3.3. Indocyanine Green Fluorescence
Indocyanine green (ICG) is a fluorescent dye with a rapid hepatic clearance largely used in hepato-biliary surgery thanks to its pharmacokinetics features
[36][37]. After portal vein or intravenous injection
[38], it allows the identification of anatomical liver vessels and biliary ducts by providing a rapid parenchymal mapping
[37]. This enables anatomical resections with lower risks of vascular injuries or bile leaks. Moreover, it increases tumor detectability, helps differentiate hepatic lesions based on their vascular patterns, and allows the detection of additional superficial hepatic lesions
[39][40][41][42]. Fluorescence software was incorporated into the Da Vinci system in 2010 and its application enhances robotic advantages in liver surgery from a different point of view. In fact, when a tumor-bearing portal vein injection is chosen—the so-called “negative staining” technique—this could be cumbersome in laparoscopic resections due to the impaired dexterity and lack of ergonomics, and, at the same time, the presence of a rigid linear transducer. The robotic system, through its delicate movements and endowrist instruments, ensures a fine and safe dissection of the hepatic pedicle, allowing it to reachthe hilum and the portal bifurcation easier in the case of a direct portal injection
[43][44][45][46][47]. Furthermore, through the dedicated probe, the transhepatic needle insertion could be also less demanding.
Another aspect to consider in ICG-guided resections is tumor clearance. Minimally invasive approaches—and robotic, in particular—lack a tactile feedback, and achieving a parenchymal free margin or performing an anatomical resection could be challenging. Furthermore, an IOUS exclusive evaluation could be insufficient because it is a user-dependent procedure and presents a heterogeneous detection rate according to tumor size and location and parenchymal stiffness
[48][49][50]. In this context, fluorescence is a precious tool in robotic surgery, with some authors reporting an enlargement of the resection area after ICG application, both in benign ad malignant lesions, in order to achieve a R0 resection
[43][51][52][53], and a significantly higher rate of margin-free specimens when comparing robotic hepatectomies with and without ICG
[53]. As in open surgery, even in robotic surgery, some series described the detection of newer superficial lesions that the dye injection missed before
[43]. This high sensitivity found is, however, limited to the liver surface because of the low penetration of the dye under 8 mm of depth, thus requiring the use of other imaging tools such as IOUS. Although no long-term results have been published, these findings have a significant impact in terms of oncological outcomes. ICG is a promising instrument of intraoperative navigation surgery, allowing rapid and easy identification of the resection plane without the inconveniences mentioned for other image-guided techniques. It can be used in combination with IOUS or AR as an additional aid rather than as a replacement
[47] and with its features, it seems to fill some gaps found in robotic surgery, making tailored and oncological surgery less challenging.
4. Future Prospective
The application of the above-described technologies is nowadays limited in the robotic
liver experience, mainly due to some technical limitations and to a relatively newborn and still debated approach
[15]. AR, for example, is a time-consuming procedure, not only for the intraoperative installation, but also for preoperative planning and liver rendering
[54]. In the context of an atypical or less demanding hepatic resection, which represent the first steps of a necessary learning curve, this time could appear exaggerated. Furthermore, AR in hepatic surgery has showed a delayed distribution compared to other surgical fields as neurosurgery, otolaryngology, orthopedics, and maxillofacial surgery
[55][56][57]. This difference comes from anatomical obstacles, such as working with a deformable soft organ that is constantly moving during operation because of respiratory cycles as well as pneumoperitoneum creation
[58]. Although some strategies have been described in this context
[23][59][60], these features make the development of AR more complex, and new software are needed for shortening modeling creation and improving the accuracy of manual, semiautomatic, and automatic images overlapping.
All the imaging techniques described must be seen, however, as a part of a puzzle rather than an independent solution towards a guided surgery; an example comes from registration accuracy in AR. IOUS and ICG have been proposed to improve overlapping quality through fluorescent markers and 3D ultrasounds used for intraoperative landmarks
[61][62]. In this scenario, the robotic platform fits perfectly by creating a unique merged environment with the possibility of using and visualizing preoperative reconstruction and intraoperative images simultaneously within the operative field (
Figure 3).
Another potential benefit of image-guided technology is minimally invasive training.
In laparoscopy, telementoring based on AR seems to speed up simple skills acquisition such as suturing
[63] or even reduce the learning curve in more complex procedures such as cholecystectomy
[64]. Similar applications in robotic training are lacking, with only a few experiences described
[65]. Hepato-biliary surgery lacks standards of training and learning curves in robotic procedures
[66], but recently, an expert panel of HPB surgeons agreed that a correct training path in hepatobiliary procedures needs different steps, starting from basic robotic skills before performing a liver resection
[67]. In this context, AR could be a useful tool to support less-experienced surgeons performing simple procedures and lower their learning curve.
5. Conclusions
The application of pre- and intra-operative imaging modalities in guiding hepatic surgery presents promising results, and the robotic ecosystem can facilitate their use and magnify their benefits. Potential advantages include reduced morbidity and improvements in oncological outcomes. However, some limitations are still present, related to limited robotic diffusion and still insufficient technological development, and most of the data in the literature come from preclinical studies or small series.