EFFECT OF TENSION AND PLACEMENT OF A PROSTHETIC ANTERIOR CRUCIATE LIGAMENT ON THE ANTEROPOSTERIOR LAXITY OF THE KNEE

We wished to determine the optimal tension required to restore normal joint laxity to anterior cruciate ligament (ACL)-deficient knees using a braided polyethylene ACL prosthesis (PACL). In 10 cadaveric specimens, we measured the anteroposterior (AP) laxity of the intact knee at 10-degrees, 30-degrees, 60-degrees, and 90-degrees of flexion. The ACL was then removed and replaced with the PACL using tunnel-tunnel (T-T) and "over-the-top" (OTT) placement techniques. In both positions, the PACL was initially tensioned to 0, 9, 18, and 27 N with the knee flexed to 30-degrees. AP joint laxity was then measured at each flexion angle. With an increase in initial tension, there was a corresponding decrease in AP laxity. At 30-degrees and 90-degrees of flexion, AP laxity was not significantly different from normal using T-T placement and an initial tension of 0 N. At 90-degrees of flexion, AP laxity was not significantly different from normal using OTT placement at 0 or 9 N of initial tension. For both positions, all other tension levels and flexion angles constrained AP laxity. No laxity differences were detected between the OTT and T-T positions at any flexion angle. The variability in AP laxity of the T-T position was significantly greater than OTT. With a 150-N anterior shear force applied to the proximal tibia, the maximum tensions developed in the PACL were not significantly different between the two positions except at 90-degrees. The results suggest that implantation of the PACL is best performed using OTT positioning with an initial tension of 0 N applied at 30-degrees of knee flexion.