Differences in the Microstructural Properties of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament

Background: Tissue properties of the anteromedial (AM) and posterolateral (PL) bundles of the anterior cruciate ligament (ACL) have not been previously characterized with real-time dynamic testing. The current study used a novel polarized light technique to measure the material and microstructural properties of the ACL. Hypothesis: The AM and PL bundles of the ACL have similar material and microstructural properties. Study Design: Controlled laboratory study. Methods: The AM and PL bundles were isolated from 16 human cadaveric ACLs (11 male, 5 female; average age, 41 years [range, 24-53 years]). Three samples from each bundle were loaded in uniaxial tension, and a custom-built polarized light imaging camera was used to quantify collagen fiber alignment in real time. A bilinear curve fit was applied to the stress-strain data of a quasistatic ramp-to-failure to quantify the moduli in the toe and linear regions. Fiber alignment was quantified at zero strain, the transition point of the bilinear fit, and in the linear portion of the stress-strain curve by computing the degree of linear polarization (DoLP) and angle of polarization (AoP), which are measures of the strength and direction of collagen alignment, respectively. Data were compared using t tests. Results: The AM bundle exhibited significantly larger toe-region (AM 7.2 MPa vs PL 4.2 MPa; P < .001) and linear-region moduli (AM 27.0 MPa vs PL 16.1 MPa; P = .017) compared with the PL bundle. Average DoLP values were similar at low strain but were significantly larger (ie, more uniform alignment) for the AM bundle in the linear region of the stress-strain curve (AM 0.22 vs PL 0.19; P = .036) compared with the PL bundle. The standard deviation AoP values was larger for the PL bundle at both transition (P = .041) and linear-region strain (P = .014), indicating more disperse orientation. Conclusion: Material and microstructural properties of the AM and PL bundles of the ACL differ during loading. The AM bundle possessed higher tissue modulus and failure stress, as well as more uniform fiber alignment under load. Clinical Relevance: These insights into native ligament microstructure can be used to assess graft options for ACL reconstruction and optimize surgical reconstruction techniques.