(H-317) Implantable Magnetohydrodynamics Pump to Generate Fluid Flow at a Femur Fracture

Introduction:: There is a lack of early mechanical intervention options to improve healing outcomes of severe femur fractures, thus contributing to the 5-10 % of delayed and non-unions cases in the 250,000 annual femur fractures in the United States [1]. Common intervention approaches such as functional loading treatments often start after a period of time post-injury to avoid disruption of the regrown callus, missing the early time points for treatment delivery. In recent years, there is increasing evidence showing that fluid shear stress, in addition to strain, are involved in bone tissue growth and maintenance, and are involved in cell differentiation following fracture formation [2] . Furthermore, increased fluid flow in femoral fractures can increase nutrient transport and improve the healing process. However, the role of fluid flow-induced shear stress in regulating the actions of bone cells in vivo has been largely unexplored due to the lack of implantable device that can isolate compressive force and fluid flow in vivo. The goal of this project is to create an implantable, non-mechanical based device that can generate fluid flow and fluid shear stress in vivo. One way to achieve this is by using a magnetohydrodynamics (MHD) pump, a device that utilizes the interaction of electric current and DC magnetic flux to generate a force that propels fluid across a channel, which in this case, the fractured bone gap.


Materials and Methods:: The implantable MHD pump (Fig. 1) consists of an implantable bone plate, a pair of electrodes, a battery-powered control board, and external magnets placed on a 3D printed platform. The fluid flow at the fracture site was controlled by activating the implanted electrodes wirelessly via Bluetooth. The fabricated pump was tested through bench top experiments, followed by a pilot in vivo feasibility study (n = 2). The in vivo feasibility study was conducted to ascertain that the fabricated device was functional and safe to use prior to a long-term animal study. Briefly, the fabricated device consisting of the bone plate along with electrodes and the transceiver pack was implanted in 14-week Wistar rats after creating a unilateral 2 mm segmental defect on the femur. The subjects were exposed to MHD-induced fluid flow for 20 minutes per day for two consecutive days followed by one day of recovery. The study lasted for two weeks and one of the rats received three total treatments and the second eight treatments. At the end of the study, the animals were euthanized using CO₂ and ex vivo µCT scans of the femurs were taken, and the amount of bone volume quantified.

Results, Conclusions, and Discussions::
Result and Discussion: Bench top experiments showed that the velocity of fluid flow can be controlled by varying the input voltage or changing the magnetic field strength. Moreover, the stiffness of the bone plate which was fabricated using PEEK was much higher than bone plates fabricated using polysulfone or UHMWPE. The higher stiffness of the fabricated fixation plate allows us to effectively reduce the effects of compressive strain and study only the influence of fluid flow-induced shear on bone regeneration. The data from the feasibility study shows that the MHD pump is functional in vivo and did not result in any adverse outcomes. Both rats were active and pain free following surgery and started using the leg that was operated on two days after surgery.  At the end of the feasibility study, the data showed that the rat with a total of eight treatments had more bone volume than the rat that received three treatments. As the goal of this study was to demonstrate the device safety, no conclusion was derived from the results due to the lack of statistical power.





Conclusion: The preliminary results obtained through the feasibility study show that an implantable MHD pump can be used to explore the role of FSS in vivo. Moreover, the device can be used to explore the potential of fluid shear stress stimulation as a treatment path to improve healing outcomes in delayed and non-union fractures.


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References (Optional): :
[1] C. M. Court-Brown and B. Caesar, “Epidemiology of adult fractures: a review,” Injury, vol. 37, no. 8, pp. 691–697, 2006.

[2] C. Wittkowske, G. C. Reilly, D. Lacroix, and C. M. Perrault, “In vitro bone cell models: impact of fluid shear stress on bone formation,” Front. Bioeng. Biotechnol., vol. 4, p. 87, 2016.