The normal human chondro-osseous junctional region: evidence for contact of uncalcified cartilage with subchondral bone and marrow spaces

Abstract

Background: The chondro-osseous junctional region of diarthrodial joints is peculiarly complex and may be considered to consist of the deepest layer of non-calcified cartilage, the tidemark, the layer of calcified cartilage, a thin cement line (between the calcified cartilage and the subchondral bone) and the subchondral bone. A detailed knowledge of the structure, function and pathophysiology of the normal chondro-osseous junction is essential for an understanding of the pathogenesis of osteoarthrosis.

Methods: Full thickness samples from human knee joints were processed and embedded in paraffin wax. One hundred serial sections (10 mum thick) were taken from the chondro-osseous junctional region of a block from the medial tibial plateau of a normal joint. They were stained with haematoxylin and eosin and photographed. For a simple physical reconstruction images of each 10th sequential tissue section were printed and the areas of the photomicrographs containing the chondro-osseous junctional region were cut out and then overlaid so as to create a three-dimensional (3D) model of this region. A 3D reconstruction was also made using computer modelling.

Results: Histochemical staining revealed some instances where prolongations of uncalcified cartilage, delineated by the tidemark, dipped into the calcified cartilage and, in places, abutted onto subchondral bone and marrow spaces. Small areas of uncalcified cartilage containing chondrocytes (virtual islands) were seen, in two-dimensional (2D) sections, to be apparently entombed in calcified matrix. The simple physical 3D reconstruction confirmed that these prolongations of uncalcified cartilage were continuous with the cartilage of zone IV and demonstrated that the virtual islands of uncalcified cartilage were cross-sections of these prolongations. The computer-generated 3D reconstructions clearly demonstrated that the uncalcified prolongations ran through the calcified cartilage to touch bone and marrow spaces and confirmed that the apparent entombment of chondrocytes was a 2D artefact.

Conclusion: This study demonstrates that the chondro-osseous junctional region is more complex than previously described. The tidemark is a clearly defined boundary delineating uncalcified from calcified cartilage. It is not a straight line across a joint, but a complex three-dimensional structure that follows uncalcified cartilage prolongations dipping down through the calcified cartilage to abut onto subjacent bone or marrow spaces.

Figure 1

Diagrammatic representation of the zones of articular cartilage and subchondral bone. (Zones as per Gardner et al. 1987 [13])

Figure 2

Histochemical staining of the COJ region of normal cartilage. (a & b) with H&E, (c) picro-sirius red and (d) toluidine blue. Prolongations of uncalcified cartilage are seen to dip down through the calcified cartilage and abut the marrow spaces at points indicated by the arrows.

Figure 3

3D reconstruction of the tidemark. Sequential sections (a-g) from a normal COJ region demonstrate the continuity of apparently discontinuous non-calcified cartilage prolongations which protrude into the calcified cartilage and abut onto a bone marrow space. The lower set of photographs has been edited to show more clearly the sections through the prolongations. Sections were 10 μm thick and every 10^th section is shown.

Figure 4

3D reconstruction of the tidemark. This shows a composite photograph made by cutting around the cartilage prolongations on the individual photographs from Figure 3 and then overlaying them.

Figure 5

Computer generated 3D reconstruction of the COJ. Panoramic view (a), lateral view (b) and superior view (c). In (a) contacts between uncalcified cartilage (UC – red), calcified cartilage (CC – dark grey) and lamellar bone (LB – grey) are evident. In (b) and (c) contact between uncalcified cartilage and lamellar bone is illustrated.

Figure 6

Computer generated image of the COJ. Uncalcified cartilage (UC) is dark blue, calcified cartilage (CC) is green and lamellar bone (LB) is red. The image represents the abutment of uncalcified cartilage onto lamellar bone.

Figure 7

Revised representation of the components of the chondro-osseous junctional region of articular cartilage. This shows a virtual island, one prolongation of the uncalcified cartilage into the calcified cartilage layer, one prolongation that abuts the bone and one prolongation that abuts the marrow space containing a blood vessel.