Skin Grafting General Principles

1. The skin is a large, indispensable, complex organ that develops both from the ectoderm (epidermis) and the mesoderm (the dermis).
2. Most of the epidermal layers survive without blood supply except at their junction with the dermis. The basal layer of the epidermis also contains melanocytes.
3. The dermis contains within it a substrate called collagen, interspersed with elastic tissue, sebaceous glands attached to hair follicles, sweat glands, blood vessels, nerves as well as lymphatics.
4. Together with the epidermis the skin as a whole is a barrier against environmental assault and also is the principal site for communication with its surroundings.
5. It serves a thermostatic function by judicious use of secretion of sweat controlled by the nervous system. The sebaceous glands play a smaller role in this regard and are not influenced by the nervous system. The two together also lubricate the skin and some of these secretions can be called as excretory in nature.
6. The skin because of its size is a huge storehouse for blood which can be diverted to other crucial areas at the time of adversity, for example in shock. A large rapidly occurring burn means depletion of a fair volume of blood as also loss of fluids due to passive evaporation of water as well as the inflammatory exudate.
7. The skin with its rich reticulo-endothelial system protects the body from invasion by bacteria and viruses. Because of its multilayered structure it is also resistant to mechanical, thermal and radiation injury up to a limit.
8. The hair follicles and sebaceous glands as well as sweat glands, which all lie in the dermis, have openings on the surface of the epidermis. The epidermal layer invaginates throughout these tubular structures and lines them from within and therefore the regeneration of the epidermis can occur from these invaginated layers as long as some of these structures are preserved in the dermis after an injury or after a skin graft is harvested. Sridhar from Chennai adds “When split skin graft is taken the donor site heals by epithelialisation from cut ends of sebaceous glands/ducts/sweat gland ducts and hair follicles. That is why the healing is fast and donor site has mottled appearance.”
9. Skin grafting means detaching (cutting) a piece of skin from one site and placing it on a bed which revascularises it.
10. The procedure of skin grafting is actually a transplantation of a complex organ which serves many vital functions.
11. A split skin graft is classified by its thickness. Purely epidermal grafts are rarely ever done at the present time. Depending upon how much dermis is taken the graft is called a thin, intermediate or a thick split skin graft. A thick graft leaves behind much less regenerative capacity in the donor area than a thin graft and consequently the donor site will take longer to heal and will be left with poorer quality skin.
12. When a full thickness graft is cut, the regenerative power to heal in the donor area is nil and therefore the donor area needs to be closed primarily (small areas) or requires to be covered with another split skin graft.
13. The thickness of the graft determines the value and quantity of the constituents that are transferred.
14. The thicker the graft the greater is the biological requirement for the graft to “take”.
15. When a graft is placed on a recipient bed, the process of “take” begins. For some hours it is bathed and nourished by plasmatic circulation or serum imbibition. Simultaneously fortuitous and accidental apposition of the vessels in the bed and those in the graft allows blood to be sucked into the graft (inosculation). Soon afterwards active penetration of the graft by blood vessels from the bed begins and is well established by the fifth day. This is when the graft regains its colour which was lost when it was harvested because of severe vasoconstriction as a result of trauma.
16. A graft that has ‘taken’ (to its bed) remodels itself over a period of several weeks or months. This process is called consolidation in which collagen gets fragmented and is re-laid, elastic fibers degenerate and form anew. No graft remains identical to what it was when it was harvested and there is always some loss of its collagen content and its elasticity. This loss is highest in thin grafts and the least in full thickness grafts. When a graft is harvested it tends to coil on itself because of its elasticity. The thicker the graft the greater the recoil. A thin graft can be spread easily and remains in that state. Those grafts which coil up on harvesting contract the least after they ‘take’. The reverse is true of thinner grafts.
17. In nature all wounds contract by the laying down of fibrous tissue through fibroblasts unless interfered with by local or systemic disease. A large granulating wound of some duration therefore has already started the process of contraction while a freshly created surgical wound which gets primarily closed shows little or no contraction. Therefore, a surgically created wound which is covered by a full thickness graft will show the minimum contraction while at the other end of the scale a large chronic, granulating wound covered by a split thickness graft, particularly if it is thin, will show maximum contraction.
18. When contraction is complete, the pathological state left behind is called a contracture (e.g. a burn contracture across the elbow or the ankle).
19. A graft fails to “take” when it cannot be revascularised or when the process of revascularization is harmed by a shearing force due to improper immobilization of the graft and the bed.
20. Failure of revascularization can occur when the bed on which the graft is placed has poor vascularity or the revascularization is prevented by a blood clot between the graft and the bed.
21. Though all granulating wounds can be considered as harbouring infection, a well prepared granulating wound will ‘take’ grafts. Suppuration is another matter. Pus and multiplying organisms have proteolytic enzymes, prevent capillary formation and an overt presence of pus is the poorest environment for ‘take’ of a graft.
22. Debilitating conditions, such as uncontrolled diabetes, hypoproteinemia or local irradiation usually have adverse implications for the recipient beds.
23. Completely normal reinnervation of grafts is extremely rare. Reinnervation depends on the bed on which the graft is placed rather than the graft. The greater the number of nerve endings in the bed, the better is the reinnervation. Reinnervation occurs by penetration of nerves into the graft but also by insinuation of hollow myelin tubes which are remnants of the remodeling process in the graft.
Dr. Sridhar adds, “When a thicker skin having hair follicle is harvested as a graft then hair can grow at the recipient site, but when thin graft is taken this does not occur. This fact must be kept in mind while harvesting thick graft for recipient areas where hair growth is not desirable.”