Essential clinical anatomy part 02

Essential clinical anatomy part 02

(Parte 1 de 8)

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Chapter 3 Pelvis and Perineum

FIGURE 3.1. Abdominopelvic cavity. A. and B. The pelvis is the space within the pelvic girdle, overlapped externally by the abdominal and gluteal (lower limb) regions and the perineum. Thus the pelvis has no unique external surface area. Light green, greater pelvis; dark green, lesser pelvis.

The pelvis (L. basin) is the part of the trunk inferoposterior to the abdomen and is the area of transition between the trunk and the lower limbs (Fig. 3.1). The perineal region refers to the area of the trunk between the thighs and the buttocks, extending from the pubis to the coccyx. The perineum is a shallow compartment lying deep to this area and inferior to the pelvic diaphragm.

The superior boundary of the pelvic cavity is the pelvic inlet, the superior pelvic aperture (Figs. 3.1 and 3.2). The pelvis is limited inferiorly by the pelvic outlet, the inferior pelvic aperture, which is bounded anteriorly by the pubic symphysis (L. symphysis pubis) and posteriorly by the coccyx.

The pelvic inlet (superior pelvic aperture) is bounded by the linea terminalis of the pelvis, which is formed by the:

Superior margin of the pubic symphysis anteriorly. Posterior border of the pubic crest. Pecten pubis, the continuation of the superior ramus of the pubis, which forms a sharp ridge. Arcuate line of the ilium. Anterior border of the ala (L. wing) of the sacrum. Sacral promontory.

The pelvic outlet (inferior pelvic aperture) is bounded by the:

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Inferior margin of the pubic symphysis anteriorly. Inferior rami of the pubis and ischial tuberosities anterolaterally. Sacrotuberous ligaments posterolaterally (Fig. 3.3B) Tip of the coccyx posteriorly

Pelvic Girdle

The pelvic girdle is a basin-shaped ring of bones that connects the vertebral column to the two femurs in the thighs. The main functions of the strong pelvic girdle are to transfer the weight of the upper body from the axial to the lower appendicular skeleton for standing and walking, and to withstand compression and other forces resulting from its support of body weight. The bony pelvis is formed by three bones (Fig. 3.2; Table 3.1):

Right and left hip bones: two large, irregularly shaped bones, each of which forms at puberty by fusion of three bones —ilium, ischium, and pubis.

Sacrum: formed by the fusion of five, originally separate, sacral vertebrae.

The hip bones are joined at the pubic symphysis anteriorly and to the sacrum posteriorly at the sacro-iliac joints to form a bony ring, the pelvic girdle.

The ilium is the superior, flattened, fan-shaped part of the hip bone (Fig. 3.2). The ala of the ilium represents the spread of the fan and the body of the ilium, the handle of the fan. The body of the ilium forms the superior part of the acetabulum, the cup-shaped depression on the external surface of the hip bone with which the head of the femur articulates. The iliac crest, the rim of the ilium, has a curve that follows the contour of the ala between the anterior and the posterior superior iliac spines. The anterior concave part of the ala forms the iliac fossa.

FIGURE 3.2. Bony pelvis. A. Articulated pelvis. B. Child's right hip bone. C. Adult's right hip bone. In the anatomical position the anterior superior iliac spine and the anterior aspect of the pubis lie in the same vertical plane.

The ischium has a body and a ramus (L. branch). The body of the ischium forms the posterior part of the acetabulum, and the ramus forms the posterior part of the inferior boundary of the obturator foramen. The large posteroinferior protuberance of the ischium is the ischial tuberosity (Fig. 3.2). The small pointed posterior projection near the junction of the ramus and body is the ischial spine.

The pubis is an angulated bone that has the superior pubic ramus, which forms the anterior part of the acetabulum, and the inferior pubic ramus, which forms the anterior part of the inferior boundary of the obturator foramen. The superior pubic ramus has an oblique ridge, the pecten pubis (pectineal line of pubis), on its superior aspect. A thickening on the anterior part of the body of the pubis is the pubic crest, which ends laterally as a swelling—the pubic tubercle (Fig. 3.3A).

The pubic arch is formed by the ischiopubic rami (conjoined inferior rami of the pubis and ischium) of the two sides. These rami meet at the pubic symphysis, and their inferior borders define the subpubic angle (the distance between the right and the left ischial tuberosities), which can

P.207 be approximated by the angle between the abducted middle and index fingers for the male, and the angle between the index finger and extended thumb for the female (Fig. 3.4).

FIGURE 3.3. Ligaments of pelvis.

The bony pelvis is divided into greater (false) and lesser (true) pelves by the oblique plane of the pelvic inlet (superior pelvic aperture) (Figs. 3.1 and 3.2). The bony edge (rim) surrounding the pelvic inlet is the pelvic brim.

The greater pelvis (L. pelvis major) is:

Superior to the pelvic inlet. Bounded by the abdominal wall anteriorly, the alea of illun laterally, and the L5 and S1 vertebrae posteriorly.

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The location of some abdominal viscera, such as the sigmoid colon and some loops of ileum.

