Fig. 1.1
Schematic representation of the gross appearance of the liver, with the macroscopical lobes. The external anatomical structures have little relation to the vascular anatomy (one exception is the round ligament)
On the inferior aspect, the hepatic pedicle widens into the hepatic hilum, drawing grooves similar to an incomplete “H” (missing the left inferior limb). The upper limbs of the H, made of the gallbladder on the right and the round ligament on the left, create the borders of the quadrate lobe; the horizontal bar of the H, made of the left portal pedicle and the lower right limb, made of Arantius’ ligament (joining the left portal vein to the confluent of the left and middle hepatic vein), create the boundaries of the caudate lobe, that encircles the vena cava.
Functional (Vascular) Anatomy
The morphological anatomy described above has little relation to the vascular anatomy of the liver, a point revealed from early anatomical drawings, but in particular from the work of Cantlie [3], Hjörtsjö [4], and Goldsmith and Woodburne [5], who identified separate vascular and biliary areas according to the branches of the portal pedicles.
It is the merit of the French anatomist Claude Couinaud [6] to have analyzed and systematized the vascular distribution of the main liver vessels into a scheme that is relatively constant and consensual. The scheme, popularized by an historical paper [2], is depicted in Fig. 1.2.
Fig. 1.2
Schematic representation of the vascular structure of the liver. The three hepatic veins and the four sectorial portal branches interdigitate like fingers of two opposing hands. The reader will notice the symmetry of the right and left liver, taking into account an anti-clockwise rotation in the disposition of segment 4 and segment 3 (maybe because the round ligament retains the segments of the left liver from taking a position symmetrical to the right?). While segment 2 is embryologically a left posterior sector, it is practical to call it a segment because two segmental branches are generally not seen within it, and because this territory is small
In Couinaud’s representation, the three hepatic veins interdigitate with the portal pedicles as the fingers of two opposite hands. The vascular territories defined this way, the right and left LIVERS, on the first division of the portal vein, and the SECTORS, on the second-order division, were further divided into SEGMENTS according to the distribution of third-order portal branches.
In extrapolating the above representation to obtain a closer fit to the morphological anatomy, Couinaud first postulated that there was only one segment (segment 2) in the lateral sector, and that there was a vertical plane rather than a horizontal plane dividing the medial sector into segment 3 and segment 4. Then, the sectors of the left liver fell into oblivion, leaving the simplified schemes with three segments in the left liver, illustrated in Fig. 1.3.
Fig. 1.3
(a) Schematic representation of Couinaud’s segments. As an anatomist, Couinaud drew the liver “flattened out” on a dissecting table. In fact, segments 6 and 7 are posterior to segments 5 and 8, as illustrated by (b), a much more faithful reproduction of the radiological and surgical reality
In this representation, the right and the left liver are defined by the bifurcation of the main portal vein. The middle hepatic vein runs in this plane, which can be approximated as the plane joining the gallbladder fossa to the vena cava, and corresponds to the main portal scissura or Cantlie’s line. On the right, the right hepatic vein separates two sectors: the anterior sector, where the right anterior sectorial pedicle (second-order division) divides into two (third-order) segmental branches, and the same for a posterior sector. On the left, the portal vein runs first in a horizontal direction, then in the direction of the umbilical ligament, with a concave side encasing one segment, and on the convex side where two segments are separated by the left hepatic vein. The caudate lobe remains on the posterior side of the portal vein and surrounds the vena cava. Indeed, this anatomy is not exactly the one described by Couinaud. Couinaud described the two sectors of the right liver as paramedian and lateral (Fig. 1.3a). This corresponds to a liver that has been flattened out on the anatomist’s table. The true three-dimensional liver was reestablished by naming the two sectors on the right as anterior and posterior, according to their position when the organ lies in the body and under the hands of the surgeon (Fig. 1.3b) [2].
Couinaud’s classification has many advantages. The classification of the main territories according to the hepatic veins, that are easily recognized in particular in modern axial imaging, is convenient; with the second and third-order portal branching that can be assumed to occur at the level of the portal bifurcation, it establishes an unambiguous system of coordinates that defines the segments (Fig. 1.4), and therefore the position of focal lesions.
Fig. 1.4
Transposition of Couinaud’s segmental representation in a modern axial radiological study (computerized axial tomography, venous phase). The vascular landmarks are easily recognized, and so are the segments (courtesy of Dr Pierre Loubeyre, Department of Radiology, University Hospitals of Geneva). IVC inferior vena cava, lhv left hepatic vein, rhv right hepatic vein, mhv middle hepatic vein, LPV left portal vein, raspv right anterior sector portal vein, rpspv right posterior dector portal vein
Also, in particular for some segments, there is a relatively good correspondence between the main anatomical planes of the common liver resections (right hepatectomy, left hepatectomy, left lobectomy) and Couinaud’s anatomical description (Fig. 1.5).
