Michael Yancey - Dr. Ting Cong - Week 2

This week I continued 3D analysis work on the femoral impaction grafting project—described in last week's post. The additional femurs analyzed this week matched the defect volume patterns established by prior femurs. Specifically, the defect volume was approximately 2000 cubic millimeters after simulated failure and compression, near 0 after repair, and between 200-800 cubic millimeters after compressing the repaired bone. 

Further analysis was done on the mechanical response of the bone during loading with the goal of obtaining the force-displacement slope and linear displacement of each femur after simulated failure and after repair by compressing the defect area to 800N in force-controlled loading on an axial tensile tester. Force-displacement slopes are calculated over the 600-800N range, because force-displacement responses are linear in this range for all samples.

- The average linear displacement at 800N was 8.38±0.87 mm for femurs after simulated failure and 4.07±0.87 mm after subsequent repair. Paired t-tests demonstrate significance (p<0.05) when comparing displacements after failure and after repair.

- The average force-displacement slope (stiffness) in the linear region was 376.27±54.06 N/mm for femurs after simulated failure and 356.06±86.69 N/mm after subsequent repair. Paired t-tests demonstrate no significance when comparing displacements after failure and after repair. However, there is a clear visual difference in force-displacement shape before and after repair, suggesting a more in-depth analysis of force-displacement characteristics may be warranted. Specifically, before repair, the bone demonstrates very low stiffness (slope) at low loads that rapidly increases as the load approaches 600-800N; after repair, the force-displacement response tends to be more linear over the whole load range from 0-800N. I plan to analyze the "toe-region" of the force-displacement responses to better quantify this difference.

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Additionally, I worked with the Khormaee lab on their alginate hydrogel project. In brief, the goals of this project are to use alginate hydrogels to biomimetically model a peri-implant hematoma, the soft microenvironment that cells must interact with before they can integrate with the stiff titanium surface of an implant. The properties of the peri-implant microenvironment affects various factors that can impact bony integration onto surgical implants—including cellular migration and osteogenic differentiation. In the short-term, this project seeks to understand how altering the microenvironment stiffness and viscoelasticity will affect cellular migration speed and whether or not cells migrate towards a stiffer titanium surface.

To achieve this goal, we create alginate hydrogels which are cross-linked through the introduction of calcium. Increasing calcium concentration increases the stiffness of the resulting hydrogels, while increasing the molecular weight of the alginate increases viscoelasticity. Previously we worked on tuning both factors to obtain four different alginate hydrogel types—each possible combination of high or low stiffness and high or low viscoelasticity.

This week, we created one of each gel type, with integrated mesenchymal stem cells, formed on the surface of lumbar interbody cage implants. Next week, we will be imaging these gels in live cell chambers using widefield microscopy to obtain a z-stack of images over time for each gel. These will later be analyzed to determine cellular migration rates and if there's any pattern of migration towards or away from the titanium implant surface.

Below is an example image of the gels we create on titanium.

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Lastly, on Thursday, I had the opportunity to attend the 2024 HSS Clinical Fellow Research Podium Presentations. Dr. Ting Cong presented on our femoral impaction grafting project, but I especially enjoyed the opportunity to see the tools that other fellows used on their various research projects. For instance, quantitative MRI was used in a few projects that led me to think about whether that tool could be useful for obtaining additional data in our femoral head project. 

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