Load sharing in augmented anterior cruciate ligament repair: a mathematical analysis based on in vitro measurements

OBJECTIVE: To quantify the stress on a reinserted anterior cruciate ligament (ACL) we studied the load sharing between the ACL and a 3.7 mm polyethylene terephthalate (PET) band in 10 knees of fresh human cadavers. DESIGN: The load sharing between the Marshall sutures-ACL complex and the PET band and between the ACL and the PET band was calculated by means of a mathematical model. BACKGROUND: Augmentation of a reinserted ACL with a synthetic band is an accepted treatment for a ruptured ACL. METHODS: After transsecting the ACL at its femoral origin and reinserting it with four sutures using the Marshall technique we augmented it with the PET band. The augmentation device was inserted by the through-the-condyle (TTC) procedure and attached without preload to the femoral and the tibial condyle with 4 mm staples. Then the length of the ACL, the lengths of the Marshall sutures, the partial lengths and the angles between the different directions, and the adherence-friction force of the PET band were measured. RESULTS: In the beginning the Marshall sutures-ACL complex takes over 40% of an externally applied load and the PET band 60%. After the hypothetical ACL healing phase the PET band takes over 27% and the ACL 73% of the load. CONCLUSIONS: An external force of 40 N acting on the knee will therefore lead to an elongation of the Marshall sutures-ACL complex and result in movement of the proximal end of the ACL away from the femoral condyle of at least 0.5 mm, which is highly undesirable in the early postoperative healing phase.