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Hip bone
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Hip bone
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Hip bone
Position of the hip bones (shown in red)
Details
Identifiers
Latinos coxae, os innominatum
MeSHD010384
TA98A02.5.01.001
TA21307
FMA16580 16585, 16580
Anatomical terms of bone

The hip bone (os coxae, innominate bone, pelvic bone[1][2] 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 parts: the ilium, ischium, and the pubis.

The two hip bones join at the pubic symphysis and together with the sacrum and coccyx (the pelvic part of the spine) comprise the skeletal component of the pelvis – the pelvic girdle which surrounds the pelvic cavity. 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 (thigh bone) (forming the primary connection between the bones of the lower limb and the axial skeleton) through the large ball and socket joint of the hip.[3]

Structure

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The skeleton of the human pelvis:
2–4. Hip bone (os coxae)
1. Sacrum (os sacrum), 2. Ilium (os ilium), 3. Ischium (os ischii)
4. Pubic bone (os pubis) (4a. corpus, 4b. ramus superior, 4c. ramus inferior, 4d. tuberculum pubicum)
5. Pubic symphysis, 6. Acetabulum (of the hip joint), 7. Obturator foramen, 8. Coccyx/tailbone (os coccygis)
Dotted. Linea terminalis of the pelvic brim.

The hip bone is formed by three parts: the ilium, ischium, and pubis. At birth, these three components 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 regions will have fused together, and by the age 25 they will have ossified. The two hip bones join each other at the pubic symphysis. Together with the sacrum and coccyx, the hip bones form the pelvis.[3]

Ilium

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Main article: Ilium (bone)

Ilium (plural ilia) is the uppermost and largest region. It makes up two fifths 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 shows clear marks of the attachment of the three abdominal wall muscles.[3]

Ischium

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Main article: Ischium
3D model of human hip bone

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: 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 colloquially 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.[3]

Pubis

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Main article: Pubis (bone)

The pubic region or pubis is the ventral and anterior of the three parts 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 unites with the other pubic bone in the pubic symphysis.[3] The fibrocartilaginous pad which lies between the symphysial surfaces of the coxal bones, that secures the pubic symphysis, is called the interpubic disc.

Pelvic brim

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The pelvic brim is a continuous oval ridge of bone that runs along the pubic symphysis, pubic crests, arcuate lines, sacral alae, and sacral promontory.[4]

False pelvis, pelvic inlet, and ramus

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The false 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]

The true pelvis is the region inferior to the pelvic brim that is almost entirely surrounded by bone.[4]

The pelvic 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] The pelvic 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]

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.

Development and sexual dimorphism

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Plan of ossification of the hip bone. Left hip bone, external surface.

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.

The male pelvis, formed by left and right hip bones, sacrum, and coccyx.
The female pelvis is wider than the male pelvis to accommodate childbirth.

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 during childbirth.

Muscle attachments

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Several muscles attach 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.

Abdominal muscles

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Back muscles

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Gluteal muscles

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Lateral rotator group

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Hamstrings

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  • 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;[5]
  • 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

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  • 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

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Clinical significance

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Fractures

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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 fractures[6] that occur in the proximal end of the femur.

Preparation for childbirth

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Pelvimetry is the assessment of the female pelvis[7] in relation to the birth of a baby in order to detect an increased risk for obstructed labor.

Evolution of the pelvis in animals

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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.[8]

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.[8]

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.[8]

In therapsids, 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 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 placental mammals, the pelvis as a whole is generally wider in females than in males, to allow for the birth of the young.[8]

The pelvic bones of cetaceans were formerly considered to be vestigial, but they are now known to play a role in sexual selection.[9]

Additional images

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  • Position of the hip bones (shown in red). Animation.
    Position of the hip bones (shown in red). Animation.
  • Right hip bone. Animation.
    Right hip bone. Animation.
  • Right hip bone. External surface.
    Right hip bone. External surface.
  • Right hip bone. Internal surface.
    Right hip bone. Internal surface.
  • Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis.
    Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis.
  • Hip bone.Medial view.
    Hip bone.Medial view.
  • Hip bone. Lateral view.
    Hip bone. Lateral view.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hip bone

View on Grokipedia
from Grokipedia
The hip bone, also known as the os coxae or innominate bone, is a large, flat, irregular bone that forms the lateral portion of the pelvic girdle, connecting the axial skeleton to the lower limbs and providing structural support for the upper body. It develops from the fusion of three primary ossification centers—the ilium, ischium, and pubis—which are separate in childhood and typically unite by late adolescence around ages 15 to 25 via the triradiate cartilage at the acetabulum. The ilium constitutes the broad, superior, fan-shaped portion, featuring the iliac crest for muscle attachments and forming the roof of the acetabulum; the ischium forms the robust, posteroinferior part with the ischial tuberosity for weight-bearing during sitting; and the pubis comprises the anterior body and rami, contributing to the pubic symphysis and obturator foramen. Together, these components create the deep, cup-like acetabulum, a socket that articulates with the femoral head to form the ball-and-socket hip joint, enabling multiaxial movement including flexion, extension, abduction, adduction, and rotation while transmitting weight and forces during locomotion. The hip bone also serves as an attachment site for key ligaments like the iliofemoral (the body's strongest), pubofemoral, and ischiofemoral, as well as major muscles such as the gluteals and iliopsoas, and it protects pelvic organs while facilitating the passage of neurovascular structures through the greater and lesser sciatic foramina (formed by the corresponding notches and ligaments). In adults, the two hip bones articulate anteriorly at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints, forming a stable yet mobile ring that is essential for upright posture, gait, and overall locomotion.

