Why Is One Knee Swollen but Not the Other

Noticing that only one knee is swollen can be alarming—especially when the other knee feels perfectly fine. While swelling in both knees often points to systemic issues like arthritis, swelling in just one knee usually indicates a localized problem. Understanding the potential causes behind unilateral knee swelling can help you decide whether it’s something minor or a sign you need medical attention.

1. Injury or Trauma

The most common reason for swelling in just one knee is trauma or overuse. You may not even remember the specific moment of injury—especially if it was a repetitive motion injury. Some possible issues include:

• Ligament sprain or tear (ACL, MCL)
• Meniscus tear
• Patellar dislocation
• Fracture or bone bruise

In these cases, swelling often appears within hours and may be accompanied by pain, stiffness, or a feeling of instability in the joint.

2. Bursitis

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