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/True pelvis, pelvic floor and perineum/TRUE PELVIS AND PELVIC FLOOR

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.

  

Fig. 63.1  Piriformis, obturator internus and the ligaments of the pelvis. Those muscles relating only to the pelvis or perineum have been omitted for clarity.
(Adapted from Drake, Vogl and Mitchell 2005.)

  

Fig. 63.2  Muscles of the female pelvis. The superior gluteal and obturator vessels and nerves as well as the pelvic viscera have been omitted for clarity.
(Adapted from Drake, Vogl and Mitchell 2005.)

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).

  

Fig. 63.3  Muscles of the female pelvis viewed from above. The sacral nerve roots have been divided close to the sacral foramina. The anorectal junction, vagina and urethra have been divided at the level of the pelvic floor.

Attachments

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

Ischiococcygeus

The ischiococcygeal part may be referred to as a separate muscle, sometimes named coccygeus. It lies as the most posterosuperior portion of levator ani and arises as a triangular musculotendinous sheet with its apex attached to the pelvic surface and tip of the ischial spine. The base of the muscle is attached to the lateral margins of the coccyx and the fifth sacral segment. Ischiococcygeus is rarely absent, but may be nearly completely tendinous rather than muscular. It lies on the pelvic aspect of the sacrospinous ligament and may be fused with it, particularly if it is mostly tendinous. The sacrospinous ligament may represent a degenerate part or an aponeurosis of the muscle since the muscle and ligament are coextensive.

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’.

Relations

The superior, pelvic surface of levator ani is separated only by fascia (superior pelvic diaphragmatic, visceral and extraperitoneal) from the urinary bladder, prostate or uterus and vagina, rectum and peritoneum. Its inferior, perineal, surface forms the medial wall of the ischio-anal fossa and the superior wall of the anterior recess of the fossa, both being covered by inferior pelvic diaphragmatic fascia. The posterior border is separated from the coccyx by areolar tissue. The medial borders of the two levator muscles are separated by the visceral outlet, through which pass the urethra, vagina, and anorectum.

Vascular supply

Levator ani is supplied by branches of the inferior gluteal, inferior vesical and pudendal arteries.

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.

Actions

Pubococcygeus is a lateral compressor of the various visceral canals which cross the pelvic floor. Puborectalis also reinforces the external anal sphincter, helps to create the anorectal angle, and reduces the anteroposterior dimension of the ano-urogenital hiatus. Iliococcygeus and, to a lesser extent, the less muscular ischiococcygeus, assist puborectalis in contributing to anorectal and urinary continence.

Levator ani must relax appropriately to permit expulsion of urine and particularly faeces, and it contracts with abdominal muscles and the abdominothoracic diaphragm to raise intra-abdominal pressure. It forms much of the basin-shaped muscular pelvic diaphragm, which supports the pelvic viscera. Like the abdominothoracic diaphragm, but unlike abdominal muscles, levator ani is also active in the inspiratory phase of quiet respiration. In the pregnant female, the shape of the pelvic floor may help to direct the fetal head into the anteroposterior diameter of the pelvic outlet.

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).

  

Fig. 63.4  Fasciae of the pelvis and perineum. Median sagittal section in the male. The parietal pelvic fascia is shown in black, the mesorectal fascia in brown, the fascial boundaries of the deep perineal space/pouch in green, the peritoneum in blue, the deep abdominal fascia in purple and the subcutaneous abdominal and perineal fascia in red. The visceral perietal fasciae have been omitted for clarity.

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.

  

Fig. 63.13  Muscles and fasciae of the male perineum – coronal view. The section passes through the bulb of the penis at the level of the urethra. The deep perineal space is continuous with the ischio-anal fossa posteriorly. The parietal pelvic fascia is shown in black, fascial boundaries of the deep perineal space/pouch in green, the deep perineal fascia in purple and the superficial perineal fascia in red. The visceral and parietal fasciae have been omitted for clarity. The pelvic fascia over the ‘pelvic’ aspect of the deep transverse perinei is very thin and does not form a distinct layer as such in places blending with the parietal pelvic fascia over the inferior aspect of levator ani.

  

Fig. 63.14  Muscles and fasciae of the female perineum – coronal view. The section passes through the bulb of the clitoris at the level of the urethra. The deep perineal space is continuous with the ischio-anal fossa posteriorly. The parietal pelvic fascia is shown in black, fascial boundaries of the deep perineal space/pouch in green, the deep perineal fascia in purple and the superficial perineal fascia in red. The visceral and parietal fasciae have been omitted for clarity. The pelvic fascia over the ‘pelvic’ aspect of the deep transverse perinei is very thin and does not form a distinct layer as such in places blending with the parietal pelvic fascia over the inferior aspect of levator ani.

Fascia over piriformis

The fascia over the inner aspect of piriformis is very thin, and fuses with the periosteum on the front of the sacrum at the margins of the anterior sacral foramina. It ensheathes the sacral anterior primary rami which emerge from these foramina: the nerves are often described as lying behind the fascia. The internal iliac vessels lie in front of the fascia over piriformis; their branches draw out sheaths of the fascia and extraperitoneal tissue into the gluteal region, above and below 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).

Presacral fascia

The presacral fascia forms a hammock-like structure posterior to the posterior portion of the mesorectal fascia. Laterally it extends to the origin of the fascia over piriformis and the fascia over levator ani (superior pelvic diaphragmatic fascia) with which it blends, and more inferiorly it extends between the white line of the parietal pelvic fascia on either side. Inferiorly, it extends to the anorectal junction, where it fuses with the posterior aspect of the mesorectal fascia and the anococcygeal ligament at the level of the anorectal junction. Superiorly, it can be traced to the origin of the superior hypogastric plexus, where it becomes progressively thinner over the promontory of the sacrum and becomes continuous with the retroperitoneal tissues. The right and left hypogastric nerves and inferior hypogastric plexuses lie on its surface and the presacral veins lie immediately posterior to it. It forms a distinct layer which can be seen both on magnetic resonance images of the pelvis and during surgery. The presacral fascia provides an important landmark because extension of rectal tumours through it significantly reduces the possibility of curative resectional surgery. Dissection in the plane posterior to the fascia may result in bleeding from the presacral veins: because the adventitia of the veins is partly attached to the posterior surface of the fascia, the haemorrhage may be severe (since the veins are unable to contract properly).

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.

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