The hip bone (os coxae) is an irregularly shaped, bilateral bone of the bony pelvis which is also known as the innominate bone, pelvic bone or coxal bone. In reality, it is a compound structure which consists of three smaller bones: the ilium, ischium and pubis. The ilium is the largest and most superior part of the bone, the ischium is located posteroinferiorly, and the pubis or pubic bone forms the anterior portion of the hip bone.
In this article, we will explore the anatomy of the hip bone and its three components.
|Ilium, ischium, pubis (united at the acetabulum)
|Body of ilium, ala, gluteal surface, sacropelvic surface, iliac fossa
|Body of ischium, ramus of ischium, ischial spine, ischial tuberosity
|Body of pubis, superior pubic ramus, inferior pubic ramus
|Sacroiliac joint, pubic symphysis, hip joint
- Obturator foramen
- Bone marrow biopsy
The three components of the hip bone develop separately and are only connected by cartilage in children. They completely fuse during puberty to form the complex and compact hip bone. The centre of this union is the acetabulum, a deep, cup-shaped socket on the lateral surface of the bone, that articulates with the femoral head at the hip joint. Anteroinferior to the acetabulum is the large obturator foramen. The right and left hip bones make up the pelvic girdle. The bony pelvis is completed posteriorly by the pelvic spine which is composed of the sacrum and coccyx. The bony pelvis connects the axial skeleton to the lower limbs, serving as a weightbearing structure. It also protects and supports the abdominopelvic viscera, provides attachment for several muscles and ligaments, and forms part of the skeletal framework of the birth canal.
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The lateral aspect of the hip bone houses the acetabulum, one of the most prominent landmarks of this bone. It bears a socket shaped articular surface that faces anteroinferiorly which articulates with the head of the femur forming the hip joint. The three components of the hip bone unit at the acetabulum, contributing to its formation.
The acetabulum itself has a number of anatomical features. The lunate surface is the moon-shaped articular surface for the head of the femur. The acetabular fossa is the non-articular portion of the acetabulum found centrally. The acetabulum is delimited by a “C” shaped acetabular margin which incomplete inferiorly at the acetabular notch. The acetabular margin is accentuated by the cartilaginous acetabular labrum and completed inferiorly by the transverse acetabular ligament.
Located anteroinferior to the acetabulum is the obturator foramen, a large opening delimited by the pubis and ischium. Most of this foramen is covered by a flat connective tissue membrane called obturator membrane. A narrow obturator canal remains superiorly between the membrane and adjacent bone through which the obturator nerve, artery and vein pass through while travelling between the pelvic cavity and the medial compartment of the thigh.
The ilium is a blade-shaped bone and the most superior component of the hip bone. It consists of two main parts: the body and ala (wing). The body is the smaller, inferior part of the bone that contributes to the formation of the acetabulum, while the ala is the superior expanded part and presents four borders and three surfaces.
The ala of the ilium has four distinctive borders: superior (iliac crest), anterior, posterior and medial.
The superior border is the prominent iliac crest which is a site of attachment for a number of muscles and fascia of the abdominal wall, back and lower limb. It ends anteriorly at the anterior superior iliac spine and posteriorly at the posterior superior iliac spine. The anterior superior iliac spine serves as a point of attachment for the inguinal ligament and is easily palpated. The posterior superior iliac spine cannot be palpated but is commonly represented by a dimple above the medial gluteal region (buttock). The iliac crest has inner and outer lips separated by an intermediate zone. The outer lip bears a prominence near the anterior end of the crest known as the iliac tubercle.
The anterior border of the ilium extends from the anterior superior iliac spine to the acetabulum. It features the anterior inferior iliac spine just superior to the acetabulum. This spine provides a point of attachment for the straight head of the rectus femoris muscle and proximal part of the iliofemoral ligament.
The posterior border of the ilium begins at the posterior superior iliac spine and extends to the posterior border of the ischium. This border contains the posterior inferior iliac spine and contributes to the superior part of the greater sciatic notch. This notch is completed inferiorly by the posterior ischial border and ischial spine. Together with the sacrotuberous and sacrospinous ligaments, the greater sciatic notch forms an enclosure known as the great sciatic foramen.
The medial border lies between the iliac fossa and the sacropelvic surface. This border includes the arcuate line, a smooth and rounded line that runs anteroinferiorly from the auricular surface to the area of the acetabulum. It marks the transition between the body and wing of the ilium.
The borders of the ilium bound its three bony surfaces: the gluteal, sacropelvic surfaces and iliac fossa.
