Femoroacetabular impingement (FAI) is caused by abnormal bone shapes in the hip joint that create harmful contact between the top of the thighbone and the hip socket during normal movement. The causes range from how your bones developed during adolescence to childhood hip conditions and genetic factors that influence hip shape. In most cases, the origin is multifactorial, meaning several causes overlap.
How Normal Hip Shape Goes Wrong
A healthy hip joint works like a ball and socket, with a smooth, rounded femoral head (the ball) gliding freely inside the acetabulum (the socket). FAI develops when either side of that joint has extra bone or an unusual shape that causes the two surfaces to collide during everyday movements like walking, squatting, or turning.
There are two main types of bone abnormality, and they often appear together. Cam morphology is a bump or flattening on the femoral head that destroys its normal roundness. When you move your hip, this bump jams into the socket rim instead of rotating smoothly. Pincer morphology is an overgrowth or misorientation of the socket itself, so the rim extends too far over the femoral head and catches the bone during motion. A high percentage of people with pincer-type FAI also have some degree of cam morphology, creating what’s called mixed FAI.
Youth Sports and Bone Development
The strongest evidence for what causes cam-type FAI points to high-intensity sports during adolescence. Between roughly ages 12 and 16, the growth plate near the top of the thighbone is still open and actively forming new bone. This growth plate is highly responsive to mechanical stress, more so than the mature skeleton. When adolescents train intensely in sports that demand repetitive, forceful hip movements, the developing bone appears to remodel in response, producing the bump characteristic of cam morphology.
The numbers are striking. In a study of adolescent basketball players, 89% had an abnormally shaped femoral head after their growth plates closed, compared to just 9% of non-athletes the same age. Among adolescent ice hockey players, 56% developed the same deformity after growth plate closure. Critically, these athletes started with normal bone shapes before puberty. The deformity formed during the window of skeletal maturation, then became permanent once growth stopped. Non-athletes in the same studies showed no change in bone shape over the same period.
Soccer, hockey, basketball, and football are the sports most frequently linked to cam development. The common thread is frequent, forceful hip flexion and rotation during the years when the skeleton is still growing. Training at least three times per week during adolescence appears to be the threshold that significantly raises risk. Once the growth plate closes and the bone hardens, the window for this type of remodeling closes too, which is why adult-onset sports participation doesn’t seem to create new cam deformities.
Genetic and Inherited Factors
Your genes likely play a role in determining hip shape, though researchers haven’t pinpointed a single “FAI gene.” Studies of specific genetic variants have found associations between certain gene markers and the shape of the proximal femur, the part of the thighbone closest to the hip. One variant in the FRZB gene has been linked to a specific femoral shape that increases the likelihood of developing hip arthritis. The current understanding is that genetics creates a predisposition, setting the baseline shape of your hip bones, while activity and mechanical loading during growth determine whether that predisposition becomes a clinically meaningful deformity.
Childhood Hip Conditions
Several childhood hip disorders can directly create the bone shapes that cause FAI later in life. Slipped capital femoral epiphysis (SCFE), a condition where the ball of the hip shifts out of its normal position on the growth plate, is one of the most recognized. In SCFE, the top of the thighbone slips backward and inward, which pushes the front of the bone into a prominent position. This prominence then collides with the front of the hip socket during normal movement, producing classic FAI symptoms. Even mild cases that heal well can leave enough residual deformity to cause impingement years or decades later.
Legg-Calvé-Perthes disease, a childhood condition where the blood supply to the femoral head is temporarily disrupted, can also reshape the ball of the hip as it heals and create a similar mismatch. In both conditions, the FAI is considered secondary, meaning it results from a prior problem rather than developing on its own.
Gender Differences in FAI Types
Cam and pincer morphologies don’t affect men and women equally. Cam deformities are more common in young, active males, which aligns with the sports-related development pathway during adolescence. When both men and women have cam morphology, men tend to have larger deformities. Studies of symptomatic patients found average alpha angles (the measurement used to quantify the bump) of about 71 degrees in men versus 58 degrees in women. For reference, anything above 55 degrees is considered abnormal.
Pincer morphology, on the other hand, is more commonly diagnosed in middle-aged women. The causes of pincer impingement are broader and include anatomical abnormalities of the socket, abnormal orientation of the acetabulum, and torsional issues in the femoral shaft. These can be constitutional (present from birth), result from previous trauma, or develop after surgery.
Having the Bone Shape Doesn’t Mean Having Symptoms
One important nuance: many people have FAI bone morphology and never experience pain. Imaging studies of adults with no hip complaints found cam-type morphology in roughly 28% of hips and pincer-type morphology in about 9%. This means the abnormal bone shape alone isn’t enough to cause problems. Symptoms typically emerge when the bone abnormality combines with activities that push the hip into impingement positions repeatedly, such as deep squatting, pivoting, or prolonged sitting with the hips flexed.
How Abnormal Contact Damages the Joint
When the misshapen bone collides with the socket rim during movement, the first structure to suffer is usually the labrum, a ring of cartilage that lines the edge of the hip socket and helps seal the joint. The repeated impact frays and tears the labrum, which is often the source of the sharp, catching pain that brings people to a doctor.
The damage pattern differs by FAI type. With cam morphology, the non-spherical femoral head pushes into the socket and generates shearing forces that peel the cartilage away from the underlying bone, a process called delamination. The cartilage peels from the outside in, starting at the labral junction. With pincer morphology, the damage is more focused at the contact point, typically the front-top portion of the socket, where repeated compression crushes the cartilage against the labrum.
Over time, this progressive damage to the labrum and cartilage can lead to hip osteoarthritis. A prospective study published in the British Journal of Sports Medicine found that symptomatic FAI with cam morphology carried nearly seven times the odds of developing hip osteoarthritis within 10 years. The absolute risk was high: 81% of people with FAI syndrome developed some degree of hip arthritis, and 33% progressed to end-stage arthritis requiring joint replacement within a decade. This makes FAI one of the most significant identified risk factors for hip arthritis in younger and middle-aged adults.
Activity and Occupation as Triggers
While the underlying bone shape typically forms during youth, the activities you do as an adult determine whether that shape becomes symptomatic. Elite athletes have higher rates of FAI than the general population, and specific movement patterns are known to provoke impingement. Deep hip flexion (bringing the knee toward the chest), internal rotation, and combined movements like squatting or lunging push the femoral head into the socket rim. Occupations or fitness routines that involve these positions repeatedly can trigger or worsen symptoms in someone with underlying FAI morphology, even if they were previously pain-free.