The dissection must be performed as a TME (total mesorectal excision) as described by Heald (1). A sharp, visually controlled, dissection is performed in the plane between the visceral and parietal mesocolic/mesorectal fascia.
Principles for circumferential dissection
This plane is identified at open operation by primarily freeing the sigmoid from the lateral abdominal wall continuing medially for the whole mesosigmoid till the central part of the posterior abdominal wall. The autonomicnerves and ureter will be left unharmed retroperitoneally on the posterior abdominal wall (behind the parietal fascia, by some called "Told fascia"). Continue in the same plane to the pelvic entrance. After the vessels/lymph nodes are divided in the indicated level, and the colon is divided and can be held up, continue exactly and visually guided dissection in the mesorectal ("holy") plane outside of the mesorectal fascia down in the pelvis. The plane is usually relative easy to define posteriorly and anteriorly because it follows the rear of vesicles and prostate gland in men, vagina in women. The proper plane should first be identified and followed far down posteriorly, then a bit down anteriorly. The plane is more difficult to define laterally, especially at 10 and 2 o’clock. The dissection here should only be performed after the initial posterior and anterior dissection. Functionally important autonomic nerves are located just on the outside of the proper plane and will be injured by an improper dissection. Dissection on the inside of the proper plane will increase the risk for local recurrence.
The quality of the resection can be visualized by inspection of the specimen. This should have an intact, even and smooth mesorectal fascia covering the mesorectal fat. In scientific studies it is customary to grade the quality of the specimen and this also ought to be performed in clinical routine and described in the surgical record:
- Quality A: Intact mesorectal fascia on the whole specimen
- Quality B: Cuts in the mesorectal fascia, and slightly into the mesorectal fat tissue, none onto the outside of the bowel
- Quality C: Cuts in the mesorectal fascia/fat tissue onto the rectal wall
The quality of the dissection can further be estimated by the integrity of the Toldt´s fascia (parietal mesocolic fascia) on the posterior abdominal/pelvic wall, this should be intact.
Distal dissection of rectum amputation (APE)
In distal parts of the rectum the mesorectal fat tissue is very thin, and if standard TME to the pelvic floor is performed, that is, dissection close to the mesorectal fascia distally, there will be a very short distance from the tumor to the resection margin, even if there is only a T2 or an early T3 tumor. Similarly, the margin from the metastatic lymph nodes may become very short. Therefore, the dissection should not be performed close to the distal part of the mesorectal fascia during an amputation, the dissection should rather be finished from above several centimeters above the levator plane.
There are three main types of rectum amputation (APE):
- Intersphincteric APE
- Extralevator abdominoperineal excision (ELAPE) (“cylindrical excision”)
- Ischio-anal APE
Intersphincteric APE is not suitable for distal cancers with short margin to the distal part of the mesorectal fascia The method is suitable for tumors treatable with low anterior resection, but where an anastomosis cannot be performed, either because of the high risk of leakage in patient's which cannot tolerate serious complications or there is an incontinence present, that will cause a poor functional outcome.
A dissection is performed on the mesorectal fascia to the pelvic floor, and the distal rectum is closed to avoid spillage. A perineal procedure, including cleaning of the analcanal is performed. The skin in the intersphincteric hollow is incised all around and the anal opening closes. A Lone Star retractor is then inserted and dissection is performed upwards in the intersphincteric plane to the level above puborectalis to the pelvic cavity, until anus and distal rectum are free. Remove the specimen. The defect in the skin- and pelvic floor is small and is closed.
Extralevator abdominoperineal excision (ELAPE) (“cylindrical excision”)
ELAPE is recommended when the distal tumor is growing near the mesorectal fascia/sphincter/levator. The procedure is relevant when an ultralow anterior resection or an intersphincteric resection would cause a risk for none or a very short distance to the mesorectal fascia, and also increase the risk of bowel perforation.
The dissection from above must stop before reaching the pelvic floor, it should only be performed down to the sacrococcygeal transition posteriorly; until distal to the inferior hypogastric plexus anterolaterally; to just under the seminal vesicles in men and the cervix uteri in women.
Ususally, it is preferred to perform the perineal part of the procedure in the prone position, after construction of the sigmoid colostomy. The patient is lying with the upper body slightly downwards, with the hip kinked 30-40 degrees and the legs spread out.
