The IS distance is a simple index for the narrow pelvis that can predict the difficulty of laparoscopic low anterior resection for rectal cancer. A narrow pelvis made the procedure difficult in terms of the time taken to clamp the distal end of the rectum, when we defined the narrow pelvis as an IS distance < 94.7 mm. Moreover, operative times were longer in the presence of a narrow pelvis. Measuring the distance between the ischial spines using an axial CT image provides a simple definition of a narrow pelvis.
Some previous studies have developed narrow pelvis indexes. Tsuruta et al. reported that a pelvic index > 13 in male patients was associated with anastomotic leakage [15]. However, complicated measurements were needed to calculate their pelvic index, because their index was defined as the difference between the interspinous distance and the diameter of the mesorectum, divided by the depth of the cavity of the lesser pelvis. Hamabe et al. reported that sacral promontory shape was related to the insertion of an additional port in reduced-port surgery [14], and Targarona et al. reported that the pubic coccyx axis was associated with prolonged operative time [13]. These narrow pelvis indexes are based on angles, and intuitively judging surgical difficulty is difficult. Shimada et al. reported the antero-posterior/transverse ratio of the pelvic inlet to be associated with prolonged operative time [12]. This index requires three-dimensional CT measurements because of the need to measure the pelvic inlet while correcting for the angle of the pelvic axis. All these previously published indexes require multiplanar measurements using MRI or reconstructed CT images. However, in the current study we found the IS distance to be a novice and simple index requiring only the axial slice of a CT scan to predict the difficulty of performing a laparoscopic low anterior resection. The axial slice of a CT scan is simple to interpret and one of the most popular images that can be available for surgeons.
In the current study, we used the time taken to clamp the distal end of the rectum as an outcome, which may not be popular. However, many surgeons may experience difficulty in managing to clamp the distal end of the rectum, especially in the pelvis at a level deeper than the peritoneal reflection. Therefore, when we designed the current study, we hypothesized that being able to clamp the distal end of the rectum may be more difficult in the presence of a narrow pelvis. As per the results of the simple linear regression and logistic regression, a shorter IS distance was significantly associated with a longer clamp time and operative time. Regarding the operative time, the narrow pelvis in our definition was not associated with a longer operative time in multivariate regression analysis. This might be because many other factors influence the operative time. Tsuruga et al. reported that the pelvic index was associated with a longer operative time but not merely the IS distance [15]. This could also be because several factors could influence the operative time. The creation of a stoma was the only independent factor that was significantly associated with a longer operative time. The creation of the stoma itself takes several minutes. Furthermore, a diverting ileostomy was created in patients where the anastomosis was located in the anal canal, patients who had received preoperative therapy, and patients with positive air leakage from the anastomosis. These are generally difficult cases [18–20] and it is natural that these operations took longer to complete. With this in mind, the process to clamp the distal end of the rectum is a simple index that directly reflects the difficulty of the pelvic aspect of the operation, and it is reasonable to use it as an index of the difficulty. One thing to consider is that the clump time may be dependent on the technical experience of the operator. However, all of the operations in the current study were performed among surgeons who had been qualified as skilled in Endoscopic Surgical Skill Qualification System (ESSQS) to eliminate the possible bias of individual surgeons’ skills. In Japan, the Japan Society for Endoscopic Surgery introduced the ESSQS [21–23] to maintain and improve the quality of laparoscopic surgery, and laparoscopic colorectal procedures performed with ESSQS-qualified surgeons have been proven to improve postoperative results [24].
In the current study, the rate of R0 resection and postoperative complication rate were too small to be reliable outcomes for operative difficulty. The cutoff point was calculated from the ROC curve. The area under the curve derived from the ROC curve was not high, and this might be because of the limited number of patients in this study. The difference in the pelvic shape between men and women is an important issue to discuss because the width of the pelvis could not be measured using the IS distance in a straightforward manner. However, multivariate regression revealed that the IS distance was an independent factor for predicting the difficulty of low anterior resection, and IS distance can be a common factor regardless of a patient’s sex.
Predicting the difficulty of low anterior resection can be useful in deciding who the operator will be and in determining the surgical approach. In difficult cases, it may be best that an expert surgeon performs the operation. For instance, a certificated surgeon by the ESSQS may be appropriate to perform difficult cases. Determining the surgeon according to the IS distance may result in better surgical outcomes because procedures beyond a surgeon’s capacity may be avoided by that surgeon. Laparoscopic surgery using the robotic assistant and the trans-anal endoscopic approach could be another option to improve the surgical outcomes in difficult cases with a narrow pelvis [25, 26], although the merit of these approaches has not been fully shown to date. IS distance may be useful in determining these appropriate surgical approaches.
The current study is weak in terms of small number, single faculty and retrospective evaluateion. Therefore, future prospective studies with a large number of patients from multiple institutions are required.