Why we use tendon to replace torn ligament? if tendon is torn, can we use ligament to replace it?

Availability: Ligaments are not available to harvest from your own body like portions of tendons or whole tendons of certain muscles that are not critical to our function. For the same reason, we do not harvest ligaments to repair tendons.

Ligaments: Ligaments go from bone to bone - usually short distance. Ligaments are usually not expendable. Tendons can be split , transferred and moved to substitute for damaged ligaments.