In vivo kinematics and ligamentous function of the knee during weight-bearing flexion: an investigation on mid-range flexion of the knee

Abstract

Purpose: To investigate the in vivo femoral condyle motion and synergistic function of the ACL/PCL along the weight-bearing knee flexion.

Methods: Twenty-two healthy human knees were imaged using a combined MRI and dual fluoroscopic imaging technique during a single-legged lunge (0°-120°). The medial and lateral femoral condyle translation and rotation (measured using geometric center axis-GCA), and the length changes of the ACL/PCL were analyzed at: low (0°-30°), mid-range (30°-90°) and high (90°-120°) flexion of the knee.

Results: At low flexion (0°-30°), the strains of the ACL and the posterior-medial bundle of the PCL decreased. The medial condyle showed anterior translation and lateral condyle posterior translation, accompanied with a sharp increase in external GCA rotation (internal tibial rotation). As the knee continued flexion in mid-range (30°-90°), both ACL and PCL were slack (with negative strain values). The medial condyle moved anteriorly before 60° of flexion and then posteriorly, accompanied with a slow increase of GCA rotation. As the knee flexed in high flexion (90°-120°), only the PCL had increasingly strains. Both medial and lateral condyles moved posteriorly with a rather constant GCA rotation.

Conclusions: The ACL and PCL were shown to play a reciprocal and synergistic role during knee flexion. Mid-range reciprocal anterior-posterior femoral translation or laxity corresponds to minimal constraints of the ACL and PCL, and may represent a natural motion character of normal knees. The data could be used as a valuable reference when managing the mid-range "instability" and enhancing high flexion capability of the knee after TKAs.

Level of evidence: Level IV.

Keywords: ACL; High flexion; In vivo knee kinematics; Mid-range instability; PCL; TKA.

Figure 1

Two views of a typical 3D knee model at different flexion angles, including the geometric center axis (GCA) of femur (nave blue) and the tibial coordinate system (red). The model included femur, tibia, ACL and PCL. The ACL and PCL were both divided into two functional bundles: anterior-medical (AM) (yellow) and posterior-lateral (PL) (pink) bundles for the ACL, anterior-lateral (AL) (blue) and posterior-medial (PM) (green) bundles for the PCL.

Figure 2

Femoral condyle motions were determined using the projection of the GCA on tibial plateau surface, including the anterior-posterior translation of the medial and lateral condyles, the internal-external rotation of the GCA along the tibial long axis. External rotation of the GCA (internal rotation of the tibia) was defined as positive.

Figure 3

a) Medial and lateral femoral condyle translation in anterior-posterior (AP) direction along the flexion arc. b) Internal-external (IE) femoral condyle rotation represented by the GCA rotation along the flexion arc.

Figure 4

Elongation of two bundles of the a) ACL and b) PCL along the flexion arc of the knee during the single-legged lunge motion.

Figure 5

Relative elongation (strain) of the ACL and PCL bundles along the knee flexion arc. The strains were calculated by setting the peak strain value of 5% at full extension for the ACL bundles and at 120° of flexion for the PCL bundles. Positive values represent tension and negative values represent relaxation of the ligament bundle.