True pelvis, pelvic floor and perineum

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CHAPTER 63 True pelvis, pelvic floor and perineum

TRUE PELVIS AND PELVIC FLOOR

The true pelvis is a bowl-shaped structure formed from the sacrum, pubis, ilium, ischium, the ligaments which interconnect these bones and the muscles which line their inner surfaces. 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 and above this the bony structures are sometimes referred to as the false pelvis. They form part of the walls of the lower abdomen. The floor or ‘outlet’ of the true pelvis is formed by the muscles of levator ani. Although the floor is gutter shaped, it generally lies in a plane between 5 and 15° up from the horizontal. This difference between the planes of the inlet and outlet is the reason why the true pelvis is said to have an axis (lying perpendicular to the plane of both inlet and outlet) which progressively changes through the pelvis from above downwards. The details of the topography of the bony and ligamentous pelvis are considered fully on page 1358.

MUSCLES AND FASCIAE OF THE PELVIS

Pelvic muscles

The muscles arising within the pelvis form two groups. Piriformis and obturator internus, although forming part of the walls of the pelvis, are considered as primarily muscles of the lower limb (Fig. 63.1). Levator ani and coccygeus form the pelvic diaphragm and delineate the lower limit of the true pelvis (Fig. 63.2). The fasciae investing the muscles are continuous with visceral pelvic fascia above, perineal fascia below and obturator fascia laterally.

Piriformis

The complete description of piriformis is in Chapter 80.

Piriformis (see p. 1371) forms part of the posterolateral wall of the true pelvis and is attached to the anterior surface of the sacrum, the gluteal surface of the ilium near the posterior inferior iliac spine, the capsule of the adjacent sacroiliac joint and sometimes to the upper part of the pelvic surface of the sacrotuberous ligament. It passes out of the pelvis through the greater sciatic foramen. Within the pelvis, the anterior surface of piriformis is related to the rectum (especially on the left), the sacral plexus of nerves and branches of the internal iliac vessels. The posterior surface lies against the sacrum.

Obturator internus

Obturator internus (see p. 1371) and the fascia over its upper inner (pelvic) surface form part of the anterolateral wall of the true pelvis. It is attached to the structures surrounding the obturator foramen, the inferior ramus of the pubis, the ischial ramus, the pelvic surface of the hip bone below and behind the pelvic brim, and the upper part of the greater sciatic foramen. It also attaches to the medial part of the pelvic surface of the obturator membrane. The muscle is covered by a thick fascial layer and the fibres themselves cannot be seen directly from within the pelvis. This fascia gives attachment to some of the fibres of levator ani and thus only the upper portion of the muscle lies lateral to the contents of the true pelvis, whilst the lower portion forms part of the boundaries of the ischio-anal fossa. In the male, the upper portion lies lateral to the bladder, the obturator and vesical vessels, and the obturator nerve. In the female, the attachments of the broad ligament of the uterus, the ovarian end of the uterine tubes, and the uterine vessels, also lie medial to obturator internus and its fascia.

Levator ani (ischiococcygeus, iliococcygeus, pubococcygeus)

Levator ani is a broad muscular sheet of variable thickness which is attached to the internal surface of the true pelvis and forms a large portion of the pelvic floor (Fig. 63.3). The muscle is subdivided into named portions according to their attachments and the pelvic viscera to which they are related. These parts are often referred to as separate muscles, but the boundaries between each part cannot be easily distinguished and they perform many similar physiological functions. The separate parts are referred to as ischiococcygeus, iliococcygeus and pubococcygeus. Pubococcygeus is often subdivided into separate parts according to the pelvic viscera to which they relate (puboperinealis, puboprostaticus or pubovaginalis, puboanalis, puborectalis). Levator ani arises from each side of the walls of the pelvis along the condensation of the obturator fascia (the tendinous arch of levator ani; see below). Fibres from ischiococcygeus attach to the sacrum and coccyx but the remaining parts of the muscle converge in the midline. The fibres of iliococcygeus join by a partly fibrous intersection and form a raphe posterior to the anorectal junction. Closer to the anorectal junction and elsewhere in the pelvic floor, the fibres are more nearly continuous with those of the opposite side and the muscle forms a sling (puborectalis and pubovaginalis or pubourethralis).

