Additional Knee Examination Tests After Positive Patellar Ballottement
After confirming joint effusion with patellar ballottement, you should immediately perform ligamentous stability tests (Lachman, posterior drawer, collateral ligament stress tests) and meniscal provocation tests (McMurray, joint line palpation), followed by patellar tracking assessment if there is concern for instability. 1
Essential Physical Examination Sequence
Ligamentous Integrity Assessment
Perform the Lachman test first - this is the most sensitive and specific test for anterior cruciate ligament (ACL) tears, which commonly present with acute knee effusion 1. The test should be performed with the knee at 20-30 degrees of flexion, applying anterior translation force to the proximal tibia while stabilizing the distal femur.
- Posterior cruciate ligament (PCL) evaluation: Perform the posterior drawer test and observe for posterior sag sign, both highly sensitive and specific for PCL injury 1
- Collateral ligament testing: Apply valgus stress (medial collateral ligament) and varus stress (lateral collateral ligament) at 0 and 30 degrees of flexion, though note that well-designed validation studies are limited 1
Meniscal Pathology Assessment
- Joint line palpation: This has fairly good sensitivity for meniscal tears but lacks specificity 1
- McMurray test: While highly specific for meniscal tears, this test has very low sensitivity, meaning a negative test does not rule out meniscal pathology 1
Patellar Tracking and Stability Evaluation
If there is any clinical suspicion for patellar instability (history of giving way, lateral knee pain, or visible malalignment):
- Moving patellar apprehension test: This has 100% sensitivity and 88.4% specificity for patellar instability 2. Start with the knee in full extension, translate the patella laterally with your thumb, flex to 90 degrees, then return to extension while maintaining lateral force. A positive test elicits apprehension and quadriceps activation 2
- Tibial external rotation test (TERT): With the knee at 90 degrees flexion, externally rotate the tibia and have the patient extend against resistance. If the patella subluxates at approximately 25 degrees of flexion with external rotation but remains stable with internal rotation, the test is positive 3
Critical Clinical Pearls
The combination of self-reported swelling and positive ballottement test has a likelihood ratio of 3.6 for confirming effusion 4, and effusion is strongly associated with internal derangement - 31 of 42 patients (74%) with MRI-confirmed effusion had internal knee pathology 4.
Common pitfall: Patellofemoral pain tests lack sensitivity when correlated with operative findings 1, so do not rely solely on these tests to rule out patellar pathology.
Imaging Considerations After Physical Examination
- Standard radiographs remain first-line: Obtain anteroposterior, lateral, and axial/patellofemoral views 5, 6
- Weight-bearing axial views provide superior assessment of patellofemoral kinematics compared to standard views 7, 6
- Cross-table lateral view can visualize lipohemarthrosis, which indicates intra-articular fracture 5
If radiographs are negative but clinical suspicion remains high for ligamentous injury, meniscal tear, or occult fracture based on your physical examination findings, MRI without contrast is the next appropriate step 8.