Fig. 1.5
The terminology of the hepatectomies consistent with Couinaud’s classification. In red are indicated the most precise denominations of liver resections; left lobectomy is the preferred term of the bisegmentectomy 2-3, and right and left extended hepatectomies for the five or six segmentectomies on the right and the left. Alternatively, hepatectomies may be defined by the number and identification of the resected segments. For one segment removed: segmentectomy + number, 1–8. Resection of two segments: bisegmentectomy + numbers. For three segments: trisegmentectomy + numbers (for instance: trisegmentectomy 8-5-1). Central hepatectomy (it is better to avoid using the term middle hepatectomy) has to be defined by the segments removed: usually segments 5-8-4, but it may also include segment 1. Resection only of segment 4 is segmentectomy 4. As segment 4 is divided into two subsegments—4a, the upper one, and 4b, the lower one—each may be removed independently: subsegmentectomy 4a or 4b. Right or left hepatectomies are well defined. For the lobectomies, left lobectomy may also be called bisegmentectomy 2-3. Right lobectomy includes five segments (segments 4-5-6-7-8). Because this extension of the right hepatectomy may remove segment 1 instead of segment 4, it is better to say right extended hepatectomy to segment …n, which makes it possible to specify to which segment the right hepatectomy is extended: segment 4 or 1. Extension may involve two segments: 4 and 1, which is the 6-segmentectomy on the right, also called right hepatectomy extended to segments 1 and 4. The left extended hepatectomy to segments 1, 5, and 8 is the mirror resection on the left: left hepatectomy extended to segments 1, 5, and 8, or 6-segmentectomy on the left
On closer study, however, there are several inconsistencies of the (simplified) Couinaud scheme that need to be resolved.
From a theoretical point of view, the conceptual and embryological symmetry of the liver is not respected: if the analogy of the interdigitating hands and the order of the portal branches defining sectors and segments were to be respected, Couinaud would have insisted on a large left medial sector composed by segments 3 and 4, separated by a vertical division, and a small left lateral sector to the left of the left hepatic vein, where two segments cannot be individualized. This discrepancy was recognized in the 1982 paper popularizing Couinaud’s classification [2] and underlined in our last paper [7].
What happens in this part of the liver is that the umbilical vein—coming from the left branch of the portal vein—is pulling out the portal branch and creates an additional division of this part of the liver, which is, in fact, if we put inside the portal branch, a single segment. The left medial sector becomes one segment, and segments 3 and 4 are indeed half-segments. The left lateral sector, only with segment 2, has to be united to the left medial sector that becomes one segment, and together they constitute one sector: one sector with two segments (Fig. 1.3b); this is in accordance with the usual description of Couinaud. In order not to change the numbers put by Couinaud, it is better to keep the numbers 3 and 4, knowing that these segments are in fact half-segments.
We arrive at the following description of the liver anatomy (Fig. 1.5):
two hemilivers (right and left)
three sectors (the right posterior, the right anterior, and the left)
seven segments
six segments (each sector divided in two segments)
right posterior sector: segments 6 and 7
right anterior sector: segments 5 and 8
left sector: half segments 3–4 (as one segment) and segment 2
plus segment 1
It is in fact fortunate for the diffusion of Couinaud’s segmental system that his obscure book was never translated, that neither he nor I (HB) insisted on this point in further papers, leading to the simplified but useful system diffusing in the world.
Alternative Representations
It is worthwhile for the experienced liver surgeon to whom this book is dedicated, to understand some points where the scheme of Couinaud does not fit the anatomical reality, better represented by alternative schemes or by a deeper understanding of Couinaud’s work. This not for a sterile anatomical discussion, but because we believe that conceptualizing these alternatives allows to perform the radical yet conservative liver resection required in modern liver surgery [8].
From an embryological point of view, a scheme recognizing the symmetry of the right and the left liver does make sense. As an embryological recall, there are at the beginning two umbilical veins entering the right and the left liver, as illustrated in Fig. 1.6 [9]. This embryological point is not only evident in the anatomical variations where the gallbladder is located on the left side, but also in all the cases where a distribution of the whole right anterior sectorial branches or of the branches of segment 5 recall the pattern of the left portal vein (Fig. 1.7 [10], in particular configurations (a) and (b), Fig. 1.8). In these cases, the surgeon can perform limited yet radical resections of territories in the anterior sector that keep the lateral branches of the traditional segments 5 and 8 well vascularized and drained by the right hepatic vein. The authors have performed, simply following the glissonean plane from the roof of the plate of the hilar bifurcation and ligating all the medial branches going to the left, some very satisfactory central hepatectomies, or resection of the posterior sector extended to part of the anterior sector and the right hepatic vein.
Fig. 1.6
Schematic representation of the development of the liver. (a) Two umbilical veins (UV) enter the right and the left portal vein (PV). (b) The right umbilical vein obliterates and the left umbilical vein remains. (c) The left umbilical vein obliterates and becomes the round ligament (RL), Arantius’ canal obliterates into a ligament (AR). PM paramedian sector, Lat postero-lateral sector (modified with permission from Makuuchi Ann Surg 2013)
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