The Anatomy of the Ischium

Forming the lower and back sides of the hip bone, the ischium is one of the three bones that make up the pelvis. It is located beneath the ilium and behind the pubis. The upper portion of the ischium forms a major part of the concave portion of the pelvis that forms the hip. As part of the hip joint, this bone plays a very important role in leg mobility, balance, standing up, and lifting tasks.

Clinically speaking, the ischium is associated with hip osteoarthritis, a common ailment characterized by erosion of necessary cartilage and wear and tear of the hip joint. In addition, fracture of the hip or pelvis is not uncommon and can be severely debilitating.

Anatomy

Structure and Location

The ischium is a major part of the hip. Along with the ilium above it and the pubic bone in front of it, this bone forms the pelvis. Importantly, the upper portion, or ramus, of this curved bone joins with a portion of the pubic bone to make up approximately two-fifths of the acetabulum: the cup-shaped, ball and socket joint that connects the femur (upper leg bone) and hip.

In human anatomy, this bone is divided into four major parts:

• Femoral surface: Facing downward and forward, this surface is bounded by the obturator foramen, one of the two large openings at both sides of the hip bone. It forms the outside limit of the ischial tuberosity, also known as the seat bone, a large swelling toward the back from the upper portion of the ischium.
• Dorsal surface: This surface of the bone runs alongside the iliac gluteal surface—the external wing of the ilium (a major part of the hip joint)—and forms the upper portion of the ischial tuberosity. Its rear (dorsal) border forms the greater and lesser sciatic notches, which are ridges separated by the spine (a protrusion) of the ischium.
• Pelvic surface: Facing the cavity of the pelvis, this flat, smooth surface lies just above ischiorectal fossa (the opening to the side of the anal canal, which is the lowest portion of the digestive system).
• Ramus of the ischium: This is the curved upper portion of the bone that completes the obturator foramen. It has a front-facing (anterior) and rear-facing (posterior) portion, the latter of which is further divided into perineal and pelvic areas. The lower portion of the ramus joins with the pubic bone to bound the pubic arch, the lowest portion of the hip bone.

Notably, the spine of ischium—a protrusion coming from the posterior side of the body—is attached to two important structures: the sacrospinous and sacrotuberous ligaments. The former connects the ischial spine to the sacrum, the triangular terminal extension of the spine (tailbone). The latter connects the sacrum to spine of the ilium (a protruding part of the uppermost hip bone).

Its positioning also ensures that it’s connected to important muscle groups, including those that make up the buttocks, such as the piriformis, the superior and inferior gemellus, and the deep and superficial transverse perineal muscles, as well as leg muscles, such as the biceps femoris, a major part of the hamstring muscles of the upper leg. 

Anatomical Variations

Most but not all variations in the structure of the ischium—and the pelvis in general—are related to childbirth and seen in women. Doctors recognize four varying shapes, which are the following:

• Gynecoid: This structure is seen in about 50% of women and is considered the standard shape of the female pelvis. It's considered especially well-suited for childbirth. The upper inlet is almost circular, the pubic arch is as wide as the pelvic outlet, and the sacrum is deeply curved. 
• Android: This is the standard shape of the pelvis in men, but it’s also observed in about 20% of women. In these cases the inlet is heart shaped, the sacral curve is more shallow, and the sidewalls closer together. This makes for a narrower overall shape, which can lead to difficulties in childbirth.
• Anthropoid: Present in only about 25% of women but very common in men, this type has an ovoid (oval-shaped) inlet and the front-facing side is significantly larger than the rear-facing side. Its pubic angle—the angle at which the ischium and pubis come together—is less than the gynecoid shape, but greater than the android pelvis, making it typically well-suited for childbirth.
• Platypelloid: This type’s inlet is wider and shallower than the others, a shape that can cause difficulties during childbirth, often making regular delivery impossible. It occurs in about 5% of women.

Function

The most important function of the ischium is in its role as part of the pelvis. As such, it’s essential in walking, jogging, and other body movements. This also means that this bone is important for balance and maintaining an erect posture with minimal muscle involvement. Importantly, the ligaments around the ischium, pubis, and ilium bones that make up the pelvis help the structure retain enough mobility for function, while also ensuring necessary limits on the kinds of motions it supports.

Associated Conditions

Fracture or injury to the pelvis, including the ischium, can vary greatly in severity, with surgery sometimes being necessary to correct the problem. Notably, in severe hip fracture, surrounding tissues, organs, and arteries can be damaged, so doctors must assume pelvic fracture in major trauma cases and take appropriate, preemptive steps. In particular, the area needs to be carefully monitored for signs of internal bleeding.

In addition, the ischium can be implicated in arthritis, in which joints are inflamed and damaged, most often due to wear and tear. Among the most common forms of painful disability is hip osteoarthritis, leading to pain within the hip, groin, thigh, or knee, limited mobility, stiffness, and a persistent limp. This condition occurs most often in adults over 50 years of age.

Rehabilitation

Hip fractures can be a serious medical problem, especially in older people. Complications and consequences of these cases can severely impact those over age 65, leading to a 33% rise in mortality in the first year after fracture for these patients. Depending on the scope of the injury, surgery may be required, and patients will also need to go through a significant amount of physical therapy.

While osteoarthritis in the hip is a chronic and irreversible condition, its symptoms can be managed medically. Also, its progression can be slowed with exercise, weight management, and physical therapy. Pain and inflammation due to the condition are treated with anti-inflammatory and pain-managing drugs. In well-advanced cases, hip replacement surgery may be recommended.