Anatomy

Overview

The hip bone, also known as the os coxae or innominate bone, is a large, irregularly shaped that forms the lateral portion of the pelvic girdle in humans. It connects the (via the ) to the (via the ), providing structural support for weight transfer from the trunk to the lower limbs during locomotion. Each hip bone is composed of three primary ossified elements—the ilium, , and pubis—that fuse together during development to create a single robust unit, essential for stability and mobility. The ilium constitutes the superior, expanded portion of the hip bone, resembling a wing-like that broadens the pelvic structure laterally. It features the prominent superiorly, which serves as an attachment site for abdominal and back muscles, and the auricular surface posteriorly for articulation with the at the . Inferiorly, the ilium contributes to the , a deep, cup-shaped socket that accommodates the head of the to form the hip joint. The ischium forms the posterior-inferior aspect, including the robust for weight-bearing when seated and the , which allows passage of major nerves and vessels to the lower limb. The pubis, the anterior-inferior component, consists of a body and superior/inferior rami; the bodies of the left and right pubes meet at the , a , while the rami border the —a large opening transmitting the and vessels. These three bones develop separate ossification centers in utero but remain distinct until they begin fusing at the triradiate cartilage during puberty, with complete fusion typically occurring by ages 16-18. The , formed by contributions from all three bones, is deepened by the and reinforced by ligaments such as the iliofemoral, pubofemoral, and ischiofemoral, ensuring a secure ball-and-socket articulation. The , bounded by the pubis and , is largely covered by the obturator membrane, except for small nutrient foramina. Overall, the hip bone's architecture balances strength for load-bearing with flexibility for bipedal gait, while also protecting pelvic viscera.

Ilium

The ilium is the largest and most superior of the three bones that fuse to form the os coxae, or hip bone, in adults. It has a broad, fan-shaped structure consisting of a superior ala () and an inferior body, which contributes to the formation of the , the socket of the hip joint. The ilium's ala flares laterally and superiorly, providing a wide base for weight transmission from the trunk to the lower limbs, while its body tapers to articulate with the and pubis. This configuration supports the pelvis's role in locomotion and stability. Key features of the ilium include the , a curved superior border that forms the iliac tuberosity medially and extends approximately 5 cm from the (ASIS) to the tubercle of the crest. The ASIS is a prominent anterior projection at the end of the iliac crest, serving as an attachment for the and , while the (PSIS) marks the posterior end, often visible as a skin dimple overlying the S2 vertebral level. Inferiorly, the (AIIS) and posterior inferior iliac spine (PIIS) provide attachments for muscles like the rectus femoris and posterior sacroiliac ligaments, respectively. The medial surface features the concave , which houses the , and the rough auricular surface for articulation with the . Laterally, the gluteal surface displays ridges for gluteal muscle attachments, and the posterior border includes the large , which transmits major neurovascular structures like the . The , a ridge on the medial aspect, forms part of the and separates the false above from the true pelvis below. The ilium articulates posteriorly with the at the via its auricular surface, stabilized by strong anterior and posterior sacroiliac ligaments that limit excessive motion and prevent hyperextension. Inferiorly, its body fuses with the and pubis to form the , while the pubis articulates anteriorly with the contralateral pubis at the , which receives the for the ball-and-socket hip joint. These articulations enable the ilium to transfer compressive forces from the spine through the to the lower extremities during activities. Functionally, the ilium serves as a key structural element in supporting upright posture and bipedal locomotion by bearing the weight of the upper body and distributing it to the femurs. It provides extensive attachment sites for muscles, including the iliacus and psoas major on the for hip flexion, gluteal muscles on the lateral surface for hip extension and abduction, and abdominal muscles like the transversus abdominis on the for core stability. Ligaments such as the anchor the ilium to the , enhancing pelvic girdle integrity. The ilium also protects underlying pelvic organs and contributes to the pelvis's basin-like shape for visceral support. Clinically, the ilium's landmarks are vital for diagnostics and procedures. The iliac crest aligns with the L4 vertebral level, guiding safe sites for lumbar punctures or bone marrow biopsies, while the ASIS is used to assess leg length discrepancies by measuring from it to the medial malleolus. PSIS dimples help locate the sacroiliac joint for evaluating sacroiliitis. Fractures of the iliac wing, often from high-impact trauma, can lead to significant hemorrhage (up to 2 liters of blood loss) and are classified using systems like Young-Burgess for stability assessment; stable lateral compression type I fractures involve the iliac wing and sacral ala but maintain pelvic ring integrity.

