40 The Hip

Ilium

The ilium is the uppermost and largest bone of the pelvis.

The ilium is the uppermost and largest bone of the pelvis. It articulates with the sacrum, ischium, and pubis.

The human ilium is divisible into the body and the ala, or wing. These two parts are separated on the superior surface by a curved line known as the arcuate line, and on the inferior surface by the margin of the acetabulum.

Ilium: The ilium is the uppermost bone of the pelvis and extends laterally.

The body contributes to the formation of the acetabulum, a concave structure where the head of the femur articulates to form the hip joint. The internal surface of the body forms part of the wall of the lesser pelvis and gives origin to some fibers of the obturator internus muscle.

The Ala

The ala or wing, is the large expanded portion that extends laterally. It has an external and internal surface, a crest (illiac crest), and an anterior and posterior border.

The external surface is smooth and divided by three lines: the posterior, anterior and inferior gluteal lines. The gluteus maximus originates from the external surface adjacent to the posterior gluteal line, the gluteus medius from adjacent to the anterior line, and the gluteus minimus from the inferior line.

The internal surface of the ala is smooth and concave and forms the iliac fossa. The sacrospinalis and multifidus muscles of the back originate in the posterior.

In humans, bi-iliac width is an anatomical term referring to the widest measure of the pelvis between the outer edges of the upper iliac bones. The bi-iliac width measure is helpful in obstetrics because a pelvis that is significantly too small or too large can have obstetrical complications. It is also used by anthropologists to estimate body mass.

Ischium

The ischium forms the lower and posterior portion of the hip bones of the pelvis.

The ischium forms the lower and back part of the hip bone. Situated below the ilium and behind the pubis, the superior portion of this bone forms approximately one third of the acetabulum, which articulates with the femoral head to form the hip joint.

Lateral view of ischium: Left hip joint, opened by removing the floor of the acetabulum from within the pelvis. The ischium is labeled at the bottom left of the ilium.

The ischium is divisible into three portions; the body, and the superior and inferior rami.

The body contains a prominent spine that is the origin for the gemellus superior muscle. Two indentations run parallel to the spine—superiorly,the greater sciatic notch and, inferiorly, the lesser sciatic notch, through which key nervous and vascular vessels pass.

The superior ramus of the ischium extends inferiorly and posteriorly from the body. It is the partial origin for the obturator internus and obturator externus muscles.

Posteriorly the ramus forms a large swelling termed the tuberosity of the ischium, or ischial tuberosity, which supports weight while sitting and is the origin for the gemellus inferior and adductor magnus muscles.

Dorsally, the ramus contributes to the obturator foramen, a large opening in the pelvis through which key nervous and vascular vessels pass.

The inferior ramus of the ischium is thin and flattened and ascends from the superior ramus of the ischium to join the inferior ramus of the pubis. It is the partial origin for the gracillis and adductor magnus muscles.

Ischium: The ischium is located below the ilium and behind the pubis.

Pubis

The pubis is the lowest and most anterior portion of the hip bones of the pelvis.

The pubic bone forms the anterior region of the pelvis and contributes to the acetabulum, which articulates with the femoral head to form the hip joint.

The pubic bone is covered by a layer of fat that is covered by the mons pubis. The two pubic bones joint anteriorly through the pubic symphysis, a cartilaginous joint.

The pubic bone articulates with the ilium and the ischium on each hip. The internal surface forms the wall of the lesser pelvis and is the point of origin for a portion of the obturator internus muscle. The pubic bone is divisible into a body, a superior ramus, and an inferior ramus.

The Body of the Pubic Bone

Body of pubic bone: (1) sacrum, (2) ilium, (3) ischium, (4) pubis: 4a-body; 4b-superior ramus; 4c-inferior ramus, (5) pubic symphisis, (6) acetabulum, (7) obturator foramen, (8) coccyx, (red dotted line) linea terminalis.

The body of the pubis is a wide, strong, medial, and flat portion of the pubic bone that unites with the pubic symphisis.

The rough superior edge of the corpus, known as the pubic crest, ends laterally in the pubic tubercle. This tubercle, found roughly 3 cm from the pubic symphysis, is a distinctive feature on the lower part of the abdominal wall and is useful when attempting to localize the superficial inguinal ring and the femoral canal of the inguinal canal.

Its internal surface enters into the formation of the wall of the lesser pelvis and gives origin to a portion of the obturator internus muscle.

Superior Pubic Ramus

The superior pubic ramus is one third of the pubic bone. It forms a portion of the obturator foramen and extends from the body to the median plane where it articulates with its counterpart from the opposite side. It is described in two portions, which are a medial flattened part and a narrow lateral prismoid portion.

Inferior Pubic Ramus

The inferior pubic ramus is a thin and flat bone that makes up one third of the pubis. It passes laterally and downward from the medial end of the superior ramus, and becomes narrower as it descends and joins with the inferior ramus of the ischium below the obturator foramen.

