Impingement

What it means

Inside a healthy joint, the moving parts glide past each other with a little room to spare. Impingement is what radiologists call it when those parts crowd one another instead — a tendon catches under a bony arch, a piece of cartilage gets nipped between two bones, or the ball of the joint bumps into the rim of its socket during certain movements. The picture on the scan often shows the structures involved and any wear they have caused over time: thickened bone, irritated tendons, frayed cartilage, or fluid.

Why it appears on a CT or MRI report

The shoulder and the hip are the two most common sites. In the shoulder, reports describe pinching of the rotator cuff or the small fluid pouch above it under the bony arch (subacromial impingement), often noting the shape of the bone above. In the hip, femoroacetabular impingement (FAI) describes a mismatch between the ball and socket — "cam" when the ball has a non-spherical bump, "pincer" when the rim of the socket extends too far. Reports usually list contributing bone shape, any cartilage or labral injury, and tendon changes nearby.

What it usually means

The picture on imaging is more common than symptoms — many people have the bone shapes or tendon changes described, with no pain at all. Pain-free adults, particularly active ones, often have hip cam morphology or rotator cuff changes on scans of joints that feel completely normal. When the condition does cause symptoms, it typically shows up as pain in specific positions: reaching overhead or behind the back for the shoulder, deep flexion or rotation of the hip (sitting deeply, getting in and out of a car, certain sports movements). Most cases improve with physical therapy focused on the muscles that control the joint, activity modification to avoid the most provoking positions, and time. Steroid injections can ease pain in the short term. Surgery to reshape the bone or repair related cartilage and tendon damage is reserved for cases that don't improve with good conservative care, and outcomes are best when symptoms and imaging clearly match.

When to follow up

If the report mentions the finding but symptoms are mild, a course of targeted physical therapy is usually the first step. See a clinician if pain is limiting sport or daily life, if the joint locks, clicks, or gives way, or if pain is worsening despite rehab. Sudden severe pain after a fall, inability to move the limb, weakness in a specific movement (such as lifting the arm), numbness, or signs of a dislocation deserve prompt care. The link between picture and symptoms matters more than the picture alone.

A plain-language way to picture it

Picture a sliding drawer in a cabinet. Open and shut it gently and it glides. If the runner is bent or the drawer front has a small bump, the drawer catches at the same spot every time — that catching is the pinch. Inside the joint, two structures meet during a particular movement that should let them slide past each other, and instead they crowd. Most of the time the catch is mild and unnoticed; sometimes it grumbles enough to need attention.