Knee Effusion

What is Knee Effusion?

The knee, which works like a door hinge, is one of the most common body parts to get injured. This joint has several small, bag-like structures (bursae) filled with a special lubricating fluid (synovial fluid) which sit between the skin and the hard bony areas. Some of these small sacs are connected to the knee joint itself. When extra fluid builds up in these sacs or in the space inside the joint, it’s called an “effusion”.

Knee effusions can happen to anyone and usually cause knee swelling, pain, stiffness and can limit how much you move your knee. There are many reasons why someone might get a knee effusion. Doctors usually identify the specific cause behind the effusion by closely looking at the symptoms, doing scans, and sometimes testing the joint fluid. The treatment often includes rest, applying ice, lifting the knee, taking anti-inflammatory medicine and physical therapy. In some cases, they might drain out the extra fluid from your knee.

What Causes Knee Effusion?

The cause of fluid buildup in the knee, or “knee effusion,” can depend on a variety of factors. These can range from immediate causes like injury or infection, to long-term health issues such as arthritis, blood disorders, immune disorders, blood vessel inflammation, or even cancer.

Small amounts of fluid buildup might not cause any symptoms and can even occur in healthy people. But larger amounts often indicate an underlying problem. Excessive fluid buildup can even cause cysts to form within the hollow area behind the knee. The type of fluid that builds up in the knee can be either synovial fluid, which helps to lubricate and cushion the joint, or blood, a condition known as “hemarthrosis.” The cause of the fluid buildup can be related to injury or not.

The main way hemarthrosis develops is through a forceful rotation of the knee while it’s bearing weight. The damage that hemarthrosis causes to the cartilage in the joint is similar in people of all ages. However, the underlying cause can vary. In adults, traumatic hemarthrosis can be due to injuries to the anterior cruciate ligament (70% of cases), dislocation of the kneecap (15%), tears in the meniscus (10%), fractures of bone and cartilage fragments (5%), and other types of injuries (5%). In young people, however, the most common cause of traumatic hemarthrosis is dislocation of the kneecap.

Risk Factors and Frequency for Knee Effusion

Visits to the emergency department for knee injuries make up over half a million cases in the United States. The knee tends to get injured more often than other joints because it carries our weight and is built differently from the hip and ankle joints.

Over a lifetime, as many as 27% of people may experience swelling in the knee. Childhood cases of knee swelling often occur in infants and teenagers, especially when blood collects in the joint (hemarthrosis) due to sports-related knee injuries.

Signs and Symptoms of Knee Effusion

When dealing with a sudden swelling of the knee, a comprehensive overview of the patient’s medical history as well as a physical examination of both knees is crucial. The medical professional will ask about how the injury occurred, when the symptoms started, how long they’ve lasted, if there were any previous issues with the same joint, what makes the symptoms worse, and if there are any related overall health symptoms. A swollen knee following an injury may suggest damage to internal knee components such as the ligament or meniscus. If the swelling occurred without an injury, it might indicate an infection or a systemic disease. Past surgeries on the knee should be noted. Patients typically report feelings of swelling and stiffness, a limited range of motion, and even an inability to fully extend the knee, especially if the swelling is extensive. In such cases, the knee may naturally rest at a 15° angle when not in use.

There are certain urgent conditions or “red flags” to watch out for, including: fever, inability to bear weight, loss of sensation or pulse below the knee, open fractures, and skin infections over the knee. These require immediate medical attention. Over half of patients with septic arthritis experience joint pain, a history of joint swelling, and fever.

During a physical examination, the medical professional will look for signs to identify the underlying cause of the swelling. Comparison of the swollen knee with the unaffected knee is necessary as it aids in identifying any abnormalities.

The first signs of swelling include the absence of natural skin folds or dimples over the knee, and this can be easily observed when comparing the affected knee with the other one. Larger swelling may lead to patellar swelling and distention of the suprapatellar bursa. Deformities and dislocations are quite noticeable. Skin color changes, such as inflammation, may suggest underlying infection. A rash might be indicative of systemic diseases, particularly those that are rheumatologic, hematologic, or vasculitic in nature. Gait issues and alignment of the lower limb should also be assessed as chronic conditions could provide a clue to the cause of the swelling.

