A lump on the side of the knee — either inside or outside — is most often a meniscal cyst. A meniscal cyst is a collection of thick fluid from a tear in the meniscal cartilage, the knee’s cushioning. But not all knee lumps are meniscal cysts. So how do we diagnose a meniscal cyst, and what do we do about it?
This page covers lumps on the side of the knee. For a lump at the back of the knee, see Baker’s cyst on ultrasound.
How does a meniscal cyst develop?
Usually, a tear forms in the meniscal cartilage from degeneration or an acute injury. Fluid from the knee joint then leaks out of the tear and forms a cyst — so a meniscal cyst isn’t a true cyst but an outpouching of displaced knee fluid. The cartilage tear can act as a one-way valve, preventing fluid from returning to the knee, so it collects as a cyst beneath the kneecap.
Meniscal cysts often form in people with a history of knee injury, cartilage damage, or contact and twisting sports. Arthritis also predisposes you to a cyst on the side of the knee.
Symptoms of a lump on the side of the knee
Not all lumps on the side of the knee are obvious or painful. When the lump is a meniscal cyst, common symptoms include pain on standing, swelling in the knee joint, and a lump on the side of the knee that changes in size.
On examination, we often find a lump close to the joint line — usually soft, and painful or painless. Twisting or rotating the knee can bring on pain. We generally use MRI to diagnose a meniscal cyst and its underlying meniscal tear, though an ultrasound can also confirm the diagnosis.
What’s the difference between a meniscal cyst and a Baker’s cyst?
A Baker’s cyst is a swelling at the back of the knee, an outpouching of the joint capsule. Like a meniscal cyst, it can form from a meniscal tear — but it also forms from anything that increases joint fluid, such as arthritis or ligament damage. We tell them apart mainly by the lump’s location (side vs back) and size.
Could a lump on the side of the knee be something else?
Yes. Other causes include housemaid’s knee, general knee joint swelling, a ganglion from the nearby tendons, and tumours such as a lipoma. If the lump is growing or painful, it’s worth a referral for an ultrasound or MRI.
Meniscal cyst treatment
Treatment mirrors that for the underlying meniscal tear, since meniscal cysts arise from degenerative tears. We start with simple measures — anti-inflammatory tablets and strengthening exercise. If the cyst is painful, an ultrasound-guided cortisone injection into the cyst can help. We suggest surgery only if pain continues despite simple treatments, or if you develop mechanical symptoms such as locking or giving way. The good news: removing the cyst itself isn’t necessary — removing the torn meniscus by arthroscopy is usually enough to stop it returning.
More on injections for a lump on the side of the knee
Injections help large, painful meniscal cysts. Under ultrasound guidance, we first numb the skin and the area outside the cyst, then place a needle into the middle of the cyst and decompress it with numbing fluid. Finally, we inject cortisone into the cyst to reduce inflammation and prevent a recurrence. Sometimes PRP injections help with meniscal tears, especially if we want to treat the tear itself.
Frequently asked questions about a lump on the side of the knee
Are all meniscal cysts painful?
No. We think about half of the meniscal cysts are painful; the rest are felt only as a lump.
Are all meniscal cysts linked to a meniscal tear?
No. Studies suggest a meniscal tear is present in about 50–75% of cases. A cyst can occasionally form without an obvious tear.
Can a meniscal cyst burst?
Yes. In some cases, the cyst bursts and disappears, though it can return if the underlying tear remains.
Do all meniscal cysts need treatment?
No. Seek treatment if the cyst grows or you develop knee pain. A small, painless, stable lump can often just be monitored.
Is a lump on the side of the knee always a meniscal cyst?
No. It’s the most common cause, but a lump on the side of the knee can also be a ganglion, housemaid’s knee, joint swelling, or, rarely, a tumour such as a lipoma. A growing or painful lump should be assessed with an ultrasound or an MRI.
My lump is on the inside of my knee and has been there since childhood — what is it?
A long-standing, painless lump may be an old Osgood-Schlatter ossicle, a ganglion, or a stable meniscal cyst. If it’s not changing or painful, it usually needs no treatment — but get any change assessed.
