HISTOLOGIC CHARACTERIZATION OF THE ANTERIOR INTERMENISCAL LIGAMENT (AIML) IN THE HUMAN KNEE: CHARACTERISTICS OF A POTENTIAL NEW CELL TYPE
BACKGROUND: The anterior intermensical ligament (AIML) is a unique ligament that has only recently been described anatomically, while its functional role remains elusive. To date, few investigations have addressed the histological composition of the AIML and none, to our knowledge, have focused on the transition zone where the AIML inserts on the medial and lateral menisci. The purpose of our study is to investigate the histological characterization of the AIML, specifically looking at the cellular and collagenous distribution within this transition region. We hypothesize that there is a transition zone between the AIML body and the meniscus that represents distinct alterations in cellular and collagenous organization.
METHODS: The AIML was dissected in total from eight cadaveric knees and fixed in 10% neutral formalin at room temperature. The patient’s age, knee side, and intra-articular pathology to include chondral changes were documented. Samples were embedded in paraffin and 5 mm sections were taken incrementally at steps of 50 mm from both the medial and lateral meniscal insertions for examination under light microscopy. Sections were taken in both transverse and longitudinal planes. These sections were stained with hematoxylin and eosin and Masson’s trichrome stain to evaluate the extracellular matrix. Cadavers that had previous surgery to the knee were excluded from our study. This project was approved by the institution and determined to be IRB exempt.
RESULTS: There were four right knees and four left knees included from patients whose age ranged from 60 to 71 (mean 66.2 ± 4.76). Histologically, the body of the AIML was composed of tight and cohesively arranged, dense collagen bundles accompanied by numerous spindle-shaped fibroblasts, which is consistent with other ligamentous structures. However, both the cellular and collagenous organization within the transition zone differ markedly from that of the ligamentous body. There was a consistent alteration in the morphology of the ligament progressing from the distal body to the proximal insertions. This transition region showed variability between samples, but overall displays abundant collagen fiber disarray where collagen bundles interdigitate with fibrocartilaginous ground matrix. Furthermore, the cellular changes were observed within the regions in which alterations in collagen organization were apparent. These cells show both nuclear rounding and lacunar formation, which becomes more marked toward the proximal insertion sites. Thus, there was a progression from proximal spindle-shaped fibroblasts to intermediate cells with more rounded nuclei and variable lacunae to overt chondrocytes within fibrocartilaginous ground matrix distally.
CONCLUSIONS: Our samples allowed us to observe and characterize the detailed histological composition of the AIML throughout both its ligamentous body and meniscal insertions. Though these results are preliminary, there appears to be an intermediate cell type between that of a spindle-shaped fibroblast and chondrocyte, with apparent nuclear rounding and variable lacunar formation. Further investigation into the origin and intracellular characteristics of these cells are required to help identify this potentially new cell type as well as its possible role in cartilaginous growth and/or regeneration.