Femoroacetabular Impingement (FAI)

The hip is a ball-and-socket joint. The ball (femoral head) at the top of the thigh bone sits in the socket (acetabulum) of the pelvis. Both surfaces are covered in smooth articular cartilage, and the rim of the socket is lined by the labrum, a ring of tissue that deepens the socket and creates a suction seal around the ball. The entire joint is enclosed by the capsule, a strong layer of ligamentous tissue that provides stability.

Normally, the femoral head is a sphere that fits smoothly into the hemispherical socket. In FAI, the shape of the ball, the socket, or both is altered, which is a bit like trying to fit a square peg into a round hole. As the hip bends and rotates, the abnormal bone contacts the rim of the socket, creating friction or a block to motion. Over time, this repeated contact damages the labrum and cartilage and can lead to early arthritis.

FAI is classified into three types based on the source of the abnormal contact:

• Cam impingement: The femoral head is not perfectly round, with a bump of extra bone where the ball meets the neck. This bump jams into the socket when the hip bends, shearing the cartilage and labrum.
• Pincer impingement: The socket has extra bone along its rim or covers too much of the ball, pinching the labrum between the rim and the femoral neck.
• Combined impingement: Features of both cam and pincer impingement, which is the most common pattern.

Many people have an FAI bone shape without ever developing pain. FAI syndrome is diagnosed when that bone shape is accompanied by symptoms and exam findings, and it is the syndrome, not the x-ray appearance alone, that guides treatment.

FAI typically develops because of the shape of the bones, which may be present from birth or develop during adolescence. Contributing factors include:

• High-level sport during growth: Intense athletic activity during adolescence, while the growth plate at the top of the femur is still open, is associated with development of a cam deformity. This pattern is especially common in hockey, soccer, and basketball players.
• Repetitive hip loading: Athletes who repeatedly flex and rotate the hip, such as cyclists, hockey players, soccer players, and dancers, place ongoing stress on the joint that can bring on symptoms. FAI is also frequently seen in patients who participate in Pilates, yoga, running, and weightlifting.
• Developmental conditions: Childhood hip conditions such as Legg-Calvé-Perthes disease or a slipped capital femoral epiphysis (SCFE) can leave the hip with an abnormal shape and increase the risk of FAI later in life.
• Genetics: Some individuals are predisposed to these bone shapes based on hereditary factors, regardless of activity level.

FAI is particularly common in active individuals and athletes, but it can affect people of all ages and activity levels. Common symptoms include:

• Groin pain: A deep aching or sharp pain in the front of the hip or groin is the hallmark symptom, often worsening with bending, twisting, or athletic activity.
• Stiffness and reduced motion: Difficulty fully flexing or rotating the hip, particularly when squatting or bringing the knee toward the chest.
• Clicking, catching, or locking: A mechanical sensation in the hip during movement may indicate an associated labral tear.
• Pain after activity: Discomfort that intensifies after exercise, running, or extended periods of hip flexion.
• Pain with prolonged sitting: Discomfort when seated for long periods, such as during car rides or desk work, or when getting in and out of a car.
• Pain during sexual activity: FAI can significantly affect sexual function and quality of life, something patients frequently experience but rarely feel comfortable raising with their provider. Research led by Dr. Rynecki Baker found that FAI-related hip pain during intercourse is particularly impactful for patients. Dr. Rynecki Baker believes no symptom should go unaddressed and is committed to creating an environment where patients feel fully comfortable discussing how their hip pain affects every area of their life.

Left untreated, FAI can cause progressive damage to the labrum and articular cartilage, potentially accelerating the development of hip osteoarthritis.

Hip pain is nuanced, and FAI is frequently misdiagnosed. Pain in the groin or hip region is not always coming from the hip joint itself. It can be referred from the lumbar spine, or it can arise from the hip flexors, the gluteal muscles, the core, and other soft tissues surrounding the hip, all of which can closely mimic FAI.

Dr. Rynecki Baker begins with a detailed history of the location, onset, and pattern of pain, along with the activities that provoke it. Her physical exam assesses hip motion, strength, and gait, and includes specific maneuvers such as the impingement test, in which the hip is flexed and rotated inward to reproduce the pain. She also examines the spine and surrounding muscles to identify other contributors.

Imaging confirms the diagnosis. X-rays, including specialized views of the hip, show the shape of the ball and socket and allow measurement of the cam and pincer deformities and the amount of remaining joint space. MRI evaluates the labrum, cartilage, and surrounding soft tissues. A CT scan with 3D reconstruction may be used to map the exact location and size of the bony deformity and to measure rotation of the femur and socket. In some cases, an image-guided numbing injection into the hip helps confirm whether the joint itself is the source of pain.

Without a provider who is skilled in advanced physical examination and experienced in interpreting specialized imaging, the true source of hip pain can easily be missed, leading to delayed treatment or interventions that fail to address the underlying problem.

Dr. Rynecki Baker has earned multiple research awards for her work on FAI, including recognition from ISHA (The Hip Preservation Society) and NYU’s Marian Frauenthal Sloane Clinical Research Award for Outcomes in Hip Arthroscopy. Her collaboration with international experts in hip preservation allows her to work through diagnostic complexity with precision, distinguishing true FAI from other causes of hip pain so that each treatment plan targets the actual source of symptoms.