Torn knee ligaments are one of the most common injuries among sports people. They occur for various reasons – a differing mechanism of injury according to which ligament is injured. Most people will not understand the function of the knee ligaments and why this knowledge is so important for correct rehabilitation.

Why rehabilitate something purely off doctor’s orders? Know why you are undergoing exercise plans and physiotherapy! A patient’s ACTIVE participation in their recovery is integral for its success!

This post will look to outline the 4 main knee ligaments and their differing functions.

Anterior Cruciate Ligament – ACL


  • Prevents the tibia (shin bone) from falling forwards in front the femur
  • Controls rotational movement of tibia under the femur

Forward Tibial Movement

Mechanism of Injury:

  • Landing, deceleration and change of direction.
  • Knee is close to full extension and typically presents as valgus stress (stress to outside of your knee directed inward).


  1. ACL is the most important ligament of the knee as it contributes to the largest amount of knee stability.
  2. Extremely important in returning to activities involving pivoting movements (sports, trail running etc)
  3. If poorly rehabilitated, you may experience regular clicking, giving way, lack of knee control and strength deficits elsewhere (hamstrings, quads etc).
  4. From this, you may also develop patellofemoral pain or foot pain (eg. plantar fascitis) due to poor lower limb movements and strength.

Posterior Cruciate Ligament – PCL


  • Primary Restraint = prevents the tibia (shin bone) from falling backwards behind the femur
  • Secondary Restraint = controls tibial rotation while the knee is bent

Blunt Force Leading to PCL Tears

Mechanism of Injury:

  • A direct blow to the front of the tibia (shin bone) pushing the tibia backwards. This stresses the PCL, leading to tears.
  • Knee Hyperextension (eg. in landing)
  • Falling onto a flexed (bent) knee.


  1. Poor PCL function will lead to poor knee stability.
  2. This leads to difficulty completing down hill walking or walking down stairs. The femur will tend to shift abnormally forwards on the tibia.

Medial Collateral Ligament – MCL

The MCL originates from the inside of the lower femur and inserts into two attachments on the inside of the upper tibia. It contains 3 main sections – superficial and deep ligaments and the posteromedial capsule. These all contribute to controlling knee laxity.


  • Provides medial (inside) stability for the knee
  • Resists valgus force (when force comes from the outside of the knee directed inwards)

Valgus Force Leading to MCL Tear


  • Valgus knee loading in slight flexion
  • Blunt force to the knee hitting the outside of the knee directed inwards.


  1. The 3 strong stabilizing structures of the MCL make distinct contributions to controlling tibiofemoral (knee) laxity.
  2. Extremely important for knee stability from 0-30deg of knee flexion (bending)
  3. If poorly rehabilitated – you should expect knee stiffness and potential development of patellofemoral pain.

Lateral Collateral Ligament – LCL

LCL is the lateral collateral ligament. It attaches from the outside of the lower femur (thigh bone) to the outside upper tibia (shin bone)

Function: provides lateral joint stability

  • Resists varus force (force hitting the inside of the knee directed outwards)
  • Resists external tibial rotation (toes pointing outward)
  • Stabilises upright knee in stance

    Varus Force – Leading to LCL Tear

Mechanism of Injury:

  • Rotation of knee (tibia) on fixed foot
  • Blunt force to the knee (tackle) – force hitting the inside of the knee directed outwards
  • Hyperextension


NOTE: LCL injuries rarely occur in isolation. They require a lot of force before disruption, thus it will be associated with a high grade injury.

  • Look for more than PCL alone – check for additional ACL, PCL or meniscus damage.