why do knee ligament injuries don’t heal by itself? eli5
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It depends a bit which knee ligament and how many were injured at a time (there are four). For the anterior and posterior cruciate ligaments (ACL and PCL), the ends of the ligaments typically aren’t touching each other after they’re torn, so there is no way for them to heal back together. Google some MRI images of torn ACLs. And even if they’re close enough together that some healing occurs, the repair is weak and the ligament is elongated and unable to perform its usual task.
For the medial collateral ligament (MCL), *if* this is the only ligament torn, it can heal well, even with complete tears, if the person has their knee immobilized in a specific way. The lateral collateral ligament (LCL) does not heal well on its own, even with immobilization.
As others have said, ligaments have poor blood supplies, but actually not terrible. After you tear your ACL, even in the absence of other injuries happening at the same time, your knee will fill with blood. That blood is coming from the torn ends of the ligament. It takes an hour or two to accumulate, sometimes more, but it does happen.
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