Hip bone
Position of the hip bones (shown in red)
Male pelvis, formed by left and right hip bones, sacrum, and coccyx
Latin Os coxae, os innominatum
Gray's subject #57 231
/ Elsevier
Hip bone

The hip bone (innominate bone, pelvic bone[1] or coxal bone) is a large, Flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three bones; ilium, ischium, and pubis.

The two hip bones joins at the pubic symphysis and together with the sacrum and coccyx, the hip bone comprises the skeletal component of the pelvis, the pelvic girdle, which surrounds the pelvic cavity. The hip bone forms the primary connection between the bones of the lower limb and the axial skeleton. They are connected to the sacrum, which is part of the axial skeleton, at the sacroiliac joint. Each hip bone is connected to the corresponding femur, which is part of the lower limb, through a large ball and socket joint, the hip joint.[2]

Human anatomy

The hip bone is formed by three bones; ilium, ischium, and pubis. At birth, these three component bones are separated by hyaline cartilage. They join each other in a Y-shaped portion of cartilage in the acetabulum. By the end of puberty the three bones will have grown together. The fusion is usually complete by the age 25. See Ossification for further details.

The two hip bones join each other at the pubic symphysis. Together with the sacrum and coccyx, the hip bones form the pelvis.[2]


Main article: Ilium (bone)

Ilium (plural Ilii) is the uppermost and largest bone. It makes up two-fifth of the acetabulum. It is divisible into two parts, the body and the ala or wing of ilium; the separation is indicated on the top surface by a curved line, the arcuate line, and on the external surface by the margin of the acetabulum. The body of ilium forms the sacroiliac joint with the sacrum. The edge of the wing of ilium forms the S-shaped iliac crest which is easily located through the skin. The iliac crest show clear marks of the attachment of the three abdominal wall muscles.[2]


Main article: Ischium

The ischium forms the lower and back part of the hip bone and is located below the ilium and behind the pubis. The ischium is the strongest of the three bones that forms the hip bone. It is divisible into three portions; The body, the superior ramus and the inferior ramus. The body forms approximately one-third of the acetabulum.

The ischium forms a large swelling, the tuberosity of the ischium, also referred to colloqially as the "sit bone". When sitting, the weight is frequently placed upon the ischial tuberosity. The gluteus maximus covers it in the upright posture, but leaves it free in the seated position.[2]


Main article: Pubis (bone)

The pubic bone or pubis is the ventral and anterior of the three bones forming the hip bone. It is divisible into a body, a superior ramus and an inferior ramus. The body forms one-fifth of the acetabulum. The body forms the wide, strong, medial and flat portion of the pubic bone which unite with the other pubic bone in the pubic symphysis.[2] The fibrocartilaginous pad which lies between the symphysial surfaces of the pubic bones, that secures the pubic symphysis, is called the interpubic disc.

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

The inferior pubic ramus is thin and flat. It passes laterally and downward from the medial end of the superior ramus. it becomes narrower as it descends and joins with the inferior ramus of the ischium below the obturator foramen.

Muscle attachments

Several muscles attaches to the hip bone including the internal muscles of the pelvic, abdominal muscles, back muscles, all the gluteal muscles, muscles of the lateral rotator group, hamstring muscles, two muscles from the anterior compartment of the thigh and even a single shoulder muscle.

Abdominal muscles

Back muscles

Gluteal muscles

Lateral rotator group


  • The long head biceps femoris arises from the lower and inner impression on the back part of the tuberosity of the ischium, by a tendon common to it and the semitendinosus, and from the lower part of the sacrotuberous ligament;[3]
  • The semitendinosus arises from the lower and medial impression on the tuberosity of the ischium, by a tendon common to it and the long head of the biceps femoris; it also arises from an aponeurosis which connects the adjacent surfaces of the two muscles to the extent of about 7.5 cm. from their origin.
  • The semimembranosus arises from the lower and medial impression on the tuberosity of the ischium

