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.
O'Donoghue's classification 
MCL injuries are classified based on the amount of torn fibres:
Grade I: few torn fibres, ligament is structurally intact
Grade II: incomplete tear, no pathological laxity
Grade III: complete tear, pathological laxity.