Knee Effusion

Etiology

The knee contains multiple bursae, which are sac-like structures consisting of synovial fluid. The bursae are located between the skin and bony prominences. Some of these bursae communicate with the knee joint. Fluid accumulation in the intra-articular space and bursae of a joint is called an effusion. Small, asymptomatic effusions can occur in healthy individuals. Larger joint effusions indicate pathology. In the knee joint, trauma, overuse, infections, systemic causes, or changes in osteoarthritis may cause the effusion. These large effusions can also result in cysts within the popliteal fossa. A knee effusion can consist of synovial fluid or hemarthrosis.

Epidemiology

The knee is more frequently injured than other joints because it is part of a weight-bearing limb, and it does not have the stability brought by the joint congruity of the hip and ankle.[2] The lifetime prevalence of knee swelling has been reported to be as high as 27%.[3] It may occur at any age in children but most commonly occurs in infants and teenagers.

Pathophysiology

Any condition that may cause knee pathology can initially present as a knee effusion from a broad differential diagnosis. A knee effusion can be caused by an acute or chronic condition which may be inflammatory or non-inflammatory, traumatic or atraumatic. The most common diagnoses in the primary care setting are osteoarthritis, trauma, and gout.[4]

Histopathology

The knee joint is comprised of synovial fluid, which is an ultrafiltrate of blood plasma and includes hyaluronic acid, glycoproteins, lubricin, proteinases, and collagenases. Inflamed synovium contains large clefts, allowing molecules of almost any size to pass through its membrane, causing an effusion. Synovial fluid also contains proteins that are identical to plasma proteins. The hyaluronic acid in the synovial fluid prevents friction and provides lubrication to the knee joint. Synovial fluid has antibacterial properties that help maintain a sterile environment for the joint.

History and Physical

The evaluation of an acutely swollen knee must begin with a very thorough history and examination of the affected knee and contralateral knee. Important questions include mechanism of injury, duration, acuity of onset, previous history of the joint, aggravating symptoms, and any associated systemic symptoms. A knee effusion with a recent injury history may suggest an internal derangement such as a ligament or meniscal tear. In contrast, an atraumatic effusion would have a higher suspicion of infection or systemic disease. A history of previous surgery should be determined in every patient who presents with knee swelling. Patients commonly complain of swelling and stiffness with decreased range of motion.[2] A large effusion can result in an inability to completely extend the knee, typically resting with 15 degrees of flexion.[1]

Red Flags

Red flags include fever, non-weight bearing, loss of distal pulses, loss of sensation distal to the knee, open fractures, and cellulitis overlying the knee. These red flags typically need immediate evaluation.

Exam

A knee joint effusion will demonstrate swelling around the patella and distention of the suprapatellar bursa. Patients may have a restricted range of motion and pain with ambulation. The exam should include evaluation of the skin, observation of gait, palpation of the external knee structures, active and passive range of motion, joint line tenderness, and special testing. Special tests include the McMurray tests and Thessaly test for the medial and lateral meniscus, Lachman's and anterior drawer tests for an ACL tear, posterior drawer test for a PCL tear, and varus and valgus stress tests for LCL and MCL injuries, respectively. The patellofemoral joint test (compression test) indicates pathology in the patellofemoral compartment.[5] 

Both the ballottement test and bulge test are done to look for knee effusion. The ballottement test is done by pressing upward on the medial aspect of the knee 2 to 3 times, then tapping the lateral patella to see if it floats outward due to effusion. Always compare the exam with the unaffected knee. In septic arthritis, the following symptoms are the only ones to occur in more than 50% of patients: joint pain, a history of joint swelling, and fever.[6] 

Evaluation

In patients presenting with an acutely swollen knee, weight-bearing radiographs in 3 planes should be ordered to look for a fracture in the case of trauma. This includes AP, lateral, and axial views.[1] A fabella, a sesamoid bone located inside the gastrocnemius, may be seen on an x-ray. It is a radio-opaque marker for the posterior border of a knee's synovium. The fabella sign or displacement of the fabella is seen with a synovial effusion and popliteal mass. The same lateral knee radiograph may show an increased opacity and widening of the suprapatellar bursa, which should be assessed if the fabella sign is seen. Another reason for plain radiographs is to detect erosive disease found in rheumatoid arthritis (RA) or joint space narrowing found both in osteoarthritis and RA.[7]

An ultrasound of the effusion can help assess a complicated effusion from a simple effusion and can also be used to perform arthrocentesis. A saline load test may be utilized to determine if a wound near a joint communicates with the joint. In the knee, 155 mL of saline is needed to reach 95% sensitivity.