Ischium

The ischium is the posteroinferior portion of the , or os coxae, which fuses with the ilium superiorly and the pubis anteriorly to form the . It consists primarily of a body and a ramus, with the body contributing to the posterior aspect of the and the ramus extending anteromedially. This V-shaped supports the body during sitting and forms key boundaries of the . The body of the ischium is the thicker, upper portion that articulates with the ilium and pubis at the , where it provides approximately the posterior two-fifths of the socket for the . It features a posterior surface continuous with the gluteal surface of the ilium and an anterior surface that faces the . Projecting from the body's inferior end is the , a robust, roughened prominence that serves as the primary weight-bearing structure when seated, often palpable with the thigh flexed. Superior to the tuberosity lies the , a sharp projection along the posterior border that marks the inferior limit of the . The ramus of the arises from the inferior aspect of the body and tuberosity, extending laterally to fuse with the inferior ramus of the pubis, thereby completing the border of the . Its surfaces include a lateral aspect facing the and a medial aspect directed toward the and , contributing to the boundaries of the ischiorectal fossa. The ramus is relatively flat and broad, with its medial border forming part of the subpubic angle in the . Key features of the ischium include the greater and lesser sciatic notches on its posterior border. The , located between the and the posterior inferior iliac spine, is a wide concavity that accommodates the passage of major neurovascular structures to the lower limb. The , situated inferior to the and superior to the tuberosity, is smaller and bridges the sacrospinous ligament to form the lesser sciatic foramen. Additionally, the often features a that can become inflamed, though this is a structural adaptation for weight distribution rather than a primary anatomical variant. In terms of articulations, the integrates seamlessly with the ilium and pubis through cartilaginous fusion during development, culminating in a single os coxae by early adulthood, while its acetabular contribution articulates directly with the head of the . The also indirectly relates to the via the formed by the ilium, influencing pelvic stability. These structural elements underscore the 's role in load transmission from the trunk to the lower limbs.

Pubis

The pubis, also known as the pubic bone, forms the anterior and inferior portion of the hip bone (os coxae), one of the three primary components alongside the ilium and . It is an angulated structure that contributes to the stability of the pelvic girdle by articulating with the contralateral pubis at the midline and fusing with the other hip bone elements at the . The pubis plays a key role in supporting the and facilitating the passage of neurovascular structures through the . The pubis consists of three main parts: the body, superior ramus, and inferior ramus. The body represents the superolateral portion of the pubis and forms approximately one-fifth of the acetabulum, the deep socket for the femoral head; its medial surface is covered with hyaline cartilage and participates in the fibrocartilaginous pubic symphysis joint, which includes an interpubic disc for slight mobility. The superior ramus extends laterally and superiorly from the body toward the acetabulum, forming part of the pelvic brim and the superior boundary of the obturator foramen; it features a prominent ridge known as the pecten pubis (or pectineal line) on its superior surface, which serves as the attachment site for the pectineal ligament and marks the linea terminalis of the pelvis. The inferior ramus projects inferolaterally from the body, fusing with the ramus of the ischium to complete the inferior border of the obturator foramen and contribute to the subpubic angle of the pelvic outlet. Key surface features of the pubis include the pubic crest, a ridge along the superior border of the superior ramus that provides attachment for the rectus abdominis and pyramidalis muscles, ending laterally at the —a small, forward-projecting process that anchors the medial end of the . On the inferior aspect of the superior ramus lies the obturator groove, which transmits the obturator vessels and nerve after being bridged by the obturator membrane. These features collectively support muscle attachments and ligamentous reinforcements essential for pelvic integrity and lower limb movement. In terms of articulations, the pubis connects anteriorly to its counterpart via the , a secondary reinforced by the superior and arcuate pubic ligaments, allowing limited movement during activities like walking or . Posteriorly and laterally, the pubis integrates with the ilium and at the and triradiate (in youth), forming a cohesive hip bone that articulates with the at the hip joint. The , bounded by the rami of the pubis and , is the largest opening in the hip bone and is mostly closed by the obturator membrane, except for the obturator canal.

Acetabulum and pelvic features

The is a deep, cup-shaped cavity located on the lateral aspect of the bone (os coxae), serving as the socket for the head of the to form the ball-and-socket . It is formed by the confluence of the three primary components of the bone: the ilium contributes the superior two-fifths, the the posterior two-fifths, and the pubis the anterior one-fifth, with these elements fusing at the by early adulthood around age 16-18. The opens laterally, inferiorly, and anteriorly, providing stability through its deep structure while allowing multiaxial movement. Key structural features of the include the lunate surface, a horseshoe-shaped articular area covered in that contacts the , and the central acetabular fossa, a non-articular roughened depression containing fibroelastic fat and the ligamentum teres. The , a ring of attached to the acetabular rim, deepens the socket by approximately 22% in surface area and 33% in volume, enhancing joint congruence and load distribution. Inferiorly, the features a U-shaped acetabular notch, bridged by the transverse acetabular ligament, which allows passage of the and contributes to the attachment of the ligamentum teres from the fovea capitis of the . The is thickest in the anterosuperior region, supporting forces. Beyond the , the hip bone exhibits several pelvic features that contribute to the overall structure and function of the bony . The , a large oval opening in the anteroinferior hip bone, is bounded by the pubis superiorly and inferiorly, serving as a passageway for vessels and nerves while reducing pelvic weight. The , a wide indentation on the posterior ilium and , is converted into the greater sciatic foramen by the , allowing exit of the and other structures from the . Adjacent to it, the smaller on the forms the lesser sciatic foramen with the , accommodating the tendon of the . Additional pelvic landmarks include the auricular surface on the medial ilium, an ear-shaped roughened area that articulates with the at the , providing stability through irregular interlocking and ligamentous support. The arcuate line of the ilium marks the boundary between the false and true , contributing to the . Anteriorly, the forms the midline articulation between the pubic bodies of the two bones, connected by and ligaments. These features collectively define the , outlet, and canal, influencing pelvic dimensions and supporting visceral organs.

