Pelvic Floor Disorders: Surgical Approach

3. Pelvic Floor Anatomy

Augusto Orlandi  and Amedeo Ferlosio

(1)

Department of Biomedicine and Prevention, Anatomic Pathology Institute, Tor Vergata University, Rome, Italy

Augusto Orlandi

Email: orlandi@uniroma2.it

Abstract

The maintenance of the correct integrity of the pelvic floor is fundamental for the physiology of this complex anatomical region, as it is involved in functions such as defecation, urination, sexual activities, especially in women, and in puerperium. In fact, the pelvic floor closes the pelvis and holds organs (uterus, rectum, urethra, bladder, and prostate) inside the body. Although there is good anatomical knowledge of the region, the neurological and biomechanical functions of the pelvic floor are not well understood and knowledge of these is continuously evolving. Consequently, correct assessment of pelvic floor anatomy is essential to understand the pathogenesis and surgical correction of pelvic disturbances.

3.1 Introduction

The maintenance of the correct integrity of the pelvic floor is fundamental for the physiology of this complex anatomical region, as it is involved in functions such as defecation, urination, sexual activities, especially in women, and in puerperium. In fact, the pelvic floor closes the pelvis and holds organs (uterus, rectum, urethra, bladder, and prostate) inside the body. Although there is good anatomical knowledge of the region, the neurological and biomechanical functions of the pelvic floor are not well understood and knowledge of these is continuously evolving. Consequently, correct assessment of pelvic floor anatomy is essential to understand the pathogenesis and surgical correction of pelvic disturbances.

For simplification, we have divided this chapter into the following three sections: bony pelvis, musculature, fasciae and aponevrosis, and somatic innervation.

3.2 The Bony Pelvis

The pelvis is divided into the major pelvis (part of abdominal cavity) and the minor or true pelvis [1]. The latter is frontally delimited by the pubic symphysis, and by the coccyx and sacrum at the back. The anatomical limits are considered to be a line from the promontory of the sacrum to the superior margin of the pubic symphysis (the superior strict of canal of partum in woman), closed laterally by the spine of the ischium and the superior ramus of the pubis forward and seen from below from ischial tuberosity (Fig. 3.1, top and bottom).

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Fig. 3.1

The human bony pelvis seen from the front (top) and from below (bottom), showing the bones and main ligament structures of the pelvis supporting the pelvic floor

The main ligamentous structures are the anterior and lateral sacrococcygeal ligaments, the sacrospinous ligament, the sacrotuberous ligament, and the arcuate pubic ligament [2].

3.3 The Musculature, Fasciae, and Aponevrosis

From a surgical point of view, when considering access to the pelvic floor starting from the cutis in a gynecological position (Fig. 3.2, top), the perineal region can be divided in two parts: the superficial and deeper planes. The superficial plane coincides with the fascia of Cruveilhier, which continues with abdominal structures and adheres to the thigh, while the deeper plane coincides with the fascia of Colles [3].

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Fig. 3.2

Pelvic floor anatomy in women (top), and the urogenital diaphragm in men (bottom)

The space between the two ischial tuberosities laterally and the pubic symphysis anteriorly is closed by the so-called urogenital diaphragm, which is composed mainly of the deep transverse perineal muscle and forms the anterior urogenital triangle (Figs. 3.1 and 3.2) together with external urethral sphincter, the urethrovaginalis (in woman) and compressor urethrae [45]. This area is covered by the inferior and superficial fascia (Colles) of the urogenital diaphragm. The urogenital diaphragm is perforated, from front to back, in order to permit the passage of the urethra and, in woman, the vagina with its sphincter muscle. Immediately behind the pubic symphysis, the space between the arcuate pubic ligament and the fusion of the deep and superficial fascia of the urogenital diaphragm (the transverse perineal ligament) identifies a tight cleft where it crosses the deep dorsal vein of penis or glans clitoridis (Fig. 3.2, bottom).

The superficial and inferior fascias are fused posteriorly to envelop the superficial transverse perineal muscles, thus contributing to the composition of the perineal body (see below).

Inferiorly to the urogenital diaphragm, and more superficially with respect to the cutaneous plane, two muscles can be identified: the ischiocavernosus and the bulbocavernosus muscles (Fig. 3.2, top). The ischiocavernosus muscles originate from the pubic symphysis, lie parallel to the ischiopubic branches, and are attached to the ischiatic tuberosities. The bulbocavernosus muscles lie medially, and there is a marked difference in these muscles between the two sexes. In women, they surround the vagina between the pubic symphysis and perineal body. In men, they run parallel to each other, separated only from a median raphe.