Anterior compartment of thigh

  • The rectus femoris muscle arises by two tendons: one, the anterior or straight, from the anterior inferior iliac spine; the other, the posterior or reflected, from a groove above the rim of the acetabulum.
  • The sartorius muscle arises by tendinous fibres from the anterior superior iliac spine,

Shoulder muscles


The hip bone is ossified from eight centers: three primary —one each for the ilium, ischium, and pubis; and five secondary —one each for the iliac crest, the anterior inferior spine (said to occur more frequently in the male than in the female), the tuberosity of the ischium, the pubic symphysis (more frequent in the female than in the male), and one or more for the Y-shaped piece at the bottom of the acetabulum. The centers appear in the following order: in the lower part of the ilium, immediately above the greater sciatic notch, about the eighth or ninth week of fetal life; in the superior ramus of the ischium, about the third month; in the superior ramus of the pubis, between the fourth and fifth months. At birth, the three primary centers are quite separate, the crest, the bottom of the acetabulum, the ischial tuberosity, and the inferior rami of the ischium and pubis being still cartilaginous. By the seventh or eighth year, the inferior rami of the pubis and ischium are almost completely united by bone. About the thirteenth or fourteenth year, the three primary centers have extended their growth into the bottom of the acetabulum, and are there separated from each other by a Y-shaped portion of cartilage, which now presents traces of ossification, often by two or more centers. One of these, the os acetabuli, appears about the age of twelve, between the ilium and pubis, and fuses with them about the age of eighteen; it forms the pubic part of the acetabulum. The ilium and ischium then become joined, and lastly the pubis and ischium, through the intervention of this Y-shaped portion. At about the age of puberty, ossification takes place in each of the remaining portions, and they join with the rest of the bone between the twentieth and twenty-fifth years. Separate centers are frequently found for the pubic tubercle and the ischial spine, and for the crest and angle of the pubis.


The hip bone first appears in fishes, where it consists of a simple, usually triangular bone, to which the pelvic fin articulates. The hip bones on each side usually connect with each other at the forward end, and are even solidly fused in lungfishes and sharks, but they never attach to the vertebral column.[4]

In the early tetrapods, this early hip bone evolved to become the ischium and pubis, while the ilium formed as a new structure, initially somewhat rod-like in form, but soon adding a larger bony blade. The acetabulum is already present at the point where the three bones meet. In these early forms, the connection with the vertebral column is not complete, with a small pair of ribs connecting the two structures; nonetheless the pelvis already forms the complete ring found in most subsequent forms.[4]

In practice, modern amphibians and reptiles have substantially modified this ancestral structure, based on their varied forms and lifestyles. The obturator foramen is generally very small in such animals, although most reptiles do possess a large gap between the pubis and ischium, referred to as the thyroid fenestra, which presents a similar appearance to the obturator foramen in mammals. In birds, the pubic symphysis is present only in the ostrich, and the two hip bones are usually widely separated, making it easier to lay large eggs.[4]

In therapsids, the hip bone came to rotate counter-clockwise, relatives to its position in reptiles, so that the ilium moved forward, and the pubis and ischium moved to the rear. The same pattern is seen in all modern mammals, and the thyroid fenestra and obturator foramen have merged to form a single space. The ilium is typically narrow and triangular in mammals, but is much larger in ungulates and humans, in which it anchors powerful gluteal muscles. Monotremes and marsupials also possess a fourth pair of bones, the prepubes or "marsupial bones", which extend forward from the pubes, and help to support the abdominal muscles and, in marsupials, the pouch. In placentral mammals, the pelvis as a whole is generally wider in females than in males, to allow for the birth of the young.[4]


Main article: Pelvic fracture

Fractures of the hip bone are termed pelvic fractures, and should not be confused with hip fractures, which are actually femoral fracture[5] that occur in the proximal end of the femur.

Additional images


This article incorporates text from a public domain edition of Gray's Anatomy.

External links

  • eMedicine Dictionary
  • hip/hip%20bones/bones3
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