The normal anterior cruciate ligament as a model for tensioning strategies in anterior cruciate ligament grafts

Background: There is some confusion about the relationship between the tension placed on the graft and the joint position used in the fixation of anterior cruciate ligament grafts. This is because of deficiency in accurate basic science about this important interaction in the normal and reconstructed anterior cruciate ligament. Hypothesis: If the normal femoral attachment of the anterior cruciate ligament can be preserved and the tibial insertion isolated and tested, an accurate force-flexion curve of the human anterior cruciate ligament can be mapped out and used as a standard for proper graft tensioning protocols in anterior cruciate ligament reconstruction. Study Design: Controlled laboratory study. Methods: In 10 fresh-frozen human cadaveric knees, an isolated bone plug containing the tibial anterior cruciate ligament insertion was connected with a custom-made tensiometer. The knees were moved through the whole range of motion; the starting point chosen was an anterior cruciate ligament tension of 10 N, which was applied at 10degrees of knee flexion and resulted in a baseline curve. This curve was compared with the results recorded when the starting point was below the baseline curve, similar to, or above it. Results: The anterior cruciate ligament showed low tension close to slackness in midflexion after starting with 10 N at 10degrees of flexion. Starting points below the baseline curve shifted the whole curve downward; those above the baseline curve increased the force in the anterior cruciate ligament, resulting in a tight anterior cruciate ligament in midflexion. Clinical Relevance: The normal anterior cruciate ligament shows a physiological laxity in midflexion. This study gives guidelines for tensioning protocols in anterior cruciate ligament grafts to replicate the force-flexion curve characteristics of the normal anterior cruciate ligament.