The Bone Cartilage Interface and Osteoarthritis

Abstract

This review describes results obtained with tissue from prior studies of equine and human osteoarthritis (OA). The main methods considered are scanning electron microscopy, novel methods in light microscopy and X-ray Micro-tomography. The same samples have been re-utilised in several ways. The tissues described are hyaline articular cartilage (HAC; or substitutes), with its deep layer, articular calcified cartilage (ACC), whose deep surface is resorbed in cutting cone events to allow the deposition of subchondral bone (SCB). Multiple tidemarks are normal. Turnover at the osteochondral (ACC-HAC-SCB) junction is downregulated by overload exercise, conversely, during rest periods. Consequent lack of support predisposes to microfracture of the ACC-SCB plate, in the resorption-related repair phase of which the plate is further undermined to form sink holes. The following characteristics contribute to the OA scenario: penetrating resorption canals and local loss of ACC; cracking of ACC and SCB; sealing of cracks with High-Density Mineral Infill (HDMI); extrusion of HDMI into HAC to form High-Density Mineral Protrusions (HDMP) in HAC which may fragment and contribute to its destruction; SCB marrow space infilling and densification with (at first) woven bone; disruption, fibrillation and loss of HAC; eburnation; repair with abnormal tissues including fibrocartilage and woven bone; attachment of Sharpey fibres to SCB trabeculae and adipocyte-moulded extensions to trabeculae (excrescences).

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

“…Instead, osteochondral junction remodelling results in gradual thinning of the ACC with age, as osteoclasts (known as chondroclasts in cartilage) erode ACC adjacent to SCB, and osteoblasts replace it with bone (Boyde, 2021). Further work by Doube et al (2007) in horses has identified the rate of chondroclastic resorption, as opposed to tidemark linear accretion rate, as the major determinant of ACC thickness in young horses.…”

“…Highly mineralised infill material is then deposited in the ACC crack and so is extruded into the HAC tear with which it is continuous (Boyde, 2021). HDMPs therefore are not surrounded by tidemark (Boyde et al, 2011).…”

“…In this way, the ACC is tightly interlocked between its neighbouring bone and HAC layers, with two interfaces; an upper non‐mineralised:mineralised interface across a collagen type II continuum and a deep collagen type II:type I interface across the mineralised ACC and SCB phases. Disruptions of the clear, upper tidemark interface are a hallmark pathological feature of OA (Boyde, 2021 ). This review will describe ACC and its related structures in detail, containing a minimal degree of natural overlap due to the close associations between different ACC components.…”

Distribution, structure, and mineralization of calcified cartilage remnants in hard antlers

“…There are, however, some exceptions to this rule. For example, a thin layer of articular calcified cartilage that is more mineralized than subchondral bone is regularly present in diarthrodial joints of the adult skeleton ( Ferguson et al, 2003 ; Boyde, 2021 ). Persistence of calcified cartilage islands also occurs in long bones of rats and mice due to the lack of bone remodeling in these small mammals ( Bach-Gansmo et al, 2013 ; Jing et al, 2017 ).…”

Osteoarthritis—Slow but Steady Steps Forward

“…Alan Boyde revisits the theme of aberrant mechanical loading, complex tissue interactions, rare diseases, biomarkers and animal models of osteoarthritis in a paper that considers what role the bone-cartilage interface plays in OA [5]. The article and the accompanying and extensive supplementary material are filled with images that should enthral and inform readers.…”