What is an MCL injury?
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The medial collateral ligament is a band of tissue on the inside of the knee which restricts inward movement and provides stability to the joint.
The ligament connects from the bottom of the thigh bone (femur), attaches to the top and inside of the shin bone (tibia), and controls the sideways movement of the knee.
In an MCL injury, there is pain on the inside of the knee, and depending on the severity the knee could feel unstable.
They are injuries often seen in professional sport.
The damage is caused when the knee is pushed inwards and towards the other knee, which can happen if there is a direct blow to the outside of the leg.
Injuries can also happen when there is repeated stress on the knee or during a fall, with severity ranging from a strain to tear.
Arsenal captain Martin Odegaard suffered an MCL injury on his left knee against West Ham.
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How long is a player out for with an MCL injury?
Physiotherapist Ben Warburton, who has worked with Cardiff City, Wales' national rugby union team and rugby union team Scarlets, describes the four grades of an MCL injury:
Grade 1 is a slight stretch to the ligament with microscopic tears. It would usually take a player between one to three weeks to return.
Grade 2 is a partial tear and a player could be out for four to six weeks.
Grade 3 is an extensive damage to the ligament, with which a player would likely return to playing after seven to eight weeks.
Grade 4 is the complete tear of the ligament which could require surgery. A player could be out for 10 to 12 weeks.
Warburton told BBC Sport that these guidelines are guidelines only, and if a player does return outside these times, there are several factors for this.
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