Skin grafting is the usage of a segment of epidermis and dermis, completely separated from the body's vascular or nervous supply, to cover a wound on the body. It is also known as a free grafting technique as opposed to flap grafting. Skin grafting is indicated whenever trauma or oncologic surgery have resulted in major loss of skin. More specifically, large surface trunk wounds or wounds at the distal limbs should be considered. But for every skin wound the simplest technique to achieve reliable healing should be used. So before considering skin grafting, alternatives should be excluded. Axial pattern grafts can be used on limbs but are mostly limited to the proximal part. Second intention healing needs to be considered, but one should realize this can result in joint dysfunction by skin contractures and might result in unwanted scar tissue and skin friability. Distant direct flaps (limb to trunk) might not be tolerated by the animal or the owner.

Contra-indications to skin grafting can be divide in local and systemic factors. Local factors contain recipient site infection and limited blood supply (presence of cartilage or tendon, or radiated tissue). Systemic factors that compromise skin grafting are: anemia, cachexia, anorexia, chronic inflammatory conditions, uremia, hypoperfusion, and infection.

Skin grafts can be divided on the origin of the donor skin. An autograft (autologous transplant) is harvested from the same animal as where the wound is located. When the donor is an identical twin or F1 hybrid this is named an isograft (isogenic transplant). Both of these situations will result in a graft that is accepted and permanently imbedded in the animal. Temporary coverage of a wound is possible by using other grafts. The donor skin of an allograft (allogenic transplant or homograft) comes from an animal belonging to the same species. In a xenograft (xenogeneic transplant or heterograft) the donor skin comes from a different animal. This type of graft is often using in human burn victims where porcine skin is used for primary wound coverage. Lastly, prosthetic grafts can also be used. Advances in medicine have led to the use of acellular dermal skin substitutes. These consist of a synthetic epidermis and a collagen-based dermis. Placed on a wound the dermis is accepted and invaded by new blood vessels, while the epidermis is later replaced by a split-thickness autograft. Cultured epithelial autografts might become important in the near future. Skin cells or stem cells from bone marrow are used to start a keratinocyte culture that later results in an epidermal sheath that can be grafted.

Skin grafts can also be divided based on the principle. An island graft is intended for partial wound covering. Second intention healing of the remaining wound is facilitated by proliferation of keratinocytes from the graft. Based on the geometric shape of the grafted skin they are called pinch, punch, stamp or strip grafts. Disadvantages of island grafts is that they are still labor intensive (requiring general anesthesia for collection and placement), demanding aftercare (expense of bandages to promote healing) and the resulting functional and cosmetic result is poor. Paw pad grafts are a type of island grafts used to create a weight bearing surface after the loss of the metacarpal or metatarsal pad. Small rectangular segments of full-thickness pad tissue are taken from remaining digital pads on the same animal and sutured to the skin at their intended location.

Skin grafts intended for use on small animal distal extremity wounds need to resist trauma, show minimal contraction and are movable around joints. Full-thickness grafts offer the best functionality (texture and elasticity) and also best cosmetics (color and hair growth). Split-thickness grafts have a greater chance of success compared to full-thickness grafts because their nutritional demand is less. But disadvantages are the need for specialized equipment for harvesting, significant donor site care requirements, lack of hair coverage and poor skin functionality.

The ideal donor site graft site has an abundance of loose skin to allow primary closure, consists of skin durable enough for anticipated wear and optionally have matching hair color, texture, length and thickness. The cranial lower lateral thoracic area, the neck and ventrum are commonly used. Based on the pattern of the wound a segment of skin is cut out at the donor site. Skin edges are manipulated with stay sutures or fine skin hooks and the level of dissection is the loose adipose tissue under the panniculus muscle. All hypodermal tissues are removed from the graft using fine scissors or scalpel blade. Scraping is to be avoided since this damages the dermal layer (hair follicles and glands). When the wound bed (recipient site) is irregular or highly exudative stab incisions are made in the graft for better conformation and allowing drainage. Meshing the graft, making multiple staggered incisions, will further increase drainage capabilities and also increases the size of the graft. However, doing so impairs functional and cosmetic characteristics. The graft is preserved in a soaked sponge (blood, saline or Ringer's).

