Knee Effusion

Article Author:
Louis Gerena

Article Editor:
Alexei DeCastro

Editors In Chief:
David Wood
Andrew Wilt
Hajira Basit

Managing Editors:
Avais Raja
Orawan Chaigasame
Khalid Alsayouri
Kyle Blair
Radia Jamil
Erin Hughes
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Navid Mahabadi
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Abbey Smiley
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beenish Sohail
Hajira Basit
Phillip Hynes
Sandeep Sekhon

5/5/2019 10:36:21 PM


The knee is a hinge joint and susceptible to injury from trauma, inflammation, infection, and degenerative changes. A knee effusion may result from acute or chronic conditions. Causes range from trauma or overuse to systemic disease. Understanding the basic anatomy and pathophysiology of knee effusions is important to make an accurate diagnosis. However, the most important information in making the diagnosis will be to correlate findings with a thorough history and physical exam.


The knee contains sac-like structures containing synovial fluid, called bursae, which are located between the skin and bony prominences. Fluid accumulation in the intra-articular space of a joint is called an effusion.  Small, asymptomatic effusions can occur in healthy individuals. Larger joint effusions indicate intra-articular pathology.  In the knee joint, trauma, overuse, infections, systemic causes, or changes in osteoarthritis may cause the effusion. These large effusions can also cause popliteal cysts. Acute swelling can be the result of a hemarthrosis from an injury to a ligament or fracture.


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.[1] The lifetime prevalence of knee swelling has been reported as high 27%.[2] It may occur at any age in children, but most commonly occurs in infants and teenagers.


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. The most common diagnoses in the primary care setting are osteoarthritis, trauma, and gout.[3]


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, which then allows molecules of almost any size to pass through its membrane. Synovial fluid also contains proteins that are identical to plasma proteins. The hyaluronic acid in the synovial fluid provides friction and lubrication to the knee joint. Synovial fluid has antibacterial properties that help maintain a sterile environment to the joint.

History and Physical

The evaluation of an acutely swollen knee must begin with a very thorough history. Important questions to ask include mechanism of injury, duration, acuity of onset, aggravating symptoms, or if swelling occurred without trauma. A knee effusion with a history of recent injury may suggest a derangement such as a ligament or meniscal tear, while an atraumatic effusion would have a higher suspicion for septic arthritis. A history of previous surgery should be determined in every patient who presents with knee swelling. Systemic symptoms of inflammation or infection should be investigated as well. Patients commonly complain of swelling and stiffness with decreased range of motion.[1]

Red Flags

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


A knee joint effusion will demonstrate swelling around the patella and distend of the suprapatellar space. Patients may have a restricted range of motion along with pain with ambulation. The exam should include observation of gait, palpation of the external knee, range of motion, joint line tenderness, McMurray tests, Thessaly test, duck walk, patellar tendon stability (ACL, PCL, valgus, varus), and a patellofemoral joint test (compression test).[4] 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.[5]


In patients presenting with an acutely swollen knee, initially plain, weight-bearing radiographs in 2 planes should be ordered to look for a fracture, in case of trauma. 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.[6]

An ultrasound of the effusion can help assess a complicated effusion from a simple effusion and can also be used to performed 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, 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.  

The synovial fluid aspirate should be analyzed for[7]:

  • Complete blood count (CBC) with differential (white blood cell [WBC], polymorphonuclear leukocytes)
  • Crystal examination of synovial fluid
  • Culture and Gram staining of synovial fluid
  • Viscosity (RA: expect decreased viscosity and poor mucin clot formation)
  • Glucose
    • 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
  • Cell count with WBC greater than 50,000 is considered diagnostic for septic arthritis. However, lower counts may still indicate infection (not sensitive)[8]
  • Prosthetic joint with WBC greater than 1100 is considered septic
  • Gram stains only identify infective organism one-third of time
  • Glucose less than 50% of serum level

Non-Inflammatory Synovial Fluid

  • Contains less than 60 to 180 cells per mL, most of which should be mononuclear
  • Synovial fluid is considered to be 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 cell 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 course of 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. Finding of greater than 90% PMNs despite 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[9]
  • 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 increased.
    • 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)[5]

Treatment / Management

  • 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, 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 debridement of necrotic material if needed.[10]
  • Ligamentous injuries: The patient can be placed in a knee brace with an outpatient referral to an orthopedic surgeon.
  • Fractures: These may need to be referred 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.
  • 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 have been ruled out first.[6]

Differential Diagnosis

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


Poor outcomes may occur in septic arthritis if the patient has any of the following[11]:

  • Sixty years old or greater
  • Affected area is the hip or shoulder joint
  • Current or history of rheumatoid arthritis
  • If the patient has been on appropriate therapy for 7 days and still has positive findings on synovial fluid cultures 
  • If greater than 7 days pass before starting treatment


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.[10]

Enhancing Healthcare Team Outcomes

Managing knee effusions requires an interprofessional team of healthcare professionals that includes a nurse, laboratory technologists, and a number of physicians in different specialties. Without proper management, the morbidity and mortality may be increased from an undiagnosed septic knee effusion. The moment the triage nurse sees an acute knee effusion, the emergency department physician must be notified to determine whether there is a high suspicion for septic arthritis.

