Your hip is the joint where your thigh bone meets your pelvis. It is called a ball-and-socket joint, because the ball-like top of your thigh bone fits into a cup-like area within your pelvis, much like a baseball fits into a glove.
Normally, the ball glides smoothly within the socket, but a problem with the ball or socket rim can interfere with smooth motion. This problem can cause hip impingement or femoro acetabular impingement (FAI). It is believed to be a major cause of early osteoarthritis of the hip, particularly in those under age 40.
Hip Impingement Symptoms
You can have hip impingement for years and not know it, because it is often not painful in its early stages.
When hip impingement causes symptoms, it may be referred to as hip impingement syndrome. The main symptoms are stiffness in the groin or front of the thigh and/or a loss of your hip's full range of motion.
At first, you may only feel pain when you move the hip near its limits. As the condition progresses, however, you may feel pain with more subtle activities, such as sitting for a long time or walking up a hill. Pain that occurs at night or when walking on flat ground suggests that the cartilage cushioning the ball and socket has begun to break down and wear away, a condition known as osteoarthritis.
Hip Impingement Causes
There are two main causes of hip impingement:
A deformity of the ball at the top of the femur (called cam impingement). If the head is not shaped normally, the abnormal part of the head can jam in the socket when the hip is bent. This may occur during activities such as riding a bicycle or tying your shoes.
A deformity of the socket (pincer impingement). If the front rim of the socket (called the acetabulum) sticks out too far, the area of the thigh bone (femur) just below the ball, called the neck of the femur, may bump into the rim of the socket during normal hip flexion movement.
In some cases, there is a problem with both the ball and the socket. Other problems that can cause hip impingement include conditions such as:
- Legg-Calve-Perthes disease, a disease in which the ball part of the hip joint doesn't get enough blood, which causes the bone to die.
- Slipped capital femoral epiphysis, a separation of the ball from the thigh bone at the upper growing end (growth plate) of the bone in adolescents. It is more common in children who are obese.
- Coxa vara, an unusual condition in which the thigh bone and ball do not grow at the same pace in children. This discrepancy leads to deformity of the hip joint.