The effect of knee flexion angle and application of an anterior tibial load at the time of graft fixation on the biomechanics of a posterior cruciateligament-reconstructed knee

Ten knees were studied using a robotic testing system under a 134-N posteri or tibial load at five flexion angles. Three knee positions were used to st udy the effect of flexion angle at the time of graft fixation (full extensi on, 60 degrees, and 90 degrees) and two were used to study the effect of an terior tibial load (60 degrees and 90 degrees). Knee kinematics and in situ forces were determined for the intact ligament and the graft for each reco nstruction. Craft fixation at full extension significantly decreased poster ior tibial translation compared with the intact knee by up to 2.9 +/- 2.9 m m at 30 degrees, while in situ forces in the graft were up to 13 +/- 35 N g reater than for the intact ligament. Conversely, posterior tibial translati on for graft fixation at 90" was significantly greater than that of the int act knee by up to 2.2 +/- 1.1 mm at all flexion angles; in situ forces decr eased as much as 33 +/- 30 N. When an anterior tibial load was applied befo re graft fixation at 90" of flexion, posterior tibial translation did not d iffer from the intact knee from 30 degrees to 120 degrees, while the in sit u force in the graft did not differ from the intact ligament at full extens ion, 60 degrees, and 120 degrees of flexion. These data suggest that graft fixation at full extension may overconstrain the knee and elevate in situ g raft forces. Conversely, fixation with the knee in flexion and an anterior tibial load best restored intact knee biomechanics.