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 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.

The relationship between hip shape and symptoms in FAI is fundamentally about mechanical load in specific positions. Biomechanical research indicates that hip and pelvis biomechanics can be altered in FAI syndrome even during tasks that don't reproduce the anterior impingement position - meaning movement patterns can change more globally, not just in deep flexion positions.

During deep hip flexion beyond approximately 90 degrees - common in activities like squatting, getting into cars, or certain yoga poses - the cam morphology (bony prominence on the femoral head-neck junction) makes premature contact with the acetabular rim and labrum. This creates abnormal shear forces on the labrum, which can lead to progressive damage over time. Research using motion capture and pressure sensors shows that in cam-type FAI, peak stress occurs during the transition from hip flexion to extension, particularly when combined with internal rotation.

The pincer morphology (acetabular over-coverage) creates a different mechanical problem: the deeper socket provides excessive anterior coverage, causing the acetabular rim to contact the femoral neck earlier in the flexion range. This can trap the labrum between the two bony surfaces, creating a pinching mechanism. Studies show that even moderate hip flexion angles (70-90 degrees) can generate impingement in pincer-type morphology.

Your body develops sophisticated compensation strategies to avoid these painful positions. One of the most common patterns I observe is excessive lumbar spine flexion during squatting - often called "butt-winking" - where instead of achieving the required hip flexion, you create extra movement through your lower back. This compensatory pattern explains why many FAI patients develop concurrent low back pain. Research demonstrates that FAI patients exhibit significantly reduced hip flexion range during functional tasks like squatting, with compensatory increases in anterior pelvic tilt and lumbar flexion.

The altered loading extends beyond the hip joint itself. Studies on gait mechanics in FAI show reduced hip extension during walking and running, leading to compensatory strategies including increased pelvic drop on the affected side and altered knee mechanics. These adaptations can create a cascade of issues up and down the kinetic chain - knee valgus collapse, reduced push-off power, and increased reliance on the quadriceps rather than the posterior chain muscles.

Activity demands significantly influence symptom development. Athletes in sports requiring repeated deep hip flexion and rotation - such as ice hockey goalies, soccer players performing kicks, and dancers - experience repetitive impingement forces that can accelerate tissue damage. A hockey goalie assumes a deep squat position thousands of times per game, each repetition creating potential impingement stress if cam or pincer morphology is present.