The subcutaneous tissue is constructed around a skeleton of connective tissue, which is filled with fat (Fig. 2.2). On ultrasound, this connective tissue is visible as white, echogenic layers which blend irregularly into the grey fat tissue in widely differing patterns (Fig. 2.3a). In the deeper zone of the subcutaneous tissue, the connective tissue fibres thicken to form a membrane which usually lies directly on the muscle fascia. This is called the membranous layer or pseudofascia. This membranous layer is not a duplication of the muscle fascia as described in the anatomy book by Tillaux in 1897, because their embryological origin is different. Muscle fascia arises from the mesoderm, while the membranous layer arises from the ectoderm (Tillaux 1897 in Appendix 2).

The muscle fascia and the membranous layer form a structural unit of connective tissue separating the muscle from the subcutaneous fatty tissue. In some parts of the leg, the two fasciae separate to form a tunnel for the veins and their accompanying arteries, nerves and lymphatic vessels. This tunnel is called the compartment of the great and small saphenous veins. In the upper part of the leg, it also contains the accessory saphenous veins. Within the fascial compartment, there are varying amounts of fat cushioning the vein (Figs. 2.3 and 2.4). Fascial compartments are found surrounding the following structures within the leg:

The membranous layer along the saphenous veins is seen as a white layer on ultrasound and can be distinguished easily from the surrounding echo-poor fat and muscle fascia. The fascia enveloping the saphenous trunks is important as a landmark for diagnosis and treatment and has only recently been discovered by duplex ultrasound. The characteristic ultrasound image of the saphenous veins between the two fasciae led to the description of the saphenous eye (Bailly [1995]) (Fig. 2.3b).

The saphenous veins lie directly on the muscle fascia. At some point along their course, the membranous layer separates from the muscle fascia to form a cover over the vein. These fasciae form an enclosed volume called the “saphenous compartment” (Caggiati and Ricci 1999; Caggiati 2001). In lean legs, this saphenous compartment is limited and contains very little fat (Fig. 2.3). The saphenous veins are tightly enclosed. However, in legs swollen by lipoedema, the saphenous compartment appears enlarged and stretched on ultrasound (Fig. 2.4). Here, a layer of fat is usually seen separating the vein from its enveloping fascia.

The membranous layer along the course of the saphenous vein was given the name saphenous fascia and has since been adopted into anatomical nomenclature.

In transverse view, two echogenic connective tissue streaks are visible which appear to anchor both sides of the saphenous trunks. They connect the adventitia of the saphenous vein, run relatively parallel to the membranous layer and the muscle fascia and finally merge into them (white arrows in Fig. 2.4). This was confirmed by anatomical dissection where two layers of connective tissue were identified connecting the saphenous adventitia to the membranous layer and the muscle fascia (green arrows in Fig. 2.2). Using ultrasound, they are easier to see in the thigh for the great saphenous vein and in the upper calf for the small saphenous vein. Caggiati called them the “saphenous ligament” (Caggiati and Ricci 1999).

The envelopment of the saphenous veins by the saphenous compartment between the membranous layer and the muscle fascia and their anchorage by the saphenous ligament appear to be responsible for the lack of tortuosity observed when they become refluxive and dilated. This is in direct contrast to the behaviour of their refluxing tributaries.

The great saphenous vein is the longest vein in the body. It originates from the confluence of the dorsal foot veins on the medial side of the dorsal vein loop and runs consistently along the front of the medial malleolus where it can be seen or felt when standing (Fig. 2.5).

It continues medial to the edge of the tibia up to the knee. At first, it runs in a slightly posterior position, but at about a hand’s-breadth below the knee, it curves further posterior behind the medial femoral condyle (Fig. 2.7).

In its course along the thigh, it returns to a more ventral position reaching the inner third of the groin where it drains into the femoral vein (Figs. 2.8 and 2.9). Throughout its course, it rests on the muscle or deep fascia. It lies deep to the arch of the saphenous fascia (Sect. 2.4.2).

At its upper end, the great saphenous vein drains into the common femoral vein at the saphenofemoral junction. Here, it curves down towards the deeper femoral vein where its path has been described as the “crosse” which is French for a bishop’s crook. The junction is located in the groin and is always distal of the inguinal ligament (Fig. 2.10).

The saphenofemoral junction is the confluence of the great saphenous vein with the common femoral vein. As shown in Figs. 2.10 and 2.11, the “crosse” refers to the region of the saphenofemoral junction rather than the point of entry into the common femoral vein.

