Effusion tests of the Knee
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Introduction
Knee joint effusion is the excessive accumulation of synovial fluid within the intra-articular space, typically caused by trauma, overuse or systemic disease.[1] Understanding the pathoanatomy of the knee is an essential part of making a correct diagnosis and generating the treatment plan. Knee joint effusion can be diagnosed by physical examination, ultrasound and MRI.[1] Various clinical tests can be conducted to assess the presence of knee effusion, which includes visual inspection of the swollen joint, palpation of the joint, and special tests such as the ballottement/patellar tap test, which is the most appropriate one for moderate to large effusions, the bulge sign for small effusions, and the sweep test for minimal effusions. At times the balloon sign is used to confirm the presence of fluid. Timely detection and treatment are essential to avoid deterioration of the joint.[2][3] [4]
Pathophysiology of Knee Effusion
Fluid Distribution and Detection
The synovial membrane is the largest synovial structure in the body, forming interconnected anterior, parameniscal, and posterior recesses around the knee and secreting the fluid that lubricates and nourishes the joint.[4] When excess fluid accumulates, its distribution is influenced by effusion volume, knee position, and the presence of a posterior joint cyst (Baker's cyst).[3] In full extension, fluid collects first in the lateral parapatellar recess; as volume increases it fills the medial parapatellar recess and subsequently the suprapatellar recess. Clinically, intra-articular fluid is generally not detectable until the volume exceeds 6–8 mL.[2]
Distinguishing between intra-articular and extra-articular swelling is essential for accurate clinical reasoning.[3] Intra-articular effusion tends to be diffuse and symmetrical, filling the suprapatellar pouch and parapatellar gutters and demonstrating a fluid wave on testing. On ultrasound, simple joint fluid appears anechoic, while haemarthrosis produces hyperechoic, more heterogeneous fluid. Clinically, haemarthrosis also tends to feel more viscous on palpation.[2] Extra-articular swelling, by contrast, is immobile, does not produce a fluid wave, and typically presents as localised firm soft tissue thickening.[2] Synovial thickening (synovitis without free fluid) may feel dense and resist displacement but similarly does not produce a palpable fluid wave.[5]
Associated Conditions
The aetiology of knee effusion may range from acute trauma and infection to chronic systemic conditions including degenerative, haematological, and inflammatory pathologies. Conditions commonly associated with knee effusion include:[2]
- Osteoarthritis - the most common cause of effusion in older adults
- Ligament or meniscal injury
- Intra-articular fractures and traumatic haemarthrosis
- Inflammatory arthritis (e.g., rheumatoid arthritis, psoriatic arthritis)
- Crystal deposition disorders (gout, pseudogout)
- Septic arthritis
- Patellofemoral pathology (e.g., plical inflammation, chondral loose bodies, osteochondral damage)
- Overuse syndromes
- Neoplastic pathology (less common)
Effusion-Synovitis
Effusion is frequently accompanied by synovitis. Synovial membrane thickening and effusion identified on MRI are termed effusion-synovitis.[3] It is readily demonstrated in the suprapatellar and parapatellar regions and is distinct from inflammatory changes in the infrapatellar (Hoffa's) fat pad.[5] Longitudinal cohort data demonstrate that effusion-synovitis is independently associated with structural osteoarthritis progression, including cartilage damage and bone marrow lesions. It also mediates the relationship between quadriceps weakness and worsening knee pain in people with knee osteoarthritis.[6]
Test Techniques
Balloon Sign/ test
Alternative names: balloon test
Indication: confirming intra-articular fluid before proceeding to ballottement.
Technique
With the person lying supine and the knees in full extension, the examiner places both thumbs in the hollows on either side of the patellar tendon (medial and lateral parapatellar gutters) and compresses the suprapatellar pouch from above with the opposite hand to drive fluid distally.
Interpretation
A positive result is a fluid impulse transmitted simultaneously to both thumbs, confirming the presence of intra-articular fluid.
A negative result is the absence of any transmitted impulse.
Statistical Analysis
Experienced physiotherapists using the balloon sign have demonstrated only fair intra-observer agreement (kappa = 0.37) and poor inter-observer agreement (kappa = −0.11), highlighting the importance of standardisation and examiner experience when interpreting results.[3] The patellar tap test is typically performed as a continuation of the balloon sign once intra-articular fluid has been confirmed.
