your meniscus 's are two "break pads" between the femur and tibia, these act like a rolling cushion to allow the bones to move freely, they are also ( quite complicated) invloved in the biomechanical movement and positioning of the knee.
It's a real shame they didn't notice this when they did your ACL, however IT IS GOOD NEWS!! the fact that the ACL is ok, and this has occured is not brilliant ( no injury is) however it doesn't have a bearing on the ligament stability. What the surgeon wil do is trim the tear and tidy up the edges so nothing catches when you move your knee. Rehab is much simpler that for an ACL. - so return to sport has a "good as any injury" chance.

On to the subject of repair, there are some very early day studies using stem cells that are grown and in lab's then placed like a patch over the area (if it's is anatomically viable), this for obvious reasons is very very "cutting edge" due to the ethics of stem cell tretments. However in little old wales (UK) it has been done (don't know if it was umbilical stem cells though), the rehab then is very carefull more so than a ACL.

Anyway I hope your surgery goes well, and that you get back training and competeing soon
physioroute