Final word from Sport Doctor London about a lump on the side of the knee
A lump on the side of the knee is most often a meniscal cyst. We treat these with simple measures first, then an ultrasound-guided injection, limiting surgery to cases that fail simple therapy. See a doctor to confirm the diagnosis and rule out other causes.
To book a one-stop knee assessment and injection with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.
Very informative
thank you. I’m pleased you found it informative.
Thank you I have a large lump beside my left knee that’s painful and swelling thanks for information
Excellent information i have had this problem for decades but never had any help from my own GPS.they deemed it quite petty.thanks for your wonderful information.
Thank you, a friend is suffering from pain in the back of her knee. Now I know where to get the information to help guide and support her through treatment.She is hoping to get cortisone injections,, ffluid removed. Her biggest fear is having a knee replacement
Behind my left knee have a painful lump
You could have a Baker’s cyst. See my blog on Baker’s cyst:
https://sportdoctorlondon.com/bakers-cyst-drainage/
Lorenzo
I got more help with this article than I did with a specialist.
I find this very informative, my mom has a lump at the side of her knee and is due for operation soon because it’s painful. Thank you for the information
I have two pea size bumps inside side of knee dr said there is not test to see what it is?
ultrasound may help clarify the cause if you can feel it. LM
wow thank you for the information … I cannot get an appointment with my GP at the moment but have a telephone consultation on 8th June … so hopefully I will get some help then … what would we do without Doctor Google tho and you of course … thank you again
Thanks, Sandy for your positive feedback.
Thank you for info, I have a lump on my right knee and now my ankle is very swollen and when I elevate my knee it’s very painful. I saw er dr they gave me a brace said it will take a longtime to go away! It hurts to walk and they didn’t do anything to see what’s wrong with my knee,
Thank you for this information. I have had a large lump on the side of my right knee since a car accident in September. I just had surgery almost two weeks ago on this knee and the lump is back. It will go away, then return.
Hi ,I have a lump on inner left knee, that hardens when flexed, I recently upped my exercise. Ultrasound diagnosis is Baker cyst usually sited at rear of knee. Symptoms are pain when kneeling siting cross legged niggling pain to side of leg. Fit 12.5 stone 57 year old fully mobile.
Would welcome general rehab tips
Paul
Please see my blog on Baker’s cyst:
https://sportdoctorlondon.com/bakers-cyst-drainage/
This cyst is secondary to excessive knee joint swelling. Treatment should be directed at reducing swelling. In the case of arthritis, we recommend the following treatment:
https://sportdoctorlondon.com/best-treatment-for-arthritis/
Thank you for your information and discussion comments. after a fall, there was no pain in knee and was not checked by emt’s called. After stepping down from a high step leaving the ambulance, after walking to car, I could not lift or bend knee to get in and knee had swelled up a lot. At emergency room, all x-rays were o.k. Went home with leg wrapped in Ace bandages. …removed after three days…two days later, skin was black from knee to lower calf. I have had a diagonsis of veinous insufficiency years ago. A fine surgeon has considered surgery, prior to covid pandemic, and we postponed. by the time I got in to see my M D as follow up on fall, the skin was normal color. The lump on right knee started after that. it is now nine weeks since fall.
Hi, you could have developed a meniscal cyst. I suggest investigations as outlined in my blog. All the best LM
But can they be on the sides of both knees? Could that be something more serious?
yes both sides are possible. I’d speak to your doctor about investigations. Lorenzo
Thank you so much for a very informative article. I have a decent sized lump on the ĺeft side of my left knee. Quite painful, though not all the time, but sometimes it gives way. I did have a new half knee 12 yrs ago on the inner side, I was worried that it might be something to do with that. Thanks again for your help.
Hi Patricia, Chances are it’s a meniscal cyst. I’d see your doctor to confirm. And thank you for the compliment. I try to write simple and unbiased articles.
LM
Hi i have a very tender and painful area on the left side of my right knee more towards the bottom of the knee. It has slight swelling but now has hardened and feels like a lump area has developed. I got this previously about 3months ago then it subsided and went away.
This time it came the next day after we had done a lot of walking around London and when i woke up the pain came down the side of the knee just as it goes to the back of the knee came again then over the cause of the next few days it hardened up and now the hardened lump area is about 50mm-75mm and still tender to touch.
What do you think i have?
You could have a Baker’s cyst
or a meniscal cyst
Hi.
