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Knee gives way or popped

Also called: Knee instability, ACL tear, knee buckling

A knee that gives way or popped during an injury, often during sports, can mean a torn ligament like the ACL. Swelling usually follows within hours. These injuries need an exam, and some need surgery. Knees can also give way from weak muscles or kneecap problems.

Common causesACL tear, meniscus tear, kneecap dislocation, weak muscles
Usually serious?Can be
Get checked ifYou heard a pop, it swelled quickly, or it keeps giving way
If you think it's an emergency, call 911.Thinking about suicide? Call or text 988.

Overview

The ACL often tears during sudden stops, pivots, or landing from a jump. Many people hear or feel a pop, then the knee swells and feels unstable.

A kneecap that slips out of place can also cause a pop and giving way. Meniscus tears can make the knee catch or buckle. Weak thigh muscles and arthritis can make the knee feel unstable over time.

When to get medical care

Call 911A knee that looks deformed, or a cold, pale, numb foot after an injury.
Get care todayA pop with fast swelling. You can't bear weight. The kneecap is out of place.
See a doctor soonThe knee keeps giving way. Swelling or pain that lasts more than a few days after an injury. You want to return to sports after an injury.

Causes

Common causes include ACL tears, other ligament injuries (MCL, PCL), meniscus tears, kneecap dislocation or instability, and weak thigh muscles. Arthritis and loose bodies in the joint can also cause buckling.

OwieClear has pages on 2 conditions that can cause knee gives way or popped. Each shows how soon to get care.

Common causes

  • Knee ligament or meniscus injurySee a doctor if it doesn't get better

    A tear or sprain of a ligament (such as the ACL) or the cartilage cushion (meniscus) in the knee, usually from twisting, pivoting, or a fall.

  • Quadriceps or patellar tendon tearOften needs care today

    A tear of the tendon above or below the kneecap, often from landing a jump or stumbling.

What you can do at home

  • Rest, ice, compress, and raise the knee.
  • Use crutches if it's hard to walk.
  • Avoid pivoting or sports until checked.
  • Do strengthening exercises as advised.

Medicines

Ibuprofen or naproxen can help pain and swelling. Physical therapy is key. Some ACL tears need surgery, especially for athletes or if the knee keeps giving way. A brace may help.

At the doctor

The provider will test the knee's ligaments.

Questions you may be asked

  • How the injury happened
  • A pop and how fast it swelled
  • Giving way
  • Sports and activities

The exam

A knee exam testing ligament stability.

Tests

  • X-rays
  • An MRI

Prevention

You can lower your risk.

  • Strengthen legs and hips
  • Practice landing and pivoting techniques
  • Warm up before sports

Check your symptoms

Most symptoms have more than one possible cause. The symptom checker looks at all of your symptoms together, plus your age, and shows how soon to get care.

Start the symptom checker

Sources

Last updated 2026-11-13.