The key to entering into the proper plane of dissection for TME surgery is to develop the plane immediately behind the IMA and superior rectal artery with identification of the hypogastric nerve plexus and sweeping the nerves posterior to the artery (Figure 6.1).[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 puts the surgeon at risk of injuring the presacral veins and as well the hypogastric nerves resulting in bladder and sexual dysfunction.[3]

Dissection further in the retro-mesorectal avascular plane leads to a condensation of connective tissue at the S4 level called the recto-sacral fascia (Waldeyer’s fascia), that should be incised sharply rather than torn bluntly to avoid bleeding. Continued dissection distally in the retro-mesorectal plane follows the curve of the sacrum and coccyx upwards towards the anorectal junction. Incision of Waldeyer’s fascia uncovers the “bare” rectal muscular tube distal to the distal end of the mesorectum and leads to the upper anal intersphincteric plane (Figure 6.2).

Identification of the lateral and anterior mesorectal planes below the cul de sac peritoneal reflection is initiated by incision of the peritoneum in the lateral para-rectal gutters. In a laparoscopic or robotic case, this plane of dissection is facilitated by "pneumo-dissection" (Figure 6.3). In open surgery, dissection is facilitated by medial retraction of the postero-lateral mesorectum and counter-traction laterally against the pelvic sidewall. The left ureter, previously identified prior to transection of the IMA, can be followed down into the pelvis and 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. 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 cul de sac peritoneal reflection. Below the cul de sac 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.[4] The lateral mesorectum in the distal third of the rectum requires dissection off the pelvic floor levator fascia (Figure 6.4). The lateral aspects of the anorectal junction are covered by Waldeyer’s fascia reflecting off the levators back onto the mesorectum. To fully release the distal mesorectum and to bare the distal rectal muscular tube, 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.[5]

Anterior dissection starts with incision of the cul de sac peritoneal reflection. The plane of dissection of the anterior mesorectum behind seminal vesicles and prostate or vagina can be anterior or posterior to the endopelvic fascia of Denonvillier depending on whether the cancer is located anteriorly and the anticipated surgical radial anterior mesorectal margin is close (Figure 6.5).[6] 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] Noteworthy is potential 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, prostate and vagina.[7] Preserving the endopelvic Denonvillier’s fascia on the posterior aspect of seminal vesicles or vagina will prevent injury to these nerves. It is important to assess and anticipate the surgical anterior mesorectal margin as anterior or posterior to Denonvillier’s endopelvic fascia and potential for injury to these nerves. Inserting an 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 cul de sac peritoneal reflection.[8]

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 assess whether the cancer involves the anterior rectal margin. 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.[9] 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 margin from the posterior vaginal wall. Similarly, the plane between the prostate and the rectum is generally easily developed and avascular. If the lesion is anterior, it is recommended to excise Denonvillier’s fascia en bloc with the tumor (Figure 6.6).[10] At the inferior apex of the prostate, dissection should be brought back through the inferior aspect of the non-adherent Denonvilliers’ fascia to reenter the proper plane. If the lesion is lateral or posterior, resection including the Denonvilliers’ fascia is not needed (Figure 6.7). If the tumor has grown into the prostate as determined from pre-operative imaging with multi-disciplinary conference, a pelvic exenteration is performed.[11] The infra-prostatic “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.[12] If 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 then absence to show a “bared” muscular rectal tube at this level.

The principles of TME dissection that should be emphasized include sharp dissection with proper traction-counter traction to expose the avascular plane and excellent lighting. Whether the operation is being performed in an open, laparoscopic, or robotic approach, these basic principles are maintained. During open procedures, the dissection is facilitated using a retractor held by an assistant to provide counter-traction to the surgeon’s traction. This coordinated maneuver under proper tension will expose the avascular tissue in the correct plane of dissection. A good headlight or lighted retractor is essential for open dissection in the deep pelvis. In a laparoscopic or 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 inferior-lateral aspect of the TME dissection is being carried out and the nervi erigentes are at greatest risk of injury.