A joint forms where two or more bones meet. The hip joint is a ball-and-socket type joint and is formed where the thigh bone (femur) meets the three bones that make up the pelvis: the ilium at the rear, the ischium at the lower front and the pubis above it. The thighbone has a ball-shaped knob on the end that fits into a socket formed in the hipbone. A smooth cushion of shiny white articular cartilage about 1/4 inch thick covers the femoral head and the acetabulum. The articular cartilage is kept slippery by fluid made in the synovial membrane (joint lining). Since the cartilage is smooth and slippery, the bones move against each other easily and without pain. Large ligaments, tendons, and muscles around the hip joint (called the joint capsule) hold the bones (ball and socket) in place and keep it from dislocating.
The weight-bearing bones in our body are usually protected with articular cartilage, which is a thin, tough, flexible, slippery surface which is lubricated by synovial fluid. The synovial fluid is both viscous and sticky lubricant. Synovial fluid and articular cartilage are a very slippery combination—3 times more slippery than skating on ice, 4 to 10 times more slippery than a metal on plastic hip replacement, and more than 30 times as slippery as metal on metal using the best petroleum-based lubricant. Synovial fluid is what allows us to flex our joints under great pressure without wear.
The hip joint is one of the largest joints in the body and is a major weight-bearing joint. Weight bearing stresses on the hip during walking can be 5 times a person’s body weight. A healthy hip can support your weight and allow you to move without pain. Changes in the hip from disease or injury will significantly affect your gait and place abnormal stress on joints above and below the hip.
It takes great force to seriously damage the hip because of the strong, large muscles of the thighs that support and move the hip. Osteoarthritis affects many people, and the brittle bones from osteoporosis in the elderly can lead to life threatening fractures.
Anatomical terms allow us to describe the body clearly and precisely using planes, areas and lines. Instead of your doctor saying “his knee hurts” she can say “his knee hurts in the anterolateral region” and another doctor will know exactly what is meant. Below are some anatomic terms surgeons use as these terms apply to the hip:
- Anterior — the abdominal side (front) of the hip
- Posterior — the back side of the hip
- Medial — the side of the hip closest to the spine
- Lateral — the side of the hip farthest from the spine
- Abduction — move away from the body (raising the leg)
- Adduction — move toward the body (lowering the leg)
- Proximal — located nearest to the point of attachment or reference, or center of the body
- Distal — located farthest from the point of attachment or reference, or center of the body
- Inferior — located beneath, under or below; under surface
Anatomy of the Hip
Like the shoulder, the hip is a ball-and-socket joint, but is much more stable. The stability in the hip begins with a deep socket—the acetabulum. Additional stability is provided by the strong joint capsule and its surrounding muscles and ligaments. It’s the need for such a high degree of stabilization of the joint that limits movement. If you think of the hip joint in layers, the deepest layer is bone, then ligaments of the joint capsule and the tendons and muscles are on top. Nerves and vessels supply the muscles and bones of the hip.
The hip joint capsule is a dense, fibrous structure which includes the iliofemoral, pubofemoral, and ischiofemoral ligaments. These ligaments along with the ligamentum teres and the labrum help give stability of the hip.
Bony Structures of the Hip
The adult skeleton is mainly made of bone and a little cartilage in places. Bone and cartilage are both connective tissues, with specialized cells called chondrocytes embedded in a gel-like matrix of collagen and elastin fibers. Cartilage can be hyaline, fibrocartilage and elastic and differ based on the proportions of collagen and elastin. Cartilage is a stiff but flexible tissue that is good with weight bearing which is why it is found in our joints. Cartilage has almost no blood vessels and is very bad at repairing itself. Bone is full of blood vessels and is very good at self repair. It is the high water content that makes cartilage flexible.
The hip is formed where the thigh bone (femur) meets the three bones that make up the pelvis: the ilium, the pubis (pubic bone) and the ischium. You can feel the arching bones of the ilium by placing your hands on your waist. The pubis attaches to the lower part of the ilium and curves forward. The ischium is slightly behind the pubis. The three bones converge to form the acetabulum, a deep socket on the outer edge of the pelvis.
The shape of the acetabulum is a half of a sphere; the femoral head is about two-thirds of a sphere. Without weight bearing, the ball-and-socket are not completely congruent. As the joint bears more weight, the contact of the surface areas increases as does joint stability. The articular cartilage is thicker on the back part of the socket where most of the force is placed on the joint with walking, running and jumping. When standing, the body’s center of gravity passes through the center of the acetabula. Obviously, injury to the acetabulum can affect its ability to distribute weight bearing.
The hip joins the leg to the trunk of the body at the hip joint. The hip joint is made up of the ball of the femoral head that fits into the cup-shaped acetabulum. The large round head of the femur rotates and glides within the acetabulum. The depth of the acetabulum is further increased by a fibrocartilagenous labrum attached to the acetabulum. The socket of the hip is much deeper than the socket in the shoulder and encompasses a greater area of the ball.
