Hip Anatomy and its Associated Problems

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 go 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. As the surface becomes increasingly rough with loss of the smooth cartilage that coats the joint, other changes begin to occur as well. 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.

Blood Supply

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.

Bursa

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.

Muscles

A number of muscles attach to the femur and the pelvis in this region, and the anatomy of all of these becomes more complex than we will be discussing here. However, a few important ones bear mentioning. The psoas muscle originates all the way up near the spine before passing down through the pelvis and attaching to a bony prominence on the inside of the thigh, the lesser trochanter. The psoas is primarily responsible for flexing the hip upward. It can sometimes become inflamed or cause pain (tendinitis). A group of large muscles over the side and back of the hip comprise the gluteus muscles, which are involved in pulling the hip backwards (extension) and out to the side (abduction). In the front of the hip, the tensor fascia lata, sartorius, and rectus femoris are involved in pulling the leg up into flexion and pass over the front, or anterior, of the thigh. A group of small muscles attach deep and behind the trochanter, collectively called the short external rotators (such as the piriformis, quadratus, obturator externus), responsible for turning the hip outward. Sometimes the piriformis can compress the sciatic nerve and cause pain (piriformis syndrome). All of these muscle groups will be revisited when discussing different surgical approaches in later chapters.

Nerves and Blood Vessels

For those readers that are interested, there are a number of nerves and blood vessels around the hip. While these are important to your surgeon, a detailed knowledge of the neurovascular anatomy is not required to understand most surgeries and diseases unless we are discussing specific injuries or problems with these structures, and so just a few of the larger and more important structures are mentioned here. More detailed discussions regarding nerve injuries are included in later chapters.

In the front of the hip, the femoral artery, nerve, and vein travel together in a neurovascular bundle along the groin and inner thigh. A branch of the femoral nerve, called the lateral femoral cutaneous nerve, can be involved in numbness or pain over the front of the thigh (sometimes called meralgia parasthetica when it is severe). It is common to have some numbness or tingling in that region for months after surgery, but it is not a motor nerve and does not affect muscle strength.

The sciatic nerve is a very large nerve that runs down the back of the leg. It can be injured particularly in surgical approaches that enter from the back of the hip, and it contains both motor and sensory fibers. In the worst case, an injury to the sciatic nerve can result in a sciatic palsy, or “foot drop,” in which it is difficult or impossible to raise the foot upwards from the ankle. Sometimes the sciatic nerve can be affected near the spine where it exits the spinal cord, typically from a bulging disc (herniated or “slipped” disc), and this can cause shooting pains and numbness down the leg.

These spine problems at the level of the sciatic nerve and nearby spinal levels can sometimes cause leg or hip pain that can mimic hip problems, necessitating special tests to differentiate and determine where the pain is actually coming from. Similarly, another nerve runs along the inner aspect of the hip joint and down to the knee – the obturator nerve – which often accounts for why patients with hip problems sometimes present with initial complaints of knee pain rather than hip pain.

The large veins around the hip are usually far from the area of surgery, but these are sometimes significant if a blood clot forms within them after surgery or injury, known as a deep venous thrombosis (DVT). It often forms farther down the leg, but the backup in blood flow leads to significant swelling and pain in the leg, especially the calf. The clot itself is not usually dangerous, but a small percentage of the time, a piece of the clot can break off and travel with the circulation to the heart and lungs, which can be serious. A blood clot that travels through the right side of the heart and then lodges in the lungs is called a pulmonary embolus (PE), which can be fatal. This is why most surgeons prescribe some sort of “blood thinner” or anticoagulant after surgery, such as aspirin, heparin (or a related low-molecular weight heparin, such as enoxaprin injections), or warfarin. This is an important topic, and an entire chapter is devoted to it.

A Tough Ball Bearing

The hip joint carries large loads, typically of more than a million steps per year for most active adults. That is a lot of potential wear and tear, but surprisingly, it actually works very well until some underlying process disrupts or overcomes its ability to make routine repairs. Like a mechanical bearing in a piece of machinery, the wear and tear usually takes a long time to gradually become apparent, and this process is called arthritis.

Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.