Chapter 31: The bones, joints and walls of the pelvis
Bony pelvis
The bony pelvis is formed by the hip bones in front and at the sides and by the sacrum and coccyx behind (figs. 31-1 and 31-2). When a subject is in the anatomical position, the anterior superior iliac spines and the pubic tubercles are in the same coronal plane. The pelvic surface of the body of the pubis, on which the bladder rests, faces more upward than backward. The pelvic surface of the sacrum faces more downward than forward (fig. 31-3).
The (lesser) pelvis has an inlet, a cavity, and an outlet, each of which has three main diameters: anteroposterior (or conjugate) (fig. 31-3), oblique, and transverse (see fig. 31-1).
The pelvic inlet.
The pelvic inlet, or brim (upper pelvic aperture), is indicated by the lineae terminales, the iliac parts of which are the arcuate lines (see fig. 31-1). The inlet is at about half a right angle to the horizontal. The anteroposterior (or true) conjugate diameter extends from the upper margin of the pubic symphysis to the middle of the sacral promontory. The obstetrical conjugate diameter, which is measured from the back ofthe pubic symphysis (fig. 31-3), is the shortest diameter through which the fetal head must pass in its course through the inlet. The diagonal conjugate diameter, between the lower margin of the pubic symphysis and the sacral promontory (fig. 31-3), is measured per vaginam (fig. 31-4). Inability to palpate the sacral promontory suggests that the conjugate diameter of the inlet is adequate for parturition, whereas palpation indicates a contracted pelvis.
The pelvic cavity.
The pelvic cavity extends backward and downward from the inlet to the outlet. It curves with the sacrum and coccyx, and hence is longer behind than in front (see fig. 31-3).
The pelvic outlet.
The pelvic outlet (lower pelvic aperture) extends from the pubic symphysis to the tip of the coccyx (its anteroposterior, or conjugate, diameter; see fig. 31-3) and, from side to side, between the ischial tuberosities; hence it is diamond shaped. The outlet is at a slight angle (10 to 15 degrees) to the horizontal.
The pubic arch formed by the conjoined rami of the pubes and ischia has its apex at the symphysis, where it forms the subpubic angle (fig. 31-5).
The path taken through the pelvic cavity by the fetal head is known as the axis of the birth canal (see fig. 31-3). The axis intersects the inlet at a right angle, turns forward at the uterovaginal angle (level of ischial spines), and follows the axis of the vagina. During parturition, the fetal head (usually the suboccipitobregmatic diameter) occupies successively the inlet (transverse diameter), cavity (oblique diameter, and outlet (anteroposterior diameter) (fig. 31-6).
Classification of Pelves.
Although pelves can be arranged by the measurements of their diameters, it is usual in obstetrics and radiology to classify pelves according to the shape of the pelvic inlet. Four main types are recognized: (1) gynecoid, a rounded inlet; (2) android, a heart-shaped inlet; (3) anthropoid, a long, narrow, oval inlet; and (4) platypelloid, an ovoid inlet with its long axis transverse, like a flat bowl (see fig. 31-1). A female pelvis may belong to any of the four types (only about half are gynecoid), but intermediate types are more frequent. The female pelvis tends to have thinner and lighter bones with less prominent muscular markings, and the subpubic angle is less acute (approximating a right angle; see fig. 31-5). Pelvic diameters and shape can be determined in vivo by radiographic pelvimetry.
Joints of pelvis
The joints of the pelvis include the lumbosacral, sacrococcygeal, and sacro-iliac, and the pubic symphysis. Associated ligaments include the sacrotuberous, sacrospinous, and iliolumbar.
The lumbosacral joint is that between L.V.5 and the sacrum. It includes an intervertebral disc and joints between the articular processes. The sacrococcygeal joint, which may undergo bony fusion, consists of an intervertebral disc between the sacrum and coccyx and accessory ligaments.
The sacro-iliac joints (fig. 31-7) are synovial articulations between the auricular surfaces of the sacrum and ilium on each side. The surfaces may be smooth and flat or reciprocally curved and irregular. The joint is strengthened posteriorly by interosseous and dorsal sacro-iliac ligaments. The weight of the body is transmitted through the sacrum and ilia to the femora during standing and to the ischial tuberosities in sitting.
The pubic symphysis (fig. 31-7) is a cartilaginous joint between the bodies of the pubic bones in the median plane. The symphysial surfaces, each covered by hyaline cartilage, are united by an interpubic disc of fibrocartilage, which may present a cleft. The ligaments around the joint become relaxed during pregnancy.
The sacrotuberous ligament (fig. 31-8) extends from the dorsal surface ofthe sacrum (as well as from the ilium and coccyx) to the ischial tuberosity. The sacrospinous ligament (fig. 31-8) extends from the lateral margin of the sacrum (and coccyx) to the ischial spine. The sacrotuberous ligament converts the sciatic notches into foramina, which are separated from each other by the sacrospinous ligament (see figs. 14-2 and 31-8). The greater sciatic foramen transmits the piriformis muscle, superior and inferior gluteal vessels and nerves, (internal) pudendal vessels and nerve, sciatic and posterior femoral cutaneous nerves, and the nerves to the obturator intemus and quadratus femoris muscles. The lesser sciatic foramen transmits the obturator intemus tendon, the nerve to the obturator intemus, and the (internal) pudendal vessels and nerve (see fig. 14-3).