Attachments

The attachments for the ischiococcygeus, iliococcygeus and pubococcygeus parts are as follows.

Iliococcygeus

The iliococcygeal part is attached to the inner surface of the ischial spine below and anterior to the attachment of ischiococcygeus and to the tendinous arch as far forward as the obturator canal (Fig. 63.2). The most posterior fibres are attached to the tip of the sacrum and coccyx but most join with fibres from the opposite side to form a raphe. This raphe is effectively continuous with the fibroelastic anococcygeal ligament, which is closely applied to its inferior surface and some muscle fibres may attach into the ligament. The raphe provides a strong attachment for the pelvic floor posteriorly and must be divided to allow wide excisions of the anorectal canal during abdominoperineal excisions for malignancy. An accessory slip may arise from the most posterior part and is sometimes referred to as iliosacralis.

Pubococcygeus

The pubococcygeal part is attached to the back of the body of the pubis and passes back almost horizontally. The most medial fibres run directly lateral to the urethra and its sphincter as it passes through the pelvic floor; here the muscle is correctly called the puboperinealis, although due to its close relationship to the upper half of the urethra in both sexes it is often referred to as pubourethralis. The muscle fibres from both sides form part of the urethral sphincter complex together with the intrinsic striated and smooth musculature of the urethra; fibres decussate across the midline directly behind the urethra. In males some of these fibres lie lateral and inferior to the prostate and are referred to as puboprostaticus (levator prostatae). In females, some fibres form the pubourethralis, others run further back to form a sling around the posterior wall of the vagina where they are referred to as pubovaginalis. In both sexes, fibres from this part of pubococcygeus attach to the perineal body; a few elements also attach to the anorectal junction. Some of these fibres, sometimes called puboanalis, decussate and blend with the longitudinal rectal muscle and fascial elements to contribute to the conjoint longitudinal coat of the anal canal. Behind the rectum, some fibres of pubococcygeus form a tendinous intersection as part of the levator raphe, and a thick muscular sling, puborectalis, wraps around the anorectal junction. Some fibres blend with those of the external anal sphincter. Pubourethralis, pubovaginalis/puboprostaticus and puboanalis are sometimes collectively referred to as ‘pubovisceralis’.

Innervation

Fibres which originate mainly in the second, third and fourth sacral spinal segments reach levator ani from below and above by a variety of routes (Wendell-Smith & Wilson 1991). Most commonly, pubococcygeus is supplied by second and third sacral spinal segments via the pudendal nerve, and ischiococcygeus and iliococcygeus by direct branches of the sacral plexus from the third and fourth sacral spinal segments.

Pelvic fasciae

The pelvic fasciae may be conveniently divided into the parietal pelvic fascia, which mainly forms the coverings of the pelvic muscles, and the visceral pelvic fascia, which forms the coverings of the pelvic viscera and their vessels and nerves (Fig. 63.4).

Parietal pelvic fascia

The parietal pelvic fascia consists of the obturator fascia, the fasciae over piriformis, and over levator ani (the pelvic diaphragm), and the presacral fascia.

Obturator fascia

The parietal pelvic fascia on the pelvic (medial) surface of obturator internus is well differentiated. Although in humans it is derived from the degenerate upper portion of the attachment of levator ani, it is usually referred to as the obturator fascia. Above, it is connected to the posterior part of the arcuate line of the ilium, and is continuous with iliac fascia. Anterior to this, as it follows the line of origin of obturator internus, it is gradually separated from the attachment of the iliac fascia and a portion of the periosteum of the ilium and pubis spans between them. It arches below the obturator vessels and nerve, investing the obturator canal, and is attached anteriorly to the back of the pubis. Behind the obturator canal, the fascia is markedly aponeurotic and gives a firm attachment to levator ani, usually called the tendinous arch of levator ani (arcus tendineus musculi levatoris ani) (see Figs 63.3, 63.13, 63.14). Below the attachment of levator ani, the fascia is thin and is effectively composed only of the epimysium of the muscle and overlying connective tissue; posteriorly it forms part of the lateral wall of the ischio-anal fossa in the perineum, and anteriorly it merges with the fasciae of the muscles of the deep perineal space (see Fig. 63.14), which is continuous with the ischio-anal fossa. The obturator fascia is continuous with the pelvic periosteum and thus the fascia over piriformis.

