Impact injury to the knee

Understanding the mechanics of how a knee is injured, its causes and the treatments is essential for prevention and rehabilitation. The aim is to use this information to help runners identify and recover from their knees injuries.


Torn ligament

The knee contains four main ligaments that connect the tibia to the femur (thighbone) and allow the knee to move forward, backward, and rotate side to side:

  • anterior cruciate ligament (ACL)
  • posterior cruciate ligament (PCL)
  • medial collateral ligament (MCL)
  • lateral collateral ligament (LCL)

A high-impact injury such as a fall or a blow to the side of the knee can cause one or more of these ligaments to tear.


Torn ACL

A torn ACL in the knee as either partial or complete and symptoms include hearing a loud pop as the ligament tears, pain, knee swelling, and difficulty walking.A torn ACL can be grouped into two categories: contact and non-contact.

Non-contact injuries would be when an athlete rapidly decelerates, followed by a sharp or sudden change in direction (cutting). Noncontact torn ACL injuries have also been linked to heavy or stiff-legged landing as well as twisting or turning the knee while landing, especially when the knee is in the valgus (knock-knee) position.

Contact injuries most commonly occur after a blow to the outside of the leg causing the knee to “buckle” and assume a valgus position.  More information on Torn ACL


Knee Sprain

A knee sprain occurs when one or more of the ligaments connecting the bones in the knee are stretched beyond their usual capacity. This can happen if you fall hard or are hit by something with a certain force, such as a football tackle. Minor sprains can be resolved at home with rest.   You should be checked out by a doctor if you have:

  • Significant swelling
  • Severe pain
  • You have a lot of difficulty moving your knee


Torn Meniscus

The meniscus is a piece of elastic cartilage situated between the femur and the tibia and works to reduce the impact and provide support for the two bones. Sharp turns can often lead to a meniscus tear, though a hard fall can sometimes be the cause too. In some cases, non-surgical treatments can be used to address a meniscus tear; however, persisting pain and swelling may suggest the need for an operation in order to manage the injury.  More information on Torn Meniscus.


When should I see a doctor?

Symptoms that indicate the need for prompt medical attention include:

  • Hearing or feeling a “pop” sound as you fall as this is often associated with a torn ligament.
  • A feeling that the knee is unstable, buckles, or gives way is also commonly associated with a torn ligament.
  • Swelling of the knee can indicate a torn ligament or fracture.
  • Your knee feeling warm to the touch after a fall could be a sign of inflammation stemming from an injured tendon or muscle. Warmth can also be a sign of infection or bursitis.
  • Being unable to put weight on your knee may mean that there’s structural damage to the joint.
  • Bleeding from a cut or scrape persisting after several minutes may require medical attention.
  • The area around a cut or scrape turning red or swollen or leaking pus some time after the injury can signal an infection.