1 Basics

1.1 Anatomy and Physiology of the Pelvic Floor

Helga Fritsch

Overview

The abdominal and pelvic cavities are bounded above by the diaphragm, anteriorly by the anterior abdominal muscles and the bony pelvis, posteriorly by the spinal column and the posterior abdominal muscles, and below by the pelvic floor (Fig. 1.1).

The striated muscles known collectively as the pelvic diaphragm close the bony outlet of the pelvic cavity incompletely. There is a common undivided opening in the pelvic floor through which the pelvic organs pass, or where they open. The anterior (ventral) part of this opening is the urogenital hiatus, and the posterior (dorsal) part is the anal hiatus. Each of these openings is sealed off or completed by muscles or fibrous tissue below or outside the pelvic diaphragm.

The organs housed in the pelvic cavity, the pelvic organs, are part of different functional systems. The terminal parts of the urinary system-namely the urinary bladder and urethra-lie anteriorly, while the terminal parts of the digestive system-the rectum and anal canal-are found posteriorly. These organs are connected to the abdominal parts of the respective functional systems above and they end in the pelvic floor. Parts of the male genital system lie below or behind the urinary bladder (Fig. 1.2) and lead into the external genitalia outside the pelvic cavity. The organs of the female genital system, lying in the frontal plane between the urinary bladder and the rectum (Fig. 1.3), are similarly linked to the external sex organs below the pelvic floor.

Fig. 1.1 Sagittal section of the abdominal and pelvic cavities.

The space between the pelvic organs and the bony wall of the pelvis is filled with connective and adipose tissue. Since this space lies below or outside the peritoneum covering the peritoneal cavity, it is sometimes comprehensively called the pelvic extraperitoneal space. However, part of this connective and adipose tissue lies directly under the peritoneum, where it covers the pelvic wall and organs. This tissue is therefore called the subperitoneal tissue. This fibroadipose tissue is continuous with the retroperitoneal connective tissue lying in the retroperitoneal space of the abdominal cavity. In general, the term "subperitoneal connective tissue" now prevails and includes the whole of the pelvic connective tissue. The vessels, nerves, and lymphatics of the pelvic organs run through the pelvic connective tissue.

As this short introduction indicates, the pelvic floor is morphologically extremely complex and functionally a very complicated region. What further complicates this region is its complex innervation by the central nervous system. There are both striated and smooth motor connections. The autonomic nervous system innervating smooth motor muscles has both sympathetic and parasympathetic neuronal wiring. The pelvic floor has all forms of sensory input, some informing the brain about skin conditions and muscle stretch relevant to the exteroceptors and muscle proprioceptors. Other forms of sensory input transmit information regarding sensory organ function. Some give the central nervous system (CNS) the specific location of sensory input or pain, while other inputs cause referred sensation or pain. The pelvic floor therefore cannot be viewed in isolation and has to be considered in connection with the surrounding structures, as well as its individual parts.

In what follows, therefore, the individual parts of the pelvic cavity and pelvic floor are assembled like building blocks. From this, we will develop an understanding of the interplay between the various organic systems and the pelvic floor.

Fig. 1.2 The male pelvic organs.

Fig. 1.3 The female pelvic organs.

The Bony Pelvis

The bony pelvis is formed by the two pelvic bones and the sacrum (Fig. 1.4a). Each pelvic bone (innominate bone, hip bone) consists of three bones that meet at the acetabulum. These are: theilium, ischium, and pubis. Important bony points in the pelvis include the iliac crest, the anterior superior iliac spine, the anterior inferior iliac spine, the pubic tubercle next to the symphysis, the ischial tuberosity, and the ischial spine, as well as the posterior inferior iliac spine and the posterior superior iliac spine. The linea terminalis marks the oblique plane of the pelvic inlet that separates the abdominal and pelvic cavities. The inferior pubic rami, the ischial rami and tuberosities, and the tip of the coccyx form the boundary of the bony pelvic outlet. The pelvic outlet is closed incompletely by the muscles of the pelvic floor, the origin and course of which lie predominantly above the plane of the pelvic outlet (Fig. 1.4b).