Fascia over levator ani (pelvic diaphragm)

The fascia over levator ani covers both of the surfaces of the pelvic diaphragm. On the lower surface, the thin inferior fascia is continuous with the obturator fascia below the tendinous arch of levator ani laterally. It covers the medial wall of the ischio-anal fossa and blends below with fasciae on the urethral sphincter and the external anal sphincter. On the upper surface, the superior fascia of the pelvic diaphragm is markedly thicker than that over the inferior surface. It is attached anteriorly to the back of the body of the pubis, approximately 2 cm above its lower border, and extends laterally across the superior ramus of the pubis, blending with the obturator fascia and continuing along an irregular line to the spine of the ischium. It is continuous posteriorly with the fascia over piriformis and the anterior sacrococcygeal ligament. Medially, the superior fascia of the pelvic diaphragm blends with the visceral pelvic fascia forming part of the endopelvic fascia.

Arcus tendineus fascia pelvis/white line of the parietal pelvic fascia

Low on the superomedial aspect of the upper fascia over levator ani, a thick white band of condensed connective tissue extends from the lower part of the symphysis pubis to the inferior margin of the spine of the ischium (see Figs 63.4, 63.13, 63.14). Although often referred to as the tendinous arch of the pelvic fascia (arcus tendineus fasciae pelvis), it is really the remnant of the degenerate tendon of iliococcygeus in humans and is best referred to as the white line of the parietal pelvic fascia (Smith 1908). It provides attachment for the condensations of visceral pelvic fascia which provide support to the urethra, bladder and vagina in females (see below).

Visceral pelvic fascia

The visceral pelvic fascia is formed from condensations of connective tissue which are closely associated with the pelvic viscera to which it relates and with the neurovascular structures supplying or running near those organs. In its most inferior and lateral extent, the visceral pelvic fascia is closely related to, and effectively derived from, the superior fascia over the attachment of levator ani (see below), whereas more superiorly and posteriorly, it is derived from part of the fascia over piriformis. The condensations where these fasciae meet are sometimes collectively referred to as the ‘endopelvic fascia’. Further accounts are given of the paravisceral portions of the visceral pelvic fascia (e.g. parametrium, paracolpium) in the chapters describing the organs to which the fascia relates.

Several different condensations of endopelvic fascia are recognized both at operation and by MRI imaging. They provide important support to the pelvic viscera and play a role in continence in both sexes.

In the female, pubourethral ligaments form from periurethral connective tissue and attach to the white line of the parietal pelvic fascia close to its pubic end. A little more laterally, similar tissue exists which may be termed the urethropelvic ligaments, and which insert into the white line over levator ani. These condensations are effectively continuous with connective tissue which runs from the paravaginal tissue to the white line, and is sometimes referred to as the vaginolevator support or attachment. Since the normal angle of the urethra during standing is nearly 45° to the vertical, these two structures combined can be viewed as providing a ‘hammock-like’ support to the midurethra (DeLancey 1994); some fibres may even decussate across the midline between the urethra and the anterior vaginal wall. The upper portion of the pubourethral ligament blends with the pubovesical and vesicopelvic ligaments (connective tissue around the neck of the urinary bladder), and may contain cholinergically innervated smooth muscle fibres, sometimes referred to as pubovesicalis. In males, pubourethral structures are augmented by the puboprostatic ligament/puboprostaticus. In both sexes, these muscles may have a role in opening the upper urethra and bladder neck during micturition.

