Hip Structure, Function and Common Problems

A joint is formed where two or more bones meet. The hip joint is a ball-and-socket type joint and is formed where the thighbone (femur) meets the hipbone (pelvis). The thighbone has a ball-shaped knob on the end that fits into a socket formed in the hipbone. A smooth cushion of articular cartilage covers the ends of the bones. 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 hold the bones (ball and socket) in place.

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 effect your gait and place abnormal stress on joints above and below the hip.

Anatomic Terms

Let’s define some anatomic terms surgeons use as these terms apply to the hip:

• Anterior — the abdominal side 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; undersurface

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.

Bony Structures of the Hip

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 depth of the acetabulum is increased by a fibrocartilagenous labrum. The socket of the hip is much deeper than the socket in the shoulder and encompasses a greater area of the ball.

The acetabulum is formed by three pelvic bones—the ileum, ischium and pubis. 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. When standing, the body’s center of gravity passes through the center of the acetabula. Obviously, injury to the acetabulum can affect its function of distributing weight bearing.

The hip joint capsule is a dense, fibrous structure which includes the iliofemoral, pubofemoral, and ischiofemoral ligaments. These ligament along with the ligamentum teres and the labrum help with the stability of the hip.

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 portion 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 very prominent and easy to feel on the lateral 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.

Muscles of the Hip

It is the muscles of the hip that allow the 4 basic movements of the hip:
• flexion – bend
• extension – straighten
• abduction – take away from the body
• adduction. – bring back toward the body

The hip muscles are divided up into three basic groups based on their location: anterior, posterior, and medial. The muscles of the anterior thigh make up the quadriceps group. 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 predominant hip extensor and helps maintain the normal tone of the iliotibial band. 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.

Adduction—bringing the leg back towards the midline—is performed by the hip adductor muscle group.

The hip also has the ability to rotate internally (medially)—turning the foot inward (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 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.

Neurovascular Structures of the Hip

The sciatic nerve is located where it could sustain injury from a posterior dislocation of the femoral head.

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.

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.

Ligaments

The stability of the hip is increased by the strong ligaments that encircle the hip. These ligaments completely encompass the hip joint. 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 artery 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.

Bursae

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 is called bursitis.

Common Problems of the Hip

• Aseptic or Avascular necrosis
• Congenital Dislocation
• Perthes’ disease
• Aplasia of the acetabulum
• Coxa valga
• Coxa vara
• Osteoarthritis
• Dislocation

Surgery of the Hip

• Hip Replacement
• Sex After Total Joint Replacement

Related posts:

1. Shoulder Structure, Function and Common Problems
2. Knee Joint Structure, Function and Problems
3. Hip Joint Replacement Surgery
4. Total Hip Replacement: a guide for surgery and recovery