Why do acl tears take so long to heal?

Blood supply: Because an acl is about 1cm thick, it takes time for the body grow into the replacement and recreate the structure of the original acl. There is no blood vessels that run through the acl graft, so the nutrients and cells that provide the replacement material must come from the joint fluid inside the knee, and the bone marrow from the ends of the graft. All this takes 6-8 months to accomplish.

The ACL: is a major stabilizer for the knee and as such if the knee is in constant movement while injured, the fibers take alot of regular amounts of stress. What you are dealing with is the constant movement of injured fibers in contact and out of contact with each other resulting in slower healing. Consider stem cell therapy to help heal faster and fully if surgery is not planned. Check Regenexx.Com.

Blood supply: The blood supply to the torn ligament is very poor, and so it is rare for a complete acl tear to ever heal. The knee after an acl tear also becomes unstable, and so there can be a lot of stress and motion at the injury site that can also limit healing. I would not hesitate to discuss with your surgeon.