There is much less condensation of connective tissue around the rectum, and no similar rectopelvic ligaments exist. The connective tissue over the longitudinal muscles of the rectum is thickened just above the anal hiatus in levator ani and it fuses with the endopelvic fascia and the anococcygeal ligament, sometimes referred to as a rectosacral ligament.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE OF THE PELVIS

The true pelvis contains the internal iliac arteries and veins and the lymphatics which drain the majority of the pelvic viscera. The common and external iliac vessels and the lymphatics which drain the lower limb lie along the pelvic brim and in the lower retroperitoneum, but are conveniently discussed together with the vessels of the true pelvis.

Arteries of the pelvis

Common iliac arteries

The abdominal aorta bifurcates into the right and left common iliac arteries anterolateral to the left side of the fourth lumbar vertebral body. These arteries diverge as they descend and divide at the level of the sacroiliac joint into external and internal iliac arteries. The external iliac artery is the principal artery of the lower limb. The internal iliac artery provides the principal supply to the walls and viscera of the pelvis, the perineum and the gluteal region.

Internal iliac arteries

Each internal iliac artery, approximately 4 cm long, begins at the common iliac bifurcation, level with the lumbosacral intervertebral disc and anterior to the sacroiliac joint. It descends posteriorly to the superior margin of the greater sciatic foramen where it divides into an anterior trunk, which continues in the same line towards the ischial spine, and a posterior trunk, which passes back to the greater sciatic foramen. Anterior to the artery are the ureter and, in females, the ovary and fimbriated end of the uterine tube. The internal iliac vein, lumbosacral trunk and sacroiliac joint are posterior. Lateral is the external iliac vein, between the artery and psoas major, and the obturator nerve lying inferior to the vein. The parietal peritoneum is medial, separating it from the terminal ileum on the right and the sigmoid colon on the left. Tributaries of the internal iliac vein are also medial (Fig. 63.5).

In the fetus, the internal iliac artery is twice the size of the external iliac artery and is the direct continuation of the common iliac artery. The main trunk ascends on the anterior abdominal wall to the umbilicus, converging on the contralateral artery, and the two arteries run through the umbilicus to enter the umbilical cord as the umbilical arteries. At birth, when placental circulation ceases, only the pelvic segment remains patent as the internal iliac artery and part of the superior vesical artery; the remainder becomes a fibrous medial umbilical ligament. In males, the patent part (usually the superior vesical artery) usually gives off an artery to the vas deferens.

Posterior trunk branches

The branches of the posterior trunk of the internal iliac artery are the iliolumbar, lateral sacral and superior gluteal arteries.

Superior gluteal artery

The superior gluteal artery is the largest branch of the internal iliac artery and effectively forms the main continuation of its posterior trunk. It runs posteriorly between the lumbosacral trunk and the first sacral ramus or between the first and second rami, then turns slightly inferiorly, leaving the pelvis by the greater sciatic foramen above piriformis and dividing into superficial and deep branches. In the pelvis it supplies piriformis, obturator internus and a nutrient artery to the ilium. The superficial branch enters the deep surface of gluteus maximus. Its numerous branches supply the muscle and anastomose with the inferior gluteal branches while others perforate the tendinous medial attachment of the muscle to supply the skin over the sacrum, where they anastomose with the posterior branches of the lateral sacral arteries. The deep branch of the superior gluteal artery passes between gluteus medius and the bone, soon dividing into superior and inferior branches. The superior branch skirts the superior border of gluteus minimus to the anterior superior iliac spine and anastomoses with the deep circumflex iliac artery and the ascending branch of the lateral circumflex femoral artery. The inferior branch runs through gluteus minimus obliquely, supplies it and gluteus medius and anastomoses with the lateral circumflex femoral artery. A branch enters the trochanteric fossa to join the inferior gluteal artery and ascending branch of the medial circumflex femoral artery; other branches run through gluteus minimus to supply the hip joint.

The superior gluteal artery occasionally arises directly from the internal iliac artery with the inferior gluteal artery and sometimes from the internal pudendal artery.