Physical examination by a doctor
Often a doctor can examine the knee and determine whether or not the ACL is torn, if the doctor gets to do the exam when the knee is not largely swollen. After the swelling sets in, examination is much more difficult. The exam that is often used is called "Lachman's exam," and the doctor basically tries to move the lower leg bone forward while holding the thighbone steady. If the bones move too much, then it may be a sign of ACL tear. ACL tears can be associated with other injuries of the knee, so it's important to have a qualified doctor fully examine the knee to determine the extent of the injury. When other injuries are present, the treatment and diagnosis options may be significantly different from those discussed in this site.
MRI
The best test to determine whether or not the ACL is torn is an MRI. During an MRI, the patient basically lays on a table with his or her leg inside a large magnet that takes sophisticated pictures of the inside of the knee. MRI usually gives a clear answer so that planning can begin even before the swelling subsides.
X-rays
Plain x- rays may sometimes some characteristic bone chips associated with ACL tears. Most of the time, however, the x-ray simply helps make sure that no bones are broken.
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