ISCHIUM

The ischium forms the lower and back region of the hip bone. Situated below the ilium and behind the pubis, the ischium is one of three regions that fuse to form the hip bone. Superiorly, it forms the posteroinferior part of the acetabulum, while inferiorly, its ramus ascends anteromedially at an acute angle to meet the inferior pubic ramus, completing the obturator foramen boundary.

The ischium is made up of three parts–the body, the superior ramus and the inferior ramus. The body contains a prominent spine. The indentation inferior to the spine is the lesser sciatic notch. Continuing down the posterior side, the ischial tuberosity is a thick, rough-surfaced prominence below the lesser sciatic notch. This part supports body weight during sitting, especially on hard surfaces, and can be felt by placing fingers beneath the body. The inferior ischial ramus joins the inferior ramus of the pubis anteriorly and is the strongest of the hip bones.

Ischium (in Red) Posterior View

Ischium (in Red) Anterior View

Body

The body enters into and constitutes a little more than two-fifths of the acetabulum which is thick and has a massive mass of bone in it. It has two ends: Upper & Lower, three borders: Anterior, Posterior & Lateral and has three surfaces: Femoral, Dorsal & Pelvic. No muscles insert on the body.

ENDS

1.     Upper end: It forms the posteroinferior two-fifths of the acetabulum.

2.     Lower end: It forms the ischial tuberosity and gives off Ischial ramus at an acute angle.

BORDERS

1.     Anterior Border: It forms the posterior margin of the obturator foramen.

2.     Posterior Border: It merges superiorly with the posterior border of the ilium and terminates at the upper end of the ischial tuberosity. It also forms the part of lower border of the greater sciatic notch and below to the notch, posterior margin shows a projection known as Ischial Spine. Below to the ischial spine, posterior border shows a concavity known as the Lesser sciatic notch.

3.     Lateral Border: It forms the lateral margin of the ischial tuberosity, except at the upper end where it’s rounded. It is indistinct above but well defined below.

SURFACES

1.     Femoral Surface: It lies between anterior and lateral borders. It faces downwards, forwards and laterally towards the thigh.

2.     Dorsal Surface: The dorsal surface continues superiorly with the gluteal surface of the ilium. From above downwards, it looks like a convex surface adjoining the acetabulum, a wide shallow groove which presents upper part of the ischial tuberosity. The ischial tuberosity is divided into two areas upper and lower by a transverse ridge. The upper area is subdivided into superolateral area and inferomedial area by an oblique ridge. The lower area is further divided into outer and inner area by a longitudinal ridge.

3.     Pelvic surface: It is smooth and faces the pelvic cavity. It forms inferiorly, a part of lateral wall of true pelvis.

Superior Ramus

The superior ramus of ischium also known as the descending ramus projects downward and backward from the body and presents for examination.

 SURFACES

1.     External Surface: It is quadrilateral in shape. It is bounded by a groove that lodges the tendon of the external obturator. Inferiorly is continuous with the inferior ramus, anteriorly by the posterior margin of the obturator foramen, posteriorly by a prominent margin separates from the posterior surface.

2.     Internal Surface: It forms part of the bony wall of the lesser pelvis. Anteriorly it is limited by the posterior margin of the obturator foramen, inferiorly it is bounded by a sharp ridge that provides attachment to a falciform prolongation of the sacrotuberous ligament.

3.     Posterior Surface: it forms a large swelling known as the ischium tuberosity.

Inferior Ramus

The inferior ramus of the ischium also known as the ascending ramus. It is thin, flattened part of the ischium which ascends from the superior ramus and joins the inferior ramus of the pubis. The junction in an adult is indicated by a raised line.

SURFACES

1.     Outer Surface: It is uneven. It gives origin to the obturator externus and some fibres of adductor magnus.

2.     Inner Surface: It forms part of the anterior wall of the pelvis which is convex and smooth.

BORDERES

1.     Medial Border: It is thick, rough, slightly everted forms part of the outlet of the pelvis and presents two ridges and an intervening space.

a.     Outer Ridge: It has attached a deep layer of the superficial perineal fascia or else known as the fascia of Colles.

b.     Inner Ridge: It has attached an inferior fascia of the urogenital diaphragm.

These two ridges trace downwards, and they join with each other just behind the point of origin of the transverse perineal muscles. The two layers of fascia are continuous behind the posterior border of the muscle.

c.     Intervening Space: It is just in front of the point of junction of the ridges, the transverse perineal attaches and in front of this portion of the ischiocavernosus and the crus penis in male or the crus clitoridis in the female.

2.     Lateral Border: It is thin and sharp and forms part of the medial margin of the obturator foramen.

MUSCLES AND LIGAMENTS ATTACHMENTS

• superior gemellus from the upper margin of the lesser sciatic notch
• inferior gemellus from the lower margin of lesser sciatic notch
• semimembranosus from the superolateral impression of ischial tuberosity
• biceps femoris long head and semitendinosus from the inferomedial impression of ischial tuberosity
• quadratus femoris originates from the femoral surface of the ischium along the lateral border of the upper part of ischial tuberosity.
• adductor magnus starts from the ischiopubic ramus and the inferolateral aspect of the ischial tuberosity
• gracilis muscle (a smaller part) originates from the inferior part of ramus and adjoining conjoined ischiopubic ramus
• deep and superficial transverse perineal muscles originate from the body of ischium
• obturator internus origins from the lesser sciatic notch which also has a deep bursa and is lined by a hyaline cartilage. 
• obturator externus muscles have part origins from the femoral surface of the ischium along the margins of the obturator foramen.
• The lateral border of the ischial tuberosity provides attachment to the ischiofemoral ligament, just below the acetabulum and forms the capsular reinforcement of the hip joint.
• The lower end of the pelvic surface of the ischium forms a part of the lateral wall of the ischioanal fossa.
• The inner lower area of the ischial tuberosity is covered with fibrofatty tissue and a bursa which supports body weight in the sitting position. While standing, this area is covered by gluteus maximus muscle.
• piriformis muscle arises from the superior aspect of the greater sciatic notch

• sacrotuberous ligament attaches to the posterior iliac spine and medial ischial tuberosity and part of it extends as the falciform ligament along the inferior ramus of ischium. It crosses posteriorly to the ischial spine by the internal pudendal vessels, pudendal nerve and the obturator internus.
• the upper border of the ischiopubic rami gives attachment to the obturator membrane
• the lower border of the ischiopubic rami gives attachment to the fascia lata and the membranous layer of the superficial fascia or Colles’ fascia of the perineum.

OSSIFICATION

Ossification of the hip bone is by 3 primary centres: one for each Ilium, Ischium and Pubis. The ischial centre appears in its body in the 4^th month. Gradual ossification of the three components of the acetabulum results in a triradiate cartilaginous stem extending medially to the pelvic surface as a Y-shaped epiphysial plate between the ilium, ischium and pubis, and including the anterior inferior iliac spine. Cartilage along the inferior margin covers the ischial tuberosity. It forms the conjoined ischial and pubic rami and continues to the pubic symphysial surface, extending along the pubic crest to the pubic tubercle. The ossifying ischium and pubis fuse to form a continuous ischiopubic ramus at the 7^th or 8^th year. Secondary centre for the ischial tuberosity in the cartilage to the inferior acetabular margin and spreading forwards. Between the age of 8 to 9 years one of the 3 major ossification centre appear in the acetabular cartilage, which is at the ischial acetabular cartilage posteriorly, fuses with ischium.