O'Donoghue classification 
Isolated grade I MCL injury (mild)
MCL has few torn fibres but no loss of ligamentous integrity.
Isolated grade II MCL injury (moderate)
MCL is partially torn. However, the fibres are still opposed. There might be mild pathological laxity, which may or may not be symptomatic.
Isolated grade III MCL injury (severe)
Integrity of the MCL is completely disrupted. There is significant pathological laxity of the knee with valgus stress.
AMA Committee on the Medical Aspects of Sports standard nomenclature of athletic injuries 
MCL injuries are classified based on the amount of medial joint opening when a valgus load is applied at 20° to 30° of knee flexion: View image
Grade I: 0 to 5 mm of opening
Grade II: 5 to 10 mm of opening
Grade III: >10 mm of opening
Other forms of MCL injury
MCL + anterior cruciate ligament (ACL) injury
The ACL is the most commonly injured ligament along with the MCL.
MCL + multiligament injury
The posterior cruciate ligament, lateral collateral ligament, and menisci are frequently simultaneously injured.
Recurring MCL injury
Recurring, or chronic, MCL injury is a complication of acute MCL injury.