The knee joint is the largest and mostcomplex weight-bearing joint in the human body that enables us to stand andcarry out our daily tasks. How much do you know your knee? Human body is themost perfect masterpiece of nature, and the knee joint inside human body has adelicate ultrathin layer of osteochondral interface between SOFT cartilage andHARD subchondral bone. In this interface, the sophisticated structures andexcellent force transfer properties allow the knee to resist fatigue damageover a lifetime of loading cycles. The interface’s secret to successfully preventingsuch damage should lie in its well-designed micro-nano structures and gradedcompositions in multiple levels. Thus, revealing this ultrathin interface iskey to understanding its superb mechanics and for the future design ofsoft-hard composite interface materials.

Against this backdrop, the team led byOUYANG Hongwei at the Zhejiang University School of Medicine conducted ahigh-resolution analysis of the osteochondral interface in human knee joints.It is the first time that researchers identified the osteochondral interfacetissue from the perspectives of microstructure, component assembly and tissuemechanics. Moreover, they discovered the mechanism for the super-strong forcetransfer and fatigue-resistant adhesion of the ultrathin osteochondralinterface. They identified an ultrathin ～20−30 μm graded calcified region with two-layered micronanostructures of osteochondral interface tissue in the human knee joint, whichexhibited characteristic biomolecular compositions and complex nanocrystalsassembly. Results from finite element simulations revealed that within thisregion, an exponential increase of modulus (3 orders of magnitude) wasconducive to force transmission.

Their research findings were published inan article entitled “Identification of an Ultrathin Osteochondral Interface Tissuewith Specific Nanostructure at the Human Knee Joint” in the journal NanoLetters.

In their study, the researchers collectednormal cartilage tissues and identified the osteochondral interface tissue viahistological staining. Using scanning electron microscopy (SEM) and energydispersive X-ray (EDX) line scan, they obtained an ultrathin graded calcifiedregion, spanning over 20-30 μm. By analyzing the transition of themicrostructure at the interface and the assembly pattern of HAp, they found theshift from the porous structure to the dense one. Meanwhile, HAp showedvariations in morphology across the interface, denoting the maturity of theassembly. The spatially graded distribution of HAp was beneficial to reducingstress concentration and promoting force transmission.

Microstructure transition of theosteochondral interface 

The schematic view of the osteochondralinterface indicated two-staged modulus increments, particularly the 3 orders ofmagnitude increment in the 30 μm spatial range. The FEA results furtherdemonstrated that this modulus transition facilitated mechanical conduction.

Biomechanics of the osteochondral interfacetissues

The tissue modulus map is intimately boundup with the two-layered micronano structure variation at the interface. Inaddition to the structure, the gradient shift of compositions at the interfacecan also modulate its mechanical function by redistributing stress. Therefore,the researchers further examined the compositional assembly of the interface atmultiple scales. Using XRD and Raman spectroscopy, they found that inorganicnanocrystals at the osteochondral interface were dominated by carbonatedsubstituted HAp. With the extension of the interface, the carbonatedsubstitution rate decreased, mineral crystallinity increased, the HApcomposition gradually increased, and the calcium-phosphorus ratio increasedfrom 1.2 to 1.6. These implied that HAp became gradually mature at theinterface. With the help of HRTEM, SAED and ELLS, the heterogeneity of HApcrystal assembly was confirmed at the nanoscale. HAp with nanoscaleheterogeneity turned out to be insensitive to cracking and thus could promoteforce conduction through energy dissipation.

Compositional analyses and nanoscaleheterogeneity of HAp at the osteochondral interface

Besides, the researchers examined theprecise protein expression profiles at the interface using LC-MS / MS and foundthat the interface tissue had a high expression of elastic-responsiveprotein-titin, which could absorb energy through reversible deformation andtransmit stress, thereby helping maintain elasticity and mechanical conductionat the interface.

Quantitative proteome analysis of thedifference in expression of osteochondral interface compared to AC and SB

“A combination ofcharacterization of microstructural, micromechanical, nanocompositional, andbiomolecular features of the interface revealed the mechanism underlyingtoughening properties of the cartilage-to-bone interface tissue,” said Prof.Ouyang. “The identified mechanism of the soft-to-hard interface allowseffective force transfer in a certain direction, thus laying a foundation forfuture approaches seeking to design biocomposite interface materials.”