Over the past 30 years, minimally invasive techniques for inguinal hernia repair (TEP and TAPP) have become established due to their low morbidity (recurrence rates comparable to open procedures and less chronic pain than open procedures) [1, 2]. To further improve these outcomes, robot-assisted surgical systems are now widely available. The surgical instruments of robotic systems have complex joints and require a standardized distance from the internal inguinal region; a distance of 20 cm between the ports and the symphysis has proven to be a practical working distance, which means, however, that the camera port is not necessarily positioned at the level of the umbilicus [3, 4]. This represents a departure from the classic aesthetic principles of minimally invasive surgery, where the umbilicus is preferred for camera insertion because of the concealed scar. One of the distinctive features of robot-assisted surgery is its high degree of standardization; the optimal visibility, the instruments’ degrees of freedom, and the ergonomics provide the flexibility to perform the operation precisely even under challenging conditions (e.g., obesity) using adjuvant interventions such as dorsal mesh fixation, suture closure of the internal inguinal ring, or suturing of the medial hernia with visible nerve dissection [3].

Several questions remain to be clarified:

What is the distance from the umbilicus to the symphysis in the uninflated state and under 12 mmHg pneumoperitoneum pressure?

Consecutive patients planned for elective robot-assisted groin hernia repair and assigned to be operated by two of the authors (UAD and MR) in the period from July 2020 to April 2021 were eligible. Demographic data were extracted from the clinical information system. Eligibility criteria were absence of abdominal scars from previous open surgery and age >18 years. Patients consented according to the General Consent Form (Ethics Committee of Northwestern Switzerland, approval number 2020-00525). The methodology of the surgical procedure followed published protocols [3, 4]. BMI was classified according to WHO: <30.0 kg/m² (normal), 30.0–34.9 kg/m² (grade I), 35.0–39.9 kg/m² (grade II), >40.9 kg/m² (grade III).

The study included 68 consecutive patients (61 male and 7 female); in total, 96 hernia sides were operated, with 28 bilateral hernias and 8 recurrent sides. On average, 4 L of CO₂ were required to achieve a pressure of 12 mmHg (range 2.8–6.8 L) (Table 1).

The elasticity of the abdominal wall is a basic prerequisite for minimally invasive surgery with pneumoperitoneum. The expansion of the abdominal cavity (abdominal compliance, C_ab) under pneumoperitoneum is the result of the elasticity of the abdominal wall and diaphragm. Abdominal compliance has limits and cannot be expanded arbitrarily by increasing pressure or neuromuscular relaxation [5]. Intra-abdominal pressure initially increases linearly with the volume of pneumoperitoneum until the limit of C_ab is reached [6]. Conversely, however, sufficient intra-abdominal working space can be gained at low pneumoperitoneum pressure by lifting the abdominal wall (lift-laparoscopy), which is also standard practice with robotic arms [7]. It is also known that the abdominal wall of women is more elastic than that of men (i.e., it has a lower Young’s modulus or is more compliant), which may be due to hormonal factors or at least related to the phenomenon of “pre-stretching,” e.g., after pregnancy [8]. The type of positioning on the operating table can also have an influence on the C_ab, especially in the anti-Trendelenburg position, but this does not apply to the patients in our study cohort, who are operated on in the Trendelenburg position [9]. According to clinical experience, abdominal scars, previous operations, previous pregnancies, intra-abdominal adhesions, CAPD, or ascites have an influence on the C_ab and may be used as predictive information for estimating in advance the grade of difficulty. In the context of robot-assisted transversus abdominis release (rTAR), the abdominal wall is weakened by detachment of the medial insertion of the transversus abdominis (MTA), which leads to an intervention-related higher compliance, as is also the case with botulinum toxin (Botox), for example.

The type of abdominal fat distribution is described as a predictive factor for the elasticity of the abdominal wall: with gynoid fat distribution, the abdominal wall has an oval shape transversely and can therefore theoretically be stretched better using pneumoperitoneum than the already round shape of the abdominal wall with android fat distribution [5]. However, experience shows that with android fat distribution, the abdominal wall is usually thin, which in turn increases its elasticity (low Young’s modulus), while a gynoid thick subcutaneous fat layer requires high pneumoperitoneum pressure to create sufficient intra-abdominal space, as demonstrated before [10].

