Alex M. Walker, BVSc, MANZCVS
Veterinary Specialist Group, Mt. Albert, Auckland, New Zealand
The veterinary surgeon is often required to close wound defects created by trauma, infection or from resection of mass lesions for diagnostic and therapeutic reasons. Successful closure of the resultant wound is reliant on adequate preplanning to meet certain important objectives:
1. To obtain closure with minimal tension to allow unimpeded wound healing
2. Allow normal function of the affected area
3. Allow the patient to experience ongoing pain-free function
There is a well-established repertoire of reconstructive options available to the surgeon, including skin grafts and skin flaps (axial pattern, local or subdermal plexus flaps, myocutaneous flaps). Critical to success is consideration and respect of the blood supply to the flap or graft.
The blood supply to the skin of dogs and cats is via direct cutaneous arteries that originate from deeper tissue and penetrate through to the skin in certain known anatomical locations. The artery then turns and runs parallel with the skin in association with the cutaneous muscle. Small branches feed the overlying dermis and epidermis. This network of vessels is called the subdermal plexus and it is essential this is protected when the skin is undermined or elevated. Dissection must be deep to the plexus, meaning the undermining is deep to the cutaneous muscle (cutaneous trunci, platysma) or between the subcutaneous tissue and deeper muscle fascia in areas of the body with a cutaneous muscle.
The integrity or strength of the blood supply to different techniques relates directly to the robustness of the repair. In order of decreasing blood supply - (most robust) axial pattern flaps, subdermal plexus flaps (sometimes called random flaps) and free skin grafts (least robust).
Guidelines for Flap Development
The aim of a local, subdermal plexus flap is to transfer tension from the edges of the original wound to the edges of a fresh wound created by raising and moving the flap of skin. Healing of a wound closed with a subdermal plexus flap is more reliable, because the skin is not involved in the original disease process and tension is minimized. Before surgery, the surrounding skin should be manipulated to determine the existing tension lines and where the most abundant skin is located and how best to move it. Spend some time doing this! Making a template of the proposed flap with cloth or rubber can help determine if the flap will move in the required direction and for required distance. Flaps are usually developed in the area adjacent to the wound defect and the surgeon must be committed to preparing a flap that is large enough to fill the defect without tension. This always involves a conflict of wanting to close the donor bed easily ( making a smaller flap) with closing the original wound without tension ( bigger flap) and ensuring the flap has adequate blood supply (width to length ratios). Preplanning with templates can help with these decisions. Because the flap is dependent on blood supply from the subdermal plexus, it is better to have a slightly wider base than tip (vice versa should be avoided). The flap should be wide enough and long enough to fill the defect, while allowing closure of the donor bed. Specific recommendations for flap length to width ratios are difficult because of variation in blood supply between individuals and anatomical locations. It has been recommended to create flaps with the base slightly wider than the width of the flap body to avoid inadvertent narrowing of the pedicle and to limit the length of the flap to that is required to cover the recipient bed without tension.
Subdermal Plexus Flaps
These flaps are sometimes called local or random flaps and derive their blood supply from the subdermal plexus of the skin still attached. Usually, they are elevated from the deeper tissue and 3 out of 4 sides of the flap are detached from the surrounding skin. Subdermal plexus flaps can be created anywhere in the body and can be classified according to the way the skin is moved or stretched (i.e., advancement flaps, rotational flaps, transpositional flaps, V-Y plasties, Z plasties).
As the name implies, these flaps have minimal rotation and are advanced into the wound by 2 incisions roughly perpendicular to the wound to create a single pedicle flap, which is then moved over the recipient wound. The length of the incision is usually equal in length to that of the defect. Two single pedicle flaps can be raised to form an H plasty. A "releasing incision" parallel to the length of the wound and about 1.5X the length can be used to create a bipedicle flap that can be advanced into the wound.
Rotation, transposition and interpolation flaps are all flaps that rotate on a pivot point. The rotation flap is a semicircular flap that rotates into the adjacent recipient bed. This flap can be used to close triangular defects and can be paired or single.