Development and Variation

Ossification process

The ossification of the hip bone, also known as the os coxae or innominate bone, primarily occurs through , beginning with three separate primary centers for the ilium, ischium, and pubis during fetal development. These centers form within cartilaginous models derived from the and somites, enabling the initial bony framework of the . Secondary ossification centers subsequently appear postnatally, primarily at apophyses and the acetabular rim, contributing to the bone's final shape and attachment sites for muscles and ligaments. The process culminates in the fusion of these components, typically completing in early adulthood, with variations influenced by sex—females generally exhibiting earlier timelines than males. The primary ossification centers emerge in utero as follows: the ilium at approximately 8 weeks of gestation (or the 56th embryonic day), the ischium at 4 to 6 months of gestation (around the 105th embryonic day), and the pubis at 4 to 5 months of gestation. Initially, these bones remain distinct, connected by hyaline cartilage, including the key triradiate cartilage at the acetabulum, which facilitates growth and eventual integration of the ilium, ischium, and pubis. Fusion begins with the and pubis uniting at the between 4 and 8 years of age. The ilium then fuses to this composite structure around the at 7 to 9 years overall, though more precisely between 11 and 15 years in females and 14 and 17 years in males. Complete closure of the triradiate cartilage, marking full unification of the hip bone, occurs between 15 and 17 years initially and fully by 20 to 25 years. Secondary centers, totaling five in the hip bone, develop at sites of muscular stress and ligamentous attachment, ossifying via endochondral mechanisms and fusing progressively. These include centers for the , anterior superior and inferior iliac spines (on the ilium), the (on the ), and the os acetabuli (on the pubis, contributing to the acetabular rim). Additionally, two centers appear specifically in the at to deepen the socket. The timings for these centers are summarized in the table below:
Bone/PartSecondary CenterAppearance AgeFusion Age
Ilium (iliac crest)Iliac crest apophysis13–15 years15–17 years
Ilium15–17 years19–25 years
Ilium13–15 years16 years
IschiumIschial tuberosity (apophysis)13–16 years20–21 years
Pubis/AcetabulumOs acetabuli9–12 yearsPuberty (12–15 years)
AcetabulumAcetabular rim centers (two)Puberty (11–14 years)20–25 years
These secondary centers enhance the hip bone's structural integrity for weight-bearing and locomotion, with fusion delays potentially indicating developmental disorders if beyond typical ranges.

Sexual dimorphism

The human hip bone, or os coxae, exhibits pronounced , reflecting adaptations to locomotion, reproduction, and body size differences between males and females. This dimorphism is most evident in the pelvis as a whole, where the female structure accommodates childbirth while maintaining bipedal efficiency, whereas the male pelvis prioritizes mechanical strength for . Key differences arise during under the influence of hormones, with females developing a broader, more gynecoid pelvis and males a narrower, android form. In females, the hip bone features a wider subpubic (approximately 90–120°), broader greater sciatic notches, and a more oval-shaped , facilitating a larger and outlet for parturition. The ilium is shorter and flares laterally, contributing to a broader bispinous width, while the pubis is relatively longer and the shorter, resulting in a shallower relative to overall size. These traits create a rounded, dorsoventrally flattened pelvic with less prominent ischial spines. In contrast, males have a narrower subpubic (around 70–90°), deeper and narrower greater sciatic notches, and a triangular , with a taller ilium, more inwardly rotated iliac blades, and a larger, deeper suited to a bigger . The male pelvis is generally heavier and thicker, with a more oval and greater overall robustness. These morphological distinctions are not solely allometric (scaling with body size), as stature explains only about 7% of shape variation; instead, most dimorphism (over 90%) is non-allometric, driven by obstetric demands and hormonal influences during . For instance, pubic length continues to elongate into early adulthood, enhancing pelvic capacity, while growth emphasizes vertical dimensions. Population-level variations exist, with larger-bodied groups showing more pronounced dimorphism (e.g., up to 100% sex classification accuracy via metrics like coxal height and acetabular diameter), influenced by and environment, but core features remain consistent across modern humans. In , these traits enable reliable sex estimation from hip bones alone, with metric analyses of dimensions like pubis length and ischial tuberosity breadth achieving 91–100% accuracy in diverse samples. The hip bone, or os coxae, undergoes continuous remodeling after its full ossification in early adulthood, typically around 20–25 years of age, leading to subtle morphological adaptations throughout adulthood. One prominent change is the gradual widening of the pelvis, observed in both sexes, with the iliac wings expanding at approximately 0.371 mm per year and the intertrochanteric region at 0.333 mm per year, resulting in over 20 mm of total increase between ages 20 and 80. This periosteal apposition and endocortical resorption contribute to a broader pelvic inlet and outlet, potentially altering gait and balance in older individuals, though the pelvic inlet diameter itself remains relatively stable. In women, pelvic widening peaks around age 40 and then reverses post-menopause, with the pelvis narrowing due to declining estrogen levels, which influence bone remodeling. Bone mineral density (BMD) in the os coxae declines progressively with age, particularly after 50 years, as resorption outpaces formation, increasing susceptibility to and fractures. Women experience a more rapid loss, with up to 15–31% reduction in BMD and cortical thickness in the proximal regions by age 85 compared to younger adults, alongside a 35% increase in ratio—a measure of structural fragility. Men show similar but less pronounced changes, with about 10% BMD loss and 22% higher ratio in advanced age. These alterations manifest as cortical thinning and increased , especially in the ilium, where cortical width decreases significantly while rises due to age-related remodeling imbalances. Overall, the proximal femur's structural , integral to hip bone stability, exhibits expanded outer diameter (up to 6% greater) but reduced in women over 85, heightening fracture risk. Specific components of the os coxae also display targeted age-related modifications. The undergoes degenerative remodeling starting around age 35–40, with surface erosion, lipping, and porosity increasing progressively, enabling forensic age estimation across the adult lifespan. The auricular surface of the ilium similarly shows granular changes, billowing, and ligamentous attachments that evolve from smooth to irregular with advancing age, reflecting degeneration. The may exhibit subchondral bone sclerosis and cyst formation as part of progression, though these are more joint-associated than purely osseous. These changes collectively reduce the hip bone's load-bearing capacity, contributing to higher incidences of pelvic fractures in the elderly, particularly in osteoporotic individuals. Recent research as of 2025 indicates that female pelvic dimensions have narrowed over the last 150 years across populations, possibly due to changes in , activity, or , with implications for obstetric outcomes.