Walls of pelvis
The wall of the pelvic cavity includes (1) superficial muscles, such as the glutei; (2) the hip bones, the sacrum and coccyx, and their associated ligaments; and (3) deep muscles, blood vessels, nerves, and peritoneum. For descriptive purposes, the pelvic wall can be subdivided into two lateral walls, a posterior wall, and a floor.
Each lateral wall (see figs. 31-8, 32-2 and 35-1), limited by the hip bone below the linea terminalis, is lined by the obturator internus muscle, medial to which are the obturator nerve and vessels and other branches of the internal iliac artery. Rarely, the intestine may protrude through the obturator canal (obturator hernia) and lie under cover of the pectineus. The lateral wall of the pelvis is crossed behind by the ureter and in front by the round ligament or the ductus deferens. The ovary lies in a slight depression on the lateral wall. The lateral and posterior walls are separated by the sacrotuberous and sacrospinous ligaments and by the greater and lesser sciatic foramina.
The posterior wall, formed by the sacrum and coccyx, is lined laterally by the piriformis and coccygeus muscles. The lumbosacral trunk and sacral plexus are situated in front of the piriformis. In the median plane is the median sacral artery (from the aorta), which ends in a vascular mass, the coccygeal body or glomus.
The pelvic floor (a term variously defined) may conveniently be considered as the main structures that support the abdominal and pelvic viscera, i.e., the peritoneum and the pelvic and urogenital diaphragms. The peritoneum descends to be reflected from the front of the rectum to the bladder (rectovesical pouch in the male) or to the uterus and vagina (rectouterine and rectovaginal pouches in the female) (figs. 31-9 and 35-4). The pouches are bounded laterally by peritoneal elevations that are frequently termed sacrogenital folds. In front, the peritoneum is reflected from the uterus to the bladder (uterovesical pouch). The blood vessels and neural plexuses to the viscera (as well as the ureter and ductus deferens) are situated in the connective tissue between the peritoneum and the pelvic diaphragm. Localized thickenings of this extraperitoneal tissue form ligaments. The pelvic floor has two median openings, one for the rectum and the other for the urethra (and vagina).
Additional reading
Steer, C. M., Maloy's Evaluation of the Pelvis in Obstetrics, 3rd ed., Plenum, New York, 1975. Readable and useful.
Questions
31-1 What is the boundary between the lesser and greater pelves?
31-2 In the anatomical position, in which direction does the visceral (pelvic) surface of the body of the pubis face?
31-3 What is the subpubic angle?
31-4 What is the diagonal conjugate diameter?
31-5 Of which type is the sacro-iliac joint?
31-6 What is the structure of the pubic symphysis?
31-7 What are the chief supports for the pelvic viscera?
31-8 How is the rectovesical pouch formed?
Figure legends
Figure 31-1 Female pelvis. A, View from above, showing inlet and anteroposterior (conjugate) and transverse diameters and surrounded by drawings of the four main types of female pelves. H, View from below, showing outlet and anteroposterior (conjugate) and transverse diameters. C, The pelvic cavity with the left hip bone removed. The anterior superior iliac spines and the pubic tubercles are in the same coronal plane (CP). The linea terminalis comprises the (1) promontory, (2) ala of the sacrum, (3) medial border of the ilium (arcuate line), (4) pectineal line, and (5) pubic crest. (After Smout and Jacoby.)
Figure 31-2 Female pelvis. Note the sacro-iliac joints, the subpubic angle, and the continuous curvature of the margin of the obturator foramen and the neck of the femur (Shenton's line).
Figure 31-3 Median section of the pelvis, showing the planes of the inlet and outlet. The true (T), obstetrical (O), and diagonal (D) conjugate diameters are indicated. The axis of the birth canal, that is, the path taken by the fetal head in its passage through the pelvic cavity, can be seen to turn at the uterovaginal angle. 5, fifth lumbar vertebra.
Figure 31-4 Measurement of the diagonal conjugate diameter by the middle finger. The indicated length on the index finger gives the true conjugate, because the index is about 1.5 cm shorter than the middle finger. (After Smout and Jacoby.)
Figure 31-5 The subpubic angle is nearly a right angle in the female and about 60 degrees in the male.
Figure 31-6 The bony pelvis and fetal head. Note how the head turns as it occupies first the inlet, then the cavity, and finally the outlet. (Based on Smout and Jacoby, after Bumm.)
Figure 31-7 The sacro-iliac and hip joints and the pubic symphysis, as seen in an oblique section through the first sacral vertebra. (After Quain.)
Figure 31-8 The muscles and ligaments of the lateral pelvic wall, pelvic aspect. (After Shellshear and Macintosh.)
Figure 31-9 The peritoneal reflections from the pelvic viscera. Note the rectovesical, uterovesical, recto-uterine, and rectovaginal pouches.