The knee is the largest joint in the body and one of the most easily injured.  
Knee sprains are classified according to a grading system:
Grade I (mild) — This injury stretches the ligament, which causes microscopic tears in the ligament. These tiny tears don’t significantly affect the overall ability of the knee joint to support your weight.Grade II (moderate) — The ligament is partially torn, and there is some mild to moderate instability (or periodic giving out) of the knee while standing or walking.Grade III (severe) — The ligament is torn completely or separated at its end from the bone, and the knee is more unstable.
Most knee sprains occur as a result of:
Direct impact on the knee from an outside forcePressure on the knee resulting from an abrupt stopping force or change in direction
Risk Factors for Knee Sprains
– Improper footwear. – Prior injury. – Cutting and contact sports. 

Anterior cruciate ligament (ACL) — typically sprains during one of the following knee movements: a sudden stop; a twist, pivot or change in direction at the joint; extreme overstraightening (hyperextension); or a direct impact to the outside of the knee or lower leg. 
Posterior cruciate ligament (PCL) —  often sprains because of a direct impact to the front of the knee, such as hitting the knee on the dashboard in a car crash or landing hard on a bent knee during sports. 
Medial collateral ligament (MCL) —  can be torn by a direct sideways blow to the outside of the knee or lower leg or by a severe knee twist. 
Lateral collateral ligament (LCL) —  caused by a blow to the inside of the knee. Least likely to be injured. 

ACL Sprain
A pop inside your knee at the moment of injurySignificant knee swelling within a few hours after injurySevere knee pain that prevents you from continued participation in your sportBlack-and-blue discoloration around the kneeKnee instability — the feeling that your injured knee will buckle or give out if you try to stand
PCL Sprain
Mild knee swelling, with or without knee instabilityMild difficulty in moving the kneeMild pain at the back of the knee that worsens when you kneelMCL Sprain
Knee pain and swellingKnee buckling toward the outsideAn area of tenderness over the torn MCL (at the inner side of the knee)LCL Sprain
Knee pain and swellingKnee buckling toward the insideAn area of tenderness over the torn LCL (at the outer side of the knee)
Expected DurationHow long a knee sprain lasts depends on the type of knee sprain, the severity of your injury, your rehabilitation program and the types of sports you play. In general, milder Grade I and Grade II MCL or LCL sprains heal within 2 to 4 weeks, but other types of knee sprains may take 4 to 12 months

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