Random subnormal plexus flaps are created to reduce tension of the wound closure. Local random subdermal plexus flaps include advancement flaps (simple/bipedicle), rotation flaps, transposition flaps, interpolation flaps and skin fold flaps. Unlike axial pattern subdermal plexus flaps (next lecture), random subdermal plexus flaps are not created from regions of skin with known angiosomes (i.e., supplied by a direct cutaneous artery and vein). Consequently the size of random flaps is limited by the blood supply within the subdermal plexus. The goal of subdermal flap creation is to produce a vascular flap of skin, from a region of loose skin to close a wound without excessive tension.

The blood supply of the dermis and epidermis originates from direct cutaneous arteries and veins which penetrate the overlying panniculus muscle and form the subdermal plexus. Consequently large flaps of skin can be elevated and still have a viable vascular supply from the subdermal plexus as long as the flap is elevated below the subdermal plexus.

Principles of subdermal flap use:

1.  Prepare wide area - clip sufficient area.

2.  Positioning - donor and recipient sites need to be accessible and in a relaxed position.

3.  Flap outline - flaps should be planned before surgery by identifying and measuring flap landmarks and marking with marker pen.

a.  Create flap with base wider than body.

b.  Avoid narrowing of pedicle.

c.  Limit flap size - two small better than one large.

d.  Create from area of ample skin.

e.  Avoid donor site of excessive motion or stress.

4.  Flap dissection and elevation - atraumatic and meticulous - flap undermined under panniculus muscle. Dissection should start at tip of flap and direct cutaneous vessels should be identified early in dissection. Use stay sutures to handle flap.

5.  Flap transposition - tension on base of flap avoided (origin of blood supply). Transposition beyond 180° avoided as may result in kinking of direct cutaneous vessels.

6.  Closure of donor site.

7.  Closure of recipient site.

8.  Manage dead space - placement of active suction drains ideal. Drain should exit percutaneously through adjacent skin rather than through flap. Tacking sutures may compromise blood supply. Bandages should be applied with care.

The most commonly used random subdermal plexus flap configurations include:

1.  Advancement flaps

2.  Bipedicle flaps

3.  Rotation flaps

4.  Transposition flaps

5.  Skin fold flaps

6.  Distant direct flaps

Advancement Flaps

Advancement flaps can be created as single or paired flaps which facilitate wound coverage by stretching skin into the wound. They are created by elevating skin under parallel skin incisions created parallel to lines of least tension. Though they are one of the most frequently considered flaps by many practitioners they are frequently the least versatile of the random subdermal flaps. They are only appropriate if the donor site is a region of ample loose skin. The classical locations of wounds that can be closed with advancement flaps is the head, muzzle and dorsal body wall.

Bipedicle Flaps

Bipedicle are created by elevating under skin between a wound and a skin incision made parallel to the wound and then transposing the skin into the wound bed. The flap is created such that is an equal width of the wound. In many circumstances the donor site is left to heal by secondary intention. This flap obtains its blood supply from both ends of the flap and hence is robust. The most common use for a bipedicle flap is to close wounds over exposed implants or bony prominences (olecranon or calcaneus) on extremities.

Rotation Flaps

Rotation flaps are created by rotation and stretching skin around an arc into the wound bed. As with advancement flaps paired flaps can be created. A large skin incision in the form of an arc is created from the wound bed and undermined to facilitate rotation of the elevated skin. The radius of the arc is equivalent to the length of the wound and the length of the arc is 4 times the width of the wound. As with advancement flaps the flap must be created from a donor site of sufficient loose skin (less than advancement flaps) Classical locations of wounds that can be closed with rotation flaps are the rump, muzzle, shoulders and body wall.

Transposition Flaps

Transposition flaps are created by creating a rectangle of skin parallel to the lines of greatest tension whose base is adjacent to the wound. The flap is rotated into the wound bed and not stretched into position like advancement and rotation flaps. As a consequence the donor site does not require significant loose skin. The flaps are very versatile and can be rotated up to 180 degrees. The base of the flap is equal to the width of the wound to be closed and the length of the flap is equivalent to the length of the pivotal arc (longest arc from the wound base to the furthest edge of the flap). Due to the versatility of transposition flaps they can be used in most locations including around natural orifices as there is no risk of orifice distortion by flap rotation if the flap is appropriately planned.

Interpolation Flaps

Interpolation flaps are modifications of transposition flaps in that the rectangular flap is not elevated adjacent to the wound but distant to the wound. Consequently the flap is rotated over healthy skin into the wound bed. There are few indications for the use of an interpolation flap over a transposition flap.

Skin Fold Flap

Skin fold flaps are created by 'unfolding' either the inguinal or axilla skin folds along either the body wall or the limb dependent on the direction of the flap required and the position of the wound. For both axilla and inguinal skin folds there are 4 permutations of flap available. Relative to other random subdermal plexus flaps the skin fold flap is large and robust. Skin fold flaps are used to close wounds over the flank, thigh, proximal forelimb and sternum.

Distant Direct Subdermal Plexus Flaps

Distant direct subdermal flaps are created by creating a pocket of skin (typically over the chest wall or flank) and transferring the wound to the flap (which is typically over the distal extremity) rather than transferring the flap to the wound as with all other random subdermal flaps. The wound is maintained within the flap of skin until revascularisation has occurred at which stage the flap is surgically excised from the donor site - at approximately 14 days after wound transfer. In most circumstances the wound is on a distal extremity which therefore requires the patient to tolerate having the limb positioned adjacent to the thorax or flank within the flap pocket during the revascularisation phase. Consequently only compliant patients are suitable. In most circumstances a skin graft is a more appropriate wound closure method for these patients.