Knee Effusion

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Continuing Education Activity

The knee is a hinge joint susceptible to injury from trauma, inflammation, infection, and degenerative changes. A knee effusion may result from acute or chronic conditions. Causes range from acute trauma to chronic systemic disease. Recognition of the basic anatomy of knee effusions and the pathophysiology is essential to make an accurate diagnosis. In the knee joint, trauma, overuse, infections, systemic causes, or changes in osteoarthritis may cause the effusion. Making the correct diagnosis requires a thorough history and physical exam. This activity reviews the evaluation and management of patients who present with knee effusion and highlights the role of the interprofessional team in improving care for those with this condition.


  • Describe the presentation and histological changes that accompany knee effusion.
  • Summarize the complete diagnostic workup for a patient presenting with knee effusion.
  • Summarize the various treatment options for knee effusion based on the underlying etiology.
  • Review the role of improving coordination and communication among the interprofessional team to address knee effusion, its sequelae, and positively direct outcomes.


The knee is a hinge joint susceptible to injury from trauma, inflammation, infection, and degenerative changes. A knee effusion may result from acute or chronic conditions. Causes range from acute trauma to chronic systemic disease.[1] Understanding the basic anatomy and pathophysiology of knee effusions is essential to make an accurate diagnosis. Making the correct diagnosis requires a thorough history and physical exam.


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.


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.


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]


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.


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] 


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


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]


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]

Deterrence and Patient Education

Patient education should focus on the underlying cause and the progressive steps to address what they may view as "water on the knee." If their treatment regimen includes physical therapy, exercise adherence must be stressed.

Enhancing Healthcare Team Outcomes

Managing knee effusions requires an interprofessional team of healthcare professionals that includes a nurse, laboratory technologists, and a number of clinicians in different specialties. Without proper management, 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 of septic arthritis.

The best outcome for a patient can be achieved with a coordinated therapy plan between multi-specialty and interprofessional teams. This starts the moment the patient is in triage in the ER or the ambulatory clinic, to the diagnosis of the septic knee, orthopedic management, and finally, appropriate treatment. Essential for a timely diagnosis is the diagnostic laboratory. Consultation with the pharmacist and infectious disease clinician 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. All members of the interprofessional team must exercise open communication with the rest of the care team to ensure optimal patient outcomes, alert appropriate personnel regarding changes in the patient's condition, and maintain accurate records so that everyone involved in the case has updated, accurate patient information from which to make clinical decisions.

Using this shared decision-making model, communication among providers is key to achieving 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 3]



Ahmed Mabrouk


Alexei DeCastro


11/15/2022 4:42:02 PM



Johnson MW. Acute knee effusions: a systematic approach to diagnosis. American family physician. 2000 Apr 15:61(8):2391-400     [PubMed PMID: 10794580]

Level 1 (high-level) evidence


Gupte C, St Mart JP. The acute swollen knee: diagnosis and management. Journal of the Royal Society of Medicine. 2013 Jul:106(7):259-68. doi: 10.1177/0141076813482831. Epub     [PubMed PMID: 23821708]


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:60(10):794-7     [PubMed PMID: 14504371]


Becker JA, Daily JP, Pohlgeers KM. Acute Monoarthritis: Diagnosis in Adults. American family physician. 2016 Nov 15:94(10):810-816     [PubMed PMID: 27929277]


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 3:286(13):1610-20     [PubMed PMID: 11585485]


Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007 Apr 4:297(13):1478-88     [PubMed PMID: 17405973]


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:69(1):12-9. doi: 10.1136/ard.2008.104406. Epub     [PubMed PMID: 19147613]


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:11(3):276-80     [PubMed PMID: 15001408]


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:25(7):749-52     [PubMed PMID: 17870475]


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:32(1):23-6     [PubMed PMID: 17239729]


Guardado KE, Sergent S. Pediatric unilateral knee swelling: a case report of a complicated differential diagnosis and often overlooked cause. Journal of osteopathic medicine. 2022 Jan 6:122(2):105-109. doi: 10.1515/jom-2020-0332. Epub 2022 Jan 6     [PubMed PMID: 34989214]

Level 3 (low-level) evidence


Maness DL, Martin M, Mitchell G. Poststreptococcal Illness: Recognition and Management. American family physician. 2018 Apr 15:97(8):517-522     [PubMed PMID: 29671499]


