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Knee Sprains and Related Injuries

(Anterior Cruciate Ligament Tear; Meniscus Injury; Medial Collateral Ligament Tear; Posterior Cruciate Ligament Tear)

By Danielle Campagne, MD, Assistant Clinical Professor, Department of Emergency Medicine, University of San Francisco - Fresno

Knee sprains occur when the ligaments that attach the thighbone (femur) to the shinbone (tibia) are torn. The cartilage pads (menisci), which act as shock absorbers in the knee, may also be injured.

  • Knee sprains are often caused by bending or twisting the knee when the foot is planted on the ground.

  • The knee is usually painful and swollen.

  • The diagnosis is usually based on results of the physical examination.

  • Resting and keeping the knee immobile are often the only treatment needed, but sometimes severe injuries must be repaired surgically.

Several ligaments help hold the knee in place:

  • Collateral ligaments: These ligaments, located on either side of the knee prevent the knee from moving from side to side too much. The medial collateral ligament is located on the inside of the leg, and the lateral collateral ligament is located on the outside of the leg.

  • Cruciate ligaments: These ligaments prevent the knee from moving forward or backward too much. The anterior cruciate ligament (ACL) crosses in front of the posterior cruciate ligament (PCL) to form an X.

Cartilage pads (menisci) fill in the space between the thighbone (femur) and shinbone (tibia). They help stabilize and cushion the knee joint.

Holding the Knee Together

Two ligaments, one on either side of the knee, prevent the knee from moving from side to side too much:

  • Medial collateral ligament, which is on the inside of the leg

  • Lateral collateral ligament, which is on the outside of the leg

Two ligaments inside the joint (cruciate ligaments) prevent the knee from moving forward or backward too much:

  • Anterior cruciate ligament (ACL)

  • Posterior cruciate ligament (PCL)

The ACL crosses in front of the PCL, forming an X.

The menisci are pads of cartilage that act as cushions between the thighbone (femur) and larger lower leg bone (tibia), which form part of the knee joint.

The most commonly injured structures in the knee are

  • The medial collateral ligament

  • The anterior cruciate ligament

Which structures are torn depends on the direction of the force against the knee:

  • Medial collateral and anterior cruciate ligaments: One or both of these ligaments can be torn the knee is hit from the side while people have their weight on one foot that is firmly planted on the ground, as occurs during a football tackle. Damage is more likely if the knee also twists.

  • Lateral collateral and anterior cruciate ligaments: These ligaments can be damaged when force against the knee is directed outward. This type of injury can occur when the leg is pushed from the inside.

  • Anterior and posterior cruciate ligaments:These ligaments can be damaged when the knee is forcefully straightened.

  • Menisci: The menisci can be damaged when people have their weight on a foot and their knee twists as it is injured.


Occasionally, people hear or feel a pop in the knee when the injury occurs. This pop usually indicates that a ligament (particularly the anterior cruciate ligament) is torn.

The knee is painful, swollen, stiff, and sometimes bruised. Where the pain is depends on which structure is injured. The knee may feel unstable, as if it will buckle. Muscle spasms—unintended contraction of muscles around the knee—may occur. When the symptoms develop and how severe they are depend on the severity of the injury:

  • Mild: Swelling develops over the first few hours but may take more than 24 hours. Pain is usually mild or moderate.

  • Moderate: Pain is moderate or severe, particularly when people move or bend their knee.

  • Severe: Pain may be severe, moderate, or mild, and some people cannot walk unaided.

Sometimes a torn meniscus prevents the knee from bending (called locking).

Sometimes the force that causes a knee sprain also causes bones to be broken and/or damages tendons in the knee (knee extensor injuries).


  • A doctor's evaluation

  • X-rays to check for broken bones

  • Sometimes magnetic resonance imaging

Doctors suspect a knee sprain when people have typical symptoms (such as a swelling) and have had an injury that is likely to cause a sprain.

Stress testing

Doctors check for a torn ligament in the knee by moving the leg in certain ways (called stress testing). A thorough examination, including stress testing, usually enables doctors to identify knee injuries.

However, stress testing is typically delayed because the knee is usually too painful to test when doctors first evaluate a person. Also, substantial swelling and muscle spasms may make evaluation of the knee difficult. Stress testing can be done a few days later, when the symptoms lessen.

Imaging tests

If the knee if very painful or swollen, doctors usually take an x-ray before stress testing to check for broken bones.

Certain findings make broken bones more likely:

  • Intense pain is felt in certain areas of the knee joint.

  • People cannot bend their knee.

  • People cannot put any weight on the ankle because of the pain.

  • The person is over 55.

Magnetic resonance imaging (MRI) is usually not done at first. It may be done if

  • A severe injury is suspected.

  • Symptoms do not resolve after several weeks of protection, rest, ice, compression, and elevation (PRICE).


  • Sometimes drainage of fluid

  • Protection, rest, ice, compression, and elevation

  • A splint or knee immobilizer and crutches

  • Sometimes surgery

If a large amount of fluid has built up in the knee, doctors sometimes drain the fluid to help relieve the pain and muscle spasms.

Most mild or moderate injuries can be treated initially with protection, rest, ice, compression, and elevation (PRICE), including immobilization of the knee with a splint or a device that supports the knee and keeps it from bending (knee immobilizer).

If the sprain is severe, some people need to wear a knee immobilizer for 6 weeks or longer. Some severe injuries of ligaments or menisci require surgical repair by an orthopedic surgeon. Surgical repair is usually done through a small incision and a small flexible tube—a procedure called arthroscopic surgery.

Exercises to strengthen the knee are usually recommended for people with mild or moderate injuries. If injuries are severe, strengthening exercises are postponed until after surgery.

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