The common femoral vein is a deep vein which runs below the deep fascia, while the great saphenous vein runs above the fascia. There is therefore an opening in the deep fascia, called the saphenous hiatus or oval foramen, through which the great saphenous vein passes immediately before it joins the deep vein. This saphenous hiatus allows the confluence of the great saphenous vein with the deep leg vein to be distinguished from confluences of other tributaries with the great saphenous vein when the saphenofemoral junction region is surgically exposed (Figs. 2.10 and 2.11).

A variable number of tributaries drain into the great saphenous vein in the groin, forming the confluence of superficial inguinal veins. Starting medial of the left great saphenous vein and going clockwise, these are the superficial external pudendal, superficial epigastric, superficial circumflex iliac, anterior accessory saphenous and posterior accessory saphenous veins (Figs. 2.10 and 2.11). The presence and form of the confluence of the individual veins are very variable and are described in more detail in Sect. 7.​1.

In 88.5 % of cases, the great saphenous vein has a terminal valve in the region of its confluence. The distance of the terminal valve from the saphenofemoral junction varies between 0.0 and 1.4 cm. A preterminal valve is present in 89.2 % of cases, 1.4–8.2 cm from the saphenofemoral junction (in Mühlberger et al. 2009).

In the saphenofemoral junction region, the superficial external pudendal artery usually crosses the great saphenous vein from lateral to medial (Fig. 2.11).

The membranous layer covers the great saphenous vein over its whole course, creating a tunnel from ankle to groin known as the saphenous compartment (Caggiati and Ricci 1999). In the distal part of the leg, the great saphenous vein is closely attached to both fasciae forming a compartment with a narrow tunnel. This broadens at the knee, and the distance over which the membranous layer is separated from the muscle fascia continues to broaden in the thigh (Figs. 2.12, 7.​32 and 7.​40). With normal amounts of fat, the great saphenous vein remains in constant contact with the two membranes. In this situation, the tunnel becomes broader with the corners of the eye further apart but the eye is no “wider open”. The broadening continues to increase up to the inguinal ligament, where the two membranes (membranous layer and muscle fascia) terminate, closing the upper end of the compartment.

In the knee region, the membranous layer is poorly defined. Using ultrasound, it appears to be composed of several layers which are loose and open in places. This disruption may result from the forces placed on the knee joint.

The saphenous fascia in an anatomical preparation can be seen in Fig. 2.13.

From descriptions and from the histology of the veins in the saphenous compartment as compared with tributaries (Sect. 2.6), it may be concluded that only veins which course through the saphenous compartment really correspond to the great saphenous vein. Duplication of the great saphenous vein is therefore only present when both vessels course through the compartment (Fig. 2.14). Using this criterion, duplication of the great saphenous vein is only found in 1 % of patients (Table 2.1) (Ricci and Caggiati 1999). This is in contrast to the considerably higher figures published previously for duplication (27 %, Kubik 1985 in Appendix 2).

In the groin region and proximal thigh, the great saphenous vein is fairly constantly seen in the fascial compartment. However, between the mid thigh and knee, the saphenous vein is absent from the saphenous eye in 15 % of legs: 12 % of patients with healthy veins and 24 % of patients with reflux (Caggiati and Mendoza 2004). This condition is described as a segmental aplasia of the great saphenous vein since histologically this vein is always present in the saphenous eye in rudimentary form (Caggiati 2000; Caggiati and Ricci 2000). In legs with segmental aplasia of the great saphenous vein, a large straight venous tributary is always visible in the subcutaneous fatty tissue, parallel to the empty saphenous eye. This joins the distal and proximal sections of the great saphenous vein and maintains the flow between them. Since this vein loop replaces the great saphenous vein functionally, it may be termed a functional great saphenous vein.

Great saphenous vein hypoplasia was defined by Caggiati when a segment of less than 3 mm in luminal diameter was found (Caggiati 2000).

The great saphenous vein does not have very many described tributaries visible on ultrasound. However, in a pathological vein, more tributaries can be seen on ultrasound or contrast phlebography. It is also known from saphenous vein harvesting that a healthy vein has few tributaries (Caggiati 2000).

The tributaries are known as anterior and posterior arch veins in the lower leg region and as the anterior and posterior accessory saphenous veins in the thigh. These named veins are fairly constant in position.

The anterior arch vein in the lower leg runs from the lateral region of the foot via the lateral malleolus, proximal along the lateral edge of the tibia, and crosses the tibia at a variable height between halfway and one-third from the top. It joins the great saphenous vein in the upper third of the calf. Two anterior arch veins are occasionally found in the calf (Fig. 2.18).

The posterior arch vein originates posterior to the medial malleolus and runs fairly straight proximally along Linton’s line on the inside of the leg. It drains into the great saphenous vein below the medial femoral condyle. Most of the important pathological perforating veins in the calf lie along Linton’s line (Fig. 2.19 and Sect. 2.4.7).