The transposition flap is a rectangular pedicle flap that is usually rotated 90 degrees into the wound bed. The transposition flap is the most useful of the rotating flaps and many specific examples of reconstruction are modifications of the transposition flap - Z- plasty (2 transposition flaps), skin fold flaps.
The interpolation flap is a rectangular flap rotated into a nearby but not adjacent defect. A portion of the flap must pass over skin between the donor site and the recipient bed. This portion can be tubed or a bridging incision made to accommodate the flap. Seldom used.
Axial Pattern Flaps
These flaps are oriented, so the base includes a known direct cutaneous artery and the flap has the network of vessels associated with that artery, thereby increasing the blood supply to that area of skin. Due to the robust blood supply, the flap can have increased length compared with a subdermal plexus flap - they can be twice as long as a subdermal plexus flap and they can be used in areas where the granulation bed is patchy (e.g., exposed bone or tendon). A number of axial pattern flaps have been described (thoracodorsal, superficial epigastric, caudal auricular, superficial brachial, reverse saphenous conduit, lateral geniculate etc.). The borders of these flaps are well defined in the literature, adding a list of reconstruction options to our armament.
Be reminded of Halsted's principles, because each one of the seven axioms has particular importance when reconstructive surgery is contemplated.
Strict aseptic technique
Gentle tissue handling
Preservation of blood supply
Obliteration of dead space
Accurate apposition of tissue planes
Minimisation of tension on tissues
Extra wide clipping is essential, because, as the skin is moved during the procedure, the haired edges can be pulled into the surgical field. A wide clip also allows an intraoperative change in plan if required - e.g., the skin does not move as freely as first thought, necessitating flap development from a different direction, or a neoplastic resection is more extensive than envisaged. Do not be restricted by your clip.
Positioning of the patient can also make life easier or harder for you. Make sure the skin you want to move is not trapped under the patient when it is positioned on the table. Make sure the skin to be used is available to you and try to support it with towels or sandbags. If the limbs are tied down, ensure this does not restrict the access or movement of the required skin. When placing the drapes, position the towel clamps so they will not limit skin movement.
Use sterile markers to draw the flap to be raised and a sterile ruler to measure the length is appropriate. Best to get this correct before the first cut. Use skin hooks (bent over 20-g needles) to minimize damage to blood supply and use sharp towel clamps to hold transposed skin in place while tension lines are assessed. Avoid placed "tacking" sutures in the deep surface of the flap to hold it in place, because this can damage fragile blood supply. Closed suction drains can be placed instead to stop fluid buildup that can interfere with flap healing to the wound bed.
Skin reconstructive surgery is painful, so the anaesthetic protocol should include adequate analgesia to minimize self-trauma by the patient. Epidural morphine is useful for the immediate 12 hours postop and systemic analgesics should be used for 24–48 hours. For large procedure, I like to use a fentanyl or morphine/lidocaine/ketamine (MLK) CRI. Patients are discharged with 10–14 days of oral analgesics.
Postoperatively, skin flaps can appear bruised or red due to some compromise of the blood flow. Failure of a flap or graft is progression of this colour to black. This usually appears in the tip of the flap first. The necrotic area of skin can be left as a "biological bandage" if there is no evidence of infection - sometimes, the necrosis is only partial thickness.
Bandages must be used with caution where a flap has been created. Advantages are they keep the incision and drain sites clean and provide some protection to the animal licking and worrying of the suture line. Bandages can also reduce dead space and seroma formation and support other concurrent orthopaedic injuries. The disadvantage is potential compromise of blood supply by compression of the capillary bed and sometimes direct occlusion of the direct cutaneous artery in an axial pattern flap (I have seen this in thoracodorsal flaps). Even a perfectly placed bandage can become a problem when the animal moves and the bandage shifts in response. The "less is more" concept is a good one here and often a stockinette tube is all that is needed. Drain exit sites should be dressed and protected.
