Overview

The key to entering into the proper plane of dissection for TME surgery is to develop the plane immediately behind the inferior mesenteric artery (IMA) and superior rectal artery with identification of the hypogastric nerve plexus and sweeping the nerves posterior to the artery.[1] The avascular plane behind the upper mesorectum is developed by dissection anterior to the superior hypogastric nerve plexus and entering into the presacral space anterior to the presacral fascia and presacral veins.[2] The surgeon should remain mindful that there is also a second plane, which is the incorrect plane, located posterior to the hypogastric nerves. This plane puts the surgeon at risk of injuring the presacral veins and the hypogastric nerves, which can result in bladder and sexual dysfunction.[3]

Dissection posterior to the distal mesorectum exposes a condensation of connective tissue at the S4 level, which is called the rectosacral (Waldeyer’s) fascia. This layer should be incised sharply rather than torn bluntly to avoid bleeding. Continued dissection distally in this plane follows the curve of the sacrum and coccyx towards the anorectal junction. Incision of the Waldeyer’s fascia leads to a full mobilization of the rectum.[4]

Identification of the lateral and anterior mesorectal planes distal to the peritoneal reflection is initiated by incision of the peritoneum in the lateral pararectal gutters. In a laparoscopic or robotic case, this plane of dissection is facilitated by "pneumodissection." In open surgery, dissection is facilitated by medial retraction of the posterolateral mesorectum and lateral countertraction. The left ureter, previously identified prior to transection of the IMA, will be lateral to dissection of the mesorectal fascia. Generally, the ureter crosses the external iliac artery 1 cm lateral to the takeoff of the internal iliac artery, but it may be more medial when prior surgery or inflammation has occurred. The left ureter lies lateral to the left superior hypogastric nerve along the pelvic brim. The lateral mesorectum is dissected away from the superior hypogastric nerves (left and right branches below the sacral promontory) above the peritoneal reflection. Below the peritoneal reflection, the lateral mesorectum is dissected away from the inferior hypogastric nerve plexus on the pelvic sidewall by dividing small nerve branches that enter the lateral mesorectum.[5] The lateral mesorectum of the distal third of the rectum requires dissection off the pelvic floor levator fascia. The lateral aspects of the anorectal junction are covered by Waldeyer’s fascia reflecting off the levators back onto the mesorectum. To free the distal mesorectum, the anorectal junction is released by incising Waldeyer’s fascia posteriorly and laterally. Branches of the middle rectal arteries may be encountered in release of the anorectal junction.[6]

Anterior dissection starts with incision of the peritoneal reflection. The plane of dissection of the anterior mesorectum behind seminal vesicles and the prostate or vagina can be anterior or posterior to the endopelvic fascia of Denonvilliers depending on whether the cancer is located anteriorly and depending on the anticipated surgical radial anterior mesorectal margin.[7] The peritoneal reflection can be incised above the actual cul-de-sac for anterior cancers in the mid-rectal location in order to dissect immediately on the posterior wall of seminal vesicles or vagina and anterior to the endopelvic fascia.[2] This incision creates the potential for injury to small nerves (bundle of Walsh branches of the nervi erigentes) that lie in the plane anterior to the endopelvic fascia in direct contact with posterior aspects of seminal vesicles and the prostate or vagina.[8] Preserving the endopelvic fascia on the posterior aspect of seminal vesicles or the vagina will prevent injury to these nerves. Inserting an endoscopic endonasal approach (EEA) sizer into the vagina can sometimes aid in this anterior dissection. Providing the anterolateral mesorectal surgical radial margin is not compromised by tumor, the nerve and vessel bundle of Walsh can be swept laterally and anteriorly and preserved during the mesorectal dissection just below the peritoneal reflection (see video below).[9]

Anterior dissection of the distal rectum is directly on the distal prostate or distal vagina where there is little to no anterior mesorectum. Preoperative clinical assessment and imaging are essential to assessing whether the cancer involves the anterior rectal margin. When the lesion is anterior, it is recommended to excise Denonvillers’ fascia en bloc with the tumor.[4]

Scarring from an episiotomy can make dissection difficult between the distal posterior vaginal wall and distal anterior rectal wall. The posterior vaginal wall does not require excision unless there is tumor adherence.[10] En bloc excision of a portion of the posterior vaginal wall to ensure a proper margin of resection is indicated for anteriorly located cancers without clear margins with respect to the posterior vaginal wall. Similarly, the plane between the prostate and the rectum is generally easily developed and avascular.[4] At the inferior apex of the prostate, dissection should be brought back through the inferior aspect of the nonadherent Denonvilliers’ fascia to reenter the proper plane. If the lesion is lateral or posterior, resection including Denonvilliers’ fascia is not needed If the tumor has grown into the prostate as determined from preoperative imaging with multidisciplinary conference, a pelvic exenteration is performed. The infraprostatic “membranous” urethra is in danger at the very distal aspect of the dissection, and the surgeon needs to remain mindful of this potential site for injury.[11] When there is a question as to the location of the urethra in this area, having an assistant pull on the urethral catheter will help the surgeon palpate and protect the urethra. Inferior to this point, the upper border of the anal canal and anorectal ring will be encountered. The surgeon will notice the mesorectum taper and then become absent, usually about 2 cm above the levator ani revealing the “bared” muscular rectal tube at this level.[12]

The principles of total mesorectal excision (TME) dissection include sharp dissection with proper traction and countertraction to expose the avascular plane; excellent lighting is important to TME dissection. These basic principles are maintained in open, laparoscopic, or robotic approaches. During open procedures, the dissection is facilitated using a retractor held by an assistant to provide countertraction to the surgeon’s traction. This coordinated maneuver under proper tension will expose the correct plane of dissection. A good headlight or lighted retractor is essential for an open dissection in the deep pelvis. In a laparoscopic approach, the retraction on the rectosigmoid to allow the exposure of the planes usually requires a good assisstant. In a robotic procedure, exposure of the avascular TME plane is accomplished using an external arm to maintain lateral and anterior traction. This crucial maneuver enables the surgeon to remain in the proper plane, particularly when the inferolateral aspect of the TME dissection is being carried out and the nervi erigentes are at greatest risk of injury.[13]

The following videos will help to illustrate the techniques described above. The first video below shows a very close view of anterior dissection.

The following two videos show a total of three male patients undergoing TME.