False and True Pelves

The false (greater) pelvis is larger and superior to the true (lesser) pelvis where the pelvic inlet is located.

There is some disagreement as to what constitutes the pelvis. Depending on what is included in the description these groupings are often termed true (lesser) or false (greater) pelves.

The True Pelvis

The true (or lesser) pelvis is bounded in front and below by the pubic symphysis and the superior rami of the pubis; above and behind, by the sacrum and coccyx; and laterally, by a broad, smooth, quadrangular area of bone, corresponding to the inner surfaces of the body and superior ramus of the ischium, and the part of the ilium below the arcuate line.

This cavity is a short, curved canal, deeper on its posterior than on its anterior wall, and contains the pelvic inlet. Some consider this region to be the entirety of the pelvic cavity. Others define the pelvic cavity as the larger space including the false greater pelvis, just above the pelvic inlet.

Greater and lesser pelvis: The greater pelvis (yellow) is larger and superior to the lesser pelvis (red) where the pelvic inlet is located.

The true pelvis contains the pelvic colon, rectum, bladder, and some of the reproductive organs. The rectum is at the back, in the curve of the sacrum and coccyx; the bladder is in front, behind the pubic symphysis.

In the female, the uterus and vagina occupy the interval between these viscera. The pelvic splanchnic nerves arising at S2–S4 are in the lesser pelvis.

The False Pelvis

The false (or greater) pelvis is bounded on either side by the ilium. In front it is incomplete, presenting a wide interval between the anterior borders of the ilia; behind is a deep notch on either side between the ilium and the base of the sacrum.

Some consider this region to be part of the pelvic cavity, while others consider it part of the abdominal cavity (hence the name false pelvis). Others compromise by referring to the area as the abdominopelvic cavity.

The false pelvis supports the intestines (specifically, the ileum and sigmoid colon), and transmits part of their weight to the anterior wall of the abdomen.

Comparison of Female and Male Pelves

The female pelvis has evolved to its maximum width for childbirth and the male pelvis has been optimized for bipedal locomotion.

Different Sex, Different Pelvis

Like the skull, the pelvis is highly useful for determining a skeleton’s biological sex. A wide pelvis is beneficial for child birth, however a narrow pelvis is beneficial for locomotion when walking upright. These conflicting demands are often termed the obstetrical dilemma.

The female pelvis has evolved to its maximum width for childbirth—a wider pelvis would make women unable to walk. In contrast, human male pelves are not constrained by the need to give birth and therefore are optimized for bipedal locomotion.

Diagnostic Criteria

There are several diagnostic criteria for differentiating a male from a female pelvis:

1. The female pelvis is larger and broader than the male pelvis, which is taller (owing to a higher iliac crest), narrower, and more compact.
2. The distance between the ischium bones is small in males. This causes the sides of the male pelvis to converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart. This results in the female inlet being large and oval in shape, while the male inlet is more heart shaped.
3. The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90–100 degrees) in women. Accordingly, the angle is called the subpubic angle in men and pubic arch in women.
4. The greater sciatic notch is wider in females.
5. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males.
6. The male sacrum is long, narrow, straighter, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.
7. The acetabula are wider apart and face more medially in females than in males. This change in the angle of the femoral head gives the female gait its characteristic swinging of hips.

Pelvic Structure and Childbearing

The human pelvis has evolved to be narrow enough for efficient upright locomotion, while still being wide enough to facilitate childbirth.

Human sex differences in pelvic shape have emerged over the course of human evolution. The human pelvis is narrower and smaller than that of our closest living relatives, the apes. These changes in the pelvis enable bipedal locomotion, or upright walking.

This narrowing of the pelvis has also affected the way humans give birth, as a narrow pelvis makes it more difficult for an infant to move through the birth canal. This problem is compounded, because as human pelvises became smaller, the heads of infants became larger to accommodate increased brain size.

Since the pelvis is vital for both efficient locomotion and childbirth, natural selection has been forced to strike a compromise between a wide pelvis to facilitate birthing large-brained infants and having a narrow pelvis to increase locomotive efficiency. This compromise has been referred to as the obstetrical dilemma. Thus, the female pelvis has evolved to be as wide as possible, to make childbirth easier, without becoming so wide as to make bipedal locomotion too inefficient.

Additionally, the female pubic symphysis, which is the cartilaginous joint connecting the left and right side of the pelvis, is remodeled by hormones released during pregnancy, allowing it to stretch during childbirth.

Male pelves are not constrained by the issue of childbirth, and thus are narrower and more optimal for bipedal locomotion. Wider hips in females cause an increased valgus angle, which is the angle between the femur and lower leg. This increases the risk of torsional knee injuries.

The Male Pelvis: The male pelvis is narrower than that of the female, as can be seen by the less than 90 degree angle of the pubic arch.

The Female Pelvis: The female pelvis is wider than that of the male, as can be seen by the greater than 90 degree angle of the pubic arch.