You are probably referring to a procedure called ligament augmentation. A braided length of polypropylene (synthetic plastic) was used. The idea was to protect and reinforce the torn ligament until it could heal. The device was designed to be absorbed by the body. As the ligament healed, mechanical forces were transferred back to the repaired ligament.
Studies didn't show an advantage to this method. The rate of surgical failures was high because the ligament augmentation device would rupture. Over time research showed that using a live tendon from another area of the knee as a graft worked much better. A simple repair of the torn ACL was abandoned as ineffective. All ACL repairs are augmented now; most are done arthroscopically without opening the knee.