Functions and Attachments

Articulations and ligaments

The hip bone, or os coxae, forms three primary articulations that connect it to the and lower limb. The is a between the auricular surface of the ilium and the corresponding surface of the , providing stability for weight transfer from the spine to the while allowing limited motion. The is a secondary uniting the anterior surfaces of the two pubic bones via a fibrocartilaginous disc, permitting slight movement to accommodate pelvic flexibility, particularly during activities like . The hip , or coxal joint, is a multiaxial ball-and-socket formed by the of the hip bone and the head of the , enabling a wide range of movements including flexion, extension, abduction, adduction, and rotation. Ligaments associated with these articulations reinforce stability and limit excessive motion. At the , the anterior sacroiliac ligament is a thin band on the pelvic surface extending from the ilium to the , while the thicker posterior sacroiliac ligament includes the deep interosseous sacroiliac —a very strong —connecting the sacral and iliac tuberosities, along with short and long posterior components that prevent anterior shear. Accessory ligaments include the , a triangular band from the and ilium to the that resists forward tilting of the , and the sacrospinous ligament, a thin triangular structure from the to the that separates the greater and lesser sciatic foramina. The further supports this region by connecting the fifth lumbar vertebra to the , enhancing lumbopelvic stability. The pubic symphysis is stabilized by four ligaments forming a fibrous sheath around the interpubic disc. The superior pubic ligament spans the pubic crests, the anterior pubic ligament covers the front of the joint, the posterior pubic ligament reinforces the back, and the inferior (arcuate) pubic ligament arches over the inferior aspect, collectively providing tensile strength and limiting separation. For the hip joint, the fibrous capsule is reinforced by three extracapsular ligaments. The iliofemoral ligament, the strongest of the lower limb, extends in a Y-shape from the anterior inferior iliac spine to the intertrochanteric line of the femur, preventing hyperextension. The pubofemoral ligament blends with the capsule inferiorly, attaching from the superior pubic ramus to the intertrochanteric line to restrict abduction and extension. The ischiofemoral ligament spirals posteriorly from the ischium to the greater trochanter, limiting internal rotation and extension while securing the femoral head. Intracapsular structures include the ligament of the head of the femur (ligamentum teres), which attaches the acetabular notch to the fovea capitis of the femur and conveys the artery to the femoral head, and the transverse acetabular ligament, which bridges the acetabular notch to complete the socket's circumference. The acetabular labrum, a fibrocartilaginous ring, deepens the acetabulum for enhanced joint congruence.

