Knee inflammation (aka knee effusion) is often your body’s attempt to heal an injury. But, other times, it can be a sign of something more serious. So, it’s essential to know what causes knee swelling in the first place.
That way, you’ll be able to treat it appropriately.
Below, I’ll explain 9 common reasons behind knee swelling. You’ll learn why each of these conditions causes inflammation and what you can do about them.
Here’s what I’ll cover, tap on any of the topics to jump straight there:
- 1) Knee sprain
- 2) Meniscus tear
- 3) Knee fracture
- 4) Tendonitis
- 5) Wear and tear
- 6) Bursitis
- 7) Baker’s cyst
- 8) Rheumatoid arthritis
- 9) An underlying disease
- How to drain the knee swelling?
Now, let’s see what causes a swollen knee, shall we?
9 common causes of knee joint swelling
1) Torn ligament
A knee sprain happens when one of this joint’s ligaments gets injured. The severity can range from a mild overstretch to a complete tear.
Here, swelling appears almost immediately after the moment of injury. It brings repairing cells to the site and kickstarts the recovery process.
Now, some people will have mild inflammation, while others may feel severe knee pain and swelling. This will depend on the type of sprain, how it happened, and other factors.
There are several ways to heal a knee sprain. The swelling will be gone once the ligament is fully recovered.
Further reading: All you need to know about knee sprains
2) Meniscus tear
A meniscus is a C-shaped cartilage within the knee joint. We have two in each knee and they work with our ligaments to keep the knee joint stable.
Meniscus tears often happen while playing sports, after twisting the knee suddenly. People with osteoarthritis are also prone to this injury.
In this case, the swelling tends to get worse a few hours after the moment of injury. Unlike a knee sprain, where it appears quickly.
Some meniscus tears can heal naturally, while others will need some kind of intervention if the symptoms are severe.
This can help: Should you walk with a torn meniscus?
3) Knee fracture
Fractures tend to cause severe pain and inflammation. As with the injuries above, swelling is there to promote healing and will improve once the bone heals.
Most knee fractures happen after:
- Falling on your knees
- A direct blow on your knee or shin bone
- A sudden and strong contraction of your thigh muscles
It’s likely you won’t be able to bear weight on that leg, as it will be severely swollen and painful. But, there are some instances in which you can walk with a kneecap fracture.
With that said, please consult with your doctor before doing any activity with a fractured knee. It’s not a minor injury.
4) Knee tendonitis
Tendons connect muscle to bone. They can get swollen if you do an effort you aren’t physically prepared for.
For example, let’s suppose you decide to run a marathon without prior training.
This can create inflammation on the tendons of your knee because they aren’t prepared for the stress of the activity.
The sudden increase in activity harms the tendon, triggering an inflammatory response. The swelling will be painful as well. This is your body telling you to rest while your tendon heals.
This makes tendinitis tricky to recover from.
You have to find the sweet spot between using the tendon to promote healing, and resting it enough to not worsen the injury.
Going to physical therapy will be your best bet for healing tendonitis properly.
5) Knee osteoarthritis
This is the process of cartilage degradation in the knee due to aging.
It sounds simple, but it’s actually a complex condition. See, some people won’t have any symptoms at all, while others will have painful knee effusion.
Researchers are still investigating why this happens but some of their findings show that the cause of swelling isn’t only due to wear and tear. (1)
There’s something called low-grade inflammation which is a process where your immune system remains activated long after the real threat is gone. People experiencing this are more prone to conditions like knee osteoarthritis. (1)
Several lifestyle choices cause low-grade inflammation.
Like smoking, drinking alcohol, sedentarism, and consuming highly processed foods. Obesity is also a risk factor for osteoarthritis. (1)
But, this also means that people with painful osteoarthritis can manage their swollen knee by changing a few habits:
- Keeping a healthy weight
- Quitting smoking
- Exercising regularly
- Eating nutritious and anti-inflammatory foods
However, people with incapacitating symptoms may need knee replacement surgery.
6) Knee bursitis
A bursa is a sac filled with fluid, located between tendons, muscles, and/or skin. It reduces the friction between these tissues so our joints can move smoothly.
But, such placement also makes bursae prone to irritation. This is when bursitis happens.