Before transferring the graft the wound bed is prepared: all tissue debris and possible bacterial contamination is removed and vascular supply for nutrition confirmed. Suitable recipient sites are fresh surgical wounds or healthy granulation tissue. In its early phase granulation tissue contains an abundant amount of growth factors, but chronic granulation tissue has decreased capillaries and should be excised. Infected granulation tissue is threated locally by antiseptic/antibiotic ointments. Epithelial edges are excised to prevent under-run of the graft or the graft is allowed to overlap the edges, with necrosis of the overlapping part expected to occur in 7–10 days.

The graft is placed on the wound respecting the direction of hair growth and fixed with simple interrupted sutures or skin staples to the surrounding skin edges. The first 1–2 days the graft will have a white color due to local vasoconstriction. Nutrition is achieved by diffusion of nutrients from the fluid layer between the graft and wound bed (plasmatic imbibition). Hemoglobin breakdown products create a purplish or cyanotic appearance and diffusion and absorption of fluid results in graft oedema which peaks at 2–3 days. Around this time new capillaries grow from the wound in the fluid layer and anastomose with existing capillaries in the graft (inosculation). Blood flow in the graft is restored by day 3–4 and fully functional at day 5–6 (revascularization). Venous and lymphatic drainage develop around day 8 and the graft's color changes at that time from purple over red to pink. Reinnervation takes up to 3 weeks.

Graft survival depends on careful asepsis, graft immobilization and minimal fluid accumulation. Pseudomonas and Klebsiella destroy the fibrin network responsible for early graft adherence and thus is devastating for graft survival. Topical and systemic antibiotics are therefore commonly used, even though infection is uncommon. Immobilization can be performed by traditional bandages with a non-adherent contact layer. Utilization of negative pressure wound therapy bandages achieves better immobilization, stimulates the adherence faze, supports plasmatic imbibition and possibly revascularization. Regardless of bandage technique, the first bandage change can be delayed until day 3–5 to prevent graft disruption. However, changing the bandage after 1–2 days allows correction of early postoperative problems: fluid accumulation under unmeshed graft can be drained, blood clots obstructing stab incisions are removed, and antiseptic/antibiotic ointments are reapplied to prevent infection. A sturdy bandage is applied during 2–3 weeks (avoid joint movement), followed by a lighter bandage for an additional 2 weeks. After removal of the bandage a moisturizing crème can be used (dry and pruritic skin) and owners are informed to be vigilant for auto-mutilation (licking induced by paresthesia from reinnervation).

References

1.  Bhandal J, Langohr IM, Degner DA, Xie Y, Stanley BJ, Walshaw R. Histomorphometric analysis and regional variations of full thickness skin grafts in dogs. Vet Surg. 2012;41(4):448–454.

2.  Kobayashi T, Enomoto K, Wang YH, Yoon JS, Okamura R, Ide K, Ohyama M, Nishiyama T, Iwasaki T, Nishifuji K. Epidermal structure created by canine hair follicle keratinocytes enriched with bulge cells in a three-dimensional skin equivalent model in vitro: implications for regenerative therapy of canine epidermis. Vet Dermatol. 2013;24(1):77–83.

3.  Prpich CY, Santamaria AC, Simcock JO, Wong HK, Nimmo JS, Kuntz CA. Second intention healing after wide local excision of soft tissue sarcomas in the distal aspects of the limbs in dogs: 31 cases (2005–2012). J Am Vet Med Assoc. 2014;244(2):187–194.

4.  Stanley BJ, Pitt KA, Weder CD, Fritz MC, Hauptman JG, Steficek BA. Effects of negative pressure wound therapy on healing of free full-thickness skin grafts in dogs. Vet Surg. 2013;42(5):511–522.

5.  Swaim SF, Bradley DM, Steiss JE, Powers RD, Buxton DF. Free segmental paw pad grafts in dogs. Am J Vet Res. 1993;54(12):2161–2170.

6.  Tong T, Simpson DJ. Free skin grafts for immediate wound coverage following tumour resection from the canine distal limb. J Small Anim Pract. 2012;53(9):520–525.