Hip Anatomy - The Acetabular Joint

The hip bone, also known as the innominate bone, coxal bone or os coxae, is a large bone that sits in the pelvis. It is formed by the bones of the pelvis including the ilium, pubis and ischium.

The Hip Joint

The hip joint, or acetabulum, is responsible for many movements including walking, bending and crouching. It is a ball and socket joint, with the femur (top of the leg bone) sitting inside the acetabulum (hip socket).

The head of the femur is a ball like bone structure that attaches to the rest of the femur by a section of bone called the femoral neck and sits inside the acetabular fossa. The greater trochanter is located at the top, outside edge of the femur and you can feel it when you push against the side of your hip. The lesser trochanter is located on the inside of the femur, just below the neck of the femur at the joint.

The hip socket and femoral head are both lined with articular cartilage that allows the bones to glide together easily with little friction. Articular cartilage in the hip joint is approximately 1/4" thick but gets thicker near the back of the socket where it takes most of the weight as you walk or run. It has a rubbery consistency making it able to absorb shock.

The acetabulum is also lined with a unique structure called the labrum which is a piece of fibrocartilage that forms a cup-like ring within the joint. It helps the femur sit properly in the socket, making the hip more stable. The hip joint is surrounded by other connective tissue including powerful muscles, ligaments and tendons that also stabilize the femur in the socket during it's wide range of powerful movements.

Ligaments of the Hip Joint

The strong ligaments of the hip attach one bone to another and are essential for stabilizing the complex movements of the hip.

The ligaments surrounding the joint (the ischiofemoral, iliofemoral, and pubofemoral ligaments) make up the joint capsule of the hip and help control the movement of the hip.

The ischiofemoral ligament attaches to the ischium of the pelvis and the intertrochanter line of the femur.

The iliofemoral ligament is the strongest ligament in the body and is an upside down "Y" shape. It starts at the anterior inferior iliac spine and the rim of the acetabulum and attaches to the anterior side of femur. It splits to join at both the neck of the femur and further down along the shaft, giving the ligament its "Y" formation. The iliofemoral ligament strengthens the hip and helps to prevent the joint from hyperextension.

The pubofemoral ligament starts at the superior ramus of the pubis and attaches to the intertrochanteric line of the femur.

There are two ligaments inside the joint capsule. One is the teres ligament, which connects the top of the femoral head to the acetabulum and contains a small artery that supplies blood to the femur. The second is the transverse acetabular ligament. It is an extension of the labrum, and is a bridge to the acetabular notch.

Bursae in the Hip

Bursae (plural for bursa) are flat, fluid filled sacs that function as cushions between your bones and the muscles (deep bursae) or bones and tendons (superficial bursae) to reduce friction and allow your soft tissue to slide over bone effortlessly during muscle contraction. They are lined with synovial cells that secret fluid that is rich in protein and collagen and act as the lubricant between areas in your body where friction is greatest.

There are approximately 160 bursae in our bodies, however, not everyone has the same number of bursae. Some bursae we are born with and some occur as a result of excess rubbing in a particular area. These bursae form initially to protect the area from the friction. The major bursae in the body are located adjacent to tendons and muscles near larger joints, such as in the shoulders, elbows, hips, and knees. However, not all tendons have bursa and bursae can also form in smaller joints like your toes.

Common bursae in the hip include the deep trochanteric bursa, traochanteric bursa, iliopsoas bursa, ischiogluteal bursa, gluteus medius bursa, and the ischial turbosity bursa.

Tendons and Muscles of the Hip Joint

The IT band (also called the iliotibial tract, ITB or iliotibial band) is a long, thick tendon that runs from the pelvis, along the outside of the hip and femur, down to the knee. Several hip muscles connect to the iliotibial band and it's essential in stabilizing your knee while walking and running.

The iliopsoas is a deep hip flexor tendon that passes along the front of the hip joint. The iliopsoas tendon attaches the iliopsoas muscle (iliacus, psoas major, and psoas minor muscles) to the femur at the lesser trochanter and is the primary hip flexor muscle.

There are four groups of muscles in the hip that are responsible for moving the leg in several directions.

The iliopsoas is a combination of 3 muscles: the psoas major, psoas minor, and iliacus. The rectus femoris is one of the four quadriceps muscles. These muscles work together to bring the leg forward (hip flexion).

The hamstring muscles (semitendinosus, biceps femoris, semimembranosus) and gluteus maximus allow you to move your leg backward, behind your body from your hip (extension).

The adductor magnus and adductor longus draw the leg toward the inside of the body.

The gluteus medius, gluteus minimus and tensor fascia latae (attached to the IT band) are used to abduct your leg away from the body. The tensor fascia latae (TFL) also assist with rotating your hip inward and hip flexion. This is also the group of muscles responsible for stabilizing the hip, especially with weight bearing activities.

The deep muscles of the hip include the piriformis, the obturator internus, the obturator externus, the gemellus superior, the gemellus inferior and the quadratus femoris.

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