Ischium

What is the Ischiad

The ischiad is one of the three bones of the hip bone, besides the ilium and pubis. It constitutes the lower and posterior portions of the hip bone.

Where is the Ischiad Bone Situated

Ischiad is positioned beneath the ilium and behind the pubis.

Quick Details

Functions of the Ischiad

• Being a component of the pelvis, it plays a significant role in various body movements, such as walking and jogging.
• It is also crucial in maintaining equilibrium and an upright posture.

Ischiad Anatomy, Components, and Osseous Landmarks

The ischiad is divided into two main components: the core and ramus. On the upper side, it forms the inferior-posterior portion of the acetabulum. On the lower side, it ascends anteromedially at an acute angle and joins the descending pubic ramus, completing the obturator foramen.

Core

It is the largest portion of the bone, projecting upward to articulate with the ilium and the upper ramus of the pubis. These articulations lead to the formation of the acetabulum. The core of ischiad constitutes a little over two-fifths of the acetabulum. The external surface of the core forms a part of the lunate surface of the acetabulum and a portion of the acetabular fossa. On the other hand, the internal surface of the core forms a part of the wall of the lesser pelvis. The anterior border of the core projects as the posterior obturator tubercle. From the posterior border of the core, a thin and pointed triangular eminence, referred to as ischial spine extends backwardly.

Ramus

The ischiad has two rami, upper ramus and lower ramus.

i. Upper Ramus: Extending downward and backward from the core, it is the partial site of origin for the obturator internus and obturator externus muscles.

ii. Lower Ramus: It is a thin, flattened part of the ischiad that extends anteromedially from the lower side of the core and articulates with the lower pubic ramus. These two rami together form the ischiopubic ramus, the lower border of the obturator foramen.

Ischial Tuberosity

The posteroinferior side of the ischiad features a large rounded bony prominence, known as the ischial tuberosity. It is the site of attachment for several thigh muscles, the adductor magnus muscle, and the sacrotuberous ligament. The ischial tuberosities of the left and right hip bones support the whole body weight while sitting.

Ischial Spine

At the junction of the upper ramus and core, there is a conical bony projection, called the ischial spine, pointing posteriorly. The sacrospinous ligament attaches here, while the gemellus superior muscle originates from here. Parallel to the spine, there are two indentations:

i. Lesser sciatic notch: Located on the lower side of the ischial spine, specifically between the ischial tuberosity and the ischial spine.

ii. Greater sciatic notch: A larger notch, located on the upper side of the spine, that is, between the posterior inferior iliac spine and the ischial spine. The lesser sciatic notch gets turned into the lesser sciatic foramen by the sacrotuberous and sacrospinous ligaments.

Surfaces

The three surfaces of ischiad are:

1. Medial Surface

As the name implies, this flat, smooth surface faces the pelvic cavity and lies just above the ischioanal or ischiorectal fossa. It forms the lateral wall of the ischiorectal fossa. The obturator internus muscle and its fascia attach here.

2. Femoral Surface

As the name implies, this surface faces the head of the femur. On the anterior side, this surface is bounded by the posterior margin of the obturator foramen. On the other hand, its lateral side is delimited by the lateral border of the ischial tuberosity.

3. Posterior or Dorsal Surface

This surface is relatively smooth on its upper aspect. It runs alongside the iliac gluteal surface and forms the upper portion of the ischial tuberosity. The surface bears the greater and lesser sciatic notches, which remain separated by the ischial spine.

Articulations

The ischiad fuses with the other two bones of the hip bone, the ilium and pubis. Therefore, it does not form any joint.

Muscle and Ligament Attachments

The ischiad offers numerous attachment points for pelvic and lower limb muscles, along with some important ligaments.

Muscle Attachments

Originating from ischiad:

1. Superior gemellus:  From the ischial spine.

2. Inferior gemellus: From the upper border of ischial tuberosity.

3. Semimembranosus: From the superolateral side of ischial tuberosity.

4. Biceps femoris long head and semitendinosus: From the inferomedial aspect of ischial tuberosity.

5. Adductor magnus: From the ischiopubic ramus and the inferolateral portion of the ischial tuberosity.

6. Obturator internus and obturator externus: From the upper part of the ramus.

7. Transverse perineal muscles: From the core of ischiad.

8. Gracilis muscle: From the ischiopubic ramus.

9. Piriformis muscle: From the upper aspect of the greater sciatic notch.

Inserting into the ischiad:

1. Coccygeus and levator ani muscles: At the ischial spine.

Ligament Attachments

1. Sacrospinous ligament: Attaches to the ischial spine, and runs from the ischial spine to the sacrum.

2. Sacrotuberous ligament: Attaches to the posterior iliac spine and medial ischial tuberosity, and travels from the sacrum to the ischial tuberosity, forming the lesser sciatic foramen.

References

1. Ischiad – Med.libretexts.org
2. Ischiad – Radiopaedia.org
3. The Hip Bone – Teachmeanatomy.info
4. Hip bone – Kenhub.com
5. Bony pelvis – Kenhub.com