The knee joint is the largest and most complex weight-bearing joint in the human body that allows us to stand and perform our daily tasks. How do you know your knee? The human body is nature’s most perfect masterpiece, and the knee joint inside the human body has a delicate ultra-thin layer of osteochondral interface between the SOFT cartilage and the bone underneath. -chondral HARD. In this interface, the sophisticated structures and excellent force transfer properties enable the knee to resist fatigue damage over a lifetime of loading cycles. The interface’s secret to successfully preventing such damage should lie in its well-designed micro-nano structures and multi-level graduated compositions. Thus, revealing this ultra-thin interface is essential for understanding its superb mechanics and for the future design of composite soft-hard interface materials.
In this context, the team led by OUYANG Hongwei at Zhejiang University Medicine School conducted a high-resolution analysis of the osteochondral interface in human knee joints. This is the first time that researchers have identified osteochondral interface tissue in terms of microstructure, component assembly and tissue mechanics. In addition, they discovered the mechanism of the super-powerful force transfer and fatigue-resistant adhesion of the ultrathin osteochondral interface. They identified a 20–30 μm ultra-thin calcified region with two-layer micronano structures of osteochondral interface tissue in the human knee joint, which exhibited characteristic biomolecular compositions and complex nanocrystal assembly. The results of finite element simulations revealed that in this region an exponential increase in modulus (3 orders of magnitude) was conducive to force transmission.
The results of their research were published in an article titled “Identification of ultra-thin osteochondral interface tissue with specific nanostructure at the human knee jointin the journal Nano Letters.
In their study, the researchers took normal cartilage tissue and identified the osteochondral interface tissue by histological staining. Using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) linear scanning, they obtained an ultra-thin graded calcified region, spanning 20-30 μm. By analyzing the transition from the microstructure to the interface and the assembly scheme of HAp, they found the transition from the porous structure to the dense structure. Meanwhile, HAp showed morphology variations across the interface, indicating the maturity of the assembly. The spatially graded distribution of HAp was beneficial in reducing stress concentration and promoting force transmission.
Osteochondral interface microstructure transition
The schematic view of the osteochondral interface indicated two-step modulus increments, specifically the 3-order-of-magnitude increments in the 30 μm spatial range. FEA results further demonstrated that this modulus transition facilitated mechanical conduction.
Biomechanics of osteochondral interface tissues
The tissue modulus map is intimately related to the variation of the two-layer micronano structure at the interface. Besides the structure, the displacement of the gradient of the compositions at the interface can also modulate its mechanical function by redistributing the stresses. Therefore, the researchers further examined the compositional assembly of the interface at multiple scales. Using XRD and Raman spectroscopy, they found that inorganic nanocrystals at the osteochondral interface were dominated by carbonate-substituted HAp. With the extension of the interface, the rate of carbonate substitution decreased, the mineral crystallinity increased, the HAp composition gradually increased, and the calcium-phosphorus ratio increased from 1.2 to 1.6. These implied that HAp gradually matured at the interface. With the help of HRTEM, SAED and ELLS, the heterogeneity of HAp crystal assembly was confirmed at the nanoscale. HAp with nanoscale heterogeneity was found to be insensitive to cracking and therefore could promote force conduction through energy dissipation.
Compositional analyzes and nanoscale heterogeneity of HAp at the osteochondral interface
In addition, the researchers examined the precise protein expression profiles at the interface using LC-MS/MS and found that the interface tissue had high expression of elastic protein-titin, which could uptake energy by reversible deformation and transmitting stress, thus helping to maintain elasticity and mechanical conduction at the interface.
Quantitative analysis of the proteome of the difference in expression of
osteochondral interface to AC and SB
“A combination of characterization of the microstructural, micromechanical, nanocompositional and biomolecular characteristics of the interface revealed the mechanism underlying the hardening properties of the cartilage-bone interface tissue,” said Professor Ouyang. “The identified mechanism of the soft-to-hard interface allows efficient force transfer in a certain direction, thus laying the foundation for future approaches to designing biocomposite interface materials.”