The ilium (/ˈɪliəm/) (plural ilia) is the uppermost and largest part of the hip bone, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.
The ilium of the human is divisible into two parts, the body and the wing; 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.
|Ilium of pelvis|
Overview of Ilium as largest region of the pelvis.
Capsule of hip-joint (distended). Posterior aspect. (Ilium labeled at top.)
|Anatomical terms of bone|
The body (Latin: corpus) forms less than two-fifths of the acetabulum; and also forms part of the acetabular fossa. The internal surface of the body is part of the wall of the lesser pelvis and gives origin to some fibers of the obturator internus.
In humans, biiliac width is an anatomical term referring to the widest measure of the pelvis between the outer edges of the upper iliac bones.
Biiliac width has the following common synonyms: pelvic bone width, biiliac breadth, intercristal breadth/width, bi-iliac breadth/width and biiliocristal breadth/width.
It is best measured by anthropometric calipers (an anthropometer designed for such measurement is called a pelvimeter). Attempting to measure biiliac width with a tape measure along a curved surface is inaccurate.
The biiliac width measure is helpful in obstetrics because a pelvis that is significantly too small or too large can have complications. For example, a large baby or a small pelvis often lead to death unless a caesarean section is performed.
The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ilium. In both saurischians and ornithischians, the ilium extends laterally to both sides from the axis of the body. The other two hip bones, the ischium and the pubis, extend ventrally down from the ilium towards the belly of the animal. The acetabulum, which can be thought of as a "hip-socket", is an opening on each side of the pelvic girdle formed where the ischium, ilium, and pubis all meet, and into which the head of the femur inserts. The orientation and position of the acetabulum is one of the main morphological traits that caused dinosaurs to walk in an upright posture with their legs directly underneath their bodies. The brevis fossa is a deep groove in the underside of the postacetabular process, the rear part of the ilium. The brevis shelf is the bony ridge at the inner side of the fossa, the bone wall forming the internal face of the rear part of the ilium, which functions as an attachment area for a tail muscle, the musculus caudofemoralis brevis. Often, close to the hip-socket the lower edge of the outer face of the postacetabular process is positioned higher than the edge of the brevis shelf, exposing the latter in side view.
The 'English' name ilium as bone of the pelvis can be traced back to the writings of anatomists Andreas Vesalius, whom coined the expression os ilium. In this expression ilium can be considered as the genitive plural of the nominative singular of the noun ile. Ile in classical Latin can refer to the flank of the body, or to the groin, or the part of the abdomen from the lowest ribs to the pubes. Ile is usually encountered as plural (ilia) in classical Latin. The os ilium can literally be translated as bone (Latin: os ) of the flanks.
More than a millennium earlier the ossa ilium were described by the Greek physician Galen, and referred to as, with a quite similar expression, τά πλατέα λαγόνων ὀστᾶ, the flat bones of the flanks, with λαγών for flank. In anatomic Latin, the expression os lagonicum can also be found, based on Ancient Greek λαγών. In modern Greek the nominalized adjective λαγόνιο is used to refer to the os ilium.
In Latin and Greek it is not uncommon to nominalize adjectives, e.g. stimulantia from remedia stimulantia or ὁ ἐγκέφαλος from ὁ ἐγκέφαλος μυελός. The name ilium as used in English can not be considered as nominalized adjective derived from the full Latin expression os ilium, as ilium in this expression is a genitive plural of a noun and not a nominative singular of an adjective. The form ilium in English is however thought to be derived from the Latin word ilium, an orthographic variant in Latin of ile, flank or groin. Whereas the expression of Andreas Vesalius os ilium appropriately expresses bone of the flanks, the sole term ilium as used in English, lacks this precision and has to be literally translated as groin or flank.
There exists however in classical Latin an adjective ilius/ilia/ilium. This adjective however means not with respect to the flanks, but Trojan. Troy is referred to in classical Latin as Ilium, Ilion or Ilios and in ancient Greek as Ἴλιον or Ἴλιος.
