this is what I wrote in another thread on the same subject:
i had several physios and doctors tell me that MRI's are not the most reliable way to tell if there is ACL damage or not. if the ligament is fully torn through the middle, then yes it is obvious. but different types of tears don't show up. such as a complete tear that is sort of diagonal through the ligament. also, a more than 50% rupture is called a complete tear, even though part of the ligament is still intact. these tears are not always obvious on MRI's.
case in point... did something to my knee skiing. went to the hospital, did xrays, it was too swollen to really move but the doctors said nothing was wrong. a few days later i saw a physio, a sports doctor, and another physio. all called it a textbook acl tear. MRI showed the same, along with partial LCL, MCL, meniscus damage, and fractured femoral condoyle and tibial plateau. went to see a surgeon, who wanted a more current MRI. had a 2nd one done, no damage whatsoever. had a scope anyways to check things out, and complete rupture of ACL, partial LCL, partial MCL, meniscus. the rest had healed by now.
so yeah. sometimes it's hard to tell without actually getting a look inside the knee.
another thing, with swelling and pain: the pain varies greatly from person to person. the acl often comes with other knee injuries. for example, fracture of the tibial plateau and femoral condoyle. this causes pretty severe pain. also a ruptured MCL is common, and very painful. other times, for other people, the only sign is a 'pop' when the injury actually occurs. others dont' notice anything.
for swelling: the acl is inside the joint capsule in your knee. this is a bag of lubricating fluid that allows your knee to flex and extend normally and without pain. the acl can tear inside the capsule, while the capsule will remain intact. this type of injury has minimal swelling. other times, the capsule can tear and the fluid seeps into the rest of the knee and causes major swelling. this type of injury is often more painful also.
in conclusion, there is no way to tell for certain whether an acl is torn until a doctor physically sees it.
however, typical signs are:
-a twisting fall
-a pop sound/feeling inside the knee
-immediate extreme pain that subsides quickly
-a feeling of instability
-aching pain in the back of the knee, which feels like upper calf pain
-and finally, an MRI/k-t test