| Hip bone | |
|---|---|
 Position of the hip bones (shown in red) | |
| Details | |
| Identifiers | |
| Latin | os coxae, os innominatum |
| MeSH | D010384 |
| TA98 | A02.5.01.001 |
| TA2 | 1307 |
| FMA | 16580 16585, 16580 |
| Anatomical terms of bone | |
Thehip bone (os coxae,innominate bone,pelvic bone[1][2] orcoxal bone) is a largeflat bone, constricted in the center and expanded above and below. In somevertebrates (including humans beforepuberty) it is composed of three parts: theilium,ischium, and thepubis.
The two hip bones join at thepubic symphysis and together with thesacrum andcoccyx (the pelvic part of thespine) comprise theskeletal component of thepelvis – the pelvic girdle which surrounds thepelvic cavity. They are connected to the sacrum, which is part of theaxial skeleton, at thesacroiliac joint. Each hip bone is connected to the correspondingfemur (thigh bone) (forming the primary connection between the bones of thelower limb and the axial skeleton) through the largeball and socket joint of thehip.[3]
The hip bone is formed by three parts: theilium,ischium, andpubis. At birth, these three components are separated byhyaline cartilage. They join each other in a Y-shaped portion of cartilage in theacetabulum. By the end ofpuberty the three regions will have fused together, and by the age 25 they will haveossified.The two hip bonesjoin each other at thepubic symphysis. Together with thesacrum andcoccyx, the hip bones form thepelvis.[3]
Ilium (pluralilia) is the uppermost and largest region. It makes up two fifths of the acetabulum. It is divisible into two parts: thebody and theala or wing of ilium; the separation is indicated on the top surface by a curved line, thearcuate line, and on the external surface by the margin of the acetabulum. The body of ilium forms thesacroiliac joint with thesacrum. The edge of the wing of ilium forms the S-shapediliac crest which is easily located through theskin. The iliac crest shows clear marks of the attachment of the threeabdominal wall muscles.[3]
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 regions that form the hip bone. It is divisible into three portions: thebody, thesuperior ramus, and theinferior ramus. The body forms approximately one-third of the acetabulum.
The ischium forms a large swelling, thetuberosity of the ischium, also referred to colloquially as the "sit bone". When sitting, the weight is frequently placed upon theischial tuberosity. Thegluteus maximus covers it in the upright posture, but leaves it free in the seated position.[3]
Thepubic region orpubis is the ventral and anterior of the three parts forming the hip bone. It is divisible into abody, asuperior ramus, and aninferior ramus. The body forms one-fifth of the acetabulum. The body forms the wide, strong, medial and flat portion of the pubic bone which unites with the other pubic bone in thepubic symphysis.[3] The fibrocartilaginous pad which lies between the symphysial surfaces of the coxal bones, that secures the pubic symphysis, is called theinterpubic disc.
Thepelvic brim is a continuous oval ridge of bone that runs along the pubic symphysis, pubic crests, arcuate lines, sacral alae, and sacral promontory.[4]
Thefalse pelvis is that portion superior to the pelvic brim; it is bounded by the alae of the ilia laterally and the sacral promontory and lumbar vertebrae posteriorly.[4]
Thetrue pelvis is the region inferior to the pelvic brim that is almost entirely surrounded by bone.[4]
Thepelvic inlet is the opening delineated by the pelvic brim. The widest dimension of the pelvic inlet is from left to right, that is, along the frontal plane.[4] Thepelvic outlet is the margin of the true pelvis. It is bounded anteriorly by the pubic arch, laterally by the ischia, and posteriorly by the sacrum and coccyx.[4]
Thesuperior pubic ramus is a part of the pubic bone which forms a portion of theobturator 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 lateralprismoid portion. Theinferior 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.
The hip bone is ossified from eightcenters: three primary, one each for the ilium, ischium, and pubis, and five secondary, one each for theiliac crest, theanterior 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 thegreater 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 proportions of the female hip bone may affect the ease of passage of the baby duringchildbirth.
Several muscles attach to the hip bone including theinternal muscles of the pelvic,abdominal muscles,back muscles, all thegluteal muscles, muscles of thelateral rotator group,hamstring muscles, two muscles from theanterior compartment of the thigh.
Fractures of the hip bone are termedpelvic fractures, and should not be confused withhip fractures, which are actuallyfemoral fractures[6] that occur in theproximal end of thefemur.
Pelvimetry is the assessment of the femalepelvis[7] in relation to thebirth of a baby in order to detect an increased risk forobstructed labor.
The hip bone first appears in fishes, where it consists of a simple, usually triangular bone, to which thepelvic fin articulates. The hip bones on each side usually connect with each other at the forward end, and are even solidly fused inlungfishes andsharks, but they never attach to the vertebral column.[8]
In the earlytetrapods, 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.[8]
In practice, modernamphibians andreptiles 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 thethyroid fenestra, which presents a similar appearance to the obturator foramen in mammals. Inbirds, the pubic symphysis is present only in theostrich, and the two hip bones are usually widely separated, making it easier to lay large eggs.[8]
Intherapsids, the hip bone came to rotate counter-clockwise, relative 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 inungulates andhumans, in which it anchors powerful gluteal muscles.Monotremes andmarsupials also possessa 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 placental mammals, the pelvis as a whole is generally wider in females than in males, to allow for the birth of the young.[8]
Thepelvic bones of cetaceans were formerly considered to be vestigial, but they are now known to play a role in sexual selection.[9]
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This article incorporates text in thepublic domain frompage 231 of the 20th edition ofGray's Anatomy(1918)
