Measuring microscale strain fields in articular cartilage during rapid impact reveals thresholds for chondrocyte death and a protective role for the superficial layer

Abstract

Articular cartilage is a heterogeneous soft tissue that dissipates and distributes loads in mammalian joints. Though robust, cartilage is susceptible to damage from loading at high rates or magnitudes. Such injurious loads have been implicated in degenerative changes, including chronic osteoarthritis (OA), which remains a leading cause of disability in developed nations. Despite decades of research, mechanisms of OA initiation after trauma remain poorly understood. Indeed, although bulk cartilage mechanics are measurable during impact, current techniques cannot access microscale mechanics at those rapid time scales. We aimed to address this knowledge gap by imaging the microscale mechanics and corresponding acute biological changes of cartilage in response to rapid loading. In this study, we utilized fast-camera and confocal microscopy to achieve roughly 85 μm spatial resolution of the cartilage deformation during a rapid (~3 ms), localized impact and the chondrocyte death following impact. Our results showed that, at these high rates, strain and chondrocyte death were highly correlated (p<0.001) with a threshold of 8% microscale strain norm before any cell death occurred. Additionally, chondrocyte death had developed by two hours after impact, suggesting a time frame for clinical therapeutics. Moreover, when the superficial layer was removed, strain – and subsequently chondrocyte death – penetrated deeper into the samples (p<0.001), suggesting a protective role for the superficial layer of articular cartilage. Combined, these results provide insight regarding the detailed biomechanics that drive early chondrocyte damage after trauma and emphasize the importance of understanding cartilage and its mechanics on the microscale.

Post‐traumatic osteoarthritis of the ankle: A distinct clinical entity requiring new research approaches

“…This reductionist approach allows investigators to gain new insight into the cartilage‐specific contributions to PTOA ex vivo, investigate mechanical thresholds for peracute cellular and subcellular responses to cartilage injury, and test targeted drugs to prevent PTOA in animal models. For example, a recent study examined microscale mechanics and corresponding chondrocyte death in articular cartilage following rapid impact injury . This new technique revealed that chondrocyte death is highly correlated with a threshold of 8% microscale strain.…”

“…When the superficial layer of the cartilage was removed, cell death penetrated deeper into the cartilage, indicating a protective role for the superficial layer. Additionally, chondrocyte death developed within 2 h of impact, suggesting a narrow window for early therapeutic intervention after injury . In summary, an overly aggressive model with rapid progression to end‐stage OA may not provide a sufficiently dynamic range of disease to evaluate therapeutic effects in a preclinical ankle model and a single, rapid impact model is most consistent with the likely etiopathogenesis of ankle PTOA.…”

Influence of the pericellular and extracellular matrix structural properties on chondrocyte mechanics

“…For such models, the assumption of continuous attachment between chondrocytes and their environment could also be updated to discrete focal cell‐matrix attachment sites . Direct validation of chondrocyte strains in the cartilage ECM have yet to be performed and may become more feasible in the near future with new developments in the field . Finally, we focused on analyzing the micromechanical environment of chondrocytes located in the middle zone of the cartilage under unconfined compression.…”

“…49 Direct validation of chondrocyte strains in the cartilage ECM have yet to be performed and may become more feasible in the near future with new developments in the field. 50 Finally, we focused on analyzing the micromechanical environment of chondrocytes located in the middle zone of the cartilage under unconfined compression. Given the depth-varying non-homogeneous strain field that was induced in both healthy and OA conditions, the effects of depth-wise variations in the chondrocyte shapes and PCM thickness and properties on chondrocyte deformations should be accounted for in future studies.…”

Hydrogels and bioreactors for cartilage research and functional tissue engineering

“…While this initially may seem biologically insignificant, it has to be considered that the baseline expression of chondrogenic marker genes of human chondrocytes in free swelling GelMA-HAMA constructs is already high. to chondrocyte death due to excessive magnitudes of mechanical stresses and strains [349], as well as high stress and strain rates in response to impact-loading [350].…”