Skip to Main Content
Research & Innovation

How To Tell If a Knee Injury Is Serious—Yale Medicine Explains

December 2, 2021

Knee injuries can range from mildly uncomfortable to devastating. If you’ve hurt your knee, it’s important to assess the damage so you can decide whether you should see a doctor.  

A key sign to look for is swelling. “Serious injuries cause a lot of swelling in the knee, especially in younger patients,” says Christina Allen, MD, division chief of Sports Medicine. This is especially so when the swelling arises quickly. “Someone could be arthritic and get some swelling the next day, but the rapid onset of swelling in which it’s hard to see where your kneecap is, is a cardinal sign that there’s something serious that’s happened to your knee.”  

According to Dr. Allen, swelling that comes on suddenly is usually caused by blood trapped in the joint; it typically occurs within three to four hours of the injury. If the injury is a torn ACL, a small blood vessel is also torn—and when that happens, the swelling can also be significant.  

“If your knee looks like a grapefruit, you should get it checked out,” Allen says.  

In this short video, Dr. Allen explains the basics of knee injuries and when to see a doctor.