Femoroacetabular implants

Understanding the hip joint

The hip is the largest weight-bearing joint other than the knee. The hip is a ball-and-socket joint in which a ball at the top of the thighbone (the femur) fits into a socket in a hollow in the pelvis. The socket, called the acetabulum, is formed by the joining of three pelvic bones.

The labrum, a rim of fibrocartilage, forms a gasket around the socket, holding the ball in place with a tight seal.

A thick layer of ligament called the capsule surrounds the joint, and both surfaces of the joint are covered with a cushioning layer of cartilage. A thin layer called the synovial membrane provides lubrication.

The joint is usually not fully hardened into bone until about age 25.

What is FAI?

FAI occurs when the bones of the femur and the pelvis don’t fit together properly, causing pain and damaging cartilage in the joint. This happens because of deformation of the ball on the femur or deformation of the socket (acetabulum), or both.

There are three types of FAI:

• Pincer, in which the socket extends too far around the ball on the femur. This causes the labrum to be crushed under the rim of the socket.
• Cam, in which the ball on the femur is not shaped properly. Rather than rotating freely, it grinds against the cartilage of the joint.
• Combined, in which both types of FAI are present.

These conditions cause abnormal contact between the hip bones that wears down the labrum and tears the cartilage. Also, the cartilage that lines and protects the socket might start to come off the bone.

Over time, the hip degenerates, leading to osteoarthritis.

Causes & symptoms

Pain and stiffness in the hip, and limping as a result, might indicate FAI. The pain might be in the groin area or toward the outside of the hip. Pain might radiate down the thigh.

FAI might flare up because of activities such as sports that apply unusual amounts of force to the hip. Genetics and pediatric hip diseases also might be to blame.

In some patients, sitting, driving or squatting can make the symptoms worse.

Diagnosis & treatment

An impingement test, in which the doctor pushes the patient’s knee toward his chest and then rotates the knee inward, can help identify FAI.

Numbing medication might be injected into the joint; if it relieves the pain, that can show that FAI is causing the pain.

X-ray, a CT scan, or MRI might help reveal the source of pain.

Treatment might include:

• Nonsurgical options such as stretching, strengthening and endurance exercises.
• Arthroscopy, in which a tiny camera and miniature surgical instruments are inserted into the hip, can be used to reshape the ball or the socket or to repair cartilage.
• Surgical dislocation of the hip, in which the ball is taken completely out of the socket so that extensive repairs can be made.
• Cutting the acetabulum completely away from the pelvis in order to move it into proper position.

After surgery, the patient might need to be on crutches for as long as two months, in addition to undergoing physical therapy.

Resources

American Academy of Orthopaedic Surgeons: orthoinfo.org