Noticing that one of your knees has suddenly become swollen — while the other feels completely normal — can be unsettling. While swelling in both knees often points to underlying conditions like arthritis, swelling in only one knee is usually a sign of a more localized issue. Knowing the possible reasons can help you decide whether rest is enough or if a doctor’s visit is in order.
Injury or Overuse
One of the most common causes of swelling in just one knee is some form of trauma — even if it’s minor or repetitive and you don’t remember exactly when it happened. Potential culprits include:
- Ligament injuries (like ACL or MCL strains or tears)
- Meniscus damage
- Dislocation of the kneecap
- Bone contusions or even small fractures
These types of injuries often cause swelling within hours, sometimes along with pain, stiffness, or the sense that your knee isn’t stable.
Bursitis
Your knee has small fluid-filled cushions called bursae. If one of them gets inflamed — often from kneeling a lot, a bump, or repetitive stress — it can result in a condition called prepatellar bursitis. This usually affects just one knee and may cause a noticeable bulge at the front.
Joint Infection
If swelling comes on quickly and is paired with redness, warmth, or sharp pain, an infection in the joint (called septic arthritis) could be the cause. This is serious — especially if there’s a fever or chills. If suspected, urgent medical care is critical, and fluid may need to be drained from the joint.
Gout or Pseudogout
