2.2.1 Introduction

The core of the continuous fascial system, which penetrates and permeates all tissue layers, is formed by bone and periosteum. The fascial system forms a 3-D scaffold responsible for the compartmentalization of the muscle-tendon unit and the differentiation of the subcutaneous tissue into a superficial and a deep layer before it merges into the reticular dermis. The dimensions and strength of these connective-tissue bands and their attachment to the dermis determine the mobility of the skin relative to the underlying structures and, thus, its stability. The main function of the fascial system lies in the maintenance of structural integrity, ie, the support and protection of the soft-tissue envelope. In case of trauma, the fascial tissue plays an essential role in tissue repair as this framework—rich in vessels—can provide a well-vascularized tissue matrix, allowing for rapid regeneration.

The fascial framework is a continuous syncytium of dense fibrous and loose areolar connective tissue, comparable to the walls of a honeycomb. The neurovascular elements are embedded in this connective tissue and follow the framework that serves as a scaffold down to the microscopic level. If the connective tissue is rigid and tear resistant, such as in intermuscular septa, periosteum, or deep fascia, (ie, muscle fascia), the neurovascular structures run beside or along it. If the connective tissue is loose, the vessels and nerves run within it as in the superficial fascia of the skin ( Fig 2.2-1 ). Occasionally neurovascular structures and tendons are found in fibrous sheaths or canals within bone, providing stability and protection against bowstringing of tendons. Loose areolar tissue within these tunnels allows the arteries to pulsate and the veins to dilate.

From a pathophysiological point of view, the fascial system can form an anatomical barrier to tumor growth and infiltration. It can also be a site of potential inflammation (eg, plantar fasciitis) or infection (eg, panniculitis, cellulitis, fasciitis,) allowing them to spread (eg, necrotizing fasciitis). Furthermore, the fascial system may constitute a strong limitation in posttraumatic and postischemic swelling, initiating a cascade, which leads to the clinical picture of compartment syndrome.

The knowledge of the vascular anatomy integrated within this fascial system is a prerequisite for any surgeon who intends to adequately deal with soft-tissue trauma or major flap surgery.

This vascular system is differentiated into:

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  vascular axes and source vessels

  
  
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  vascular arrangement

  
  
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  vascular plexuses

  
  
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  microcirculation

  
  
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  venous drainage.

This is essential in order to illustrate the different levels of blood flow from the heart to the muscle, subcutaneous tissue, and skin as well as backflow in the venous system, and to appreciate the vascular organization within these tissues, especially the physiologically most active zones for the exchange of metabolites, ie, the microcirculation.

2.2.2 Vascular axes and source vessels

Starting at the level of the groin and axilla, respectively, the arborization of the major arteries and veins provides source vessels for different flaps that can be isolated for specific indications. The anatomical details of the vascular tree, in particular the one of the lower extremity, is described in chapter 10.3.

2.2.3 Vascular arrangement

Arterial injection studies of source vessels (segmental and distributing arteries) have proven the existence of 3-D units consisting of skin and underlying deep tissue, forming vascular territories [1, 2]. These are called angiosomes ( Fig 2.2-1 , 2.2-2a–b ). On the next level embedded within such vascular territories, the blood supply to the skin is provided by two main sources:

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  a direct cutaneous vascular system

  
  
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  a musculocutaneous vascular network [3].

The cutaneous vascular system runs through structures such as fascia or septa of muscles. The musculocutaneous vascular network consists of three types of vessels:

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  segmental arteries, which are a direct extension of the aorta. They generally run beneath the muscles and are accompanied by a single large vein and often by a peripheral nerve [4].

  
  
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  perforating vessels, passing through septa or muscles (true muscle perforators). They serve as connections from segmental vessels to the cutaneous circulation. These vessels or perforators form ramifications to supply the muscles with blood [5].

  
  
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  cutaneous vessels consisting of musculocutaneous arteries which run perpendicular to the skin surface, and direct cutaneous vessels, which run parallel to the skin. The latter can be divided into a fascial, subcutaneous, and cutaneous plexus ( Fig 2.2-3 ).

Interconnections exist at all levels between adjacent vascular territories and can be true anastomosis or choke vessels ( Fig 2.2-1 ). Bidirectional perfusion via vascular connections between adjacent angiosomes may occur, depending on local pressure changes. Thus, compensation is possible in case one branch should be occluded, which permits the transfer of more than one angiosome on a single pedicle, based on its source vessel respectively. In general, the anatomical territory of each tissue in the adjacent angiosome can be included without endangering distal flap perfusion. It is of the utmost importance to know these safe anatomical boundaries of tissue in each layer that can be transferred separately or combined as a composite flap.

2.2.4 Vascular plexuses

Beside connecting superficial with deep structures, fascial frameworks present also a horizontal orientation, and so do their accompanying vascular plexuses. The deep muscle fascia is well vascularized with a pre- and subfascial plexus. In contrast, subcutaneous tissue is relatively poorly vascularized with the exception of the superficial fascial layer. Even more superficially, a subdermal, dermal, and subepidermal plexus guarantee an appropriate perfusion of the skin ( Fig 2.2-3 ).

2.2.5 Microcirculation

The vascular network between arteries and veins, ie, arterioles, capillaries and venules, forms the zone of the vascular system where most of the tissue oxygenation, nutrition, and metabolite exchange occurs. Insufficient arterial inflow pressure, venous outflow obstruction due to trauma, vascular insufficiency, inadequate surgical tissue manipulation (ie, incision, undermining), hematoma, seroma, and kinking of the pedicle can jeopardize microcirculatory tissue perfusion in zones furthest from the source vessels. This may lead to ischemic tissue damage, resulting in delayed wound healing, dehiscence and necrosis ( Fig 2.2-4a–b ). This, in turn, increases the risk of infection, which will further aggravate the situation. Maintenance of tissue perfusion in critical situations ( Fig 2.2-4c–e ) by hemodilution or pharmacological substances has been investigated intensively as has preoperative tissue protection, ie, tissue preconditioning [6, 7].

2.2.6 Venous drainage system

There are two systems of perforating veins. Communicating veins are large veins that pierce the deep fascia and connect the superficial venous plexus to the deep venous system. Concomitant veins are small, usually paired, and accompany the cutaneous arterial perforators often present within the fascial system. Another important distinction lies in the distribution of valves. Oscillating, avalvular veins permit the bidirectional flow usually found in smaller, horizontally organized veins, whereas the bigger veins mentioned above are equipped with valves that direct venous return toward the heart. Insufficiency of the venous outflow can be detrimental to the delicate equilibrium of the arteriovenous blood flow. Venous stasis can lead to edema formation and increases the risk of infection, wound-healing disorders, and tissue necrosis.

2.2.7 Innervation

Cutaneous nerves, like the arteries, pierce the deep fascia at certain fixed skin sites and follow the connective tissue framework. For muscle innervation, the nerve enters the tissue at the neurovascular hilum of the muscle and follows the intramuscular connective tissue to reach the muscle bundles. Usually, nerves take the shortest extra- and intramuscular routes compatible with the function of each muscle. Tendon innervation is largely, if not entirely, afferent.