Human Anatomy - Kotsan I. Ya. 2009

Skeleton of the Lower Limb
The Pelvis as a Whole

The pelvis is formed by the right and left hip bones, the sacrum, and the coccyx, which are interconnected by joints and ligaments to form a single sturdy bony ring. It is divided into an upper section, the greater pelvis (pelvis major), and a lower, smaller section, the lesser pelvis (pelvis minor). The greater pelvis serves as a support for the internal abdominal Organs and as an attachment site for the abdominal wall Muscles. The lesser pelvis resembles a canal with two openings: a superior and an inferior one. It houses the Urinary Bladder, the rectum, and, in women, the FEMALE REPRODUCTIVE ORGANS as well.

The greater pelvis is bounded laterally by the wings of the iliac bones, posteriorly by the lumbar vertebrae, and anteriorly it lacks a bony wall. It is separated from the lesser pelvis by the linea terminalis, which runs along the upper edge of the Pubic Symphysis, the pubic crests, the arcuate lines of the iliac bones, and the sacral promontory. This line defines the pelvic inlet (apertura pelvis superior), also known as the entrance to the lesser pelvis. Below the inlet lies the cavity of the lesser pelvis. The anterior wall of the lesser pelvis cavity, formed by the joined pubic bones, is very short (50–55 mm in height). Conversely, the posterior wall is long and consists of the sacrum and coccyx. The lateral walls of the lesser pelvis are formed by the areas of the hip bones corresponding to the acetabula, as well as the ischial bones and ligaments extending to them from the sacrum. At the bottom, the pelvic cavity terminates in the pelvic outlet (apertura pelvis inferior), which is bounded by the inferior rami of the pubic bones, the ischial tuberosities with ligaments running from the sacrum to the ischial bones, and, finally, the coccyx.

The human pelvis is one of those skeletal regions where Sexual Dimorphism is most pronounced. In women, the pelvis is wider and shorter than in men. The wings of the iliac bones in the female pelvis are flared outward, whereas in males they are positioned more vertically. The BONES OF THE female pelvis are somewhat thinner, with less prominent roughness and tubercles. The sacrum in males is narrow and curved, with a prominent forward-projecting promontory. In females, the sacral bone is broader and flatter, and the promontory is less pronounced. The most striking difference lies in the size of the subpubic angle. In men, the inferior rami of the pubic bones converge near the pubic symphysis to form an acute subpubic angle (angulus subpubicus) of 70–75°. In women, this angle approaches a right angle or is even obtuse, measuring 90–100° and forming the so-called pubic arch (arcus pubicus). The shape of the pelvic inlet in males resembles a playing card Heart, while in females it is round or oval. The cavity of the lesser pelvis is cone-shaped in men and approaches a cylindrical shape in women. The dimensions of the male and female pelvis also differ; in women, they are approximately 1.5–2 cm larger than in men. Familiarity with the average dimensions of the female pelvis is of great importance in obstetric practice.

When measuring the greater pelvis, three transverse dimensions are determined (Fig. 66):

1) spinous distance (distantia spinarum) — the distance between the anterior superior iliac spines, averaging 22–23 cm in men and 25–27 cm in women;

2) crest distance (distantia cristarum) — the maximum distance between both iliac crests, averaging 26 cm in men and 28–29 cm in women;

3) trochanteric distance (distantia trochanterica) — the distance between the greater trochanters of the femurs, measuring 26–29 cm in men and 30–32 cm in women.

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Fig. 66. Dimensions of the greater pelvis

1 — crest distance, 2 — spinous distance, 3 — trochanteric distance

When measuring the lesser pelvis, the dimensions of the pelvic inlet and the pelvic outlet are distinguished.