A skin graft is a piece of skin that is totally removed from a healthy area of the body and transferred to the wound. The graft is devoid of blood supply, so it is reliant on the wound bed for its nutrition until new vascular connections are established. Initially, for the first 2–3 days, nutrition is by plasmatic imbibition whereby wound fluid is moved into the graft via the transected capillary ends by capillary action. At this time, the graft has no venous or lymphatic drainage, so the graft will look oedematous and dark blue during this phase. Towards the end of the first week, the capillaries in the granulation bed will link up with the capillaries in the graft - this is called inosculation. Nutrition and drainage will improve and the graft will be less oedematous and become redder. Finally, new vessels will cross from the wound bed to the graft (neovascularization) and at the same time, fibrous connective tissue forms that holds the graft firmly in place.
Healthy pink granulation tissue is the ideal wound bed for a skin graft, but healthy muscle, periosteum or peritenon can support a skin graft.
The donor site is chosen by accessibility when the patient is in position for graft application, expendability of skin, ability to close the donor bed and matching the nature of skin (thick or thin), colour and length to the recipient site. Note direction of hair growth before clipping, so the graft is oriented correctly - it is often difficult after the hair is clipped.
The recipient bed is often an irregular shape, so it is good to get an imprint of the bed on to sterile paper (glove paper), and then cut that pattern out to get a trace onto the donor site. Note the "up" side on the paper, so the pattern is correctly traced onto the donor site - if the paper is the wrong way round, the graft will be the mirror image of the recipient site.
The graft is prepared by removal of all subcutaneous tissues, because this interferes with plasmatic imbibition and inosculation. Removal of this tissue can be done by pinning the graft on to a block of sterile wood or cardboard and the subcutaneous tissue scraped off with a scalpel blade. Alternatively, the graft can be wrapped over the fingers of the left hand (for right handers) and Metzenbaum scissors are used to remove the subcutaneous tissue until the hair follicles of the dermis are visible. The graft should be almost luminous when held up to the light.
The graft can be applied as pinch/punch grafts, strip grafts, stamp grafts, or most commonly mesh grafts. A mesh graft is created by cutting multiple rows of holes in the graft, allowing the graft to expand in size and contour perfectly to the wound bed. The graft is sutured in place with 3/0 or 4/0 monofilament nonabsorbable sutures placed from graft to skin edge. The graft must be in contact with the wound bed in all places so tension must be avoided if the graft is stretched over a hollow creating a 'drum skin' effect. Tacking the graft to the wound bed should be avoided as well, because this may induce haematoma formation. Any fluid (pus, seroma, blood) that forms between the graft and recipient bed will delay or prevent graft revascularization.
Punch or pinch grafts are indicated in areas of high motion where it would be difficult to limit movement between sheet or mesh graft and recipient bed. These grafts can be used when donor site is limited and small punches of skin are harvested with a standard skin biopsy punch. The subcutaneous tissue is removed with a scalpel and the graft is placed in a small cut in the recipient bed made with a #15 scalpel. No sutures. The disadvantage of punch or pinch grafts is sparse hair growth and less than ideal cosmetics.
Bandages are essential for skin grafts. Survival of the graft eventually requires revascularization and the capillaries involved in this process are very fragile. Any movement between the graft and the recipient bed will shear off the new vessels resulting in graft failure. Even the movement during bandage changes is avoided for the first 2–3 days unless absolutely necessary. The patient should also be minimized by cage confinement and sedation if necessary. For grafts near joints, splints or split casts can be used to further restrict motion. The use of non-adherent primary dressings helps atraumatic removal of the bandage - soaking the dressing off with saline can help minimize the damage to the graft during dressing changes. The graft should gradually change from oedematous/blue to red and then normal skin colour over 2 weeks. Some areas may become necrotic and black and these can be left for the rest of the graft to heal as long as no infection is present. The necrotic portion will slough leaving the underlying granulation bed to mature and epithelialize.