The elasticity of the abdominal wall is higher in the longitudinal than in transversal direction [5, 11, 12]. According to our results, the supraumbilical longitudinal elasticity is significantly higher than the infraumbilical elasticity (15.92% vs. 12.13%, respectively, p = 0.0202); in the subgroup analysis of women, no significant difference was found; explanations for this difference may be the small number of female patients, but probably the overstretching of the abdominal wall as a whole during pregnancy may contribute to increased infraumbilical elasticity also. Why the abdominal wall is more elastic supraumbilically than infraumbilically in men is not clear, maybe it is related to the higher prevalence of type I rectus diastasis according to Corvino in males, but this was not investigated in the current study [13]. Another explanation may be related to the different anatomical configuration of the supraumbilical linea alba, due to the physiological umbilical hernia, due to the physiological umbilical defect in the eighth embryonic week, which arises supraumbilically, while the embryonal Linea alba remains closed in the infraumbilical region to protect the urinary bladder [14]. Probably the infraumbilical region must be considered as a distinct part of the abdominal core. Other possible explanations for the longitudinal elasticity are the muscular anatomy (the rectus muscles are easier to stretch than the complex lateral abdominal wall) and the cylindrical shape of the abdominal cavity (tangential tension is expected to be higher than longitudinal tension) [5, 11, 12]. From a statistical point of view, nevertheless, measurements of the supraumbilical distances failed to pass the normality test based on the standard deviation (Shapiro-Wilk test, inconsistent with a Gaussian distribution); probably a larger population would be needed to achieve a smaller standard deviation, but since the xiphoumbilical length after pneumoperitoneum is secondary for the positioning of the ports (the distance to target is measured from the pubic bone), this result is not relevant in the context of rTAPP.

The infraumbilical (umbilicopubic) distance after pneumoperitoneum is–according to the present measurements–not sufficient for an optimal positioning of the ports at the umbilicus, which should be aimed at 20 cm distance to target; as a consequence, the umbilicus is not useful as orientation for port placement; according to our results, in more than 50% of patients (median of 16.25 cm), the port for the endoscope should be placed cranially to the umbilicus and ideally slightly lateral to the linea alba. For the sake of completeness, it should be noted that due to the longitudinal elasticity of the abdominal wall, the port position in rTAPP is to be marked with a ruler after reaching the pressure of 12 mmHg pneumoperitoneum.

The transversal elasticity after pneumoperitoneum (11.90%) is less than the longitudinal elasticity (13.80%) (Figure 3); as a consequence, the distribution of the intraabdominal pressure is higher in the transverse direction than in the longitudinal direction [7, 8, 11]. This difference in elasticity or compliance of the abdominal wall in the longitudinal and transverse directions is called anisotropy. Whereas the anisotropy may not be relevant in rTAPP, it needs to be considered for closing defects of the Linea alba and for choosing the proper mesh in ventral hernia repair. For example, closing an umbilical hernia with 4 cm diameter in transverse fashion protects the suture line from tension; closing it in longitudinal way, exposes the scar to the higher radial tension forces [12, 15]. Furthermore, studies show that surgical meshes with anisotropy comparable to the abdominal wall integrate better because they match the natural tension conditions of the abdominal wall [16] and the anisotropy of the abdominal wall needs also to be taken into consideration not only when implanting meshes but also in the process of designing new meshes [15, 17]. For the TARUP procedure, other anthropometric data would be important: the Carbonell formula can be calculated from the CT images, and the visceral fat tissue (android or gynoid fat distribution) and thickness of the musculature of the abdominal wall could likely provide clues regarding the AEA or abdominal wall compliance, thereby optimizing port placement and, if necessary, helping to choose between a caudal and a lateral approach [18]. The umbilical position is not relevant for TARUP.

