Femoroacetabular impingement, often called FAI or hip impingement, is a condition where abnormally shaped bones in the hip joint collide during movement. Extra bone along the femoral head (the ball) or the acetabulum (the socket) creates friction that damages cartilage and soft tissue over time. It’s surprisingly common: imaging studies find signs of FAI in roughly 23% of the general population and up to 55% of athletes, though many of these people never develop symptoms.
How the Hip Joint Normally Works
Your hip is a ball-and-socket joint. The rounded top of the thighbone fits snugly into a cup-shaped cavity in the pelvis, and a ring of tough, flexible tissue called the labrum lines the rim of that socket to create a seal. This seal keeps lubricating fluid inside the joint, distributes pressure evenly, and helps hold the ball in place. In a healthy hip, the ball glides smoothly within the socket through a wide range of motion.
In FAI, bony irregularities on either the ball or the socket (or both) cause the two surfaces to collide, especially when you bend the hip, bring your knee inward, or rotate it. Over time, this repeated contact damages the labrum, grinds down cartilage, and can set the stage for arthritis.
Cam, Pincer, and Combined Types
FAI comes in two distinct patterns, and some people have elements of both.
Cam type involves a bump or flattening at the junction where the femoral head meets the neck of the thighbone. Instead of being perfectly round, the ball is shaped more like a cam on a rotating shaft. When the hip flexes, this extra bone jams into the socket and shears the cartilage from the inside out. Cam-type FAI primarily damages the cartilage surface rather than the labrum, and research shows it develops more often in people who were highly active during adolescence. Structurally, cam hips tend to be slightly shallower than normal.
Pincer type is the opposite problem. Here, the socket itself is too deep or angled too far forward, so its rim extends farther over the ball than it should. When the hip moves, the neck of the thighbone levers against this overhanging rim, crushing the labrum and eventually causing wear on the cartilage at the back of the joint. Pincer hips are measurably deeper than normal at every point around the socket.
Combined type features both a cam bump on the femur and overcoverage on the acetabulum. Many people diagnosed with FAI have some degree of both, though the cam and pincer components are distinct problems with different damage patterns rather than a single blended condition.
What FAI Feels Like
The hallmark symptom is a deep, dull ache in the hip that can spread into the groin, buttock, or thigh. Many people describe it as feeling like a deep bruise that someone is constantly pressing on. The pain typically worsens with activities that push the hip into deep flexion or rotation: squatting, lunging, jumping, or pivoting. Sitting for long periods or lying on the affected side can also flare it up, sometimes shifting the sensation from a dull ache to something sharper and more stabbing.
Stiffness is common, particularly after periods of inactivity. Some people notice a limp developing gradually, and certain movements that were once easy, like tying shoes or getting in and out of a car, start to feel restricted or painful.
Other Conditions That Cause Similar Pain
Deep groin and hip pain has a long list of potential causes, and several conditions can look and feel a lot like FAI. Labral tears can occur independently. Stress fractures of the pubic bone or femoral neck produce overlapping symptoms. Tendon strains in the hip flexors, adductors, or abdominal muscles are common mimics, especially in athletes. Bursitis around the hip, nerve entrapment, and even inguinal hernias can all cause groin-area pain. In some athletes, FAI and athletic pubalgia (sometimes called a sports hernia) coexist, which complicates diagnosis. Proper imaging is essential for sorting these out.
How FAI Is Diagnosed
Diagnosis starts with a physical exam. The classic provocation test involves flexing the hip, bringing the knee toward the opposite shoulder, and rotating the leg inward. If this reproduces your pain, it strongly suggests impingement.
X-rays remain the first-line imaging tool and can reveal the bony abnormalities responsible for each type. For cam impingement, clinicians measure the alpha angle, which quantifies how round the femoral head is at the head-neck junction. An alpha angle above roughly 50.5 degrees, confirmed on MRI, is the standard threshold for diagnosing a cam deformity. MRI provides more detail about the labrum and cartilage and helps determine how much soft tissue damage has already occurred.
One important nuance: many people have FAI-type bone shapes on imaging but no symptoms at all. Asymptomatic FAI morphology shows up in about 30% of the general population, with higher rates in men (around 46%) than women (around 17%). The diagnosis of FAI syndrome specifically requires both the abnormal bone shape and the clinical symptoms to match.
Long-Term Risk of Arthritis
Left unaddressed, symptomatic FAI significantly increases the risk of hip osteoarthritis. A prospective study published in the British Journal of Sports Medicine followed middle-aged individuals over 10 years and found that FAI syndrome with cam morphology carried nearly seven times the odds of developing hip osteoarthritis compared to hips without impingement. The absolute numbers were striking: 81% of people with FAI syndrome developed some degree of hip arthritis within a decade, and 33% progressed to end-stage disease severe enough to potentially warrant joint replacement. This makes early recognition and management more than just a matter of comfort.
Conservative Treatment
The good news is that about 70% of people with FAI syndrome respond well to structured, non-surgical treatment combining physical therapy, activity modification, and rest. The goals of conservative care center on four principles: improving postural alignment, building core endurance, strengthening the hip muscles, and increasing flexibility in the muscles that attach to the hip and pelvis.
Hip abductor weakness is a consistent finding in people with FAI. The three main muscles on the outside of the hip tend to be underpowered, and another muscle (one that runs along the outer thigh) compensates by working overtime, which changes movement patterns and can perpetuate symptoms. Supervised physical therapy programs that target hip and core strength consistently produce better outcomes than unsupervised exercise.
Day-to-day modifications also help. Avoiding sitting cross-legged, limiting deep squats, and breaking up long periods of sitting can all reduce irritation. The key is minimizing positions that combine hip flexion with inward rotation, since that’s the motion pattern that drives impingement.
When Surgery Becomes the Answer
If several months of dedicated conservative treatment haven’t resolved symptoms, hip arthroscopy is the standard surgical approach. The procedure is minimally invasive, performed through small incisions with a camera and specialized instruments. What the surgeon does depends on the type of impingement.
For cam deformities, the surgeon shaves down the bony bump at the femoral head-neck junction to restore its spherical shape, a procedure called osteoplasty. For pincer impingement, the overhanging rim of the socket is trimmed back. If the labrum has been torn, it can often be repaired by reattaching it to the rim with small anchors and sutures. In cases where the labrum is too damaged to salvage, reconstruction using a graft is an option.
Recovery follows a structured rehabilitation protocol, typically spanning about 24 weeks across five phases. Early phases focus on protected weight-bearing and gentle range of motion. Later phases progressively add strengthening, sport-specific movements, and cardiovascular conditioning. In a study of NCAA Division I athletes, return to sport evaluation occurred at an average of about two years after surgery, reflecting the time needed for full recovery and confidence in the joint. For non-athletes, the timeline to full, pain-free activity is often shorter, but the rehabilitation process still requires patience and consistency.