Muscle attachments

The hip bone provides attachment sites for numerous muscles that facilitate movement of the lower limb, stabilize the , and support functions. These attachments are distributed across the ilium, , and pubis, with the ilium serving as a primary origin for gluteal and trunk muscles, the for posterior thigh muscles, and the pubis for medial thigh adductors and structures. On the ilium, the on the internal surface gives origin to the , which combines with the psoas major to form the complex responsible for hip flexion. The (ASIS) serves as the origin for the and the straight head of the rectus femoris, both key extensors and flexors of the and . The (AIIS) attaches the reflected head of the rectus femoris. The external gluteal surface features ridges for the and minimus muscles, which originate along the anterior, posterior, and inferior gluteal lines and insert on the of the to abduct and medially rotate the ; the originates from the posterior ilium, , and , inserting on the and for hip extension. The provides broad attachments for trunk muscles, including the external oblique and internal oblique aponeuroses anteriorly, latissimus dorsi and erector spinae posteriorly, and the tensor fasciae latae at its outer lip. The ischium hosts origins for powerful posterior lower limb muscles, particularly at the ischial tuberosity, which gives rise to the hamstring group: the long head of the biceps femoris, semitendinosus, and semimembranosus, all inserting distally on the tibia and fibula to extend the hip and flex the knee. The adductor magnus (hamstring portion) also originates here, inserting on the adductor tubercle of the femur for hip extension and adduction. The gluteus maximus partially attaches to the ischial tuberosity, enhancing its role in hip extension. The ischial spine provides origin for the gemellus superior and coccygeus muscles, with the former aiding external rotation of the hip via the obturator internus tendon. The body of the ischium attaches the quadratus femoris (for hip lateral rotation) and contributes to the obturator internus origin, which exits the pelvis through the lesser sciatic foramen. Pelvic floor muscles such as the levator ani (including pubococcygeus) originate from the ischial spine and body, supporting visceral structures. Attachments to the pubis primarily involve medial muscles and components. The pubic body and crest serve as origins for the adductor longus, adductor brevis, and gracilis, which insert on the and medial , respectively, to adduct the and flex the . The adductor magnus (adductor portion) and obturator externus originate from the and pubic body, aiding adduction and external . The pectineal line on the superior ramus attaches the , which flexes and adducts the . The superior pubic ramus gives origin to the (puborectalis and pubococcygeus portions) and the , both contributing to pelvic support and stabilization. The pubic crest also anchors the rectus abdominis and pyramidalis for abdominal compression.

Clinical Significance

Fractures and injuries

The hip bone, or os coxae, forms a critical component of the alongside the and , and fractures involving this structure are collectively termed pelvic fractures. These injuries disrupt the stability of the , which supports weight-bearing and protects vital organs, often resulting from high-energy trauma such as accidents or falls from height. Low-energy mechanisms, including falls in older adults with , can also cause fractures, particularly in the pubic rami or iliac wing of the hip bone. Avulsion fractures, where muscle contractions pull off fragments, occur more commonly in adolescents during activities. Pelvic fractures are classified based on stability and mechanism to guide treatment. Stable fractures involve a single break in the pelvic ring, such as isolated fractures of the pubic or ischial rami or the iliac wing, and typically do not disrupt ligamentous integrity. Unstable fractures feature multiple breaks or disruption of the posterior sacroiliac complex, leading to potential displacement of the hemipelvis. The Young-Burgess classification further categorizes them by injury pattern: lateral compression (LC) types involve inward forces causing ramus or sacral fractures; anteroposterior compression (APC) types result from frontal impacts widening the symphysis pubis; and vertical shear injuries arise from axial loads displacing the hemipelvis upward. Acetabular fractures, specifically involving the socket formed by the ilium, , and pubis, often accompany these and require specialized assessment due to their impact on function. Symptoms of hip bone fractures include severe pain in the , , or lower back, exacerbated by movement or , along with swelling, bruising, and difficulty ambulating. In unstable cases, patients may experience numbness from nerve compression or hemodynamic instability due to hemorrhage from disrupted pelvic vasculature. begins with anteroposterior pelvic radiographs to identify fractures, followed by computed tomography (CT) scans for detailed evaluation of displacement, acetabular involvement, and associated injuries. Additional tools like focused assessment with sonography for trauma (FAST) help detect intra-abdominal bleeding. Treatment prioritizes hemodynamic stabilization, pain control, and restoration of pelvic alignment. Stable fractures of the hip bone are managed nonoperatively with , analgesics, and assistive devices like crutches, allowing healing over 8-12 weeks. Unstable injuries require urgent intervention, such as pelvic binders or to reduce volume and bleeding in the acute phase, followed by definitive including open reduction and (ORIF) with plates and screws for the os coxae components. Vertical shear or high-grade APC/LC fractures often necessitate sacroiliac screw placement or anterior plating of the pubis. Rehabilitation involves to regain mobility, with full weight-bearing typically achieved by 3 months post-injury. Complications from hip bone fractures are significant, particularly in high-energy cases, including massive blood loss (up to 10-20% of total volume), thrombosis, and at surgical sites. Long-term issues encompass , (affecting up to 56% of women and 61% of men), and reduced , with unstable fractures carrying higher risks of or in the hip joint. Prognosis varies: stable fractures heal with minimal sequelae, while unstable ones may require prolonged recovery and multidisciplinary care, though early fixation improves outcomes.

