The Hip Joint
Good Hips and Bad Hips
This page examines the typical changes that occur when a normal hip joint is damaged.
Healthy Hip Joint Structure
The pelvis is a bony structure which extends from the waist to the thigh (Fig. 2a). It consists of two large bones separated from each other in the back by the sacrum, an extension of the spinal column. The hip joints are located in the lower half of the pelvis.
The upper side of the femur (thighbone) tapers into a neck that is topped with a ball-shaped head which is capped with a shiny, slippery tissue called articular cartilage(Fig. 2b). The head of the femur fits snugly into the cartilage-lined acetabulum, a round, half-spherical indentation in the pelvis, with just enough space to rotate smoothly. The hip joint is a good example of a ball-and-socket joint.
The head of the femur is longer than the depth of the cavity of the acetabulum into which it fits. However, a fibrocartilage lip, or labrum, which extends around the rim of the cup-shaped cavity, increases the depth of the acetabulum (Fig. 2a). This labrum contributes to hip joint stability by providing a snug fit for the femoral head within the acetabulum.
The head of the femur and its acetabular cavity are both enclosed by a fibrous sac or joint capsule (Fig. 2a) which has a smooth specialized lining (the synovium). The joint capsule unites the margin of the acetabulum with the neck of the femur.
Tendons of the hip muscles cross the joint and help to stabilize it. They insert along the femur and/or into the strong deep fascia lata of the thigh. Muscle tendons surround the capsule and some blend with it.
Several ligaments, which strengthen the capsule, also stabilize the hip joint (Fig. 2b). The iliofemoral ligament is a very strong ligament that reinforces the upper part of the joint capsule. It, together with the pubofemoral and the ischiofemoral ligaments, limits how far the leg can be lifted.
Another ligament, called the ligament of the head of the femur, runs from a notch in the acetabulum to a notch on the head of the femur. It is stretched when the thigh is raised in front, raised by the side, or rotated outward. Additional ligaments, located within the acetabular cavity, unite the femoral head to the acetabulum (not shown).
The hip joint has several bursae. These are small pockets of fibrous tissue lined with synovial membrane and filled with a fluid that lubricates the joint to reduce friction.
The femoral nerve, sciatic nerve, and obturatur nerve cross the hip joint and supply the muscles acting on the hip joint as well as the joint itself. Because these nerves also supply the knee joint, hip pain can be referred to the knee and misinterpreted as a knee problem.
Hip Joint Movement
The hip joint is capable of more movements than many other joints, being only slightly less versatile than the shoulder joint. A large number of muscles, arranged in layers, surround the hip joint and interact to lift the leg forwards and backwards, out to the side and back, and to rotate the leg.
The Damaged Hip
Various types of arthritis commonly cause damage to the hip that is sufficient to require a replacement. Although they arise in different ways, this diverse set of diseases involves pain, stiffness, joint deformities, restricted joint motion, and disability.
Osteoarthritis or degenerative joint disease is the most common reason for hip replacement. This non-inflammatory progressive disorder results from the way we age. When fully developed, it reduces mobility and produces severe pain.
Osteoarthritis involves deterioration of articular cartilage and new, but abnormal, bone formations or "osteophytes" in subchondral regions at joint margins. The cartilage covering the surface of affected bone progressively erodes, becoming more irregular in thickness, unusually thin or absent in some areas and thickened in other areas. X-rays may reveal cysts below the cartilage.
When the head of the femur is severely affected by osteoarthritis, its cartilage covering is often eroded (Fig. 2c) and it can no longer smoothly glide within the acetabulum. The femoral head may then migrate upward, or less frequently inward, with respect to its previous location. This shows up on X-rays as progressive and asymmetric loss of joint space, a fundamental pathological characteristic of this disease. Some natural repair attempt takes place but it is less than perfect.
The actetabulum may also be affected by osteoarthritic change. Osteophytes develop around the rim of the osteoarthritic acetabulum and the articular cartilage within the acetabulum erodes (Fig. 2d), further impairing hip movement.
Either or both hip joints may be affected. Pain may be confined to the hip region (in the groin), or it may be referred to the knee, buttock, thigh, or the greater trochanter, a ridge on the outer side of the femur. Initially noticeable during standing and walking, the pain may eventually be present even during rest. Marked restriction of motion is common, particularly for rotation and extension.
The second most common reason for hip replacement is rheumatoid arthritis. This chronic illness is systemic with widespread involvement of connective tissues. Affecting two-three females for every male, its onset is usually in the fourth or fifth decade. The major characteristic of rheumatoid arthritis is that many limb joints are inflamed, usually in a symmetrical distribution. If unchecked, progressive inflammation causes irreversible destruction of both cartilage and bone, resulting in joint swelling, deformity, stiffness, and crippling disability.
Some patients will develop an avascular necrosis of the femoral head. This condition involves bone loss due to an insufficient blood supply. It is often accompanied by a secondary painful arthritis even in the absence of trauma or infection. In some cases, avascular necrosis has been associated with steroid pills taken for 6 weeks or more, kidney transplantation, alcoholism, and various other diseases. In other cases, there may be no identifiable association or cause. Total hip replacement is likely to be warranted when the secondary arthritic changes are advanced.
Other Hip-Damaging Diseases and Injury
Other main candidates for hip replacement are people who have suffered from: