Introduction to Femoro-Acetabular Impingement FAI by Mauricio Herrera, M.D. Orthopedic Surgery & Sports Medicine

Femoroacetabular
is the term to describe the location where the femur (thigh bone) and the acetabulum (hip socket) come together. Impingement is a term that refers to pinching. Therefore, a diagnosis of femoroacetabular impingement indicates a pinching or pain in the hip joint where the femur meets the hip socket. A patient will point to their groin. Pain near the buttocks or back usually is NOT hip related, but related to the back instead.

Introduction 
In a normal hip, the head of the femur moves smoothly within the hip socket enabling movement without pain. With femoroacetabular impingement or FAI, the femur pinches against the cartilage in the hip socket impeding smooth movement and causing pain. In order to learn more about FAI, it helps to understand the normal anatomy of the hip.

Normal Hip Anatomy 
The thigh bone, femur, and the pelvis, acetabulum, join to form the hip joint. The hip joint is a “ball and socket” joint. The “ball” is the head of the femur, or thigh bone, and the “socket” is the cup shaped acetabulum. The joint surface is covered by a smooth articular surface that allows pain free movement in the joint.
The cartilage cushions the joint and allows the bones to move on each other with smooth movements. This cartilage does not show up on X-ray, therefore you can see a “joint space” between the femoral head and acetabular socket. The labrum is a ring of dense fibrocartilage tissue lining the outer rim of the hip socket giving depth and stability to the hip joint.

Pelvis 
The pelvis is a large, flattened, irregularly shaped bone, constricted in the center and expanded above and below. It consists of three parts: the ilium, ischium, and pubis.
The socket, acetabulum, is situated on the outer surface of the bone and joins to the head of the femur to form the hip joint.
Femur
The femur is the longest bone in the skeleton. It joins to the pelvis, acetabulum, to form the hip joint. The upper part is composed of the Femoral head, Femoral neck, and Greater and Lesser trochanters.

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