Arthrocentesis and subsequent synovial fluid analysis should be done in all cases of unexplained knee effusion. The aspirated fluid should be analyzed for cell counts, Gram stain, bacterial cultures, and crystal analysis. Hemarthrosis is commonly caused by joint trauma. Fat droplets (detected by polarized microscopy) also indicate an articular fracture. Other clotting disorders like hemophilia can cause hemarthrosis in the absence of trauma.  

Synovial Fluid Aspirate Analysis

• Complete cell count with differential (white blood cell [WBC], polymorphonuclear leukocytes)
• Crystal examination of synovial fluid
• Bacterial Culture and Gram staining of synovial fluid
• Viscosity (RA: expect decreased viscosity and poor mucin clot formation)[8]
• Glucose
  • A low level of synovial-fluid glucose is suggestive of an infected joint, but low glucose levels are present in only about 50% of patients with septic joints and can also occur in rheumatoid arthritis.
  • Fasting glucose levels are usually reduced to less than half of the simultaneously obtained blood levels.
• The presence of crystals cannot exclude septic arthritis with certainty.[8] Septic arthritis occurs concurrently with gout or pseudogout in less than 5% of cases.

Septic Arthritis

• Joint fluid appears cloudy or purulent.
• A cell count with WBC greater than 50,000 is considered diagnostic for septic arthritis. However, lower counts may still indicate infection (not sensitive).[9]
• A prosthetic joint with WBC greater than 1100 is considered septic.
• Gram stains only identify infective organisms one-third of the time.
• Glucose will be less than 50% of the serum level.

Non-Inflammatory Synovial Fluid

• It contains less than 60 to 180 cells per mL, most of which should be mononuclear.
• Synovial fluid is considered non-inflammatory if it contains less than 2000 cells/mL, but most samples of synovial fluids from pts with osteoarthritis contain less than 500 cells per ml.
• The most common cause of non-inflammatory effusions of the knee (synovial fluid white blood cell count less than 2000 cells/mcL) is osteoarthritis; other causes include osteonecrosis, Charcot arthropathy, sarcoidosis, amyloidosis, hypothyroidism, and acromegaly. Inflammatory arthritis (synovial fluid white blood cells greater than 2000 cells/mL) can be caused by infection, autoimmune disease, and crystal-induced arthritis. Aspiration of dark brown serosanguinous fluid should raise the possibility of pigmented villonodular synovitis.

Inflammatory Synovial Fluid

• Greater than 2000 leukocytes/mL
  • Traumatic: Less than 5000  (w/RBCs)
  • Toxic Synovitis: 5000 to 15,000 and less than 25% polymorphs
  • Acute Rheumatic Fever: 10,000 to 15,000 and 50% polymorphs
  • JRA 15,000 to  80,000 and 75% polymorphs
• Greater 50,000 leukocytes/mL;
  • Although other diseases, including trauma, may produce WBC cells in joint fluid, levels greater than 50,000/mm3 are usually due to infectious arthritis.
  • Usually causes most intense synovial fluid leukocytosis, w/ 50,000 to 200,000 cells/mL and usually over 90% PMNs.
  • Lower leukocyte counts are more common early in bacterial arthritis and in patients with disseminated gonococcal infection.
  • Non-infectious conditions such as gout, pseudogout, acute rheumatic fever, reactive arthritis, and RA can cause a markedly inflammatory synovial effusion. A finding of greater than 90% PMNs despite a relatively low total leukocyte count should prompt concern about infection or crystal-induced disease. However, the presence of crystals cannot exclude septic arthritis with certainty[10]
• Septic arthritis 80,000 to 200,000 and greater than 75% polymorphs
  • In synovial fluid, WBC count and percentage of polymorphonuclear cells from arthrocentesis are the most powerful predictors for septic arthritis. The LR is increased as the synovial fluid WBC count increases.
  • For counts greater than 50,000/microL (LR, 7.7; 95% CI, 5.7-11.0) and for counts greater than 100,000/microL (LR, 28.0; 95% CI, 12.0-66.0). On the same synovial fluid sample, a polymorphonuclear cell count of at least 90% suggests septic arthritis with an LR of 3.4 (95% CI, 2.8-4.2), while a PMN cell count of less than 90% lowers the likelihood (LR, 0.34; 95% CI, 0.25-0.47)[6]

