[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical exam](https://durgical.com/en/clinical-exam/) › [Musculoskeletal system](https://durgical.com/en/clinical-exam/#a-musculoskeletal) › Knee effusion: patellar tap and bulge sign [Clinical exam](https://durgical.com/en/clinical-exam/) · [Musculoskeletal system](https://durgical.com/en/clinical-exam/#a-musculoskeletal) # Knee effusion: patellar tap and bulge sign - Maneuver Two maneuvers to tell whether there is fluid inside the knee: the kneecap that bounces when there is a lot and the bulge that appears when there is little. Updated 2026-11-01 **See it in 3D:** [Patella](https://durgical.com/en/anatomy/patella/) · [Suprapatellar bursa](https://durgical.com/en/anatomy/suprapatellar-bursa/) · [Joint capsule](https://durgical.com/en/anatomy/articular-capsule-of-knee-joint/) · [Femur](https://durgical.com/en/anatomy/femur/) ## Key points - Patellar tap: moderate to large effusions; bulge sign: small effusions. - Effusion within the first hours after trauma: hemarthrosis; think of ACL tear. - Acute hot single knee: septic arthritis until the fluid analysis says otherwise. - Fluid collects in the suprapatellar bursa, which communicates with the joint. ## What it is A joint effusion is extra fluid inside the synovial cavity of the knee. The patellar tap (ballottement) detects moderate to large effusions and the bulge (sweep) sign detects small ones. Knowing that there is an effusion, and when it appeared, points to traumatic hemarthrosis, meniscal injury, osteoarthritis, crystal arthritis or infection. ## The anatomy it tests - The synovial cavity of the knee continues upward as the suprapatellar bursa, under the quadriceps tendon. - Squeezing that bursa gathers fluid under the patella and lifts it off the femur, which is why the patella floats and bounces. - In small effusions the fluid shifts between the medial and lateral recesses, and the bulge sign makes it visible. ## How to do it - Patient supine, quadriceps relaxed, knee straight. - Inspection: look for loss of the hollows beside the patella and fullness above it, compared with the other knee. - Patellar tap: with one hand, squeeze the suprapatellar pouch from the thigh down toward the patella and hold it there. - With two fingers of the other hand, push the patella sharply against the femur and release. - Positive tap: the patella sinks, knocks on the femur and springs back; this means a moderate to large effusion. - Bulge sign: stroke upward along the medial side of the knee to move fluid into the suprapatellar pouch and the lateral side. - Then stroke downward along the lateral side of the patella and watch the medial hollow. - Positive bulge: within a second or two a bulge appears on the medial side; this detects small effusions. ## What a positive result means - Effusion within the first hours after trauma: hemarthrosis (ACL tear, tibial plateau fracture, patellar dislocation). - Effusion that appears hours later or the day after a twist: meniscal injury. - Hot, painful effusion, with or without fever: septic arthritis, gout or pseudogout; the fluid is examined. - Cool, recurrent effusion with crepitus: osteoarthritis. - Swelling in front of the patella with no fluid inside the joint: prepatellar bursitis. - A very large, tense effusion can give a negative bulge sign, so both tests are combined. ## Usefulness and limits - Neither test measures the effusion well; ultrasound confirms it. - Arthrocentesis with fluid analysis (cells, crystals, Gram stain and culture) defines the cause of an unexplained effusion. - A contracted quadriceps or a knee with a lot of soft tissue can give false negatives. ## Test yourself Think of the answer, then tap the question to see it. ## Which test best detects a small knee effusion? The bulge sign: the medial side is emptied upward, the lateral side is stroked and a medial bulge appears. ## How is the patellar tap performed? One hand squeezes the suprapatellar bursa toward the patella and the other pushes the patella against the femur; if it bounces back, there is a moderate or large effusion. ## What does an effusion appearing within the first hours after a knee twist suggest? Hemarthrosis, whose most common cause is an anterior cruciate ligament tear. ## Which test defines the cause of an acute hot effusion? Arthrocentesis with synovial fluid analysis: cell count, crystals, Gram stain and culture. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-exam/knee-effusion-patellar-tap-and-bulge-sign/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Patella](https://durgical.com/en/anatomy/patella/) - [Suprapatellar bursa](https://durgical.com/en/anatomy/suprapatellar-bursa/) - [Joint capsule](https://durgical.com/en/anatomy/articular-capsule-of-knee-joint/) - [Femur](https://durgical.com/en/anatomy/femur/) ## Related - [Joint examination: inspection, palpation and range of motion](https://durgical.com/en/clinical-exam/joint-examination-inspection-palpation-and-range-of-motion/) - [Lachman test and anterior drawer](https://durgical.com/en/clinical-exam/lachman-test-and-anterior-drawer/) - [Meniscal tests: McMurray and Apley](https://durgical.com/en/clinical-exam/meniscal-tests-mcmurray-and-apley/) ## Related conditions - [Septic Arthritis of the Knee](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/) - [Gout of the Knee](https://durgical.com/en/conditions/gout-of-the-knee/) - [Pseudogout (CPPD) of the Knee](https://durgical.com/en/conditions/pseudogout-of-the-knee/) - [Knee Osteoarthritis](https://durgical.com/en/conditions/knee-osteoarthritis/) - [ACL Tear](https://durgical.com/en/conditions/acl-tear/) - [Prepatellar Bursitis](https://durgical.com/en/conditions/prepatellar-bursitis/) [← All of the clinical exam](https://durgical.com/en/clinical-exam/) Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis. Sources: [Musculo-Skeletal Examination – Knee Exam](https://sites.ucsd.edu/clinicalmedguide/musculo-skeletal-examination-the-knee-exam/) (UC San Diego School of Medicine, Practical Guide to Clinical Medicine, 2026); [Musculo-Skeletal Exam – Practical Guide to Clinical Medicine](https://sites.ucsd.edu/clinicalmedguide/musculo-skeletal-exam/) (UC San Diego School of Medicine, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/clinical-exam/knee-effusion-patellar-tap-and-bulge-sign/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-exam/knee-effusion-patellar-tap-and-bulge-sign/