Balloon Sign[7]
Patellar Tap Test (Ballottement Test)
Also known as: ballottement patellae, ballottement test, patellar tap, effusion test
Indication: moderate to large effusions.
Technique
With the person lying supine and the knee in full extension, the examiner places one hand on the suprapatellar pouch and applies firm downward pressure to milk fluid distally into the intra-articular space beneath the patella.[3] Maintaining this pressure, the examiner uses the index finger or thumb of the other hand to press the patella sharply downward against the femur.
Interpretation
A positive result is the patella felt to displace downwards through fluid and then rebound, indicating a moderate to large intra-articular effusion. A negative result is when the patella is already in direct contact with the femoral condyles and no ballottement is detected.
Statistical Analysis
Both knees must be tested and compared, as false positives may occur.[3] The patellar tap has demonstrated inter-observer kappa agreement ranging from −0.02 to 0.75, with sensitivity of 18.2%–85.7% and specificity of 35.3%–93.3%, both higher with larger effusions.[3]
Patellar Tap Test[8]
Fluid Displacement Test (Bulge Sign / Stroke Test)
Also known as: bulge sign, wipe test, stroke test, fluid displacement test
Indicated for: small to moderate effusions.
Technique
With the person lying supine and the knee in extension, the examiner strokes firmly upwards along the medial aspect of the knee (approximately 10 cm proximal to the patella) using the edge of the hand to displace fluid from the medial gutter into the lateral compartment, then strokes downward on the lateral side while observing the medial aspect of the knee.[9]
Interpretation
A positive result is a visible fluid wave or bulge returning to the medial aspect within seconds, indicating intra-articular effusion.[9]
A negative result is the absence of visible fluid redistribution.
Statistical Analysis
When compared against ultrasound as the reference standard, visual inspection of swelling demonstrated the highest sensitivity at 90.54%, while the bulge sign demonstrated the highest specificity at 65.71% among clinical tests.[9] Of all single clinical tests, the bulge sign has also demonstrated the highest reliability, with a reliability coefficient of 0.97.[3]
Fluid Displacement Test[10]
Sweep Test
Also known as: knee hydrops test, effusion wave test, brush test, patella sweep test
Indicated for: minimal or trace effusions (not visible on inspection)
For details of the test, see Sweep test.
Sweep Test[11]
Statistical Analysis
Using a modified stroke test graded on an operationally defined 5-point scale, physiotherapists have demonstrated high inter-rater reliability (kappa = 0.75), with 54 of 75 pairs of assessors in perfect agreement and only 5 pairs disagreeing by two grades.[12]
Diagnostic Accuracy and Reliability
The diagnostic accuracy of clinical effusion tests is variable and depends on effusion volume, clinician experience, and the degree of standardisation applied.[3]
Across the individual tests, the bulge sign shows the widest range of inter-observer kappa (−0.04 to 0.97), reflecting the impact of standardisation, but achieved the highest reliability coefficient (0.97) when conditions were controlled.[3] The patellar tap similarly shows variable kappa (−0.02 to 0.75), with performance markedly influenced by effusion prevalence within the tested sample.[3] The sweep test, graded using the 5-point modified stroke scale, demonstrates the strongest inter-rater reliability of any clinical effusion test at kappa = 0.75.[12]
Given this variability, imaging is required for accurate confirmation of knee effusion. Where imaging is unavailable, a combination of clinical tests is recommended to maximise detection rates.[3]
The Role of Ultrasound
Musculoskeletal ultrasound is now widely regarded as the preferred bedside imaging adjunct for confirming effusion presence and grading its severity.[13] A meta-analysis of 14 studies demonstrated a pooled sensitivity of 0.90 and specificity of 0.86 for synovial effusion against MRI as the reference standard, with an area under the receiver operating characteristic curve of 0.94 and strong correlation between ultrasound- and MRI-defined effusion (r = 0.63).[13] Unlike MRI, ultrasound additionally permits aspiration of joint fluid and can target small-volume focal collections that may not be accessible under fluoroscopic guidance.[5]
Clinical Application for Physiotherapists
Accurate detection of knee effusion is a foundational component of the physiotherapeutic knee assessment. Clinicians should always compare both knees bilaterally, as effusion volume and contour are best appreciated relative to the unaffected side.[3] Test selection should follow effusion volume: the sweep test or fluid displacement test is the appropriate first choice when effusion is subtle, while the patellar tap is reserved for moderate to large effusions.[3]
Findings should be documented systematically using a validated grading scale such as the 5-point modified stroke test scale, which supports objective monitoring of effusion over time and informs decisions regarding rehabilitation progression.[12] Because effusion-synovitis mediates the relationship between quadriceps weakness and worsening knee pain, rehabilitation should address neuromuscular impairments alongside the effusion itself.[6] Where clinical tests are inconclusive, the aetiology is uncertain, or clinical management decisions (such as aspiration or intra-articular injection) depend on confirmation of effusion volume and location, onward referral for imaging is recommended.[13]
Summary
Three main clinical tests used by the physiotherapist to diagnose the knee joint effusion include the bulge sign, patellar tap and sweep test. Based on the volume of effusion the appropriate test can be chosen. Clinical tests are useful for diagnosis and monitoring but imaging is needed for confirmation, particularly when it will change management. None of the clinical tests detect the cause, for which imaging is essential.
