Laparoscopic Anatomical Resection of Liver Segment 8 via a Hepatic Parenchymal Transection-rst Approach Guided by the Middle Hepatic Vein

Background Although recent technological developments and improved endoscopic procedures have further extended the application of laparoscopic liver resection, pure laparoscopic anatomic resection of liver segment 8 (S8) is still rarely performed due to the lack of an appropriate surgical approach. This article discusses the technical tips and operation methods for laparoscopic anatomical resection of liver S8 via a hepatic parenchymal transection-rst approach. Methods Clinical data of 14 patients who underwent laparoscopic anatomical resection of liver segment 8 via a hepatic parenchymal transection-rst approach guided by the middle hepatic vein (MHV) in the Second Aliated Hospital, Third Military Medical University (Army Medical University) from May 2017 to December 2019 were retrospectively analyzed. The operation time, intraoperative blood loss, postoperative complications, and hospitalization duration were observed. Results The operation was successful with no complications. No other abnormality was noted during outpatient follow-up examination. Conclusions pedicle (P8d) them accordingly. (4) Dissection of the left side transection plane and caudal side transection plane liver parenchyma: Hepatic resection is begun from the caudal side of the liver at the intersection of the S8 Glissonean pedicle level which was positioned by ultrasound and the Cantlie line via a hepatic parenchymal transection-rst approach. The resection is continued from the caudal to cranial side along the markings of MHV (left of the vein), until the critical separation of S8 from S4a was completed. Then, the hepatic resection was continued from the above-depicted starting point toward the right direction of caudal side transection plane and the caudal side liver transection plane parenchyma was divided 1-2 cm for better visualization. The left side transection plane resection and caudal side transection plane resection continued alternately to expose the root of the MHV. S8 was retracted cephalically during this procedure. The resection was continued until the S8 Glissonean pedicle or P8v and P8d were naturally exposed on the resected liver surface. (5) Management of the S8 Glissonean pedicle: The S8 Glissonean pedicle or P8v and P8d is clamped, and the liver surface ischemic line is marked along the resulting discoloration using electrocautery. Then, the right side transection plane hepatic parenchyma was initially transected along the ischemic line, followed by ligated and divided of the S8 Glissonean pedicle. (6) Management of the ventral branch of the draining vein from S8 (V8v) and intermediate vein for S8 (V8i) branches: When hepatic resection is continued further cranially along the MHV, the S8 branches of the middle hepatic vein, including the V8v and V8i branches and small ducts, are ligated and divided. (7) Dissection of the right side transection plane liver parenchyma: To expose the root of the RHV,


Background
Liver segment 8 (S8) corresponds to the anterosuperior portion of right paramedian sector, which lies between the middle hepatic vein (MHV) and the right hepatic veins (RHV) [1]. Because of the absence of anatomical landmarks on the liver surface of S8 and the many variations and deep location of the Glisson pedicle, laparoscopic anatomical liver resection (LALR) for lesions in S8 can be very challenging and is considered one of the most di cult laparoscopic surgeries. However, with increasing experience and developments in surgical techniques and instruments, limited reports in the last few years have shown the feasibility and safety of this surgery [2]. There are many technical tips for LALR of S8, and the core technical tip is how to choose an appropriate laparoscopic approach, which is a main determinant of surgical success. To date, the approaches for LALR of S8 roughly include ultrasound-guided S8 segmental portal branch puncture, a hepatic hilum Glisson pedicle approach, and a left and right hemihepatic splitting approach [3][4][5]. However, all these approaches have certain drawbacks. Through

Methods
The patient was placed in a reversed Trendelenburg and left semilateral position with head up 30° and leg splitting (Fig. 1). The surgeon stood on the right side of the patient, the camera assistant stood between the spread legs, and the assistant and monitor were on the left side of the patient, facing the surgeon.