Other Labs

Blood Work

• Infectious processes: 
  • Lyme disease (especially in the pediatric population with atraumatic unilateral knee effusion) [11]
  • Antistreptolysin-O for post-streptococcal infection [12]
  • Rapid plasma reagin (RPR) for syphilis
  • Elevated WBC in a complete blood count (CBC) for osteomyelitis, inflammation, malignancy
• Inflammatory markers, including C-reactive protein and sedimentation rate
• ANA for possible autoimmune disorders

Urine studies check for urinary tract infections and sexually transmitted diseases, including gonorrhea, chlamydia, and trichomonas, which can cause reactive arthritis.

Polymerase chain reaction tests of affected mucosal sites for gonorrhea and chlamydia should also be considered.

Stool cultures can detect salmonella, yersinia, campylobacter, and shigella, which can all cause reactive arthritis.[13]

Treatment / Management

For acute pain and swelling, treatment should be individualized. This includes splints, cold or ice packs, partial or non-weight-bearing braces, non-steroidal anti-inflammatories (NSAIDs), or other analgesics. If the joint has a large effusion causing pain, drainage may be an effective treatment. The fluid sample should be obtained and tested/cultured before starting antibiotics. Intra-articular steroids should be held until infection, or other contraindications are first ruled out.[7]

• Septic joint: Once the lab studies have been sent, start intravenous (IV) antibiotics for the suspected infective agent. The most common bacterial causes include staphylococci (40%), streptococci (28%), gram-negative bacilli (19%), mycobacteria (8 percent), gram-negative cocci (3%), gram-positive bacilli (1%), and anaerobes (1%). Also, an orthopedic consult may be necessary. Drainage of the joint is associated with rapid recovery and low morbidity. Arthroscopy allows visualization of the joint, provides the ability to lyse adhesions, drains any purulent pockets, and can facilitate the debridement of necrotic material if needed.[14]
• Ligamentous injuries: The patient can be placed in a knee brace with an outpatient referral to an orthopedic surgeon.
• Fractures: These may need a referral to a surgeon depending on severity. The Salter-Harris classification can be used for pediatric fractures involving the growth plate.
• Rheumatologic conditions: Anti-inflammatories (NSAIDs) or acetaminophen can be used, and the patient should be referred to a rheumatologist.
• Reactive Arthritis: The primary infection should be treated with the appropriate antibiotic. NSAIDs and acetaminophen can be used for pain.

Differential Diagnosis

• Infection
  • Bacterial
  • Mycobacterial
  • Spirochete (Lyme, syphilis)
  • Viral
• Crystal (gout and pseudogout)
• Spondyloarthritis
  • Reactive arthritis
  • Inflammatory bowel disease
• Hemarthrosis
• Acute injury
• Osteoarthritis
• Osteonecrosis
• Malignancy
• Autoimmune disorders

Prognosis

Poor outcomes may occur in septic arthritis if the patient has any of the following:

• Eighty years old or greater
• The affected area is a larger joint, including the hip or shoulder
• If the patient has been on appropriate therapy for seven days and still has positive findings on synovial fluid cultures 
• If greater than seven days pass before starting treatment
• Comorbidities, including diabetes mellitus, bacteremia, chronic kidney disease, or RA[15][16]

Complications

Risks of delaying treatment beyond 24 to 48 hours include a permanent limitation in joint function and subchondral bone loss. Bacterial invasion can lead to permanent damage to articular cartilage. An infection in a prosthetic joint may result from a local infection that is left untreated and has spread to the prosthesis via blood flow to the area.[14]