Pelvic Anatomy
Radu Apostol
Farr Nezhat
INTRODUCTION
The obstetrician/gynecologist needs a thorough understanding of the female pelvic anatomy. The etiology and pathogenesis of clinical problems should be studied in the context of normal anatomy, as considered in other chapters. Sound surgical technique is based on accurate anatomic knowledge. In this chapter, we will describe some important anatomic relations that are critical during open as well as minimally invasive procedures.
EXTERNAL GENITALIA
The external genitalia also known as the vulva includes the mons pubis, labia majora and minora, the vaginal vestibule, the clitoris, and the greater vestibular glands. The mons pubis is a fibrofatty pad covered by hair-hearing skin that covers the bony pubic ramus. The bony pelvic outlet is bordered anteriorly by the ischiopubic rami, posteriorly by the coccyx, and laterally by the ischial tuberosities and sacrotuberous ligaments.
The labia majora (Figure 1.3.1) are two folds of skin with underlying adipose tissue lying on either side of the vaginal opening and contain sebaceous and sweat glands as well as a few specialized apocrine glands. It is also where the round ligament inserts into.
The labia minora are hairless skin folds that split anteriorly to form the prepuce and frenulum of the clitoris, also known as the clitoral hood. Posteriorly, it divides to form a fold of skin called the fourchette at the back of the vaginal introitus. Labia minora contains sebaceous glands but has no adipose tissue. The vestibule is the cleft between the labia minora and contains the openings of the urethra, the Bartholin glands, and the vagina.
The clitoris is an erectile structure ˜0.5 to 3.5 cm in length, consisting of the glans, a shaft that is attached to the pubis by a subcutaneous ligament, and paired crura that stem from the shaft and attach to the inferior aspect of the pubic rami. The blood supply to the clitoris is from branches of the internal pudendal artery. The deep artery of the clitoris supplies the clitoral body whereas the dorsal artery of the clitoris supplies the glans and prepuce.
 Figure 1.3.1. Vulva and perineum. |
The hymen is a thin fold of mucous membrane across the entrance to the vagina. After childbirth, only a few remnants are visible, also referred to hymenal caruncles, and they delineate the vulva from the vagina.
The Bartholin glands, also known as greater vestibular glands, are bilateral and about 0.5 cm in diameter on each side of the vestibule, posterolateral to the vaginal orifice. They open via a 2 cm duct into the vestibule below the hymen and contribute to lubrication during intercourse.
PERINEUM
The perineum represents the inferior boundary of the pelvis. It is bounded superiorly by the levator ani muscles and inferiorly by the skin. Anteriorly, it extends to the pubic symphysis and the inferior borders of the pubic bones. Posteriorly, it is limited by the ischial tuberosities, the sacrotuberous ligaments, and the coccyx. The superficial and deep transverse perineal muscles cross the pelvic outlet between the two ischial tuberosities and come together at the perineal body (Figure 1.3.2). They divide the space into the urogenital triangle anteriorly and the anal triangle posteriorly. The urogenital diaphragm is a fibromuscular sheet that stretches across the pubic arch (1,3).
PELVIC FLOOR
The pelvic floor is comprised of the perineal membrane and the muscles of the pelvic diaphragm. It helps support the pelvic contents above the pelvic outlet. The muscles of the pelvic diaphragm comprised of the levator ani and coccygeal muscles (Figure 1.3.3). The levator ani muscles comprise the puborectalis, pubococcygeus, and iliococcygeus muscles. The puborectalis arises from the inner surface of the pubic bones and inserts into the rectum. Some fibers form a sling around the posterior aspect of the rectum. The pubococcygeus arises from the pubic bones and inserts into the anococcygeal raphe and superior
aspect of the coccyx. The iliococcygeus arises from the arcus tendinous levator ani and inserts into the anococcygeal raphe and coccyx (1,2,3,4).
aspect of the coccyx. The iliococcygeus arises from the arcus tendinous levator ani and inserts into the anococcygeal raphe and coccyx (1,2,3,4).
