Pelvic inlet, pelvic floor and perineum

Perineum

The pudendal nerve supplies sensory fibres from the perineum (external genitalia and anal skin) and motor fibres to the pelvic floor muscles. It can be compressed and damaged in childbirth. It can be blocked with local anaesthetic or stimulated by an electrode, and can be accessed from either the posterior or the perineal approach. The injection needle for local anaesthesia is inserted perpendicular to the perineum between the ischial tuberosity and the anus ( Fig. 71.1 ). The surgeon's finger is placed into the rectum or the vagina to guard against inadvertent bowel or vaginal puncture during needle insertion and is used to guide the needle to the ischial spine. The needle is then tilted medially and dorsally to reach the recto-ischial fossa until it is located below and behind the ischial spine in the pudendal canal (Alcock's canal) . Always aspirate before injecting the anaesthetic agent to avoid direct vascular injection into the internal pudendal vessels that run in the pudendal canal.

During transobturator stress incontinence surgery in both males and females, it is useful to mark out surface anatomy in the perineum to aid trocar passage through the obturator foramen. With the patient in the lithotomy position, in either sex, a polypropylene mesh is typically inserted in either an out-to-in or in-to-out technique, depending on the product used. In men, the landmark for trocar placement in an out-to-in technique is two fingers’ breadths below the inferior border of adductor longus in the groin crease ( Fig. 71.2 ). In women undergoing an in-to-out transobturator tape procedure, the markings of the exit position of the trocar are 2 cm above a line drawn parallel to the external urethral meatus and 2 cm lateral to the groin crease.

Sacrum

The sacrum is made up by the fusion of five vertebrae and forms the posterosuperior wall of the pelvic cavity between the two iliac bones. The borders and anatomical landmarks of the sacrum can be identified by following the iliac crest medially and by palpating the sacrococcygeal articulation. The median sacral crest can be palpated in the dorsal midline. The sacral foramina are located approximately 2 cm lateral to the median sacral crest. These landmarks are surgically relevant in sacral neuromodulation, a procedure originally described by Schmidt et al that involves stimulation of the S2–4 sacral nerve roots. The technique has been used to treat a variety of lower urinary tract and pelvic floor dysfunction disorders, such as urinary and faecal incontinence, overactive bladder, bladder/pelvic pain and urinary retention. The technique involves implantation of an electrode adjacent to the sacral nerve roots; the electrode is placed percutaneously with or without the use of X-ray guidance. When X-ray is not available, surface anatomy landmarks are critical for correct placement of leads into the S3 foramina. The patient is placed prone and a point identified 10 cm from the coccyx in the midline and 2 cm lateral and 3 cm superior to this point. The curvature of the sacrum means that the electrode must be inserted at an angle of 30–60° to facilitate entry into the S3 foramen ( Fig. 71.3 ). Correct placement is confirmed by stimulating the S3 nerve root and observing a bellows contraction of the pelvic floor (an ‘anal wink’) and flexion of the great toe. Plantar flexion of the entire foot with heel rotation suggests stimulation of the S2 nerve root, and bellows contraction in isolation suggests S4 nerve root stimulation.

Muscles and fasciae

The muscles within the pelvis form two groups: piriformis and obturator internus form part of the walls of the pelvis, and levator ani is the largest muscle of the pelvic floor. The fasciae investing the muscles are continuous with the obturator fascia laterally, perineal fascia inferiorly and visceral fascia superiorly. Obturator internus and the fasciae over its upper and inner surface form part of the anterolateral wall of the true pelvis and are attached to the structures surrounding the obturator foramen. Piriformis forms part of the posterolateral wall of the true pelvis and has attachments to the anterior surface of the sacrum, the gluteal surface of the ilium and the capsule of the adjacent sacroiliac joint. Levator ani is subdivided into pubococcygeus, iliococcygeus and puborectalis. Ischiococcygeus lies cranial to levator ani and is contiguous with it. These muscles act as the principal support mechanism of the pelvic floor and, through its attachments and relations around the vagina, prostate and rectum, play an important physiological role during micturition, defaecation and parturition.