Fig. 1.4 a The bony pelvis. b The dashed line (- - -) indicates the plane of the pelvic inlet; the solid line (--) indicates the plane of the pelvic outlet.

Posteriorly and inferiorly, two bands-the sacrospinous and sacrotuberal ligaments-stabilize the bony pelvis. These bands complete the foramina formed by the greater and lesser sciatic notches and in part give rise to the muscles of the pelvic floor and the gluteus maximus.

Muscles of the Pelvic Floor

Several successive muscular layers close off the bony frame of the pelvic outlet. Its essential component is the pelvic diaphragm, which is composed of the following muscles: the ischiococcygeus (coccygeus) and the levator ani (Fig. 1.5).

Fig. 1.5a, b The muscles of the pelvic floor.

Ischiococcygeus (Coccygeus)

This muscle arises from the ischial spine and the supraspinous ligament. It is inserted into the lateral aspect of the coccygeal vertebrae. This muscle develops irregularly-i. e., it may be present or only in rudimentary form. Posteriorly, it blends into the levator ani. It is the most dorsal of the muscles of the pelvic floor. The ischiococcygeus is supplied by separate branches arising from the sacral plexus (S3-S4).

Levator Ani

The levator ani is a composite muscle. It is composed of several bilaterally symmetrical parts. Although there is some dispute in the literature concerning the organization and nomenclature of these parts, the classic division into three parts (pubococcygeus, iliococcygeus, puborectalis) has prevailed and proved reliable when applied to function.

The pubococcygeus arises from the internal surface of the superior pubic ramus and is inserted into the lower sacral and the coccygeal vertebrae.

The iliococcygeus adjoins the pubococcygeus posteriorly. It does not have a direct bony origin, but arises from a reinforced fascial band that extends in an arc as the tendinous arch of the levator ani, over the pelvic aspect of the fascia of the obturator internus, to the ischial spine. It is inserted into the lower sacral and the coccygeal vertebrae, where it interdigitates with the pubococcygeus.

In life, the parts of the levator-namely, the pubococcygeus and iliococcygeus, together with the ischiococcygeus-are layered in a flat muscular plate that seals the pelvic outlet posteriorly and laterally.

The puborectalis has its bony origin below the pubococcygeus on the inner surface of the pubic bone. It is not inserted into bone. Rather, the puborectalis muscles of both sides interdigitate behind the rectum and in this manner form a muscular sling posterior to the rectal wall at the level of the anorectal flexure. The muscular sling is continuous posteriorly, while anteriorly it opens to form the anal and urogenital hiatuses. The puborectalis is supplied by separate branches from the sacral plexus and the pudendal nerve [Roberts et al. 1988].

The levator ani, like all other skeletal muscles, is surrounded by fascia. It is termed the superior fascia of the pelvic diaphragm on the side facing the pelvic cavity, and the inferior fascia of the pelvic diaphragm on the side facing outward.

The bony pelvis presents characteristic sex-specific differences; for instance, in the female the pelvic cavity is wider and the ischial tuberosities further apart than in the male. Consequently, the muscles of the pelvic floor also differ. The muscles surrounding the levator arch are further apart in the female pelvis than in the male. In the latter, the muscles are more strongly developed and, because of the narrower bony pelvis, take on more of a funnel shape than in the female (Fig. 1.6). In the female pelvis, the muscles of the pelvic floor are often infiltrated by fibrous tissue, and this can frequently be seen even during prenatal stages of development [Fritsch and Frohlich 1994].

Fig. 1.6 The muscles of the pelvic floor.

Urogenital Diaphragm

It is generally thought that the opening between the limbs of the levator is closed off externally and below by a muscular layer, termed the urogenital diaphragm. This has been debated, especially in the morphological literature [Oelrich 1983]. In the male pelvis, the anterior opening between the limbs of the levator ani, the urogenital hiatus, is closed off by a thin sheet of muscle and connective tissue. This muscle, known as the deep transverse perineal muscle, is the inferior part of the external urethral sphincter. Its fibers first surround the urethra in semicircular loops, then run horizontally through the perineum and so spread out as the deep transverse perineal muscle. These striated muscles are innervated by the pudendal nerve,...