Skin flap design and principles

Physiology of flaps

• Perfusion is paramount - a poorly-perfused flap actually has worse survival than a skin graft
• Haematoma also compromises a flap. Both pressure and local inflammatory mediators seem to be involved.
• A perforator included in the flap will usually reliably supply skin from its own territory and the next territory, but beyond that it is doubtful.
• Blood supply can come from either horizontal or vertical vasculature
  • Perforators need to be considered for island flaps, and flaps in continuity with length:breadth ratio >1
  • Simplest skin flaps are supplied by horizontal component
    • Need the dermis and some subcutaneous tissue to include the subdermal plexus, and the skin will be supplied for some distance.
    • If the deep fascia is included too, the pre-fascial plexus will be captured, and the blood supply will be significantly better (fasciocutaneous flap). Flaps can be made longer and larger with more safety.
  • Island flaps can rely on pre-fascial plexus or solely perforators, depending on how deeply they are divided
• Length-breadth ratio
  • For flaps in continuity, if the length:breadth ratio is <1 (that is, broader than it is long) then the flap is generally safe
  • That rule can be broken, especially in the face, if a perforator included or if a known large vessel is included (axial flap)
• Delay phenomenon - divide the vessels that will not be useful for the lap a week prior to surgery, to allow the important vessels to expand. Useful but difficult to exploit due to needing an extra procedure.

General design principles

• Try to follow contour/aesthetic units of face
• Err on the side of a bigger flap rather than smaller - easier to trim down than to fix a small flap. Traditional advice was to 'think of a flap then double it' but this is probably excessive.

Flaps in continuity

• Defined as blood supply for the flap coming through an intact skin bridge
• Transposition flaps
  • Donor site runs radial to the defect
  • Donor site is closed directly, and the flap crosses over intervening normal skin to enter the defect
  • The point at the base of the flap opposite the defect is called the pivot point and this does not move
• Rotation flaps
  • Do not cross intervening tissue, but advance directly into the defect
  • Roughly semicircular with the pivot point at the far end of the incision from the defect
  • Takes up tension over the whole incision
  • Can use a back-cut at the far end to allow further advancement of the tip of the flap. Usually you can set up the flap so that the back cut is made in an area with loose skin.
• Advancement flaps
  • Either one limb or two limb
  • Two limb advancement flaps are susceptible to pin-cushioning
  • Takes up skin in the direction of closure, parallel to the incision
  • Advancement flaps in continuity are most useful for reducing dog ears, whereas island advancement flaps will help close wounds that would not otherwise close
• Rhomboid flaps
  • Subtype of transposition flaps
• Bilobed flap
  • Transposes skin through an angle up to 180 degrees while avoiding dog ears

Pivot points

• Pivot point - the fixed point at one side of the base of the flap in rotation and transposition flaps. If there is tightness in the closure, there will usually be a tension line beginning at the pivot point and extending to the far side of the defect. If this tightness is seen, the flap is high-risk for ischaemia, particularly venous ischaemia. Avoid this by ensuring the distance from the pivot point to tip of flap is the same as the distance from the pivot point to the far side of the defect.

Island flaps

• Defined as the flap being circumferentially incised, and all blood supply comes through the base. The subdermal plexus is disconnected from surrounding skin, so blood must come from deeper tissue.
• Large island flaps are safer than small ones - more likely to include a perforator, and less tension on the wound.
• More mobile than flaps in continuity due to released dermal tethering.
• No pivot point - entire island moves, and tension is evenly distributed.
• Quite mobile in the cheek with just dermal incision, but at many other sites, fascia must be divided to sufficiently mobilise it.
• Can be rotated or transposed, but usually moved by advancement.
• Lateral pedicle island flap
  • For use with well-defined deeper fascial layer restricting mobility of the island
  • Identify side of blood supply and incise deep fascia on other side
  • Often done as V-Y island advancement with concave side on blood supply side (horn flap)
• Fasciocutaneous island flap (perforator flap)
  • Divide skin and deeper fascia circumferentially
  • Requires identification of perforator
• Island flap on isolated vascular pedicle
  • Dissection of neurovascular pedicle to increase mobility but preserve sensation and blood supply