The hip provides the area of attachment for muscles of the back, abdomen, gluteals, quadriceps, hamstrings, and inner and outer thigh muscles. With the assortment of tendons and a strong ligament involved with these muscle groups, the hip joint is the strongest and one of the most stable joints in the body. The hip bone itself provides the basic foundation for the formation of the pelvic region. Both bones in the hip are characterized by three components or separate bones. At birth, these three components are distinctive and separated by cartilage. The bones fuse together by early adulthood and begin to function as a single bone.
Structure of the Ilium
The largest of the three components, the ilium, provides one of the more commonly known regions of the hip, the iliac crest. The iliac crest is the superior, or upper, region of the ilium. It is the area commonly visible in lean individuals and is noticed by its bony protusion in the hip area. It is also the area commonly injured during contact sports. Viewed from the front, the ilium has a winged shape that extends to the side and a concave middle section. This open section is where the tendon of the iliacus muscle is attached. The ilium is the primary site for muscle attachment.
The Ischium
The ischium is the lowest part of the hip bone and angles slightly to the back. This bone fuses with the pubis in the front to form the bottom foundation of the pelvis, with the ilium at the side, and fuses to the back as well. Prominent areas on the ischium include the ischial spine extending out towards the rear of the pelvis, the lesser sciatic notch allowing for passage of the sciatic nerve to the lower part of the body, and the ischial tuberosity, which is the bone you feel if someone sits on your thigh or lap. The ischium is the area of the hip bone that bears the weight of the body when sitting.
The Pubis
The pubis, or pubic bone, is the smallest of the three that make up the hip bone. It extends towards the midline of the body. The two pubes of each hip bone join together in the center of the pelvic region to form the pubic symphysis. The pubic symphysis is considered a joint consisting of a disc of fibrocartilage that connects the two hip bones together at the lower region of the pelvis. This joint is a vital one for women during pregnancy and delivery. A hormone, relaxin, is released that increases the flexibility of the pubic symphyis and provides greater ease during delivery of a baby.
Significance
Collectively, the ilium, ischium and pubis join together to form two vital structures of the hip and pelvic region. The first is the hip joint. The hip joint is composed of the acetabulum, or the socket for the head of the femur. The head of the femur, shaped like a ball, fits into the acetabulum and is attached by strong ligaments and tendons of surrounding muscle. This is known as a ball-and-socket joint, which provides for an increased range of motion compared to other joints of the body. It is formed by the fusing of all three components of the hip bone. The other area, known as the obturator foramen, is formed by the joining of only the ischium and the pubis. The upper and lower connections form a foramen, or a circular opening, in each hip bone. Foramens are important because they allow safe passage of blood vessels and nerves throughout the body.
Functions
The hip bones are connected to the axial skeleton--the torso, by way of the vertebral column--at the sacrum. The sacrum is the lowest region of the vertebral column. Where the hips bones connect on either side is known as the sacroiliac joint. The hip bones along with the sacrum form the pelvis. Functions of the pelvis include transferring weight from the axial skeleton to the lower extremities when standing, and to the ischial tuberosities when sitting down. The pelvic region is also important for housing the reproductive system, parts of the urinary and digestive tracts, and for providing attachments for various muscle groups.
References
- Introduction to Sports Medicine and Athletic Training. France, Robert 2004
- Principles of Anatomy and Physiology, 12th ed. Tortora, Gerard and Derrickson, Bryan. 2009
