The ischium is a foundational part of the human pelvis, acting as one of the three large bones that fuse together to form the hip bone, or os coxae. It is found on the lower and posterior side of the hip. The ischium joins with the ilium and the pubis to create the complete structure of the pelvic girdle.
Anatomical Location and Context
The ischium is positioned as the lower and rear-most segment of the hip bone, situated beneath the ilium and behind the pubis. It provides structural support to the pelvis from the back and bottom. The superior part of the ischium joins with the ilium, the large, flaring upper part of the hip bone.
Anteriorly, the ischium meets the pubis. The junction point where the ilium, ischium, and pubis meet is the acetabulum, a deep, cup-shaped socket. The acetabulum is where the head of the femur articulates to form the hip joint. The ischium contributes approximately two-fifths of the acetabulum’s surface area, providing a stable base for the hip joint.
Key Structures of the Ischium
The ischium has several distinct features that serve specific anatomical roles. The largest component is the Ischial Tuberosity, a thick, roughened prominence on the lower, posterior surface. This feature is commonly referred to as the “sitting bone” because it directly contacts the surface when a person is seated.
Superior to this prominence is the Ischial Spine, a pointed projection extending backward from the body. This spine separates two indentations in the bone’s posterior edge: the Greater Sciatic Notch above and the Lesser Sciatic Notch below.
The bone is composed of a main Body and two bony extensions called Rami. The Body is the thick, proximal portion that contributes to the acetabulum. The Superior and Inferior Rami project forward to connect with the pubis, completing the bony ring that defines the obturator foramen.
Primary Functions and Muscle Attachments
The primary function of the ischium is to support the weight of the upper body during sitting. When a person is seated, the load from the trunk rests directly on the Ischial Tuberosities, which are designed to withstand this pressure. This weight-bearing role is fundamental to maintaining posture and stability.
The ischium also serves as a point of origin for several powerful lower body muscles. The rough surface of the Ischial Tuberosity is the attachment site for the hamstring muscle group (biceps femoris, semitendinosus, and semimembranosus). These muscles are responsible for extending the hip and flexing the knee.
The ischium provides attachment for muscles that control hip rotation and adduction. Portions of the adductor magnus muscle, which draws the leg toward the midline, originate from the ischial tuberosity and the inferior ramus. Smaller, deep external rotator muscles also anchor to the ischium, contributing to the rotation and stabilization of the hip joint.
Common Conditions and Injuries
The Ischial Tuberosity is a common site for certain clinical conditions due to its weight-bearing and muscle attachment roles.
Ischial Bursitis
Ischial Bursitis, sometimes called “weaver’s bottom,” involves the inflammation of the bursa, a fluid-filled sac that cushions the tuberosity. This condition is caused by repetitive friction or prolonged sitting on hard surfaces. It leads to a deep, dull ache that worsens with pressure.
Avulsion Fracture
An Avulsion Fracture of the Ischial Tuberosity is another injury seen in this area. This injury is most common in adolescents and young athletes because the bony growth plate has not yet fully fused. A sudden, forceful contraction of the hamstring muscles, such as during sprinting, can pull a fragment of the bone away. Symptoms include acute pain, tenderness, and difficulty walking. While conservative treatment is often effective, surgery may be required for fractures with significant displacement.