Pelvic fasciae can be divided into parietal (coverings of the pelvic muscles) and visceral (coverings of the pelvic organs and their neurovascular supply). Parietal fasciae include obturator fascia, fascia over piriformis, fascia over levator ani (pelvic diaphragm) and presacral fascia. The visceral pelvic fascia connects the pelvic walls to the urogenital organs and is made up of neurovascular mesenteric condensations ensheathed by loose connective tissue and adipose tissue lying above the perineal membrane. The most lateral attachments constitute the endopelvic fascia. These attachments provide a course for neurovascular structures and also help to support and retain the pelvic organs in place. In females, loose connective tissue separates the bladder and vagina, and the vagina and rectum; in males, it separates the bladder, prostate and seminal vesicles from the rectum. There are important condensations of connective tissue within the endopelvic fascia. In the female, uterosacral ligaments pass from the posterolateral aspect of the cervix posteriorly flanking the rectum, and cardinal ligaments surround the cervicovaginal junction (parametrium), extending to mid-vaginal level where the vagina is attached to the tendinous arch of the pelvic fascia (paracolpium). Superiorly, this connective tissue is traversed by the ureters. The pubocervical fascia extends forwards from the cardinal ligaments to the pubis on either side of the bladder. A layer of fascia between the rectum and seminal vesicles in the male, and the rectum and vagina in the female – the rectovesical or rectovaginal septum, respectively – is less prominent. Approximately 1 cm above and lateral to the inferior border of the pubic bone, the anterior end of the tendinous arch of the pelvic fascia is attached to the paravaginal or prostatic tissues. This band, sometimes referred to as the ‘white line’, extends to the superior margin of the ischial spine, lying on the superomedial aspect of the upper fascia covering levator ani. The attachment of the anterior vaginal wall to this tendinous arch of pelvic fascia helps to support the vagina, urethra and bladder.

Loss of the integrity of these structures can contribute to herniation of pelvic organs and prolapse into the vagina. The causes of pelvic organ prolapse (POP) are often multifactorial and defects may involve more than one compartment of the vagina (anterior, posterior and apical/vault). The paracolpium provides two levels of support ( Fig. 71.4 ). Level I or upper support suspends the vagina via the utero­sacral ligaments; damage will result in an enterocele or uterine prolapse, depending on whether the uterus is still present. Level II support is derived from the mid vagina; damage to either the pubocervical fascia or to the rectovaginal septum will result in anterior (cystocele) or posterior defects (rectocele), respectively. Surgery to correct POP aims to reconstruct these defects and restore normal anatomy. When considering transvaginal anterior compartment repairs, it is important to recognize that the defect may be midline sagittal, transverse, paravaginal (where the vagina is separated from its attachment to the arcus tendineus fasciae pelvis) or site-specific ( Fig. 71.5 ). The surgeon must be able to recognize these defects at the time of surgery and tailor repair accordingly. The repair involves plication of the pubocervical fascia, which is repaired in the midline when there is a central defect, typically with interrupted delayed absorbable sutures. Apical support must be addressed to ensure a durable native tissue repair with incorporation of the fascial repair into the cervix or cardinal/uterosacral ligament complex. In paravaginal defects, the pubocervical fascia becomes separated in part from the arcus tendineus fasciae pelvis. The approach is identical initially to a central defect repair in terms of initial exposure, and concomitant central defects can be repaired. At the site of the defect, the bladder is retracted using a suitable retractor and the ‘white line’ of the arcus tendineus fasciae pelvis exposed. Four to five interrupted permanent sutures are then placed into the ‘white line’ and aponeurosis of levator ani, from the bladder neck down to the ischial spine; at each level they are placed into the lateral aspect of the vaginal repair. Anterior compartment repairs can be augmented with the use of biological or synthetic material; however, their use is currently controversial and there is a move towards traditional native tissue repairs. Each case needs to be considered carefully on its merits. Augmentation with mesh or biological material is used following traditional repair; the implant is secured to firm structures such as the arcus tendineus fasciae pelvis and the sacrospinous ligaments.

