Structural clues to articular calcified cartilage function: A descriptive review of this crucial interface tissue

Abstract

Articular calcified cartilage (ACC) has been dismissed, by some, as a remnant of endochondral ossification without functional relevance to joint articulation or weight‐bearing. Recent research indicates that morphologic and metabolic ACC features may be important, reflecting knee joint osteoarthritis (OA) predisposition. ACC is less investigated than neighbouring joint tissues, with its component chondrocytes and mineralised matrix often being either ignored or integrated into analyses of hyaline articular cartilage and subchondral bone tissue respectively. Anatomical variation in ACC is recognised between species, individuals and age groups, but the selective pressures underlying this variation are unknown. Consequently, optimal ACC biomechanical features are also unknown as are any potential locomotory roles. This review collates descriptions of ACC anatomy and biology in health and disease, with a view to revealing its structure/function relationship and highlighting potential future research avenues. Mouse models of healthy and OA joint ageing have shown disparities in ACC load‐induced deformations at the knee joint. This raises the hypothesis that ACC response to locomotor forces over time may influence, or even underlie, the bony and hyaline cartilage symptoms characteristic of OA. To effectively investigate the ACC, greater resolution of joint imaging and merging of hierarchical scale data will be required. An appreciation of OA as a ‘whole joint disease’ is expanding, as is the possibility that the ACC may be a key player in healthy ageing and in the transition to OA joint pathology.

Cryogenic non-invasive 3D X-ray phase-contrast imaging of unfixed, intact mouse joints reveals shifting chondrocyte hypertrophy across the endochondral interface

“…Our data also indicate STR/Ort trans-zonal and ACC chondrocytes are larger than in CBA. Excessive hypertrophy in OA has been reported, and is linked with decreased capacity to form structurally-sound matrix and increased MMP-13 expression (18,33,55) .…”

“…Our data also indicate STR/Ort trans-zonal and ACC chondrocytes are larger than in CBA. Excessive hypertrophy in OA has been reported, and is linked with decreased capacity to form structurally-sound matrix and increased MMP-13 expression (18, 33, 55) . Larger STR/Ort lacunae described here may indicate exuberant hypertrophy, but may alternatively reflect greater fluid uptake, more cells in a stage of chondroptosis, differences in cytoskeletal organisation, or a combination (18) .…”

Investigating the interfaces of the epiphyseal plate: An integrated approach of histochemistry, microtomography and SEM

“…intermediate between soft cartilage and bone (Boyde, 2021;Mente & Lewis, 1994) and it seems to be essential for a stronger mechanical junction between the two tissues (Evans & Pitsillides, 2022), but on the epiphyseal interface the thickness of calcified cartilage here seems minimal and the cartilage-bone contact is limited to discontinuous, widely spaced trabeculae.…”

Osteochondral Interface: Regenerative Engineering and Challenges

“…31 It is believed to prevent excessive diffusion of nutrients from the marrow cavity to the articular cartilage. 36 The base of the OC tissue is the subchondral zone. "Subchondral zone" is used interchangeably along with "subchondral bone plate" to specify the cortical end plate or bony lamella that is found below the calcified zone of the articular cartilage tissue.…”

“…The cement line is between 9 and 277 μm distance from the tidemark, and exhibits a wavier nature relative to tidemark . It is believed to prevent excessive diffusion of nutrients from the marrow cavity to the articular cartilage …”