Adhesive capsulitis (frozen shoulder)

What it means

Adhesive capsulitis, almost universally known as frozen shoulder, happens when the flexible capsule of tissue surrounding the shoulder joint becomes inflamed, thickens, and tightens. Instead of stretching easily as the arm moves, the capsule turns stiff and scarred, so the joint physically cannot move through its normal range no matter how hard the muscles try. It is a problem with the joint lining itself, not with the bones, cartilage, or rotator cuff tendons, although those structures can look normal on the same scan.

Why it appears on a CT or MRI report

MRI is the imaging test most likely to show frozen shoulder directly, since it can capture the thickened joint capsule and the tightened axillary pouch — a normally loose fold of tissue at the base of the joint — that are the hallmark of the condition. Reports may describe capsular thickening, loss of the normal axillary recess, or enhancement of the coracohumeral ligament and rotator interval. CT and X-ray are less sensitive to these soft-tissue changes but are sometimes used to rule out other causes of a stiff, painful shoulder, such as arthritis or a fracture.

What it usually means

Frozen shoulder tends to unfold in three overlapping stages that together can last one to three years: a painful "freezing" stage where movement becomes increasingly limited, a "frozen" stage where pain often eases but stiffness remains severe, and a "thawing" stage where motion gradually returns. It is more common in people between 40 and 60, in women, and in people with diabetes or thyroid conditions, and it can follow a period of shoulder immobility such as an injury, surgery, or a stroke. The good news is that the large majority of cases eventually resolve, even without surgery, though recovery can be slow and the shoulder may never regain quite the same flexibility it had before.

When to follow up

Talk to your doctor if shoulder stiffness is limiting everyday tasks like reaching overhead, behind your back, or into a car seat, especially if it has been building for weeks rather than days. Early treatment with physical therapy, anti-inflammatory medication, or a corticosteroid injection can shorten the painful stage and preserve motion, so earlier evaluation tends to work better than waiting it out. Seek prompt care if the stiffness follows an injury, comes with a fever, or is accompanied by significant swelling, since these point toward a different cause that needs its own workup.

A plain-language way to picture it

Picture the shoulder capsule as a stretchy sleeve of fabric wrapped around the joint, normally loose enough to let the arm swing freely in every direction. In frozen shoulder, that sleeve shrinks and stiffens like fabric that has been left to dry too tight, so the arm underneath can only move as far as the fabric allows. Just as shrunk fabric can loosen again with gentle, repeated stretching over time, the capsule in frozen shoulder gradually relaxes through its stages, and movement slowly comes back.