The liver is macroscopically divided by the falciform ligament and the attachment of the round ligament of the liver on the diaphragmatic surface, as well as the sagittal fissure on the visceral surface, into a larger right lobe and a smaller left lobe. However, this does not correspond to the functional structure of the liver (1). The functional structure is based on the portal branching into individual, independent subunits, the liver segments (1). According to Couinaud, eight liver segments are distinguished. These are numbered clockwise and begin with the caudate lobe as segment I (2).
The liver accounts for a total of 20 – 30 % of the cardiac output. The blood is transported into the liver via arterial (10 – 20 % of the blood supply) and portal venous vessels (80 – 90 % of the blood supply) in a three-dimensional network (3). Blood is drained from the liver via the hepatic veins. Other vessels draining from the liver are the bile ducts (3). Due to a higher content of collagen and elastin, these duct systems differ significantly in their structure and resilience from the liver parenchyma (4). The bile ducts are the most resilient structures. These properties can be utilized in liver resection. Dissection methods that exploit this different tissue composition are referred to as selective. These primarily include blunt dissection, the ultrasonic aspirator (CUSA®), and the water-jet dissector (Water-Jet) (3, 4, 5).
In contrast, non-selective resection methods are distinguished. In these, no distinction is made between liver parenchyma and duct structures. Examples are mechanical instruments such as the scalpel, scissors, and with limitations the staple suturing device, as well as thermal instruments such as the high-frequency coagulator, the laser, or the UltraCision® scissors, which work both thermally and mechanically (3).
Decisive parameters for the postoperative outcome and the patient's survival are the level of intraoperative blood loss and transfusion requirements. This results in the demand that modern liver surgery should employ methods that are as parenchyma-sparing and low-bleeding as possible (4, 6, 7). Through continuous improvement of dissection techniques, the liver resection-associated mortality is currently 2 – 4 % (4).
In the following, selected selective dissection options will now be discussed. The technique of parenchyma dissection is strongly dependent on the habits and school of the surgeon.
Blunt Dissection
Lin et al. first described the technique of finger fragmentation in 1958 (8). In this, the liver parenchyma is crushed between the fingers. This allows larger vessels to be isolated and subsequently ligated. This technique is very archaic and unsuitable for modern, blood- and parenchyma-sparing, segment-oriented liver surgery (3, 8). This original form of dissection is still mentioned in a few textbooks, but is obsolete in clinical practice today (8).
Blunt dissection with a clamp is a further development. In this, the liver tissue is crushed between clamps and the more resilient blood vessels and bile ducts are mechanically isolated from the parenchyma. The clamp technique is still used, but the blood loss and dissection time are unsatisfactory (3, 5). In principle, however, all variants of liver resection are possible with clamp dissection (5).
A modification of this clamp technique is blunt scissor dissection. The liver parenchyma is gently pushed apart with the closed scissors, thereby isolating the duct structures. The smaller duct structures are subsequently closed with metal clips, the larger vessels are oversewn or ligated (9). Blunt scissor dissection is a frequently used method that can be performed quickly and cost-effectively. Lesurtel et al. showed in a randomized study that blunt dissection in a standardized operation (hemihepatectomy) was on the one hand the most cost-effective and fastest to perform, and on the other hand also showed the lowest intraoperative consumption of blood products and the lowest intraoperative total blood loss (10). In numerous centers, this type of dissection is still the standard procedure in non-cirrhotic, non-cholestatic livers (4, 5, 10).
Ultrasonic Aspirator (CUSA®)
The principle is the conversion of electrical energy through ultrasound into mechanical energy (3). The function of the CUSA® – Cavitron Ultrasonic Surgical Aspirator – is based on the combination of ultrasound fragmentation with aspiration and irrigation. The energy generated by the ultrasound triggers fragmentation of the liver tissue due to the water content dissolved in the tissue (3). Due to the different tissue composition, selective fragmentation of the various structures of the liver tissue is enabled. Tissue with high water content (parenchyma) is fragmented faster than tissue with higher tissue content (vessels, bile ducts) (3). Irrigation cools the device, brings the fragmented tissue into suspension, and thereby achieves a combined aspiration function (3, 4, 11). The aspirate can subsequently be subjected to histopathological examination in addition to the resected tissue (3). Another advantage of simultaneous aspiration is the reduced risk of intraoperative tumor cell dissemination during tumor resection (3, 12, 13). In studies, liver resections with the ultrasonic aspirator showed a significant reduction in intraoperative blood loss, transfusion requirements, operating time, mortality and morbidity, as well as the length of hospital stay (14, 15). However, to achieve this, a relatively long ischemia time (Pringle time) is necessary intraoperatively (15).
Water-Jet
The Water-Jet dissector uses a high-pressure water jet for cell fragmentation (3). The high-pressure liquid jet operates at pressures of 20 – 50 bar and nozzle diameters of 0.1 – 0.2 mm (3). This allows the liver parenchyma to be washed away from vascular and biliary duct structures according to their hardness gradient. Liver dissection with the Water-Jet can also be performed laparoscopically (3). Contrary to the results of Lesurtel et al. (10), Loss et al. and Rau et al. showed that compared to liver resection by blunt dissection or using the CUSA®, with liver resection using the Water-Jet, the intraoperative blood loss, liver resection time, and liver ischemia time can be significantly reduced (16, 17, 18).
In studies, the additional application of high-frequency current or laser energy could significantly increase the dissection speed while maintaining selectivity. Larger vessels are thereby preserved, smaller ones (diameter up to 1mm) are coagulated (3). To avoid or reduce the risk of tumor cell dissemination, the jet solution can be mixed with cytotoxic drugs (3).
Liver dissection with the Water-Jet is the standard procedure in our center for both open and laparoscopic liver resections due to the advantages mentioned above.
Due to the continuous improvement of dissection techniques, surgical resection of liver tissue is a safe and standardized operation, especially in appropriate centers (4, 17). Currently, the open surgical approach is the method of choice for extensive oncological liver resections (17). However, the development of suitable instruments for efficient and safe liver surgery has led to a decisive advance in laparoscopic liver surgery (5).
In the current literature, both laparoscopic and open liver resections show low postoperative complication rates (17, 19, 20, 21). With appropriate selection (benign liver lesions, smaller, peripherally located carcinomas), a laparoscopic liver resection should be performed primarily, as it results in shorter hospitalization and lower minor complication rate, with identical major complication rate (16, 17, 19, 20). Critically, it should be noted that extended liver resections are currently still more frequently performed in open technique, and for these procedures, both higher morbidity and longer hospitalization are to be expected. The literature lacks larger, prospective, randomized studies on the oncological value of extended liver resections in laparoscopic and open technique. Such studies should also include a comparison regarding mortality, morbidity, and hospitalization. In smaller studies, it has already been shown that hemihepatectomies can also be performed safely laparoscopically (17, 20, 22). Currently, the performance of extended laparoscopic and laparoscopically assisted liver resections is still critically discussed in the literature (17, 19, 20, 22).
In laparoscopic liver resections, disadvantages are particularly evident in extensive, central findings in the exact three-dimensional orientation of the surgeon, for example in preparation at the large vessels. Bleeding complications are the most common reason for conversion to open liver resection (19, 20, 22, 23). Other disadvantages of laparoscopic procedures are the often higher time expenditure, higher costs, and greater dependence on the respective surgeon (16). Nevertheless, laparoscopic liver resections will increasingly become the gold standard in liver surgery by experienced surgeons in the future (19, 20, 22, 23).