Naranje S, Kelly DM, Sawyer JR. A Systematic Approach to the Evaluation of a Limping Child. American family physician. 2015 Nov 15:92(10):908-16     [PubMed PMID: 26554284]

Level 1 (high-level) evidence


Horowitz DL, Katzap E, Horowitz S, Barilla-LaBarca ML. Approach to septic arthritis. American family physician. 2011 Sep 15:84(6):653-60     [PubMed PMID: 21916390]


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:40(5):884-92     [PubMed PMID: 9153550]


Earwood JS, Walker TR, Sue GJC. Septic Arthritis: Diagnosis and Treatment. American family physician. 2021 Dec 1:104(6):589-597     [PubMed PMID: 34913662]


Davis DL, Vachhani P. Traumatic Extra-capsular and Intra-capsular Floating Fat: Fat-fluid Levels of the Knee Revisited. Journal of clinical imaging science. 2015:5():60. doi: 10.4103/2156-7514.170729. Epub 2015 Nov 30     [PubMed PMID: 26713176]


McCaig LF, Nawar EW. National Hospital Ambulatory Medical Care Survey: 2004 emergency department summary. Advance data. 2006 Jun 23:(372):1-29     [PubMed PMID: 16841785]

Level 3 (low-level) evidence


Mathison DJ, Teach SJ. Approach to knee effusions. Pediatric emergency care. 2009 Nov:25(11):773-86; quiz 787-8. doi: 10.1097/PEC.0b013e3181bec987. Epub     [PubMed PMID: 19915432]


Jansen NW, Roosendaal G, Bijlsma JW, Degroot J, Lafeber FP. Exposure of human cartilage tissue to low concentrations of blood for a short period of time leads to prolonged cartilage damage: an in vitro study. Arthritis and rheumatism. 2007 Jan:56(1):199-207     [PubMed PMID: 17195222]


Gelber AC, Hochberg MC, Mead LA, Wang NY, Wigley FM, Klag MJ. Joint injury in young adults and risk for subsequent knee and hip osteoarthritis. Annals of internal medicine. 2000 Sep 5:133(5):321-8     [PubMed PMID: 10979876]


Roosendaal G, Vianen ME, van den Berg HM, Lafeber FP, Bijlsma JW. Cartilage damage as a result of hemarthrosis in a human in vitro model. The Journal of rheumatology. 1997 Jul:24(7):1350-4     [PubMed PMID: 9228136]


Maffulli N, Binfield PM, King JB, Good CJ. Acute haemarthrosis of the knee in athletes. A prospective study of 106 cases. The Journal of bone and joint surgery. British volume. 1993 Nov:75(6):945-9     [PubMed PMID: 8245089]

Level 3 (low-level) evidence


Lombardi M, Cardenas AC. Hemarthrosis. StatPearls. 2023 Jan:():     [PubMed PMID: 30252255]


Adalberth T, Roos H, Laurén M, Akeson P, Sloth M, Jonsson K, Lindstrand A, Lohmander LS. Magnetic resonance imaging, scintigraphy, and arthroscopic evaluation of traumatic hemarthrosis of the knee. The American journal of sports medicine. 1997 Mar-Apr:25(2):231-7     [PubMed PMID: 9079180]


Olsson O, Isacsson A, Englund M, Frobell RB. Epidemiology of intra- and peri-articular structural injuries in traumatic knee joint hemarthrosis - data from 1145 consecutive knees with subacute MRI. Osteoarthritis and cartilage. 2016 Nov:24(11):1890-1897. doi: 10.1016/j.joca.2016.06.006. Epub 2016 Jun 29     [PubMed PMID: 27374877]


Abbasi D, May MM, Wall EJ, Chan G, Parikh SN. MRI findings in adolescent patients with acute traumatic knee hemarthrosis. Journal of pediatric orthopedics. 2012 Dec:32(8):760-4. doi: 10.1097/BPO.0b013e3182648d45. Epub     [PubMed PMID: 23147616]


Wang JH, Lee JH, Cho Y, Shin JM, Lee BH. Efficacy of knee joint aspiration in patients with acute ACL injury in the emergency department. Injury. 2016 Aug:47(8):1744-9. doi: 10.1016/j.injury.2016.05.025. Epub 2016 May 18     [PubMed PMID: 27262773]