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A swollen knee is one of the most common problems that walks into our clinic, along with frequent related questions including: what causes knee swelling, and is this serious? In general, swelling means fluid, and fluid means the knee is reacting to something.
I’m Dr. Cory Calendine, a board-certified orthopaedic surgeon at the Bone and Joint Institute of Tennessee in Franklin. I perform more than 700 joint replacements a year, and after more than twenty years I can tell you the story behind the swelling matters more than its size.
What causes knee swelling?
Knee swelling is caused by fluid collecting inside or around the knee joint. That fluid is normal joint fluid produced in excess, blood from a torn structure, or inflammation in the soft tissue surrounding the joint.
Your knee is lined by a thin layer of tissue called the synovium. It makes a small amount of slippery fluid that keeps the joint gliding. Irritate that lining, and it produces far more than you need. Doctors call the result an effusion; patients call it water on the knee.
What is the difference between fluid in the knee and swelling around the knee?
Fluid inside the joint makes the whole knee feel tight and full, and can limit full bending and full straightening. Swelling outside the joint feels more like a soft lump over one spot.
Here is what I tell my patients: a swollen bursa (a small fluid-filled cushion over the joint) sits on the surface and usually still lets the knee move.
What are the most common causes of knee swelling?
The most common causes of knee swelling are knee ligament and cartilage injuries, osteoarthritis, inflammatory arthritis, bursitis, gout, and joint infection. Overuse and extra body weight make all of them more likely.
Can a knee injury cause knee swelling?
Yes, and knee injury is the single most common reason a previously healthy knee swells. Damage to the anterior cruciate ligament (ACL), the meniscus (the C-shaped cartilage shock absorber), or the bone itself lets blood and joint fluid pour into the space.
The classic ones I see are ACL tears, meniscus tears, kneecap dislocations, and fractures around the joint. Twisting on a planted foot is the usual mechanism.
Does arthritis cause knee swelling?
Arthritis is the leading cause of knee swelling that appears without any injury at all. Osteoarthritis wears down the smooth cartilage covering the ends of the bones, and the raw surfaces irritate the joint lining into producing extra fluid.
Inflammatory arthritis, including rheumatoid arthritis and psoriatic arthritis, attacks the joint lining directly. Those knees often swell on both sides and feel stiffest first thing in the morning.
What about bursitis, gout, and infection?
These three explain most of the remaining cases. Bursitis is inflammation of the cushioning sacs around the joint; kneeling for work is a frequent trigger.
Gout and pseudogout happen when sharp crystals form in the joint fluid. Gout affects roughly 9 million US adults, and a gouty knee can go from normal to red, hot, and barely bendable overnight.
Infection inside the knee is uncommon, but it is the one I never want missed. A hot, red, severely painful knee with a fever needs evaluation the same day, not next week.
Can extra weight or overuse cause knee swelling?
Both do, usually by aggravating something already there. Every extra pound of body weight adds roughly three to four pounds of force through the knee with each step, which speeds cartilage wear.
Overuse works the same way. Ramping up mileage, or a weekend of yard work, can inflame a joint that was quietly coping.
Why does the timing of knee swelling matter?
How fast the knee swelled tells me what is likely torn. This is the question I ask before I ask anything else.
Swelling that appears within the first one to two hours after an injury is almost always blood in the joint, and studies of acute blood-filled knees show roughly 70 percent involve an ACL tear. Blood means something with a rich blood supply was damaged.
Swelling that shows up the next morning is more typical of a meniscus tear or a cartilage injury. Swelling that creeps in over weeks, with no injury you can name, usually points to arthritis or an inflammatory condition.
What does a swollen knee feel like?
Most patients describe a swollen knee as tight rather than painful, like a band wrapped around the joint. That tightness is the joint capsule stretching to hold fluid it was never built to hold. Other signs usually travel with it:
- Loss of full bending and straightening
- A puffy look around the kneecap compared with your other knee
- Warmth or skin color change over the joint
- A feeling that the knee might give way
When should you see a doctor for a swollen knee?
See a doctor if the swelling followed a significant injury, if it has not improved after a few days of home care, or if it keeps returning. Waiting rarely makes a structural problem easier to fix.
Which symptoms need same-day care?
Some presentations should not wait. Get seen the same day, or go to an emergency department, if you have:
- Fever along with a hot, red, or discolored knee
- Severe pain, or a knee you cannot bend or straighten at all
- Inability to put weight on the leg after an injury
- An obvious deformity after a fall or a crash
- Calf pain, or swelling of the whole leg rather than just the knee
That last one matters. Swelling of the entire leg can signal a blood clot, a different and more urgent problem than a swollen joint.