The trocars were inserted according to the 5-port-method. The initial trocar (10-mm) is placed at a site 2 cm right of the umbilicus and was used as observation port. Two 12-mm ports were inserted at 5 cm below the costal margin on the right midclavicular line (for the right main working) and below the xiphoid (for the left main working), respectively. The 5-mm port was placed at the midpoint of xiphoid process and the umbilicus as the left subsidiary port. Additional 5-mm port was placed at the subcostal area that meets the right anterior axillary line as the right subsidiary port (Fig. 2). To prepare for extracorporeal Pringle's maneuver, a 3-mm length incision was made between left two ports through which a selfdesigned tube would be inserted for holding a cotton tape around the hepatoduodenal ligament.
Intraoperative procedure: (1) Liver mobilization: the round and falciform ligaments were divided to the second hepatic hilum to expose the root of the MHV and RHV and the crypt between MHV and RHV. The right coronary ligament next to the second hepatic hilum was also divided. and caudal side transection plane liver parenchyma: Hepatic resection is begun from the caudal side of the liver at the intersection of the S8 Glissonean pedicle level which was positioned by ultrasound and the Cantlie line via a hepatic parenchymal transection-rst approach. The resection is continued from the caudal to cranial side along the markings of MHV (left of the vein), until the critical separation of S8 from S4a was completed. Then, the hepatic resection was continued from the above-depicted starting point toward the right direction of caudal side transection plane and the caudal side liver transection plane parenchyma was divided 1-2 cm for better visualization. The left side transection plane resection and caudal side transection plane resection continued alternately to expose the root of the MHV. S8 was retracted cephalically during this procedure. The resection was continued until the S8 Glissonean pedicle or P8v and P8d were naturally exposed on the resected liver surface. (5) Management of the S8 Glissonean pedicle: The S8 Glissonean pedicle or P8v and P8d is clamped, and the liver surface ischemic line is marked along the resulting discoloration using electrocautery. Then, the right side transection plane hepatic parenchyma was initially transected along the ischemic line, followed by ligated and divided of the S8 Glissonean pedicle. (6) Management of the ventral branch of the draining vein from S8 (V8v) and intermediate vein for S8 (V8i) branches: When hepatic resection is continued further cranially along the MHV, the S8 branches of the middle hepatic vein, including the V8v and V8i branches and small ducts, are ligated and divided. (7) Dissection of the right side transection plane liver parenchyma: To expose the root of the RHV, the transection of the liver parenchyma was continued from the root of the MHV toward right side, in the crypt between MHV and RHV and the RHV direction. Subsequently, the RHV is exposed. Then, liver resection is performed toward the caudal direction e along the RHV and ischemic line, and the main trunk of the RHV is exposed. At that point, the surgeon was standing at the left side of the patient and performed the parenchyma transection via a cranial approach. After the completion of S8, the following structures are exposed at the resected liver surface: MHV, stump of S8 Glissonean pedicle, and RHV. (8) Bagging of the resected specimen: A protective bag was inserted intra-abdominally into which the resected specimen was placed. (9) Management of the resection margin: The surgical eld was irrigated. The margin was carefully checked for any bleeding or bile leakage. Hemostasis was achieved by using bipolar electrocoagulation or Prolene sutures ligation. Any bile leakage from the suspicious bile ducts was ligated using 5-0 Prolene sutures. Hemostatic products were routinely used. (10) Specimen removal: After the specimen was resected, the incision for extracorporeal Pringle's maneuver was extended to the incision for the left subsidiary port for removal of the specimen. (11) Drainage and wound closed: After a silastic drain placed under the right diaphragm, the wound was closed in layers with absorbable sutures (Fig. 3A-I).

Results
All 14 patients underwent blood biochemistry and tumor markers analyses, imaging examination ( Fig. 4), indocyanine green clearance test, and 3-dimensional reconstruction (Fig. 5) before the operation. After surgery, all patients were diagnosed with HCC. All patients completed the operation successfully without conversion to open surgery. The median operation time was 220 ± 52 min, median blood loss was 350 mL (200-500 mL), and blood transfusion was not needed. Pathology showed free surgical margins. The postoperative median hospital stay was 8 d (7-12 d). All the patients recovered well without severe complications.