The medical professional will then gently examine the bone and soft tissue landmarks around the knee. Joint line tenderness could imply meniscal damage. Tenderness over a tendon or a ligamentous insertion may suggest tendonitis or apophysitis. Pain directly over the femur may indicate osteochondritis dissecans. If the patient reports joint pain but the doctor doesn’t find any tenderness over bony or soft tissue structures, this could indicate arthralgia.

A swollen knee could limit the range of motion and cause pain while walking. Excess fluid could create pressure and irritate the knee, leading to pain and discomfort during weight-bearing activities like walking.

Special tests like the balloon test, ballottement test, and bulge test can confirm knee effusion. The balloon test involves applying pressure to the extended pouch above the kneecap with the thumb and fingers for better detection of the swelling. The ballottement test is a continuation of the balloon test and involves pushing the floating kneecap downward against the femur. A gentle patellar tap should be felt. The bulge test is performed when only a minimal swelling is apparent on inspection where the fluid is milked laterally until a medial bulge is formed.

If specific injuries are suspected, the following specific tests may be performed:

• McMurray and Thessaly tests for tears in the medial and lateral meniscus
• Lachman and anterior drawer tests for tears in the anterior cruciate ligament
• Posterior drawer test for tears in the posterior cruciate ligament
• Varus and valgus stress tests for injuries in the lateral and medial collateral ligaments, respectively
• Patellofemoral joint compression test for pathology in the patellofemoral compartment

Swelling of the knee could lead to reduced blood flow and nerve compression, requiring immediate medical attention. The following structures, in particular, could be affected by swelling:

• Popliteal artery, which would consequently affect downstream vessels such as the anterior and posterior tibial arteries and dorsalis pedis
• Peroneal nerve
• Posterior tibial nerve

Arterial flow in the lower limb can be assessed by palpation. The functioning of the posterior tibial nerve can be evaluated by testing the strength of ankle plantar flexion. The function of the superficial peroneal nerve can be checked by examining the strength of ankle eversion. Testing the strength of great-toe extension can help test the function of the deep peroneal nerve. Sensory tests on the lower limb must also be conducted.

Testing for Knee Effusion

If your knee suddenly swells, your doctor will likely start with X-rays from three different angles. X-rays can help to see if you have a fracture, a common cause of knee swelling, especially if you have recently injured your knee. X-rays can also detect signs of rheumatoid arthritis, such as damage to your joints, or osteoarthritis, which might show up as a decrease in the space between your joints.

Sometimes, X-rays might reveal a small bone near the knee called a fabella. This bone is not present in everyone but can be used to identify swelling in the knee or a lump behind the knee, also called a popliteal mass.

An ultrasound is another useful tool to examine your swollen knee, especially to distinguish between simple fluid buildup or a more complicated issue. It can also guide a procedure called arthrocentesis, which involves removing fluid from the knee for testing.

Knee arthroscopy, a surgical technique that allows doctors to see inside the knee, is considered the best method for identifying and treating issues within the knee. However, it’s important to note that medical examinations or MRI scans are not always 100% accurate in ruling out knee problems.

To analyze the cause of a swollen knee, doctors often perform arthrocentesis, a procedure to collect and examine knee fluid. This fluid is analyzed for the number of cells, presence of bacteria, and crystal particles. Other factors such as thickness, glucose levels, and complete cell count are also checked.

Various results from this analysis can signal different conditions. For example, joint trauma can cause blood in the joint space or hemarthrosis. Fat droplets can indicate a joint fracture. Joint infection or septic arthritis can sometimes occur alongside conditions like gout or pseudogout, revealed by the presence of crystals in the fluid. Septic arthritis can also show up as a cloudy or purulent (or pus-filled) joint fluid. Various other conditions can also affect the number and type of cells in the knee fluid.

In addition to these tests, other blood and urine tests can help identify the causes of knee swelling. These might include tests for Lyme disease, post-streptococcal infection, syphilis, inflammation, autoimmune disorders, and urinary tract infections. For example, a test for the presence of a specific type of antibody can help diagnose Lyme disease, especially in children.

Finally, if you have a wound close to your knee, your doctor might conduct a saline load test to check if the wound is connected to the knee. In this test, a specific amount of normal saline is injected into the knee.

Treatment Options for Knee Effusion

If you experience acute pain and swelling, there are several treatment options. These include splints for support, applying cold or ice packs to reduce swelling, wearing braces that limit your weight-bearing, and taking medicine to relieve pain, such as nonsteroidal anti-inflammatory drugs (NSAIDs). If there’s excessive fluid causing pain in the joint, draining the fluid might help. If an infection is suspected, a fluid sample should be taken and tested before any antibiotics are administered. Steroids should not be used until an infection or other potential issues have been ruled out.

