vascular proliferation.
Secondary adherence is mediated through fibrovascular ingrowth. The new vascular connections between graft and bed, as well as the new fibrous connections, solidify graft adherence.

Properties of Skin Grafts

Skin grafts have been used to provide both temporary and permanent coverage, offering the inherent benefit of protection of the host bed from additional trauma while also providing an important barrier to infection.
Split-thickness grafts tend to adhere to wound beds more easily and under adverse conditions that would not typically support full-thickness graft viability. This characteristic of split-thickness skin grafts provides a considerable advantage in managing difficult wounds; however, certain disadvantages can arise from their use. Once healed, split-thickness skin grafts undergo secondary contraction which, under uncontrolled conditions, can lead to pathologic contracture.
- Contracture refers to a disability in function that arises from secondary contraction.
- Additional disadvantages arising from the use of split-thickness skin grafts include dyschromia, poor elasticity, and reduced durability when referenced against their full-thickness counterparts.
Full-thickness skin grafts include the full thickness of the dermis, along with the epidermis. In the initial phases, full-thickness skin grafts tend not to show the hardy “take” often seen with split-thickness skin grafts. To ensure full-thickness graft success, their use should be limited to well-vascularized recipient beds only.
- Once established, full-thickness grafts offer distinct advantages; specifically, secondary contraction is far less problematic. Their thickness offers more resistance to external trauma and tends to be less likely to experience the dyspigmentation often associated with split-thickness grafts. They have much better inherent elasticity than split-thickness grafts, and for this reason, they are the graft of choice for use over and around joints.

Contraction

As mentioned earlier, split-thickness skin can undergo a process of secondary contraction that ultimately may lead to pathologic contracture. Immediately on harvest, full- and split-thickness skin grafts behave differently.
- The phenomenon of primary contraction refers to the tendency of a graft to shrink on elevation from the donor site. Substantial primary contraction is more often associated with full-thickness skin grafts than with split-thickness skin grafts. Full-thickness skin grafts contain the entire dermal layer and have more elastin than split-thickness skin grafts.
- It is clinically important to remember that the immediate- and long-term elasticity of full-thickness skin grafts is much greater that in split grafts. It is this elastic property that makes full-thickness skin grafts an ideal choice for use around joints.
- Once skin grafts have healed in place, the secondary process of contraction occurs more than in split-thickness grafts.
- Full-thickness grafts tend to remain about the same size and, for practical purposes, show little to no secondary contraction. Full-thickness skin grafts have the capacity to increase their surface area with limb growth over time, whereas split-thickness grafts tend to decrease in size by a process of contraction or, alternatively, their size remains static.

Reinnervation

The restoration of sensation in skin grafts is mediated through both peripheral ingrowth and direct growth into the graft from the bed.
Factors affecting reinnervation of skin grafts include the location and quality of the recipient bed as well as the choice of full- versus split-thickness skin graft.
Timing of recovery is variable, with some sensory recovery at between 4 and 6 weeks postgrafting. The return of normal sensation occurs between 12 and 24 months.
The speed with which sensory recovery is realized depends on the accessibility of graft neural sheaths to wound bed nerve fibers. Accessibility of neural sheaths is improved in full-thickness grafts over their split-thickness counterparts, and, therefore, sensory recovery in full-thickness grafts is both more rapid and more complete.

Dyspigmentation

The harvest of a graft disrupts its normal circulation, causing a loss of melanoblast content. This reduction results in a significant decrease in the number of pigment-producing cells within the graft.
After graft revascularization, the initial hypoxia is corrected, and the melanocyte population recovers to a normal level.

Skin Substitutes

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