Subchondral Cyst in the Knee

Where it sits. Subchondral means just under the cartilage, in the layer of bone that supports the joint surface. A subchondral cyst is a small pocket in that bone. On an X-ray it shows as a small darker area in the white of the bone, just under the joint line. An MRI shows it along with the cartilage and the bone marrow around it. The report usually lists it with the other changes of osteoarthritis.

The words that come with it. The same finding is written in several ways. These are the terms you are most likely to see around it.

It comes with other signs of arthritis. On 806 X-rays of arthritic knees awaiting knee replacement, cysts showed on 30.6 percent, while joint space narrowing showed on 99.5 percent, bony spurs on 98.1 percent and subchondral sclerosis on 88.3 percent. Cysts were more common in women and in more bow-legged knees, and the authors concluded that cysts should not be counted as a core X-ray sign of osteoarthritis (Audrey et al., 2014). Most of the arthritic knees in that study showed no cyst on X-ray, and a cyst does not make the diagnosis on its own.

On MRI, it usually sits with a bone marrow lesion. In 109 people with painful knee osteoarthritis whose MRI could be assessed, 52 had cysts, and 51 of those 52 also had bone marrow lesions (Tanamas et al., 2010).

It tends to grow out of a bone marrow lesion. Two explanations have been put forward for how cysts form: joint fluid forced into the bone through worn cartilage, or bruising of the bone under the joint surface (bone contusion). In 1,283 knees scanned twice, 30 months apart, a new cyst was far more likely to appear where there had been a bone marrow lesion, and much less linked with full-thickness cartilage loss above it. The authors read this as support for the bone bruising explanation (Crema et al., 2010). The bone marrow edema guide explains why those lesions often mark the part of the joint carrying the most load.

It changes over time. Of the 88 who had a second scan two years later, 6 of the 46 without cysts developed one, and of the 42 with cysts, 15 had more cystic change, 10 had less (in 6 the cysts had gone) and 17 were unchanged (Tanamas et al., 2010). So a cyst that is a little larger on a later scan, and that the radiologist describes as fitting the arthritis, is not on its own a sign that something else is going on. A lesion described as expansile or atypical is a different matter, covered further down.

Its link with pain is mixed. A systematic review found the evidence linking cysts with knee pain limited or conflicting, unlike bone marrow lesions and joint fluid or inflammation of the joint lining, where it was moderate (Yusuf et al., 2011). In 1,145 older adults in Japan, most with arthritis on X-ray, knee pain went with bone marrow lesions mainly when a cyst, or flattening of the bone surface, sat alongside them (Tomura et al., 2025). The cyst is one piece of the picture, not a measure of how much the knee hurts.

It looks more like a sign of arthritis than a cause. In a community study that followed people at risk of knee osteoarthritis for seven years, cysts on the first MRI did not predict who developed arthritis on X-ray, but knees with cysts were more likely to develop arthritis with frequent pain. That link was no longer clear once other MRI findings were taken into account, though it held with a stricter definition of persistent pain. The authors concluded that cysts are more likely a consequence of the arthritis than its trigger, and may predict symptoms in knees that already have it (Perry et al., 2022).

It can mark a more active joint. In the same study, those with cysts lost cartilage faster than those with bone marrow lesions alone or neither, and the risk of knee replacement over four years rose from neither finding, to bone marrow lesions alone, to cysts (Tanamas et al., 2010). That is a pattern across a small group, not a forecast for one knee. The decision to refer for surgery is still based on symptoms and how much they affect your life, not on the scan findings (NICE, 2022).

In the hip. Cysts are common in arthritic hips too. On the X-rays of 100 people having a hip replacement for osteoarthritis, 82 had cysts in the socket (acetabulum), all in its upper, weight-bearing part (Rees et al., 2004). Whether hip cysts relate to pain has been studied less, and a systematic review of nine studies found the evidence limited or conflicting (Fang et al., 2024). The plan for an arthritic hip is on the hip osteoarthritis page.

