Fiedler Lukas Sebastian, M.D. MBA^1,2, Daaloul Houda, PhD ^3,41 Corresponding author, ENT and Head and Neck Surgery, plastic operations, SLK Kliniken Heilbronn, Heilbronn, Germany, OrcidID: 0000-0001-9319-8260, l.fiedler@gmx.at

² University of Heidelberg, Faculty of Medicine, Heidelberg, Germany

³ Department of Neurology, Klinikum Rechts der Isar, Medical Faculty, Technical University of Munich, Munich, Germany, houda.daaloul@caire.ai

⁴ Caire Health AI GmbH, Neherstraße 1, c/o TUM Venturelabs, 81675 München

Keywords: Cutaneous Perfusion assessment, assessment, Flap, flaps, perfusion, monitoring

In cutaneous reconstructive surgery, we count on different workhorses. Depending on the extent of the defect, the reconstructive ladder in Plastic surgery is used for evaluation and method selection. This ladder starts with the primary closure, followed by the free skin grafts, local flaps, pedicled flaps and ends with free flaps. [1, 2] A skin graft does not have its vascular supply, whereas a flap is always connected to a vascular system. [3] Flaps can be of different supply patterns and are regularly used for defect reconstruction in larger skin and subcutaneous defects.

Best medical care needs preoperative planning and peri- and postoperative assessment of flap perfusion. A key to successful reconstructive surgery is the necessity for preoperative planning and intra- and postoperative assessment of flap perfusion to limit necrosis or flap loss. [4] Preoperative flap planning has evolved in recent times. In the past, flap planning was based on surgical and anatomical landmarks and flap proportions. However, modern techniques, such as the use of CT-angiography that enables perforator mapping as well as pre-/and intraoperative assessment, have enhanced surgical outcomes in individual cases. [5, 6] Despite progressive medical and technical developments, there is currently no available method that permits a uniform, simple, valid and cost-effective flap perfusion assessment. The gold standard methods mainly rely on the surgeon’s clinical assessment [7, 8], which involves observation of skin colour, temperature of the flap, capillary refill, and bleeding pattern [7, 9, 10]. Similarly, surgical re-exploration is a valid way to clinically assess perfusion, especially in the case of local and pedicled flaps. [11]

In an era of rapid technological advancement, as the field of reconstructive surgery continues to evolve, it becomes imperative for the surgeon to navigate through a multitude of technical solutions for assessing flap perfusion without feeling inundated. It is essential to possess a discerning understanding of when and how each method proves effective, is economically viable, and possesses the necessary sensitivity to genuinely inform clinical decisions in flap surgery.

This paper aims to furnish a thorough analysis of modern techniques employed in the assessment of flaps in cutaneous surgery. For this paper, we classify investigative modalities as non-invasive and invasive. Any necessary insertion of contrast medium, probes or needles into the patient is classified and treated here as an ”invasive” method.

As demonstrated in Figure 1, the skin (cutis) consists of two layers of different thicknesses, the epidermis and the dermis. [12] Underneath are subcutaneous tissue, fascia and muscle. [13] The epidermis comprises the covering epithelial layer, this is also where skin colour is controlled by the density of the resident melanocytes, while the dermis is the carrier of the vascular/nerve supply and collagen fibres. The superficial part of the epidermis is interlocked with the dermis via the stratum papillare so that a tangential displacement of these layers is not possible in a separable way. [12] In this respect, displacements and thus, flap mobilization takes place in the subcutis. While the subcutis can be prominent in terms of thickness in regions such as the cheek, there are areas such as the eyelids or the auricular anterior surface, as well as the lateral columella, where this layer is virtually absent. [12] The vessels of the skin consist of two vascular plexuses parallel to the surface, which serve not only for supply but also for thermoregulation. The superficial Plexus is located at the border between the reticular and papillary dermis. The deeper plexus is located between the cutis and subcutis. Both vascular systems are connected by vertical vessels. [12, 14]Mostly on muscles, larger defined arteries (with their accompanying veins) run parallel to the skin surface and send vertical vessels (in addition to vessels from the subdermal plexus) to the skin. Examples here are the superficial temporal artery [15], the supraclavicular artery [16] and the angular artery [17]. From these defined vessels, axial pattern flaps can be formed, i.e. those flaps that are supplied from a defined subdermal vessel. [14]