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Common Causes of Knee Swelling

Knee swelling is a common presenting condition in both primary care and the emergency department. Knee swelling can be due to trauma, overuse or systemic disease. However, it is particularly common in sporting populations due to the high risk of traumatic injury. The knee is injured more often than any other joint because it has an important role in weight-bearing and it does not have the same kind of strong stability and joint congruity (i.e., how well the joint surfaces connects to one another) as seen in the hip and ankle.

What is knee swelling?

Knee swelling, or effusion, is defined as excess synovial fluid (i.e., non-Newtonian fluid found in the cavities of synovial joints) that has accumulated in or around the knee joint. As with all presenting conditions, a thorough history and orthopaedic and neurological examination usually provides enough pertinent information to make a provisional diagnosis, or whether there is a need for referral for further investigations.

What are important clinical considerations with knee swelling?

There are a few important clinical considerations to identify during the history taking when a patient presents with knee swelling. These include:

  1. Location of swelling and pain. Where the swelling and pain is located about the knee will give the clinician a good understanding of the tissues in lesion. For example, swelling and pain on the inner joint line of the knee strongly suggests meniscal or medial collateral ligament injury; whereas pain and swelling behind the patella (i.e., kneecap) strongly suggests prepatellar bursitis.
  2. Timeframe/onset. Acute (i.e., 0-4 days) injures will present differently and infer different diagnoses than sub-acute (i.e., 5-14 days) or chronic (i.e., >14 days) injuries. For example, an acute sporting injury like an anterior cruciate ligament (ACL) injury will present with severe pain, joint laxity and hesitancy to weight-bear, and severe swelling that begins within a few hours; whereas with chronic disorders like osteoarthritis, swelling may worsen over a longer timeframe.
  3. Age. The age of the patient also plays an important factor in the differential diagnosis of knee swelling. Older patients typically present with chronic disorders like osteoarthritis spontaneous osteonecrosis of the knee (SONK); whereas, younger patients often present with more acute or sport-related injuries including Osgood-Schlatter disease and ACL injuries.
  4. Mechanism of injury. How did the joint become swollen? Was it due to something obvious like a sports injury? If so, how did it happen — did it occur when the patient twisted suddenly with a planted foot? Did they hear a pop/snap? Or perhaps the mechanism of injury was not so obvious? Did it come on insidiously? Is it associated with other signs and symptoms? All these questions and many more help clinicians differentially diagnose knee swelling even before performing any physical observation. In fact, up to 80% of the information required to make a sound provisional diagnosis comes from a thorough history.
  5. Aggravating factors. Factors that exacerbate (i.e., make the condition worse) or bring on the pain are also important. For example, if the pain is particularly worse when the patient is twisting or turning at pace, this may indicate a meniscal injury or instability due to an ACL injury.

What are the signs and symptoms of knee swelling?

The signs and symptoms of knee swelling will depend on the exact cause of the effusion, but the most common include:

  1. Pain. As the knee is a weight-bearing joint, pain usually occurs while standing or during any upright activity, but most commonly when rapidly changing direction. The excess fluid that has effused into the joint can cause chemical irritation to surrounding pain-sensitive tissue structures. Some patients suffer severe pain, while others report no discomfort, even despite severe swelling.
  2. Swelling. One knee may appear larger than the other. Puffiness or bogginess around the bony parts of the knee appear prominent when compared with the other knee.
  3. Stiffness. When the knee joint contains excess fluid, it can become difficult or painful to move the knee through its full range. Fluid can also show under the knee when straightened.
  4. Bruising. If the patient has injured their knee, they may note bruising about the knee. Bruising often appears pink or red first, then blue or dark purple. As the tissues begin to clean up the injured capillaries (i.e., small blood vessels supplying nutrients to surrounding tissues) the colour turns pale green, then yellow and brown, until the bruise has completely abolished.

Serious injury can cause blood to flood into the knee joint, causing significant swelling, warmth, stiffness, and bruising. This condition is referred to as haemarthrosis and warrants urgent medical attention. A patient should also seek medical attention if knee pain is severe, if they are unable to weight-bear on the affected leg or if there is a suspected fracture or dislocation.

What are the common causes of knee swelling?

