Designing a facial flap
Authors: Liv Schöllhammer, MD, Frederik Gulmark Hansen, med.stud. and Magnus Balslev Avnstorp, MD
It is crucial to understand the skin anatomy, physiology and blood supply of the operated area. A Careful analysis of the skin defect e.g., the depth of the lesion, tissue available in the region and distortion of this and vascularity determines which flap or combination of flaps are appropriate.
It is important to ensure that thickness of the skin, color and texture match the excised skin and adjacent skin when using distant flaps
Random cutaneous skin flap
The vascular plexuses become reliant on the blood supply from the basis when raising a random cutaneous flap. The perfusion pressure in the arterioles decreases with increasing length, and necrosis will occur when the perfusion pressure falls below a critical point.
Random cutaneous flaps were once considered to have a max length-to-width ratio of 3:1. This has proven more complex and unpredictable.
A ratio of 2:1 yields a successful flap in well-vascularized areas as the head and neck in most cases.
Axial skin flaps
The flap may be longer compared to a random flap – as blood supply is more consistent depending on the known specific artery. This allows for reconstruction in the immediate vicinity and in the surrounding tissue. An example of this is the nose reconstruction with the paramedian forehead flap based on the supratrochleal artery.
The sustainable size of an arterial flap is related to the length of the artery and the area of skin directly supplied by the artery. An additional area of skin can be included in the flap but is then classified as a random flap in extension of the arterial flap (supplied by the subdermal plexuses).
Biomechanics of skin flaps
Extensibility and quality of the skin varies significant between individuals depending on age, gender, body weight, previous traumas/surgery, medical treatment and overall physical health. There is great variation in skin quality within the same individual depending on the anatomical region. As an example, the skin around the eyes is thin and lax as opposed to the skin of the lower back which is thick with high skin tension.
Taking into account for three essential properties when preparing for and designing a skin flap is crucial:
The strain-stress curve (Fig. 3) of skin allowing stretching of a skin flap to a certain degree determining acceptable wound closure tension for adequate blood flow and flap survival.
The directional variability of skin tension. The highest tension in the skin runs parallel to the relaxed skin tension lines (RSTLs) and when possible in flap design, incisions should be made accordingly parallel to minimize wound closure tension and thereby seeking to reduce scaring.
The ability of skin to expand when placed under constant stress over time. This is utilized when doing a serial excision of the skin.
When planning reconstructive surgery – involving a local flap – all of the above must be taken into account to choose the optimal design and donor site for the best result.
Illustrations: Christian Kaare Paaskesen, med.stud.