Femoroacetabular impingement (FAI)

What it means

The hip is a ball-and-socket joint: the rounded head of the thigh bone (femur) sits inside a cup formed by the pelvis (the acetabulum). In a small but common share of people, one or both of these surfaces develops a shape that doesn't leave quite enough clearance, so the ball and the rim of the socket catch against each other during deep or rotational movement. Radiologists describe two patterns, often present together: "cam" impingement, where the femoral head has a bony bump instead of being perfectly round, and "pincer" impingement, where the socket rim extends further over the ball than usual.

Why it appears on a CT or MRI report

MRI, sometimes combined with a joint injection (MR arthrogram), gives the most detail, while CT is often used to map bone shape precisely when surgery is being planned. Reports describe the shape of the femoral head-neck junction (often using a measurement called the alpha angle), the depth and coverage of the socket, and — importantly — whether the labrum, the ring of cartilage lining the socket rim, shows a tear or the joint cartilage shows early wear. The combination of cam or pincer shape plus labral or cartilage damage is what ties the finding to hip pain.

What it usually means

Cam and pincer shapes are surprisingly common in the general population, including in people with no hip symptoms at all — studies of young athletes regularly find this bone shape on scans of pain-free hips, especially in sports involving repetitive hip flexion like football, hockey, and dance. When it is symptomatic, the typical pattern is groin pain that builds with deep hip flexion — sitting low, getting out of a car, tying shoes, deep squats — sometimes with catching, clicking, or a feeling of instability. Most people improve with physical therapy aimed at hip and core control, activity modification, and time. Surgery to reshape the bone and repair labral damage (often done arthroscopically) is considered for those who don't improve with a genuine trial of conservative care, and results tend to be best when imaging findings and symptoms line up clearly, rather than the shape being an incidental finding.

When to follow up

If the report notes cam or pincer morphology but your hip doesn't bother you, this is frequently an incidental anatomical variant that needs no action. If you do have groin pain, clicking, or reduced range of motion, a course of physiotherapy focused on hip mechanics is a reasonable first step. See an orthopaedic or sports medicine specialist if pain persists despite several months of rehab, limits sport or daily activity, or if the hip catches, locks, or gives way. There is ongoing debate about whether untreated FAI raises the long-term risk of hip osteoarthritis, so it's worth mentioning to your clinician even if symptoms are mild.

A plain-language way to picture it

Picture a doorknob that's meant to spin freely inside its socket plate. If the knob has a slightly flattened edge, or the plate's opening is a touch too narrow, turning the knob all the way catches at one point in its rotation instead of moving smoothly. The hip works the same way — most of the time you never reach that catching point in daily movement, but deep bending or twisting can find it, and repeated catching is what wears at the soft tissue around the rim over time.