Ischium

The ischium constitutes the posteroinferior part of the hip bone and consists of a body and a ramus.

A. Body of the ischium: This dense bone mass is situated below and behind the acetabulum. Its upper end articulates with the ilium and the superior ramus of the pubis. The lower end thickens to form the ischial tuberosity.

The body of the ischium has three borders: anterior, posterior, and lateral:

• The anterior border forms the posterior boundary of the obturator foramen.

• The posterior border extends upwards and joins the posterior border of the ilium, forming part of the greater sciatic notch. Below, the posterior border features the ischial spine and the concavity known as the lesser sciatic notch. Further down, the lower border culminates at the ischial tuberosity.

• The lateral border descends along the lateral aspect of the ischial tuberosity.

The body of the ischium also possesses three surfaces: femoral, pelvic, and dorsal:

• The femoral surface faces outward and lies between the anterior and lateral borders.

• The pelvic surface faces inward and lies between anterior and posterior borders. It is smooth and contributes to the lateral wall of the true pelvis. Above, it continues as the sacropelvic surface of the ilium.

• The dorsal surface resides between the lateral and posterior borders. It continues above as the gluteal surface of the ilium.

The ischial tuberosity contains a transverse ridge, dividing it into upper and lower halves. The upper half is further divided by an oblique ridge into superolateral and inferomedial parts, while the lower half is subdivided by a longitudinal ridge into outer and inner regions.

B. Ramus of the ischium: It projects anteriorly from the lower end of the body of the ischium and joins with the posterior ramus of the pubis to form a conjoined ischiopubic ramus. For details click here: ischiopubic ramus.

Several important muscles and ligaments attach to or relate to the ischium, including:

• The sacrospinous ligament and posterior fibers of the levator ani attach to the ischial spine (on its pelvic surface). Moreover, the dorsal aspect of the ischial spine serves as a crossing point for the internal pudendal vessels and nerve as they pass through the gluteal region into the perineum.

• The superior and inferior gemelli muscles originate from the upper and lower margins of the lesser sciatic notch, respectively. Between the gemelli, the tendon of the obturator internus muscle crosses over the notch and enters the gluteal region.

• The femoral surface of the ischium provides the origin of the obturator externus muscle along the margin of the obturator foramen. Additionally, the quadratus femoris muscle arises near the lateral margin of the ischial tuberosity.

• The pelvic surface of the ischium gives rise to the obturator internus muscle along the margin of the obturator foramen. It also forms the lateral wall of the ischioanal fossa.

• The hamstring muscles originate from the ischial tuberosity: the semimembranosus from the superolateral area (above the oblique ridge), the semitendinosus and long head of the biceps femoris from the inferomedial area (below the oblique ridge), and adductor magnus from the outer lower area (lateral to the longitudinal ridge). The inner lower area (medial to the longitudinal ridge) is covered by fibrofatty tissue, providing support during sitting. Additionally, the sacrotuberous and ischiofemoral ligaments arise from the medial and lateral margins of the ischial tuberosity, respectively.