Tapping a knee, also known as knee arthrocentesis or joint aspiration, is performed to extract fluid from the knee joint. This procedure is indicated in various situations to diagnose and/or relieve symptoms. The references provided suggest the following scenarios for when to tap a knee:
Indications for Knee Arthrocentesis
| Indication | Description |
|---|---|
| Monoarticular Arthritis | To evaluate the cause of inflammation in a single joint, helping to differentiate between various types of arthritis. (Reference 1) |
| Suspected Septic Arthritis | To rule out or confirm a bacterial infection within the joint. This is crucial to prevent joint destruction. (Reference 2) |
| Joint Effusion | To relieve pain and pressure caused by excessive fluid buildup in the knee joint, regardless of the underlying cause. (Reference 3) |
| Intra-articular Fracture | To identify the presence of fat droplets in the aspirated fluid, which is suggestive of a fracture within the joint. (Reference 4) |
| Crystal Arthropathy | To diagnose conditions like gout or pseudogout by identifying crystals (e.g., monosodium urate or calcium pyrophosphate dihydrate) in the synovial fluid. (Reference 5) |
Detailed Explanations:
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Diagnosis of Monoarticular Arthritis: If a patient presents with inflammation in only one knee joint, tapping the knee allows for analysis of the synovial fluid to determine the underlying cause, such as infection, crystal deposition, or inflammatory arthritis.
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Ruling out Septic Arthritis: Septic arthritis is a medical emergency. A knee tap is essential to obtain fluid for Gram stain and culture to identify the presence of bacteria and guide antibiotic treatment.
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Relief of Joint Effusion: Large effusions can cause significant pain and limited range of motion. Aspiration can relieve pressure and improve comfort. The fluid can then be sent for analysis to determine the cause of the effusion.
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Identification of Intra-articular Fracture: If a patient has suffered a knee injury and a fracture within the joint is suspected, aspiration can help identify fat droplets in the fluid, which indicates that bone marrow has leaked into the joint space due to a fracture.
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Diagnosis of Crystal-Induced Arthropathy: Patients with sudden, severe knee pain may have gout or pseudogout. Synovial fluid analysis under polarized light microscopy can reveal the presence of characteristic crystals.
