The ischium is one of the three large bones that form the hip bone (os coxae) on each side of the body. These three components—the ilium, the pubis, and the ischium—begin as separate structures in childhood and fuse into a single, complex bone structure during the late teenage years. The ischium is commonly referred to as the “sit bone” because it is the primary part of the skeletal system that makes contact with a surface when a person is seated. The ischium contributes to the structural integrity of the pelvic girdle, the basin-shaped complex of bones that connects the trunk to the lower limbs.
The Ischium’s Place in the Pelvic Girdle
The ischium is situated as the posteroinferior portion of the hip bone. It is positioned beneath the ilium, the large, fan-shaped upper part of the hip, and behind the pubis, the most anterior part. This location makes the ischium a fundamental component of the pelvic ring.
The upper part of the ischium, known as the body, contributes significantly to the formation of the acetabulum, the deep, cup-shaped socket that receives the head of the femur to form the hip joint. Specifically, the ischium forms approximately two-fifths of the acetabulum’s surface, articulating with both the ilium and the pubis at this junction. The ischium also connects with the pubis anteriorly through a narrow segment of bone called the ischial ramus, which joins with the inferior pubic ramus.
Distinct Anatomical Features of the Ischium
The ischium is an irregularly shaped bone composed of a body and a ramus, featuring several recognizable landmarks. The most prominent feature is the Ischial Tuberosity, a large, roughened bony prominence located on the lowest, posterior part of the bone. This structure is easily palpable, especially when sitting on a hard surface, and it is the physical landmark that earns the ischium its common name, the “sit bone.”
Superior to the tuberosity along the posterior border is the Ischial Spine, a pointed, triangular projection that extends medially into the pelvic cavity. This spine is an important anatomical reference point for ligaments and nerves, including the pudendal nerve. The Ischial Spine also serves to separate two concave indentations on the posterior margin of the bone: the greater sciatic notch, which is superior to the spine, and the lesser sciatic notch, which is inferior to it.
The Ischial Ramus is a projection of bone that extends forward and upward from the body and tuberosity, where it meets and fuses with the inferior ramus of the pubis. This fusion creates the ischiopubic ramus, which forms the lower boundary of the large opening in the hip bone called the obturator foramen.
The Ischium’s Role in Weight Bearing and Movement
The primary functional importance of the ischium is its role in supporting body weight when the trunk is upright but not standing. Upon sitting, the weight of the head, torso, and upper limbs is channeled through the pelvis and directly onto the ischial tuberosities, which contact the sitting surface. This transfer of load is a biomechanical necessity that stabilizes the body in a seated posture.
Beyond bearing weight, the ischium serves as the origin or attachment site for several major muscle groups that facilitate lower limb movement and stability. The prominent ischial tuberosity is where the hamstring muscles—the biceps femoris, semitendinosus, and semimembranosus—originate, connecting the hip to the lower leg. Other muscles, such as the adductor magnus and the quadratus femoris, also attach to the tuberosity and ramus, contributing to thigh movement and hip joint function.
The Ischial Spine provides an anchoring point for the sacrospinous ligament, which helps to stabilize the pelvis by connecting the ischium to the sacrum. The muscle attachments and ligament connections on the ischium are directly involved in movements like hip extension, knee flexion, and adduction of the thigh.