All 14 patients were followed for 13.5-31.8 months, with a median follow-up time of 16.7months. During the follow-up period, none of them developed hemorrhage, bile leakage, and other complications. There was no reoperation or perioperative mortality during the follow-up. Imaging examination scans showed portal branch of segment 5 (P5) and portal branch of posterior segment (PP) were clearly exposed in almost their entirety and then preserved and the main trunk of the MHV was also preserved. All patients survived tumor-free.

Discussion
Anatomical hepatectomy primarily refers to liver resection according to liver segmentation, including single hepatic resection and multiple segmental hepatic (half hepatic) resection. Theoretically, anatomical liver resection has oncologic bene t compared with nonanatomical resection, because it systemically removes potentially tumor-bearing portal and venous tributaries around the malignant lesion. Limited resection of hepatic parenchyma can save postoperative functional hepatic reserve and enhance the possibility for future repeat hepatectomy [6, 7]. Since Azagra [8] and Kaneko [9] et al. reported laparoscopic anatomical liver resections in 1996, advancements in laparoscopic surgical techniques have led to increasing reports of laparoscopic anatomical liver resections. However, due to the lack of anatomic landmarks on the liver surface, anatomical variations and cross distribution of intrahepatic Glisson and hepatic venous systems, laparoscopic anatomical resection of liver S8 is considered a di cult and challenging surgical procedure by the minimally invasive surgery community. In laparoscopic anatomical resection of liver S8, the main di culty lies in the choice of surgical approach. The choice of laparoscopic surgical approach for anatomical resection of liver S8 is not simply a "road of entry" but a series of strategic decisions on how to accomplish the surgical goals while ensuring the safety and effectiveness of the surgery [10].
To date, the approaches for laparoscopic anatomical resection of liver S8 include an ultrasound-guided S8 portal branch puncture and localization approach, hepatic hilum Glisson pedicle approach, and left and right hemi-hepatic splitting approach. The ultrasound-guided S8 portal vein puncture and localization approach refers to ultrasound-guided injection of the methylene blue and ICG dye into the S8 portal vein branches to determine the extent of resection during surgery. The puncture is different from the one under Moreover, the portal vein is deeply positioned in S8 and is usually divided into ventral and dorsal branches. Thus, puncturing the corresponding portal veins one by one is di cult, and the borders of the S8 area with dye staining may not display accurately. The above factors lead to a low puncture success rate, and therefore, this approach has been di cult to popularize in a short time [3]. The Glisson pedicle approach refers to removal of S8 after isolation of S8 Glissonean pedicle branch from the right anterior and posterior pedicles. This approach is essentially an extra-Glissonian approach and is safer. However, this approach is di cult and not practical because of the deep location of the S8 Glissonean pedicle, anatomical variation, the small space in the liver hilum area, the di culty in exposing the hilar plate, the long initial transection plane, the requirements of the dissection technique and the limitations of laparoscopic instruments [11,12]. This approach is useful and reasonable when S8 Glissonean pedicle rami es from the right anterior Glissonean pedicle near the hepatic hilum. The left and right hemi-hepatic splitting approach refers to liver resection that is performed from the caudal to cranial side along the Cantlie line, superiorly to the second hepatic hilum and inferiorly to the rst hepatic hilum, to complete the mid-split of the liver parenchyma and expose the MHV and then to separate the S8 Glissonian pedicle and remove S8. This approach is associated with a large wound, a risk of biliary leaks and bleeding and may damage right anterior Glissonean pedicle, Glissonean branches of liver segment 5 (S5) and S5 hepatic vein (V5), which increases the di culty of application [13].
Laparoscopic anatomical resection of liver S8 should follow the principle of "simpli cation of complicated surgery". Ome and colleagues found that hepatic parenchyma transection from the root of MHV toward the periphery, called cranial approach is feasible and safe when used in LALR of S8. This approach can avoid complex anatomical separation, accurately determine the transection plane in a simple and convenient manner, and simplify the operation steps [14]. Since the direction of endoscopic view and liver dissection are from the foot to the head side, most surgeons have not got used to perform the cranial approach. Moreover, we believe that the critical problems of cranial approach are thoracic organ injuries and postoperative pneumothorax using intercostal trocars.
