Editing Skin flap design and principles (section)

== 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.