MCL Sprains Part 1 – Anatomy, Function, Mechanism of Injury & Diagnosis

 In Lower Limb

The Medial Collateral Ligament (MCL) is one of four major ligaments in the knee (illustrated to the right). It originates on the medial femur (inner aspect of the thigh bone), crosses the knee joint and attaches onto the medial tibia (inner aspect of the shin bone). The MCL’s function is to prevent valgus forces on the knee joint.

MCL Sprains


What is a valgus force?

A valgus force (pictured below) is an increase of the angle on the inside of the knee. Some people refer to valgus forces as ‘the knee collapsing in’. As the MCL’s primary function is to resist valgus forces, it is often injured in situations where the knee collapses in.

MCL Sprains

Mechanism of Injury

MCL injuries can be classified into two groups – 1) Contact and 2) Non-contact.

MCL Sprains


The video below depicts NBA star Stephen Curry sustaining a contact grade II MCL injury (more on grading injuries later). Steph’s teammate Javale collides into steph’s outer left leg, causing a valgus force on Steph’s knee. As the MCL is the primary restraint to valgus force, it has ultimately led to a grade II injury in this situation.

 mcl sprains


Non-contact injuries of the MCL are typically more common than contact injuries, and can be attributed to poor functional hip stability and/or poor jumping/landing/stepping biomechanics. The video below depicts women’s AFL athlete Kate Sheahan sustaining a non-contact knee injury. Notice how when Sheahan steps hard off her left leg to evade her defender, her knee falls into a valgus position.


The most reliable way of assessing acute knee injuries is through an MRI machine. If that’s the case, then shouldn’t everyone that has a sore knee get an MRI to find out exactly what’s wrong? Not necessarily. The downside of getting an MRI is that they are expensive and can often take days to organise. In a lot of cases, an MRI may only confirm what a qualified physio/sports doctor suspects and will have no affect on how the knee injury is managed.

It is not at all uncommon for people that aren’t high level athletes with suspected low grade knee injuries to avoid undergoing an MRI. So how can physiotherapists and doctors be confident that their diagnosis is correct and that an MRI isn’t necessary? A physiotherapist or doctor will decide whether an MRI is justified based on the individuals accounts of what exactly happened during the injury and through thorough objective testing.

What do objective tests look like?

With an MCL sprain, there are two objective tests that will likely be positive.

1. Palpation

If pressing on the inside of the knee reproduces pain, that may indicate that their is some sort of injury to the MCL.

2. Valgus Stress Test

The video below demonstrates a valgus stress test. As the MCL’s function is to prevent valgus forces on the knee. If the foot moves away from the midline of the body moreso than the uninjured leg, injury to the MCL is suspected. The extent of laxity often determines the extent of the injury. If slightly more laxity is felt than the uninjured leg but there is a clear end-feel (stop), then a grade 1 or 2 may be suspected. If the foot keeps going and going and no end-feel is felt, a grade 3 is suspected and an MRI will likely be considered.

Interestingly, level of pain does not correlate well with valgus stress testing. Grade 1 strains may reproduce mild to moderate pain levels with testing. Grade 2 strains may reproduce moderate to high levels of pain. In grade 3 strains, there may likely be no pain felt with testing. This is because the nerve endings on the MCL aren’t able to send pain signals up to the brain, and thus no pain is reproduced.


The following estimates on return to sport are based on Brukner and Khan’s Clinical Sports Medicine Guidelines. These estimates are made on the assumption that no other injuries were sustained to any other structures in the knee during the injury.

  • Grade I MCL Sprain       Partial fiber disruption                         0-2 weeks
  • Grade II MCL Sprain      ~ 50% fiber disruption                         2-6 weeks
  • Grade III MCL Sprain     Full rupture                                           6-12 weeks

MCL Sprains


In Part 2, we’ll delve a little deeper into the treatment and management of MCL sprains.

George Dooley (APAM)
Master Physiotherapist

George Dooley

Click Here to book an appointment with George today.

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