Fig. 67. Dimensions of the pelvic inlet

1 — transverse diameter, 2, 4 — oblique diameter, 3 — true, or gynecological conjugate

Among the dimensions of the pelvic inlet, the following are determined (Fig. 67):

1) direct (anteroposterior) dimension of the pelvic inlet, or anatomical conjugate (conjugata anatomica) — the distance between the sacral promontory and the upper edge of the pubic symphysis, measuring 10.5 cm in men and 11.5 cm in women;

2) direct dimension — the true or gynecological conjugate (conjugata vera s. gynaecologia) — the distance between the sacral promontory and the most posteriorly protruding part of the pubic symphysis, measuring 10.5–11.0 cm. If this conjugate in a pregnant woman is smaller than the size of a full-term fetal HEAD, a Cesarean Section is necessary;

3) diagonal conjugate (conjugata diagonalis) — the distance between the sacral promontory and the lower edge of the pubic symphysis, measuring 12.5–13 cm;

4) transverse dimension of the pelvic inlet, or transverse diameter (diameter transversa) — the distance between the furthest points of the linea terminalis separating the lesser pelvis from the greater pelvis, measuring 12.5 cm in men and 13.5 cm in women;

5) oblique dimension of the pelvic inlet, or oblique diameter (diameter obliqua) — the distance between the sacroiliac joint on one side and the iliopubic eminence on the other, measuring 12 cm in men and 13 cm in women.

The dimensions of the pelvic outlet exhibit the most pronounced sexual differences. When measuring the pelvic outlet, the following two dimensions are determined:

1) direct dimension of the pelvic outlet, or straight diameter (diameter recta) — the distance from the tip of the coccyx to the lower edge of the pubic symphysis. It is 7.5 cm in men and 9.5 cm in women; however, since the coccyx is mobile, this dimension in women can increase by an additional 2 cm;

2) transverse dimension of the pelvic outlet, or transverse diameter (diameter transversa) — the distance between the ischial tuberosities, measuring 8 cm in men and 11 cm in women.

The line connecting the midpoints of the direct dimensions of the superior and inferior pelvic apertures (i.e., the pelvic inlet and outlet) and running almost parallel to the pelvic surface of the sacrum is called the axis of the pelvis (axis pelvis).

In its natural position, the pelvis is tilted significantly forward, such that the plane of the pelvic inlet forms an angle with the horizontal plane — the pelvic inclination angle, which is greater in women (55–60°) than in men (50–55°) (Fig. 68). In a standing position, the pelvic inclination is greater than when sitting. In a sitting position, the pelvis is positioned almost horizontally, resulting in an angle of only 7°. Comparative anatomical studies indicate that pelvic inclination depends on the vertical posture of The Human Body, which is the cause of the spinal curves closely associated with the pelvis. The structural resistance of the pelvis to external loads is very high. According to P.F. Lesgaft, the pelvis can withstand pressures exceeding 1200 kg.

The shape and dimensions of the pelvis reflect its function. In quadrupeds, where the pelvis does not bear the weight of the upper body and does not serve as a support for Internal Organs, it is relatively small and has a narrow, elongated shape, with the anteroposterior dimension of the lesser pelvis strongly prevailing. In anthropoid apes, which have developed a functional distinction between arms and legs, the pelvis has become considerably wider and shorter, although the anteroposterior dimension still exceeds the transverse one. In Neanderthals, the pelvis already exhibits all the characteristics of a human pelvis—indicating an upright posture and bipedal locomotion—yet it remains somewhat narrower than that of modern humans, in whom upright walking has resulted in a shorter and wider pelvis. In males, both dimensions (anteroposterior and transverse) are nearly equal, whereas in females, whose pelvis is adapted for gestation and childbirth, the transverse dimension even exceeds the anteroposterior one.

Reflecting this evolutionary process, during human ontogeny the pelvis initially (in the fetus) has a narrow shape typical of quadrupeds, then (in the newborn) it resembles the pelvis of anthropoids (simian pelvis), and finally, as bipedalism is mastered, it gradually acquires the shape characteristic of humans.

Fig. 68. Dimensions of the female pelvis (sagittal section)

1 — anatomical conjugate, 2 — true (obstetric) conjugate, 3 — direct dimension (pelvic outlet), 4 — diagonal conjugate, 60° — angle of pelvic inclination

Rapid growth of the pelvis occurs during the prepubertal period. Sexual differences in the pelvis begin to emerge at 9–10 years of age. At 14–16 years, the three bones forming the hip bone fuse together, and the final Formation of the pelvis is completed by age 22–25. In elderly individuals, the pelvic bones become thinner, and due to some degree of visceral ptosis, the inclination of the iliac wings increases.