  1. Knee injury. Traumatic injury to the knee is perhaps the most common cause of swelling about the knee. Injury can be acute (e.g., sudden tissue tear or joint dislocation) or chronic (e.g., overuse degeneration), blunt (i.e., not breaking skin) or penetrating (i.e., breaking through skin), intra-articular (i.e., within the knee joint) or extra-articular (i.e., outside of knee joint), and external (e.g., sporting collision) or internal forces (e.g., ACL rupture). Therefore, there is significant variability in the cause of knee swelling due to injury. Injury can be to bones, ligaments, tendons, bursae, meniscus or articular cartilage.
  2. Knee osteoarthritis. Degeneration of the articular cartilage lead to an overproduction of joint fluid, causing the knee to swell. Static fluid can also begin to degrade tissue causing pain and stiffness, particularly in the morning as fluid pools into the joint throughout the night.
  3. Baker’s cyst. This is an encapsulated sac of fluid at the back of the knee. The cyst may have no other symptoms or may be accompanied by pain and stiffness.
  4. Non-septic bursitis. Throughout the body are tiny, thin, fluid-filled sacs called bursa that normally protect joints. An inflamed knee bursa can fill with excess fluid, causing swelling, or water on the knee. The swollen knee may feel “squishy” and may or may not be painful. The most common types of knee bursitis are prepatellar bursitis and pes anserine bursitis.
  5. Gout. An accumulation of microscopic uric acid crystals in the joint that can aggregate and impact on the pain-sensitive articular cartilage. Knee swelling may occur rapidly and be accompanied by excruciating pain, redness, and warmth.
  6. Pseudogout. This is an accumulation of calcium pyrophosphate crystals in a joint that presents similarly to gout, but is less common than gout.
  7. Osgood-Schlatter Disease. This is most common in active and sporting adolescent patents and is an inflammation of the patellar tendon where it attaches to the tibial shaft. Usually it can be treated at home and will resolve as the adolescent grows.

What are more serious causes of knee swelling?

  1. Septic bursitis. This is a bursa that has been infected with a microorganism and becomes inflamed and filled with pus. Causes can be due to a dirty penetrated wound and in immunocompromised patients. The swollen knee may appear red and feel hot. Patients should seek immediate medical attention if they suspect symptoms are caused by septic bursitis.
  2. Rheumatoid arthritis. This is an autoimmune (i.e., body doesn’t recognise self and attacks self) disease that affects joint lining causing knee swelling, stiffness, pain, tenderness, and redness. Symptoms often occur on both sides of the body, so if the right knee is affected it’s likely the left knee is also affected. It is more common in the hands and feet.
  3. Juvenile rheumatoid arthritis. Signs of juvenile rheumatoid arthritis include aching, swollen joints causing the child to limp or seem clumsy, and a fever or rash. If signs and symptoms persist for a week or more, caregivers should contact their medical doctor.
  4. Septic arthritis. Bacteria or other microorganisms can penetrate the joint lining infecting the joint and potentially causing it to fill with pus. Sudden knee swelling, intense knee pain, and fever are signs of septic arthritis. Patients should seek immediate medical attention.
  5. Reactive arthritis. Certain types of bacterial infections (e.g., chlamydia and gastrointestinal infections) can spur an inflammatory immune response in the body that may cause pain and swelling in joints. The classic triad of symptoms includes arthritis, conjunctivitis/iritis (i.e., infection of conjunctivae), and either urethritis or cervicitis (i.e., infection of the urinary tubes of cervix)
  6. Tumor. These are relatively uncommon, but a benign or malignant tumor can cause a swollen knee. A tumor may be accompanied by night pain (that wakes you up at night), night sweats, fever, unrelenting pain with no mechanical cause, and unexplained weight loss.

Conservative management for knee swelling

First and foremost, patients should seek immediate medical attention if they suspect a serious pathological cause to their knee swelling and pain. If there is a more obvious mechanism of injury, like an acute mechanical injury (e.g., ACL rupture), the knee should be immediately treated using the P.O.L.I.C.E protocol (i.e., protect the joint, optimally load the joint, ice, compression and elevation. If symptoms persist, it would be reasonable to seek medical advice.

The role of physical therapy in knee swelling will depend on the cause of knee swelling. Clinicians will take a thorough history, specific physical examinations (e.g., orthopaedic and neurological) to rule out possible pathology and functional examination to ultimately come to a provisional diagnosis. From there, further investigations (e.g., MRI, radiograph etc) may or may not be warranted, and physical therapy can begin straight away.

The role of physical therapy for knee swelling will depend on the cause of knee swelling. As you now know, there are several causes of knee swelling. The most common types we see in the clinic, and those that physical therapy is indicated for, include all types of knee injuries (e.g., ACL/PCL ruptures, meniscal tears, lateral/medial collateral ligament ruptures, bursitis, tendonitis/tendinopathies, osteoarthritis and Osgood-Schlatters. There is good-quality evidence that physical therapy can help reduce symptoms, improve strength, mobility and endurance and reduce risk factors for knee swelling.

At Sydney Spine & Sports Centre (S3C), our clinicians are highly trained to differentially diagnose and conservatively manage a range of mechanical knee complaints causing swelling. If you suspect knee swelling or injury, come and see us at our Balmain clinic for an expert opinion.

More information

Please visit our blog page and website for more information about knee and lower leg complaints and conditions:

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Chris Knee

Chris is an experienced and qualified chiropractor, sports chiropractor, McKenzie Credentialed practitioner, nutritionist and Certified Strength and Conditioning Specialist (CSCS) and is finishing of his Doctor of Physiotherapy at Macquarie University.

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