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Knee ligaments connect the leg bones and give support to the knee joint. A sprain affecting the knee occurs if a person tears or overstretches a ligament.

The ligaments in a person’s knee are the bands of tough, elastic connective tissue that link the thighbone to the lower leg bones. They also give support and help to limit the joint’s movement.

In this article, we discuss the types of ligament sprains that affect the knee, as well as the causes, treatment options, and when to seek medical help.

a man in a doctors officer holding his sprained kneeShare on Pinterest
A person can sprain a knee ligament if they overstretch it.

There are four main ligaments in the knee, and each one plays a vital role in keeping the knee stable and enabling a person to have a full range of movement.

Where the two cruciate ligaments control the back and forth of the knee, the two collateral ligaments control the sideways movement of the knee.

The type of knee sprain a person may be experiencing depends on which ligament they have injured:

Anterior cruciate ligament

Healthcare professionals typically refer to this ligament as the ACL. It is on the inside of the knee joint.

The two cruciate ligaments control the back and forth motion of the knee. They form an ‘X’ shape together. The anterior cruciate ligament is the ligament in front.

Posterior cruciate ligament

This ligament is referred to as the PCL and is the back counterpart to the anterior cruciate ligament. The PCL controls backward movement of the tibia, or shin bone.

Medial collateral ligament

This ligament is known as the MCL and is the ligament that gives stability to the inner knee.

Collateral ligaments are on the sides of the knee. The medial collateral ligament is on the inside of the knee.

Lateral collateral ligament

This is the ligament that gives stability to the outer knee. The lateral collateral ligament, or LCL, is on the outside of the knee.

Classification system

Doctors consider any injury to a ligament in the knee as a sprain, and they use a grading system to diagnose the severity of the sprain:

  • Grade I: In a mild grade 1 sprain, a person has overstretched the ligaments, but the ligaments are still able to keep the knee joint stable.
  • Grade II: A moderate grade II sprain shows signs of a person having partially torn a ligament. The ligament is loose, and therefore, if a person puts weight on the affected leg, the knee may feel unstable and painful.
  • Grade III: A grade III sprain is the most severe type of sprain. This is a complete tear of the ligament. It comes with considerable instability and loss of function and range of motion. Swelling and bruising can be severe, and it may be difficult or impossible to put weight on the leg.

The symptoms of a sprained knee ligament may differ, depending on which ligament a person has damaged.

If an injury affects one of the collateral ligaments, the knee may pop and buckle, which can cause pain and swelling.

If a person has injured a cruciate ligament, they typically hear a popping sound as the injury occurs. The leg may then buckle as a person attempts to stand on it. Swelling may also occur, usually within 24–36 hours.

According to the American Academy of Orthopaedic Surgeons, if an MCL injury occurs, the pain is on the inside of the knee. The pain is on the outside of the knee if an LCL injury happens.

An ACL injury can involve multiple knee ligament sprains, which doctors call the “unhappy triad.”

A sprain affecting the knee occurs when the knee experiences a hard muscle contraction or direct contact with something. These situations can happen when a person changes direction too quickly when running, or by playing sports.

Cruciate ligaments

The ACL is the most common knee ligament to experience an injury.

An ACL injury can occur when a person makes a sharp change in direction, typically at high speed. It can also occur when the knee experiences a blunt force, for example, during a football tackle.

The PCL often becomes injured during direct contact to the front of the knee during activities, such as a football tackle.

Collateral ligaments

These ligament injuries typically occur when a force pushes the knee sideways.

MCL ligament injuries can occur when the outside of the knee receives a direct blow.

MCL injuries can also occur if an individual lands incorrectly. This can result in a “valgus” force on the knee.

If there is a blow on the inside of the knee, it can push the knee outwards, injuring the LCL.

Collateral ligament injuries are common in contact sports.

Depending on the severity of the sprain, home remedies could be all that an individual needs to heal the knee successfully. However, if the knee does not recover after a few weeks, they should follow up by visiting the doctor.

Painkillers, such as acetaminophen or ibuprofen, may help ease the pain and reduce inflammation.

If the sprain is severe, it may need medical intervention.

A person should get treatment, as soon as possible if they experience the following:

  • a popping noise
  • the knee gives out when the injury occurs
  • an inability to move the knee
  • limping occurs
  • swelling occurs at the injured site
  • the joint appears deformed
  • intense pain

Home treatment

At home, people can follow the R.I.C.E method:

  • Rest: Give the knee time to heal.
  • Ice: Apply ice packs to the area for 20 minutes, four to eight times daily.
  • Compression: Using compression socks to help reduce swelling.
  • Elevate: Elevating the knee may also help to reduce swelling.

Learn more about the R.I.C.E method here.

People can buy compression socks online here.

Physical therapy may also be an option to help treat knee sprains and strengthen the muscles.

Medical treatment

In some cases, such as grade III sprains, surgery may be a viable option.

This typically involves reattaching the torn ligament or replacing it with a piece of healthy tendon from another part of the body.

Repaired ACLs typically fail over time, and so a surgeon may use a substitute graft made of tendon.

A doctor will look at the injured knee and compare it with the other knee.

An MRI may provide images of soft tissues, including the injured knee ligament, and help the doctor diagnose the problem.

The recovery will depend on the severity and type of sprain. Typically, mild knee sprains may recover after 2 weeks. If the sprain is severe, it can take months.

During recovery, a therapist may prescribe exercises to strengthen the muscles. These exercises may progress for 6–10 months after the injury occurs.

A person may also need to do neuromuscular stimulation for 6–8 weeks to help improve the function and strength of the muscles.

Before returning to playing any sport, a person should wait until their range of motion is back to normal, and they can walk without limping. They may have to do this gradually.

People should always follow the advice of a doctor before returning to normal physical activities. Depending on how severe the sprain was, they may have to wear a knee brace temporarily.

Although people may not be able to prevent knee ligament sprains entirely, they can minimize the chance of a sprain occurring.

In sport, they should warm up before practice or competition. A person should also wear appropriate footwear and build leg strength by using knee and leg exercises.

If a knee sprain causes more than slight pain and swelling, or for a prolonged time, it is important to see a doctor.

Call a doctor if any of the following symptoms occur:

  • inability to bear weight on the knee
  • a popping sound at the time of injury
  • severe knee swelling
  • inability to fully extend or flex the knee
  • a fever

The knee is crucial for mobility and movement, and so it is essential to get sprains seen by a clinician.

A knee ligament sprain can occur due to an injury or tear to the ligaments in the knee.

It is important that people take the appropriate steps to allow the knee sprain to heal fully before resuming normal activity.