The lesser pelvis (L. pelvis minor) is:

Between the pelvic inlet and the pelvic outlet (Fig. 3.3B). The location of the pelvic viscera—urinary bladder and reproductive organs, such as the uterus and ovaries. Bounded by the pelvic surfaces of the hip bones, sacrum, and coccyx. Limited inferiorly by the musculomembranous pelvic diaphragm (levator ani) (Table 3.2, Fig. 3.1B).

FIGURE 3.4. Comparison of male and female.

TABLE 3.1 COMPARISON OF MALE AND FEMALE BONY PELVES Bony PelvisMale (ʑ)Female (ʐ)

General structureThick and heavyThin and light Greater pelvis (pelvis major)DeepShallow

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Lesser pelvis (pelvis minor)Narrow and deepWide and shallow

Pelvic inlet (superior pelvic aperture)Heart-shapedOval or rounded

Pelvic outlet (inferior pelvic aperture)Comparatively smallComparatively large

Pubic arch and subpubic angle ()Narrow (<70°)Wide (>80°)

Obturator foramen Round Oval Acetabulum Large Small

Sexual Differences in Bony Pelves

The male and female bony pelves differ in several respects (Fig. 3.4 and Table 3.1). These sexual differences are related mainly to the heavier build and larger muscles of men, and to the adaptation of the pelvis, particularly the lesser pelvis, in women for childbearing. Hence, the male pelvis is heavier and thicker than the female pelvis and usually has more prominent bone markings. In contrast, the female pelvis is wider and shallower and has a larger pelvic inlet and outlet. The shape and size of the pelvic inlet (and the pelvic brim) are significant because it is through this opening that the fetal head enters the lesser pelvis during labor. To determine the capacity of the pelvis for childbirth, the diameters of the lesser pelvis are noted during a pelvic examination. The ischial spines face each other and the interspinous distance between them is the narrowest part of the pelvic canal (the passageway traversing the pelvic inlet, lesser pelvis, and pelvic outlet through which a baby's head must pass at birth).

Pelvic Fractures

Pelvic fractures can result from direct trauma to the pelvic bones, such as may occur during an automobile accident, or from forces transmitted to these bones from the lower limbs during falls on the feet. Pelvic fractures may cause injury to pelvic soft tissues, blood vessels, nerves, and organs.

Joints and Ligaments of Pelvic Girdle

The primary joints of the pelvis are the sacro-iliac joints and the pubic symphysis, which link the skeleton of the trunk and the lower limb (Fig. 3.2A). The lumbosacral and sacrococcygeal joints are directly related to the pelvic girdle. Strong ligaments support and strengthen these joints (Fig. 3.3).

The sacro-iliac joints are strong, weight-bearing, compound joints, consisting of an anterior synovial joint (between the ear-shaped auricular surfaces of the sacrum and ilium covered with articular cartilage) and a posterior syndesmosis (between the tuberosities of the same bones) (Figs. 3.2C and 3.5). The articular (auricular) surfaces of the synovial joint have irregular but congruent elevations and depressions that interlock. The sacro-iliac joints differ from most synovial joints in that they have limited mobility, a consequence of their role in transmitting the weight of most of the body to the hip bones.

The sacrum is suspended between the iliac bones and is firmly attached to them by posterior and interosseous sacro-iliac ligaments. The thin anterior sacro-iliac ligaments form the anterior part of the fibrous capsule of the synovial joint. The interosseous sacro-iliac ligaments occupy an area of about 10 cm2 each and are the primary structures involved in transferring the weight of the upper body from the axial skeleton to the two ilia and then to the femurs during standing and to the ischial tuberosities during

P.210 sitting. The posterior sacro-iliac ligaments are posterior external continuations of the interosseous sacro-iliac ligaments.

Usually movement is limited to slight gliding and rotary movements, except when subject to considerable force such as occurs after a high jump (or during late pregnancy—see next Blue Box). Then the weight of the body is transmitted through the sacrum anterior to the rotation axis, tending to push the superior sacrum inferiorly, thereby causing the inferior sacrum to rotate superiorly. This tendency is resisted by the strong sacrotuberous and sacrospinous ligaments (Fig. 3.3). These ligaments allow only limited upward movement of the inferior end of the sacrum, thus providing resilience to the sacro-iliac region when the vertebral column sustains sudden weight increases (Fig. 3.5C).

The pubic symphysis is a secondary cartilaginous joint that is formed by the union of the bodies of the pubic bones in the median plane (Figs. 3.3 and 3.5D). The fibrocartilaginous interpubic disc is generally wider in women than in men. The ligaments joining the pubic bones are thickened superi they orly and inferiorly to form the superior pubic ligament and the inferior (arcuate) pubic ligament, respectively. The decussating fibers of tendinous attachments of the rectus abdominis and external oblique muscles also strengthen the pubic symphysis anteriorly.

The L5 and S1 vertebrae articulate anteriorly at the anterior intervertebral joint, formed by the L5/S1 intervertebral (IV) disc between their bodies posteriorly and (Fig. 3.1A) at two zygapophysial joints (facet joints) between the articular processes of these vertebrae (Fig. 3.3B). The facets on the S1 vertebra face posteromedially, interlocking with the anterolaterally facing inferior articular facets of the L5 vertebra, preventing L5 from sliding anteriorly. Iliolumbar ligaments unite the transverse processes of L5 to the ilia.

(Parte 1 de 8)

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