We have also carried out related research and explored feasible and safe approach. During laparoscopic right and left hepatectomies, we used a hepatic parenchymal transection-rst approach guided by the MHV to control the hepatic Glissonean pedicle and achieved good outcomes [15,16]. Precise parenchymal transection of the liver is necessary to locate the Glissonean branch of S8 because it is located in the deep parenchyma. Moreover, the main trunk of the MHV is a landmark in anatomical resection of liver S8 and the location of the MHV is relatively stable [17,18]. Therefore, we wondered whether the hepatic parenchymal transection-rst approach guided by the MHV can be extended for laparoscopic anatomical resection of liver S8. After clinical practice, we proposed laparoscopic anatomical resection of liver S8 via the hepatic parenchymal transection-rst approach guided by the MHV and made it a standardized and streamlined procedure after continuous exploration and improvement. Different from the cranial approach, our method used caudal approach which makes the operation line with the operation habits of most surgeons. Our method is simple, fast, safe and accurate and has higher clinical application value in laparoscopic anatomical resection of liver S8.
The precautions for laparoscopic anatomical resection of liver S8 are listed as follows: (1) Preoperative and intraoperative high resolution thin-sliced enhanced CT scanning, helical CT arterial portography and 3D reconstruction visualization system can be used to make accurate assessments of the location and courses of Glissonean pedicles, MHV, and RHV in S8 to avoid damaging the vessels that need to be preserved during surgery [19]. (2) This technique is not dependent on liver staining by injection of special dye. Meanwhile, there was no ischemic line to guide parenchyma transected before S8 Glissonean pedicle ligated and divided. Therefore, intraoperative ultrasonography is a useful and convenient step to accurately locate the S8 Glissonean pedicle (caudal side transection plane), the MHV (left side transection plane) and the RHV (right side transection plane) and determine their relationship with the tumor and to locate the larger branches of the MHV and RHV. The parenchymal transection along the anatomic landmark veins results in the anatomical segmentectomy of S8. According to the intraoperative situation, it is possible to repeatedly adjust the transection plane under guidance of intraoperative ultrasound. To reduce air interference in IOUS, saline can be injected on the transection plane [20]. Without IOUS instrument or lack of technical experience in operating IOUS, a more thorough comprehension of the LALR techniques and imaging data are required. The boundary between S8 and S4a can also be determined by right or left Glissonean pedicle temporary clamping to form ischemic line.
(3) The starting point of the parenchymal dissection generally starts from the intersection of the S5 and S8 boundaries and the Cantlie line, but during surgery, this point is di cult to determine. To avoid excessive or insu cient parenchymal transection, we generally use the intersection between the caudal side transection plane where the S8 Glissonean pedicle is at and the Cantlie line as the starting point for parenchymal transection. (4) Exposure of the intrahepatic MHV and anatomical separation of the S8 Glissonean pedicle are the key steps for success of the laparoscopic anatomical resection of liver S8 guided by the MHV. We use IOUS to locate the middle hepatic vein to determine the left transection plane. The transection starts from the starting point on the caudal side, superiorly to the crypt between MHV and RHV, and then returns to the starting point and turns to the right for further transection. The dissection of left side transection plane and the caudal side transection plane are continued alternately (Fig. 6). The liver parenchyma separation does not go deeper than the level of the MHV (the area beyond this is the right caudal lobe), and then, the MHV and the S8 Glissonean pedicle can be exposed in the liver parenchyma. When the S8 Glissonean pedicle is controlled, segment resection can be performed relatively accurately along the ischemic line and RHV. In order to full exposure, partial hepatic parenchyma of S5 and/or S4a could be dissected [21]. (5) During surgery, attention should be paid to not damage the venous that drained S5, which leads to congestion of liver S5. If hepatic vein injury occurs, hemostatic gauze compression, vessel clipping, and suturing under the endoscope can be used as rescue according to the type of hepatic vein injury. (6) S8 Glissonean pedicle have many variations and are generally divided into trunk and branch types. The branches include ventral, dorsal, and lateral branches.
