Hip impingement happens when the bones of the hip joint don’t fit together smoothly, causing them to collide during movement. The underlying cause is always structural: either the ball of the hip (femoral head), the socket (acetabulum), or both have an abnormal shape that creates friction and pinching where the joint should glide freely. What makes those bones develop that way involves a mix of genetics, how the hip grows during adolescence, and the physical demands placed on it during those critical years.
The Three Structural Patterns
Hip impingement comes in three forms, defined by where the abnormal bone sits. In a cam deformity, there’s an extra bump of bone at the junction of the femoral head and neck, usually along the front and top of the ball. Instead of being round, the femoral head is more egg-shaped, so it jams against the socket rim when you move your hip into certain positions, especially deep flexion combined with inward rotation.
A pincer deformity is the opposite problem. The socket itself has too much bony overhang along its front edge, covering more of the femoral head than it should. This over-coverage means the rim of the socket catches the neck of the femur during normal movement. Most people who develop symptoms actually have both patterns at once, a combination called mixed morphology, which accounts for roughly 70 to 75% of symptomatic cases.
How Bone Shape Develops During Growth
The shape of the femoral head and neck is largely established during adolescence, when the growth plate at the top of the thighbone is still open and actively forming new bone. The way forces travel through that growing bone plays a major role in whether a cam bump develops. Research using biomechanical modeling has shown that different loading patterns on the immature femur can trigger extra bone formation right at the spot where cam morphology typically appears, the front-outside edge of the head-neck junction.
The orientation of the growth plate itself matters, too. When the growth plate extends farther toward the femoral neck, it creates a stimulus for bone to build up in that anterolateral zone. The angle of the femur (whether it tilts more toward varus or valgus) also influences stress distribution through the growing bone, and a more varus-oriented femur appears to increase the risk. A five-year follow-up study found that these changes in femoral anatomy don’t happen before cam morphology appears. They develop at the same time, suggesting that the same biomechanical stresses driving cam formation also shape the broader geometry of the proximal femur.
Because the hip joint develops as an interplay between the femoral head and the socket, acetabular coverage can influence the process as well. A socket that provides more or less coverage changes how forces are distributed across the growing ball, potentially steering the development of cam morphology one way or the other.
Genetics and Family Risk
Bone shape runs in families. Siblings of people diagnosed with hip impingement have nearly three times the prevalence of cam deformity and clinical symptoms compared to the general population. The condition also varies by ethnicity, with differing prevalence between Asian and white populations, pointing to a genetic component beyond shared environment or activity levels.
Researchers have identified several genes linked to hip impingement and related conditions, including genes involved in collagen production, cartilage breakdown, and bone growth signaling. The inheritance pattern isn’t simple or predictable, but the familial clustering is strong enough that having a close relative with hip impingement meaningfully raises your own risk.
Sports and Repetitive Loading
The connection between high-impact sports during adolescence and the development of cam morphology is well established. Activities involving repeated sprinting, cutting, kicking, and deep squatting place large, repetitive loads on the hip during the exact window when the growth plate is most responsive to mechanical stress. Sports like soccer, hockey, basketball, and football are commonly associated with higher rates of cam deformity in young athletes.
The issue is specifically about timing. The same activities performed after the growth plate has closed (typically by the late teens or early twenties) don’t reshape the bone. But during the growth years, heavy and repeated loading can essentially “build in” a cam bump that remains for life. Among athletes, the prevalence of cam deformity on imaging is around 55%, compared to about 23% in the general population.
Gender Differences in Impingement Type
Men and women tend to develop different patterns. In a study of over 1,300 symptomatic hips, about 21% of men had a pure cam deformity compared to only 5% of women. Pincer-type impingement showed the reverse: 24% of women versus just 4.5% of men. Mixed morphology was the most common pattern in both sexes, affecting around 71 to 75% regardless of gender.
These differences likely reflect the underlying differences in pelvic and femoral anatomy between men and women, as well as differences in the types and intensity of physical loading during adolescence. Men tend to have a larger, less spherical femoral head-neck junction, while women tend to have greater acetabular coverage, which predisposes each group to its respective pattern.
Abnormal Bone Shape Doesn’t Always Mean Symptoms
One of the most important things to understand about hip impingement is that the bone shape alone is extremely common and often causes no problems at all. A systematic review of over 2,100 pain-free hips found that 37% had cam morphology on imaging, and 67% had pincer morphology. Among those asymptomatic hips, 68% even had labral changes visible on MRI.
This is why the international consensus known as the Warwick Agreement established that a diagnosis of femoroacetabular impingement syndrome requires three things to be present together: symptoms (typically motion-related or position-related pain in the hip or groin), clinical signs on physical examination (reproducing your familiar pain during specific tests), and imaging findings showing cam or pincer morphology. Bone shape on an X-ray, by itself, is not a diagnosis. The panel specifically emphasized that the presence of cam or pincer features without matching symptoms and exam findings does not constitute impingement syndrome.
What Happens Inside the Joint
When impingement does become symptomatic, the damage follows a predictable pattern. With each collision between the misshapen bone surfaces, the labrum (the ring of cartilage that lines the rim of the socket) gets caught and gradually torn. Cam deformities tend to lever the labrum up and away from the acetabular rim, peeling it off over time. This process also damages the smooth articular cartilage that covers the joint surfaces underneath.
The repetitive trauma is what links hip impingement to early osteoarthritis. Over a mean follow-up of nearly 25 years, about 14% of hips with impingement morphology developed symptomatic arthritis, and 4% eventually required hip replacement. The strongest risk factors for progressing to arthritis were male sex (roughly doubling the risk), a BMI above 29 (also doubling the risk), and increasing age. Interestingly, the specific shape of the bony deformity on X-ray did not predict who would develop arthritis, suggesting that factors like body weight and cumulative joint stress matter more than the size or type of the bump itself.
Positions and Movements That Provoke Pain
The positions that cause the most trouble are those that compress the femoral head-neck junction against the acetabular rim. Deep hip flexion combined with the knee moving inward (adduction and internal rotation) is the classic pain-provoking position. In daily life, this translates to pain with prolonged sitting, getting in and out of a car, squatting, or climbing stairs. In sports, sprinting, cutting, and kicking are common triggers.
The most widely used clinical test, called FADIR, reproduces exactly this position by bringing the hip into flexion, pulling it across the body, and rotating it inward. It’s sensitive, meaning it catches most cases, but not highly specific, so a positive test alone doesn’t confirm the diagnosis without supporting imaging.