Hip bone

Borders

The ala of the ilium has four distinctive borders: superior (iliac crest), anterior, posterior and medial.

Superior border

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.

Anterior border

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.

Posterior border

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.

Medial border

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. 

Surfaces

The borders of the ilium bound its three bony surfaces: the gluteal, sacropelvic surfaces and iliac fossa.

Gluteal surface

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.

Sacropelvic surface

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.

Iliac fossa

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. 

Body

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.

Ramus

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.

Ischial tuberosity

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.

Ischial spine

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.

Body

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. 

Anterior/pectineal surface

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.

Obturator surface

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.

Pelvic surface

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).