Role in childbirth and pelvic disorders

The hip bones, or ossa coxae, form the lateral and anterior portions of the , providing the structural framework for the during . In females, the broader configuration of the hip bones—characterized by a wider (approximately 13.5 cm transverse diameter) and outlet (11 cm transverse diameter)—facilitates the passage of the through the . The pubic bones of the hip bones meet at the , forming an obtuse that aids in accommodating the fetal head during descent and rotation. During late , the relaxin softens the ligaments connecting the hip bones to the and each other, allowing slight separation at the sacroiliac joints and , which can increase the circumference by 10–15% to ease delivery. This adaptability is crucial for vaginal birth, as variations in hip bone shape (such as the gynecoid ) influence fetal positioning and the likelihood of uncomplicated labor. Pelvic disorders often arise from the mechanical stresses on the bones during and . Symphysis pubis dysfunction (SPD), also known as , involves inflammation and instability at the due to excessive laxity from relaxin, affecting about 1 in 4 pregnant individuals and causing in the front and back of the that radiates to the hips. , linked to the articulation between the ilium of the hip bone and the , is exacerbated by pregnancy weight gain and hormonal changes, leading to lower back and hip that can hinder mobility during labor. Postpartum, damage to the muscles and attached to the hip bones—such as tears in the —can result in or , with studies indicating higher risks in vaginal deliveries involving prolonged pushing or use. Pre-existing conditions of the bones can complicate and contribute to disorders. Developmental dysplasia of the (DDH), where the fails to fully cover the , may distort pelvic alignment and reduce canal dimensions, increasing the risk of —a mismatch between fetal size and pelvic capacity that necessitates cesarean delivery. Pelvic fractures involving the bones, often from trauma, can lead to or instability during , potentially requiring surgical stabilization to support safe . Management of these disorders typically involves to strengthen attachments around the bones, pelvic supports, and in severe cases, interventions like symphysiodesis to fuse the temporarily.

Surgical and prosthetic considerations

Surgical management of acetabular fractures, which form a critical part of the bone, typically involves open reduction and (ORIF) for displaced injuries to restore congruity and prevent . Indications for surgery include articular displacement greater than 2 mm, roof-arc angles of 45° or less medially, 25° or less anteriorly, or 70° or less posteriorly, and persistent after closed reduction. Nonoperative treatment is reserved for nondisplaced fractures or those with an intact and congruent coverage, particularly in patients with severe comorbidities or extreme . Surgical timing is ideally within 72 hours of injury to minimize complications like heterotopic ossification, though delays up to one week may be necessary for swelling reduction. Common surgical approaches depend on fracture pattern: the Kocher-Langenbeck approach for posterior wall or column s, the ilioinguinal approach for anterior column or both-column injuries, and extensile combined approaches for complex transverse or multifragmentary s requiring access to both columns. Fixation employs 3.5-mm reconstruction plates and lag screws, with provisional stabilization using K-wires or cerclage wires; specialized implants like spring plates address comminuted posterior walls, while buttress plates support quadrilateral surface defects. Anatomic reduction is prioritized, proceeding centripetally from peripheral fragments to the articular surface, with intraoperative imaging to confirm stability. Postoperative outcomes show good to excellent results in 70-80% of cases when reduction quality is anatomic (step-off <2 mm), though complications include infection (2-5%), nerve injury (sciatic, 3-5%), and osteoarthritis (20-50% at 10 years). Prosthetic considerations in hip bone surgery center on total hip arthroplasty (THA), where the acetabulum is reamed and fitted with a press-fit metal shell (typically titanium or cobalt-chrome) at 35-40° inclination and 15-20° anteversion to optimize stability and load distribution. The acetabular component interfaces with the hip 's triradiate cartilage fusion site, formed by the ilium, ischium, and pubis, and requires medialization to a healthy bony bed; under-reaming by 1-2 mm enhances initial fixation, but excessive under-reaming (>2 mm) risks intraoperative (0.4% incidence). Bearing surfaces include metal-on-polyethylene for cost-effectiveness, ceramic-on-polyethylene for reduced in younger patients, or ceramic-on-ceramic for minimal generation, selected based on activity level and quality. In cases of acetabular deficiency from malunion or , reconstruction uses grafts or augments to restore stock, preventing loosening (aseptic loosening rate 1-2% at 10 years). Surgical approaches for THA influence acetabular exposure: the posterior approach (most common, 50-60% of cases) allows wide access but risks posterior (2-3%), while anterior or anterolateral approaches minimize abductor disruption at the expense of visibility. Complications specific to the acetabular side include periprosthetic during impaction, particularly in osteoporotic bone, and requiring reinforcement rings. Long-term success hinges on achieving primary stability and avoiding leg length discrepancy (>1 cm increases risk), with revision rates for acetabular loosening around 5-10% at 15 years in primary THA.

