Subchondral cyst

What it means

Cartilage covers the ends of bones inside a joint, cushioning movement and spreading load evenly. Just underneath that cartilage sits a thin layer of bone called the subchondral bone. When a joint experiences abnormal stress over time — most often from cartilage that has thinned or worn away — small pockets can form in this layer, filled with thick fluid, gel-like material, or fibrous tissue. These are subchondral cysts, sometimes called geodes, and they're one of the classic bony changes radiologists watch for alongside cartilage loss and bone spurs.

Why it appears on a CT or MRI report

On X-ray and CT, a subchondral cyst shows up as a small round or oval lucent (darker) area in the bone just under the joint surface, sometimes with a thin rim of denser bone around it. MRI shows it as a well-defined pocket of fluid signal in the same location, and is more sensitive at picking up small ones before they're visible on X-ray. Reports usually note the joint involved (knee, hip, and shoulder are common sites), the size, and whether it sits alongside other signs of joint wear — narrowed cartilage space, osteophytes (bone spurs), and subchondral bone thickening (sclerosis). Together, these findings build the imaging picture of osteoarthritis.

What it usually means

A subchondral cyst is not a tumor and does not spread — it's a reactive change tied to how the joint is handling load, most commonly seen as part of osteoarthritis. One theory is that joint fluid gets pushed into small cracks in worn cartilage and settles into the bone underneath, forming the pocket over time. Small cysts found incidentally, especially without other signs of joint wear, are often of little consequence and simply reflect normal variation or very early, localized stress. When several cysts appear together with cartilage thinning and bone spurs, they usually confirm a diagnosis of osteoarthritis that's likely already suspected from symptoms like stiffness and activity-related pain. The cyst itself rarely needs direct treatment; management targets the underlying joint wear, through weight management, targeted exercise, activity modification, and pain control as needed.

When to follow up

An isolated small subchondral cyst mentioned on a report, without significant joint pain, generally doesn't require any specific action beyond routine follow-up of the joint. If the report ties the cyst to broader osteoarthritis changes, or if you have joint pain, stiffness, or swelling, it's worth discussing management options with your doctor — physical therapy and activity adjustments are usually the first line. Rapidly increasing joint pain, sudden swelling, redness and warmth around a joint, or inability to bear weight deserve a prompt medical evaluation, as these point to other causes rather than a simple subchondral cyst.

A plain-language way to picture it

Picture the underside of a well-worn stone step. Where feet have pressed down the same spot for years, tiny pockets and pits can form just beneath the surface, even though the step overall still holds weight fine. A subchondral cyst is similar — a small pocket forming in the bone right under the joint surface where load has concentrated over time. One small pocket rarely means the step is failing; several together, alongside a visibly worn surface, tell a fuller story about how much the joint has been used.