There are many well recognized hip and groin injuries that affect athletes. One hip injury you hear about more recently is femoroacetabular impingement, often referred to as FAI. So what exactly is FAI? Dr. Struan Coleman, orthopedic surgeon at Vincera, explains the anatomy of the hip and what exactly occurs when you have FAI.
Anatomy of the Hip Joint
The hip joint is a "ball and socket" joint located where the thigh bone (femur) meets the pelvic bone. The upper segment ("head") of the femur is a round ball that fits inside the cavity in the pelvic bone that forms the socket, also known as the acetabulum. The ball is normally held in the socket by very powerful ligaments that form a complete sleeve around the joint capsule.
Both the ball and socket are covered with a layer of smooth cartilage, each about 1/8 inches thick. The cartilage acts as a sponge to cushion the joint, allowing the bones to slide against each other with very little friction. Additionally, the depth of the acetabulum (socket) is increased by a fibrocartilaginous rim called a labrum that lines the rim of the socket and grips the head of the femur, securing it in the joint. The labrum acts as an "o-ring" or a gasket to ensure the ball fits into the socket.
Hip Mobility and Femoro-acetabular Impingement
What is femoro-acetabular impingement?
Femoro-acetabular impingement (FAI) occurs when the ball (head of the femur) does not have its full range of motion within the socket (acetabulum of the pelvis).
Impingement itself is the premature and improper collision or impact between the head and/or neck of the femur and the acetabulum. This causes a decreased range of hip joint motion, in addition to pain. Most commonly, FAI is a result of excess bone that has formed around the head and/or neck of the femur, otherwise known as "cam"-type impingement. FAI also commonly occurs due to overgrowth of the acetabular (socket) rim, otherwise known as "pincer"-type impingement, or when the socket is angled in such a way that abnormal impact occurs between the femur and the rim of the acetabulum.
A) Normal Hip
B) Cam impingement
C) Pincer impingement
D) Combination of cam and pincer impingement
What happens inside a hip joint that has impingement?
When the extra bone on the femoral head and/or neck hits the rim of the acetabulum, the cartilage and labrum that line the acetabulum can be damaged.
The extra bone can appear on x-rays as a seemingly very small "bump." However, when the bump repeatedly rubs against the cartilage and labrum (which serve to cushion the impact between the ball and socket), the cartilage and labrum can fray or tear, resulting in pain. As more cartilage and labrum is lost, the bone of the femur will impact with the bone of the pelvis. This "bone on bone" notion is most commonly known as arthritis.
Tears of the labrum can also fold into the joint space, further restricting motion of the hip and causing additional pain. This is similar to what occurs in the knee of someone with a torn meniscus.
How does femoroacetabular impingement occur?
The extra bone that leads to impingement is often the result of normal bone growth and development. Cam-type impingement is when such development leads to the bump of bone on the femoral head and/or neck.
Normal development can also result in the overgrowth of the acetabular rim, or pincer-type impingement. Hip trauma (falling on one's hip) can also lead to impingement. The tears of the labrum and/or cartilage are often the result of athletic activities that involve repetitive pivoting movements or repetitive hip flexion.
What are the common symptoms associated with impingement?
Impingement can present at any time between the teenage years and middle age. Many people first realize a pain in the front of their hip (groin) after prolonged sitting or walking. Walking uphill is also found to be difficult.
The pain can be a consistent dull ache or a catching and/or sharp, popping sensation. Pain can also be felt along the side of the thigh and in the buttocks.
Provided by Struan Coleman, MD, PhD