The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 93

In the synovial joint, mineral crystals within the ZCC largely align with the collagen
fibrils. Similar structures observed in bone and dental enamel are highly anisotropic
with the highest modulus in directions parallel to the long axes of mineral crystals
[ 65 , 66 ]. Unmineralized hyaline cartilage and the ZCC form a continuous, aniso-
tropic, and multilayered structure (Figs. 5.1 and 5.2 ).
During postnatal development, the ZCC forms via calcification of hyaline
cartilage and is a late event in the terminal differentiation of chondrocytes [ 67 ,
68 ]. Mineralization of the hyaline cartilage is initiated within territorial matrix near
the cells and spreads throughout the broader extraterritorial matrix [ 69 , 70 ].
Some chondrocytes survive within the ZCC [ 71 - 73 ]. These cells likely regulate
mineralization of the surrounding tissue, as evidenced by highly mineralized
regions surrounding necrotic chondrocytes in osteoarthritic human joints [ 20 ].
While blood vessels rarely penetrate the interface between ZCC and SCB, pores
up to 200 m in diameter enable some fluid movement across the osteochondral

interface [ 98 ]. However, due to the poor permeability of the subchondral plate, the
deep zones of hyaline cartilage are largely reliant on diffusion of nutrients and gases
from the synovial fluid of the joint and interstitial fluid within the neighboring bone.
Endplate mineralization increases, while nutrition delivery is diminished, with age
in this region, which may contribute to degenerative joint changes in aging and
osteoarthritis [ 49 , 99 , 100 ].
Interplay exists between the synovial fluid and the less viscous, mineral-containing
interstitial fluid that is contained within the adjacent bones. At the mineralizing front
of the ZCC, mechanical strains imparted upon the bony tissues may force mineral-
containing, watery interstitial fluid into the adjacent unmineralized cartilage and
extend the region occupied by the ZCC. Such mineral apposition into the hyaline
cartilage is denoted by a tidemark.
Dense subchondral tissues, including the ZCC and a SCB plate, limit fluid
movement between the bone and cartilage [ 70 ] (Table 5.2 ). The dense mineralized
endplate structure restricts movement of the bony interstitial fluid—thus preventing
mineral deposits within the bulk of healthy soft cartilage and the joint space.
However, the ZCC and its tidemarks provide evidence of mineral-filled fluid
intrusions into the soft cartilage, and nodules of mineral are commonly found to
intrude within osteoarthritic hyaline cartilage and joint spaces [ 20 ].
The tidemark, as first described by Fawns and Landells in 1953 [ 101 ], forms an
undulating surface and represents the most recently calcified border of the ZCC
(Fig. 5.2 ). Tidemark advancement may occur with aging [ 41 , 62 , 102 , 103 ], altered
loading states [ 104 , 105 ], and exercise [ 21 , 106 ]. The waviness of the tidemark may
represent differential growth rates of the mineralizing front [ 62 ] and reduce stress
concentrations as unmineralized collagen fibrils penetrate the ZCC.
Reduplication of the tidemark represents advancement of the ZCC into and
thinning of the overlying hyaline cartilage [ 107 ]. Multiple basophilic tidemarks
are present in healthy tissues, yet excessive tidemark numbers may indicate joint
degeneration or osteoarthritis [ 62 ]. Thus, the tidemark appears to indicate a miner-
alization pattern that “starts” and “stops” during normal activities [ 108 ] and may
be linked with resorption activity within the underlying SCB [ 109 ].

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