There is no routine treatment aimed at the cyst itself. The plan is the plan for the arthritic knee or hip it sits in, and the main guideline is clear about what that is.

• Information, exercise and weight. Information about the condition, tailored therapeutic exercise for everyone with osteoarthritis, and support with weight loss where weight is a factor (NICE, 2022). For the knee that usually means strengthening the quadriceps and hip, with walking, cycling or water exercise, built up over weeks, as set out on the knee osteoarthritis page. A cyst on the report does not change that advice.
• What has not been tested. I could not find a trial that looked at exercise in knees with cysts as a separate group, so the advice rests on the trials in osteoarthritis as a whole. That is one more reason the plan is progressed gradually and judged by how the knee responds.
• Medication and injections. For the knee, topical anti-inflammatories are a common next step, and a steroid injection can give short-term relief. These are arranged through your doctor (NICE, 2022).
• Surgery. A cyst is not a reason for surgery on its own. A surgical opinion is worth asking for when symptoms substantially affect your quality of life and non-surgical care has not been enough. Your family doctor can arrange the referral.
• Follow-up scans. Repeat scans to watch a typical arthritic cyst are not part of the usual plan, because the plan is guided by how the joint feels and works. A lesion the radiologist has asked to see again is the exception.

Most cysts on a report sit among other signs of osteoarthritis and are part of it. Holes in the bone next to a joint have other causes too, including rheumatoid arthritis, crystal arthritis, the joint damage of hemophilia and, less often, tumors (Bullough and Bansal, 1988), so the radiologist reads each one in its setting. These situations need your family doctor before going further.

• The report calls the lesion expansile, aggressive, atypical or suspicious. Ask your family doctor to go through it with you promptly. If it only recommends a follow-up scan for a cyst that fits arthritis, your family doctor can arrange it at the interval suggested.
• The lesion sits away from the joint surface, or there is little sign of arthritis around it. In the hip, a cyst in the lower, inner wall of the socket is not assumed to be osteoarthritis, even when osteoarthritis is present, and warrants further investigation (Rees et al., 2004).
• You have pain at rest or at night that is new, unexplained or quickly getting worse, unexplained weight loss, or a history of cancer. Arthritis can cause some pain at rest or at night, but alongside a bone lesion any of these needs a medical diagnosis before physiotherapy; see your family doctor promptly.
• The pain came on suddenly and severely, or you cannot put weight through the leg. Keep weight off it and get medical assessment today, so that a subchondral insufficiency fracture can be ruled out.
• Several joints are swollen, or the knee is stiff for well over half an hour each morning. That pattern can point to an inflammatory arthritis, which your family doctor can check for.
• The knee is newly hot, red and swollen, with or without a fever, or you feel unwell. Seek same-day medical care, or go to emergency if you feel unwell.

I start with how the knee behaves: where the pain sits and whether it matches the part of the joint the report names, how long the knee is stiff in the morning, whether it swells, gives way or locks, and how the pain came on. I ask about pain at night or at rest, weight loss and any history of cancer, and I read the radiologist’s wording about the lesion itself. Anything that points away from a typical arthritic cyst goes to your family doctor first. I cannot order imaging; when further imaging would change the plan, I flag it and refer you.

On examination I look at alignment and walking, check for swelling and warmth, measure range of motion, and test quadriceps and hip strength, balance and single-leg control. I screen the hip and lower back, because both can send pain to the knee. I record a few measures I can repeat, such as a 30-second sit-to-stand and a timed walk, so you can see whether the plan is working.

The plan is the plan for the arthritic knee: strengthening, especially the quadriceps and hip, with walking or cycling and balance work, progressed as the knee tolerates it. I adjust the load if discomfort is more than mild or symptoms are still worse the next day. Joint mobilization and soft tissue work can sit alongside the exercise where they make it easier. A plan for flare-ups is part of it, and your family doctor comes in when medication, an injection or a surgical opinion is worth discussing.