The falciform ligament and the origin of the round ligament of liver on the phrenic aspect and the sagittal fissure on the visceral aspect macroscopically divide the liver into a larger right lobe and a smaller left lobe. However, this does not correspond with the functional anatomy of the liver (1). The functional structure of the liver rests on the portal vein ramifying into individual, completely independent subunits, the liver segments (1). Couinaud structures the liver into eight segments. They are numbered in clockwise direction and start with the caudate lobe as segment I (2).
In general, the liver accounts for 20% to 30 % of the cardiac output. Blood is transported into the three-dimensional vasculature of the liver via arterial (10% – 20 % of the blood supply) and portal venous vessels (80% – 90 % of the blood supply) (3). The blood drains from the liver through the hepatic veins. Other structures leaving the liver include the bile ducts (3). Due to their higher content of collagen and elastin these duct systems markedly differ in their structure and resistance compared with the parenchyma of the liver (4). The bile ducts are those structures with the highest resistance. These characteristics may be used in liver resection. Dissection techniques utilizing these differences in tissue composition are known as "selective". This primarily includes blunt dissection, ultrasonic surgical aspirator (CUSA®) and water jet dissection (3, 4, 5).
These must be distinguished from the nonselective resection techniques. The latter do not differentiate between hepatic parenchyma and duct structures. Examples include mechanical instruments such as scalpel, scissors and – with some reservation – staplers and thermal instruments, e.g., as radiofrequency coagulation, laser and the Ultrafidian® harmonic scalpel which employ heat and mechanical force (3).
Vital parameters of the postoperative outcome and survival in patient is the extent of intraoperative blood loss and the need for blood transfusion. In modern liver surgery this calls for parenchymal sparing and bloodless surgical techniques (4, 6, 7). With the continued improvement in dissection techniques the current mortality associated with liver resection is 2% – 4% (4).
Some selective dissection options are detailed below. The technique of parenchymal dissection heavily depends on the habits and surgical school of the surgeon.
Blunt dissection
Lin et al. in 1958 were the first to describe the technique of finger fracture (8). Here, the parenchyma of the liver is fractured between the fingers. This allows for isolation of larger vessels which may then be ligated. This technique is quite antiquated and unsuitable for modern segment-oriented liver surgery sparing blood and parenchyma (3, 8). This prototype of dissection is only mentioned in few textbooks today but is obsolete in present clinical practice (8).
Next step in the development of blunt liver dissection was the use of a hemostat/clamp. Here, the liver tissue is crushed between the branches of a dissector/clamp, thereby mechanically isolating the more resistant blood vessels and bile ducts in the parenchyma. Crush-clamping is still used today, but the blood loss and duration of dissection associated with it are deficient (3, 5). In principle, crush-clamping may be employed in all variants of liver resection (5).
One modification of crush-clamping is blunt scissors dissection. Here, the hepatic parenchyma is carefully split with the closed branches of the scissors, leaving the duct structures isolated. Smaller ducts are then transected between metal clips, while larger vessels are ligated, or suture ligated (9). Blunt scissors dissection is a common rapid and cost-effective technique. In their randomized trial Laurel et al. demonstrated that blunt dissection in standardized surgery (hemihepatectomy) was not only fastest and most cost-effective, but also excelled with the least intraoperative administration of blood products and lowest total intraoperative blood loss (10). In many centers, this type of dissection in non-cirrhotic, non-cholestatic livers is still the standard technique today (4, 5, 10).
Ultrasonic surgical aspirator (CUSA®)
The fundamental principle here is the transformation of electrical energy into mechanical energy by ultrasound (3). CUSA® – Cavitron Ultrasonic Surgical Aspirator – works by combining ultrasound fragmentation with aspiration and irrigation. Due to the water present in the tissue the energy generated by the ultrasound fragments the liver tissue (3). The difference in tissue composition allows selective fragmentation of the various structures in the hepatic tissue. Tissue with a high water content (parenchyma) is fragmented more rapidly than tissue with denser structures (vessels, bile ducts) (3). The irrigation cools the unit and suspends the fragmented tissue, thereby resulting in a combined aspiration function (3, 4, 11). The aspirate and the resected tissue may then be studied by histopathology (3). One other benefit of simultaneous irrigation and aspiration is the reduced risk of intraoperative tumor cell dissemination during resection of the tumor (3, 12, 13). Studies were able to demonstrate that in liver resection the ultrasound aspirator significantly lowered intraoperative blood loss, need for transfusion, operating time, mortality, and morbidity, as well as the length of stay in the hospital (14, 15). However, to achieve this does require a rather long intraoperative duration of ischemia (Pringle time) (15).
Water-Jet
Water jet dissection uses a high-pressure water jet for cell fragmentation (3). The high-pressure water jet works with pressures of 20bar – 50bar and jet diameters of 0.1mm – 0.2mm (3). This flushes the soft hepatic parenchyma from the harder vascular and biliary structures. Liver dissection with the water jet may also be performed laparoscopically (3). Unlike the results by Laurel et al. (10), Loss at al. and Rau et al. both demonstrated that liver resection by blunt dissection or with the CUSA® resulted in significantly higher intraoperative blood loss, longer resection time and hepatic ischemia than with the water jet (16, 17, 18).
Studies also demonstrated that the additional application of radio frequency current or laser energy significantly speeded up dissection while still maintaining its selectivity. This spares larger vessels while coagulating those with smaller diameter (up to 1mm) (3). The addition of cytotoxic agents to the jet solution will prevent/lower the risk of tumor cell dissemination (3).
Due to the benefits detailed above, liver dissection with the water jet is the standard technique in our center in both open as well as laparoscopic liver resection.
The continued improvement in dissection technique has made surgical resection of liver tissue a safe and standardized procedure, particularly in centers with appropriate experience (4, 17). At present, the standard approach in extensive oncological liver resection is open surgery (17). However, the development of appropriate instruments for efficient and safe liver surgery has resulted in decisive advances in laparoscopic hepatic procedures (5).
Current literature notes a low rate of postoperative complications in both laparoscopic and open liver resections (17, 19, 20, 21). Appropriately selected cases (benign hepatic lesion, small peripheral cancer) should primarily undergo laparoscopic liver resection because it will shorten the hospital stay and lower the rate of minor complications, with identical rate of major complications (16, 17, 19, 20). A critique of these results must point out that extended liver resections are often still performed in open technique today, and for these procedures a higher morbidity and longer hospital stay should be expected. No large prospective randomized trials comparing the oncological significance of laparoscopic and open surgery in extended liver resections have been published to date. These trials should also compare mortality, morbidity and hospitalization. Smaller trials were able to demonstrate the safety of laparoscopic hemihepatectomy (17, 20, 22). At present, extended laparoscopic and laparoscopically assisted liver resections still are very much under discussion (17, 19, 20, 22).
Laparoscopic liver resection, especially in extensive central findings, still suffers from drawbacks in the precise three-dimensional orientation of the surgeon, e.g., when dissecting the large vessels. Bleeding complications are the most common reason for converting to open liver resection (19, 20, 22, 23). Other drawbacks of laparoscopic procedures include the often longer duration of surgery, higher costs and the greater dependency on the surgeon performing the procedure (16). Nevertheless, in the future laparoscopic liver resections by experienced surgeons will become the gold standard in liver surgery (19, 20, 22, 23).