What Does a Positive Ballottement Test Mean?
A positive patellar ballottement test indicates the presence of knee joint effusion (fluid accumulation within the knee joint). This finding suggests intra-articular pathology and warrants further evaluation to determine the underlying cause.
Clinical Significance
The ballottement test detects moderate to large knee joint effusions by assessing whether the patella can be displaced downward against the femur and then rebounds when pressure is released, creating a palpable "tap" sensation 1.
When combined with patient-reported swelling, the diagnostic accuracy improves substantially, with a positive likelihood ratio of 3.6 for confirming effusion on imaging 1.
Effusion is strongly associated with internal knee derangement: in traumatic knee injuries, 74% of patients with effusion on MRI had internal derangement of the knee (meniscal tears, ligament injuries, or fractures) 1.
Diagnostic Performance
The ballottement test alone has modest diagnostic value with a positive likelihood ratio of 1.6, meaning it increases the probability of effusion but is not definitive by itself 1.
Sensitivity and specificity are moderate when used in isolation, which is why combining it with patient history (self-noticed swelling) significantly enhances diagnostic accuracy 1.
Clinical Context and Differential Diagnosis
When a ballottement test is positive, consider these underlying causes:
- Traumatic injuries: meniscal tears, ligament ruptures (ACL/PCL), or intra-articular fractures
- Inflammatory conditions: septic arthritis, crystalline arthropathy, or inflammatory arthritis
- Post-surgical complications: particularly after total knee arthroplasty where effusion may indicate infection, loosening, or particle disease 2
- Hemarthrosis: particularly with acute trauma and intra-articular fractures
Important Clinical Caveats
A negative ballottement test does not rule out effusion, especially with small effusions that may still be clinically significant 1.
The test requires adequate technique: the suprapatellar pouch should be compressed to displace fluid into the joint space before attempting to ballot the patella 1.
In post-arthroplasty patients, effusion seen on imaging does not reliably distinguish between infection and non-infectious complications such as mechanical loosening or particle disease, requiring additional diagnostic workup including aspiration 2.
Recommended Follow-up
Standard radiographs should be obtained as the first-line imaging modality when effusion is detected in the setting of trauma or suspected fracture 3, 4.
Joint aspiration is indicated when infection is suspected (particularly in post-arthroplasty patients or those with systemic signs), as it provides definitive diagnosis with sensitivity of 75-100% and specificity of 100% for infection 2.
MRI may be warranted when internal derangement is suspected based on mechanism of injury and clinical findings, given the strong association between effusion and soft tissue injuries 1.