The Science of Femoroacetabular Impingement (FAI)
Femoroacetabular Impingement (FAI) syndrome involves abnormal contact between the femoral neck and acetabular rim during hip movement, specifically during deep hip flexion and internal rotation. The biggest misconception is that having a certain hip shape (a "cam" or "pincer" morphology on an X-ray) automatically means you will have pain - this is false. Many elite athletes and pain-free individuals have these shapes. FAI is a syndrome, which means it's the combination of a specific hip shape plus symptoms plus clinical signs. The shape itself is not the problem; the problem is how you are loading that shape. Two main types exist: CAM (extra bone on femoral head-neck junction creating a "bump" that makes contact with the socket during deep flexion) and PINCER (deep acetabular socket where the socket is too deep, causing the rim to contact the femoral neck). Mixed types with both morphologies are common. The cam shape can make contact with the socket and labrum (cartilaginous ring around the socket) during deep hip flexion, leading to a pinching sensation and potential stress on the labrum. To avoid the pinching sensation, the body develops clever compensation strategies. A common one is to create extra movement through the low back and pelvis - instead of flexing at the hip to squat, a person might excessively round their lumbar spine (butt-winking). Over time, this can lead to low back pain. Similarly, a stiff and painful hip can cause the knee to collapse inwards during activities like running or landing, potentially contributing to knee pain. When a specific movement consistently causes sharp pain, the brain learns to fear and avoid it. This leads to protective muscle guarding, where muscles around the hip (like hip flexors and adductors) become chronically tense in anticipation of pain. This tension can then become a secondary source of pain itself.