Hip
What is the hip?
How does the hip joint allow movement?
What conditions can affect the hip joint?
Hip, in anatomy, the joint between the thighbone (femur) and the pelvis; also the area adjacent to this joint.
In humans, the hip joint is a ball-and-socket joint, in which the round head of the femur rests in a cavity (the acetabulum) that allows free rotation of the limb. The surfaces of the femur and the pelvis are protected by a layer of articular cartilage; the smoothness of this tissue reduces friction and helps to absorb force produced during movement. Surrounding the joint socket is the acetabular labrum, a layer of fibrocartilage that serves to stabilize the hip joint.
As a significant weight-bearing joint, the hip both supports the upper body during standing and transfers forces between the upper body and lower limbs during movement. The free-rotation structure of the hip further allows movement in multiple directions, including forward and backward, laterally (side to side), and rotationally. Such movements enable a wide range of activities, from walking and running to climbing and sitting.
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The strong ligaments surrounding the hip joint, which include the iliofemoral (located anteriorly) and ischiofemoral (located posteriorly) ligaments, help maintain stability and prevent excessive movement. In addition, the gluteal muscles, hip flexors, and thigh muscles that surround the hip generate a full range of lower-limb movements and further contribute to core body stability.
The hip joint is affected by injury and disease, which often are debilitating, owing to the substantial forces that the hip bears in order to maintain stability and enable movement. For example, osteoarthritis, in which joint cartilage deteriorates, can cause significant pain and severely reduce mobility over time. Fractures, particularly in the portion of the femoral that rests in the joint socket, are a major concern in individuals with osteoporosis. Hip dislocation and hip dysplasia, in which the joint forms improperly, are other conditions affecting the hip joint.
Amphibians and reptiles have relatively weak pelvic girdles, and the femur extends horizontally. This does not permit efficient resistance to gravity, and the trunks of these animals often rest partially on the ground. In mammals (including humans) the hip joint allows the femur to drop vertically, thus permitting the animal to hold itself off the ground and leading to specializations for running and leaping. See also pelvic girdle.