Large skin deficits that can't be closed by simply undermining the skin locally arise for many reasons (e.g., large tumour excision, burn wounds or abrasion and degloving injuries). Also, relatively small wounds can be difficult to close if there is little surrounding spare skin (e.g., distal limb wounds).

Various techniques can be employed to cover the exposed tissues and thus prevent prolonged healing, scarring and contracture associated with secondary healing. Skin flaps and skin grafts are two such techniques that can be used.

Skin Flaps

The base of the flap (i.e., the area where the flap remains attached) contains the blood supply that allows the flap to survive. Skin flaps can be classified in a number of ways:

 Composition. A basic skin flap consists of epidermis and dermis, a compound skin flap additionally has associated underlying tissues (usually muscle). To be absolutely correct, a skin flap raised with the underlying panniculus muscle from the trunk region is actually a composite or musculocutaneous flap.

 Blood supply:

 Subdermal plexus flap. The survival of these flaps depends on the deep or subdermal plexus of blood vessels entering the base of the flap.

 Axial pattern skin flap. This is a flap which incorporates a blood vessel (a direct cutaneous artery) in its base. These arteries are located at predictable anatomical locations and axial pattern skin flaps can be reliably elevated by following guidelines and anatomical landmarks that are reported in surgical textbooks. By following these guidelines the surgeon should be able to close the resulting defect without tension. These flaps have a good blood supply so larger flaps can be raised and more of the flap is likely to survive.

 Location relative to the wound:

 Local skin flaps. These flaps are simple and practical methods of closing wounds and are commonly used. The flap is raised from a neighbouring area to the wound where the skin is loose and pliable. The defect created by raising the flap can be closed primarily. These flaps can be further divided into two broad categories: rotating flaps--these rotate or pivot about a fixed point to close an adjacent deficit; advancement flaps--these flaps travel in a forwards direction to close a skin deficit.

 Distant flaps. These flaps are used to close skin deficits on the distal limbs. The skin flap is raised at a distance from the wound, and the skin flap can be transferred directly or indirectly to the defect. They are not used as commonly as local flaps.

Patient Preparation (Also Applicable to Skin Grafting)

When clipping and preparing an anaesthetised patient for reconstructive surgery it is important to prepare an adequate area, particularly when the wound is large or in a position where there is little 'spare' skin and more advanced reconstructive techniques may be necessary. An insufficient clip will compromise sterility and the surgeon's ability to extend their surgery.

The patient should be positioned on the operating table ensuring that 'spare' or loose skin on the other side of the body away from the wound is not 'trapped' under the patient.

Any reconstructive surgery is likely to be very painful and the patient should receive appropriate analgesia. This should be given preemptively; analgesic drugs should be included in the premedication and continued into the postoperative period, usually for several days.

Patient Care

The patient should not be allowed to interfere with the surgical wound so it may be necessary to use an Elizabethan collar. Hobbles can be a useful way of ensuring that the patient does not excessively strain a reconstructed wound of the hind-quarters. Any drains must be covered with a sterile dressing and the volume of fluid exiting the drain should be recorded.

It is hoped that the entire skin flap will survive but this is not always the case; if the blood supply is too severely compromised the skin flap, or part of it, will die. The usual criteria with which we assess the viability of damaged skin (pain sensation, warmth, colour and bleeding) are all unreliable for assessing skin flap viability. The flap should be assessed at least once daily initially. The flap will appear swollen at first, the colour may alter from red to purple and the flap can still survive, depending on the degree and outcome of the vascular compromise. There is little that can be done postoperatively to alter the outcome other than ensure good basic patient care (i.e., analgesia, adequate nutrition, a clean and comfortable padded bed and prevention of self-trauma).

If the flap becomes necrotic then it is best to remove this portion only when the borders are clearly demarcated. The necrotic skin will not be painful and removal of this portion is possible under sedation. The underlying wound can be allowed to heal by second intention, or further reconstructive techniques may be necessary depending on the size of the exposed wound.

Skin Grafts

A skin graft is a piece of epidermis and dermis that is taken from one site, removed as a free piece of tissue and transferred to a site elsewhere on the body. The survival of a skin graft depends on the development of a new blood supply from the wound bed. Skin grafts are classified depending on the amount of dermis that is transferred. A full-thickness skin graft consists of the epidermis and entire dermis, a split-thickness graft is composed of epidermis and varying amounts of dermis.

Additionally, grafts can be classified depending on their size (stamp and strip grafts) and whether they are meshed (have slits cut into them) or not. Meshed grafts have the advantage that the harvested skin can be 'expanded' to cover a much larger wound than the size of the original graft and the slits in the graft allow fluid to drain from the graft bed. The disadvantage of meshed grafts is that the cosmetic result may not be as good as a non-meshed graft because granulation tissue can grow up between the slits.

The commonest use of skin grafts is to cover wounds on the distal limbs.

The donor site is chosen so that it has similar hair colour, texture, length and thickness as that surrounding the wound. A common donor site is the lower lateral thorax where the skin is haired but thin, as there is plenty of mobile skin in this area for closure of the resulting wound.

Skin grafts can be placed on fresh surgical wounds that are vascular enough to provide granulation tissue, or on a healthy granulation tissue bed. They will not 'take' over stratified squamous epithelial surfaces, bone or cartilage because these tissues are unable to provide an adequate vascular bed.

Patient Care

Fluid accumulation under the graft and movement of the graft must be prevented because they will disrupt blood vessel formation between the wound and the graft; without developing a blood supply the graft will die.

Fluid accumulation is prevented/reduced by meshing the graft, or placing a drain under an unmeshed graft. Bandaging is used to prevent movement of the graft. It is essential that the bandage is applied so that it will not slip or rub. In addition the patient must be managed post-operatively so that a good bandage is not challenged by the patient's behaviour! Patients should receive restricted exercise (cage rest), they must not be allowed to interfere with the bandage and it must be kept clean and dry so that it is only changed at scheduled intervals.

The choice of bandaging materials will vary between clinicians but the aim of the bandage is to absorb fluid away from the graft bed, stabilise the graft on the wound bed and, usually, immobilise the limb (to reduce the chance of graft movement).

Bandage changes should be performed under sedation or general anaesthesia until the graft is adherent on the wound bed and healing well, because the graft can easily become disrupted whilst changing a bandage in a less compliant patient. The first bandage change is performed between 24 and 72 hours postoperatively. Re-bandaging at 24 hours allows detection of fluid under the graft, this fluid can then be drained to reduce the likelihood of graft failure. However, with meshed grafts, drainage of fluid is usually adequate and changing a dressing at 48-72 hours avoids disrupting the graft in the early stages of new blood vessel formation. If the primary layer has adhered to the graft it must be soaked in saline and gently removed. Further bandage changes are usually required every 3-4 days for at least 3 weeks.

When first placed the graft will be pale; over the next 48 hours it become a dark bluish colour. By 72-96 hours the dark colour should fade and the graft should change to a red colour. By the 14th day the colour should be pink/normal.

The commonest reasons for failure are fluid accumulation under the graft, movement of the graft and infection. Grafts of questionable viability should not be removed until it is certain that the tissue has not survived.