 Figure 1.3.2. Perineum showing superficial structures. |
 Figure 1.3.3. Sagittal section through the pelvis. The levator ani takes its origin from the thickened obturator internus fascia (arcus tendineus) and the pubic ramus. The bifurcation of the common iliac vessels is seen. Note the pudendal neurovascular bundle enters a fascial canal created within the lowest portion of the obturator internus muscle, Alcock canal. (Courtesy of Dr. Vlad Apostol.) |
VAGINA
The vagina is a fibromuscular canal lined with nonkeratinized stratified squamous epithelium starting at the hymenal ring and ending at the fornices surrounding the cervix. The posterior wall has an average length of 9 cm and the anterior wall of ˜7 cm. The vaginal vault is divided into four fornices: anterior, posterior, and two laterals. The vagina has no mucous glands and no hair follicles. Mesonephric duct remnants in the lateral vaginal wall can result in Gartner duct cysts.
Anteriorly, the vagina is in direct contact with the base of the bladder, whereas the urethra runs down the lower half in the midline to open into the vestibule. At the vaginal apex, its fibromuscular layer fuses to create the cardinal and uterosacral ligaments. The fan-shaped cardinal ligament creates a sheath that envelops the uterine artery and vein, fusing medially with the paracervical ring. The uterosacral portion inserts into the posterior and lateral aspects of the paracervical ring and then curves laterally along the pelvic sidewall to attach to the presacral fascia that overlies the sacral vertebrae (S-2, 3, and 4). The cardinal and uterosacral ligaments support the upper part of the vagina. The fibromuscular layer (endopelvic fascia) of the anterior vaginal wall is referred to as the pubocervical fascia, whereas the posterior fibromuscular layer is known as the rectovaginal fascia. The pubocervical fascia attaches to the cervix and the cardinal-uterosacral support of the vaginal apex, while laterally it fuses with the fascia of the obturator internus muscle to create the arcus tendinous fascia pelvis, also referred to as the white line. The pubocervical fascia attaches inferiorly to the pubic symphysis (1,2,3,4).
UTERUS
The uterus is a fibromuscular organ divided into the following parts: body and cervix. It is where the embryo and fetus develop. The disappearance of maternal estrogen from the circulation after birth causes the uterus to decrease in length and weight. The cervix is then twice the length of the uterus. During childhood, the uterus grows slowly in length, in parallel with height and age. After the onset of puberty, the anteroposterior and transverse diameters of the uterus start to increase, leading to a sharper rise in the volume of the uterus. The increase in uterine volume continues well after menarche, and the uterus reaches its adult size and configuration by the late teenage years. After menopause, the uterus atrophies, the mucosa becomes thin, the glands almost disappear, and the wall becomes relatively less muscular.
Uterine Body (Corpus)
The uterine body forms the upper two-thirds of the uterus and has two parts: the fundus and the isthmus. It is a thick, pear-shaped organ somewhat flattened anteroposteriorly, consisting of interlacing smooth muscle fibers. The endometrial lining of the uterine corpus is the innermost lining of the uterus made
up of columnar epithelium and specialized stroma. The endometrial lining varies from 2 to 10 mm in thickness, depending on the stage of the menstrual cycle. The superficial layer of the endometrium contains hormonally sensitive spiral arterioles that shed during each cycle. The deeper basal layer is preserved with each cycle and has its own arterial supply. The myometrium contains interlacing smooth muscle fibers, whereas the serosa of the uterus is formed by peritoneal mesothelium. Myometrial muscle cells also grow during pregnancy, with contractile activity that is present both during labor and during the menstrual cycle and female orgasm. The fundus is the rounded part of the body of the uterus that lies superior to the orifices of the fallopian tubes (Figure 1.3.4). The isthmus is the constricted region of the uterine body, ˜1 cm in length, just above the cervix.
up of columnar epithelium and specialized stroma. The endometrial lining varies from 2 to 10 mm in thickness, depending on the stage of the menstrual cycle. The superficial layer of the endometrium contains hormonally sensitive spiral arterioles that shed during each cycle. The deeper basal layer is preserved with each cycle and has its own arterial supply. The myometrium contains interlacing smooth muscle fibers, whereas the serosa of the uterus is formed by peritoneal mesothelium. Myometrial muscle cells also grow during pregnancy, with contractile activity that is present both during labor and during the menstrual cycle and female orgasm. The fundus is the rounded part of the body of the uterus that lies superior to the orifices of the fallopian tubes (Figure 1.3.4). The isthmus is the constricted region of the uterine body, ˜1 cm in length, just above the cervix.
