The structural and compositional transition of the meniscal roots into the fibrocartilage of the menisci

Abstract

The meniscal roots, or insertional ligaments, firmly attach the menisci to tibial plateau. These strong attachments anchor the menisci and allow for the generation of hoop stress in the tissue. The meniscal roots have a ligament-like structure that transitions into the fibrocartilagenous structure of the meniscal body. The purpose of this study was to carry out a complete analysis of the structure and tissue organization from the body of the meniscus through the transition region and into the insertional roots. Serial sections were obtained from the meniscal roots into the meniscal body in fixed juvenile bovine menisci. Sections were stained for collagen and proteoglycans (PG) using fast green and safranin-o staining protocols. Unstained sections were imaged used a backlit stereo microscope. Optical projection tomography (OPT) was employed to evaluate the three-dimensional collagen architecture of the root-meniscus transition in lapine menisci. Tie-fibres were observed in the sections of the ligaments furthest from the bovine meniscal body. Blood vessels were observed to be surrounded by these tie-fibres and a PG-rich region within the ligaments. Near the tibial insertion, the roots contained large ligament-like collagen fascicles. In sections approaching the meniscus, there was an increase in tie-fibre size and density. Small tie-fibres extended into the ligament from the epiligamentous structure in the outermost sections of the meniscal roots, while large tie-fibre bundles were apparent at the meniscus transition. The staining pattern indicates that the root may continue into the outer portion of the meniscus where it then blends with the more fibrocartilage-like inner portions of the tissue. In unstained sections it was observed that the femoral side of the epiligamentous structure surrounding the root becomes more fibrous and thickens in the inferior inner portion of the posterior medial root. This thickening changes the shape of the root to more closely resemble the meniscus wedge shape. These observations support the concept of root continuity with the outer portion of the meniscus, thereby connecting with the hoop-like structure of the peripheral meniscus. OPT identified continuous collagen organization from the root into the meniscal body in longitudinal sections. In the radial direction, the morphology of the root continues into the meniscal body consistent with the serially sectioned bovine menisci. Blood vessels were prevalent on the periphery of the root. These blood vessels then arborized to cover the anterior femoral surface of the meniscus. This is the first study of the structural transition between the insertional ligaments (roots) and the fibrocartilagenous body of the menisci. These new structural details are important to understanding the meniscal load-bearing mechanism in the knee.

Keywords: Knee; Meniscal Roots; Meniscus; Optical Projection Tomography.

Figure 1

Top left: photo of a medial meniscus (dashed red lines: orientation of sections). Bottom: serial sections of the anterior insertion showing the transition in shape and staining pattern from the ligament to the meniscus. Breakout images: identify blood vessels (dashed arrows) and tie-fibres (solid arrows) in sections.

Figure 2

Serial sections from the posterior lateral root into the meniscal body. There is an increase in the number and density of the tie-fibres in the meniscus compared with the root. Tie-fibres were observed in the root. The breakout image shows a blood vessel in the meniscal root surrounded by tie-fibres (solid arrow).

Figure 3

Right: photo of a lateral meniscus. Left: serial sections of the anterior insertion. Note the increase in PG staining as sections approach the meniscus. Portions of the ligament appear to be preserved as it approaches the meniscus (dashed ellipses).

Figure 4

Serial sections of the posterior medial root into the meniscal body. In sections approaching the meniscus, there is a thickening of the epiligamentous of the root to an epimeniscal structure on the femoral and tibial surfaces. This thickening is associated with increased tie-fibre density and branching from the epimeniscal structure. There is also a marked increase in PG staining on the inferior inner portion of the transition into the meniscal body.

Figure 5

Backlit images taken on a stereo-microscope of unstained, transverse sections of the meniscal root as it transitions into the meniscus. Sections move from the root into the meniscal body (1–3). The outer structure (left side) of the tissue structure appears to change very little from the root into the meniscus. The epiligament on the femoral side of the tissue appears to thicken and increase in branching into the meniscus approaching the meniscus (breakout images 1–3).

Figure 6

Schematic identifying the regions of the meniscus imaged using optical projection tomography (OPT; top left). 3D reconstruction of the meniscal body and meniscal root (top right). Brightly fluorescing region on the surface of the ligament and meniscal body identify blood vessels. (Bottom) Radial and longitudinal sections identifying the discrete structural changes from the outer meniscus to the inner body of the meniscus. Solid red arrows denote the discrete change from the outer collagen organization to the inner organization. A collagen-sparse area can be seen at the junction.