Reconsidering Reciprocal Fundation of Anterior Cruciate Ligament Bundles During In Vivo Gait

Abstract

Some cadaveric studies have indicated that the anterior cruciate ligament (ACL) consists of anteromedial (AM) and posterolateral (PL) bundles that function reciprocally with motion. However, several in vivo imaging studies have indicated that ACL subdivisions elongate during knee extension. Whether the ACL is a single structure or consists of different bundles with distinct function is debated. We hypothesized that both the AM and PL ACL subdivisions would undergo relative elongation with knee extension during gait. In 6 healthy subjects, 3D models of the femur, tibia and associated ACL attachment sites were created via segmentation of magnetic resonance (MR) images. The footprints of the AM and PL bundles were estimated and the bone models were registered to high-speed biplanar radiographs in order to reproduce the in vivo positions of the knee throughout gait. The overall length of the ACL was measured as the distance between ACL attachment site footprint centroids for each knee position. 1000 simulated ACL bundles were generated for each subject by randomly sampling locations on the ACL attachment site surfaces, and measuring the distances between each pair of locations. Spearman rho rank correlations were used to determine relationships between simulated bundle lengths and the overall ACL lengths in flexion and extension. Simulated bundle lengths and overall ACL length were highly correlated (rho = 0.91±0.04, mean±standard deviation across subjects, P < 5×10-5, Figure 1). The lengths of the bundles that originated and terminated in the AM footprints were positively correlated (rho=0.81±0.09, P < 5×10-5) with the lengths of the bundles that originated and terminated in the PL footprints. Lengths of all ACL subdivisions were highly positively correlated with overall ACL length throughout gait, suggesting absence of reciprocal bundle function.