Low rectal cancer can be defined as any malignant tumor that arises in 4-6 cm distal part of the rectum between the anal verge and the puborectal muscle. Recently, it has been possible to provide gastrointestinal continuity at the levels below the puborectal muscle, thanks to various technological developments such as the use of staplers and new surgical techniques such as transanal access, and sphincter-sparing surgical procedures performed at these levels are called ultra-low anterior rectal resection [ 1 ]. Despite the decrease observed in the proportion of patients who underwent permanent colostomy due to the development of sphincter-sparing surgical techniques, still 20% of patients with rectal cancer require abdominoperineal excision. Sphincter-sparing surgical methods for cancers localized in the lower rectum have higher rates of rectal amputation, recurrence, and fecal incontinence compared to cancers in other regions of the rectum, due to the difficulties encountered in surgical access due to the anatomical localization of the lower rectum, as well as its close proximity to the anal sphincteric muscles and pelvic floor [ 2 ].
Anatomically, after passing through the rectum, abdominal and pelvic cavities, it opens into the anatomical anal canal after forming an important part of the surgical anal canal in the ischioanal fossa below the level of the puborectal muscle. The 2/3 distal part of the lower rectum, which is located in the ischioanal fossa and opens into the anal canal, together with the external anal sphincteric musculature surrounding it, constitutes an important component of the surgical anal canal. In this sense, it can be roughly thought that the surgical anal canal is formed by two intertwinning cylindrical muscular tubes. The outer cylindrical muscular tube is formed by the external anal sphincteric muscle and has a reel-like structure that expands at the lower and upper poles between the puborectal muscle and the anal verge. The inner muscular tube is formed by the 2/3 distal part of the lower rectum below the level of the puborectal muscle and the anatomical anal canal [ 3 ]. The potential space between both cylindrical muscular tubes is called the intersphincteric space and the intersphincteric dissection technique is performed in this potential space. Intersphincteric dissection can usually be performed using perabdominal and peranal accesses [ 4,5 ]. Coloanal anastomoses following distal rectal resections performed in the intersphincteric plane below the level of the puborectal muscle are usually performed peranally with the aid of a circular stapler or hand-sewn following anal dilatation with the help of retractors [ 6 ]. In the recently developed transanal total mesorectal excision technique, the abdominopelvic cavity is reached after performing distal rectal dissection in the intersphincteric plane by peranal way [ 7 ].
While rectal dissection is performed in the intersphincteric plane, the external anal sphincteric musculature remains outside the surgical field of vision. The surgical anal canal is anatomically located in the ischioanal fossa. With anterior or posterior perineal access, direct surgical access can be achieved in the ishioanal fossa where there is no direct surgical access with the intersphincteric dissection technique, and on the surgical anal canal in the extrasphincteric plane. Surgical interventions for the lower rectum at this level are performed in the transsphincteric plane. In transsphincteric rectal resection techniques that can be performed with anterior or posterior perineal access, coloanal anastomosis can be performed by resection of the distal rectum in two different ways, after surgical vision is obtained in the ischioanal fossa over the external anal sphincteric system on the extrasphincteric plane . As in Mason's original description, after providing of the surgical dissection in the extrasphincteric plane with perineal access and the transition to the intersphincteric plane by cutting the external sphincteric muscle, the dissection line in the intersphincteric plane is combined with the abdominopelvic dissection line extending to the same plane. Subsequent distal resection and coloanal anastomosis are performed in the intersphincteric plan with perineal access, and following the coloanal anastomosis, the external anal sphincter muscle is repaired and the surgical procedure is completed (Figure 1) [ 8 ]. Or, after dissection in the extrasphincteric plane in the ischioanal fossa by perineal access, the distal resection line is determined. Afterwards, abdominopelvic and perineal dissection lines are connected by releasing the fibrous ligaments between the puborectal muscle and the external anal sphincteric muscles. In this way, the distal rectum is mobilized to the level of the subcutaneous external anal sphincter in the extrasphincteric plane, together with external anal sphincter surrounding it. When the rectosigmoid resection is completed in the transsphincteric plane from the determined distal resection line, the lower rectal segment between the puborectal muscle level and the distal resection level as distal part of the resected specimen is excised together with the proximal external sphincteric segment surrounding it, and a coloanal anastomosis is made by using perineal access(Figure 2)[ 9,10].
Currently, sphincter-sparing surgical interventions for lower rectal cancers are generally performed in the intersphincteric dissection plane using perabdominal or peranal accesses. For sphincter-sparing surgery performed in the intersphincteric space, absolute contraindications are T4 tumors, invasion of external anal sphincter, fixed tumors in digital examination, poorly differentiated tumor, poor preoperative sphincter function, distant metastases and presence of mental disease [ 11,12 ]. Anal sphincteric muscle invasion is usually seen 3 in lower rectal cancers localized below the level of the puborectal muscle. The 2/3 distal part of the lower rectum, which forms the inner tubular component of the surgical anal canal, is completely surrounded by the external anal sphincteric musculature (EAS). In this localization, there is no fatty tissue or mesorectal tissue around the lower rectum, and it is directly adjacent to the EAS via the potential intersphincteric space. Cancers originating from the lower rectal mucosa, after spreading radially in the intestinal wall, may invade the EAS locally by direct proximity. Therefore, EAS invasion can be encountered in locally advanced cancers located in the lower rectum, unlike cancers localized in other regions of the rectum. EAS invasion constitutes one of the most important contraindications for the intersphincteric dissection technique [ 11,12,13 ]. In cases of lower rectal cancer encountered with EAS invasion, the cylindrical rectal amputation technique performed in the extrasphincteric plane is the prominent surgical intervention in current treatment [ 14,15,16 ]. However, recently, sphincter-sparing surgical techniques performed in the transsphincteric plane by combining anterior or posterior perineal access with abdominal access have attracted attention in the surgical treatment of cancers localized in the lower rectum [ 17,18 ]. With sphincter-sparing surgical techniques performed in the transsphincteric plane using combined abdominal and perineal accesses, the distal surgical dissection is performed in the extrasphincteric plane as in cylindrical rectal amputation. In this way, it is possible the excision of the invading proximal sphincteric segment together with the lower rectal segment surrounded it by using the surgical advantages of the extrasphincteric dissection plane unlike the intersphincteric dissection technique [ 9,10 ].