Herniation of the rectum into the vaginal canal results in a recto­cele. Correction can involve a transvaginal rectocele repair, where the posterior vaginal epithelium must be dissected from the rectum in the rectovaginal fascial plane. The rectovaginal fascial plane is thinned and the rectum will be bulging into the vagina; care must therefore be taken not to buttonhole the rectum. Dissection should be completed cranial to the defect; caution is required because if the patient has a concomitant enterocele (when the small bowel prolapses into the pouch of Douglas), there is a risk of small bowel injury. Lateral dissection should be completed to the lateral side wall (lateral vaginal sulcus) to expose the levators.

Weakness or damage to the cardinal and uterosacral ligaments can lead to apical/vault prolapse. Currently, one of the most common transvaginal procedures to repair post-hysterectomy vault prolapse or complete procidentia is sacrospinous fixation. Surgery involves posterior wall vaginal dissection and development of the pararectal space. Typically, two non-absorbable sutures are placed through the sacrospinous ligament on one side (although bilateral fixation is possible) and secured to the underside of the vaginal apex ( Fig. 71.6 ). The first suture is placed 2 cm medial to the ischial spine and the second suture 1 cm medial to the first. The sutures should be placed through the ligament and not around it, to avoid injury to the pudendal neurovascular bundle, which sits just behind the ligament.

The other common procedure to treat this condition is sacro­colpopexy, which uses an abdominal approach and can be performed open, laparoscopically or with robotic assistance; this also allows anterior and posterior repairs. Either biological or synthetic material is attached to the vaginal vault and to the longitudinal ligament overlying the sacrum distal to the sacral promontory. Ventral mesh rectopexy utilizes the same approach as sacrocolpopexy but the dissection is focused on the posterior compartment through the pouch of Douglas, and the posterior vagina is dissected off the rectum in the rectovaginal fascial plane to a point just cranial to the anal sphincters ( Fig. 71.7 ). Sacrocolpopexy and ventral mesh rectopexy can be combined for multi­compartment prolapse.

The pelvic inlet as viewed surgically from above is formed by the sacrum, pubis, ilium, ischium, the ligaments that interconnect these bones, and the muscles that line their inner surfaces ( Fig. 71.8 ). The true pelvis is considered to start at the level of the plane passing through the promontory of the sacrum, the arcuate line on the ilium, the iliopectineal line and the posterior surface of the pubic crest. This plane, or ‘inlet’, lies at an angle of between 35° and 50° up from the horizontal. During sacrocolpopexy, the surgeon must be able to recognize the bifurcation of the aorta, the middle sacral vessels, the right ureter and the sigmoid colon. The dissection around the sacrum and initial anatomical exposure is particularly important and care must be taken to identify the major pelvic vessels (described below), right ureter and autonomic nerves. The presacral soft tissues are dissected to expose the vessels overlying the sacrum; care should be taken to avoid injuring these vessels and causing bleeding. Once the mesh is in place, the cul-de-sac posterior to this is often closed off to avoid enterocele formation.

During a vaginal hysterectomy, a McCall culdoplasty is typically performed at the end of the procedure to prevent vaginal vault prolapse. The goal is to appose the apical endopelvic fascia to the cardinal–uterosacral ligament complex at the apex. The uterosacral ligaments are tied in to the midline with intervening cul-de-sac peritoneum and full-thickness apical vaginal wall. When placing the sutures through the uterosacral ligaments, caution must be taken to avoid injury to the ureters, which lie 1–2 cm lateral at the level of the cervix.