This injury is a total disruption of the most important stabilizing ligament in the knee: the anterior cruciate ligament (ACL). The ACL is located centrally within the knee behind the kneecap, making it difficult to specifically localize the injury. The most common mechanism of injury is a knee hyperextension with a twist. This may occur while the knee is momentarily unprotected by the surrounding musculature, especially the hamstrings (muscles in the back of the thigh). The injury typically occurs during cutting and twisting sports such as skiing, basketball, or soccer, and may occur with surprisingly low force.
An x-ray can rule out any fractures. The physical exam is most helpful in determining knee instability, but your physician may wish to order an MRI to evaluate the ACL. MRI scans are a good diagnostic tool for a ruptured ACL.
Typical symptoms may include the dreaded "pop" at the time of initial injury, followed by instability and swelling which appear within the first 24 hours. Pain may be minimal initially if the ACL alone is damaged. Athletes who attempt to return to action may experience a second episode of instability, or a sensation that their knee "pivoted."
ACL injuries may be prevented by maintaining consistent lower extremity strength. Hamstrings in particular should be focused on to offset the naturally more powerful quadriceps. When possible avoid knee positions that expose the ACL to injury. A slight bent knee position is preferable to a fully extended knee.