Swelling in the knee most commonly develops after an injury but there are a number of different possible causes to consider. The diagnosis will depend on the cause so, to get an idea, it’s important to consider events leading up to the swelling as well as how quickly it came on.
Broadly speaking, there are two types of swelling within the knee.
The knee lining, or synovium, produces fluid containing nutrients and lubricants to keep knee cartilage (the firm, flexible tissue that acts as a shock absorber) nice and healthy.
The first type of knee swelling, sometimes called an effusion, is where the synovium manufactures excess fluid, which makes the knee appear to be swollen.
The second type is caused by bleeding in the knee that can follow some kind of trauma (physical injury) to the ligaments, the tough bands of tissue that connect the femur (thighbone) to the tibia (shin bone).
Bleeding in the knee can also follow a bone fracture or a trauma to one of the two meniscus cartilages that lie between the major bones of the knee.
Here are the five main instances of swelling to the knee:
- Rapid knee swelling after an injury
A knee that swells within a few hours after an injury is the most common type of swollen knee.
These cases often involve non-contact injuries caused by things like stopping suddenly, changing direction very quickly, twisting, turning or slipping.
High-energy injuries caused by, for example, falling off a motorbike are often very major injuries to the ligaments or fractures and you should seek immediate medical attention. In the case of elderly patients with thin bones (osteoporosis), simply tripping over can cause fractures.
The vast majority of patients who present with a knee that has swollen very rapidly have injured a ligament.
The most common such injuries are to the Anterior Cruciate Ligament or ACL (to the front of the knee), the Medial Collateral Ligament or MCL (inner knee) and, more rarely, the patella (kneecap).
Classically, a torn (ruptured) ACL is a non-contact injury that may be accompanied by a snap, pop or tearing sensation and may feel unstable.
- Rapid knee swelling without an injury
In these cases, there will often be a known diagnosis or you will have past experience of this problem.
For example, it could be a flare-up (in which the knee suddenly gets worse with more pain and swelling making it harder to move around) of osteoarthritis, gout or pseudogout.
Osteoarthritis is when the joint cartilage and the bone underneath wear down, which means your knee can feel painful, warm and swollen. Gout and pseudogout are types of arthritis in which crystals deposit in the knee and can cause sudden intense pain.
More rarely, rapid swelling can be caused by infection within the knee especially, but not exclusively, in the case of people who have health problems.
If you are suffering from increasing pain and stiffness, your knee is feeling hot and you are feeling unwell, for example if you have a temperature and you are feeling shivery, please seek urgent medical attention.
If left untreated, infections can lead to very serious illness.
- Gradual knee swelling after an injury
When a knee swells overnight or within a couple of days, it usually relates to a minor injury, for example tweaking a ligament such as the MCL or tearing a meniscus.
It could also indicate a flare-up of arthritis after training too hard, playing a sport you’re not used to or suffering a minor injury.
- Gradual knee swelling without an injury
This could include inflammatory, autoimmune and degenerative conditions such as osteoarthritis, gout, pseudogout and, rarely, rheumatoid arthritis.
- Rare causes of knee swelling
Although possible, tumours and cancers are extremely rare around the knee.
When should I contact a doctor?
Contact your doctor if the knee is not settling, if you are a feeling unwell or if it’s a recurring problem.
What is the best treatment for a swollen knee?
Basic treatment for a swollen knee should start with the RICE formula – Rest, Ice, Compression (with a knee support, if available, or a bandage), Elevation – and analgesics such as paracetamol and ibuprofen.
Please visit this page for more on the RICE formula.
Many people will quickly begin treatments such as physiotherapy, chiropractic or osteopathy, without getting a clear knee pain diagnosis.
If you take this approach, you may have to go through a great deal of pain and spend lots of time and money on the wrong treatment without getting any better (or things could get worse).
Why you should get a clear knee pain diagnosis
At Wimbledon Clinics, we keep things scientific, which means we give you the right treatment from the start so you don’t have to waste your precious time and money.
We do this by giving you a clear diagnosis (a clear label for your condition) and an assessment of the severity of the problem, which may include an MRI scan. Only then do we recommend the treatment options to help you get back to your best as soon as possible.
These may include rest, anti-inflammatory tablets and, on occasion, steroid injections.
For more significant injuries, such as those to ligaments, some form of knee support or knee brace may be required but this would normally only be fitted after a clear knee pain diagnosis has been made.
Or check our blog “To brace or not to brace”
For people with osteoarthritic knees, treatment can include injections of hyaluronic acid (such as Ostenil, Durolane) or PRP (Platelet Rich Plasma).
These injections can reduce pain and knee swelling with improvements sometimes lasting up to a year.