Initial tension and anterior load-displacement behavior of high-stiffness anterior cruciate ligament graft constructs.

BACKGROUND Because the tension that exists in an anterior cruciate ligament graft when the knee is unloaded (the initial tension) affects the surgical outcome and because high initial tension has a number of adverse consequences, the primary purpose of this study was to determine quantitatively how much less initial tension was required for a high-stiffness construct than for a low-stiffness construct. A secondary purpose was to determine how the stiffness of the graft construct affects the anterior load-displacement behavior of the knee from 0 degrees to 90 degrees of flexion. METHODS Anterior-posterior load-displacement was measured in each of ten intact cadaveric knee specimens, the anterior cruciate ligament was excised, and the anterior cruciate ligament was reconstructed with a double-loop bovine tendon graft. Graft constructs of different stiffness were created with use of six springs, ranging in stiffness from 25 to 275 N/mm to simulate the fixation stiffness. After adjusting the initial tension of the graft so that the anterior-posterior laxity of the reconstructed knee matched that of the intact knee, the 0-N posterior limit and the 225-N anterior limit were measured at 0 degrees, 30 degrees, 60 degrees, and 90 degrees of flexion. RESULTS The highest stiffness fixation (275 N/mm) required an average of 73 N of initial tension, which was more than three times less than the average of 242 N of initial tension required by the lowest stiffness fixation (25 N/mm). The 225-N anterior limit was overconstrained an average of 1.0 mm with the highest stiffness fixation (275 N/mm), which was 3.6 mm less than the overconstraint with the lowest stiffness fixation (25 N/mm). Likewise, the posterior limit was overconstrained an average of 2.6 mm with the highest stiffness fixation (275 N/mm), which was 3.8 mm less than the overconstraint with the lowest stiffness fixation (25 N/mm). CONCLUSIONS The initial tension for a high-stiffness graft construct is more than three times less than that for a low-stiffness construct. The initial tension for a high-stiffness graft construct better restores both the 225-N anterior limit and the 0-N posterior limit to normal than the initial tension for a low-stiffness graft construct over the range of flexion from 0 degrees to 90 degrees.