Acetabulum

The acetabulum lies on the outer surface of each side of the pelvis, a little below the bony area you can feel when you place your hands on your hips. In the Chinese characters used for its Korean name, “bi” means thigh and “gu” means mortar. The name describes a bowl that receives the rounded head of the femur like a mortar receiving a pestle.

The acetabulum is not formed from a single bone, but from the meeting of three bones. The ilium, ischium, and pubis, which make up the pelvis, form its upper, lower rear, and lower front portions, respectively. In childhood, these three bones are separated by Y-shaped cartilage, then fuse into one through adolescence. This is why no seams are visible in the adult pelvis.

A look inside reveals a fairly intricate structure.

• Lunate articular surface — This portion curves around like a horseshoe. It is covered with smooth articular cartilage, allowing it to bear weight and glide smoothly.
• Acetabular fossa (floor of the socket) — This is the hollow in the center. It has no cartilage; fatty tissue and an attached ligament help hold the head in place.
• Labrum — This is a rim of fibrocartilage that circles the edge of the acetabulum. It deepens the bowl and holds the head in place with a suction-like effect to keep it from slipping out.

The femoral head is nearly spherical, so it can move in every direction within this bowl: forward and backward, side to side, and in rotation. The shoulder is also a ball-and-socket joint, but unlike the shoulder's shallow hollow, the acetabulum deeply encloses more than half of the head. It therefore sacrifices a little range of movement in exchange for the stability to withstand weight-bearing activities, even running.

The acetabulum opens outward, downward, and slightly forward rather than straight ahead. This angle varies a little from person to person. Some people have a particularly deep bowl or one that opens farther forward, while others have a shallow bowl that covers less of the head. These differences are understood to affect whether spreading the legs or squatting feels comfortable or produces a catching sensation, as well as differences in natural flexibility.

Some hip problems also arise from differences in how the acetabulum and the head come into contact. A typical example is femoroacetabular impingement. This refers to a condition in which the shape of the head or the acetabular rim causes the two bones to bump against each other when the hip is deeply bent.

The Integrative Manual Therapy (IMT) book treats the acetabulum as a starting point for interpreting the posture of the pelvis and lower limbs. Its view is that when the pelvis tilts forward or backward and rotates to one side while standing, the orientation of the acetabulum changes along with it, as does the position of the femoral head within the bowl. The explanation is that this shifts tension toward one side of the muscles around the hip, producing compensatory postures that extend to the knee and foot.

For this reason, IMT does not examine the hip in isolation, but considers pelvic position in three planes along with articular balance. This involves checking whether the space between the head and the acetabulum is evenly open and whether muscles on one side are pulling the head too far inward into the acetabulum.

Discomfort around the acetabulum is usually felt deep in the groin or at the side of the hip, and sometimes at the front of the thigh. Because the acetabulum lies deep within the body, it can be difficult to pinpoint the exact location even when it hurts. These are some moments when you may become aware of your hip socket.

• Your groin feels achy when you stand up after sitting for a long time.
• You feel catching on one side when sitting cross-legged or squatting.
• Lifting your leg feels uncomfortable when climbing stairs or getting into or out of a car.
• You feel a “click” around the hip when walking.

The hip socket is most comfortable when the surrounding muscles move evenly. If you spend many days sitting for long periods, get up and walk about once an hour. Even gently moving the hips back and forth circulates the lubricating fluid within the joint, helping movement feel smoother.

Gentle stretching of the buttock and thigh muscles, along with regular weight-bearing exercise such as walking, also helps nourish the articular cartilage. However, avoid forcing your legs farther apart while pushing through pain. The shape of the acetabulum differs from person to person, and stretching cannot increase the range permitted by the bone structure. The best care is to keep using it consistently within a comfortable range of movement.