After the patient is turned around and the area is cleaned, a purse-string suture is used to close the anus. A boat shaped insicion is performed around anus (there is no need to make it wide), and the dissection is performed just outside of sphincter externus and up underneath the levator muscle. The dissection is then performed laterally along the underside of levator out to the lateral pelvic wall so that the division of the muscles can be performed completely out on the sides. The integrity of the distal mesorectum, which is covered by a cuff of muscle/fat tissue, will in this way be preserved. The coccyx is resected to improve access to the pelvis and the pelvic cavity is entered right in the front edge of the bone and the mesorectal fascia is identified. This is difficult, and to avoid damage on the mesorectal fascia and fat pad, the procedure requires great attention. Inside the pelvic cavity, it is important to ensure that the dissection is performed in the correct direction (along the sacrum).
The levator muscles must be divided properly out on the sides, from behind and in a forward direction. When both sides are loose, gently pull the oral bowel end out through the opening and twist the specimen. How early in the progress this may be performed depends on the thickness of the tumor/mesorectum and the available opening. There must not be used any force when the bowel is twisted down and out. When this is performed the rectum is still located in the anterior structures, in which there are now excellent visibility. The anterior dissection must always be performed with great caution and under continuous control by following the plane against prostate/vagina and avoiding damage of these and the urethtra distally from prostate. The dissection anteriorly, on the side and posteriorly should be performed incrementally, and the specimen should be held in different directions for control of a correct dissection plane. If the tumor/lymph nodes some places are threatening the mesorectal fascia, it may be necessary to dissect outside the plane to achieve R0 resection (extended TME).
Anteriorly the dissection plane for standard TME and cylindrical APE will be identical, but on the sides the dissection plane is much further out. Withdrawing of a complete cylinder (the levators are divided as far out as possible all around) is leading to a large defect in the pelvic floor which cannot be closed, this may cause problems with the healing and later on perineal hernia. Therefore, the cylindrical excision may in some cases be adapted to tumor growth. For example, if the tumor only threatens the resection margins on the left side, removal of a complete cylinder on the right side is not nessesary.
If tumor is infiltrating or perforating the pelvic/levator (may be associated with abscess under levator, or fistula into the rectum/skin) an extended excision of skin and ischiorectal tissue must be performed to ensure R0 resection.
Principles for regional lymph node dissection
- For cancer in the rectosigmoid flexure the vessels/lymph nodes should be resected as in sigmoid cancer, which means at the origin of the inferior mesenteric artery at the aorta.
- For cancer in the upper rectum it is probably adequate to resect the inferior mesenteric artery just distal to the origin of the left colic artery with removal of the local lymph nodes. Fat and lymph nodes in the apical station may be removed even if the artery is resected distal to the left colic artery.
- For cancer in the middle and lower rectum the vessels and lymph nodes are resected just distally to the origin of the left colic artery.
Reconstruction of the pelvic floor/perineum after cylindrical rectal amputation
After a cylindrical rectum amputation a large defect arises in the pelvic floor muscles. This cannot be closed with simple sutures. There is also a defect in subcutaneous fat and in skin which can be closed, but tensioning must be expected. There are often problems with the healing of the perineal wound, both because it is an unclean area where infection easily can occur and because there is a tensioning of tissue after closing. In patients who have undergone preoperative radiotherapy, the problems are bigger and occur more frequently.
There are several alternative methods to close the defect in the pelvic floor after rectal amputation:
- The pelvic floor is adapted in the best possible way with simple sutures.
- A net equivalent to what is used in hernia operations is sewn into the defect in the levator plate.
- Musculo-cutaneous swing flap with the rectus muscle from the abdominal wall. Vertical rectus abdominus plasty (VRAM). Rectus musculature with skin are dissected with the patient in supine position. The blood supply to the flap have to be ensured. The flap is pivoted around its own distal anchoring down in the small pelvis.The abdomen is closed and the patient is moved to prone position. The VRAM flap is sewn to the edges of the remaining levator musculature. The skin on the flap is sewn on the perineal skin.
- Musculo-cutaneous swing flap with gluteus maximus muscle.
There are advantages and disadvantages with the methods, and there are different views of what is best. The larger defects the better indication to use swing flap. When performing swing flap, plastic surgical expertise must be present.
Postoperative complications are wound infection, infection of the small pelvis, slow healing, persistent sinus, necrosis of the swing flaps and perineal hernia.