To our knowledge, the abdominal elevation angle (AEA) was not previously described in the literature. The median difficulty score was 6.00 in the group of patients with an abdominal elevation angle of 20°–39°; although the median was higher at >40° (8.00) and <19° (7.50), this was not statistically significant (p = 0.1289). A similar, non-significant result was also found for BMI (p = 0.0930). Although from a subjective point of view both a narrow and a wide abdominal elevation angle appear to make the endoscopic operation of groin hernia more difficult, this hypothesis could not be confirmed in the current study. In the one-way ANOVA statistics, the abdominal elevation angle did not correlate with the degree of difficulty of the procedure (p = 0.1289). One possible explanation for this is the small number of patients in the outlying groups (11 with narrow or wide AEA and 6 with obesity grade I and II), which is too small to be statistically significant. Another explanation may be due to the optimal working conditions of the robotic platform, which, thanks to the degrees of freedom of the instruments, facilitates working at angles that are too narrow or too wide and allows optimal access to the anatomical structures of the abdominal wall, compensating for the difficulties associated with obesity. A further explanation may be the high level of experience of the surgeons involved. A comparative study between conventional laparoscopy and robot-assisted surgery would possibly reveal a difference here, although this will likely not be performed.

Despite the difficulty of the operations (as shown in the previous table), the degree of procedural satisfaction at the end of the operation was very high regardless of the angle (Tables 6, 7). The grade of satisfaction is based on the final result of the treatment of the hernia orifices and the anatomical preparation of the myopectineal orifice as well as the creation of the landing zone for the mesh. Despite the anisotropy of the abdominal wall and the different grades of difficulty due to the abdominal elevation angle and the BMI, the robotic platform enabled the surgeons to accomplish the expected results. Since the positioning of the ports was standardized at 20 cm to the pubic bone, port positioning may have contributed to this result also. Multiple linear regression showed a significant effect of the longitudinal elasticity on the procedural difficulty (high sum of squares of 18.97 and high F of 7.144, p = 0.0139). BMI, transversal elasticity, abdominal CO₂ volume and abdominal elevation angle were not independent factors for increased procedural difficulty, probably due to the low number of included patients.

AEA, abdominal wall elasticity, and insufflated CO₂ volume are not surrogates for planning the expected duration of surgery, as they can only be measured intraoperatively. Further studies are needed to generate a reliable and useful preoperative predictive score, for optimization of OR capacity as soon as during preoperative consent consultation [19].

The current study has several limitations: differences between males and females could not be confirmed due to the small number of patients; the decisive factor for the lower proportion of women is that the incidence of inguinal hernia in women is 10-fold lower than in men. Another limitation is the computation of the operating time, which is influenced by many independent variables: expertise, complexity of the case, anatomy of the hernia, concomitant findings, recurrences, unilateral or bilateral, previous operations such as prostatectomy, age (soft or stiff tissue), time lost due to contemplation of the amazing anatomy, and many others. The surgical team involved had experience with at least 500 rTAPPs (experience may be a source of bias). Therefore, operating time was not used to assess difficulty in the current study. To group the AEA following the mean and SD was used to compare the extremes in relation to the norm; the main limitation of this approach is the greater number of patients in the normal group in comparison with the outlier groups (narrow and wide AEAs); this may be a bias in the interpretation of the results; otherwise to create groups with comparable number of patients for the AEA, an algorithm to reduce the number of the normal population (exclusion criteria for the “some of the normals”) would be necessary, creating another source of bias.

In conclusion, the mean distance from the umbilicus to the pubic bone under 12 mmHg pneumoperitoneum pressure is less than the 20 cm port-to-target-organ distance recommended for rTAPP; the Abdominal Elevation Angle (AEA) under 12 mmHg of pneumoperitoneum pressure in males had a wide range, from 17.72° to 51.96°; and finally, neither AEA nor BMI showed a statistically significant correlation with procedural difficulty. However, multiple linear regression analysis identified longitudinal abdominal wall elasticity as an independent predictor of procedural difficulty (p = 0.0139). Our data failed to define preoperative anthropometric information useful to predict the degree of difficulty of the procedures.

The results of our study are partially generalizable, with the recommendation for port placement 20 cm cranial to the pubic bone instead of using the umbilicus as reference, the definition of the AEA and the anisotropy being the most relevant. The results aim to standardize the rTAPP and further refine clinical outcomes, but further prospective studies involving a larger number of patients and surgeons with different levels of experience are needed.