The hip is a ball and socket joint. At its simplest, the hip functions as a ball bearing. This design allows the hip to flex, extend, move from side to side (called abduction and adduction), and rotate (called internal rotation and external rotation). Although not quite as complex as the knee, the hip joint does its job wonderfully and allows us to get around with surprising agility (when it works properly!).
Bones And Cartilage
Normally the “ball” (femoral head) and “socket” (acetabulum) are very smooth, and both sides of the joint (as with most joints in the body) are covered with a smooth layer of cartilage that is called articular cartilage. This layer is about 1/8th of an inch thick on both sides of the joint. It is analogous to having Teflon that coats a frying pan; similarly, if that smooth coating is scraped or worn off, it leaves the underlying bone surface exposed. This is where the term “bone on bone” arthritis comes from.
There is a cartilage “bumper” or gasket that surrounds the rim of the socket, called the labrum. This gasket can develop tears and cause problems, typically in the form mechanical difficulties and pain from the torn piece (a labral tear).
The socket itself (acetabulum) can sometimes be formed incorrectly, leading to congenital problems that may be apparent as an infant or that goes undetected until adulthood. Most commonly, this happens if the socket is too shallow, and the hip has a tendency to “pop out” or dislocate as an infant. Even if the hip does not ever completely dislocate, the abnormal shape shallow acetabulum (called hip dysplasia) can cause problems years or even decades later. This is a common reason for adults to wear out their of a hip joint at an early age and require surgery.
Arthritis simply means there is an inflammation of the joint (there are multiple types of arthritis, as will be discussed in the next chapter), and this accompanies the loss of the smooth cartilage surface. The surface becomes increasingly rough with the smooth cartilage that coats the joints causing other changes to also begin occurring. The body may react by forming large spurs around the joint, called osteophytes. The underlying bone surface becomes more dense and hard in order to resist the forces against the exposed bone surface. These dense changes are called subchondral sclerosis. The bone around the joint may also commonly develop cysts that fill with joint fluid, known as subchondral cysts. Eventually, the bone surfaces begin to erode as arthritis worsens.
The "ball" of the femoral head itself is supplied with a blood supply from several sources. Some of these include the circumflex arteries around the base of the femoral neck. Injuries to this blood supply such as a traumatic hip dislocation, or clotting disorders that prevent blood flow in this region such as Sickle Cell Disease, can lead to a loss of the bone called avascular necrosis (discussed in the next chapter).
The hip joint is surrounded by a tough covering called the capsule. Injections into the hip joint are actually injections inside this capsule. Similarly, infections often involve the joint fluid and space within the capsule. If this capsule becomes contracted and tight, either with disuse or aging, it can lead to a flexion contracture of the hip where it is difficult to fully lay the leg flat while laying down. This often requires a release at the time of hip surgery to restore range of motion. Loose bodies or small nuggets of bone or cartilage (sometimes called “joint mice”) are usually found in this joint space within the capsule.
The prominent bony area over the side of the hips is called the greater trochanter (and there is in fact a lesser trochanter also, but it is located deep on the inside of the thigh). This area can be involved in hip fractures, particularly when someone lands directly on to their side. More commonly, many patients develop pain in this area due to bursitis. The bursa is a small sac that fits between muscle layers, similar to two layers of plastic wrap with olive oil in between. These sacs are located all over the body, usually over joints where they assist in allowing muscle layers to slide smoothly over one another. If the sac becomes inflamed, it is referred to as bursitis, and this is commonly seen over the side of the hip, shoulders, elbows, and the front of the knees.
The most common cause of pain over the side of the hip is from trochanteric bursitis, which causes pain when pressing on or laying on the side of the hip.
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