The femur is the longest bone in the body. The neck of the femur connects the femoral head with the shaft of the femur. The capsular ligament of the hip joint attaches to the posterior part of the femoral neck. The neck ends at the greater and lesser trochanter prominences. The greater trochanter serves as the site of attachment for the abductor muscles. The lesser trochanter is the site of the iliopsosas tendon.
The greater trochanter is a very prominent bump on the femur and easy to feel on the outside of your thigh. It is the widest part of the lower legs and is where the tendons of several muscles attach including the gluteus, obturator, gemelli and piriformis muscles. The lesser trochanter serves as the attachment for the iliopsoas and iliacus muscle tendons.
The stability of the hip is increased by the strong ligaments that encircle the hip (the iliofemoral, pubofemoral, and ischiofemoral ligaments). These ligaments completely encompass the hip joint and form the joint capsule. The iliofemoral ligament is the strongest ligament in the body. Damage to the ligamentum teres can result in avascular necrosis because of injury to the small artery within the ligament that supplies most of the blood to the head of the femur. Death of the bone in the femoral head is one cause for hip replacement.
Muscles of the Hip
The muscles of the thigh and lower back work together to keep the hip stable, aligned and moving. It is the muscles of the hip that allow the 4 basic movements of the hip:
- flexion – bend
- extension – straighten
- abduction – take the leg away from the body
- adduction – bring the leg back toward the body
The hip muscles are divided up into three basic groups based on their location: anterior muscles (front), posterior (back), and medial (outside). The muscles of the anterior thigh make up the quadriceps group (vastus medialis, intermedius, lateralis and rectus femoris muscles). The quads make up about 70% of the thigh’s muscle mass. The purpose of the quads is flexion (bending) of the hip and extension (straightening) of the knee.
The gluteal, hamstring and piriformis muscles are located in the buttocks. The gluteus maximum is the main hip extensor and helps keep up the normal tone of the iliotibial band. The gluteus maximus also keeps the head of the femur from sliding forward in the hip socket; if it can’t do this, pain results form the femoral head pressing against the soft tissues in the front of the hip joint. The gluteal and sartorius muscles also help abduct the hip—that is, move the leg away from the midline of the body (using the spine as a midline reference point). It is abduction that allows us to walk sideways. When the glutes are weak, it is the hamstrings that pick up the slack. Hamstrings can be constantly strained injuries can take awhile to heal when the glutes (gluteus maximus and gluteus medius) are weak.
Adduction—bringing the leg back towards the midline—is performed by the hip adductor muscle group (gracilis muscle, pectineus muscle).
The hip also has the ability to rotate internally (medially)—turning the foot in (pigeon-toed) and externally (laterally)—turning the foot out. Medial rotation is needed for squatting. The piriformis muscle assist in lateral rotation of the hip. Lateral rotation is needed for crossing the legs.
The hip muscles do not attach right at the hip joint, thereby giving the hip more stability. The gluteus medius muscle connects to the greater trochanter, a bony prominence on the neck of the femur. The gluteus medius helps keep the pelvis level when you walk.
The facia lata, which is not a muscle but the deep fascia of the thigh, is known as the iliotibial band. The function of this band is to prevent dislocation of the hip. If this band is too tight, it can cause hip and knee problems.
Blood Vessels and Nerves of the Hip
The nerves in the hip supply the various muscles in the hip. These nerves include the femoral nerve, lateral femoral cutaneous nerve, and obturator nerve . The obturator nerve is also responsible for sensation over the thigh. The sciatic nerve is the most commonly recognized nerve in the hip and thigh. The sciatic nerve is large—as big around as your thumb—and travels beneath the gluteus maximus down the back of the leg and then branches on down to the foot. Hip dislocation can cause injury to the sciatic nerve. Nerves carry signals from the brainto the muscles to move the hip and carries signals from the muscles back to the brain about pain, pressure and temperature.
The blood supply to the hip is primarily from the internal and external iliac, femoral, obturator, and superior and inferior gluteal arteries. The femoral artery is well-known because of its use in cardiac cath; it travels from deep within the hip down the leg to the knee. The main blood supply for the femoral head comes from vessels that branch off the femoral artery.
Bursae are fluid filled sacs lined with a synovial membrane which produce synovial fluid. The synovial fluid is similar in consistency to raw egg white. Bursae are often found near joints. Their function is to lessen the friction between tendon and bone, ligament and bone, tendons and ligaments and between muscles. There are as many as 20 bursae around the hip. Inflammation or infection of the bursa called bursitis.
The greater trochanteric bursa is located between the greater trochanter (the bony prominence on the femur) and the muscles and tendons that cross over the greater trochanter. This bursa can get irritated if the iliotibial band is too tight. Two other bursa that can get inflamed are the iliopsoas bursa, located under the iliopsoas muscle and the bursa located over the ischial tuberosity (the bone you sit on).
Common Problems of the Hip
- Aseptic or Avascular necrosis
- Congenital Dislocation
- Perthes’ disease
- Aplasia of the acetabulum
- Coxa valga
- Coxa vara
- Dislocation (see image above of simple dislocation)
- Legg-Perthes disease
- Bone tumor