A few hours after a 16km run my right knee felt a bit restricted when crouching down onto my honkers. I then noticed some swelling in it but it has not had any pain.
48 hours on there is still swelling to the knee, above knee cap towards the outside of the leg and below the knee cap towards the outside. When I tense my quads you can see a small finger length, squashy lump on the lower part.
I run regular and 16km was not a far run for me generally. I damaged the meniscus twice (4 and 10 years ago).
I have been icing this the last two days. I am training for a half marathon and reluctantly decided to rest a few days in hope this settles.
Any advice as to what this may be or if I should seek advice from a physio etc would be very much appreciated!
I suspect the ‘lump’ is, in fact, swelling from the knee joint. Swelling is indicative of your joint reacting to the load – so I’d suggest a few more days rest and reducing your volume by about 20-30%. If you continue to struggle, you should see a physio.LM
Thank you for this helpful information.
I have a swelling on the outside of my left knee and have no pain, just a stiffness when climbing stairs. This only appeared a couple of days ago.
I had an ultra sound done yesterday and read the Radiologist report on my Patient Portal. He says Bakers cyst and I am no doctor but I feel certain from all I have read here and elsewhere that this is a meniscal cyst.
I have a referral to see an Orthopedic surgeon who is a knee specialist. I would however like to hear your opinion.
if the lump is outside of the knee, then it’s more likely a meniscal cyst.
LM
Hello,
Yesterday a 3″ round lump has developed on the left side of my left leg, just above the kneecap area. It is very painful if I try to kneel on that knee or bend it too far. I can walk on the leg without a lot of pain but I do feel pressure in that area. I have done nothing to cause it. As a matter of fact I walked for 45 minutes yesterday am and did not notice the lump until later in the morning yesterday. As the lump developed the pain got worse. I do have Lupus (with no flare in about 9 years) and I take Plaquenil for that, I also have slow developing Parkinson’s (only tremor on the right side right now) which is why I try to walk 45 minutes a day, 5 days a week. What do you think this is? Do you think it will go away on its own if I take it easy? If not, what type of Dr. would you recommend I see?
Hi Loretta, The lump might be knee joint swelling. I’d suggest you see a musculoskeletal doctor to assess your knee.
LM
Hello, Thank you for this information, I have a large lump on the inside of my left knee, I’ve had it for several years now, all my GP has said is that it’s probably a sebaceous cyst, but in my opinion it looks nothing like a sebaceous cyst its skin coloured, occasionally looks like its bruised and once a clear watery fluid leaked out, it is now very painful especially walking and bending knee, I have made another appointment with my GP and I will ask about meniscal cyst, it’s about the size of a ping pong ball now what is your opinion about it, once again thank you.
Agree – you could have a meniscal cyst arising from the knee joint or a ganglion – which is a cyst-like structure arising from the tendons on the outside of the knee. https://sportdoctorlondon.com/bakers-cyst-on-ultrasound/ Ultrasound or MRI will clarify.
LM
Hi I have a solid fluid semi sphere ~2″ diameter mass (confirmed via MRI) on medial side of knee starting at bottom of knee cap and extending downward. My knee doesn’t really bother me, but this mass is bothering me and causing tightness.
MRI showed no obvious meniscus tears
Thoughts?
You could have a cyst without a meniscal cyst – although this is less common. Treatment is similar to a meniscal cyst: consider drainage if increases in size or causes pain.
My husband has swelling on the out side of his rught knee. It causes severe pain if he kneels on it and his leg aches if he is up on it for a long time , carries anything or when he is laying down at night. What could this be?
it could be a meniscal cyst – you should see a doctor to get a diagnosis
LM
Thank you your helpful information. For 10 years I went to orthopedic he said it is not your knee it is your back.long story short, it took 5doctors and one finally MRI severe pain.I now have a meniscus tear and cyst, arthritis and bone on bone. Getting a total joint in march
Thank you for this informative article! I’m wondering, does this meniscal cyst can causing weight loss?
Weight loss doesn’t usually affect the size of the cyst.
Thanks very much,I have had this problem for a long time since 2015 and i was about to panic,when i saw this post i know right it could be treated… sometimes/somedays i get pain on this lump which only lasts 3 minutes,then am okay again