Effects of initial graft tension on the tibiofemoral compressive forces and joint position after anterior cruciate ligament reconstruction

Abstract

Background: The initial tension applied to an anterior cruciate ligament graft at the time of fixation modulates knee motion and the tibiofemoral compressive loads.

Purpose: To establish the relationships between initial graft tension, tibiofemoral compressive force, and the neutral tibiofemoral position in the cadaveric knee.

Study design: Controlled laboratory study.

Methods: The tibiofemoral compressive forces and joint positions were determined in the anterior cruciate ligament-intact knee at 0 degrees , 20 degrees , and 90 degrees of knee flexion. The anterior cruciate ligament was excised and reconstructed with a patellar tendon graft using graft tensions of 1, 15, 30, 60, and 90 N applied at 0 degrees , 20 degrees , and 90 degrees of knee flexion. The compressive forces and neutral positions were compared between initial tension conditions and the anterior cruciate ligament-intact knee.

Results: Increasing initial graft tension increased the tibiofemoral compressive forces. The forces in the medial compartment were 1.8 times those in the lateral compartment. The compressive forces were dependent on the knee angle at which the tension was applied. The greatest compressive forces occurred when the graft was tensioned with the knee in extension. An increase in initial graft tension caused the tibia to rotate externally compared with the anterior cruciate ligament-intact knee (1.5 degrees and 7.7 degrees of external rotation when tensioned to 90 N at 0 degrees and 90 degrees of knee flexion, respectively). Increases in initial graft tension also caused a significant posterior translation of the tibia relative to the femur (0.9 and 5.3 mm of posterior translation when tensioned to 90 N at 0 degrees and 90 degrees of knee flexion, respectively).

Conclusion: Different initial graft tension protocols produced predictable changes in the tibiofemoral compressive forces and joint positions.

Clinical relevance: The tibiofemoral compressive force and neutral joint position were best replicated with a low graft tension (1-15 N) when using a patellar tendon graft.

Figure 1

Lateral and anterior view of custom jig for quasi-static knee flexion.

Figure 2

Mean total tibiofemoral (T-F) compressive force (medial + lateral) versus knee flexion angle for the intact and ACL reconstructed knee at 1, 15, 30, 60 and 90 N of initial graft tension. (* = significantly different than that of the ACL intact specimen).

Figure 3

Graft tension versus compressive load (pooled by knee flexion angle) showing that the medial compressive loads are greater than those of the lateral compartment at all initial graft tension levels (p<0.001).

Figure 4

An increase in initial graft tension caused the tibia to rotate externally at all knee flexion angles.

Figure 5

An increase in initial graft tension caused a posterior neutral shift of the tibia relative to the femur at 0°, 20° and 90° flexion.