Anterior view of a torn anterior cruciate ligament (ACL) of the knee. ACL tears can cause swelling, pain, and instability in addition to other symptoms.

A person who experiences an acute ACL tear often reports some combination of the following knee symptoms:

  • Feeling a “pop” at the time of injury
  • Immediate swelling after the injury
  • Deep pain in the knee
  • Instability: the knee feels like it is “giving out”
  • Restricted range of motion, with particular difficulty straightening the affected knee
  • The knee may feel warm to the touch, due to bleeding within the knee joint
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Instability may be especially noticeable during activities that strain the knee joint, such as walking downstairs and pivoting on one leg.

See Common Running Injuries: Knee Pain

Unlike progressive knee conditions, such as knee osteoarthritis, symptoms of an ACL tear are sudden onset and can almost always be traced back to a specific incident or injury.

Without treatment, the swelling and pain in the knee generally subside over a period of several weeks; however the knee instability often persists, limiting the person’s ability to engage in athletics and even daily activities. In particular, it may be difficult for the person to stand up from a sitting position or going downstairs.

Additional symptoms

Other structures in the knee, such as the meniscus and medical collateral ligament (MCL), are often damaged with ACL injuries. These other injuries may cause additional symptoms, such as knee locking, to occur.