A flap is a unit of tissue that is transferred from a donor to a recipient site with its blood supply. Numerous classification schemes exist. Flaps may be characterized by their component parts (e.g., cutaneous, musculocutaneous, and osteocutaneous), their special relationship to the defect (local, regional, distant, or free), the nature of the blood supply (random vs. axial), and finally by the movement that is required for the flap to fill the desired defect (e.g., advancement, pivot, transposition, and interpolation). This chapter will focus on the blood supply and classification systems for muscle flaps and their associated uses. The blood supply to the muscle is intimately involved with the overlying fascia and skin. In general, muscle flaps are “axial” pattern flaps, with a known blood vessel oriented longitudinally within the flap. These vessels will have perforating branches that then supply contiguous territories including the overlying skin. With the advancement of and enthusiasm for perforator flaps, the knowledge of the muscle blood supply has expanded into blood supply for territories of tissues (Chapter 4). These territories can be harvested as composite flaps incorporating all of the tissue layers or individual components can be taken separately based on the perforator blood supply.

The most universally accepted system of muscle flap blood supply was developed by Mathes and Nahai. Every muscle, in part or as a whole, has the potential for use as a muscle flap. Muscle circulation is based on specific pedicles that enter the muscle between its origin and insertion and consist of an artery and single or paired venae comitantes.10 The position, number, and size of the vascular pedicles influence both the likelihood of flap survival and the flap design. The relative importance of each vascular pedicle to muscle circulation has been determined in cadavers by colored latex and barium injections, allowing evaluation of each vascular pedicle with regard to its length, diameter, location, and regional source. Subsequent use of the muscles as flaps has confirmed the relative importance of each pedicle to muscle survival and the potential for various flap modifications. If a pedicle to a muscle is critical to muscle survival based on its size and distribution to the internal vascular architecture of the muscle, it is specified as a dominant (where multiple pedicles are present) or major (where more than one pedicle is dominant) pedicle. Nondominant pedicles are labeled as minor pedicle(s). When a series of segmental smaller vessels are identified that may support muscle flap survival despite ligation of dominant or major pedicles, these minor pedicles are considered secondary pedicles. Variations in major and dominant pedicle anatomy are uncommon, although the location and number of minor pedicles is quite variable.

Five patterns of circulation to the muscle have been identified and are the basis of the classification system (Figure 5.2): Types I to V.

As noted above, Taylor et al. performed injection studies leading to the angiosome concept of the body. They noted that vessels frequently accompany nerves and described a classification system based on these observations.11 These studies revealed that many of the currently used flaps can be considered neurovascular flaps. Muscles were classified into four types according to their extrinsic and intrinsic neurovascular supplies. Type I muscles are supplied by a single unbranched nerve. In Type II muscles, the nerve branches before entering the muscle. Type III muscles receive multiple motor nerves from the same nerve trunk, and Type IV muscles are supplied from multiple nerve trunks. This system provides the clinical information necessary to divide muscles into functional neurovascular units for local and distant transfer.

Each muscle and myocutaneous flap has a limited arc of rotation when transferred as a pedicle flap. The distance from the point where the pedicle enters the flap to the distal end of the flap defines the capability of that flap. A muscle flap that can be based on a dominant vascular pedicle can reach adjacent areas that fall within the radius created by the pedicle and the most distal portion of muscle that is supplied by that circulation. Generally, the muscle is released from either its origin or its insertion. The muscle is then mobilized on the major or dominant pedicle being utilized. In pedicle flap elevation, the pedicle is not usually skeletonized in order to avoid vascular injury and kinking. These rotational limitations should be incorporated into the surgical plan so that defect coverage will be maximized. With progressive mobilization of the pedicle, the arc of rotation of the flap can be increased. Release of the bony attachments overlying the point of entry of the vascular pedicle will also allow the muscle to be elevated as an island flap based only on its vascular pedicle with subsequent increase in its arc of rotation (Figure 5.3A-C).

Specific knowledge of anatomic landmarks including muscle insertion and origin and where the vascular pedicle enters the muscle will allow for better planning. A template can be made of the defect and then the arc of rotation of potential regional muscles can be plotted. Certain defects require two or more regional flaps, but knowledge of the muscular anatomy will allow reliable planning Muscle flap elevation based on the dominant pedicle is designated as the standard flap. If a muscle flap is elevated on its secondary pedicle, which requires division of the dominant pedicle, the flap is classified as a “reverse” flap. An example of this is a pectoralis muscle flap that is normally elevated on its dominant axial pedicle, the thoracoacromial vessels. The flap can also be raised as a turnover flap based on the secondary vessels from the internal mammary circulation, to cover a midline sternal defect.