Abstract
Summary
The presence or absence of crimp within the anterior cruciate ligament (ACL) sub-bundle anatomy was correlated with knee flexion angle changes and provided a measure of differential loading within its sub-bundle microstructure.
Introduction
Previous studies have shown that macroscopically the anteromedial (AM) and posterolateral (PL) bundles of the ACL tighten/slacken differently with knee flexion angle. This research used fibre crimp morphology, revealed following in situ fixation of the intact ligament structures, to investigate patterns of differential fibre recruitment across each ACL sub-bundle.
Methods
Twelve mature ovine knees were divided into four test-groups of three: control, hyper-extension (8°), neutral position (50°), and deep flexion (170°). For the control group, ligament-bone (tibia) samples were isolated and chemically fixed, unconstrained, in 10% formalin. For the flexed groups the whole joints were first positioned in the relevant angle of flexion using a custom-built rig and formalin-fixed in-situ before undergoing dissection. All ligament-bone samples were decalcified using 10% formic acid and cryo-sectioned in the sagittal plane to obtain 20μm serial slices. These were then examined in their fully hydrated state using differential inference contrast (DIC) microscopy. The degree of crimping, taken as an approximate indicator of elongation and thus loading, was classified as either substantial, intermediate or minimal. Crimp coarseness or wavelength, crimp angle and number of visible apices per unit fibre length were measured and analysed statistically using both ANOVA and post hoc analysis (significance, p < 0.05).
Results
Control group. Substantial crimping was present in all regions of both AM and PL bundles. However two distinct crimp morphologies were discerned, coarse and fine. Coarse crimp dominated both the small anterior region of the AM bundle proximal to the tibial plateau and the entire PL bundle. Fine crimp was present in the remaining part of the AM bundle. The wavelengths and maximum angles of the coarse and fine crimp were significantly different at 44.8 ± 5.9 µm/45° and 16.2 ± 3.1 μm/15° respectively. This difference in crimp morphology was consistent along the serial sections of the ACL. From the serial sections, a three dimensional interpretation of the degree of crimping was obtained for the three different loading positions. A summary of the main findings is presented as follows: Hyper-extension. A small antero-medial portion of the AM bundle was substantially crimped (indicating an unloaded state) and the remaining region exhibiting only intermediate crimping; the bulk of the PL bundle was largely free of crimp (indicating a relatively loaded state) except for a substantially crimped region in its postero-lateral aspect. Neutral position. The anterior portion of the AM bundle exhibited minimal crimp but elsewhere was intermediate. The entire PL bundle was substantially crimped. Deep flexion. The substantially crimped medial portion of the AM bundle transformed progressively into an intermediate crimp morphology as the lateral aspect of the bundle was approached. Both the postero-medial and antero-lateral aspects of the PL bundle were largely free of crimp whereas the intervening region was intermediately crimped.
Conclusion
Through serial sectioning, this study has revealed differential patterns of fibre recruitment, following loading, in all four quadrants of the sub-bundles of the ovine ACL at different knee flexion angles.
