Isometric versus tension measurements. A comparison for the reconstruction of the anterior cruciate ligament.

This study was designed to compare the displacement patterns of an isometer, used to determine graft placement during reconstruction, with the actual tensions on an anterior cruciate ligament substitute. In cadaveric specimens, a Kevlar anterior cruciate ligament substitute was implanted in three separate femoral sites, each of which was subsequently fixed to two different tibial sites. The initial tension of the Kevlar substitute was set to 22 or 33 N at 20 degrees of knee flexion. The displacement patterns for each position were recorded during passive flexion-extension using the isometer. Using a custom-designed tensiometer, the tensile forces on the substitute after rigid fixation at the tibia and femur were measured. During passive flexion-extension, the maximum change in tension of the anterior cruciate ligament substitute, measured by the tensiometer, was correlated with the maximum change in displacement between attachment sites, measured by the isometer. The coefficient of determination was equal to 0.15, indicating that the isometer may not accurately predict the tensions developed in the substitute.