There are two main types:
- Septic bursitis, meaning there’s an infection
- Non-septic knee bursitis
Although each type of bursitis is treated differently, they always involve some level of inflammation.
In septic bursitis, the inflammation is there to fight off bacteria. While in non-septic bursitis, swelling helps the bursa heal.
7) Baker’s cyst
Our joints are surrounded by soft tissue, which creates a capsule. That capsule contains the synovial fluid, a liquid within some joints that lubricates and nurtures them.
Sometimes, the backside of the capsule in the knee joint weakens. This causes a fluid buildup, where the excess fluid accumulates in the weakened area. The result is a baker’s cyst.
This is often asymptomatic. But, if it causes symptoms, the fastest way to reduce the water on the knee is with joint aspiration.
8) Rheumatoid arthritis (RA)
RA is an autoimmune disease. Here, the immune system attacks healthy cells in the fingers, hands, and/or knee joints.
The attack causes swelling and pain in the affected joint. It can also limit its functionality, making it hard to bend and/or straighten the knee.
Rheumatoid arthritis is more common in women with a family member with this disease. It’s also more common in smokers. (2)
Sadly, this condition has no cure. If you have swollen knees due to RA, the treatment consists of medication and physical therapy to overcome the flare-ups.
9) An underlying cause
Although rare, knee effusion can be a sign of an underlying medical condition. This can be the case if your knee suddenly got swollen without a traumatic event to explain it.
If this happened to you, please seek medical attention. You may have water on the knee joint because of the following (3):
- An infection
- A blood clot
- Some kind of nerve issue
- A tumor
Your doctor will probably ask for an MRI and blood tests to determine what’s going on and give you the right treatment.
How to drain the excess fluid in a swollen knee?
Cold therapy is best for injuries like tears or sprains. Just wrap the ice pack with an elastic bandage and raise the leg above your heart. This will decrease swelling effectively.
Hot therapy promotes blood flow and can be effective for managing swelling in chronic conditions such as knee joint osteoarthritis.
Nonsteroidal anti-inflammatory drugs
An NSAID is an over-the-counter medication that can help reduce knee pain and swelling. Common types include aspirin, ibuprofen, and celecoxib, among others.
We, physical therapists, are trained to identify the root cause of your swelling and treat it. We use non-invasive techniques to boost your healing process, reduce knee pain, and get you back on track.
Going to a physical therapist in your area will help you recover and reduce your risk of re-injury.
This becomes an option for people with chronic swelling and other symptoms, like severe knee pain. The procedure will depend on the cause of the swelling.
The doctor may recommend doing a joint aspiration in the affected knee if it’s from a baker’s cyst, for example. Or a knee replacement if it’s due to knee osteoarthritis.
What is the most common cause of a swollen knee?
Knee injuries – sprains, meniscus tears, or fractures.
Why is my knee swelling without injury?
You may have a knee injury without pain, creating the perception that there isn’t damage when there actually is. Common examples include mild tears or osteoarthritis.
But a swollen knee can be a sign of a metabolic issue if the knee joint is intact. Please go to a doctor in that case.
Can a swollen knee be serious?
Having excess water on the knee can be serious if there are other symptoms, like:
- Severe pain and/or instability
- Inability to fully extend/bend your knee
- Pain that wakes you up at night
- Sudden weight loss
- Swelling on other joints at the same time
Conclusion: Why do I have a swollen knee?
Several knee problems can cause water on the knee. Most of them will heal with some rest, medication, and physical therapy. But, certain conditions can also cause a swollen knee.
If you’re not sure what caused the inflammation, please visit a doctor to find out the reason and treat it properly.
- Goldring, Mary B, and Miguel Otero. “Inflammation in osteoarthritis.” Current opinion in rheumatology vol. 23,5 (2011): 471-8. DOI: 10.1097/BOR.0b013e328349c2b1
- Chauhan K, Jandu J S, Goyal A, et al. Rheumatoid Arthritis. [Updated 2021 Oct 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Retrieved on December 2021 from: https://www.ncbi.nlm.nih.gov/books/NBK441999/
- Gupte, Chinmay, and Jean-Pierre St Mart. “The acute swollen knee: diagnosis and management.” Journal of the Royal Society of Medicine vol. 106,7 (2013): 259-68. DOI: 10.1177/0141076813482831