The first editions of the official Latin nomenclature, Nomina Anatomica of the first 80 years (first in 1895) used the Vesalian expression os ilium. In the subsequent editions from 1983 and 1989 the expression os ilium was altered to os ilii. This latter expression supposes a genitive singular of the alternate noun ilium instead of a genitive plural of the noun ile. Quite inconsistently, in the 1983 edition of the Nomina Anatomica the genitive plural of ile (instead of ilium) is still being used in such expressions as vena circumflexa ilium superficialis. In the current 1998 edition of the Nomina Anatomica, rebaptized as Terminologia Anatomica, the expression os ilium is reintroduced and os ilii deleted.
The anterior gluteal line (middle curved line) refers to bony line on the hip bone. It is the longest of the three gluteal lines, begins at the crest, about 4 cm. behind its anterior extremity, and, taking a curved direction downward and backward, ends at the upper part of the greater sciatic notch.
The space between the anterior and posterior gluteal lines and the crest is concave, and gives origin to the gluteus medius muscle.
Near the middle of this line a nutrient foramen is often seen.Anterior inferior iliac spine
The anterior inferior iliac spine (abbreviated: AIIS) is a bony eminence on the anterior border of the hip bone, or, more precisely, the wing of the ilium (i.e. the upper lateral parts of the pelvis).Anterior superior iliac spine
The anterior superior iliac spine (abbreviated: ASIS) is a bony projection of the iliac bone and an important landmark of surface anatomy.
It refers to the anterior extremity of the iliac crest of the pelvis, which provides attachment for the inguinal ligament, and the sartorius muscle. The tensor fasciae latae muscle attaches about 5 cm away at the iliac tubercle.
The anterior superior iliac spine provides a clue in identifying some other clinical landmarks, including:
True leg length (see unequal leg length)Arcuate line of ilium
The arcuate line of the ilium is a smooth rounded border on the internal surface of the ilium. It is immediately inferior to the iliac fossa and Iliacus muscle.
It forms part of the border of the pelvic inlet.
In combination with the pectineal line, it comprises the iliopectineal line.The arcuate line marks the border between the body (corpus) and the wing (ala) of the ilium, and, running inferior, anterior, and medial from the auricular surface to the area corresponding to the acetabulum, it also indicates where weight is transferred from the sacroiliac joint to the hip joint.Iliac
Iliac can refer to one of the following:
Iliac fasciaIliac crest
The crest of the ilium (or iliac crest) is the superior border of the wing of ilium and the superiolateral margin of the greater pelvis.Iliac fossa
The iliac fossa is a large, smooth, concave surface on the internal surface of the ilium (part of the 3 fused bones making the hip bone). The fossa is bounded above by the iliac crest, and below by the arcuate line; in front and behind, by the anterior and posterior borders of the ilium.
The fossa gives origin to the Iliacus muscle and is perforated at its inner part by a nutrient canal; below this there is a smooth, rounded border, the arcuate line, which runs anterior, inferior, and medial.
When the "left" or "right" adjective is used (e.g "right iliac fossa"), the iliac fossa usually means one of the nine regions of the abdomen.Iliac spine
The ilium is a bone of the pelvic girdle with four bony projections, each serving as attachment points for muscles and ligaments:
Anterior superior iliac spine
Anterior inferior iliac spine
Posterior superior iliac spine
Posterior inferior iliac spineIliac tubercle
The iliac tubercle is located approximately 5 cm (2 in) posterior to the anterior superior iliac spine on the iliac crest in humans. The transverse plane that includes each of the tubercles (one from the left iliac tubercle and one from the right iliac tubercle) is called the transtubercular plane. The origin of the iliotibial tract is the iliac tubercle. The iliac tubercle is also the widest point of the iliac crest, and lies at the level of the L5 spinous process.Iliac tuberosity
Behind the iliac fossa is a rough surface, divided into two portions, an anterior and a posterior. The posterior portion, known as the iliac tuberosity, is elevated and rough, for the attachment of the posterior sacroiliac ligaments and for the origins of the Sacrospinalis and Multifidus.Inferior gluteal line
The inferior gluteal line (inferior curved line), the least distinct of the three gluteal lines, begins in front at the notch on the anterior border, and, curving backward and downward, ends near the middle of the greater sciatic notch.Oyster (fowl)
Oysters are two small, round pieces of dark meat on the back of poultry near the thigh, in the hollow on the dorsal side of the ilium bone. Some regard the "oyster meat" to be the most flavorful and tender part of the bird, while others dislike the taste and texture.