If there’s a chance your joint is infected, a type of bacteria is likely the cause. The most common ones include staphylococci, streptococci, and gram-negative bacilli. If a bacterial infection is suspected, it’s important to get a fluid sample from the joint for testing. After this, intravenous antibiotics should be initiated. Depending on the situation, you may need to see an orthopedic specialist. Draining the joint can lead to faster recovery and lower risk of other problems. In some cases, arthroscopy might be done. This procedure uses a small camera to look inside your joint, break down scar tissue, drain pus pockets, and clean out dead tissue.

If you suffer an injury to your ligament, a knee brace can be placed on your knee as a temporary solution before seeing an orthopedic surgeon for more definitive treatment. If the ligament is completely torn or the knee is unstable, surgery might be needed.

Fractures should be evaluated by an orthopedic specialist to decide if conservative management or surgery is best. In children, fractures that involve the growth plate can be classified using the Salter-Harris system.

In cases of rheumatologic conditions, which are diseases affecting the joints, muscles, and ligaments, common pain medications such as NSAIDs or acetaminophen may be used for pain relief. However, a rheumatologist, a doctor who specializes in these conditions, will determine the best approach for management.

For reactive arthritis, which is joint pain and swelling triggered by an infection in other parts of your body, treating the primary infection with the appropriate antibiotic is important. NSAIDs and acetaminophen may also be used for pain relief.

What else can Knee Effusion be?

A swollen knee, or knee effusion, could be caused by several different conditions. To identify the exact cause, doctors will carry out a detailed health assessment along with effective diagnostic tests. Here are some potential causes:

• Infections which could be bacterial, mycobacterial, spirochete (such as Lyme disease or syphilis), or viral
• Crystals, as in cases of gout and pseudogout
• Spondyloarthritis, which includes reactive arthritis and inflammatory bowel disease
• Hemarthrosis, or bleeding into the joint space
• Acute injuries
• Osteoarthritis, or ‘wear and tear’ arthritis
• Osteonecrosis, or death of bone tissue due to lack of blood supply
• Cancer
• Autoimmune disorders, where your body’s immune system attacks healthy cells

By considering these possible reasons for knee swelling, doctors can work towards identifying the right treatment plan.

What to expect with Knee Effusion

Several conditions can influence the recovery rate and outcome in patients who have fluid buildup in their knee, a condition called knee effusion:

• Being 80 years of age or older
• Having the same problem in a larger joint like the hip or shoulder
• Bacterial infection in the joint fluid, even after one week of suitable antibiotic treatment
• Delaying treatment for longer than one week
• Having other health conditions like diabetes,blood-borne bacterial infection, chronic kidney disease, or rheumatoid arthritis.

Additionally, factors such as the severity of the symptoms, the patient’s ability to stick to the medical advice, and the state of the patient’s immune system can also affect the outcome of knee effusion.

Possible Complications When Diagnosed with Knee Effusion

If treatment is delayed for more than 24 hours, it can cause lasting damage to joint function and loss of bone beneath the joint surface. Infections caused by bacteria can permanently damage the cartilage in the joint. If left untreated, these bacterial infections can also spread to an artificial joint.

Potential Consequences of Delayed Treatment:

• Lasting damage to joint function
• Loss of bone beneath the joint surface
• Permanent damage to joint cartilage from bacterial infections
• Bacterial infections spreading to artificial joints

Preventing Knee Effusion

Preventive measures are taken to lower the chances of getting injured or inflamed, which can lead to the development of knee swelling, also known as knee effusion. Patients can be taught about keeping a healthy body weight, strengthening their muscles, enhancing their flexibility, and preventing too much use of the knee, which can help protect them from osteoarthritis, a common form of arthritis that can cause knee damage.

Using the right techniques during exercise, wearing protective gear like knee pads, and staying away from dangerous activities can prevent knee injuries. Regular check-ups with your doctor and sticking to their treatment advice can help manage long-term conditions that can affect the knees. Drinking plenty of water can help keep your knees lubricated and cushioned.

While these steps might not prevent all cases of knee effusion, they can certainly lower your risk of ending up with this condition.