The management method should be based on ndings of 3D reconstruction visualization system and other imaging assessments to individualize a method. Either branch ligation or main trunk clipping can be used. (7) There may be problems, such as inadequate exposure during initial transection of the liver parenchyma. The exposure can be improved by S8 suspension using suture and adjusting patient positioning during the procedure or retracting the liver toward the left and inferior side. The exposure will be su cient after splitting of the liver parenchyma. (8) When parenchymal transection along the RHV to the caudal side, the surgeon should move from the right side of the patient to the left, and the parenchyma is transected from the cephalic side to the caudal side. Alternatively, two-surgeon technique can be used in which the surgeon and the assistant exchange roles as needed [22].
The clinical value of laparoscopic anatomical resection of liver S8 via the hepatic parenchymal transection-rst approach guided by the MHV is mainly re ected in the following aspects: (1) The resection range can be easily determined with accuracy. IOUS can be used to locate the S8 Glissonean pedicle, MHV and RHV to determine transection plane. Exposing the hepatic veins with anatomical landmarks in the transection plane can improve the accuracy of liver resection and achieve precise S8 resection.
(2) The MHV is a landmark vein in liver resection. Due to its special anatomical position, it is easy to locate with IOUS. Therefore, in anatomical resection of liver S8, the MHV is a good landmark for the hepatic parenchymal transection-rst approach, which is direct, convenient, and capable of reducing the length of the initial transection plane. (3) The use of the hepatic parenchymal transection-rst approach can avoid the need to perform an elaborate hilar dissection approach, bypassing the surgical obstacle caused by the complicated anatomic variation of hepatic hilar area. By dissection the left and caudal side transection plane of the hepatic parenchyma in relatively nonvascular planes, S8 can be fully lifted cephalically to expose the S8 Glissonean pedicle and MHV. The "dead liver" lacking in ow and out ow could be completely removed. Doing so can avoid damage to blood vessels and bile ducts in the residua liver and reduce the incidence of postoperative complications such as bile leakage, infection, and early tumor recurrence. (4) In our approach, the major hepatic ssure do not need completely divided. The risk of right anterior Glisson pedicle, Glisson branches of S5 and V5 laceration can be avoided [23].

Conclusion
Laparoscopic anatomical resection of liver S8 is still quite challenging at present, and it is our goal to design a reasonable procedure with accurate e cacy and high safety. We use hepatic parenchymal transection-rst approach guided by the MHV for laparoscopic anatomical resection of liver S8. This technique overcomes the problem of high technical risk, greatly reduces the surgical di culty and achieves technological breakthroughs, but there are still many problems worth further exploration: (1) The safety of this technique still requires multicenter, large-sample-sized, prospective, randomized controlled studies to verify. (2) Whether selective hemihepatic or total hepatic blood ow occlusion is needed requires continuous improvement based on the actual situation and technological developments. (3) Whether patients with malignant liver tumors will bene t in the long term from this technique is still controversial. Therefore, long-term survival bene ts need to be further studied [24].

Declarations
Availability of data and materials The raw data of the current study are not publicly available due to the protection of participants' personal information but are available from the corresponding author on reasonable request.  Diagrams of trocar placement for LALR of S8. Two 12-mm trocars, two 5-mm trocars and one 10-mm trocar are used. The incision marked with p was made 3 mm in length for insertion of extracorporeal Pringle's maneuver.  Preoperative CT (A) and MRI (B) of the liver. A 3.2 cm × 3.0 cm sized mass (arrows) was found on S8.  Diagram of hepatic parenchymal transection process. Site A indicates the starting point of the parenchymal dissection.
Step 1. Hepatic resection is begun at Site A and continued further cranially along the MHV; and the trunk of the MHV was also exposed; Step 2. Next, the hepatic resection was continued from the Site A toward the right direction of caudal side transection plane; the Glissonian pedicles to S8 was exposed and divided; Step 3. Subsequently, the liver parenchyma between S8 and S7 was dissected along the RHV and ischemic line from the root side to the peripheral side; with this, transections nished.