Compared to dark meat found in other parts of the bird, the oyster meat has a somewhat firm/taut texture which gives it a distinct mouth feel. It is also customary for the cook to be given first preference to the oyster meat.
In French, this part of the bird is called sot-l'y-laisse which translates, roughly, to "the fool leaves it there", as unskilled carvers sometimes accidentally leave it on the skeleton.Posterior gluteal line
The posterior gluteal line (superior curved line), the shortest of the three gluteal lines, begins at the iliac crest, about 5 cm in front of its posterior extremity; it is at first distinctly marked, but as it passes downward to the upper part of the greater sciatic notch, where it ends, it becomes less distinct, and is often altogether lost.
Behind this line is a narrow semilunar surface, the upper part of which is rough and gives origin to a portion of the Gluteus maximus; the lower part is smooth and has no muscular fibers attached to it.Posterior inferior iliac spine
The posterior inferior iliac spine is an anatomical landmark that describes a bony "spine", or projection, at the posterior and inferior surface of the iliac bone.
It is one of two such spines on the posterior surface, the other being the posterior superior iliac spine. These two spines are separated by a bony notch. They appear as two dimples in the skin, at the level of the lower back.
The posterior inferior iliac spine corresponds with the posterior extremity of the auricular surface.Posterior superior iliac spine
The posterior border of the ala, shorter than the anterior, also presents two projections separated by a notch, the posterior superior iliac spine and the posterior inferior iliac spine. The posterior superior iliac spine serves for the attachment of the oblique portion of the posterior sacroiliac ligaments and the multifidus.Risser sign
The Risser sign is an indirect measure of skeletal maturity, whereby the degree of ossification of the iliac apophysis by x-ray evaluation is used to judge overall skeletal development. Mineralization of the iliac apophyses begins at the anterolateral crest and progresses medially towards the spine. Fusion of the calcified apophyses to the ilium then progresses in opposite direction, from medial-to-lateral.A typical five-point grading scale is as follows:
Grade 1 is given when the ilium (bone) is calcified at a level of 25%; it corresponds to prepuberty or early puberty.
Grade 2 is given when the ilium (bone) is calcified at a level of 50%; it corresponds to the stage before or during growth spurt.
Grade 3 is given when the ilium (bone) is calcified at a level of 75%; it corresponds to the slowing of growth.
Grade 4 is given when the ilium (bone) is calcified at a level of 100%; it corresponds to an almost cessation of growth.
Grade 5 is given when the ilium (bone) is calcified at a level of 100% and the iliac apophysis is fused to iliac crest; it corresponds to the end of growth.Risser grading is traditionally used to estimating the future growth potential of the adolescent spine, particularly in the setting of spinal scoliosis. Risser originally recognized that ossification of the iliac apophyses approximately parallels the ossification of the vertebral apophyses. The earlier the stage of growth, the greater the likelihood of a scoliosis progressing and potentially needing intervention. Note that although Risser first described his findings during a 1948 lecture and published the eponymous paper in 1958, formalized staging systems were developed at a later time.Wing of ilium
The wing of ilium (or ala) is the large expanded portion which bounds the greater pelvis laterally. It presents for examination two surfaces—an external and an internal—a crest, and two borders—an anterior and a posterior.