Ischium

Ischium, together with ilium and pubis, is one of the three bones that make up the pelvis. Located in the lower and posterior part of the hip bone, it contributes to the formation of the acetabulum and constitutes the base of the pelvic girdle.

1.0Anatomy of Ischium

1. Body of Ischium

• The ischial body forms a considerable portion of the acetabulum, the cavity for the reception of the head of the femur.
• At the junction of the ischial body with the ilium and pubis, a deep notch is formed known as the acetabular notch, and this forms an important part in the stability of the hip joint.
• Attachment of various muscles, such as obturator internus, to the body and gemellus muscles.

2. Ramus of Ischium

• The ramus of ischium extends anteriorly and superiorly, meeting up with the inferior ramus of the pubis to form the ischiopubic ramus.
• This structure provides attachment for the adductor magnus and gracilis muscles, which are important in the movement of the thigh.

3. Ischial Tuberosity

• The ischial tuberosity is a large roughened area located on the posterior part of the ischium.
• It is a weight-bearing structure, particularly when sitting, and it serves as an attachment site for the hamstring muscles that include the biceps femoris, semitendinosus, and semimembranosus, the adductor magnus, and the sacrotuberous ligament.
• The ischial tuberosity serves as an attachment point for the quadratus femoris muscle.

4. Ischial Spine

• Lying between the greater and lesser sciatic notches, the ischial spine projects medially and slightly posteriorly.- It provides attachment to the sacrospinous ligament and it separates the opening known as the greater sciatic foramen from the lesser sciatic foramen.
• The ischial spine serves as an important landmark in pelvic anatomy, particularly with regard to the course of the pudendal nerve, which passes rather close to the ischial spine.

2.0Functions of the Ischium

1. Support and Stability

• Ischium, along with other bones in the pelvis, contributes to the structural integrity of the latter and provides a weight-bearing platform while sitting, and for stability of the hip joint.

2. Muscle Attachment

• It also forms the origin for the greater part of locomotion muscles, which include those responsible for posture and movement of the thigh and lower leg.

3. Pelvic Support

• The ischium, with the other bones of the pelvis, forms a protective basin for the pelvic organs, which include but are not limited to the bladder, the rectum, and in females, the reproductive organs.

3.0Clinical Relevance

1. Ischial Bursitis

• Inflammation of the bursa overlying the ischial tuberosity due to prolonged sitting on hard surfaces or repetitive movements may present with pain and discomfort.

2. Ischial Fractures

• Fractures of the ischium may be traumatic, often due to a fall or accident, and are usually associated with pelvic fractures.

3. Hamstring Injuries

• Given that the origin of the hamstring muscles is from the ischial tuberosity, any form of injury to these muscles, including strains or avulsions, will have some effects on the ischium.