People who have hip impingement, also known as femoroacetabular impingement, experience hip pain and notice a decrease in the hip’s range of motion. Anyone can have hip impingement, though it is often diagnosed in active people in their 20s, 30s and 40s.
Hip impingement occurs when there is abnormal contact between the hip’s bones. This page discusses hip anatomy, the different types of hip impingement, and how hip impingement may lead to hip osteoarthritis and labral tears.
The hip is a ball-and-socket joint.
- The ball (femoral head) is the rounded top of the femur, commonly known as the thighbone.
- The socket (acetabulum) is located in the pelvis.
The femoral head fits into the socket. The ball-and-socket anatomy allows the leg to move forward, backward, and side-to-side. It also allows for internal and external rotation (pointing the toes inward and outward).
Cartilage helps stabilize the hip joint and facilitate hip movement.
- Articular cartilage. Both the femoral head and the acetabulum are lined with a strong, slippery material called articular cartilage. This cartilage allows the surfaces of the ball and socket to glide against each other during hip movement.
- Labrum. Another piece of cartilage, called the labrum, rings the outer edge of the acetabulum. The labrum deepens the socket, making the hip joint more stable, and its elasticity allows for flexibility.
Understanding hip anatomy is necessary to understanding hip impingement and its possible long-term effects.
3 Types of Hip Impingement
In most active adults, hip impingement is caused by abnormal bone growth. The abnormal growth can occur near the femoral head, the acetabulum, or both. The location of the abnormal growth determines what type of impingent occurs:
1. Cam. A cam impingement occurs when excessive bone grows at the edge of the femoral head, where it meets a part of the femur called the femoral neck. The excessive bone growth forms a bump, which can do one or both of the following:
- Prevent the femoral head from fully rotating in its socket.
- Rub against the cartilage located inside the hip’s socket, causing damage to the cartilage.
Cam impingement is most common in young men.1
2. Pincer. A pincer impingement occurs when excessive bone grows at the edge of the hip’s socket. The excess bone creates an overhang, making the socket too deep in certain places. The excessive bone tissue can
- Prevent the femoral head from rotating in its socket.
- Cause the hip’s labrum to become pinched.
3. Combined. Many people have both cam and pincer hip impingement.1
A doctor can use medical imaging to determine which type of hip impingement a patient has.
Hip Impingement and Osteoarthritis
When the hip’s ball and socket do not fit together properly, the articular cartilage that covers their surfaces can undergo excessive mechanical friction. This friction wears away the cartilage. In some cases, the hip cartilage may become partially separated from the bone, creating a flap. The degeneration of articular cartilage is a defining characteristic of hip osteoarthritis.
Not everyone who has impingement will develop arthritis. In fact, one study suggests that pincer impingement protects the hip from developing osteoarthritis.3
Most experts agree that there is a close association between hip impingement and hip osteoarthritis. Why most people with hip impingement develop osteoarthritis but others do not is not well understood.
Hip Impingement and Labral Tears
The labrum is a ring of cartilage that deepens the hip socket. When the femoral head (ball) or acetabulum (socket) grows excess bone, movement between the two bones can cause the labrum to become pinched.
Repetitive pinching can cause the hip labrum to fray or tear, which can result in pain and decreased range of motion. In addition, damage to the labrum can destabilize the hip joint and lead to further joint damage.