Table of Contents

• What is Ischium?
• Properties of Ischium
• Structure of Ischium
• Body
• Superior Ramus
• Inferior Ramus
• Clinical Significance
• Frequently Asked Questions

What is Ischium?

The pelvis region is the connection between the axial skeleton (composed of bones of neck, vertebrae and head) with the lower appendage. The pelvic girdle, also known as os coxae, means ‘bone of the hip’ in Latin. The hip bone or the coxal bone is made up of two innominate bones, each consisting of three parts: ilium, ischium and pubis. Ischium is found situated below the ilium and behind the pubis. Each innominate bone articulates with its pair to form the pelvic girdle. The superior portion of ischium forms almost one thyroid portion of the acetabulum (the concave surface of the pelvis).

Properties of Ischium

• The ischium is divided into a body, superior ramus and inferior ramus.
• Each ischium forms an inferoposterior aspect for its respective innominate.
• Towards the front to the border of the ischium is the obturator foramen through which major nerves and vessels pass through.
• A round prominence, called tuberosity, projects from the body of the ischium that bears the weight of the upper body while sitting.
• The superior part of the body of the acetabulum makes the floor of the acetabulum and an articular surface of the hip joint.
• The ischium is associated with a number of important muscular attachments such as the hamstrings, obturator externus, quadratus femoris and adductor magnus.

Structure of Ischium

Body

The body of ischium enters into and forms almost two-fifths of the acetabulum. The external surface forms the semicircular surface of the acetabulum and also the acetabular fossa. The lesser pelvis is formed by the internal surface and gives rise to the internal obturator. No muscles are associated with the body of ischium.

The anterior border projects to form a posterior obturator tubercle. The ischial spine, a thin and pointed triangular eminence projects forms the posterior surface of the body.

Superior Ramus

The superior ramus or the descending ramus of the ischium extends backwards and downwards from the body and presents three surfaces: internal, external and posterior.

The external surface is a quadrilateral shaped portion. It houses the tendon of the external obturator by a groove. From the bottom, it is continuous with the inferior ramus. The front surface gives rise to quadratus femoris and the lower part gives rise to adductor magnus.

The internal surface forms parts of the bony wall of the lesser pelvis. It is limited by obturator foramen in the front. At the bottom, it provides attachment to the falciform prolongation of the sacrotuberous ligament.

The posterior surface forms a large swelling (tuberosity) from where the hamstrings originate.

Inferior Ramus

The inferior ramus or the ascending ramus of the ischium is a thin and flattened part which arises from the superior ramus and meets the inferior ramus of the pubis.

The outer surface is rough because obturator externus and fibres of adductor magnus arise from it. The internal surface forms parts of the anterior wall of the pelvic girdle.

Similar to the inferior ramus of the pubis, two ridges are continuous on the inferior ramus of ischium that join with each other at the origin of transverse perineal muscles.

The lateral border is sharp and thin and forms medial margin of the obturator foramen.

Clinical Significance

1. Ischial bursitis: Commonly known as weaver’s bottom, it is caused due to long hours of sitting. The main cause is inflammation of the synovial bursa that is present between the ischial tuberosity and gluteus maximus muscle.
2. Avulsion Fracture: The ischial tuberosity tears away in such conditions in adults during sports that require sudden forces such as kicking football, jumping hurdles and martial arts.

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