There are four paired sets of ligaments that are attached to the uterus. Each round ligament inserts on the anterior surface of the uterus just in front of the fallopian tube, passes to the pelvic sidewall in a fold of the broad ligament, traverses the inguinal canal, and ends in the labium majora. It contains the artery of Sampson. The uterosacral ligaments are formed by thickening of the endopelvic fascia that arises from the sacral fascia and inserts into the posterior-inferior portion of the uterus at the level of the isthmus. Other than providing structural support to the uterus, the uterosacral ligaments also contain sympathetic and parasympathetic nerve fibers that supply the uterus. The cardinal ligaments are other important supporting structures of the uterus that prevent prolapse. They extend from the pelvic fascia on the lateral pelvic walls and insert into the lateral portion of the cervix and vagina, reaching superiorly to the level of the isthmus. The pubocervical ligaments pass anteriorly around the bladder to the posterior surface of the pubic symphysis. Additionally, there are two broad ligaments that each pass from the side of the uterus to the lateral wall of the pelvis. Between the two leaves of each broad ligament are the fallopian tube, the round ligament, the ovarian ligament as well as nerves, blood vessels, and lymphatics (1,2,4,5).
CERVIX
The cervix measures ˜2 to 3 cm in length and is divided into supravaginal, lying above the vagina, and the vaginal part that protrudes into the vagina. It contains dense fibrous connective tissue and is surrounded circularly by a small amount of smooth muscle into which the cardinal and uterosacral ligaments and pubocervical and rectovaginal fascia insert. The cervix contains a longitudinal canal connecting the endometrial cavity with the vagina, known as the endocervical canal. The internal os of the cervix is at the junction of the endocervical canal with the endometrial cavity, whereas the external os is the distal opening in the vagina. The epithelium of the endocervix is columnar and also ciliated in its upper two-thirds. This changes to stratified squamous epithelium around the region of the external os, and the junction of these two types of epithelium is called the squamocolumnar junction (1,2,4,5).
 Figure 1.3.4. Panoramic view of the pelvis. |
FALLOPIAN TUBES (OVIDUCTS)
The fallopian tubes are bilateral muscular tubes, ˜10 cm in length, with lumina that connect the uterine cavity with the peritoneal cavity (Figure 1.3.4). They are enclosed by the broad ligament. The fold of the broad ligament containing the fallopian tubes is referred to as the mesosalpinx. The fallopian tubes are lined by ciliated, columnar epithelium that is thrown into branching folds. They convey the ovum from the ovary toward the uterus and promote oxygenation and nutrition for sperm, ovum, and zygote, should fertilization occur.
The fallopian tube has four regions from proximal to distal: interstitial, isthmic, ampullary, and infundibular. The interstitial portion lies within the wall of the uterus, whereas the isthmus is the narrow portion adjoining the uterus. This passes into the widest and longest portion, the ampulla. The opening of the tube into the peritoneal cavity is surrounded by finger-like processes known as fimbria. The inner surface of the fimbria is covered by ciliated epithelium that is similar to the lining of the fallopian tube itself. One of the fimbria is longer than the others and extends to and partially embraces the ovary. The muscular fibers of the wall of the tube are arranged in an inner circular and an outer longitudinal layer. The ampullary and fimbriated segments of the fallopian tube are suspended from the broad ligament by the mesosalpinx and are quite mobile. The mobility of the fimbriated end of the tube plays an important role in fertility. The ampullary portion of the tube is the most common site of ectopic pregnancies (1,2,4,5).
OVARIES
The ovaries are oval, flattened, compressible organs ˜3 cm long, 2 cm wide, and 2 cm thick. The size and appearance of the ovaries depend on both age and stage of the menstrual cycle. The ovary is the only intra-abdominal structure not to be covered by the peritoneum (Figure 1.3.4). Each ovary is attached to the cornu of the uterus by the ovarian ligament and at the hilum to the broad ligament by the mesovarium. Laterally, the ovary is attached to the suspensory ligament of the ovary, also known as the infundibulopelvic ligament (Figure 1.3.4). The blood supply to the ovaries is provided by the ovarian arteries that arise from the abdominal aorta. The ovaries also receive blood supply from the uterine artery through the uterine-ovarian arterial anastomosis. The venous drainage from the right ovary is directly into the inferior vena cava, whereas that from the left ovary is into the left renal vein (1,2,4,5).
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