Authors: Liv Schöllhammer, MD, Christian Kaare Paaskesen, MD, Frederik Gulmark Hansen, med.stud. and Magnus Balslev Avnstorp, MD

When designing a flap, several factors must be considered to ensure optimal results. The donor site should be selected from an area with sufficient skin laxity, allowing closure with minimal tension and acceptable secondary tissue movement. If closure of the donor site causes visible traction on surrounding structures, an alternative donor site or reconstruction method should be chosen.

The suture line from closing the donor site should ideally follow the relaxed skin tension lines or align with facial subunit borders to make it as inconspicuous as possible. Additionally, when the flap is sutured into the defect, it must be tension-free, ensuring that secondary tissue movement does not distort adjacent structures. An ideal flap design connects the donor site to the defect in a manner that positions suture lines discreetly and avoids unacceptable secondary tissue movement during closure.

Understanding the skin’s anatomy, physiology, and blood supply in the operated area is also essential. A careful analysis of the defect—including lesion depth, available tissue, distortion risks, and vascularity—determines the most appropriate flap or combination of flaps. When using distant flaps, matching the thickness, color, and texture of the flap with the excised and adjacent skin is critical for optimal aesthetic and functional outcomes.

Random Cutaneous Skin Flap

A random cutaneous skin flap relies on vascular plexuses at its base for blood supply. As the length of the flap increases, perfusion pressure in the arterioles decreases. Necrosis occurs if the perfusion pressure falls below a critical threshold.

Historically, a maximum length-to-width ratio of 3:1 was considered safe for random flaps, but this has proven unpredictable. In well-vascularized areas like the head and neck, a 2:1 ratio is generally more reliable for successful flap survival. 

Axial Skin Flaps

Axial skin flaps are supplied by a specific named artery, allowing for a longer flap compared to random flaps. This consistent blood supply supports reconstruction in nearby and surrounding tissues. A notable example is the paramedian forehead flap for nasal reconstruction, which is based on the supratrochlear artery.

The sustainable size of an axial flap depends on the artery’s length and the area of skin directly supplied by it. Additional skin beyond the arterial supply can be included in the flap but is then classified as a random flap, relying on subdermal plexuses for blood supply.

Biomechanics of Skin Flaps

The extensibility and quality of skin vary significantly between individuals, depending on factors such as age, gender, body weight, previous trauma or surgery, medical treatments, and overall health. Within the same individual, skin properties also vary by anatomical region—for example, the skin around the eyes is thin and elastic, while the skin on the lower back is thick and has high tension.

Three key properties must be considered when designing a skin flap:

Viscoelasticity: Skin can expand under constant stress over time, a property utilized in serial excision techniques.

Nonlinearity: The biomechanical behavior of skin under stress is crucial in flap design. As shown in Figure 1, the skin’s stress-strain curve reflects collagen fiber alignment under tension. Initially, fibers straighten with minimal resistance (A), then progressively align and stiffen (B). Beyond this, excessive strain causes fibers to surpass their yield point, leading to irreversible damage (C and D). Since blood vessels also contain collagen, overstretching a flap can compromise vascular integrity, compressing capillaries and reducing perfusion. This increases the risk of ischemia and necrosis, making it essential to avoid excessive tension during flap design and closure.

Anisotropy: Skin tension varies directionally, with the highest tension running parallel to the relaxed skin tension lines (RSTLs). Incisions should follow these lines to minimize closure tension and reduce scarring.

Conclusion

When planning reconstructive surgery involving a local flap, careful consideration of the donor site, flap design, vascular supply, and skin biomechanics is essential. These factors ensure the best possible functional and aesthetic outcomes for the patient.

Acknowledgements

Illustrations: Christian Kaare Paaskesen, MD.