Ultrastructural Assessment of the Anterolateral Ligament

Abstract

Background: The anterolateral ligament (ALL) has been identified as a structure on the lateral side of the knee, but debate exists regarding whether it is a capsular thickening or a ligament.

Hypothesis: A detailed ultrastructural characterization of the ALL and its ultrastructure collagen arrangement will reveal it more closely resembles ligamentous tissue than joint capsule.

Study design: Descriptive laboratory study.

Methods: Eight paired knee samples from 4 fresh-frozen male cadavers were used for this study. Samples were harvested from the ALL, the joint capsule, and the medial collateral ligament (MCL). All samples were evaluated with light microscopy (LM), transmission electron microscopy (TEM), and variable pressure scanning electron microscopy (VP-SEM). With LM, the 3 tissues were analyzed and their morphology described. With TEM, the ultrastructure and collagen characteristics were quantified and compared among specimens. Then, the 3-dimensional characteristics were compared with VP-SEM.

Results: Ultrastructure analysis demonstrated similar morphology between the ALL and MCL, with significant differences in these 2 structures as compared with the joint capsule. On LM, the ALL and MCL were characterized by the presence of a dense collagen fiber oriented in the longitudinal and transversal directions of the fiber bundles, while the joint capsule was found to have a more disorganized architecture. On TEM, the collagen fibers of the ALL and MCL demonstrated similar ultrastructural morphology, with both having collagen fibers in parallel, longitudinal alignment. A quantitative analysis was also performed, with the mean (± SD) diameter of fibrils in the ALL and MCL being 80 ± 2.66 nm and 150 ± 3.35 nm, respectively (all P < .001). The VP-SEM highlighted that ALL and MCL morphology demonstrated arrangements of fiber bundles that are densely packed and organized, in contrast to the disorganized fibers of the joint capsule.

Conclusion: The ALL and MCL have comparable ultrastructures that are distinctly different from the joint capsule, as visualized on LM, TEM, and VP-SEM.

Clinical relevance: The ALL should be considered a distinctive structure of the knee, although strictly connected to the surrounding capsule.

Keywords: anterior cruciate ligament; anterolateral ligament; electron microscopy; ultrastructure.

Figure 1.

Lateral aspect of the knee shows a structure that attaches slightly posterior and proximal to the lateral epicondyle (LE) and courses distally to the tibia between the Gerdy tubercle (GT) and the fibular head (FH). This structure has previously been described as the deep capsulo-osseous layer of the iliotibial band and as the anterolateral ligament. The circle indicates the area of the ligament that was sampled, with an approximate size of 2 × 2 mm.

Figure 2.

Histological images of the ALL and MCL ligaments and the joint capsule of connective tissues from fresh-frozen male cadavers. (A) ALL and (B) MCL collagen fiber sectioning. (C) Compacted collagen fibers with lipid drops (gray circle) of a joint capsule. Insets: identically sized magnified areas of dense collagen fiber oriented in the longitudinal and transversal directions of the corresponding figures. Scale bar = 60 µm. ALL, anterolateral ligament; MCL, medial collateral ligament.

Figure 3.

Ultrastructural morphology of the ALL, MCL, and joint capsule. (A) ALL collagen and (B) MCL collagen fibers. Insets: identically sized magnified areas of aligned fibrils of the corresponding figures. (C) Tissue of the joint capsule. (D) Synoviocyte cells of the joint capsule. ALL, anterolateral ligament; MCL, medial collateral ligament.

Figure 4.

Three-dimensional surface morphology of the ALL, MCL, and joint capsule. (A) ALL collagen and (B) MCL bundle fibers. (C) Microcavities of the joint capsule. ALL, anterolateral ligament; MCL, medial collateral ligament.