A sprain is a joint injury that causes a stretch or a tear in a ligament. Ligaments are strong bands of tissue that connect one bone to another. The ACL is one of the major ligaments in the knee. It connects the thighbone to the shinbone. This ligament, along with the PCL (posterior), helps keep the knee stable and protects the femur from sliding or turning on the tibia.

Sprains are graded 1,2,3 depending on their severity.

1= pain with minimal damage to the joint
2= more ligament damage and mild looseness of the joint
3= the ligament is completely torn and the joint is loose and unstable

The ACL is frequently injured in forced twisting motions of the knee. It may also become injured when the knee is straightened further then it should, it sometimes occurs when the thigh bone is forcefully pushed across the shin bone, such as with a sudden stop while you are running or a sudden transfer of weight while you are skiing.

There is usually a loud, painful pop when the joint is first injured. This is followed by a lot of swelling of the knee in the first few hours after the injury. This swelling is called an effusion and is made up of blood with in the joint.

If you have an old ACL that hasn't been repaired the knee may feel unstable during twisting or pivoting movements. It is important to strength train properly with the specific protocol. Several treatment programs are: ice, physical therapy, proper supports to hold the knee firm, closed chain exercise and of course crutches.

The goal of rehabilitation is to return you to sports or activity as soon as is safely possible. If you return too soon you may worsen your injury, which will lead to permanent damage. Everyone recovers at a different rate. Return to your activity will be determined by how your knee recovers not by how many days or weeks it has been since your injury occurred.

You may safely be able to return to your activity when you are able to:

1. You can fully straighten and bend the knee without pain.
2. Your strength is equal on both legs.
3. The effusion is gone.
4. You can jog without limping.
5. You are able to sprint straight ahead without limping.
6. You are able to do 45 degree and 90 degree cuts
7. You are able to do 10 to 20 yard figure eight runs.
   
Unfortunately most ACL injuries happen during accidents that are not preventable. However, you may be able to avoid this injury by having strong thigh and hamstring muscles and maintain a good leg stretching routine. In activities such as skiing, make sure your ski bindings are set correctly by a trained professional so that your skies will release when you fall.