Abstract
Recent insights suggest that the osteochondral interface plays a central role in maintaining healthy articulating joints. Uncovering the underlying transport mechanisms is key to the understanding of the cross-talk between articular cartilage and subchondral bone. Here, we describe the mechanisms that facilitate transport at the osteochondral interface. Using scanning electron microscopy (SEM), we found a continuous transition of mineralization architecture from the non-calcified cartilage towards the calcified cartilage. This refurbishes the classical picture of the so-called tidemark; a well-defined discontinuity at the osteochondral interface.Using focused-ion-beam SEM (FIB-SEM) on one osteochondral plug derived from a human cadaveric knee, we elucidated that the pore structure gradually varies from the calcified cartilage towards the subchondral bone plate. We identified nano-pores with radius of 10.71 ± 6.45 nm in calcified cartilage to 39.1 ± 26.17 nm in the subchondral bone plate.The extracted pore sizes were used to construct 3D pore-scale numerical models to explore the effect of pore sizes and connectivity among different pores. Results indicated that connectivity of nano-pores in calcified cartilage is highly compromised compared to the subchondral bone plate. Flow simulations showed a permeability decrease by about 2000-fold and solute transport simulations using a tracer (iodixanol, 1.5 kDa with a free diffusivity of 2.5 × 10−10 m2/s) showed diffusivity decrease by a factor of 1.5. Taken together, architecture of the nano-pores and the complex mineralization pattern in the osteochondral interface considerably impacts the cross-talk between cartilage and bone.
Highlights
The osteochondral interface bridges the articular cartilage to the subchondral bone plate and balances the un-matching mechanical properties of non-calcified soft cartilage and the stiff⇑ Corresponding author at: Department of Orthopaedics, University Medical subchondral bone plate
A large amount of studies has focused on solute transport across articular cartilage, since it is vital for the many biological activities of chondrocytes (Quinn et al, 2001)
Using FIB-SEM, we observed that the calcified cartilage contains segregated non-mineralized regions near the non-calcified cartilage, which enhances transport of solutes due to the presence of gel-like material comprising of water, collagen and glycosaminoglycans (Region I, Fig. 2) (Sophia Fox et al, 2009)
Summary
In conjunction with other studies, confirmed the transport of solutes across the osteochondral interface (Burstein et al, 2001; Pan et al, 2012; Pan et al, 2009; Pouran et al, 2016a). This factor may play a role in the cross-talk between cartilage and bone and highlights a possible signal transduction mechanism in articulating joints involved in the development and recovery processes of cartilage and related diseases, such as osteoarthritis (Pan et al, 2009; Weinans et al, 2012). The existence and the exact role of porosity inside the deep cartilage layers, the extracellular matrix of calcified cartilage and subchondral bone plate have not been fully clarified
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