The following topics were newly elucidated in the present preliminary research:

1) The use of lithotomy stirrups-2 significantly reduced the pressure applied on the lower limb muscles in various lithotomy positions compared with lithotomy stirrups-1. 2) The most pressured lower limb muscle with the use of both lithotomy stirrups-1 and -2 was the central part of the soleus muscle, which is the most common site for the development of WLCS and DVT, as reported in previous studies [13, 14]. 3) In addition, when using the conventional lithotomy stirrups-1, the pressure was predominantly applied to the proximal soleus muscle; however, when using lithotomy stirrups-2, the pressure was shifted to the more distal soleus muscle.

Compartment syndrome is a condition in which the internal pressure in the fascia-covered muscle compartment (compartment) increases owing to edema or hemorrhage, causing necrosis and fibrosis of the muscle due to microcirculatory or sensory disturbance, nerve paralysis, or renal damage due to elevated myoglobin levels [14].

Delayed diagnosis and treatment can lead to permanent disability, limb amputation, or death. Acute compartment syndrome is defined as intramuscular hemorrhage caused by fracture or trauma or by prolonged compression or ischemia such as cast immobilization [15, 16] and is distinguished from chronic compartment syndrome [17, 18], which is caused by damage resulting from the overuse of muscle groups owing to prolonged exercise. Among the acute compartment syndromes, WLCS is caused by improper intraoperative positioning of the lower extremities and is mostly associated with prolonged surgery in the lithotomy position [19,20,21].

Halliwill et al. [22] reported that WLCS occurs in 1 out of every 3500 lithotripsy procedures (0.028%); however, in recent years, several studies have reported a higher incidence. In 2009, Tomassetti et al. [23] reported the incidence of WLCS in 373 laparoscopic endometriosis surgeries (0.8%), and, in 2014, Bauer et al. [24] reported that the incidence of WLCS in gynecologic surgery ranged from 0.067–0.28%. Since 2009, we have observed two cases (0.8%) of WLCS in 240 robotic-assisted rectal cancer surgeries at Fujita Health University Hospital. Therefore, to prevent the occurrence of these fatal complications, not only the surgeon but also other paramedical staff must pay close attention to preoperative positioning, intraoperative tilt angles, and monitoring of the lower limb and must perform decompression procedures appropriately. However, the effects of these efforts cannot be quantified, making it difficult to provide clear evidence. In addition, WLCS is highly infrequent, and it is impossible to obtain statistical evidence from observational studies at a single institution. The present study is the first study to scientifically quantify and demonstrate the effect of the use of new lithotomy stirrups-2 on lower limb decompression in lithotomy positions. As shown in Fig. 3, the pressure applied on the lower limb muscle when using lithotomy stirrups-1 increased to 26.5, 29.8, and 31.1 mmHg when the lithotomy position was changed from neutral to Trendelenburg position and to the Trendelenburg position with a 10° right inferior tilt. As most rectal cancer surgeries are usually performed in the Trendelenburg position with a 10° right inferior tilt, we believe that this increase in pressure is one of the causes of WLCS. Furthermore, we quantitatively demonstrated that the use of lithotomy stirrups-2 significantly reduces this pressure.

The present study also revealed that the central soleus muscle is most frequently subjected to the pressure load. Postoperative incidence of WLCS and DVT are most often associated with the soleus muscle, and pressure loading in this area must be considered during rectal surgeries [13, 25, 26]. However, as pressure application on the soleus muscle is not avoidable when using lithotomy stirrups, it is important to improve the quality of the stirrups to reduce the pressure load on this muscle.

We showed that the use of both lithotomy stirrups-1 and -2 resulted in a high frequency of pressure loading on the central soleus muscle, but the actual pressure was significantly reduced with the use of lithotomy stirrups-2 in most lithotomy positions. Furthermore, the use of lithotomy stirrups-2 shifted the pressure load from the soleus muscle to the more distal soleus muscle. DVT is more likely to occur in intramuscular veins that primarily depend on muscle pumping and venous valves than in those that primarily depend on arterial beat. The soleus vein is reported to be more likely to develop thrombosis than the gastrocnemius vein because of the structure of the vein and the muscle. Moreover, soleus vein thrombosis often extends along the drainage veins toward the proximal veins and has been reported to develop lethal pulmonary emboli more frequently than in the gastrocnemius vein [27]. Therefore, prevention of the formation of thrombus in the proximal soleus vein is clinically essential to prevent sudden death due to pulmonary embolism after rectal carcinoma surgery. Based on the results obtained in the present study, we believe that the introduction of the new lithotomy stirrups-2 may reduce WLCS and DVT caused by the lithotomy position.

The present study has some limitations. First, the sample size was small, and the study should be regarded only as an observational study. Second, this article does not prove that new lithotomy stirrups reduce the frequency of WLCS and DVT after surgery, and therefore further large-scale prospective or randomized study is needed to show the true effect of new lithotomy stirrups. Third, the average age of the participants was 35 years old, which is too young age as a candidate for rectal cancer. Fourth, since the diameter of the Palm Q pad is approximately 10 cm and each sensor is equally spaced and the soleus and gastrocnemius muscles have many overlapping areas, it is difficult to completely segregate each muscle of the lower leg to measure pressure load. Forth, However, the pressure load on the lower leg cannot be completely eliminated even with the use of newer stirrups2 as shown in the Table 3. Thus, in the near future, it is desirable to develop a boot pad that is tailor-made to suitably fit the individual patient's body shape (obese or skinny etc.) and its lower legs.