The achilles tendon is the big tendon at the back of the ankle.

The Achilles tendon is very important in the lower limb, but in relation to the knee it iis importance is its availability as a cadaveric donor graft.
Achilles tendon harvest
Tissue banks may 'harvest' this tendon from human donors and it is freeze-dried and stored for transplant (eg to the knee), where it can be used for a variety of purposes to strengthen areas in the recipient patient where their own natural body tissues have failed. Such a graft of human tissue from another person is called an allograft.
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CLINICAL PUBLICATIONS
Survivorship of allograft ACL reconstruction in adolescent patients. Soneru A and Sarwark JF. J Orthop. 2018 Dec 6;16(1):11-13. doi: 10.1016/j.jor.2018.11.004. eCollection 2019 Jan-Feb.