How is knee swelling diagnosed?
Diagnosis starts with your history and a hands-on exam, not with a scan. I want to know how it started, how fast it swelled, and what you have already tried.
Next come weight-bearing X-rays, and sometimes an MRI if I suspect a ligament or meniscus tear. Standing X-rays matter: a knee can look fine lying down and show bone-on-bone contact once you stand on it.
If the cause is unclear or infection is a concern, we draw a fluid sample with a needle. Clear straw-colored fluid, bloody fluid, and cloudy fluid each point in a different direction.
How is knee swelling treated?
Treatment targets the cause, not the fluid. Draining a knee without addressing why it filled up simply buys a few weeks.
What can you do at home for a swollen knee?
Start with rest, ice, compression, and elevation. Ice for about 15 minutes at a time with a cloth barrier against the skin, wrap the knee in an elastic bandage, and prop the leg above heart level when you sit.
Over-the-counter anti-inflammatory medicine such as ibuprofen or naproxen reduces both pain and swelling. Check with your own physician first if you have kidney disease, ulcers, or take a blood thinner.
What treatments does an orthopaedic surgeon offer?
Most of what I do for a swollen knee is not surgery. Physical therapy to rebuild quadriceps strength is the workhorse, because a strong quad protects the joint and helps keep fluid from returning.
Other options include aspiration, a cortisone injection to quiet inflammation, hyaluronic acid injections, bracing, and activity changes. Many patients get years of relief from nonsurgical joint pain treatment alone.
When a mechanical problem drives the swelling, such as a torn meniscus flap catching in the joint, knee arthroscopy can address it through small incisions.
When does a swollen knee mean it is time for knee replacement?
Surgery becomes the honest conversation when arthritis is advanced, the swelling keeps returning, and nonsurgical care has stopped working. The trigger is not the X-ray; it is what the knee is doing to your life.
I look for night pain, a knee that swells after ordinary activity, and a shrinking list of things you still enjoy. At that point we discuss total knee replacement or, for arthritis confined to one side of the joint, partial knee replacement. Swelling and knee stiffness after surgery are expected for a time.
How can you keep knee swelling from coming back?
Prevention comes down to reducing load and building the muscles that share it. None of it is glamorous, and all of it works.
- Strengthen your quadriceps and hips; strong muscles unload the joint
- Choose low-impact exercise: cycling, swimming, or an elliptical
- Lose weight if you carry extra, since every pound removed takes three to four pounds off the knee per step
- Treat flares early instead of waiting for the knee to shut down
Talk with a knee specialist in Franklin, Tennessee
If your knee keeps swelling or keeps limiting what you do, it is worth a proper diagnosis. Knowing the cause turns a frustrating symptom into a plan.
My team and I see patients at the Bone and Joint Institute of Tennessee, 3000 Edward Curd Lane, Franklin, TN 37067. Call (615) 791-2630 or schedule a consultation online. We care for patients from Franklin, Nashville, Brentwood, Spring Hill, Murfreesboro, and across Middle Tennessee.
Medical disclaimer: This article is general education about knee swelling, not medical advice, and is not a substitute for evaluation by a qualified orthopaedic provider. Individual results vary; treatment decisions belong with your own physician.
References
- Gupte C, St Mart JP. The acute swollen knee: diagnosis and management. J R Soc Med. 2013;106(7):259-268. PMC3704066
- Nguyen US, Zhang Y, Zhu Y, Niu J, Zhang B, Felson DT. Increasing prevalence of knee pain and symptomatic knee osteoarthritis: survey and cohort data. Ann Intern Med. 2011;155(11):725-732. PMC3408027
- Chen-Xu M, Yokose C, Rai SK, Pillinger MH, Choi HK. Contemporary prevalence of gout and hyperuricemia in the United States and decadal trends: the National Health and Nutrition Examination Survey, 2007-2016. Arthritis Rheumatol. 2019;71(6):991-999. PMC6536335
- Messier SP, Gutekunst DJ, Davis C, DeVita P. Weight loss reduces knee-joint loads in overweight and obese older adults with knee osteoarthritis. Arthritis Rheum. 2005;52(7):2026-2032. PubMed 15986358
- American Academy of Orthopaedic Surgeons. Common knee injuries. OrthoInfo. orthoinfo.aaos.org. Accessed August 2026.
- National Institute of Arthritis and Musculoskeletal and Skin Diseases. Osteoarthritis. niams.nih.gov. Accessed August 2026.
