How the fixation method stiffness and initial tension affect anterior load-displacement of the knee and tension in anterior cruciate ligament grafts: a study in cadaveric knees using a double-loop hamstrings graft.

There were two objectives to this study. The first was to investigate the relationship of graft fixation stiffness and graft initial tension on the anterior load-displacement behavior of knees reconstructed with a double-loop hamstrings tendon graft. The second was to determine the corresponding graft tensions at 225 N of anterior force applied to the knee. To satisfy these objectives, the anterior-load displacement curves were measured for seven cadaveric knees with the ACL intact at flexion angles ranging from 0 degrees to 90 degrees. The ACL was reconstructed in the same knees using a double-loop hamstrings graft. A/P load-displacement curves of the knee and graft tension were measured as the fixation method stiffness and the initial tension applied at full extension were varied (25-326 N/mm and 25-300 N). The 0 N posterior limit (unloaded position of tibia) and the anterior laxity (difference between the 0 N posterior limit and 225 N anterior limit) were computed to characterize the A/P load-displacement of the intact and reconstructed knees. The key results were that the 0 N posterior limit of the tibia was insensitive to changes in stiffness (p>0.6503) but that increasing initial tension caused increasing posterior subluxation of the tibia with respect to the femur (p=0.0001). The tibia was subluxed posteriorly by 5-6 mm on average at high levels of initial tension. Both initial tension and stiffness significantly affected the anterior laxity (p=0.0001 for both factors). Anterior laxity was restored closely to normal (i.e. <1 mm difference) by relatively high initial tension of 200 N in combination with low stiffness of 25 N/mm and by low initial tension of 25 N in combination with higher stiffness ranging between 94 and 326 N/mm. When anterior laxity is restored to normal using a high initial tension-low stiffness combination however, the tibia undergoes a large posterior subluxation with respect to the femur in the unloaded state (approximately 5 mm) and a relatively high graft tension of 275 N is developed at 225 N of anterior force. Both the tibial subluxation and graft tension are reduced substantially with low initial tension-higher stiffness combinations because the amount of initial tension required to restore anterior laxity to normal is reduced by about 200 N.