The gluteal surface faces posterolaterally and provides various points of attachment for gluteal and thigh muscles. It is bounded superiorly by the iliac crest, inferiorly by the upper part of the acetabular margin and by the anterior and posterior borders. This surface features three curved ridges known as the anterior, posterior and inferior gluteal lines. The anterior gluteal line travels obliquely along the gluteal surface from the tubercle of the iliac crest towards the greater sciatic notch. The posterior gluteal line descends from the external lip of the crest and ends anterior to the posterior inferior iliac spine. The inferior gluteal line runs superior to the acetabular margin. Between the inferior gluteal line and the acetabular margin is the supraacetabular groove, which is the attachment point for the reflected head of the rectus femoris muscle.
On the medial aspect of the ilium, posterior to the iliac fossa is the sacropelvic surface, which is composed of the iliac tuberosity, auricular and pelvic surfaces. The auricular surface is an ear-shaped surface that articulates with the sacrum to form the sacroiliac joint. The iliac tuberosity is the posterior portion of the sacropelvic surface; it is a roughened, elevated area that provides a point of attachment for ligaments of the sacroiliac joint. The narrow pelvic surface is found anteroinferior to the auricular surface and contributes to the lateral wall of the lesser pelvis.
The iliac fossa, is the large concave anteromedial surface of the wing of the ilium. This fossa forms the smooth posterolateral wall of the greater pelvis and is bounded superiorly by the inner lip of the iliac crest, inferiorly by the arcuate line, anteriorly by the anterior border of the ilium and posteriorly by the medial border which separates it from the sacropelvic surface.
The ischium is the posterior and inferior part of the hip bone. It is divided into two main parts: the body and the ramus.
The body of the ischium is the largest portion of the bone. It projects superiorly to join the ilium and the superior ramus of the pubis, and contributes to the acetabulum.
The ramus of the ischium, extends anteromedially from the inferior aspect of the body to join the inferior pubic ramus. Together, these bony rami form the ischiopubic ramus which is the inferior border of the obturator foramen.
On the posteroinferior aspect of the ischial ramus is the most prominent feature of the ischium, the ischial tuberosity. It supports the weight of the body in the seated position and is a place of attachment for posterior thigh muscles (semimembranosus, semitendinosus and long head of biceps femoris), adductor magnus muscle and the sacrotuberous ligament.
The posterior margin of the ischium displays a conical prominence pointing posteromedially known as the ischial spine. It is the attachment site for the sacrospinous ligament. Immediately below the ischial spine is a small ‘C’ shaped concavity known as the lesser sciatic notch. The lesser sciatic notch is converted into the less sciatic foramen by the sacrotuberous and sacrospinous ligaments.
The anterior and inferior part of the hip bone is the pubis or pubic bone. This bone is the smallest component of the hip bone. It is divided into three main parts: body, superior ramus, and inferior ramus. The body is located anteromedially and the two rami extend posterollatery from the body: the superior ramus extends superiorly to the acetabulum, while the inferior ramus continues inferiorly to join the ischial ramus. It should be noted that some sources consider the body of the pubis to be the superolateral part of the bone which contributes to the formation of the acetabulum, however this is not in accordance with the Terminologia Anatomica.
The body of the pubis has anterior, posterior and symphysial (medial) surfaces. The rounded upper border is known as the pubic crest. This crest ends laterally as the pubic tubercle which provides attachment to the medial end of the inguinal ligament. The symphyseal surface of the body faces medially to articulate with its counterpart at the pubic symphysis.
Superior pubic ramus
The superior pubic ramus projects posterolaterally from the body towards the acetabulum where it joins the ilium and ischium. It is triangular in cross section and has three borders and three surfaces.
The anterior/pectineal surface of the superior pubic ramus extends from the pubic tubercle to the iliopubic eminence (where the pubis meets the ilium). This surface is limited by the round obturator crest anteriorly and by pecten pubis (pectineal line) posteriorly. The pecten pubis is continuous anteromedially with the pubic crest and posteromedially with the arcuate line of the ilium. Together these three landmarks, along with the promontory of the sacrum make up the pelvic brim, a line that delineates the pelvic inlet, which is an imaginary planar surface that separates the greater from the lesser pelvis.
The obturator surface faces posteroinferiorly and is bounded by the obturator crest and the sharp inferior border. This surface is crossed by the obturator groove.
The pelvic surface of the pubis is smooth and faces posterosuperiorly. It is limited by the pecten pubis above and the inferior border of the superior pubic ramus below.
Inferior pubic ramus
The inferior pubic ramus projects posteroinferiorly and laterally from the pubic body and unites with the ischial ramus. It provides attachment for several of the adductor muscles of the thigh (adductor magnus/minimus/brevis and gracilis).
Bone marrow biopsy
A bone marrow biopsy is a useful procedure in the diagnosis and staging of certain conditions, especially hematologic diseases. As the iliac crest lies close to the surface of the body and is easily palpated, it is often used for this biopsy. It is performed by injecting anesthetic in the skin and passing a cutting needle through the cortical bone of the iliac crest. The bone marrow tissue is removed and sent to the laboratory to be analyzed under a microscope.
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