The best outcome for a patient can be achieved with a coordinated therapy plan between multi-specialty and interprofessional teams. This includes the moment the patient is in triage in the ER, to the diagnosis of the septic knee, to orthopedic management, and finally post-hospitalization treatment. Essential in timely diagnosis is the diagnostic laboratory. Consultation with the pharmacist and infectious disease physician about the choice and administration of antibiotics will increase efficacy and patient safety. If the patient requires any resources for home, the social worker and case manager should be notified. Finally, due to knee pain and decreased mobility, a physical therapist must be consulted for early ambulation.

Using this shared decision-making model, communication among providers is a key element for the best result. This interprofessional approach must use evidence-based medicine and a unique, integrated care pathway. The best prognosis and outcomes depend on the early recognition of potential complications. (Level III)

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Knee Effusion - Questions

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A 45-year-old male presents with left knee pain after stepping off a curb awkwardly. He states he heard a pop and had immediate swelling. The pain is mostly on the medial side of his knee. On the lateral radiograph of the knee, it is observed that the distance from the fabella to the femur is markedly increased. The joint space is preserved. No lytic or sclerotic areas are seen. Which of the following is the most likely diagnosis of this patient?

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A patient with pain and swelling of the left knee and positive ballottement and bulge tests has which one of the following?

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A 17-year-old male patient presents with large acute swelling of his right knee. He denies any trauma. Arthrocentesis on his knee effusion yields synovial fluid analysis with 80,000 WBCs/mL and greater than 90% polymorphonuclear neutrophils (PMNs). What is his likely diagnosis?

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A 65-year-old female with insulin-dependent type 2 diabetes mellitus, hypertension, peripheral arterial disease, and chronic kidney disease stage 3b presents with chronic right knee pain. She reports swelling within the knee. The swelling comes and goes. The pain has gradually worsened over a period of years. She denies fever, redness, or warmth to the knee. She used to be a house cleaner and was constantly on her feet. She has not tried any medications for the pain yet. A physical exam shows a small palpable effusion on her right knee and decreased joint line demarcation. Lachman's, McMurray's, valgus, varus are all normal. Flexion of the knee is slightly limited. Crepitus can be felt with extension of the knee. Hip range of motion is normal. X-ray shows moderate osteoarthritis of her right knee. Recent blood work shows decreased glomerular filtration rate (GFR), elevated creatinine, and normal liver function tests. She does not want surgery. What would be the best medication for her right knee osteoarthritis?

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A 17-year-old running back with no medical history presents with left knee pain and swelling after feeling a "pop" while running during a football game. There was no contact involved in the mechanism of the injury. He states he had immediate pain and swelling after the injury. A physical exam of the affected knee shows a positive Lachman's test, negative McMurray's, negative valgus and varus maneuvers. What is the next best step in management?

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Knee Effusion - References


Gupte C,St Mart JP, The acute swollen knee: diagnosis and management. Journal of the Royal Society of Medicine. 2013 Jul     [PubMed]
Baker P,Reading I,Cooper C,Coggon D, Knee disorders in the general population and their relation to occupation. Occupational and environmental medicine. 2003 Oct     [PubMed]
Becker JA,Daily JP,Pohlgeers KM, Acute Monoarthritis: Diagnosis in Adults. American family physician. 2016 Nov 15     [PubMed]
Solomon DH,Simel DL,Bates DW,Katz JN,Schaffer JL, The rational clinical examination. Does this patient have a torn meniscus or ligament of the knee? Value of the physical examination. JAMA. 2001 Oct 3     [PubMed]
Margaretten ME,Kohlwes J,Moore D,Bent S, Does this adult patient have septic arthritis? JAMA. 2007 Apr 4     [PubMed]
Landewé RB,Günther KP,Lukas C,Braun J,Combe B,Conaghan PG,Dreinhöfer K,Fritschy D,Getty J,van der Heide HJ,Kvien TK,Machold K,Mihai C,Mosconi M,Nelissen R,Pascual E,Pavelka K,Pileckyte M,Puhl W,Punzi L,Rüther W,San-Julian M,Tudisco C,Westhovens R,Witso E,van der Heijde DM, EULAR/EFORT recommendations for the diagnosis and initial management of patients with acute or recent onset swelling of the knee. Annals of the rheumatic diseases. 2010 Jan     [PubMed]
Li SF,Henderson J,Dickman E,Darzynkiewicz R, Laboratory tests in adults with monoarticular arthritis: can they rule out a septic joint? Academic emergency medicine : official journal of the Society for Academic Emergency Medicine. 2004 Mar     [PubMed]
McGillicuddy DC,Shah KH,Friedberg RP,Nathanson LA,Edlow JA, How sensitive is the synovial fluid white blood cell count in diagnosing septic arthritis? The American journal of emergency medicine. 2007 Sep     [PubMed]
Shah K,Spear J,Nathanson LA,McCauley J,Edlow JA, Does the presence of crystal arthritis rule out septic arthritis? The Journal of emergency medicine. 2007 Jan     [PubMed]
Horowitz DL,Katzap E,Horowitz S,Barilla-LaBarca ML, Approach to septic arthritis. American family physician. 2011 Sep 15     [PubMed]
Kaandorp CJ,Krijnen P,Moens HJ,Habbema JD,van Schaardenburg D, The outcome of bacterial arthritis: a prospective community-based study. Arthritis and rheumatism. 1997 May     [PubMed]


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