Evolutionary History

Human pelvis evolution

The evolution of the human pelvis reflects a complex interplay of selective pressures favoring bipedalism, obstetric demands, and thermoregulation, transforming it from the elongated, narrow structure of early hominoids to the short, broad, bowl-shaped form characteristic of modern Homo sapiens. In Miocene hominoids, such as Ekembo and Sivapithecus (approximately 17.9–9.0 million years ago), the pelvis resembled that of quadrupedal apes, with tall ilia and a mediolaterally narrow inlet suited for arboreal locomotion. The transition to bipedalism in early hominins, beginning around 6–7 million years ago, drove fundamental changes: the ilium shortened cranio-caudally and widened mediolaterally, the sacrum became more curved and shorter, and the ischium reduced in length to reposition gluteal muscles for efficient upright walking. These adaptations, evident in fossils like Ardipithecus ramidus (4.4 million years ago) with its short, broad ilium, enhanced balance and energy efficiency during terrestrial bipedality. Fossil evidence from the (4.4–0.08 million years ago) illustrates a mosaic of pelvic traits across hominin species, highlighting gradual refinements. , exemplified by the "" specimen (A.L. 288-1, ~3.2 million years ago), retained a platypelloid (flattened) birth canal with flaring ilia, balancing bipedal stability against obstetric needs for relatively small-brained infants. Later species like (e.g., KNM-WT 15000, ~1.6 million years ago) showed a taller, more human-like ilium but primitive wide ilia, while (~1.98 million years ago) displayed a mix of australopith and -like features, including a short and tall despite small brain size. By ~200,000 years ago, anatomically modern in evolved a narrower, circular birth canal with a more transverse , accommodating larger neonatal heads through fetal rotation during delivery—a configuration that persisted despite increased encephalization. Discoveries like (~250,000 years ago) and (~100,000 years ago) further reveal variability, with platelike ilia suggesting retained primitive traits in some lineages. Genetic studies underscore how developmental shifts shaped these morphological changes, particularly during embryonic weeks 6–8 when the ilium forms. Regulatory enhancements in genes like SOX9, PTH1R, RUNX2, and FOXP1/2 delayed ossification and reoriented growth plates perpendicularly, promoting a wider, shorter ilium for bipedal support while expanding the birth canal. Human-specific variants in EMX2 and DLX5 further influenced iliac breadth and parasagittal alignment, reducing variability compared to apes and enabling upright posture. These adaptations occurred in stages: transverse ilial expansion around 8–5 million years ago, growth plate fixation 5–2 million years ago, and delayed internal ossification post-2 million years ago to accommodate encephalization. Thermoregulation also played a role, as the broader pelvis in early hominins facilitated heat dissipation in open savannas. However, the "obstetric dilemma"—the tight fit between bipedal narrowing and large-brained fetuses—remains debated, with recent evidence suggesting nutritional and locomotor factors contributed to pelvic variation beyond childbirth alone.

Comparative anatomy in animals

The hip bone, or os coxae, forms part of the in vertebrates, providing attachment for the hind limbs and supporting locomotion, with variations reflecting adaptations to different environments and gaits. In basal vertebrates like , the is a free-floating cartilaginous or bony structure unattached to the , consisting of a puboischiac bar that fuses the left and right sides, often supporting a basipterygium for the pelvic fin. In amphibians, the evolves into a more robust form with iliac processes articulating with the sacral ; for instance, in anurans, the ilium elongates while the and pubis remain small, and the puboischia form a median for weight support during jumping. In reptiles, the os coxae comprises distinct ilium, , and pubis bones, with pubic and ischial symphyses providing stability; squamates like exhibit a preacetabular process on the ilium in some species, enhancing muscle leverage for terrestrial movement, while snakes often lack a functional due to limblessness. Birds display a specialized where the ilium fuses with the (fused sacral vertebrae), eliminating symphyses to accommodate egg passage and flight; the and pubis are reduced and slender, and the features a prominent anti-trochanter process in fowl, deepening the socket for weight-bearing during perching and takeoff. The in birds and higher vertebrates includes an isolated round ligament, contrasting with amphibians' unique meniscoid structure that aids in joint lubrication and stability. Among mammals, the os coxae fuses into a single innominate comprising ilium, , and pubis, with variations tied to locomotion and ; monotremes and marsupials retain epipubic bones for pouch support, absent in placentals. In domestic mammals, the features a large, triangular ilium, with a trifid ischii, and a deep, hemispherical suited for weight-bearing in grazing, though shallower than in carnivores, increasing luxation risk. The horse's os coxae has a wide, shallow ischial arch and prominent tuber coxae with four tuberosities, optimizing for speed and endurance, with a deep dorsolateral and medial pubic groove. Pigs exhibit a long, narrow os coxae with ilium and nearly aligned, two gluteal fossae, and an elliptical adapted for omnivorous foraging. In carnivores like dogs, the ilium is vertical with a concave gluteal surface, the twisted without a lesser notch, and the deep for agile predation; the is dorsomedial and slightly hemispherical with an elongated fovea capitis. Sheep and show ilium and in a straight line, a large deep , and non-ossified ischial , supporting quadrupedal agility on uneven terrain. Laboratory mammals reveal further nuances in hip bone microstructure: mice and rats have small femoral heads (0.19 mm³ and 2.59 mm³ bone volume, respectively) with thin cortical bone (0.23 mm and 0.44 mm), ideal for rapid modeling in but less representative of load-bearing; rabbits feature short femoral necks and limited bone volume (3.22 mm³), making them less suitable for orthopedic studies; dogs and cynomolgus macaques approximate geometry more closely, with larger femoral heads (59.72 mm³ and 43.96 mm³) and thicker cortices (14.3 mm and 19.5 mm), though bone mineral density varies slightly (e.g., 1206.4 g/cm² in mice vs. 1167.5 g/cm² in rabbits). Across mammals, the acetabulum's meniscoid labrum and transverse form a functional unit with the round , enhancing congruence from amphibians to .

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