(L-454) Use of a novel magnetically actuated compression system to study the accumulation of strain during dynamic loading in human osteochondral explants

Introduction:: The high water content of articular cartilage allows this biphasic tissue to withstand large compressive loads through fluid pressurization. The compressive loading system presented here, termed the “MagnaSquish”, provides new capabilities for quantifying the effect of rehydration on cartilage behavior during cyclic loading. Compression of cartilage drives fluid out of local regions of the issue while simultaneously deforming the solid extra-cellular matrix. If the tissue does not sufficiently recover its fluid content in between loading cycles, strain can persist and accumulate to higher levels with each successive cycle. Typical experimental systems for cartilage biomechanics use continuous contact between the platen and sample, which affects tissue rehydration by compressing the top layer of cartilage that is critical for fluid re-entry. To more accurately investigate strain accumulation during dynamic loading, we developed a magnetically actuated device that provides full lift-off of the platen in between loading cycles.


Materials and Methods:: A total of 93 osteochondral explants were harvested from the tali of 13 cadaveric donors with no clinical history of osteoarthritis or macroscopic cartilage damage. The 3 mm diameter explants were press-fit into custom restraining rings, bathed in cell culture media, and situated into the “MagnaSquish” within a tissue culture incubator. The lid of the sample dish is replaced by a custom 3D-printed carriage that contains the magnetic actuator and directly threads onto the dish bottom to maintain sterility. The z-position of a large neodymium magnet below the sample determines whether there is sufficient attractive force to bring the platen into contact with the sample. Magnet position was controlled by PID logic implemented on a System-on-Chip (SoC) on the BeagleboneBlack®. We applied 750 cycles of compressive loading using a pattern that mimics walking, with 1.5 MPa peak stress in the form of a Haversine function. For “Haversine-1”, the active phase of loading took ~600 msec and the dwell period took ~400 msec. For “Haversine-2”, the dwell period was extended an additional second to provide longer recovery time and additional fluid re-entry. Ten minutes of recovery were provided after cyclic loading was complete. We captured two-dimensional profiles of explants at 30 frames per second throughout the entirety of loading and recovery. Strain measurements were extracted from the tissue profiles using a U-Net-based deep learning algorithm to circumvent manual tracing.


Results, Conclusions, and Discussions:: Images of the tissue were captured at 30 frames per second, allowing for detailed quantification of the loading and recovery behavior during each cycle. A representative trace of cartilage strain is shown in Figure 1. The maximum strain during a given cycle is denoted as the “total strain”, whereas the “accumulated strain” refers to the residual strain that is still present right before application of the next loading cycle. Figure 2 shows the accumulated and total strain at each of 750 loading cycles for a single explant. The representative two-dimensional profiles show that the explant has accumulated ~5% strain by cycle 750. The explant nearly returns to the original tissue volume by the end of the ten-minute recovery period. Figure 3 plots the axial strain for a set of explants challenged with the Haversine-1 (standard recovery time) or Haversine-2 (extended recovery time) loading protocol. The additional dwell time between loading cycles with Haversine-2 led to a decrease in strain accumulation beginning at 150 cycles. These data are consistent with the concept that efficient fluid re-entry is a critical component of load resistance. Importantly, the loss of glycosaminoglycans with aging reduces the fixed negative charge that facilitates fluid re-entry. Further, changes to gait that occur after joint injury or during osteoarthritis progression can alter the distribution of load across the cartilage. Future work will take advantage of the maintained sterility during compressive loading to perform repeated loading experiments to assess the effects of aging and distinct loading profiles on cartilage health. These insights will be used to support the development of novel therapeutic approaches for osteoarthritis.


Acknowledgements (Optional): : We thank the Gift of Hope Tissue and Donor Network, donor families, and Mrs. Arnavaz Hakimiyan for tissue procurement.


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