References
- ↑ 1.0 1.1 Meyer R, Lin C, Yenokyan G, Ellen M. Diagnostic utility of ultrasound versus physical examination in assessing knee effusions: a systematic review and meta‐analysis. Journal of Ultrasound in Medicine. 2022 Jan;41(1):17-31.
- ↑ 2.0 2.1 2.2 2.3 2.4 Johnson MW. Acute knee effusions: a systematic approach to diagnosis. American family physician. 2000 Apr 15;61(8):2391-400.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 Maricar N, Callaghan MJ, Parkes MJ, Felson DT. Clinical assessment of effusion in knee osteoarthritis—A systematic review. InSeminars in arthritis and rheumatism 2016 Apr 1 (Vol. 45, No. 5, pp. 556-563). WB Saunders.
- ↑ 4.0 4.1 Won H, Lee HS, Youn D, Park D, Eo T, Kim W, Hwang D. Deep learning-based joint effusion classification in adult knee radiographs: A multi-center prospective study. Diagnostics. 2024 Aug 29;14(17):1900.
- ↑ 5.0 5.1 5.2 Pandya S, Melville DM. Evaluation of the knee joint with ultrasound and magnetic resonance imaging. Journal of Ultrasonography. 2023 Nov 23;23(95):e239.
- ↑ 6.0 6.1 Gong Z, Ao D, Li T, et al. Mediating role of effusion-synovitis in knee pain worsening following quadriceps weakness: data from the osteoarthritis initiative. BMC Musculoskelet Disord. 2025;26:847.
- ↑ Balloon Sign. Available from: http://www.youtube.com/watch?v=1Mt8oyPpqQs [last accessed 28/7/2026]
- ↑ Patellar Tap Test. Available from: http://www.youtube.com/watch?v=f76eVYmCmvg[last accessed 28/7/2026]
- ↑ 9.0 9.1 9.2 Elsawy NA, Ibrahiem AH, Younis GA, Meheissen MA, Abdel-Fattah YH. Clinical examination, ultrasound assessment and aspiration of knee effusion in primary knee osteoarthritis patients. J Orthop Surg Res. 2023;18:422.
- ↑ Fluid Displacement Test. Available from: http://www.youtube.com/watch?v=LHOmmWB_PFo [last accessed 28/7/2026]
- ↑ Sweep Test. Available from: http://www.youtube.com/watch?v=DLzvaHG35z4 [last accessed 28/7/2026]
- ↑ 12.0 12.1 12.2 Sturgill LP, Snyder-Mackler L, Manal TJ, Axe MJ. Interrater reliability of a clinical scale to assess knee joint effusion. J Orthop Sports Phys Ther. 2009;39(12):845–9.
- ↑ 13.0 13.1 13.2 Liu K, Li X, Weng Q, Wang Y, Wei J, Zeng C, et al. Diagnostic accuracy of ultrasound for assessment of synovial abnormalities among patients with knee pain: a meta-analysis. Arthritis Care Res. 2024;76(2):295–303.