In the urogenital diaphragm, the anal triangle can be identified [5] (Fig. 3.2, top). The anococcygeal ligament originates from the apex of the coccyx, and it is inserted into the perineal centrum, interrupted by the passage of anal canal with the external anal sphincter muscle (Fig. 3.2, top). The central ligament of perineum or perineal body is histologically composed of smooth and skeletal muscle, as well as collagen and elastic fibers [6]. The perineal body represents the fundamental structure maintaining the anatomical and functional integrity of the pelvic floor, and the point of insertion of the anal sphincter muscle, the transverse superficial perineal muscles (originating from ischiatic tuberosities), and the bulbocavernosus muscle (Fig. 3.2, top).

Once the urogenital diaphragm is removed, the pelvic diaphragm becomes visible. The latter is composed of the ischiococcygeous and levator ani muscles (Fig. 3.3). The ischiococcygeous muscle (or simply coccygeus) originates from the coccyx and sacrum and extends bilaterally extends towards the ischiatic spines and sacrospinous ligaments.

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Fig. 3.3

Pelvic floor anatomy seen from cranial axis, showing the pelvic diaphragm and the relation between the muscles and the pelvic organs

The levator ani muscle comprises the pubococcygeus, puborectalis and iliococcygeus muscles (Fig. 3.3) [78]. Moreover, a thickening of the obturator internus, known as the arcus tendineus levator ani muscle, is stretched between the ischial spine to the superior pubic branch. From the arcus tendineus, the iliococcygeus originates, connecting to the coggyx posteriorly and to the perineal body medially, contributing to the creation of the anococcygeal ligament. Some authors divide the pubococcygeus into three regions called: the puboperineus, the pubovaginalis (in women), and the puboanalis [4]. The puboperineus is attached to the perineal body and contributes to its formation; the pubovaginalis and puboanalis muscle bundles fuse with those of the vagina and external anal sphincter, respectively.

The levator ani muscle is crossed by the urethra, vagina, and rectum through the urogenital hiatus. Indeed, the rectum is not considered to be part of the hiatus because the levator ani muscle attaches directly to the perineal body. Both the pubococcygeus and the puborectalis originate from the inferior pubic branch. The latter determines the closure of the urogenital hiatus since it surrounds the vagina and rectum, and terminates in the perineal body. Finally, the levator ani is covered by the superior and inferior fascia of the same name (i.e., superior and inferior fascia of the levator ani). In particular, all the connective tissue covering the pelvic diaphragm and viscera is called the endopelvic fascia [9], which attaches to the bony pelvis [10]. The endopelvic fascia has different names according to anatomical relationships: the pubocervical fascia (between the bladder and the vagina) and the rectovaginal fascia (between the vagina and rectum). Moreover, the parametrium and paracolpium are the tracts of the fascia extending from the cervix and vagina, respectively, to the pelvic walls [11]. Indeed, the term fascia is erroneous since microscopic study failed to determinate this [4].

Further support ligaments in the pelvic floor are the periurethral, paraurethral, and pubourethral ligaments, which help to maintain the structure of the urethra and bladder [912].

3.4 Somatic Innervation

The bulbocavernosus, ischiocavernosus and superficial transverse perineal muscles are innervated by the perineal branches of the pudendal nerves [13]. Historically, the levator ani muscle has been assumed to be innervated by two nerves: the pudendal and the sacral nerves. More recently, the role of the pudendal nerve has been questioned. In fact, experimental studies have demonstrated that a nerve originating from S3, S4, and S5 innervates the levator ani and coccygeus, and it has been called levator ani nerve [414]. It is probably correct that there are anastomotic branches between the pudendal and levator ani nerves [14]. Sometimes, the puborectalis receives a direct contribution from S5 [4]. From their sacral origins, the pudendal nerves exit from the pelvis across the great sciatic foramen, and enter again across the lesser sciatic foramen running in the so-called Alcock (or pudendal) canal [15]. At this level, posteriorly, the inferior rectal and perineal nerves, and the dorsal nerve of clitoridis (or penis in man), originate in succession from the pudendal nerve. The latter also provides the sensitive innervation of the external genitalia.

3.5 Conclusions

In conclusion, stability and correct anatomical function of the pelvic floor is maintained from the pelvic diaphragm caudally and the endopelvic fascia cranially. When the structures of pelvic diaphragm lose their integrity, as a result of traumatic or senile causes, there is only the endopelvic fascia to maintain the position of the organs. This fascia can rapidly lose its capacity to function and the organs may then prolapse.

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